PART III Recommendations Q.400 to Q.490 SPECIFICATIONS OF SIGNALLING SYSTEM R2 MONTAGE: PAGE 32 = PAGE BLANCHE SIGNALLING SYSTEM R 2 INTRODUCTION General Signalling System R2 is used as an international signalling system within international regions (world numbering zones). More- over, System R2 can be used for integrated international/national signalling if it is employed, in conformity with the present specifications, as a signalling system in the national networks of the region concerned. Suitable for both automatic and semi-automatic working it offers a high reliability in transmission of the information necessary for setting up a call. It allows for rapid call set-up and provides sufficient signals in both direc- tions to permit the transmission of numerical and other information relating to the called and calling subscribers' lines and to increase routing facilities. System R2 is specified for one-way operation on analogue and digital transmission systems and for both-way operation on digital transmission systems. Distinction is made between line signalling (supervisory signals) and interregister signalling (call set-up control signals). Versions of line signalling are specified for use on 4-wire carrier or PCM circuits. Nevertheless, the interregister signalling specified can also be used on 2-wire circuits. System R2 is suitable for use on satellite links and 3 kHz spaced carrier circuits. It is not designed for use on transmission systems with speech interpolation but it may be used on digital transmission systems with speech interpolation as long as transparency for pulsed interregister signals is guaranteed. The interregister signalling is a compelled multifrequency code system. This offers the possibility of end-to-end signalling and to take advantage of modern switching systems by providing a sufficient number of signals in both directions. System R2 is capable of normal interworking with other CCITT signalling systems. Line signalling The following versions of line signalling are specified: - line signalling for carrier systems called the analogue version; - line signalling for PCM systems called the digi- tal version. Since multifrequency signalling permits exchange of a large amount of information between registers, the quantity of informa- tion that has to be transmitted in the form of line signals is small. The versions of line signalling for System R2 have been designed accordingly. The analogue line signalling version stand- ardized originally for use on international circuits is also suit- able for national working. An analogue (in-band) line signalling version is only described for the specific use on 3 kHz spaced car- rier circuits of international submarine cables. The analogue version | s link-by-link using an out-band, low-level continuous tone-on-idle signalling method. Signal transfer simply involves the transition from one signalling condi- tion to the other, except for the clearing sequence which is based on additional timing criteria. It is necessary to have a device for protection against the effect of interruption in the signalling channel since interruption of the signalling frequency simulates a false seizure or answer signal (interruption control). The signal- ling frequency is transmitted at a low level which avoids overload- ing of the transmission system when the frequency is sent continu- ously in both directions on all circuits in the idle state. The digital version | s link-by-link using two signalling channels in each direction of transmission per speech circuit. The signalling channels are two of the four provided for channel asso- ciated signalling in a 2048 kbit/s primary multiplex (see Recommendation G.732). Protection against the effects of faulty transmission is provided. The analogue version and the digital version of the line sig- nalling can be converted to each other by a transmultiplexer or other conversion equipment. Such equipment forms a conversion point between analogue (FDM) transmission on one hand and digital (PCM) transmission on the other hand. The System R2 line signalling, how- ever, must be treated separately because for both transmission sys- tems different line signalling versions are specified. Although both versions are described in some detail, a description of the actual conversion is given in Recommendation Q.430. Although these versions do not include a forward-transfer signal, forward-transfer signalling may be introduced for international working by bilateral agreement. Interregister signalling The interregister signalling is performed end-to-end using a 2-out-of-6 in-band multifrequency code with forward and backward compelled signalling. The signalling frequencies do not therefore overlap with the line signalling frequency and differ according to the direction of transmission with a view to possible use of the multifrequency part of the system on 2-wire circuits. System R2 is designed to use six signalling frequencies (1380, 1500, 1620, 1740, 1860 and 1980 Hz) in the forward direction and six signalling frequencies (1140, 1020, 900, 780, 660 and 540 Hz) in the backward direction. Nevertheless for national application less signalling frequencies may be used. End-to-end signalling is a method for signalling between registers over two or more links in tandem without signal regenera- tion in intermediate exchanges (see Figure 1). Figure 1, p. With this signalling method, in general only the address information needed for routing the call through an intermediate (transit) exchange is transferred from the outgoing register to the incoming register. In the intermediate exchange the speech path is immediately through-connected and the incoming register released. Then the outgoing register can exchange information directly with the incoming register of the next exchange. Such end-to-end signal- ling is advantageous as it reduces the interregister signalling equipment needed and minimizes the holding time of registers in transit exchanges. System R2 has been designed to allow end-to-end interregister multifrequency signalling over several links in tandem. However, in circumstances where transmission conditions do not comply with the requirements specified for System R2 and might consequently jeop- ardize the exchange of the interregister signals, or in case of using System R2 via a satellite link, the overall multi-link con- nection is divided into sections, each with its individidual inter- register signalling (signals being then relayed and regenerated by a register at the point where the division is made). In the case of satellite working, the register at the incoming end of the satellite link must act as an outgoing R2 register (see also Recommendation Q.7). Generally transmission conditions in a national network comply with the requirements specified for System R2 and as such allow end-to-end signalling over complete connections between local exchanges. In the case of international System R2 working transmission conditions impose division into at least two signalling sections, when both the outgoing and incoming countries use System R2 in the national network. The division must be made in an exchange in the outgoing country. The relaying or regenerating register situated at the point where the division is made is called the outgoing inter- national R2 register . In the case when System R2 is not used in the outgoing country the outgoing international R2 register receives address information via a national signalling system and it controls the call set-up over the outgoing System R2 signalling section. The outgoing R2 register | s generally defined as a register situated at the outgoing end of a signalling section on which System R2 interregister signalling, according to the present specifications, is used. It controls the call set-up over the whole signalling section. It sends forward interregister signals and receives backward interregister signals. The outgoing R2 register receives information via the preceding links of the connection in a form used by the signalling system applied over the last of these links; this system may be System R2, a decadic pulse system, or any other system. The preceding link may also be a subscriber's line. When an R2 register in a transit exchange | s working accord- ing to the definition above it is also called an outgoing R2 regis- ter outgoing international R2 register is the special case of the outgoing R2 register when the outgoing signalling section comprises at least one international link. The incoming R2 register | s defined as a register situated at the incoming end of a link on which System R2 interregister sig- nalling, according to the present specification, is used. It receives forward interregister signals via the preceding link(s) and sends backward interregister signals. The information received is used completely or in part for the control of selection stages and may be sent in its entirety or in part to the succeeding equip- ment, in which case the signalling used for retransmission is never System R2. Interworking then takes place between System R2 and the other System. Thus every register not situated at the outgoing end of a System R2 signalling section is called an incoming R2 register independently of the exchange type. The following operational features are provided by using all the 15 forward and 15 backward combinations of the multifrequency code: - capability of transferring address information for automatic, semi-automatic and maintenance calls; - indicators in order to identify transit and maintenance (test) calls; - language digits or discriminating digit(s) for international calls; - transfer of the next digit or repetition of the last but one, the last but two, or the last but three digit on request by the register at the incoming end; - forward and backward signals for control of echo suppressors; - information concerning the nature and the origin of the call (calling party's category) can be transferred to the register at the incoming end, i.e. whether national or interna- tional, whether from an operator or from a subscriber, whether from data-transmission, maintenance or other equipment, etc.; - information concerning the nature of the circuit can be requested by the register at the incoming end, i.e. whether a satellite link is already included or not; - information on congestion, unallocated number and on the condition of the called subscriber's line can be transferred to the outgoing register, i.e. whether free, busy or out of service, etc.; - a restricted number of signals left to the dis- cretion of Administrations for allocation of national meanings. The interregister signalling specified for System R2 can also be used together with any non-standardized line-signalling system. The resulting combination is not considered to be System R2. System R2 gives potential for a short post-dialling delay using overlap operation with dialling and end-to-end signalling. The outgoing R2 register starts call set-up as soon as it has received the minimum requisite information. Therefore signal transfer starts before the complete address information is received, i.e. before the caller finishes dialling. This practice particularly applies at an outgoing R2 register where the complete address information from a subscriber or operator is stored (e.g. local registers). This is in contrast to en bloc register signalling i.e. the transmission of all the address information as a whole in one sequence starting only after complete reception of the address information. Unused signalling capacity provides potential for development and allows for future requirements yet undefined. This spare capa- city may be used for increasing the number of signals and signal- ling procedures e.g. for new services to be offered to sub- scribers. SECTION 1 DEFINITIONS AND FUNCTIONS OF SIGNALS Recommendation Q.400 1.1 FORWARD LINE SIGNALS 1.1.1 seizing signal A signal sent at the beginning of the call to initiate transi- tion of the circuit at the incoming end from the idle state to seized state. At the incoming exchange it causes the association of equipment capable of receiving register signals. 1.1.2 clear-forward signal A signal sent to terminate the call or call attempt and to release in the incoming exchange and beyond it all switching units held on the call. The signal is sent when: a) in semi-automatic working the operator of the out- going international exchange takes the plug out or performs an equivalent operation; b) in automatic working, the calling subscriber clears or performs an equivalent operation. This signal is also sent by the outgoing international exchange upon receiving a backward register signal requesting the outgoing international R2 register to clear the connection, or in the case of forced release of the connection as mentioned in Recommendation Q.118. This signal may also be sent as the result of abnormal release of the outgoing international R2 register. 1.1.3 forward-transfer signal A signal sent on semi-automatic calls when the outgoing inter- national exchange operator wants the help of an operator at the incoming international exchange. The signal will usually bring an assistance operator (see Recommendation Q.101) into the circuit. If the call is completed via an incoming or delay operator at the incoming international exchange, the signal indicates that recall of this operator is wanted. 1.2 BACKWARD LINE SIGNALS 1.2.1 seizing-acknowledgement signal _________________________ This signal is not provided in either the analogue or digital version of System R2 line signalling. Informa- tion about possible arrangements for such a signal and signalling procedures involved are contained in Annex A to the present Specifications. This signal is only used in the digital version of System R2 line signalling. A signal sent to the outgoing exchange to indicate the transi- tion of the equipment at the incoming end from the idle state to seized state. Recognition of the seizing acknowledgement signal at the outgoing end causes the state of the circuit to change from seized to seizure acknowledged. 1.2.2 answer signal A signal sent to the outgoing international exchange to indi- cate that the called party has answered the call (see Recommendation Q.27). In semi-automatic working this signal has a supervisory function. In automatic working this signal is used: - to start metering the charge to the calling sub- scriber, unless the register signal indicating no charge has been sent previously; - to start measurement of the call duration for international accounting purposes. 1.2.3 clear-back signal A signal sent to the outgoing international exchange to indi- cate that the called party has cleared. In semi-automatic working, this signal has a supervisory function. In automatic working, arrangements must be made in accordance with Recommendation Q.118, and the Notes of Recommendation Q.120, S 1.8 also apply. 1.2.4 release-guard signal A signal sent to the outgoing exchange in response to a clear-forward signal to indicate that the latter has been fully effective in returning the switching units at the incoming end of the circuit to idle condition. An international circuit is pro- tected against subsequent seizure as long as the release operations initiated by the clear-forward signal have not been completed at the incoming end. 1.2.5 blocking signal A signal sent on an idle circuit to the outgoing exchange to cause engaged conditions (blocking) to be applied to this circuit, guarding it against subsequent seizure. 1.3 FORWARD REGISTER SIGNALS 1.3.1 address signal A signal containing one element of information (digit 1, 2, . | | , 9 or 0, code 11, code 12 or code 13) about the called or calling party's number or the end of pulsing indication (code 15). For each call a series of address signals is sent (see Recommendations Q.101 and Q.107). 1.3.2 country-code and echo-suppressor indicators Signals indicating: - whether or not the country-code is included in the address information (international transit or terminal call); - whether or not an outgoing half-echo suppressor should be inserted in the first international exchange reached; - whether or not an incoming half-echo suppressor should be inserted (an outgoing half-echo suppressor having already been inserted in the connection). 1.3.3 language or discriminating digit A numerical signal occupying a predetermined position in the sequence of address signals indicating: - in semi-automatic working, the service language to be used in the incoming international exchange by the incoming, delay and assistance operators when they come in the circuit (language digit); - the automatic working or any other special characteristic of the call (discriminating digit). 1.3.4 test call indicator A signal occupying the position of the language digit when the call is originating from test equipment. 1.3.5 nature of circuit indicators Signals only sent on request by certain backward signals and using a second meaning of some signals, to indicate whether a satellite link is already included in the connection or not. 1.3.6 end-of-pulsing signal An address signal sent indicating (in semi-automatic service) that no other address signal will follow or (in automatic service) that the transmission of the code identifying the origin of the call is completed. 1.3.7 calling party's category signals A special group of signals providing, in addition to the information contained in the language or discrimination digit, sup- plementary information concerning the nature of the call (i.e. whether national or international) and its origin. Typical categories are: - operator capable of sending the forward-transfer signal; - ordinary subscriber or operator with no forward-transfer facility; - subscriber with priority; - data transmission call; - maintenance call. 1.3.8 Signals for use on the national network Some of the Group II forward signals (see Recommendation Q.441, S 4.2.3.2) have been allocated for national use. When the outgoing international R2 register receives them, it must react as specified in Recommendation Q.480. 1.4 BACKWARD REGISTER SIGNALS 1.4.1 Signals requesting transmission of address signals Five backward signals without particular names are provided; four of them are interpreted with reference to the latest address signal sent: - signal requesting the transmission of the address signal following the latest address signal sent; - signal requesting repetition of the address sig- nal preceding the latest address signal sent (last but one); - signal requesting the repetition of the last but two address signals sent; - signal requesting the repetition of the last but three address signals sent; - signal requesting the transmission or repetition of the language or discrimination digit. 1.4.2 Signal requesting information about the circuit A backward signal is provided to request the nature of the circuit. 1.4.3 Signals requesting information about the call or cal- ling party Three backward signals without particular names are provided for this purpose: - signal inquiring the calling party's category; - signal requesting the repetition of the country-code indicator; - signal inquiring whether or not incoming half-echo suppressor should be inserted. 1.4.4 Congestion signals Two congestion signals are provided: - a signal indicating international congestion, i.e. that the call set-up attempt has failed owing to congestion of the group of international circuits, or congestion in the interna- tional switching equipment, or to time-out or abnormal release of an incoming R2 register in an international transit exchange; - a signal indicating national congestion, i.e. that the call set-up attempt has failed owing to congestion in the national network (excluding a busy called subscriber's line) or to time-out or abnormal release of an incoming R2 register in a termi- nal international exchange or a national exchange. 1.4.5 address-complete signals Signals indicating that it is no longer necessary to send another address signal, and - either cause immediate passage to the speech position to enable the calling subscriber to hear a tone or a recorded announcement of the national incoming network; - or announce the transmission of a signal indicat- ing the condition of the called subscriber's line. 1.4.6 Signals indicating the condition of the called subscriber's line Six signals sent in the backward direction are provided to give information about the called subscriber's line and to indicate the end of interregister signalling. These signals are: - send special information tone a signal sent in the backward direction indicating that the special information tone should be returned to the calling party. This tone indicates that the called number cannot be reached for reasons not covered by other specific signals and that the una- vailability is of a long term nature. (See also Recommendation Q.35); - subscriber line busy a signal indicating that the line or lines connecting the called subscriber to the exchange are busy; - unallocated number a signal indicating that the number received is not in use (e.g. an unused country code or an unused trunk code or subscriber number that has not been allocated); - subscriber line free, charge a signal indicating that the called subscriber's line is free and that the call is to be charged on answer; - subscriber line free, no charge a signal indicating that the called subscriber's line is free and that the call is not to be charged on answer. This signal is used only for calls to special destinations; - subscriber line out of order a signal indicating that the subscriber's line is out-of-service or faulty. 1.4.7 Signals for use in the national network Some of the backward signals have been allocated for national use. Since not all incoming registers can know the origin of the connection and since end-to-end signalling is used, it may happen that the above-mentioned signals are sent to the outgoing international R2 register. When this register receives them it must react as indicated in Recommendations Q.474 and Q.480. SECTION 2 LINE SIGNALLING, ANALOGUE VERSION Recommendation Q.411 2.1 LINE SIGNALLING CODE 2.1.1 General The System R2 line signalling, analogue version, is intended for use on carrier circuits. The line signals are transmitted link-by-link. The code for the transmission of line signals is based on the tone-on-idle signalling method employed are equipped in each direction of transmission with a signalling channel outside the speech frequency band. When the cir- cuit is in the idle state, a low-level signalling tone is sent con- tinuously in both directions over the signalling channels. The tone is removed in the forward direction at the moment of seizure and in the backward direction when the called subscriber answers. The connection is released when the signalling tone is restored in the forward direction; release causes the tone to be restored in the backward direction. If the called party is the first to clear, the signalling tone is restored in the backward direction first. It is then restored in the forward direction either when the caller clears or when a certain interval has elapsed after recognition of the signalling tone in the backward direction. This signalling method, requiring only simple equipment, provides rapid signal recognition and retransmission. The signal transfer speed provided by continuous type signalling compensates for the need of signal repetition inherent in link-by-link transmission. The signalling system is specified for one-way operation of 4-wire carrier circuits. 2.1.2 Line conditions Tone-on or tone-off denotes a certain line signalling condi- tion. The line thus has two possible conditions in each direction, i.e. a total of four line signalling conditions. Taking into account the time sequence, the circuit may resume one of the six characteristic states shown in Table 1/Q.411. TABLE [1/Q.411] p. The transition from one signalling condition to another corresponds to the transfer of a line signal according to the definitions in S 1. To change from the release state to the idle state additional criteria (timing) are necessary to ensure a defined sequence corresponding to the transfer of the release-guard signal (see S 2.2.2.6 below). Recommendation Q.412 2.2 CLAUSES FOR EXCHANGE LINE SIGNALLING EQUIPMENT 2.2.1 Recognition time for transition of signalling condi- tion The recognition time trfor a changed condition (transition from tone-on to tone-off or vice versa) is 40 _ 10 ms | duration that the presence or absence of a direct current signal must have _________________________ Although the signalling condition (tone-on or tone-off) physically only appears in transmission equipment, it is used in this section as a reference criterion to specify functions of exchange equipment. Originally this value was (20 _ 7) ms. Since there will be no problem in the interworking between equipment having the original recognition time tr = (20 _ 7) ms, and equipment having the new recognition time tr = (40 _ 10) ms, existing equipment need not neces- sarily be changed to the value tr = (40 _ 10) ms. at the output of the signal receiver in order to be recognized as a valid signalling condition by the exchange equipment. Thus the specified value does not include the response time tr\dsof signal- ling receivers (see Recommendation Q.415). However, it is deter- mined on the assumption that there is interruption control (see Recommendation Q.416). 2.2.2 States and procedures under normal conditions 2.2.2.1 Seizure The outgoing end removes the tone in the forward direction. If seizure is immediately followed by release, removal of the tone must be maintained for at least 100 ms to make sure that it is recognized at the incoming end. 2.2.2.2 Answering The incoming end removes the tone in the backward direction. When another link of the connection using tone-on-idle continuous signalling precedes the outgoing exchange, the tone-off condition must be established on this link immediately after it is recognized in this exchange. When another signalling system is used on the preceding link, the rules for interworking are applicable. 2.2.2.3 Clear-back The incoming end restores the tone in the backward direction. When another link of the connection using tone-on-idle continuous signalling precedes the outgoing exchange the tone-on condition must be established on this link immediately after it is recognized in this exchange. When another signalling system is used on the preceding link, the rules for interworking are applicable. The pro- visions set forth in S 2.2.2.6 below must also be taken into con- sideration. 2.2.2.4 Clear-forward procedure The outgoing end restores the tone in the forward direction (see S 2.2.2.1 above). The forward connection is released and the release-guard sequence begins as soon as the changed signalling condition is recognized at the incoming end. In the outgoing exchange the circuit remains blocked until the release-guard sequence is terminated (see S 2.2.2.6 below). 2.2.2.5 Blocking and Unblocking procedure At the outgoing exchange the circuit stays blocked so long as the tone remains off in the backward direction. Restoration of the tone in the backward direction - accom- panied by the presence of the tone in the forward direction - restores the circuit to the idle state. The circuit may then be seized for a new call. 2.2.2.6 Release and release-guard sequence Release-guard must be ensured whatever the state of the cir- cuit at the moment the clear-forward signal is sent - seized prior to answer, answered or cleared by the called party. It may also happen that answering or clearing by the called party occurs when release has already begun at the outgoing exchange. The cases are described below and shown in the Figures 2/Q.412 to 4/Q.412. The exact timing is shown in Figure 5/Q.412. a) Release prior to answered state The clear-forward signal is sent from the outgoing end by res- toring the tone in the forward direction (see Figure 2/Q.412). Recognition of this tone has the following consequences at the incoming end: i) the tone in the backward direction is removed; ii) the release of the switching units is ini- tiated; iii) the release-guard sequence starts. When release operations at the incoming end are complete, but not before an interval T2has elapsed after the removal, the tone is again restored at the incoming end in the backward direction. FIGURE 2/Q.412 p. When T1has elapsed the outgoing end must recognize that the tone-off condition in the backward direction is established. After this recognition the restoring of the tone in the backward direc- tion returns the circuit to the idle state and completes the release-guard sequence. At the incoming end the sending of an answer signal can only be prevented after the clear-forward signal has been recognized. To avoid any false operation should answer coincide with release, transition from tone-on to tone-off in the backward direction must not be interpreted, at the outgoing end, as part of the release-guard sequence during an interval T1. The interval T1starts with the sending of the tone in the forward direction. It is long enough to make quite sure that the clear-forward signal is recog- nized and the tone-off condition established at the incoming end. For the calculation of the intervals T1and T2see S 2.2.2.7 below. b) Release in answered state In this case, the release operations differ only from a) above in so far as i) does not apply. At the incoming end, however, send- ing of a clear-back signal can only be prevented after the clear-forward signal has been recognized. Should there be a clear-back signal the specified interval T1enables any resultant difficulties to be obviated (see Figure 3/Q.412). Figure 3/Q.412 p. c) Release in clear-back state The release operations are identical to those described in a) above. Should there be a second answer signal, the specified interval T1enables any resultant difficulties to be obviated (see Figure 4/Q.412). 2.2.2.7 Calculation of the intervals T1and T2specified for release and release-guard operations Figure 5/Q.412 shows the factors involved in calculating the intervals T1and T2. At the outgoing end when the interval T1has elapsed (Figure 5/Q.412, point D) the tone-off condition in the backward direction can be expected and recognized with certainty in all cases mentioned in S 2.2.2.6 above. Similarly, the interval T2preceding the restoring of the tone in the backward direction may be applied in all cases. To avoid false operation in the event of coinciding forward and backward signals or an irregular sequence of signals, the time interval T2must also be observed with the release operation [see S 2.2.2.6 b) above]. Figure 4/Q.412, p.11 Figure 5/Q.412, p.12 The minimum value of T1is the sum of the maximum values of the times required for the various operations which take place between application of the tone in the forward direction and recognition of the absence of tone in the backward direction. T2delays release of the circuit. It must therefore be as short as possible. However, it must be long enough to ensure recognition of the tone-off condition in the backward direction at the outgoing end when T1is at its maximum, even when this condition begins as early as possible. a) Exclusive terrestrial circuits The calculation is based on a maximum one-way transmission delay of 30 ms for a circuit. Thus, for circuits on high-velocity transmission systems via terrestrial lines (including submarine cables) the maximum operating range of the specified line signal- ling system is 4800 km Values used for the calculation of T1and T2: 0 < tp < 30 ms 0 < to < 30 ms (see S 2.3.2.4) 30 ms < tr < 50 ms 0 < ti < 20 ms Calculation of intervals T1and T2: T 1 > AC T 1 > 2 (t p + t o ) max. + t r max. + t i max. T 1 > (2 x 60 + 50 + 20) ms T 1 > 190 ms Allowing a safety margin of 10 ms and a tolerance of _ | 0%, the specified value of T1is (250 _ 50) ms. T 2 > BF T 2 > T 1 max. + t r max. + t i max. - 2 (t p + t o ) min. - t r min. - t i min. T 2 > (300 + 50 + 20 - 0 - 30 - 0) ms T 2 > 340 ms _________________________ See Recommendation G.114, Fascicle III.1. Allowing a safety margin of 20 ms and a tolerance of _ | 0%, the specified value of T2is (450 _ 90) ms. b) Circuits including a satellite link The calculation is based on the assumption that the complete connection includes two terrestrial sections with a maximum one-way transmission delay of 15 ms each and a satellite section with a one-way transmission delay of (270 _ 20) ms. Values used for the calculation of T1and T2: 250 < tp < 320 ms 0 < to < 30 ms (see S 2.3.2.4) 30 < tr < 50 ms 0 < ti < 20 ms Calculation of intervals T1and T2: T 1 > AC T 1 > 2 (t p + t o ) max. + t r max. + t i max. T 1 > (2 x 350 + 50 + 20) ms T 1 > 770 ms Allowing a safety margin of 30 ms and a tolerance of _ | 0%, the specified value of T1is (1000 _ 200) ms. T 2 > BF T 2 > T 1 max. + t r max. + t i max. - 2 (t p + t o ) min. - t r min. - t i min. T 2 > (1200 + 50 + 20 - 2 x 250 - 30 - 0) ms T 2 > 740 ms Allowing a safety margin of 60 ms and a tolerance of _ | 0%, the specified value of T2is (1000 _ 200) ms. c) Terrestrial circuits and circuits including a satellite link The situation in which equipment can serve terrestrial cir- cuits as well as circuits including a satellite link is not to be preferred because for the case of terrestrial circuits the release sequence is unduly prolonged. The calculation is based on the same assumption as in b) above, but with a minimum for (tp + to) = 0. This does not affect the value of T1, so also in this situation T1 = 1000 _ 200 ms. Values used for the calculation of T2: 0 < tp < 320 ms 0 < to < 30 ms (see S 2.3.2.4) 30 < tr < 50 ms 0 < ti < 20 ms Calculation of interval T2: T 2 > BF T 2 > T 1 max. + t r max. + t i max. - 2 (t p + t o ) min. - t r | min. - t i min. T 2 > (1200 + 50 + 20 - 0 - 30 - 0) ms T 2 > 1240 ms Allowing a safety margin of 40 ms and a tolerance of _ | 0%, the specified value of T2is (1600 _ 320) ms. 2.2.3 Abnormal conditions The situations described below are those in which interruption control of signalling channels (see Recommendation Q.416) does not function and which occur only during interruptions of individual channels or in the event of a fault in a line-signalling equipment. In addition, the situations described in SS 2.2.3.3 and 2.2.3.4 below may also result from operation of interruption control at the incoming end of the circuit. In that case the circuit automatically returns to normal at the command of interruption control. 2.2.3.1 If an exchange recognizes tone-off condition in the backward direction (premature answer) before the outgoing R2 register has received a signal A-6 or a Group B-signal, the con- nection must be released. Congestion information is then sent back- wards or a repeat attempt is made to set up the call. 2.2.3.2 In the case of non-reception of the answer signal, of delay in clearing by the calling subscriber in automatic working, or of non-reception of the clear-forward signal by the incoming exchange after the clear-back signal has been sent, the provisions of Recommendation Q.118 apply. 2.2.3.3 If, in the cases given in S 2.2.2.6 a) or c) above, the tone in the backward direction is not removed, the circuit will remain blocked, since it cannot return to the idle state of its own accord. The action to be taken in such cases is described in S 6.6. 2.2.3.4 If after sending of the clear-forward signal the sig- nalling tone in the backward direction is not restored, the circuit stays blocked, as described in section S 2.2.2.5 above. The same occurs when, in the idle state, the signalling tone in the backward direction is interrupted by a fault. 2.2.3.5 When the signalling tone in the forward direction of an idle circuit is interrupted owing to a fault, the incoming end recognizes seizure and connects multifrequency signalling equip- ment, but no interregister signalling follows. a) When the interruption is greater than the incoming R2 register time-out (see Recommendation Q.476) the regis- ter will release and the circuit must be brought into blocked state by removal of the signalling tone in the backward direction. As soon as the fault is cleared and the tone in the forward direction consequently restored, the circuit returns to the idle state in accordance with S 2.2.2.6 b) above. b) When the interruption is shorter than the time-out, restoration of the signalling tone in the forward direction will return the circuit to the idle state in accordance with S 2.2.2.6 a) above. 2.2.4 Alarms for technical staff According to Recommendation Q.117, an alarm should in general be given to technical staff as soon as an abnormal condition is recognized as probably due to a fault. It is recommended that a delayed action alarm should be operated at the outgoing end for the conditions described in SS 2.2.2.5, 2.2.3.3 and 2.2.3.4 above, i.e. when the circuit does not revert to the idle state after sending of the clear-forward signal or receipt of the blocking signal. Arrangements for the operation of the alarm will be made by each Administration. At both the outgoing and the incoming end, when interruption control (see Recommendation Q.416) functions, alarm condition must first be established for the transmission equipments. However, in this case a delayed-action alarm may also be given to the technical staff of the exchange. 2.3 CLAUSES FOR TRANSMISSION LINE SIGNALLING EQUIPMENT Recommendation Q.414 2.3.1 SIGNAL SENDER 2.3.1.1 Signalling frequency The nominal value of the signalling frequency is 3825 Hz. Measured at the sending point, the frequency variation from the nominal value must not exceed _ | Hz. 2.3.1.2 Send level The send level of the signalling frequency, measured at the group distribution frame or an equivalent point, must be -20 _ | dBm0. 2.3.1.3 Leaks The level of the signal frequency which may be transmitted to line as a leak current (e.g. when static modulators are used), must be at least 25 dB below the level of the signalling tone. 2.3.1.4 Phase distribution of the signalling frequencies As the signalling frequency is sent on any circuit in idle state, the addition of these tones in moments of low traffic may give rise to the following phenomena on certain transmission sys- tems: - high peak voltage on the line caused by the sig- nalling tones and involving the possibility of overloading the sys- tem; - intelligible crosstalk due to third-order intermodulation; - unwanted tones coming from second-order intermo- dulation products and occurring within sound-programme circuits. The following special measures must be taken to avoid these effects: One method recommended is to inject the signalling frequencies with random 0 and ~~ radian phases in the channels. An equivalent method is to use carrier frequencies of which the phases are ran- domly distributed 0 and ~~ radians. With these methods the proba- bility of occurrence of 0 and ~~ radian phases should be 0.5 Other methods may be used provided they give comparable results. 2.3.1.5 Protection of the signalling channel at the sending end This signalling channel must be protected at the sending end against disturbance from the associated and the adjacent speech channel. When a sinewave at 0 dBm0 level is applied to the audio-frequency input of the associated channel, the level measured at the group distribution frame or at an equivalent point must not exceed the levels shown in Figure 6/Q.414. When a sinewave of frequency f is applied to the audio-frequency input of the adjacent channel it produces two sig- nals that appear on the frequency scale of Figure 6/Q.414 as having the frequencies (4000 + f ) and (4000 - f ). The level of the (4000 + f ) signal, measured at the group distribution frame or at an equivalent point, shall not be higher than -33 dBm0 when the sinewave with frequency f is applied to the audio-frequency input of the adjacent channel at a level shown in Figure 6/Q.414 for the frequency of (4000 + f ). The level of the (4000 - f ) signal, measured at the group distribution frame or at an equivalent point, _________________________ For further details on the method of random distribu- tion of the phases of frequency 3825 Hz, see: Ekholm, O. and Johannesson, N.O.: "Loading Effects with Con- tinuous Tone Signalling", English edition of TELE , No. 2, 1969. For further details on a systematic method of phase distribution, see: Rasch, J. and Kagelmann, H.: "On Measures for Reducing Voltage Peaks and Distortion Noise on Carrier Transmission Paths with Single Channel Supervision", Nachrichten- technische Zeitschrift (NTZ) , 22 (1969), No. 1, pp. 24-31. shall not be higher than -33 dBm0 when the sinewave with frequency f is applied to the audio-frequency input of the adjacent channel at any level below the value shown in Figure 6/Q.414 for the frequency (4000 - f ). FIGURE 6/Q.414, p. When the Go path is looped to the Return path at the group distribution frame or an equivalent point, the signal receiver must not change condition when: - the click generator shown in Figure 7/Q.414 is connected to the associated speech channel or to the adjacent speech channel at the very point where this channel is connected to the switching equipment; - to take the most difficult circumstances possi- ble, the channel level adjusting devices are set to such values encountered in practice which give rise to the worst disturbance; - gain is introduced in the loop at the group dis- tribution frame or at the equivalent point, so that the receive level at the point in question is +3 dBm0. FIGURE 7/Q.414, p. 2.3.1.6 Response time The response time of the signal sender is defined as the interval between the instant when the change signalling condition command is applied to the sender and the instant at which the envelope of the signalling frequency, measured at the group distri- bution frame or at an equivalent point, reaches half of its value in the steady state. For each of the two possible changes of sig- nalling condition the response time must be less than 7 ms. Recommendation Q.415 2.3.2 SIGNAL RECEIVER 2.3.2.1 Recognition of the tone-on condition The receiver must have assumed or assume the tone-on condition when at the group distribution frame or at an equivalent point: - the level of the received frequency has risen to -27 dBm0 or more; - its frequency lies between 3825 _ 6 Hz. The level of -27 dBm0 specified above does not preclude the use of individual adjustments in the channel translating equipment to compensate for constant level deviations. 2.3.2.2 Recognition of the tone-off condition The receiver must have assumed or assume the tone-off condi- tion when the level of the test frequency, at the group distribu- tion frame or at an equivalent point, has dropped to the values shown in Figure 8/Q.415. FIGURE 8/Q.415, p. 2.3.2.3 Protection against near-end disturbances The signal receiver must not change state when any one of the following disturbing signals is applied at the 4-wire output of the associated speech channel looped at the group distribution frame or at an equivalent point: - a sinusoidal signal whose level as a function of the frequency is shown in Figure 9/Q.415, - a transient signal produced by the click genera- tor (described in S 2.3.1.5 above) applied at the point where the channel is connected to the switching equipment, all level adjust- ing devices being set to such values encountered in practice which give rise to the worst disturbance. FIGURE 9/Q.415, p. 2.3.2.4 Overall response time of signal sender and receiver When the modulation equipment is looped at the group distribu- tion frame or at an equivalent point, the overall response time is defined as the interval between the instant when a change signalling condition command is applied to the sender and the moment when the changed signalling condition appears at the receiver output. For each of the two possible changes of signalling condition, the overall response time must be less than 30 ms. 2.3.2.5 Interference by carrier leaks The requirements stated in SS 2.3.2.1, 2.3.2.3 and 2.3.2.4 above must be fulfilled in the presence of carrier leaks. It is assumed that: - when the receive level of the signalling tone is at its nominal value at the group distribution frame or an equivalent point, each carrier leak is present at a level of - 26 dBm0; - the level of the carrier leak varies proportion- ally with any variations in the level of the signalling tone. 2.3.2.6 Interference by pilots The specified signalling system is not intended to work in the presence of those pilots specified by CCITT having a frequency differing by 140 Hz from the nearest multiple of 4 kHz (see Recommendation M.460). On the other hand, the requirements stated in SS 2.3.2.1, 2.3.2.2, 2.3.2.3, 2.3.2.4 and 2.3.2.5 above must be met in the presence of any other pilot recommended by the CCITT. It is assumed that variations in level of the pilot and of the signalling tones are correlated. Recommendation Q.416 2.4 INTERRUPTION CONTROL 2.4.1 General In System R2, removal of the tone corresponds to the sending of the seizing and answer signals. Steps must be taken, therefore, to guard against unwanted interruption of the signalling channels resulting in false signalling. Special devices monitor a number of circuits and transmit an indication to each individual equipment as soon as an interruption occurs. The whole protection system against the effect of interruptions is designated by the term interruption control . In each case, the response time of the interruption control must be based on the time required to recognize the signalling con- dition. The interruption control systems in the two directions of transmission operate independently of each other. The interruption control specified uses the group pilot to detect interruptions. 2.4.2 Mode of operation of interruption control For each direction of transmission of a carrier circuit con- nection the equipment for interruption control comprises: - a group pilot generator at the outgoing end; - a pilot receiver and a wiring system for signal- ling the interruption at the incoming end. In principle, the existing pilots of the carrier system will be used. The receiver at one end supervises the pilot transmitted by the other end. When a considerable fall in the level of the pilot is detected it is assumed that an interruption has occurred on the signalling channels associated with the carrier circuits. The interruption control equipment then reacts to prevent the unwanted transmission of certain signals on those circuits which have already been seized or to ensure that idle circuits are blocked. Figure 10/Q.416 shows functionally an arrangement where the pilot receiver controls the relay sets of interrupted circuits. To ensure proper interruption control, it is essential that the individual transmission or switching equipments should not react to any change of signalling state due to a fault. The action initiated by the interruption control must therefore be completed in less time than the sum of the response time of the signalling receiver and the recognition time for the tone-off condition caused by interruption of the signalling channel. Again, to prevent the unwanted transfer of certain signals, interruption control, during re-establishment of the pilot, must return to alarm off after an interval long enough for the signalling equipment to revert to nor- mal. To operate independently for each direction of transmission the incoming end interruption control supervises only the forward direction and, if necessary, initiates an operation at the outgoing end via the line signalling system. Conversely, interruption con- trol at the outgoing end supervises the backward direction of transmission only. Blocking of a circuit at the outgoing end therefore takes place in two different ways: - immediate blocking by intervention of interrup- tion control at the outgoing end; - blocking on recognition in the backward direction of the tone-off condition caused by interruption control interven- tion at the incoming end. When the transmission system is re-established, interruption control reverts to normal and the signalling equipment must automatically revert to normal operating. Since the action to be taken on the individual circuits differs according to their state at the time the fault occurs, the different possibilities are dealt with in detail below. Figure 10/Q.416, p.5 2.4.2.1 Mode of operation of interruption control at the incoming end (transmission interrupted in the forward direction) a) Circuit in idle state Transition of interruption control to alarm brings about: i) removal of the tone in the backward direction by locking of the sending unit in the tone-off condition; ii) locking of the receiving unit in its position, i.e. in the tone-on condition. The effect of operation i) is to block the circuit at the out- going end against possible seizing; operation ii) prevents incorrect recognition of seizing of the incoming circuit. Return of interruption control to normal ensures return to the idle state of the circuits affected by the fault, by switching sending units at the incoming end to the tone-on condition. b) Circuit seized prior to answered state Transition of interruption control to alarm brings about: i) locking of the sending unit in its position, i.e. in the tone-on condition; ii) locking of the receiving unit in its position, i.e. in the tone-off condition; iii) start of a time-out device which after a cer- tain interval clears the chain beyond the faulty circuit; this tim- ing arrangement may be the one specified in Recommendation Q.118, S 4.3.3. Operation i) prevents the transfer of an answer signal while interruption control is in action. If the called subscriber answers before the time out delay mentioned in iii) above has elapsed, then the timer is stopped. For existing equipment this requirement may not apply. If the called subscriber clears while interruption con- trol is active, the part of the connection beyond the faulty cir- cuit must be released immediately. Operation iii) prevents blocking of the called subscriber's line if the fault persists; short breaks, on the other hand, have no effect. When the caller clears, operations i) and ii) block the faulty circuit against any new seizure even when the backward signalling channel is still intact; since the release-guard signal has not been sent the outgoing circuit cannot return to the idle state. When interruption control reverts to normal before the called subscriber has answered, the call may still mature normally, pro- vided the caller is holding. If the called subscriber has answered during the time-out delay and the interruption control reverts to normal with both the calling and called subscribers holding, the answer signal is sent immediately. If at the moment when interruption control reverts to normal the called subscriber has already cleared, operation ii) ensures that in all cases the release-guard sequence takes place as in S 2.2.2.6 a) above (either immediately if the outgoing exchange has already sent the clear-forward signal or when the caller clears). If, on the other hand, the called subscriber is still holding and the outgoing exchange is already sending the clear-forward signal when interrup- tion control reverts to normal the circuit returns to the idle state at the outgoing end as described in S 2.2.2.6 b) above. c) Circuit in answered state Transition of interruption control to alarm brings about: i) locking of the sending unit in its position, i.e. in the tone-off condition; ii) locking of the receiving unit in its position, i.e. in the tone-off condition. When the caller clears, operation i) blocks the faulty circuit against any new seizure, even when the backward signalling channel is still intact; since the release-guard signal has not been sent, the outgoing circuit cannot return to the idle state. When the called subscriber clears, the part of the connection beyond the faulty circuit (including the called subscriber's line) must be released immediately. When interruption control reverts to normal with both sub- scribers still on the line, the connection is maintained. When the caller has already cleared by the time the interrup- tion control reverts to normal, the release-guard sequence is car- ried out as in Recommendation Q.412, S 2.2.2.6 b) or c). d) Circuit in clear-back state Transition of interruption control to alarm causes: i) locking of the sending unit in its position, i.e. in the tone-on condition; ii) locking of the receiving unit in its position, i.e. in the tone-off condition; iii) immediate release of the part of the connec- tion beyond the faulty circuit (including the called subscriber's line). When interruption control reverts to normal, the release-guard signal is sent as in Recommendation Q.412, S 2.2.2.6 c) as soon as the clear-forward signal is recognized. e) Circuit in release When interruption control functions after a clear-forward sig- nal has been recognized at the incoming end, it causes: i) locking of the sending unit in the tone-off | ondition; if at the instant interruption control operates, the tone-on condition exists in the backward direction, it will be switched to the tone-off condition following recognition of the clear-forward signal and locking in the tone-off condition can take place as prescribed; ii) locking of the receiving unit in its position, i.e. in the tone-on condition. The effect of operation i) is to guard the faulty circuit from a new seizure at the outgoing exchange. Operation ii) ensures the release of the part of the connec- tion beyond the faulty circuit (including the called subscriber's line). When interruption control reverts to normal the tone-on | ondition is established in the backward direction and causes the circuit at the outgoing exchange to return to the idle state. 2.4.2.2 Mode of operation of interruption control at the outgoing end | transmission in the backward direction interrupted) a) Circuit in idle state Transition of interruption control to alarm is immediately followed by blocking of the outgoing circuit. b) Circuit seized but not in answered state (including clear-back) i) Transition of interruption control to alarm causes locking of the receiving unit in its position, i.e. the tone-on condition. This operation prevents recognition of an answer signal or return to the answered state should the called subscriber have cleared. ii) As soon as a clear-forward signal is sent on the part of the connection preceding the faulty circuit, it must be retransmitted; the tone must therefore be established in the for- ward direction to ensure, assuming that the forward signalling channel is left intact, that the part of the connection beyond the faulty circuit is released. iii) When interruption control reverts to normal, the tone may already have been sent in the forward direction as a clear-forward signal. If the forward signalling channel has remained intact, recognition at the incoming end of the tone-on condition will have caused generation of the release-guard sequence which, because of the fault, will not have been received at the outgoing end. Exceptionally, therefore, return of the outgoing circuit to the idle state must take place simply on recognition of tone-on in the backward direction without necessarily taking into account time-out T1. c) Circuit in answered state In this case transition of interruption control to alarm does not cause immediate action. A clear-forward signal sent on the part of the connection preceding the faulty circuit must be repeated forward to ensure that, if the forward signalling channel is left intact, the part beyond the faulty circuit is cleared. Once the interruption control reverts to normal the connection is maintained provided the caller and the called subscriber are still holding. On the other hand, by the time the interruption con- trol reverts to normal the clear-forward signal may already have been sent and the situation will be the one described in S 2.4.2.2 b), iii). d) Circuit in release [See S 2.4.2.2 b), iii).] 2.4.3 Clauses on interruption control equipment Adoption of thresholds with widely differing levels makes for economy in the design of interruption control equipment. Against this must be set the fact that the device cannot cope with the effects of certain slow drops in level. However, the probability of these occurring in practice is very small. 2.4.3.1 Pilots Interruption control uses the 84.08 kHz group pilot or by bilateral agreement and, at the request of the receiving end coun- try, the 104.08 kHz group pilot. However, if the ends of the supergroup link coincide with the end of the five group links it is carrying, the supergroup pilot may also be used. 2.4.3.2 Alarm-on threshold Interruption control must pass to alarm-on | hen the pilot level, measured at the group distribution frame or at an equivalent point, drops to -29 dBm0. 2.4.3.3 Alarm-off threshold Interruption control must revert to alarm-off | i.e. normal when the pilot level, measured at the group distribution frame or at an equivalent point, rises to -24 dBm0. 2.4.3.4 Response time for a drop in level Interruption control must pass from normal to alarm-on within an interval t v such that: 5 ms t v tr\ds m\di\dn + 13 ms when the pilot level, measured at the group distribution frame or at an equivalent point, suddenly drops from its nominal level to -33 dBm0. In the above formula, tr\dsm\di\dnis the minimum response time of the signalling receiver for a drop in level, taking into account a possible variation of _ | dB in the signalling tone level from its nominal value, the level being measured on the receiving side of the group distribution frame or at an equivalent point. If the value (40 _ 10) ms is exclusively applied, it is possi- ble to use the minimum value of 30 ms instead of 13 ms for the interruption control device. The figure of 13 ms in the above formula is derived on the assump- tion that the output of the interruption control equipment acts upon the input of the device which regulates the recognition time for the tone-on and tone-off conditions (20 _ 7 ms) , i.e. absence of a direct current signal at this input for a period of up to 13 ms has no relevance. 2.4.3.5 Response time for rise in level Interruption control must revert from the alarm-on to normal in an interval t ^ such that: tr\ds m\da\dx - 13 ms t ^ 500 ms when the pilot level, measured at the group distribution frame or at an equivalent point, suddenly rises from its nominal level to -33 dBm0. In the above formula, tr\dsm\da\dxis the maximum response time of the signalling receiver for a rise in level, taking into account a possible variation of _ | dB in the signalling tone level from its nominal value, the level being measured on the receiving side of the group distribution frame or at an equivalent point. The figure of 13 ms in the above formula is derived on the assumption that the output of the interruption control equipment acts upon the input of the device which regulates the recognition time for tone-on and tone-off condition (20 _ 7 ms) i.e. absence of a direct current signal at this input for a period of up to 13 ms has no relevance. 2.4.3.6 Precautions against noise An interruption may produce increased noise on the group link. Interruption control must be capable of distinguishing between the pilot itself and a high level noise simulating the pilot. Interruption control must not revert to normal in the presence of white noise having a spectral power density of not more than -47 dBm0 per Hz. To facilitate the design of interruption control equipment operating satisfactorily at high noise levels, the upper limit of 500 ms for t ^ has been specified. SECTION 3 LINE SIGNALLING, DIGITAL VERSION Recommendation Q.421 3.1 DIGITAL LINE SIGNALLING CODE 3.1.1 General Primary PCM multiplexes (see Recommendations G.732 and G.734) economically provide more than one signalling channel per speech circuit in each direction of transmission. By making use of the increased signalling capacity, simplification of the outgoing and incoming switching equipment can be achieved since the timing con- ditions necessary for the System R2 line signalling, analogue version, are not required. For this rea- son the digital version of System R2 line signalling is recommended for use on PCM systems in national and international public switched networks and is specified below. Note - The continuous line signalling scheme specified for FDM systems may also be used on PCM systems by utilizing one sig- nalling channel only in each direction. In this case relay sets designed for the continuous line signalling system on FDM channels can be used provided that the functions specified for the interrup- tion control on FDM circuits (see Recommendation Q.416) are per- formed by use of the local alarm facility provided by PCM equip- ment. This method of line signalling on PCM systems is not recom- mended for use on international circuits. The digital version of System R2 line signalling uses two sig- nalling channels in each direction of transmission per speech cir- cuit. These signalling channels are referred to as afand bffor the forward direction (i.e. the direction of call set-up) and aband bbfor the backward direction. Under normal conditions: - The afchannel identifies the operating condition of the outgoing switching equipment and reflects the condition of the calling subscriber's line. - The bfchannel provides a means for indicating a failure in the forward direction to the incoming switching equip- ment. - The abchannel reflects the condition of the called subscriber's line (on hook or off hook). - The bbchannel indicates the idle or seized state of the incoming switching equipment. The line signals are transmitted link-by-link. The digital version of System R2 line signalling also speci- fies a means for appropriate action in the case of faulty transmis- sion conditions on the PCM multiplex, see Recommendation Q.424. The signalling system is specified for one-way operation, but both-way operation is also possible (see S 3.2.7 below). 3.1.2 Signalling code Table 2/Q.421, shows the signalling code on the PCM line under normal conditions. H.T. [2/Q.421] TABLE 2/Q.421 __________________________________________________ Signalling code State of the circuit Forward Backward a f __________________________________________________ Idle/Released 1 0 1 0 Seized 0 0 1 0 Seizure acknowledged 0 0 1 1 Answered 0 0 0 1 Clear-back 0 0 1 1 Clear-forward | | | | | | | | | 1 | | | | | | | 0 | | | | | | | | 0 | | | | | | | 1 or 1 1 Blocked 1 0 1 1 __________________________________________________ | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | Table [2/Q.421], p. Recommendation Q.422 3.2 CLAUSES FOR EXCHANGE LINE SIGNALLING EQUIPMENT 3.2.1 Recognition of a change of signalling code 3.2.1.1 Signalling channel transitions The recognition time for a transition from 0 to 1 or vice versa on a signalling channel is 20 _ 10 ms. This value presupposes the existence of protection against the effects of faulty transmis- sion conditions on the PCM multiplex. The recognition time is defined as the duration that the sig- nals representing 0 or 1 must have at the output of the terminal equipment of a signalling channel in order to be recognized by the exchange equipment. 3.2.1.2 Change of signalling code Recognition of a change of signalling code is thus defined as either of the following: a) Recognition of a transition detected on one sig- nalling channel with no transition detected on the second signal- ling channel during the recognition period. b) Recognition of a transition detected on the second signalling channel during the recognition period already being applied to the first signalling channel. In this case, a change of signalling code is recognized only when both recognition timing periods have elapsed. 3.2.2 Sent signal time tolerance The time difference between application of transitions intended to be simultaneous on two signalling channels in the same direction of transmission must not exceed 2 ms. 3.2.3 States and procedures under normal conditions | see Table 2/Q.421) In the forward direction bf= 0 is established permanently. 3.2.3.1 Idle state In the idle state the outgoing end sends af= 1, bf= 0. At the incoming end this results in sending ab = 1, bb= 0 in the backward direction, provided that the switching equipment at the incoming end of the circuit is idle. 3.2.3.2 Seizing procedure i) Seizure Seizing should occur only if ab = 1, bb = 0 is recognized. The outgoing end changes af = 1 into af = 0. The code af = 0, bf = 0 must be maintained until the seizing acknowledgement signal is recognized. In this way the outgoing switching equipment will only be able to send the clear-forward signal after recognition of the seizing acknowledgement signal. ii) Seizure acknowledgement After having recognized the seizing signal, the incoming end sends ab = 1, bb = 1 as an acknowledgement. 3.2.3.3 Answering The off-hook condition of the called subscriber's line pro- vokes the incoming switching equipment to send ab = 0, bb = 1. The answered state must be established on the preceding link immediately after it is recognized: see also S 3.2.3.6 below. 3.2.3.4 Clear-back The on-hook condition of the called subscriber's line provokes the incoming switching equipment to send ab = 1, bb = 1. The clear-back state must be established on the preceding link immedi- ately after it is recognized: see also S 3.2.3.6 below. 3.2.3.5 Clear-forward procedure The cleared condition of the calling subscriber's line or the release of the outgoing switching equipment will normally result in sending af = 1, bf = 0. The outgoing switching equipment will not be restored to the idle state until recognition of the code ab = 1, bb = 0: see also SS 3.2.3.2, 3.2.3.6 and Table 3/Q.422. 3.2.3.6 Release procedure Recognition of the clear-forward signal in the incoming switching equipment initiates the release of the succeeding link even though answering or clearing by the called party has occurred. Upon complete release of the incoming switching equipment, the code ab = 1, bb = 0 is established on the circuit. This will cause the circuit to be restored to the idle state and the outgoing switching equipment to become available for another call. 3.2.3.7 Blocking and unblocking procedure Blocking of an idle circuit to new calls at the outgoing end must occur as soon as ab = 1 and bb = 1 is recognized: see also Tables 3/Q.422 and 4/Q.422. The recognition of ab = 1, bb = 0 restores the circuit to the idle state. 3.2.4 Actions appropriate to various signalling conditions In addition to normal conditions described in Table 2/Q.421 other conditions due to faults may be encountered. Tables 3/Q.422 and 4/Q.422 indicate the states appropriate to each signalling code recognized and the actions to be taken at the outgoing and incoming end respectively of a circuit operated with the digital version of System R2 line signalling. 3.2.4.1 Outgoing end H.T. [3/Q.422] TABLE 3/Q.422 ________________________________________________________________________________________________________________________________________ Received code { a b = 0, b b = 0 a b = 0, b b = 1 a b = 1, b b = 0 a b = 1, b b = 1 ________________________________________________________________________________________________________________________________________ Idle/Released a f = 1, b f = 0 Abnormal, see Note 1 Abnormal, see Note 1 Idle Blocked ________________________________________________________________________________________________________________________________________ Seized a f = 0, b f = 0 Abnormal, see Note 2 Abnormal, see Note 2 Seized see Note 2 Seizure acknowledged ________________________________________________________________________________________________________________________________________ Seizure acknowledged a f = 0, b f = 0 Abnormal, see Note 3 Answered Abnormal, see Note 3 Seizure acknowledged ________________________________________________________________________________________________________________________________________ Answered a f = 0, b f = 0 Abnormal, see Note 4 Answered Abnormal, see Note 4 Clear-back ________________________________________________________________________________________________________________________________________ Clear-back a f = 0, b f = 0 Abnormal, see Note 4 Answered Abnormal, see Note 4 Clear-back ________________________________________________________________________________________________________________________________________ Clear-forward a f = 1, b f = 0 Abnormal, see Note 1 Clear-forward Released = Idle Clear-forward ________________________________________________________________________________________________________________________________________ Blocked a f = 1, b f = 0 Abnormal, see Note 1 Abnormal, see Note 1 Idle Blocked ________________________________________________________________________________________________________________________________________ | | | | | | | | | | | | | | | | | | | | | | | | | | | | Note 1 - In these abnormal conditions the outgoing end must prevent a new seizure of the circuit. A delayed alarm should also be given (see S 3.2.6). Note 2 - Non-recognition of the seizing acknowledgement signal 100 ms-200 ms after sending the seizing signal on a terrestrial link or 1-2 seconds after sending the seizing signal on a satellite link results in an alarm and either congestion information being sent backward or a repeat attempt being made to set up the call. The outgoing end must prevent a new seizure of the circuit. When the seizing acknowledgement signal is recognized after the time-out period has elapsed, the clear-forward signal must be sent. Note 3 - Receipt of b b = 0 by the outgoing switching equipment for 1-2 seconds after recognition of the seizing acknowledgement signal and prior to recognition of the answer signal, results in an alarm and either congestion information being sent backward or a repeat attempt being made to set up the call. The outgoing end must prevent new seizures of the circuit. When b b reverts to 1 after the 1-2 seconds timeout period has elapsed, the clear-forward sig- nal must be sent. Note 4 - In the case of recognition of b b = 0 whilst in the answered or clear-back state, immediate action is not necessary. On receipt of clearing from the preceding link, the clear-forward sig- nal (a f = 1, b f = 0) must not be sent until b b is restored to 1. A delayed alarm should also be given. Tableau [3/Q.422] + Remarques, p.7 Blanc 3.2.4.2 Incoming end H.T. [4/Q.422] TABLE 4/Q.422 ________________________________________________________________________________________________________________________________________________________ Received code { a f = 0, b f = 0 a f = 0, b f = 1 a f = 1, b f = 0 a f = 1, b f = 1 ________________________________________________________________________________________________________________________________________________________ Idle/Released a b = 1, b b = 0 Seized Fault see Note 1 Idle Fault see Note 1 ________________________________________________________________________________________________________________________________________________________ Seizure acknowledged a b = 1, b b = 1 Seizure acknowledged Fault See Note 2 Clear- forward Fault See Note 2 ________________________________________________________________________________________________________________________________________________________ Answered a b = 0, b b = 1 Answered Fault see Note 3 Clear- forward Fault see Note 3 ________________________________________________________________________________________________________________________________________________________ Clear-back a b = 1, b b = 1 Clear-back Fault see Note 4 Clear- forward Fault see Note 4 ________________________________________________________________________________________________________________________________________________________ Clear-forward { a b = 0, b b = 1 or a b = 1, b b = 1 } Abnormal seized see Note 7 Fault see Note 7 Clear- forward see Note 7 Fault see Note 7 ________________________________________________________________________________________________________________________________________________________ Blocked a b = 1, b b = 1 Abnormal seized see Note 5 Fault see Note 6 Blocked Fault see Note 6 ________________________________________________________________________________________________________________________________________________________ | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | Note 1 - When in the idle/released state b f changes to 1, b b must be changed to 1. Note 2 - In these cases a time-out device is started, which after a certain interval clears the connection beyond the faulty circuit: this timing arrangement may be the one specified in Recommendation Q.118, Section 4.3.3. If the answer signal is recog- nized during the time-out delay, the timer is stopped but the answer signal is not sent on the preceding link until recognition of a f = 0, b f = 0. If the clear-back signal is recognized while the fault persists, the connection beyond the faulty circuit must be released immediately. Additionally, when the incoming register has not started to send the last backward signal, the rapid release procedure described in Note 5 may be used. Note 3 - In these cases no action is taken until the clear-back signal is recognized, at which stage the connection beyond the faulty circuit is immediately released. Note 4 - Under these conditions the succeeding link must be released immediately. Note 5 - In this case immediate action is not necessary. However, rapid release of the circuit should occur if the incoming end simu- lates answer by sending a b = 0, b b = 1. Note 6 - Under these conditions no action is taken. Note 7 - After clear-forward signal is recognized and until the code a b = 1, b b = 0 is sent, all transitions in the forward direction shall be ignored. Tableau [4/Q.422] + Remarques, p.8 Blanc 3.2.5 Abnormal conditions 3.2.5.1 Special release arrangements a) If an exchange where an outgoing R2 register is situated recognizes ab = 0, bb = 1 (premature answer) before an address-complete signal A-6 or a Group B signal is received, the connection must be released. Congestion information is then sent backward or a repeat attempt is made to set up the call. b) In the cases of non-receipt of the answer sig- nal, of delay in clearing by the calling subscriber in automatic working and of non-receipt of the clear-forward signal by the incoming exchange after the clear-back signal has been sent, the provisions of Recommendation Q.118 apply. 3.2.5.2 Safeguard against failures The PCM equipment and the exchange line signalling equipment should be designed in such a way that at least those faults which are most likely to occur in this equipment or in the interconnect- ing cables, result in blocking of the circuit at the outgoing end and in the ultimate clearing of the connection beyond the incoming switching equipment. This can be achieved, as far as possible, by ensuring that a = 1, b = 1 is sent on line upon: - removal of PCM or switching equipment by mainte- nance personnel; - occurrences of abnormal conditions (e.g. open wire, low voltage) in switching equipment. 3.2.6 Alarms for technical staff According to Recommendation Q.117, an alarm must in principle be given to the technical staff upon recognition of abnormal condi- tions. Arrangements for these alarms are to be specified by the Administrations. It is recommended that a delayed alarm be given at the outgo- ing end for the procedure described under S 3.2.3.7 above (block- ing) and for the following reasons: - when the abnormal conditions covered by Note 1 to Table 3/Q.422 are applicable; - when the seizing acknowledgement signal is not recognized within the time specified in Note 2 to Table 3/Q.422 after sending the seizing signal; - when, after recognition of the seizing ack- nowledgement signal and prior to recognition of the answer signal, bb = 0 is received for 1-2 seconds; - when the abnormal conditions covered by Note 4 to Table 3/Q.422 are applicable. It is also recommended that a delayed alarm be given under PCM failure conditions specified in Recommendations G.732 and G.734. 3.2.7 Both-way working System R2 is specified for one-way working, but in principle the line signalling code detailed in Recommendation Q.421 is also suitable for use on both-way circuits. Where Administrations have undertaken, by bilateral agreement, to use both-way working, the clauses and additional specifications for exchange signalling equipment detailed in SS 3.2.7.1 and 3.2.7.2 below, must be observed. 3.2.7.1 Procedures under normal conditions a) Double seizure Double seizure is assumed if the outgoing equipment is in a seized state and the signalling code ab = 0, bb = 0 is recognized instead of ab = 1, bb = 1 (seizure acknowledgement). In such a situation the connection must be released at both ends and conges- tion information sent to the calling subscriber or a repeat attempt must be made. On recognition of double seizure the line signalling equipment at both ends must maintain the seized state for a minimum of 100 ms after which the clear-forward signal af = 1, bf = 0 must be sent. 100 ms after sending the clear-forward signal and on recognition of ab = 1, bb = 0 each end may assume the idle state. The clear-forward state af = 1, bf = 0 must be maintained for at least 100 ms to ensure that it is recognized at the other end. In the sense of preventive action it is recommended that an opposite order of circuit selection be used by each exchange of a both-way circuit group to minimize double seizure. b) Requirements for circuit release When a both-way circuit is released, the end which acted as the incoming end must maintain the signalling code ab = 1, bb = 0 for at least 100 ms to ensure that the signal is recognized at the other end after which the circuit becomes idle. c) Blocking and unblocking procedure When a both-way circuit is blocked manually in its idle state at one end (e.g. end B) the blocking signal must be sent to the other end (A). The circuit must then be kept blocked locally (at end A) against all calls in the A to B traffic direction as long as the blocked state persists in the B to A direction. In order to avoid permanent blocking, end A should maintain the signalling code a = 1, b = 0 in the direction A to B. When the blocked state is removed, end B must send the clear-forward signal and must maintain that state for at least 100 ms, before assuming the idle state. 3.2.7.2 Special arrangement The physical realization of signalling equipment terminating a bothway circuit may allow that part of the equipment concerned with outgoing calls to be removed without preventing the remaining equipment from being used for incoming calls. In this case it is only necessary to block the circuit locally against outgoing calls and a blocking signal need not be sent to the other end. Recommendation Q.424 3.3 PROTECTION AGAINST THE EFFECTS OF FAULTY TRANSMISSION Faulty transmission conditions in PCM systems can lead to degradation of the speech channels and erroneous signalling. In the case of primary PCM multiplex equipment operating at 2048 kbit/s failures due to loss of frame or multi-frame alignment and/or failure of any other important function results in both PCM termi- nals going into alarm condition in accordance with Recommendations G.732 and G.734. Thus both PCM terminals apply the state corresponding to state 1 on the PCM line on each "receive" signalling channel at the interfaces with the switching equipment, as indicated in Table 4 of Recommendation G.732. In this way, the incoming switching equipment receives the equivalent of af = 1, bf = 1 on the PCM line and the outgoing switching equipment receives the equivalent of ab = 1, bb = 1. These characteristics are taken into account in the present specifications (see S 3.2.4) so that: - at the outgoing end (see Table 3/Q.422) a PCM fault results in a blocked state, seizure acknowledged state or clear-back state. This means that all circuits in the idle state of a faulty PCM multiplex will be blocked against seizure and that seized circuits will go to or remain in the seizure acknowledged or clear-back state; - at the incoming end (see Table 4/Q.422) a PCM fault can be identified and appropriate actions can be taken. When the signalling equipment is a part of a digital exchange, it may receive alarm indications in a form other than both signal- ling bits in state one. The failure may be detected by the signal- ling equipment or an indication be received from the PCM terminal according to Recommendation G.734. When the signalling equipment recognizes a failure it must: - block the detection of signalling transitions to avoid recognition of erroneous signalling codes caused by the failure. This action must be taken as soon as possible and at least within 3 ms as specified in Recommendation G.734 for a PCM termi- nal, - react as specified in Tables 3/Q.422 and 4/Q.422 when a signalling code a = 1, b = 1 is detected at the input of signalling equipment situated at the analogue access of a PCM ter- minal equipment complying with Recommendation G.732. Recommendation Q.430 3.5 CONVERSION BETWEEN ANALOGUE AND DIGITAL VERSIONS OF SYSTEM R2 LINE SIGNALLING This Recommendation is applicable to a conversion equipment placed on the circuit between two switching exchanges each using one of the two versions specified for System R2 line signalling. Owing to this particular use, all the specifications for the two versions of the line signalling may not be fully observed. Nevertheless, the diagrams which follow are based on the principles of CCITT Recommendations Q.411, Q.412 and Q.416 for the analogue version and on Recommendations Q.421, Q.422 and Q.424, for the digital version. The only time conditions taken into account in this Recommendation are those set out in the aforementioned Recom- mendations. The operation of the interruption control device is also, wherever possible, that speci- fied in Recommendation Q.416. As stated in that Recommendation, the receiver is blocked immediately whenever pilot tone off is detected: since this is a routine operation, it has not been represented explicitly in the diagrams. The conversion diagrams have been divided into four parts: - for conversion between the analogue version at the incoming end and the digital version at the outgoing end, into: - incoming analogue, - outgoing digital; - for conversion between the digital version at the incoming end and the analogue version at the outgoing end, into: - incoming digital, - outgoing analogue. It should be noted, however, that this Recommendation can be simplified with respect to alarm processing when the conversion equipment is connected directly at the input or output of a switch- ing exchange; the processes necessary for this application are shown on the diagrams with thickened lines. 1 Drawing conventions - tone tf = 1 tone on forward tf = 0 tone off forward tb = 1 tone on backward tb = 0 tone off backward When recognized p = 1 pilot tone on p = 0 pilot tone off - signalling bits The conventions are those of Recommendation Q.421. 2 Conversion incoming analogue version to outgoing digital version 2.1 Incoming analogue Figure CCITT-55980, p. List of timers: T2: Recommendation Q.412 (S 2.2.2.7) T3: 2 to 3 mn Recommendation Q.118 (S 4.3.3) In sheets 2 and 4 the diagram for group pilot supervision is given. In sheets 3 and 5 the diagram for supergroup pilot supervision is given. Sheet 1 is for group and supergroup pilot supervision. Blanc FIGURE CCITT-60881 (feuillet 1), p.10 FIGURE CCITT-60891 (feuillet 2), p.11 FIGURE T1115760-88 (feuillet 3), p. FIGURE CCITT-60901 (feuillet 4), p.12 FIGURE T1115770-88 (feuillet 5), p. 2.2 Outgoing digital Figure CCITT-55990, p. List of timers: T6: Recommendation Q.422 (S 3.2.4.1, Note 2 to Table 3) Blanc Figure CCITT-60911 (Sheet 1 of 3), p. Figure CCITT-60920 (Sheet 2 of 3), p. Figure CCITT-60930 (Sheet 3 of 3), p. 3 Conversion incoming digital version to outgoing analogue version 3.1 Incoming digital Figure CCITT-56000, p. List of timers: T3: 2 to 3 mn Recommendation Q.118 (S 4.3.3) Blanc Figure CCITT-60940 (Sheet 1 of 3), p. Figure CCITT-60950 (Sheet 2 of 3), p. Figure CCITT-60960 (Sheet 3 of 3), p. 3.2 Outgoing analogue Figure CCITT-50610, p. List of timers: T1: Recommendation Q.412 (S 2.2.2.7) T5: 100 ms Recommendation Q.412 (S 2.2.2.1) Automatic restoration of an abnormally blocked circuit Recommendation Q.490 (S 6.6) n: number of attempts made T4: 30 s to 2 mn T7: 2 to 3 s Blanc Figure CCITT-60971 (Sheet 1 of 5), p. Figure CCITT-60981 (Sheet 2 of 5), p. Figure CCITT-60991 (Sheet 3 of 5), p. Figure CCITT-70001 (Sheet 4 of 5), p. Figure, p. SECTION 4 INTERREGISTER SIGNALLING Recommendation Q.440 4.1 GENERAL Interregister signals are of the multifrequency type using a 2-out-of-6 in-band code in both directions. The multifrequency com- binations are sent and received by multifrequency signalling equip- ment which is assumed to be associated with the registers used to control the switching equipment at both ends of the inter-exchange link. 4.1.1 End-to-end signalling method In System R2 interregister signalling is generally performed end-to-end by a compelled procedure between the outgoing register and the incoming registers which come into action one after another. Signalling takes place over one or more links in tandem without signal regeneration in intermediate exchanges. With this signalling method only the address information needed for routing the call through an intermediate exchange is transferred from the outgoing register to the incoming registers. In an intermediate exchange the speech path is immediately through-connected and the incoming register released, then the outgoing register can exchange information directly with the incoming register of the next exchange (see Figure 11/Q.440). If any intermediate exchange has to carry out charging operations, additional address information can be transferred from the outgoing register to the incoming register of that exchange as required. FIGURE 11/Q.440, p. In circumstances where transmission conditions do not comply with the requirements specified for System R2 (see Recommendation Q.457) or where System R2 is used in a satellite link, and thus the exchange of the interregister signals over the complete System R2 connection is not permitted, the overall multi-link connection is divided into end-to-end signalling sec- tions. When the number of multifrequency combinations available is not the same on all parts of the connection, the division should be made at an exchange between links having different numbers of mul- tifrequency combinations to improve the transfer of information. The outgoing R2 register is situated at the outgoing end of a signalling section on which System R2 interregister signalling according to the present specifications is used. It controls the call set-up over the whole signalling section. It sends forward interregister signals and receives backward interregister signals. The outgoing R2 register receives information via the preced- ing links of the connection in a form used by the signalling system applied over the last of these links; this system may be System R2, a decadic pulse system, or any other system. The preceding link may also be a subscriber's line. When an R2 register in a transit exchange is working according to this definition, it is also called an outgoing R2 register. Special cases of the outgoing R2 register are the outgoing international R2 register (see S 4.1.2) and the outgoing R2 regis- ter at the incoming end of a satellite link. The incoming R2 register is situated at the incoming end of a link on which System R2 multifrequency interregister signalling according to the present specification is used. It receives forward interregister signals via the preceding link(s) and sends backward interregister signals. The information received is used completely or in part for the control of selection stages and may be sent in its entirety or in part to the succeeding equipment, in which case the signalling used for retransmission is never System R2. Inter- working then takes place between System R2 and the other system. Thus every register not situated at the outgoing end of a System R2 signalling section is called an incoming R2 register independently of the exchange type. The term incoming R2 register | ay be applied also to devices controlling one or more selection stages, for example markers etc. It should be noted that in such equipment no provision is made for further transfer of the information received. 4.1.2 International end-to-end working The inclusion of a special outgoing international R2 register | s shown in Figure 11/Q.440 is dictated by a number of considera- tions. An outgoing international R2 register is an outgoing R2 register which controls the call set-up on a signalling section which comprises at least one international link and possibly national links in tandem. Even when System R2 is used in the national network of the originating country an outgoing international R2 register must always be provided in the country of origin to divide the connec- tion into two signalling sections for the following reasons: - to prevent the maximum number of links in a sig- nalling section over which System R2 can function from being exceeded on international calls; - to permit the use of fewer signalling frequencies in the country of origin (e.g. 5 forward and 5 or 4 backward sig- nalling frequencies); - to enable non-standardized meanings to be allo- cated to the spare national multifrequency combinations for use solely in the country of origin; - to provide criteria for distinction, at the incoming end, between national and international calls; - to have the facilities for routing and charging of international calls concentrated in one place if it is more economical than to equip every normal outgoing R2 register to per- form these functions. In general the outgoing international R2 register is situated at the outgoing international exchange. However, it may be situated in a national exchange preceding the international exchange pro- vided that the transmission requirements specified are fulfilled. End-to-end signalling from the outgoing international R2 register can be extended beyond the incoming international exchange even if a reduced number of signalling frequencies is used. How- ever, such international/national end-to-end signalling requires: - that the transmission conditions in the national destination network comply with the System R2 specifications, and - that the routing principles employed in the national destination network are compatible with the System R2 sig- nalling procedures for end-to-end transmission of address digits from the outgoing international R2 register to an incoming R2 register in a national exchange. 4.1.3 Register requirements Apart from signalling procedures specified in Section 5 the following characteristics of R2 registers are specified. 4.1.3.1 Outgoing R2 registers The multifrequency signalling equipment associated with outgoing R2 registers in national exchanges may be of the 2-wire or 4-wire type (see Recommendation Q.451). The multifrequency signalling equipment associated with outgo- ing international R2 registers must be of the 4-wire type. This is dictated by transmission constraints (see Recommendation Q.452). The outgoing R2 register controlling a given multi-link sig- nalling section must be able to recognize at least all the backward signals used on that section. The outgoing international R2 register must be able to send all 15 forward multifrequency combinations with the meanings speci- fied for international working. It must be able to receive all 15 backward multifrequency combinations and act upon the received information in the specified manner. In the case when System R2 is used on the preceding link the calling party's category signals have to be translated by the outgoing international R2 register as specified (see Recommendation Q.480). The outgoing R2 register must start call set-up as soon as it has received the minimum requisite information. Therefore signal transfer starts before the complete address information is received, i.e. before the caller finishes dialling. This overlap- ping interregister signalling particularly applies at an outgoing R2 register where the complete address information from a subscriber or operator is stored (e.g. local registers). This is in contrast to en bloc register signalling i.e. the transmission of all the address information as a whole in one sequence starting only after the complete reception of the address information. 4.1.3.2 Incoming R2 registers The multifrequency signalling equipment associated with incom- ing R2 registers may be of the 2-wire or 4-wire type (see Recommendation Q.451). The multifrequency signalling equipment associated with the incoming R2 register in an international exchange must be of the 4-wire type and equipped to send and receive all 15 multifrequency combinations. Every incoming R2 register in a multi-link signalling section must be able to recognize at least the forward signals used on that section and directed to that register. 4.1.4 System R2 compelled signalling method The compelled signalling operates as follows (see Figure 12/Q.440): - on seizure of a link, the outgoing R2 register automatically starts sending the first forward interregister sig- nal; - as soon as the incoming R2 register recognizes this signal, it starts sending a backward interregister signal which has its own meaning and at the same time serves as an ack- nowledgement signal; - as soon as the outgoing R2 register recognizes the acknowledging signal, it stops sending the forward interregis- ter signal; - as soon as the incoming R2 register recognizes the cessation of the forward interregister signal, it stops sending the backward interregister signal; - as soon as the outgoing R2 register recognizes the cessation of the acknowledging backward interregister signal it may, if necessary, start sending the appropriate next forward interregister signal. When the duration of the forward and backward signals is not controlled by the compelling mechanism described above, it is either limited by the time-out delay for release of the register or determined by the nature of the pulse imposed on them (see Recommendation Q.442). Figure 12/Q.440 shows a basic compelled signalling cycle. Figure 18/Q.457 shows in detail the build-up and time sequence of a compelled signalling cycle, including the operating and release times of multifrequency signalling equipment and other internal operating times of exchange equipment. Besides being a functional part of the compelled procedure, the acknowledging backward signals serve to convey special informa- tion concerning the required forward signals, to indicate certain conditions encountered during call set-up or to announce switchover to changed meanings of subsequent backward signals. Changeover to secondary meanings allows information concerning the state of the called subscriber's line to be transferred (see Recommendation Q.441, S 4.2.4). Therefore a choice of acknowledging backward signals is provided. Figure 12/Q.440 p.2 Recommendation Q.441 4.2 SIGNALLING CODE 4.2.1 Multifrequency combinations Each interregister signal consists of the simultaneous sending of 2 out of a range of 6, 5 or 4 in-band frequencies (multifre- quency combination). The band of the interregister signalling fre- quencies does not overlap with the frequency band generally used for line-signalling. This 2-out-of-n | ode allows erroneous signals consisting of less or more than two frequencies to be detected and identified as faulty. To make the system suitable for application on 2-wire links, two different sets of 6 frequencies are defined for the composition of the forward and the backward signals. Table 5/Q.441 shows all multifrequency combinations which can be derived from the maximum of 6 signalling frequencies per direc- tion provided by the system. For reference purposes each multifre- quency combination of a given direction is identified by a serial number. The numerical value of this serial number can be calculated by adding the respective index and weight allocated to the two fre- quencies making up the combination. The number of multifrequency combinations depends upon the number of signalling frequencies used. When the maximum of 6 sig- nalling frequencies are used, 15 multifrequency combinations are available. System R2 is designed for operation on international links with 15 multifrequency combinations in each direction. However, it can be used in national networks with a reduced number of signal- ling frequencies and still permit international/national end-to-end working of System R2 signalling in the case of incoming interna- tional traffic (see Figure 13/Q.441). The reduction naturally cuts the number of multifrequency com- binations available but has the economic advantage of effecting savings in equipment. The resulting reduction in facilities is of less importance in automatic than semi-automatic service. Tableau 5/Q.441 p.3 Figure 13/Q.441 p.4 4.2.2 Allocation of interregister signals The signal coding consists of the association of the defined meaning of interregister telephone signals to the multifrequency combinations transmitted over the links. Certain combinations are left spare for the allocation of national or international signals. (For signalling procedures, see Recommendations Q.460 to Q.480.) 4.2.2.1 Multiple meaning The meaning of both the forward and backward multifrequency combinations can change after transmission of certain backward sig- nals. The changed meaning is specific to the signal which caused or announced the change. In certain cases a change back to the origi- nal meaning is possible. The meaning of certain forward multifre- quency combinations may also vary depending on their position in the signalling sequence. 4.2.2.2 Meanings of the forward multifrequency combinations There are two groups of meanings allocated to the forward mul- tifrequency combinations. The Group I meanings are indicated in Table 6/Q.441 and the Group II meanings in Table 7/Q.441 . The change from Group I to Group II meanings takes place when requested by the backward signals A-3 or A-5. Change back to Group I meanings is only possible when the change to Group II meanings was in response to signal A-5. The first forward signal transmitted in international working is used for additional routing information. It enables a distinc- tion to be made between terminal and transit calls. In the case of terminal calls it carries the language or discriminating digit whereas, for transit calls, it serves the dual purpose of providing a country code indicator and to indicate whether an echo suppressor is required or not. The above arrangements obviate the need for two different seizing signals (line signals) to distinguish between transit and terminal traffic on links terminated in a transit exchange. 4.2.2.3 Meanings of the backward multifrequency combina- tions There are two groups of meanings allocated to the backward multifrequency combinations. The Group A meanings are indicated in Table 8/Q.441 and the Group B meanings in Table 9/Q.441. The change to Group B meanings is announced by the backward signal A-3. No change back is possible once a change to the Group B meanings of the backward multifrequency combinations has been indicated. 4.2.2.4 Integration of national and international signal- ling codes The utilization of Signalling System R2 in national networks is facilitated by the provision, in the specified signalling code, of signals specially assigned for national use. Under the specified code, specific national meanings are allocated to some of these signals, others are available for the allocation of national mean- ings at the discretion of each Administration. National allocations must not conflict with the present specifications in order to ensure end-to-end interregister signal- ling i.e. the direct dialogue between the outgoing international R2 register (in the originating country) and incoming R2 registers, in the national network of the destination country. The specified signalling code allows for a reduction of sig- nalling frequencies in national networks (see S 4.2.1 above). 4.2.3 Forward signals 4.2.3.1 Group I forward signals Compelled interregister signalling must always begin with a Group I forward signal. For the signal codes see Table 6/Q.441. TABLE 6/Q.441, p. The signals I-1 to I-10 | re numerical signals indicating: a) the address | equired for setting up the call (country code, national significant number); such address signals are sent by an outgoing R2 register or an outgoing international R2 regis- ter, either spontaneously and immediately after the seizure of the link or in response to one of the backward signals A-1, A-2, A-7 or A-8; b) the country code (and possibly the area code as well) of the location of the outgoing international R2 register , in response to signals requesting the origin of the call. For national traffic, the telephone number of the calling subscriber's line (see Recommendation Q.480); c) for automatic working the discriminating digit | r, in the case of semi-automatic working, the service language to be used by the operator (i.e. language digit ). The signal I-11 | s a non-numerical address signal. The mean- ing of the signal is dependent on its position within the sequence of address signals specified in Recommendation Q.107. a) Country code indicator, outgoing half-echo suppressor required When signal I-11 is transmitted as the first forward signal it indicates that: i) a country code will follow (international tran- sit); ii) the call requires echo suppressors; iii) the outgoing half-echo suppressor has to be inserted. The use of this signal in international working is subject to bilateral agreement and is made in conformity with Recommendation Q.479. b) Access to incoming operator (code 11) When signal I-11 is preceded by the language digit (and possi- bly by one further address digit) it indicates the address of the incoming operator's position and is then always followed by signal I-15 alone. For international working this signal is to be used only in conformity with Recommendation Q.107 | fIbis . It can be used in national traffic only if the incoming R2 registers are equipped to receive all six forward frequencies. The necessary specifications are then the responsibility of the Administration concerned. The signal I-12 | s a non-numerical address signal. The mean- ing of the signal is dependent on its position in the sequence of address signals specified in Recommendation Q.107. a) Country code indicator, no echo suppressor required When signal I-12 is transmitted as the first forward signal it indicates that: i) a country code will follow (international tran- sit); ii) the call may not require any echo suppressor (see Recommendation Q.479). b) Access to delay operator (code 12) When signal I-12 is preceded by the language digit (and possi- bly by one further address digit) it indicates that the call must be routed to the delay operator's position, either to a particular operator, or one of those operating a particular group of posi- tions. It is then followed by further digits and the signal I-15 or by the signal I-15 alone. In international working this signal is to be used in confor- mity with Recommendation Q.107 | fIbis . It can be used in national traffic only if the incoming R2 registers are equipped to receive all six forward frequencies. The necessary specifications are then the responsibility of the Administrations concerned. c) Request not accepted An outgoing international R2 register which receives a signal A-9 or A-10, the use of which is exclusively national, or which receives by signal A-13 a request to which it is unable to reply, should indicate by transmitting signal I-12 that it cannot answer the query (see Recommendation Q.480). This signal may be similarly used in national traffic to indicate that response to signal A-9 or A-10 is not possible. The meaning of the non-numerical signal I-13 is dependent on its position in the sequence of address signals specified in Recommendation Q.107. a) Test call indicator When in international working the signal I-13 is transmitted as the first forward signal it occupies the position of the language or discriminating digit. It serves then as a test call indicator and must be followed by the complete test equipment address information as specified in b) below. b) Access to test equipment (code 13) To access automatic test equipment the second signal I-13 (the address digit) must be followed by two digits xy and the signal I-15. c) Satellite link not included In response to signal A-13, the meaning of the signal I-13 is that up to the outgoing R2 register no satellite link is included. The meaning of the non-numerical signal I-14 | s dependent on its position in the sequence of address signals specified in Recommendation Q.107. a) Country code indicator, outgoing half-echo suppressor inserted When signal I-14 is transmitted as the first forward signal it indicates that: i) a country code will follow (international tran- sit); ii) the call requires echo suppressors; iii) the outgoing half-echo suppressor has already been inserted. This signal is to be used in international working and only in conformity with Recommendation Q.479. b) Incoming half-echo suppressor required In response to signal A-14, the meaning of the signal I-14 is that an incoming half-echo suppressor is necessary. c) Satellite link included In response to signal A-13, the meaning of the signal I-14 is that up to the outgoing R2 register a satellite link is included in the connection. The non-numerical signal I-15 | ndicates the end of a sequence of forward interregister signals. It is never sent as the first signal on an international link. a) End-of-pulsing In international working, signal I-15 is used to indicate that there are no more address signals to follow (see Recommendations Q.107 and Q.473). b) End of identification In national traffic, signal I-15 may be used to indicate that transmission of the sequence identifying the calling subscribers line is terminated (see Recommendation Q.480, S 5.8.2). 4.2.3.2 Group II forward signals The Group II forward signals are calling party's category sig- nals sent by outgoing R2 registers or by outgoing international R2 registers in reply to the backward signals A-3 or A-5 and give information whether national or international working applies. For the signal codes see Table 5/Q.441. It is useful to identify calls according to type or function: i) to indicate whether the forward transfer facil- ity is required in international traffic; ii) for adequate control of switching operations; iii) to enable any additional meanings for the signals A-5 used in a national network but not internationally accepted (for example, to change the meanings of one or more fol- lowing signals, forward or backward) to be made inoperative for incoming international calls; iv) for maintenance purposes. The meanings of the calling party's category signals are detailed below: a) The signal II-1, subscriber without priority | ndicates that the call is set up from a subscriber's line and is non-priority. b) The signals II-2 and II-9, subscriber with priority | ndicate that the call is set up from a subscriber's line to which priority treatment of calls has been accorded. Signal II-2 is specified for national working only since there are no Recommenda- tions regarding priority calls in automatic international working (see Recommendation Q.480). c) The signal II-3, maintenance equipment | ndicates that the call comes from maintenance equipment. d) The signal II-5, operator | ndicates that the call is set up from an operator's position. e) The signals II-6 and II-8, data transmission | ndicate that the call will be used for data transmission. f ) The signal II-7, subscriber | ndicates that the call is set up from a subscriber's line, an operator's position or from maintenance equipment and that no forward transfer signal will be used. g) The signal II-10, operator with forward transfer facility | ndicates that the call is set up from an operator's position with possibility of recourse to the forward transfer facility. Its use must be subject to bilateral agreement (see Annex A to the present Specifications). The signal II-4 and the signals II-11 to II-15 are spare. The meaning of signal II-4 will be decided later by international agreement. TABLE 7/Q.441, p. 4.2.4 Backward signals 4.2.4.1 Group A backward signals Group A backward signals (for signal codes see Table 8/Q.441) are required to acknowledge Group I forward signals and under cer- tain conditions, Group II forward signals. Besides that functional part of the compelled procedure, the Group A signals convey signal- ling information as detailed below: a) The signal A-1, send next digit (n + 1) | equests transmission of the next digit (n + 1) after reception of digit n . The latest address signal sent is assumed to have the rank n within the signal sequence specified in Recommendation Q.107. b) The signal A-2, send last but one digit (n - 1) | equests the transmission of digit (n - 1) after reception of digit n . The latest address signal sent is assumed to have the rank n within the signal sequence specified in Recommendation Q.107. This signal must not be used on a satellite link. TABLE 8/Q.441, p. c) The signal A-3, address-complete, changeover to reception of Group B signals | ndicates that the incoming R2 register at the incoming end needs no additional address digit and is about to go over to transmission of a Group B signal conveying information about the condition of the equipment at the incoming exchange or the condition of the called subscriber's line (see Recommendation Q.442). d) The signal A-4, congestion in the national net- work | ndicates: i) congestion of national links; ii) congestion in selection stages of terminal international or national exchanges; iii) occurrence of time-out or abnormal release of a System R2 register produced for any reason. For exception to these rules see n) below. See also signal B-4 and Recommendation Q.442. e) The signal A-5, send calling party's category | equests transmission of a Group II signal. f ) The signal A-6, address-complete, charge - set-up speech conditions | ndicates that the R2 register at the incoming end needs no additional digit, but will not send Group B signals. The call has to be charged on answer (see Recommendation Q.442). g) The signal A-7, send last but two digit (n - 2) | equests the sending of digit (n - 2) after reception of digit n . The latest address signal sent is to have the rank n within the signal sequence specified in Recommendation Q.107. This signal must not be used on a satellite link. h) The signal A-8, send last but three digit (n - 3) | equests the sending of digit (n - 3) after reception of digit n . The latest address signal sent is to have the rank n within the signal sequence specified in Recommendation Q.107. This signal must not be used on a satellite link. i) The signals A-9 to A-10 | re spares available for allocation of national meanings. The use of signals A-9 and A-10 in national networks can be decided by each Administration. These signals must not be used on international satellite links. j) The signal A-11, send country code indicator | equests the country code indicator (transit indication) in ack- nowledgement of any forward signal. The signal is used in international transit calls only (see Recommendation Q.462). This signal must not be used on a satellite link. k) The signal A-12, send language or discriminating digit | equests the language digit or the discriminating digit in ack- nowledgement of any forward signal. This signal must not be used on a satellite link. l) The signal A-13, send nature of circuit | equests information regarding the nature of the circuits involved in the connection so far, i.e. satellite link (see Recommendation Q.480). This signal should only be used on a satel- lite link by bilateral agreement. m) The signal A-14, request for information on use of echo suppressor | is an incoming half-echo suppressor required?) indicates that an incoming international exchange ack- nowledges the discrimination digit or the language digit and that it is possible to insert an incoming half-echo suppressor in that incoming international exchange if required. This signal must not be used on a satellite link. n) The signal A-15, congestion in an international exchange or at its output | ndicates: i) congestion on international links; ii) congestion in selection stages at an interna- tional transit exchange or at a terminal international exchange and/or its outgoing links; iii) occurrence of time-out or abnormal release of a System R2 register produced for any reason (see Recommendation Q.442). 4.2.4.2 Group B backward signals Any Group B backward signal (for signal codes see Table 9/Q.441) acknowledges a Group II forward signal and is always preceded by the address-complete signal A-3 which indicates that the incoming R2 register has received all the Group I forward sig- nals it requires from the outgoing international R2 register. Besides that functional part of the compelled procedure the Group B signals convey information about the condition of switching equip- ment in the incoming exchange, or about the condition of the called subscriber's line, to the outgoing international R2 register which can then take the necessary action as specified in Recommendation Q.474. TABLE 9/Q.441, p. The following Group B signals are specified: a) The signal B-1 is spare | or national use and its meaning must be compatible with that of signal B-6 (see Recom- mendation Q.474). b) The signal B-2, send special information tone | ndicates that the special information tone should be returned to the calling party. This tone indicates that the called number can- not be reached for reasons not covered by other specific signals and that the unavailability is of a long term nature (see also Recommendation Q.35). c) The signal B-3, subscriber's line busy | ndi- cates that the line or lines connecting the called subscriber to the exchange are engaged. d) The signal B-4, congestion | ndicates that congestion condition is encountered after the changeover from Group A signals to Group B signals. The signal B-4 shall be sent in the conditions specified for signal A-4 [see S 4.2.4.1, d) and Recommendation Q.474, S 5.3.5.1]. e) The signal B-5, unallocated number | ndicates that the number received is not in use (e.g. an unused country code, an unused trunk code or subscriber number that has not been allocated). f ) The signal B-6, subscriber's line free, charge | ndicates that the called party's line is free and that the call has to be charged on answer. g) The signal B-7, subscriber's line free, no charge | ndicates that the called party's line is free but is not to be charged on answer. This signal permits non-chargeable calls without the need for transferring "no charge" information by line signals. h) The signal B-8, subscriber's line out of order | ndicates that the subscriber's line is out-of-service or faulty. i) Signals B-9 and B-10 | re spare for national use. Their meaning must not be incompatible with the sending of special information tone to the calling party (see Recommendation Q.474). j) Signals B-11 to B-15 | re spare for national use. Their meaning can be allocated as required. Sending of these signals into the international network must be prevented. To deviate from this rule will result in the actions defined in Recommendation Q.474. Recommendation Q.442 4.3 PULSE TRANSMISSION OF BACKWARD SIGNALS A-3, A-4, A-6 OR A-15 Under certain conditions it may prove necessary or desirable to send one of the signals A-3, A-4, A-6 or A-15 without prior reception of a forward signal. This can occur when the incoming R2 register, after acknowledging a recognized forward signal, is unable to complete the call (for example during congestion) and the next forward signal does not appear on the line; or when the address-complete signal must be sent after the last forward address signal has been acknowledged. It may be desirable to deliberately suspend compelled signalling by acknowledging the last address digit, and signal I-15 if received, with signal A-1 to avoid pro- longing the transmission time of certain interregister signals. Such a course should cer- tainly be considered when there is a possibility that a relatively long period may elapse between reception of the last digit and detection of the condition of the called subscriber's line. The average duration of such periods during the busy hour must be lim- ited to 3 seconds in view of the load on the carrier systems in the case of international calls. The following conditions must be observed in transmitting pulsed interregister signals (see Figure 14/Q.442): - the minimum delay between the end of transmis- sion of the last signal of the compelled cycle and the start of transmission of the pulse signal must be 100 ms; - the pulse duration must be 150 _ 50 ms. Reception of a pulse signal must cause interruption of any forward signal in course of transmission at the outgoing R2 regis- ter. It is sometimes impossible, however, to prevent a forward sig- nal from being sent by the outgoing R2 register at the very moment when one of the backward signals A-3, A-4, A-6 or A-15 is sent in pulse form by the register at the incoming end. To reduce the operating difficulties which may result, the incoming R2 register must be so designed that no forward multifre- quency combination can be recognized during and after the transmis- sion of signals A-4, A-6 or A-15 in pulse form or during 300 _ 100 ms from the start of transmission of the address-complete signal A-3 in pulse form (900 _ 180 ms when signal A-3 is transmit- ted over a satellite link) (see Figures 14/Q.442 and 15/Q.442). When the end of a pulse signal A-3 has been recognized in the outgoing R2 register, a Group II signal must be sent forward. The incoming R2 register will acknowledge this signal by a Group B sig- nal. On recognizing signal A-4, A-6 or A-15 no forward signal is sent by the outgoing R2 register. The end of these backward signals must cause the dismissal of the outgoing and incoming R2 registers in accordance with Recommendation Q.475. The conditions under which pulse transmission of the backward signals A-3, A-4, A-6 or A-15 apply are specified in Section 5. Figure 14/Q.442, p. 4.4 MULTIFREQUENCY SIGNALLING EQUIPMENT Recommendation Q.450 4.4.1 GENERAL Since System R2 can provide, in international working, end-to-end signalling from the outgoing international R2 register to an incoming R2 register at the called subscriber's local exchange (see Recommendation Q.440), the specifications for mul- tifrequency signalling equipment take account of transmission con- ditions in both the international and national networks. The incoming national network may include both 4-wire and 2-wire links. However, it is assumed in the following specifications for multifrequency signalling equipment for outgoing international R2 registers and incoming R2 registers in international exchanges including the incoming international exchange that the registers are directly connected by four wires to the virtual switching points of the links. The registers thus contain a multifrequency signalling equipment with a transmitting part and a receiving part, each separately connected to the GO and RETURN path of the 4-wire circuit respectively (see Figure 16/Q.451). When the outgoing international R2 register is situated in a national exchange preceding the outgoing international exchange, or when the incoming R2 register is situated in a national exchange following the incoming international exchange, special conditions apply (see Recommendation Q.457). The upper and lower limits specified for the sending level and for the national extension attenuation leave a degree of freedom, thus simplifying the problem of interworking among different net- works. The outgoing international R2 register relays the interre- gister signals by the method described in Recommendation Q.478. The present specification ensures that the system has an adequate range. Recommendation Q.451 4.4.2 DEFINITIONS 4.4.2.1 Multifrequency signalling equipment During the exchange of multifrequency combinations the link or multi-link section is terminated at both ends by equipment allowing multifrequency combinations to be sent and received under mutual, compelled control, as indicated in Recommendation Q.440. Generally speaking, the equipment involved in the signal transfer at each end of the multi-link section has the following functions: - reception of multifrequency combinations; - protection against disturbances (e.g. 2-out-of-n -check, 0-out-of-n -check, see Recommendation Q.458); - transfer of signals to and from the registers or equivalent equipment; - sending of multifrequency combinations. This equipment as a whole can be considered as a single func- tional unit, henceforth called the multifrequency signalling equip- ment. The functions of the multifrequency signalling equipment may be distributed among a number of sub-units, according to the design principles adopted in each particular case; these, within certain limits, are open to choice. For the purpose of the present specification, the multifre- quency signalling equipment is divided into a sending part and a receiving part. a) Four-wire multifrequency signalling equipment Multifrequency signalling equipment connected to the speech path by two pairs of wires is called 4-wire multifrequency signal- ling equipment (see Figure 16/Q.451). In such cases, the speech path itself is generally 4-wire too. In Figure 16/Q.451, point B is the output of the sending part and point C the input to the receiving part of the 4-wire multifre- quency signalling equipment. The latter includes the devices for protection against disturbances and the equipment for transferring signals to the register or equivalent equipment. b) Two-wire multifrequency signalling equipment Because different frequencies are used for interregister sig- nalling in the two directions of transmission, a single pair of wires connecting the speech path to the multifrequency signalling equipment can be used for simultaneous sending and reception of multifrequency combinations. Multifrequency signalling equipment connected to the speech path by a single pair of wires is called 2-wire multifrequency sig- nalling equipment (see Figure 17/Q.451). In Figure 17/Q.451, point A is the output of the sending part and, at the same time, the input to the receiving part. The latter includes the devices for protection against disturbances and for transferring signals to the register or equivalent equipment. 4.4.2.2 Operate and release times of the receiving part of the signalling equipment The speed and reliability of the compelled interregister sig- nal transfer is dependent on the operation and release times T0, TR, T `0and T ` Rdefined below. These times include the minimum recognition times referred to in S 4.4.5.3 below. FIGURE 16/Q.451, p. a) Operation time If the two frequencies making up a multifrequency combination are applied simultaneously to the input of the receiving part of the multifrequency signalling equipment, the time interval between the application of both frequencies and recognition of the mul- tifrequency combination is called operation time T0. If one of the two frequencies making up a multifrequency com- bination is applied to the input of the receiving part of the mul- tifrequency signalling equipment somewhat after the other fre- quency, the time interval between the application of the second frequency and recognition of the multifrequency combination is called operation time T `0. b) Release time If the two frequencies making up a multifrequency combination are simultaneously cut off from the input of the receiving part of the multifrequency signalling equipment, the time interval between the cut-off and recognition of the end of the multifrequency combi- nation is called release time TR. If one of the two frequencies making up a multifrequency com- bination is cut off from the input of the receiving part of the multifrequency signalling equipment somewhat after the other fre- quency, the time interval between the cut-off of the second fre- quency and recognition of the end of the multifrequency combination is called release time T ` R. 4.4.2.3 Internal operation time a) If the equipment at a switching centre has to analyse one or more of the signals received and if it has to carry out routing and switching operations before being able to determine which back- ward signal should be sent, it can delay completion of the com- pelled signalling cycle. Such delay depends on the function the centre has and on the design of the switching equipment used in it. The delay cannot therefore be specified. The time required by an incoming R2 register to determine which backward signal to send in reply to a forward signal is denoted as T int 1 must therefore be kept to a minimum. Its value will be zero if the signal to be sent is already determined. b) An outgoing R2 register sends a forward signal after inter- preting the backward signal it has received. The time required to determine the appropriate signal is denoted as T int 2 c) At the end of a forward signal the incoming R2 register may have to perform certain functions in order to be ready to receive the next signal. The time required for these functions is denoted as T int 3 Recommendation Q.452 4.4.3 REQUIREMENTS RELATING TO TRANSMISSION CONDITIONS 4.4.3.1 Impedances The impedance ZAmeasured at the terminals A (see Figure 17/Q.451) of a 2-wire multifrequency signalling equipment will have a nominal value equal to the nominal terminating impedance ZTof the links at the exchange under consideration and will be balanced to earth. In most cases this value ZTwill be 600 ohms non-reactive. The impedance ZAwill then satisfy the condi- tions: 20 log | |00 - Z AfR __________| | _" 10 dB (1) in the 300-3400 Hz band, and 20 log | |00 - Z AfR __________| | _" 16 dB (2) in the 520-1160 Hz and 1360-2000 Hz bands. The impedances ZBand ZCmeasured at the terminals B and C (see Figure 16/Q.451) of a 4-wire multifrequency signalling equipment will have a nominal value equal to the nominal terminating impedance ZTat the exchange under consideration and will be bal- anced to earth. In most cases this value ZTwill be 600 ohms non-reactive. The impedances ZB and ZCwill then satisfy condi- tion (1) above in the 300-3400 Hz band and condition (2) above in the 520-1160 Hz or 1360-2000 Hz bands, according to the set of frequencies generated by the equipment concerned. All the above requirements must be met, whether signalling frequencies are being transmitted or not. 4.4.3.2 Echoes The balance return loss presented to the international link by different national networks varies considerably. Recommendation G.122 gives the desirable values based on transmis- sion requirements and these values could hardly be improved on sim- ply in order to facilitate signalling, especially as the balance return loss would then have to be very high. The multifrequency signalling equipment must therefore func- tion with an echo signal. Allowance is made for this in the test conditions (see Recommendation Q.455). In order to eliminate disturbing and unwanted double-echo effects, outgoing international R2 registers must be provided with 4-wire signalling equipment; the 4-wire loop will then remain open at the outgoing end of the multi-link section during signalling. For the same reason, it is recommended that 4-wire signalling equipment be provided whenever 4-wire switching equipment is used. 4.4.3.3 Location and connection of mutifrequency signalling equipment associated with outgoing international R2 registers The multifrequency signalling equipment of the outgoing inter- national R2 registers is assumed to be directly connected to the virtual switching point of the links in an international exchange. This equipment may be situated in a national switching centre of the country of origin on the national network side of the outgoing international exchange provided that the 4-wire extension links between this centre and the outgoing international exchange have the same upper limit for the standard deviation of transmission loss variations with time (1 dB) as the international links. Should the national extension links not have a nominal transmission loss of 0.5 dB between the national exchange and the virtual switching point in the outgoing international exchange, there must be suitable compensation of the power level of the mul- tifrequency combinations. Recommendation Q.454 4.4.4 THE SENDING PART OF THE MULTIFREQUENCY SIGNALLING EQUIPMENT 4.4.4.1 Signalling frequencies The composition of the multifrequency combinations is speci- fied in Recommendation Q.441. The frequencies in the forward direction are: f0 = 1380, f1 = 1500, f2 = 1620, f3 = 1740, f4 = 1860, f5 = 1980 Hz. The frequencies in the backward direction are: f0 = 1140, f1 = 1020, f2 = 900, f3 = 780, f4 = 660, f5 = 540 Hz. The frequency variation at the sending point must not exceed _ | Hz relative to the nominal value. 4.4.4.2 Absolute power level transmitted a) Four-wire multifrequency signalling equipment for outgoing international R2 and incoming R2 registers assumed to be directly connected to the virtual switching point in an international exchange: i) The absolute power level of each non-modulated signalling frequency transmitted by the sending part of the mul- tifrequency signalling equipment in the international exchange con- cerned will have a nominal value of -8 dBm0 with a tolerance _ | dB. At an analogue input of the exchange, this corresponds to -11.5 dBm _ 1 dB. ii) The difference in level between the two signal- ling frequencies making up a multifrequency combination must be less than 1 dB. The tolerances specified apply to the sending point itself, i.e. terminal B in Figure 16/Q.451. b) Four-wire multifrequency signalling equipment for outgoing international R2 registers situated in a national exchange further down in the network of the country of origin: i) The sending-level conditions mentioned above under a) are applicable in such national exchanges, provided that the national 4-wire extension links between the national exchange and the outgoing international exchange have the same nominal transmission loss (0.5 dB) and the same upper limit of standard deviation of transmission loss variations with time (1 dB) as the international links. ii) If these national links do not have a nominal transmission loss of 0.5 dB, the level of interregister frequen- cies must be compensated as required. c) Multifrequency signalling equipment for incoming R2 registers in national exchanges: The nominal absolute power level N | f a single signalling frequency transmitted by the sending part of the multifrequency signalling equipment (2-wire or 4-wire) in any national exchange must be chosen within the limits: N _" A b + 0,5 m + 2.3 \| _______________________ (m + k)+(m + k + 1)0.04 - 31 dBm (1) .sp 1 and N A b - 11.5 dBm (2) .sp 1 or N A b + 0.5 m - 2.3 \| _______________________ (m + k)+(m + k + 1)0.04 - 9 dBm (3) .sp 1 whichever of (2) or (3) yields the lower value. In order to avoid marginal operation, it is recommended that levels higher than the minimum levels given by formula (1) be specified. In these formulae: - m = the number of 4-wire links switched in tan- dem between the outgoing international R2 register and the incoming international exchange. The number m thus includes the national 4-wire extension links in the country of origin (see Recommendation Q.457) and the international links. In formula (1) m should always be given the maximum value 4. In formula (3), m should be given all its values, from minimum to maximum, and the lowest value so obtained or derived from formula (2) is to be taken as the upper limit of N . - k = the number of national 4-wire extension links switched in tandem between the incoming international termi- nal exchange and the national exchange. The maximum value of k will be 4. - Ab = the nominal transmission loss at 800 Hz in the backward direction between the output terminals of the sending part of the multifrequency signalling equipment of the incoming R2 register in the exchange under consideration and the send side of the virtual switching point of the backward speech path in the incoming international exchange. Formulae (1), (2) and (3) were derived as shown in Annex A to Section 4. The variation in level relative to the nominal value N | hosen for a given exchange must not exceed _ | dB. However, the difference in level between the two signalling frequencies making up a multifrequency combination must not exceed 1 dB. 4.4.4.3 Signalling frequency leak level The total power level of the leak current transmitted to line must be: a) at least 50 dB below the nominal level of one signalling frequency when no multifrequency combination is being sent; b) at least 30 dB below the level of either of the signalling frequencies when a multifrequency combination is being sent. Furthermore, any single leak current must be at least 34 dB below the level of either of the signalling frequencies when a mul- tifrequency combination is being sent. 4.4.4.4 Harmonic distortion and intermodulation The total power level of all frequencies due to harmonic dis- tortion and intermodulation within the frequency band 300-3400 Hz must be at least 37 dB below the level of one signalling frequency. 4.4.4.5 Time tolerance for multifrequency combinations The time interval between the start of sending of each of the two frequencies constituting a multifrequency combination must not exceed 1 ms. The time interval between the cessation of sending of each of the two frequencies must not exceed 1 ms. Recommendation Q.455 4.4.5 THE RECEIVING PART OF THE MULTIFREQUENCY EQUIPMENT 4.4.5.1 Sensitivity range The power levels given below relate to the nominal impedance of the receiving part of the multifrequency signalling equipment. The receiving part of the multifrequency signalling equipment shall have a sensitivity range of -31.5 dBm0 to -5 dBm0 _________________________ For equipments already in service the sensitivity range is -35 dBm to -5 dBm. 4.4.5.2 Operate and release time requirements The operating and release times of the receiving part of the multifrequency signalling equipment depend on the design and, for a given design, on: - the time difference between the instants of reception of each of the two frequencies making up a multifrequency combination; - the level of each of the two frequencies; - the difference in level between the two frequen- cies; - the level, spectrum and instant of onset of the noise. The factors vary with transmission conditions. With certain types of switching equipment it may prove advisable to embody dev- ices to counteract low-frequency disturbances in the multifrequency signalling equipment. Time requirements have been specified for two types of mul- tifrequency test combinations , A and B, applied to the input of the receiving part of the multifrequency signalling equipment in the presence of disturbing signals as specified below. When test combinations and disturbing frequencies as specified under a) to c) below are applied to the terminals A (see Figure 17/Q.451) of 2-wire multifrequency signalling equipment or to the terminals C (see Figure 16/Q.451) of 4-wire multifrequency signalling equipment, the following time requirements must be met: - for type A test combinations: T 0 + T R 70 ms - for type B test combinations: T 0 + T R 80 ms - for test combinations of types A and B: (T ` 0 + T ` R ) (T 0 + T R ) + 5 ms For definitions of T0, T `0, TR, and T ` R see Recommendation Q.451. For the third requirement, the only cases to be considered are those in which the frequency first appearing is also the first to disappear (transmission delay effect). The T ` 0 and T ` R time requirements are specified to ensure proper functioning of the multifrequency signalling equipment when the received multifre- quency combination is affected by group delay distortion, for exam- ple. The appropriate procedure for this test depends on the design of the equipment to be tested; in many cases it will be convenient to assume that a delay of one second or more for the second fre- quency constitutes the most unfavourable case. When a multifrequency combination has caused the receiving part of the multifrequency signalling equipment to operate, the latter cannot release if the signal frequencies are interrupted for not more than 7 ms. A method of improving the system reliability in case of interrupted signals is described in Annex B of Section 4. a) Multifrequency test combinations type A - The multifrequency test combination consists of any 2-out-of-n combination of the n signalling frequencies; - each frequency differs from the nominal fre- quency by not more than _ | Hz; - the absolute power level of each of the two fre- quencies of the multifrequency combination lies between -5 dBm and -20 dBm; - the difference in level between the two frequen- cies is not greater than 3 dB. b) Multifrequency test combinations type B - The multifrequency test combination consists of any 2-out-of-n combination of the n signalling frequencies; - each frequency differs from the nominal fre- quency by not more than _ | 0 Hz; - the absolute power level of each of the two fre- quencies of the multifrequency combination lies between -5 dBm and -35 dBm; - the difference in level between the two frequen- cies is not greater than 5 dB for adjacent frequencies and 7 dB for non-adjacent frequencies. c) Disturbing frequencies The disturbing frequencies to be applied are: - in all cases one or more of the n | requencies for which the receiving part under test is designed, with a total power level of -55 dBm or less, when no multifrequency test combi- nation is being applied; - when a multifrequency test combination is applied, one or more of the (n - 2) remaining frequencies with a total power level 20 dB below the highest test combination fre- quency level during application of the test combination; - for testing the receiving part of 4-wire mul- tifrequency signalling equipment in an outgoing international R2 register: any multifrequency combination consisting of two frequen- cies out of the forward group of frequencies, each of these two frequencies having a level of 13.5 dB above the lowest test-combination frequency level in the backward direction; an upper limit of -12.5 dBm is nevertheless specified for the level of the disturbing signal. System R2 interregister signalling may be used in the country of origin on the national links preceding the out-going international R2 register. In that case, the receiving part of 4-wire multifrequency signalling equipment connected to the forward speech path of the national 4-wire links may have to function in the presence of frequencies used in the backward direction. No gen- eral specification can be given for the level of these disturbing frequencies; it is recommended that Administrations formulate their own specifications. - for testing the receiving part of 2-wire mul- tifrequency signalling equipment: any multifrequency combination at the highest level used in operational signalling (measured at point A, Figure 17/Q.451) by the sending part of the 2-wire multifrequency signalling equipment concerned. 4.4.5.3 Non-operate and non-recognition requirements The receiving part of the multifrequency signalling equipment must remain in the non-operate state when the following distur- bances, singly or together, are the only signals that are applied to the terminals A or C (see Figures 17/Q.451 and 16/Q.451): - any single pure sine wave or any combination of two pure sine waves, each with a power level of -38.5 dBm0 within the 300-3400 Hz band; - any single pure sine wave or any combination of two pure sine waves, each with a power level of -42 dBm within the 300-3400 Hz band; - any combination of two pure sine waves, each with a power level of -5 dBm within the 1300-3400 Hz band for the set of frequency receivers used in the backward direction; and within the 330-1150 Hz and 2130-3400 Hz bands for the set of frequency receivers used in the forward direction. Furthermore, when signalling tones have activated the receiv- ing part of the multifrequency signalling equipment, this must assume the non-operate state when the signalling tones are removed in the presence of these same disturbances, singly or together, at the above-mentioned terminals. The receiving part of the multifrequency signalling equipment must not recognize a combination consisting of two signalling fre- quencies out of the set of frequencies normally used in the transmission direction considered, each having a level not exceed- ing -5 dBm and a duration of less than 7 ms. The receiving part of the multifrequency signalling equipment must not recognize a combination consisting of two signalling fre- quencies used in the transmission direction considered, having a difference in level of 20 dB or more. 4.4.5.4 Influence of transient disturbances | see also Recommendation Q.458) The recognition of faulty signals due to short-lived transient conditions can largely be avoided if a multifrequency combination is recognized only after a specified minimum time, during which two, and only two, of the individual receivers are active and if the absence of multifrequency combinations is recognized only after a minimum time, during which all individual receivers are at rest. These times are included in the operating and release times T0and TR. Typical transient disturbances, such as clicks, change of polarity, etc., generated by the switching equipment, must not change signals transferred from the receiving part of the signal- ling equipment to the register. It is recommended that Administrations formulate their own specification for a test method according to the type of distur- bances encountered in their switching equipment. _________________________ For equipments already in service, the non-operation level is -42 dBm. 4.5 RANGE, SPEED AND RELIABILITY OF INTERREGISTER SIGNALLING Recommendation Q.457 4.5.1 RANGE OF INTERREGISTER SIGNALLING 4.5.1.1 Number of international links The number of international links switched in tandem for establishing an international connection using System R2 must not exceed 4 (see Recommendation Q.440). Assumptions for the transmission loss in 4-wire international links: i) nominal transmission loss at 800 Hz: 0.5 dB (Recommendation G.101, S 5); ii) standard deviation of transmission loss varia- tions with time must not exceed 1 dB [Recommendation G.151, S 3, a)]; iii) the difference between the mean value and the nominal is assumed to be 0 [as in Recommendations G.122, S 1.2), and G.131, S 1]. 4.5.1.2 Number of national links a) Outgoing international R2 register and number of national 4-wire extension links in the country of origin The outgoing international R2 register is always provided with 4-wire multifrequency signalling equipment and the 4-wire loop will be open during interregister signalling. The outgoing international R2 register must be placed in an exchange from where the incoming international exchange in the country of destination is reached by not more than four 4-wire links switched in tandem (see Recommendation Q.440). It is understood that the national 4-wire links in the country of origin shall have the same standard deviation of transmission loss variations with time (1 dB) as the international links and that, if these national links do not have the same nominal transmission loss as the international links (0.5 dB), appropriate compensation of the multifrequency com- bination levels will be made in both directions of transmission. b) Number (k) of national 4-wire extension links in the country of destination Not more than four national 4-wire extension links may be used. Forward transmission loss in the country of destination: i) The standard deviation of transmission loss variations with time in the national 4-wire extension links in the country of destination must not exceed 1 dB. ii) The nominal transmission loss at 800 Hz in the forward direction (Af) between the virtual switching point in the incoming international exchange and any incoming R2 register in the country of destination must not exceed: 11.4 dB for a country using 3 national 4-wire extension links at the most or 11.0 dB for a country using 4 national 4-wire extension links at the most, and must never be less than: A f min. = -2.5 - 0.5 m + 2.3 \| ________________________ (m + k) +(m + k + 1)0.04 dB. The values for the minimum forward transmission loss A f min. resulting from this formula are shown in Table 10/Q.457 (for the definitions of m and k see Recommendation Q.454). In practice the forward transmission loss must not fall below these values. The formula has been determined as shown in Annex C to Sec- tion 4. When System R2 end-to-end signalling is applied in a national network, the 4-wire links involved may not conform to the charac- teristics specified by the CCITT for international circuits. Also, national transmission plans may be based on principles different from those of the international transmission plan. Consequently, the range over which end-to-end signalling is possible must be established using calculations, e.g. as shown in Annex C to Section 4 (see also SS 4.5.1.3 and 4.5.1.4 below). Table 10/Q.457, p. 4.5.1.3 Total attenuation distortion It has been assumed that at all frequencies within the 530-1990 Hz band the overall attenuation distortion relative to 800 Hz between the outgoing international R2 register and any incoming R2 register will not exceed _ | dB. Attention is drawn to the fact that on some national connections these assumptions might not be fulfilled. As type B test signals (see Recommendation Q.455) allow for a 5 dB difference in level between two adjacent signalling frequen- cies, and a 7 dB difference between two non-adjacent signalling frequencies, a 4 dB attenuation distortion of the multi-link sec- tion can be allowed for two adjacent frequencies and a 6 dB distor- tion for two non-adjacent frequencies, provided that the level of the weakest signalling frequency is not lower than -35 dBm at the terminals of the receiving part of the multifrequency signalling equipment. The values 4 dB and 6 dB were obtained by allowing for a 1 dB difference in sending level. 4.5.1.4 Intermodulation A multifrequency signalling system in conformity with the above specifications will allow satisfactory working over a multi-link section introducing intermodulation products from two signalling frequencies and falling within the 520-1160 Hz and 1360-2000 Hz bands, the level of each of such products being at least 24 dB below the highest signal frequency level. 4.5.2 Build-up and time specification of a complete forward compelled signalling cycle Figure 18/Q.457 shows in detail the build-up and time sequence of a compelled signalling cycle. Figure 18/Q.457 p. In this figure: TP\dFdenotes the transmission delay of the slower of the two frequencies of a forward multifrequency combination; TP\dBdenotes the transmission delay of the slower of the two frequencies of a backward multifrequency combination; T0and T `0 denote the operating times as defined in Recommendation Q.451; TRand T ` Rdenote the release times as defined in Recommendation Q.451; T int 1 , T int 2 and T int 3 denote the internal opera- tion times as defined in Recommendation Q.451. If the values of T int 2 and T int 3 lie within certain lim- its, they do not contribute to the total duration of the compelled signalling cycle, as can be seen from Figure 18/Q.457. TS\d1and TS\d2denote respectively the time required for starting and stopping the sending of a multifrequency combination (switching-on or switching-off times, exclusive of logic opera- tions). It thus appears that the total duration T of a complete com- pelled signalling cycle is given by the formula: T = 2 (T PF + T PB ) + | | | (T ` 0 + T ` RfR) DfR +(T ` 0 + T ` RfR) AfR T 0 + T RfR) DfR + (T 0 + T RfR) AfR | | | + T int 1 + (T S 1 + T S 2 ) D + (T S 1 + T S 2 ) A or The subscripts D and A apply respectively to the outgoing and the incoming registers. TP\dFand TP\dBdepend on the propagation characteristics of the forward and backward speech paths respectively and therefore cannot be specified. A value of 10 ms for TP\dFand TP\dBcan be considered as typi- cal, e.g. for average terrestrial regional connections and 320 ms for circuits including a satellite link. The maximum value of T0 + TRhas been fixed at 70 ms. A value of 35 ms could be taken in a certain number of cases as represent- ing the minimum duration of T0 + TR. T int 1 , T int 2 and T int 3 are dependent on the type of exchange and therefore cannot be specified; but their contribution to the total duration of the compelled signalling cycle must be kept as small as possible. If the influence of T int 1 , T int 2 and T int 3 is ignored, and if the extreme values of T0 + TRand for TS\d1 + TS\d2are assumed to be identical for the outgoing and the incoming register, the extreme values of TS\d1 + TS\d2being taken as 5 ms TS\d1 + T2 10 ms and if the value of 10 ms indicated above as typical is adopted for TP\dFand TP\dB, the probable extreme values of the compelled signalling cycle T would be: for terrestrial connections: 120 ms T 200 ms. for circuits including a satellite link: 1080 ms T 1440 ms. The signalling rates would be between approximately 8 and 5 signalling cycles per second for terrestrial circuits. These values are not absolute limits; the signalling cycle could, for example, be longer on a complex connection or in the presence of noise or other conditions approaching those of type B test combina- tions (see Recommendation Q.455). Recommendation Q.458 4.5.3 RELIABILITY OF INTERREGISTER SIGNALLING 4.5.3.1 General In signal transmission, reliability and speed are to some extent conflict requirements: the slower the signalling, the more reliable it is likely to be. System R2 combines the two essential requirements of speed and reliability, since it is a compelled sys- tem, adapting its signalling speed to the working conditions with minimum loss of reliability. System R2 is protected against the acceptance of faulty infor- mation (multifrequency combinations consisting either of one fre- quency only or of more than two frequencies) by means of the 2-out-of-n method of checking the number of frequencies received. This protection is inoperative in the case of disturbances (noise, clicks, etc.) activating two, and only two, of the single frequency receivers; nor does it prevent the release of all the receivers (thus wrongly indicating the end of a multifrequency com- bination) in the event of an interruption caused by disturbances during transmission of a multifrequency combination. Disturbances causing the operation of two single frequency receivers or the release of all the receivers are due mainly to short-lived, transient conditions. The recognition of faulty infor- mation due to such conditions can therefore largely be avoided by so designing the receiving part of the multifrequency signalling equipment that a multifrequency combination will be recognized only after a specified minimum time. During this time two, and only two, of the individual receivers are active and the absence of multifrequency combinations will be recognized only after a specified minimum time, during which all individual receivers are at rest. The degree of protection against faults due to the above-mentioned conditions is dependent on these times, which are included in the operating and release times defined for the receiving part of the multifrequency signalling equipment in Recommendation Q.451. 4.5.3.2 Error rate for compelled working The test of the multifrequency signalling equipment as a whole consists in continuous, compelled transmission of multifrequency combinations. It must be ensured that all possible combinations of the for- ward and backward multifrequency combinations have equal probabil- ity of occurrence during the test period. The error rate is observed at the receiving part at both ends of the link and is defined, for each end, as the number of errors divided by the number of combinations sent by the corresponding sending parts at each end of the link. It is for each Administration to define the sources of per- manent and impulsive noise to be applied to the interface between the sending and receiving parts in the light of its experience and local conditions. The compelled working may be tested, on the one hand, by using test combinations of Type A (see Recommendation Q.455) in the presence of noise at a power level of -40 dBm and a uniform power distribution in the 300-3400 Hz band (filtered white noise) and, on the other hand, by using Type B test combinations in the presence of noise at a power level of -45 dBm and a uniform power distribu- tion in the 300-3400 Hz band. The error rates in these conditions will be: - for Type A test combinations and noise at -40 dBm: 10DlF2615; - for Type B test combinations and noise at -45 dBm: 10DlF2614. ANNEX A (to Section 4) (see Recommendation Q.454) Development of formulae for the power level of signalling frequencies The formulae (1), (2), and (3) of Recommendation Q.454 describing the nominal absolute power level N of a transmitted sin- gle signalling frequency were derived as follows: 1. The nominal transmission loss in the backward direction between the incoming R2 register and the outgoing international R2 register is given by (see Recommendation Q.457): Ab + 0.5 m dB. 2. On the assumption that the contribution of the national 2-wire links to transmission loss variations with time is negligi- ble and with allowance for the standard deviation of transmission loss variations with time of the international and national 4-wire circuits and in the exchanges (see CCITT Recommendation Q.45: ~ = 0.2 dB), the total transmission loss variation for a 1% proba- bility of it being exceeded is given by: _ 2.3 \| _________________________ (m + k)+(m + k + 1)(0.2)2 dB. 3. Given a total attenuation distortion of _ | dB relative to 800 Hz (see Recommendation Q.457) and a tolerance of _ | dB on the nominal power level N , the lower and upper limits of the level received in the outgoing international R2 register will be: N - A b - 0.5 m - 2.3 \| _______________________ (m + k)+(m + k + 1)0.04 - 3 - 1 dBm and N - A b - 0.5 m + 2.3 \| _______________________ (m + k)+(m + k + 1)0.04 + 3 + 1 dBm. respectively. 4. The receiving part of the multifrequency signalling equip- ment has a sensitivity range between -35 dBm and -5 dBm (see Recommendation Q.455). The minimum value of N | s thus given by: N - A b - 0.5 m + 2.3 \| _______________________ (m + k)+(m + k + 1)0.04 - 4 = - 35 dBm hence N _" A b + 0.5 m + 2.3 \| ________________________ (m + k)+(m + k + 1)0.04 - 31 dBm (1) The maximum value of N | s given by: N - A b - 0.5 m + 2.3 \| _______________________ (m + k)+(m + k + 1)0.04 + 4 = - 5 dBm hence N A b + 0.5 m - 2.3 \| ________________________ (m + k)+(m + k + 1)0.04 - 9 dBm (3) 5. The nominal level of the backward signals at the virtual switching point of the incoming international exchange must not be higher than -11.5 dBm; thus N - A b - 11.5 dBm hence N A b - 11.5 dBm. (2) ANNEX B (to Section 4) (see Recommendation Q.455) Possible method of improving protection from interruptions If the receivers are locked through an internal function of the registers until the times Q and S (see Figure B-1) the inter- vals during which there is a risk that interruptions may cause their release are reduced by PQ and RS respectively. This method can be used on both terrestrial and satellite cir- cuits. To ensure that the duration of the compelled signalling cycle is not prolonged on terrestrial circuits (short transmission delay). PQ and RS must be calculated with: TP\dF= TP\dB = 0 ms: PQ Ti\dn\dt\d1 + TS\d1 + T0+ TS\d2 and RS TS\d2 + TR+ TS\d2 To ensure that the duration of the compelled signalling cycle is not prolonged on satellite circuits (long transmission delay) PQ and RS must be calculated with: TP\dF+ TP\dB = 250 ms: PQ 500 ms + Ti\dn\dt\d1 + TS\d1+ T0+ TS\d2 and RS 500 ms + TS\d2+ TR+ TS\d2.bp Figure B-1/Q.455, p. ANNEX C (to Section 4) (see Recommendation Q.457) Development of a formula for the admissible forward transmission loss in a country of destination The formula used in Recommendation Q.457 for the calculation of the minimum forward transmission loss A f min. a country of destination was determined as follows: 1. According to Recommendation Q.454 the lowest and highest sending levels will be, respectively, -11.5 - 1 = -12.5 dBm and -11.5 + 1 = -10.5 dBm. 2. Let Afbe the nominal transmission loss at 800 Hz in the forward direction, between the virtual switching point in the incoming international exchange and the incoming R2 register con- cerned. The total nominal transmission loss between the outgoing international R2 register and the incoming R2 register will then be: A f + 0.5 m dB. 3. On the assumption that the contribution of the national 2-wire links to transmission loss variations with time is negligi- ble, and that the standard deviation of transmission loss variation in the exchange is 0.2 dB (see Recommendation Q.45), the total transmission loss variation, calculated for a 1% probability of it being exceeded will be: _ 2.3 \| ________________________ (m + k)+(m + k + 1)0.2 2 dB. 4. With allowance for a total attenuation distortion of _ | dB relative to 800 Hz (see Recommendation Q.457) the lower and upper limits of the level of each frequency at the input of the incoming R2 register will be respectively: -12.5 - A f - 0.5 m - 2.3 \| _______________________ (m + k)+(m + k + 1)0.04 - 3 dBm and -10.5 - A f - 0.5 m + 2.3 \| _______________________ (m + k)+(m + k + 1)0.04 + 3 dBm. 5. The receiving part of the multifrequency signalling equip- ment has a sensitivity range of -35 dBm to -5 dBm. 6. The maximum value of Afis thus given by: -12.5 - A f max. - 0.5 m - 2.3 \| _______________________ (m + k)+(m + k + 1)0.04 - 3 = - 35 dBm hence A f max. = 19.5 - 0.5 m - 2.3 \| _______________________ (m + k)+(m + k + 1)0.04 dB. 7. The minimum value for Afis given by: -10.5 - A f min. - 0.5 m + 2.3 \| _______________________ (m + k)+(m + k + 1)0.04 + 3 = - 5 dB hence A f min. = -2.5 - 0.5 m + 2.3 \| _______________________ (m + k)+(m + k + 1)0.04 dB. According to Recommendation Q.457, the maximum number of 4-wire links with international characteristics is m = 4. The maximum number of national 4-wire extension links is k = 4. MONTAGE: PAGE 114 = PAGE BLANCHE SECTION 5 SIGNALLING PROCEDURES A major characteristic of System R2 is the inherent flexibil- ity in the interregister signalling procedures which allow effi- cient signal information transfer, adapted to the particular requirements relating to different call types, traffic conditions and switching equipments. This flexibility is primarily achieved by the principle that the interregister signalling sequence is con- trolled by the incoming R2 register by means of backward signals. In this context the function of an outgoing R2 register is basically to respond with the forward signal requested by the incoming R2 register. At the incoming R2 register the determination of the backward signal to be sent, thus controlling the signalling sequence, is closely related to the analysis necessary to determine the routing and handling of the call. In this Section the signalling procedures are specified. Although System R2 in principle allows a high degree of freedom in the combination of these procedures within the signalling sequence for call set-up, the procedures to be used for normal international calls connected via terrestrial links are described below. These procedures may vary when a satellite link is employed in a connec- tion because the register at the incoming end of a satellite link also acts as an outgoing R2 register (see Recommendation Q.440, S 4.1.1). For national application of System R2 the signalling sequences to be applied have to be defined in accordance with the limitations of the present specifications by the Administration concerned. Recommendation Q.460 5.1 NORMAL CALL SET-UP PROCEDURES FOR INTERNATIONAL WORKING 5.1.1 General During call set-up an outgoing international R2 register is connected to an international link. This register signals to at least one incoming R2 register situated in an international exchange and possibly, depending upon the routing, to as many as 4 successive incoming R2 registers in international exchanges and 4 successive incoming R2 registers situated in national exchanges in the destination country where System R2 is employed. The duration of the normal call set-up procedure can vary because different phases of the signalling sequence between the outgoing international R2 register and successive incoming R2 registers may be repeated. Recommendation Q.462 5.1.2 SIGNALLING BETWEEN THE OUTGOING INTERNATIONAL R2 REGISTER AND AN INCOMING R2 REGISTER IN AN INTERNATIONAL EXCHANGE The address signals from an operator or a subscriber must be stored in an outgoing international R2 register. When a sufficient number of digits is available an outgoing link is selected and a seizing (line) signal is sent. When the seizing signal is recog- nized an incoming R2 register is associated with the link. Immediately the outgoing link is seized the outgoing international R2 register sends the first interregister signal. 5.1.2.1 Signalling to an international transit exchange When the outgoing link is to an international exchange from which a transit routing is required to the destination country, the first interregister signal sent is a country code indicator. This is one of signals I-11, I-12, I-14 depending upon the requirement for echo suppressors (see Recommendation Q.479). On recognition of a country code indicator the incoming R2 register determines that the call is to be internationally transit switched. The incoming R2 register sends signal A-1 when it requests the first digit of the country code. The outgoing international R2 register sends this address digit (a signal I-1 to I-10). The incoming R2 register may send signal A-1 to request the next digit. The incoming R2 register examines the address digit(s) and if a further digit (or digits) is required for routing, signal A-1 is sent to request the next digit. When sufficient digits are stored at the incoming exchange to permit the call to be routed to the next exchange, the backward signal is determined by the nature of the signalling system employed on the outgoing link. a) If the outgoing link employs System R2, one of two backward signals is sent immediately the outgoing link is seized: i) If the outgoing link is to another international transit exchange from which a transit routing is required to the destination country, signal A-11 is sent to request repetition of the country code indicator. On recognition of signal A-11 the outgoing international R2 register sends a country code indicator as the first signal to be received by the incoming R2 register in the next international transit exchange. This is one of signals I-12 or I-14. If signal I-11 was sent initially, signal I-14 is sent subsequently. On recognition of a country code indicator the incoming R2 register determines that the call is to be internationally transit switched. The signalling procedure which follows is identical to that described above. ii) If the outgoing link is to an incoming interna- tional exchange in the destination country, signal A-12 is sent to request a language or discriminating digit. In both these cases after the backward signal is sent and the compelled signalling sequence is complete, the transit exchange releases the incoming R2 register and through-connects the speech-path between the outgoing exchange and the next exchange. b) If the outgoing link employs a signalling system other than System R2, then the acting incoming R2 register is the last incoming R2 register. The exchange seizes an outgoing international link to a further international transit exchange or an incoming international exchange in the destination country. Signalling con- tinues between the outgoing international R2 register and the last incoming R2 register and interworking takes place with the other signalling system. If congestion is encountered signal A-15 is sent, if necessary in pulse form. After the compelled signalling sequence is complete or after the end of the pulse signal the transit exchange dismisses the register. On recognition of signal A-15 the outgoing exchange may ini- tiate a repeat attempt, a re-routing or cause the return of conges- tion information to the calling subscriber. In all cases the outgo- ing (multi-)link section is released. 5.1.2.2 Signalling to an incoming international exchange in the destination country When the outgoing international link is a direct link to an incoming international exchange in the destination country the first interregister signal sent is a language or discriminating digit. Alternately, if the outgoing multi-link section is routed via one, two or three international transit exchanges then on recogni- tion of signal A-12 the outgoing international R2 register sends a language or discriminating digit as the first signal to be received by the incoming R2 register in the terminal international exchange in the destination country. The first signal A-12 received from an international transit exchange by the outgoing international R2 register informs it that an international link terminating at an incoming international exchange has been added to the (multi-)link section. In both cases, on recognition of a language or discriminating digit (a signal I-1 to I-10), the incoming R2 register determines that the call is to be routed to the national network, and selects the next backward signal: i) The incoming R2 register may send signal A-14 to request information about the requirement for echo suppression. - If an incoming half-echo suppressor is required the outgoing international R2 register sends signal I-14. In response to signal I-14 the incoming R2 register sends signal A-1 to request the first digit of the national (significant) number. In response to signal A-1 the outgoing international R2 register sends the first digit of the national (significant) number. - If no echo suppressor is required the outgoing international R2 register sends the first digit of the national (significant) number. ii) Alternatively, if it is known that no echo suppressor is required to be inserted the incoming R2 register may send signal A-1 to request the first digit of the national (signi- ficant) number. In response to signal A-1 the outgoing international R2 register sends the first digit of the national (significant) number. The incoming R2 register examines the first digit of the national (significant) number and if a further digit (or digits) is required for routing, signal A-1 is sent to request the next digit. When sufficient digits are stored at the incoming exchange to permit the call to be routed to the next exchange the backward sig- nal (if any) is determined by the nature of the signalling system employed on the outgoing link and the national routing principles. a) If the outgoing national link employs System R2 and international/national end-to-end signalling is used, a backward signal may be sent after the outgoing link is seized to request the address digit required as the first signal to be received by the incoming R2 register in the following national exchange. The appropriate signal is one of signals A-1, A-2, A-7, A-8 or A-12. These signals may be sent after any digit, and can be repeated provided they do not conflict with the logic procedure. However, if the address digit on-line | s the digit required as the first signal to be received by the incoming R2 register in the next exchange, it is possible for the transit exchange to release the incoming R2 register and through-connect the speech-path after the outgoing link is seized without sending a backward signal. Otherwise, after the appropriate backward signal is sent and the compelled signalling sequence is complete the transit exchange releases the register and through-connects the speech path. b) If the outgoing national link employs System R2 but end-to-end international/national signalling cannot be used, the register in the incoming international exchange relays the mul- tifrequency signals: it acts as an outgoing R2 register. It requests the remainder of the address digits by repetitive use of signal A-1. The digits received by the outgoing R2 register are retransmitted over the outgoing national link at the request of the incoming R2 register in the following national exchange(s) (see Recommendation Q.478). c) If the outgoing link employs a signalling system other than System R2 then the acting incoming R2 register is the last incoming R2 register. The exchange seizes a national link. Signal- ling continues between the outgoing international R2 register and the last incoming R2 register and interworking takes place with the other signalling system. If congestion is encountered, congestion signal A-4 or A-15 is transmitted (if necessary in pulse form) and the incoming R2 regis- ter is dismissed. On recognition of the congestion signal A-4 the outgoing exchange releases the outgoing link or connection and causes the return of congestion information to the calling subscriber. On recognition of the congestion signal A-15 the outgoing exchange may initiate a repeat attempt, re-routing or cause the return of congestion information to the calling subscriber. In all cases the outgoing link or connection is released. It is desirable to use the congestion signal A-15 solely in the case of incoming international terminal traffic when repeat attempt or re-routing may be expected to be successful. Recommendation Q.463 5.1.3 SIGNALLING BETWEEN THE OUTGOING INTERNATIONAL | R2 REGISTER AND AN INCOMING R2 REGISTER IN A | NATIONAL EXCHANGE IN THE DESTINATION COUNTRY 5.1.3.1 Signalling to a national transit exchange The outgoing international R2 register sends the requested address digit as the first signal to be received by the incoming R2 register in the national transit exchange in the destination coun- try. The incoming R2 register examines the digit and if a further digit (or digits) is required for routing, signal A-1 is sent to request the next digit. When sufficient digits are stored at the incoming exchange to permit the call to be routed to the next exchange the backward sig- nal (if any) is determined by the nature of the signalling system employed on the outgoing link and the national routing principles. a) If the outgoing national link employs System R2 a backward signal may be sent after the outgoing link is seized to request the address digit required as the first signal to be received by the incoming R2 register in the next exchange. The signalling procedure which occurs is similar to that described in S 5.1.2.2 a) above. b) If the outgoing link employs System R2 but end-to-end international/national signalling cannot be used, the register in the national exchange relays the interregister signals: it acts as an outgoing R2 register. The digits received by this outgoing R2 register are retransmitted over the outgoing link at the request of the incoming R2 register in the following exchange(s) (see Recommendation Q.478). c) If the outgoing link employs a signalling system other than System R2 then the acting incoming R2 register is the last incoming R2 register. The exchange seizes an outgoing national link to the next national exchange. Signalling continues between the outgoing international R2 register and the last incoming R2 regis- ter and interworking takes place with the other signalling system. If congestion is encountered signal A-4 is sent (if necessary in pulse form) and the incoming R2 register is released. On recognition of congestion signal A-4 the outgoing exchange releases the outgoing multi-link section and causes the return of congestion information to the calling subscriber. 5.1.3.2 Signalling to a national exchange to which the called subscriber is connected When the outgoing multi-link section is routed to the national exchange to which the called subscriber is connected the acting incoming R2 register is the last incoming R2 register: the outgoing international R2 register sends the requested address digit as the first signal to be received by the last incoming R2 register and signalling continues as described below. Recommendation Q.464 5.1.4 SIGNALLING BETWEEN THE OUTGOING INTERNATIONAL R2 REGISTER AND THE LAST INCOMING R2 REGISTER 5.1.4.1 General The usual System R2 signalling procedure is to request in suc- cession the remainder of the address digits stored in the outgoing international R2 register by the repetitive use of signal A-1 until it is determined at the incoming end that the complete address information has been received by the incoming exchange or that the call cannot be routed. System R2 enables transmission of information concerning a large number of different conditions of called subscriber's line or reasons why call set-up has failed. But this can only be sent if the switching systems and other signalling systems employed on the remaining links of the connection offer the possibility of dif- ferentiating several line conditions. Group B-signals are provided for this purpose. Transition from Group A to Group B meanings is indicated by means of address-complete signal A-3. However, if the incoming exchange is unable to send any signals concerning the condition of the called subscriber's line it is unnecessary to send signal A-3 followed by a Group B signal. In such cases the address-complete signal A-6, provided for the purpose is used. If congestion is encountered after transmission of the address-complete signal A-3 the congestion signal B-4 is sent in place of signal A-4 or A-15. 5.1.4.2 When the last incoming R2 register is able to transmit the condition of the called subscriber's line When the condition of the called subscriber's line can be determined the incoming R2 register can send signals conveying this information after receipt of the address digits. As soon as the last address digit is received the last incoming R2 register sends the address-complete signal A-3 to announce changeover to the transmission of Group B signals. In response the outgoing international R2 register sends the appropri- ate calling party's category signal (II-7 to II-10). The last incoming R2 register acknowledges this with the relevant Group B signal indicating the condition of the called subscriber's line. Immediately the compelled signalling sequence is complete the incoming R2 register is released and depending upon the Group B signal sent the speech-path may be through-connected. When the condition of the called subscriber's line is deter- mined by an electrical signal to the last incoming R2 register and if the called subscriber's line is free the address-complete signal A-6 can be sent instead of signal A-3, possibly in pulse form. This will be the last interregister signal. After signal A-6 is sent the incoming R2 register is released, the speech-path is through-connected and ringing tone returned to the calling subscriber. Outgoing international R2 registers must be able to interpret all Group B signals. On recognition of the last backward signal the outgoing exchange releases the outgoing international R2 register and either through-connects the speech-path or releases the outgoing connec- tion and causes the return of an appropriate tone or special announcement or both alternately to the calling subscriber. 5.1.4.3 When the last incoming R2 register is not able to transmit the condition of the called subscriber's line In this case the last incoming R2 register sends the address-complete signal A-6, possibly in pulse form, as the last interregister signal. After this is sent, the last incoming R2 register is released and the speech-path is through-connected. When the last incoming R2 register is in the exchange to which the called subscriber is connected, an appropriate tone is returned to the calling subscriber from this exchange. On recognition of address-complete signal A-6, the outgoing exchange releases the outgoing international R2 register and through-connects the speech-path. The calling subscriber will then hear ringing tone, busy tone, special information tone or a recorded announcement alternately with special information tone sent by the incoming equipment. Recommendation Q.465 5.1.5 PARTICULAR CASES 5.1.5.1 Unallocated number When, after the reception of any digit, the incoming R2 regis- ter determines that the address information corresponds to an unallocated number, the address-complete signal A-3 is sent immedi- ately without requesting all the address digits. In response the outgoing international R2 register sends the appropriate Group II signal. This is then acknowledged by signal B-5 unallocated number . 5.1.5.2 Congestion in the national network If congestion is encountered in the national network the incoming R2 register sends congestion signal A-4, possibly in pulse form. However, if address-complete signal A-3 has already been sent, congestion signal B-4 is sent in acknowledgement of the Group II signal which commences the last compelled signalling sequence. 5.1.5.3 Operator calls The procedures described in Recommendations Q.462-Q.464 are also valid for a semi-automatic call. However, in this case the address information is always terminated by the end-of-pulsing signal I-15. For code 11 or code 12 calls only a limited number of backward signals can be employed as the last interregister signal (e.g. signal A-4, A-6 or B-6). 5.1.5.4 Request of calling party's category At any time calling party's category information may be requested by the incoming R2 register interrupting the normal transmission of address information. The incoming R2 register sends signal A-5 in acknow ledgement of a Group I signal and the outgoing international R2 register sends the appropriate Group II signal (a signal II-7 to II-10) in reply. If this Group II signal is ack- nowledged by a signal different from address-complete signal A-3 or signal A-5 the next forward signal sent will be one of the Group I. Recommendation Q.466 5.1.6 SUPERVISION AND RELEASE OF THE CALL An answer signal is sent when the called subscriber's off-hook condition is detected. Each transit exchange in the connection, relays this line signal. At the outgoing international exchange, receipt of the answer signal normally causes call charging to take place and metering for international accounting purposes to start. When the called subscriber's on-hook condition is detected a clear-back signal is sent and relayed by each transit exchange. When the calling subscriber's on-hook condition is detected a clear-forward signal is relayed by the outgoing international exchange over the outgoing international link. On receipt of the clear-forward signal on an incoming link, release operations are initiated and the clear-forward signal is repeated forward on the outgoing link. Immediately release operations at an exchange are complete (although the outgoing circuit may still be held) a release-guard sequence is initiated on the incoming link. After recognition of the end of the release-guard sequence at the outgoing exchange the link reverts to the idle state. Supervision of a call must be in accordance with Recommendation Q.118. Recommendation Q.468 5.2 ROUTING AND NUMBERING FOR INTERNATIONAL WORKING (See Recommendations Q.107 and Q.107 | fIbis , Fascicle VI.1.) 5.3 TERMINATION OF INTERREGISTER SIGNALLING Recommendation Q.470 5.3.1 AT AN INCOMING R2 REGISTER SITUATED IN A TRANSIT EXCHANGE 5.3.1.1 Successful routing Normally, interregister signalling is terminated in one of the following ways: a) The last forward interregister signal received by the incoming R2 register at the transit exchange is not ack- nowledged from that register. After an outgoing link to the next exchange is seized, the register is dismissed and the speech-path is through-connected. The forward signal remains on-line and is the first signal received by the next incoming R2 register. Provision must be made for the switching operations to be completed so as to ensure that the signal will remain on-line and be received by the succeeding register. b) The last forward interregister signal received by the incoming R2 register at the transit exchange is acknowledged by a backward signal (A-1, A-2, A-7, A-8, A-11 or A-12) requesting transmission of a clearly specified signal as the first to be received by the next incoming R2 register. When the compelled sig- nalling sequence is complete the register is dismissed and the speech-path is through-connected. Because it is not possible to send signals A-2, A-7, A-8, A-11 or A-12 in pulse form, precautions are necessary to avoid acknowledging the last address digit (on automatic calls) until the signalling system employed on the outgoing link is known. If signal A-1 is sent in acknowledgement of the last address digit and if the outgoing link employs System R2 it may not then be pos- sible to send one of these signals and therefore end-to-end signal- ling to the next exchange is no longer feasible (see also Recommendation Q.474). In international working signals A-2, A-7 and A-8 may be used to acknowledge receipt of any forward signal. Signal A-11 must be used to request a country code indica- tor. Signal A-12 must generally be used to request the language or discriminating digit. However, in the case where an incoming R2 register is equipped with only 5 backward signalling frequencies only one of the signals A-2, A-7 or A-8 can be used for this pur- pose. Attention is drawn to the fact that in this case the language or discriminating digit may not be sent by all outgoing international R2 registers. Although it is not normally necessary to request a repeat of a digit n because it remains on the line until acknowledged (but see Recommendation Q.476) such repetition may prove necessary after an interruption of digit transmission (e.g. to request infor- mation concerning the calling party's category) or if the comple- tion of switching operations referred to in method a) above, can- not be guaranteed with a time compatible with the time-out of the outgoing (international) R2 register (see Recommendation Q.476). The procedure is then as follows: Signal A-2 is sent, eliciting the digit n -1; this is immediately acknowledged by signal A-1 to elicit the required digit n . When the digit n in question is the first digit in the store of the outgoing R2 register this procedure is not applicable. At an international transit exchange method b) above, must be used and only one of the signals A-11 or A-12 applies. When the outgoing link connects to a further international transit exchange signal A-11 must be used to request a country code indicator. On recognition of signal A-11 the outgoing international R2 register must send a country code indicator (signal I-12 or I-14, see Recommendation Q.479) as the first for- ward signal to be received by the next incoming R2 register. Signal I-12 or I-14 may be requested as many times as neces- sary by sending signal A-11. When the outgoing link connects to an incoming international terminal exchange signal A-12 must be used to request the language or discriminating digit. On recognition of signal A-12 the outgoing international R2 register must send the language or discriminating digit (a signal I-1 to I-10) as the first forward signal to be received by the next incoming R2 register. On recognition of signal A-12 (sent from an international transit exchange) an outgoing international R2 register is informed that an international link connected to an incoming international terminal exchange has been added to the (multi-)link section and that call set-up is now in progress in the national destination network. This is of importance when signal A-9 or A-10 are, in the originating country, amongst those used to set-up international calls. 5.3.1.2 Congestion If it is impossible to set-up the desired connection in the transit exchange the incoming R2 register terminates interregister signalling by transmission of the congestion signal A-4 or A-15. The backward signal may serve as acknowledgement of the last for- ward signal received by the incoming R2 register at the transit exchange or is sent in pulse form. The two congestion signals A-4 and A-15 are provided to enable an outgoing international R2 register to determine whether conges- tion is occurring in the international network or in that of the destination country so that provision for repeat attempt or re-routing can be made in the former case: - Signal A-15 is sent from an international exchange. - Signal A-4 is sent from a national exchange or possibly from a terminal international exchange. [See also S 5.1.2.2 c).] Because receipt of congestion signal A-15 by an outgoing international R2 register may initiate repeat attempt or re-routing it is possible to transmit congestion signal A-4 from international exchanges where repeat attempt or re-routing may be expected use- less. Recommendation Q.471 5.3.2 AT THE LAST INCOMING R2 REGISTER SITUATED IN THE EXCHANGE TO WHICH THE CALLED SUBSCRIBER IS CONNECTED 5.3.2.1 Successful routing When a call has been completely set up by means of System R2 interregister signalling, the incoming R2 register terminates interregister signalling immediately on receipt of the entire number. The following criteria are used to determine whether the number received by the incoming R2 register is complete: a) analysis - to determine the last digit; b) electrical conditions given by the switching equip- ment succeeding the incoming R2 register; c) receipt of the end-of-pulsing signal (I-15); d) the assumption, after a specified time has elapsed, that no further digits will be sent (see Recommendation Q.476). When criterion a) (analysis) is applied: - if the incoming R2 register is equipped to send Group B signals to provide information about the condition of the called subscriber's line the address-complete signal A-3 is transmitted on receipt of the last digit. As soon as it is known whether the connection with the subscriber's line can be esta- blished, only the appropriate Group B signal needs to be sent. The use of the Group B signals is detailed in Recommendation Q.474; - if the incoming R2 register is not equipped to receive information about the condition of the called subscriber's line, the address-complete signal A-6 is sent immediately after reception of the last digit and no Group B signal will be transmit- ted. In both cases the time interval between the end of a signal A-6 or a Group B signal and the start of the subsequent answer signal must not be less than 75 ms. When criterion b) (electrical conditions) is applied: It is recommended that, to avoid delay in sending the answer signal, no Group B signal should be sent when the called subscriber's line is free, and that the setting-up of speech condi- tions be ensured by sending address-complete signal A-6 immediately the electrical conditions are recognized. The time interval between the end of signal A-6 and the start of transmission of the subse- quent answer signal must be not less than 75 ms (see also Recommendations Q.412 and Q.475). Criterion c) (end-of-pulsing) can be applied only if the incoming R2 register is equipped to receive the 6 forward signal- ling frequencies (see also Recommendation Q.473). When signal I-15 is received and recognized the last incoming R2 register can per- form in the way described under criterion a) . When criterion d) (time-out) is applied: Address-complete signal A-6 must be sent in pulse form as soon as the specified time has elapsed. The time between the end of signal A-6 and the start of transmission of the subsequent answer signal must be, as indicated above for criterion b) , not less than 75 ms (see also Recommendations Q.412 and Q.472). It may happen, however, that the called subscriber answers before the specified time has elapsed. In such exceptional cir- cumstances the pulse signal A-6 must be sent immediately the answer signal is recognized. In this case the time between the end of signal A-6 and the start of the transmission of the subsequent answer signal must be 75 ms or more but less than 150 ms. The cal- ling subscriber will not hear the ringing tone. This disadvantage can be avoided by not using a received digit to set up the call until after the following digit has been received or until a certain time has elapsed. This procedure, how- ever, may give rise to difficulties if the time-out devices pro- vided in the switching equipment succeeding the incoming R2 regis- ter are set for too short a delay [see also Recommendation Q.120, S 1.5.5.2 b) iv)]. 5.3.2.2 Congestion An incoming R2 register should terminate interregister signal- ling immediately any conditions preventing complete setting-up of a call have been recognized. If congestion is encountered congestion signal A-4 is sent, possibly in pulse form. However if address-complete signal A-3 has already been sent then congestion signal B-4 is sent in ack- nowledgement of the Group II signal which commences the last com- pelled signalling sequence. Recommendation Q.472 5.3.3 AT THE LAST INCOMING R2 REGISTER SITUATED IN A TRANSIT EXCHANGE 5.3.3.1 Successful routing Interregister signalling to such a register can be terminated after all the address digits are received. In order to determine that the number is complete the same criteria as described in Recommendation Q.471 are used. When criterion a) (analysis) is applied: 1) if the signalling system employed on the outgo- ing link enables the condition of the called subscriber's line to be sent backwards within an acceptable period compared to the time-out of the outgoing international R2 register, the last incom- ing R2 register can react in accordance with one of the following methods: i) address-complete signal A-3 is sent in ack- nowledgement of the last address digit followed by the relevant Group B signal depending upon the condition of the called subscriber's line; ii) signal A-1 is sent in acknowledgement of the last address digit, and signal I-15 if received, to deliberately suspend compelled signalling and later, - when the condition of the called subscriber's line is known, address-complete signal A-3 is sent in pulse form followed by the relevant Group B signal. This makes it possible to avoid maintaining on-line the Group II signal sent after reception of signal A-3, - when the called subscriber's line is free, it is preferable to send the address-complete signal A-6 in pulse form, 2) if the signalling system employed on the outgo- ing link does not enable the condition of the called subscriber's line to be transmitted backwards or if such information is only available at the last incoming R2 register after a delay incompati- ble with the time-out of the outgoing international R2 register, then address-complete signal A-6 is sent in acknowledgement of the last address digit. When criterion b) (electrical conditions) is applied: Only signal A-1 can be used to acknowledge each address digit. Upon receipt of an electrical signal on the outgoing link, the last incoming R2 register acts in one of the following ways: - if the condition of the called subscriber's line is not known or is known to be free , address-complete signal A-6 is sent in pulse form; - if the condition of the called subscriber's line is known to be other than free , address-complete signal A-3 is sent in pulse form, followed by the relevant Group B signal. Criterion c) (end-of-pulsing) is only applicable if the last incoming R2 register can receive 6 forward signalling frequencies (see also Recommendation Q.473). In this case, when end-of-pulsing signal I-15 is received and recognized, the last incoming R2 register can perform in the way described under criterion a) . When criterion d) (time-out) is applied: Address-complete signal A-6 is sent in pulse form after the specified time has elapsed (see Recommendation Q.476). 5.3.3.2 Congestion If congestion is encountered, the procedure described in Recommendation Q.470 is followed. However, if address-complete sig- nal A-3 has already been sent then congestion signal B-4 is sent in acknowledgement of the Group II signal which commences the last compelled signalling cycle. Recommendation Q.473 5.3.4 USE OF END-OF-PULSING SIGNAL I-15 IN INTERNATIONAL WORKING In international working, the end-of-pulsing signal I-15 is used in accordance with Recommendation Q.468. Signal I-15 (end-of-pulsing) is sent immediately after the last digit. In national working, signal I-15 may be used too. In semi-automatic operation, calls to operators' positions are always terminated by transmission of signal I-15. This signal can only be interpreted if the incoming R2 register is equipped for reception of 6 forward frequencies. However, when an incoming R2 register is equipped for reception of only 5 forward signalling frequencies no provision can be made for recognition of signal I-15. Such an incoming R2 register will then act as though signal I-15 had not been sent. Consequently the next interregister signal is only one of the signals A-3, A-4 or A-6 sent in pulse form. 5.3.4.1 Procedures to be followed after receipt of end-of-pulsing signal I-15 by the last incoming R2 register (situated in a transit exchange or in the exchange to which the called subscriber is connected) Incoming R2 registers equipped for the reception of all 6 for- ward signalling frequencies can acknowledge receipt of the end-of-pulsing signal I-15 by sending an appropriate backward sig- nal to complete the compelled signalling cycle. Interregister sig- nalling is terminated according to the procedures specified in Recommendations Q.471 or Q.472 when criterion a) (analysis) is applied. In particular, if the last incoming R2 register acknowledges signal I-15 with signal A-1 the outgoing international R2 register will not send any signal and the incoming R2 register can only send one of signals A-3, A-4, A-6 or A-15 in pulse form. Since there is no compulsion to equip incoming R2 registers in national exchanges with receivers for all 6 forward signalling fre- quencies, the signal I-15 sent by an outgoing international R2 register may not be recognized by the incoming R2 register. In such cases other criteria can be used to determine whether the number received by the incoming R2 register is complete. When the incoming R2 register determines that the number received is complete using criteria other than c) (end-of-pulsing) it may be that the last digit of the subscriber number is ack- nowledged by signal A-3, A-4, A-6 or A-15. In this case interregis- ter signalling is terminated without request for the signal I-15, in the normal way according to the procedures specified in Recommendations Q.471 or Q.472 (a compelled signalling cycle including signal I-15 is saved). 5.3.4.2 Procedure to be followed after receipt of signal I-15 by an incoming R2 register situated in a transit exchange Signal A-1, A-2, A-7, A-8, A-11 or A-12 may be sent in ack- nowledgement of signal I-15 (end-of-pulsing). However, at a transit exchange precautions are necessary to avoid acknowledgement of signal I-15 before the signalling system employed on the outgoing link has been identified. If signal A-1 is sent in acknowledgement of signal I-15 and if the outgoing link employs System R2, it is not then possible to send backward signal A-2, A-7, A-8, A-11 or A-12 after the end of the compelled signalling cycle because these signals cannot be sent in pulse form. Therefore end-to-end signal- ling to the next exchange is no longer possible. Recommendation Q.474 5.3.5 USE OF GROUP B SIGNALS Group B signals are used to transmit information about the condition of switching equipment in the incoming exchange or the called subscriber's line, to the outgoing international R2 regis- ter, which can then take the necessary action. The address-complete signal A-3 is sent by an incoming R2 register to announce changeover to sending Group B signals. In addition, signal A-3 indicates that the incoming R2 register has received all the Group I forward signals it requires from the out- going international R2 register. On recognition of signal A-3 an outgoing R2 register sends a Group II signal. The incoming R2 register may interpret the Group II signal giving information about calling party's category, in order to control the switching operations accordingly (e.g. suppression of automatic ringing for operator initiated calls or to prevent subscriber access to data transmis- sion terminals). Finally, a Group II forward signal is ack- nowledged by any Group B backward signal. In cases where outgoing R2 registers are able to interpret all Group B signals there is generally no need to provide equipment at the incoming end able to send, in addition to Group B signals, the tones and/or announcements corresponding to those signals, except in the case of ringing tone. In general, outgoing R2 registers must be equipped with dev- ices permitting, after reception of signal A-3: - the exchange of an additional cycle of interre- gister signals before the register is released; - changeover from Group A to Group B meanings of backward signals. Outgoing international R2 registers, however, must be able to interpret all Group B signals. 5.3.5.1 Procedures to be followed by an outgoing international R2 register on receipt of Group B signals Signal B-1 is interpreted by an outgoing international R2 register as signal B-6: the register is released and the speech-path is through-connected. A subsequent answer signal ini- tiates call charging. Signal B-2 send special information tone is sent by the incoming R2 register: - when the number of the called party has been changed, - when the three following conditions are fulfilled together: i) the called party's line condition does not fit with one of the meanings of the present Group B signals, ii) it does not lead to setting up the speech path, iii) it is not incompatible with return of special information tone to the calling party. After recognizing signal B-2, the outgoing international R2 register clears forward and causes the transmission of only special information tone. The subscriber's line busy signal B-3 is sent by the incoming register when the called subscriber's line is engaged. On recogni- tion of this signal, the outgoing register releases the connection and causes transmission of the busy tone. When the congestion condition is encountered following the changeover from Group A signals to Group B signals, the congestion signal B-4 shall be transmitted on the conditions specified for congestion signal A-4. In all cases recognition of the signal causes release of the call and transmission of congestion informa- tion. After recognizing unallocated number signal B-5, the outgoing international R2 register clears the call and causes transmission of special information tone or a recorded announcement and special information tone, alternately, to the calling party. After recognizing signal B-6, the outgoing international R2 register sets up speech conditions so that a caller may hear the ringing tone. In this case, a subsequent answer signal activates the call-charging mechanism. After recognizing signal B-7 the outgoing international R2 register sets up speech conditions, so that a caller may hear the ringing tone. In this case, a subsequent answer signal does not activate the call-charging mechanism. However, in an outgoing international R2 register signal B-7 may be interpreted as B-6 if there is no international agreement on non-chargeable calls. After recognizing signal B-8 subscriber's line out of order, the outgoing international R2 register clears forward and causes the transmission of a special information tone or recorded announcement and special information tone alternately to the cal- ling party. Receipt of signal B-9 or B-10 by an outgoing international R2 register causes release of the outgoing connection and return of special information tone to the calling subscriber, i.e. these sig- nals should be interpreted as signal B-2. If an outgoing international R2 register receives one of signals B-11 to B-15 the call should be released and indication of this returned to the calling subscriber or operator, i.e. this sig- nal should be interpreted as signal B-4. 5.3.5.2 Special procedures for national working Outgoing R2 registers in national exchanges may not be able to recognize and interpret Group B signals. In such networks it is essential that the equipment at the incoming end must transmit not only the Group B signals but also the corresponding tones and/or announcements, when the incoming R2 register does not know whether the outgoing R2 register is capable of interpreting Group B sig- nals. In the case where the incoming R2 register is able to make only two or three distinctions of conditions of the called subscriber's line the following procedure may be adopted: When the only distinction that can be made is between subscriber's line free and subscriber's line engaged, i) signal B-3 is sent if the line is engaged; ii) otherwise signal B-6, or signal A-6 alone is sent so the caller can hear the ringing tone sent by the incoming equipment. In the case where outgoing R2 registers in national exchanges are only able to interpret a limited number of Group B signals or are indeed not able to interpret any, it is essential that such equipment be able to respond to signal A-3 in the specific manner and at least to recognize the next backward signal (which is a Group B signal) as indicating the end of interregister signalling. 5.3.5.3 Possible application of signal B-1 in national working Signal B-1, for example, may be used to indicate that the call must remain under the control of the incoming equipment, insofar as this is possible (e.g. for maintenance reasons, to trace malicious calls, etc.). The incoming exchange sets up speech conditions, so that the calling subscriber can hear the ringing tone. At present such a use of signal B-1 in international working is not envisaged. Recommendation Q.475 5.4 NORMAL RELEASE OF OUTGOING AND INCOMING R2 REGISTERS An outgoing international R2 register must be released once it has received an appropriate backward interregister signal terminat- ing the interregister signalling, or on receipt of a clear-forward (line) signal from the preceding link. An incoming R2 register must be released once it has accom- plished the necessary switching control and terminated the interre- gister signalling required, or on receipt of a clear-forward (line) signal from the preceding link. The last recognized interregister signal preceding setting up of speech conditions will normally be a backward signal: e.g. address-complete signal A-6, subscriber's line free signals B-6 or B-7. The multifrequency combination receivers at the two ends must be disconnected before the switching equipment passes to the speech condition; this procedure eliminates any possibility of their operating or being held under the influence of speech or line sig- nals. The following conditions regarding the duration of the vari- ous phases should be observed (see Figure 20/Q.475): a) the multifrequency signalling equipment of the incoming R2 register must be disconnected within 30 ms after recog- nition of the end of transmission of the last backward signal; b) the multifrequency signalling equipment of the outgoing R2 register must be disconnected within 30 ms after recog- nition of the end of the last backward signal; c) at the outgoing exchange speech conditions must be set up between 30 and 60 ms after recognition of the end of the last backward signal. However, at the exchange where the outgoing international R2 register is situated, the setting-up of speech conditions depends upon the signalling system employed on the preceding link; d) at the incoming exchange at least 75 ms must elapse between the end of transmission of the last backward signal and the setting up of speech conditions. Figure 20/Q.475 p.13 Recommendation Q.476 5.5 ABNORMAL RELEASE OF OUTGOING AND INCOMING R2 REGISTERS To limit the holding time of R2 registers, when interregister signalling is interrupted either by a fault or by any other cause, all R2 registers must be equipped with devices for continuous supervision of the time taken by the various phases of interregis- ter signalling. The time-out delay of these devices must be as short as possible, but long enough not to interrupt normal opera- tion. 5.5.1 Time-out of outgoing international R2 register In an outgoing international R2 register, the intervals during which a forward multifrequency combination is transmitted and the intervals during which no such combination is transmitted are supervised separately. 5.5.1.1 Supervision during sending of forward multifre- quency combinations The lower limit of the time-out delay is a function of the time required for the switching procedures in a transit exchange. On this basis, the time-out delay is specified as 15 _ 3 seconds. The supervision device will start functioning at the beginning of the transmission of a forward multifrequency combination and be reset with the deactivation of the senders involved. It will start again at the beginning of the transmission of the next forward mul- tifrequency combination. 5.5.1.2 Supervision during intervals when no forward mul- tifrequency combination is being sent The lower limit of the time-out delay is a function of: a) the maximum permissible time interval between dialling of two successive digits by the subscriber; b) the time-out delay specified for incoming R2 registers (see S 5.5.2 below). On this basis, the time-out delay is specified to be longer than 24 seconds (a longer delay and an upper limit may be specified by each Administration). If this specification is observed an incoming R2 register, which has acknowledged the last received digit with the signal A-1, is bound to be released before the supervision device of the outgo- ing international R2 register initiates the alarm condition. 5.5.1.3 Procedure to be followed if time-out occurs If time-out occurs, the time supervision devices mentioned in SS 5.5.1.1 and 5.5.1.2 above will bring about operations producing: - return of an appropriate signal and/or audible tone to inform the calling party, - release of the outgoing international R2 register and of the connection as far as the latter is not necessary for the above-mentioned operation. Fault recording equipment may start functioning and/or a delayed alarm may alert the technical staff. 5.5.1.4 Time-out of outgoing R2 register It is recommended that the same principles outlined in SS 5.5.1.1 to 5.5.1.3 above be applied by analogy to outgoing R2 registers. 5.5.2 Time-out of incoming R2 register The time-out device shall supervise the interval elapsing between seizure of the register and recognition of the first for- ward multifrequency combination as well as the interval elapsing between the recognition of two successive multifrequency combina- tions in the forward direction. 5.5.2.1 Time-out delay The lower limit of the time-out delay is a function of: a) the maximum permissible time interval between the recognition of 2 successive forward multifrequency combina- tions; this time interval may in certain cases be influenced by the maximum permissible time interval between dialling of 2 successive digits by the subscriber; b) the maximum time required for setting-up the call under conditions which slow down the interregister signalling. In view of the desirability expressed in S 5.5.1.2 above that the incoming R2 register be released before expiry of the time-out delay specified for the outgoing international R2 register, an upper limit should be fixed as well. On this basis the time-out delay should be specified within the range of 8-24 seconds. A minimum delay of 15 seconds corresponding to the time-out delays in other CCITT standardized signalling systems is to be preferred. For incoming R2 registers using the criterion d) (time-out) indicated in Recommendation Q.471 to determine completion of the number, the time referred to there as the specified time may excep- tionally be shorter than 8 seconds, but never less than 4 seconds. 5.5.2.2 Procedure to be followed if time-out occurs If time-out occurs, the time supervision device will bring about operations producing: - sending of a congestion signal (A-4 or A-15) in pulse form; - release of the incoming R2 register and other equipment in the incoming exchange; - on time-out of the initial digit: i) line signalling, analogue version: establishment of the blocked state on the incoming circuit until recognition of the clear-forward signal (see Recommendation Q.412, abnormal condi- tions); ii) line signalling, digital version: no further action required. Fault recording equipment may start functioning and/or a delayed alarm may alert the technical staff. Recommendation Q.478 5.6 RELAY AND REGENERATION OF R2 INTERREGISTER SIGNALS BY AN OUTGOING R2 REGISTER IN A TRANSIT EXCHANGE When the overall multi-link connection is divided into end-to-end signalling sections, regeneration of R2 interregister signals is required by either an outgoing international R2 register or an outgoing R2 register (see Recommendation Q.440). There are three possible procedures for relaying interregister signals by an outgoing R2 register: a) the outgoing R2 register acknowledges each signal received over the incoming link by transmitting the appropriate backward signal; this operation is independent of the operations entailed in retransmitting on the outgoing link; b) the forward address signal with rank n + 1 is ack- nowledged on the incoming link as soon as the forward address sig- nal with rank n is acknowledged on the outgoing link; c) as soon as a forward signal is received on the incoming link, a signal is sent on the outgoing link; the ack- nowledgement signal is sent on the incoming link only when such a signal has been received on the outgoing link. Methods a) and b) provide the most rapid transfer of informa- tion and are therefore the preferable methods of relaying the information necessary for setting up the call. It is essential, however, that the outgoing R2 register be provided with adequate storage capacity in the case of method a) . Method b) can only be applied after method a) . Method c) should be used for relaying information relating to the procedures for the termination of interregister signalling. The changeover from method a) or b) to method c) may require transmission of an address-complete signal A-3 in pulse form as indicated in Recommendation Q.442 (see Figure 21/Q.478). The procedure for disconnecting the multifrequency combination receivers and setting-up speech conditions for each link, incoming and outgoing, is given in Recommendation Q.475. When method a) or b) is used, the time-outs of the R2 regis- ters associated with the first signalling section may expire if the signalling on the second signalling section is too slow. Relatively long time-outs are recommended (see Recommendation Q.476). Note - The use of signal A-3 on circuits with very long transmission delay, e.g. satellite circuits, for relaying such information (method c) may result in premature release in certain incoming local exchanges with very short time guard. The problem may be avoided if the outgoing R2 register immediately following the satellite link uses signal A-5 to obtain the calling party's category information previously to the receipt of signal A-3 from those incoming local exchanges. Figure 21/Q.478 p.14 Recommendation Q.479 5.7 ECHO-SUPPRESSOR CONTROL - SIGNALLING REQUIREMENTS 5.7.1 Introduction Recommendations Q.42 and Q.115 give fundamental requirements with which the employment of echo suppressors must comply. There are two methods of connecting echo suppressors to a cir- cuit. One method is by use of permanently connected echo suppres- sors and the other is by the insertion of echo suppressors from a pool, when required. In System R2 (see Recommendation Q.441) two forward signals (I-12 and I-14) and one backward signal (A-14) are provided to indicate whether or not an incoming half-echo suppressor (IHES) is required. A third forward signal (I-11) is also available for use by bilateral agreement to indicate that an outgoing half-echo suppres- sor (OHES) must be employed. The following principles must be taken into account. 5.7.2 Principles of echo-suppressor control 5.7.2.1 The outgoing international exchange may be able to determine the need for echo suppressors in the connection by ana- lysing the received country code. 5.7.2.2 In direct traffic between 2 countries the use of echo suppressors generally takes place according to fixed rules. Echo-suppressor control signalling is not, therefore, necessary in these cases unless it is applied for the sake of uniformity. 5.7.2.3 In international transit traffic the outgoing half-echo suppressor (OHES) will normally be employed at the outgo- ing international exchange, and the incoming half-echo suppressor (IHES) at the incoming international exchange, unless other arrangements are agreed upon with the Administration of the inter- national transit exchange(s) (see S 5.7.2.5 below). However, in cases where an outgoing satellite link is selected by a transit exchange without the knowledge of the outgoing inter- national exchange, the signal I-12 may be sent over the satellite link. In such cases half-echo suppressors are required and will normally be permanently fitted at each end of the satellite link. 5.7.2.4 An incoming international exchange, equipped for the connection of IHES, must ask the outgoing international exchange whether or not an IHES must be employed unless this is known from other criteria (e.g. line classification). It will do this by send- ing the backward signal A-14 as an acknowledgement of the discrim- inating or language digit (Z-digit). When a satellite link is used in the connection the exchange situated at the incoming end of the satellite link responds to the A-14 signal instead of the outgoing international exchange (see Recommendation Q.7). 5.7.2.5 It may be agreed upon bilaterally that in interna- tional transit traffic the OHES (IHES) will not be employed in the outgoing (incoming) international exchange, but in the interna- tional transit exchange, e.g. in the case where most of the traffic on the circuit group between the outgoing exchange and the transit exchange (between the transit exchange and the incoming exchange) needs no echo-suppressor. a) When echo suppressors are needed and the OHES must be employed at the international transit exchange the outgoing international exchange sends the signal I-11 as a country code indicator. If an international connection is routed via two or more transit exchanges, signal I-11 must not be sent beyond the first transit exchange. Therefore the outgoing exchange, after having sent the signal I-11 once, must send the signal I-14 if the country code indicator is requested again (signal A-11). b) When echo suppressors are needed and the IHES must be employed at the international transit exchange this is known by the international transit exchange. In this case the incoming international exchange will not send signal A-14. 5.7.2.6 When echo suppressors are employed they will remain disabled until the answer signal is received. This condition is necessary in order to permit compelled interregister signalling to (e.g. national) exchanges further on in the connection. Alternatively, when the ability exists to detect that interre- gister signalling has been completed, the echo-suppressor may be enabled at that time without awaiting the answer signal. 5.7.3 Examples of echo-suppressor control signalling The following situations may arise: 5.7.3.1 Direct interregister signalling between two countries A and B. a) No echo-suppressor control signalling is applied. This may be for one of two reasons. Either the connection does not normally require echo suppressors or echo suppressors are required and are permanently connected to the circuit. The signalling procedure is indicated in Table 11/Q.479, column a. If echo suppressors are necessary, the OHES is employed at A and the IHES is employed at B. b) Echo-suppressor control signalling is applied. There are two cases: i) No echo suppressors are required (see column b of Table 11/Q.479). ii) Echo suppressors are required (see column c of Table 11/Q.479). The OHES is employed at A and the IHES is employed at B. 5.7.3.2 Traffic between two countries A and D via two international transit exchanges B and C There are two cases: 5.7.3.2.1 The connection is via terrestrial circuits only. a) No echo suppressors are required (see column a of Table 12/Q.479). b) Echo suppressors are required (see columns b, c, d, e of Table 12/Q.479). The following examples may be mentioned: - OHES in A: IHES in D (column b); - OHES in B: IHES in D (column c). The country code indicator I-11 is used by bilateral agreement and indicates that B must employ the OHES. When the country code indicator is to be sent to C, B asks A to do this by sending signal A-11. A, now sends signal I-14 instead of signal I-11, because B has employed the OHES. The signal A-14, which is sent from D as an acknowledgement of the Z-digit will, of course, also be answered by signal I-14. - OHES in A: IHES in C (column d). In accordance with S 5.7.2.5 b) above, C knows that D cannot employ an IHES and C itself will therefore make the connection. D will naturally not send signal A-14. - OHES in B: IHES in C (column e). 5.7.3.2.2 The connection includes a satellite link. The following examples are given as typical situations that may arise: a) The first link in the connection is via satel- lite. In Table 13/Q.479, P and Q both know that echo suppressors are required. If Q has a permanently connected IHES then: - OHES in P; IHES in Q (column a). When R or S is able to employ an IHES then: - OHES in P; IHES in R (column c); - OHES in P; IHES in S (column b). b) A later link in the connection is via satellite. SECTION 6 TESTING AND MAINTENANCE Recommendation Q.490 TESTING AND MAINTENANCE 6.1 General In international working the guiding principles and testing arrangements for maintenance as defined in Recommendations M.700 to M.728 and Q.134 also apply to Signalling System R2. The organi- zation of routine maintenance, tests and measurements of signalling and switching should comply with Recommendations M.716, M.718, M.719, M.728 and M.732. The analogue line signalling of System R2 differs from other CCITT signalling systems in two significant respects: - line signals are sent over out-band signalling channels; - an "interruption control" protects the line sig- nalling from the consequences of interruptions of the transmission path. These two features of System R2 require special attention from a maintenance point of view. 6.2 Automatic procedures for transmission measurements and signalling tests Circuits operated with System R2 require elaborate transmis- sion measurements and signalling tests and also rapid and simple testing of transmission and signalling. Both needs are preferably met by means of automatic devices. The specification for ATME-2 as adopted by CCITT makes it applicable to the testing of international circuits using System R2. The necessary information for its use on such circuits is contained in Recommendation O.22. A description of a simplified programme for rapid testing of signalling and checking the transmission quality of a circuit is given in S 6.3. Generally speaking, the arrangements for automatic testing consist of outgoing test equipment connected at the outgo- ing end of the circuit and incoming test equipment connected at the incoming end. 6.3 Automatic test procedures for test equipments Automatic test procedures provides a means for rapid testing of signalling and also checking the transmission quality of cir- cuits operated with Signalling System R2. 6.3.1 Numbering of access to test equipment In international working, to set up a call to maintenance equipment via circuits operated with System R2, the following mul- tifrequency signals must be sent: - I-13 (replacing the language digit, in accordance with Recommendation Q.133), - I-13, - two digits "XY" which will be associated with the type of test equipment and the procedure for testing to be employed (see Recommendation Q.107, Table 7), - I-15 (if requested by the incoming equipment). Provision is made for repetition of signal I-13 to avoid com- plications in the incoming R2 register in the country of destina- tion. The second signal I-13 is stored in the place where the first digit of the routing information is normally recorded. In this way, access to the test equipment requires no analysis, for routing pur- pose, of the signal which takes the place of the language digit. When calls are set up to the test equipment, it is desirable to avoid repetition of the request for the access code or for any other digit. This is because the calls may come from equipment which is not normally designed to interpret signals A-2, A-7 or A-8. The address complete signal to be sent on calls to test equip- ment must be one of the following: - A-6 or A-3 followed by B-6 when incoming test equipment is free, - A-4 or A-3 followed by B-3 or B-4 when incoming test equipment is busy. Precautions should be taken that signal A-6 is only sent when it is sure that the incoming test equipment is available for that call. When receiving signal A-3, the outgoing test equipment sends signal II-7 in response. Note - In national working, or in international working where the language digit is omitted by bilateral agreement, the following multifrequency signals must be sent: - I-13. - Two digits "XY". - I-15 (if necessary). 6.3.2 Test sequence for simplified test The test sequence is as follows: a) seizing of the automatic incoming test equip- ment; b) transition to answer state; c) sending backward of a composite identification signal 1020 + 1140 Hz; this signal will be acknow ledged in a compelled manner by the signal mentioned under d); d) recognition of a composite acknowledgement sig- nal 1380 + 1980 Hz, sent in the forward direction; e) on the disappearance of the acknowledgement sig- nal the incoming test equipment passes to the clear-back state; f ) on recognition of the clear-back signal, the outgoing equipment will send in a normal manner the clear-forward signal which will clear the connection and release incoming test equipment. After release of the incoming line circuit the release-guard signal will be sent in the normal way. Detection of failure is made by timing out at the outgoing equipment. The frequencies mentioned under c) and d) are those for System R2 interregister signalling; transmission and reception of these frequencies in the incoming test equipment must be in accor- dance with Section 4. Attenuation pads may be inserted in the send and receive paths of the outgoing test equipment to shift the receive level at the input of the multifrequency receivers of the outgoing and incoming test equipment toward the lower operational limit. This makes it possible to diagnose abnormal loss on the circuit under test from defective multifrequency signal exchange between outgoing and incoming test equipment. For testing international System R2 circuits, the additional attenuation produced by the pads should be 10 _ 1 dB. 6.3.3 Good/no good transmission test equipment In addition to the tests described in SS 6.3.1 and 6.3.2 a good/no good transmission test may be provided as a simple means for fast error localization. Such a test is described in Recommendation Q.137 for System No. 4 (i.e. and the frequency of the test signal, the tolerances and the deviation from the nominal value, the test signal generators and receivers would all be the same) but the sending level being -10 dBm. It is to be noted that loop transmission measurements of the kind specified in Recommendation Q.136 cannot be made on System R2 circuits. 6.4 Testing of analogue line signalling equipment under abnormal conditions The specification of the analogue line signalling equipment contains clauses concerning operation under abnormal conditions, including the action to be taken in case of interruption control alarm. The testing equipment described in S 6.2 is not applicable to such conditions and therefore the functioning of the analogue line signalling equipment under abnormal conditions should be tested internally at each end of a circuit either manually or automatically with special equipment. The detailed programme for this testing will be specified by each Administration. The design and construction of the line signalling equipment should be such as to permit both operational and limit testing in normal and abnormal conditions. 6.5 Alarms for the technical staff Certain abnormal conditions in the signalling equipment should cause alarms to be set off for the technical staff (see also Recommendation Q.117). The relevant requirements are found in Sec- tion 2 (line signalling equipment) and in Section 5 (time-out in multifrequency registers). As indicated in S 2.2.3, a fault occurring during release of a circuit may result in an abnormal blocking condition. In this case there is a "tone-on" condition in both signalling directions, yet the circuit is not in the idle condition since the release-guard signal has not been received. If no special action is taken, a tem- porary fault may therefore result in the circuit's being out of service until it is manually restored by the maintenance staff, after receipt of an alarm (see S 2.2.4). It may accordingly be desirable to arrange for automatic res- toration of abnormally blocked circuits. For Administrations wish- ing to introduce this function, the recommended arrangement is described below. 6.6 Recommended method for automatic restoration of an abnormally blocked circuit When an outgoing link is abnormally blocked, periodic sending on the outgoing link of the seizing signal, followed shortly after- wards by the clear-forward signal, is initiated. Clearance of the fault which caused the abnormal blocked con- dition will initiate a release-guard signal at the incoming end whereupon the outgoing end restores the link to the idle condition. The intervals, at which the periodic sequence described above is repeated, should be between 30 seconds and 2 minutes. The first operation of the automatic device should be per- formed as soon as possible, but not before 2-3 seconds have elapsed, after recognition of the abnormally blocked condition at T1 (see S 2.2). After a period of three to six minutes a delayed alarm should be given in accordance with Recommendation Q.412, S 2.2.4. In the event of a backward tone-off condition being detected, other than in response to a periodic clear-forward signal, the periodic sequence is suspended until the backward tone is again recognized whereupon the periodic sending sequence is restarted. If interruption control at the outgoing end occurs during the abnormally blocked condition, the periodic sending sequence is suspended until the interruption control reverts to normal, whereu- pon the periodic sending sequence is restarted. 6.7 Instructions for the maintenance of channels and cir- cuits using System R2 line signalling system at 3825 Hz The analogue line signalling equipment specified in Section 2 is closely associated with the channel translating equipment and its operation may be a function of the group and supergroup translating and through-connection equipments. Maintenance of the circuits and groups which support them is governed by the principles and Recommendations of Volume IV. However, the introduc- tion of out-band signalling calls for a few complements to these Recommendations, as described below. 6.7.1 Bringing into service of group, supergroup, master- group or supermastergroup links a) SS 2.1 and 7.6 of Recommendation M.460 It should be noted that group and supergroup pilots placed at 140 Hz from a virtual carrier frequency are incompatible with sig- nalling at 3825 Hz. Hence, the pilot on 84.140 kHz should not be applied to groups in which channel 6 is to be operated with this out-band signalling. Similarly, the pilot on 411.860 kHz should not be applied to supergroups in which channel 1 of the group in the group 3 position is to be operated with signalling at 3825 Hz. If the channels of a group are to be operated with System R2, each extremity of the group should be equipped, at the receiving end, with a device to give protection against faulty signalling conditions which may result from an interruption in the transmis- sion channels (interruption control). This equipment, which is based on pilot level detection; must comply with the conditions specified in S 2.4.3 of Recommendation Q.416. Note - If the channels of a supergroup which are operated with System R2 have the same extremities as the supergroup, a dev- ice based on monitoring of the supergroup pilot can be used instead of one based on monitoring of the group pilot. It will have to meet the same specifications. b) S 7.2 of Recommendation M.460 The group-translating and through-connection equipments are specified with a passband extending from 60.600 kHz to 107.700 kHz. If it is wished to use channels 12 with signalling at 3825 Hz, it is necessary to ensure when the group is set up, that the corresponding frequency (60.175 kHz) is transmitted satisfactorily from end to end of the group link. Provisionally, in view of the operating margin of the receiv- ing part of the signalling equipment, it is desirable to check that attenuation at this frequency does not exceed the attenuation at the group pilot frequency by more than 3 dB. A similar precaution should be taken on setting up group links when signalling is to be used at 3825 Hz on channel 12 of the group transmitted in position 5 on the supergroup. 6.7.2 Setting-up and lining-up the channels of an interna- tional group 6.7.2.1 Setting up the out-band signalling channel for the System R2 Testing of the sending equipment: - The sending level of the signalling frequency corresponding to 3825 Hz if the carrier is taken as the frequency of origin must be lined up at -20 _ 1 dBm0. When this frequency is not to be sent, its leak transmitted to line should not exceed -45 dBm0. Testing of the receiving equipment: - The signalling receiver must operate in the con- ditions described in SS 2.3.2.1 and 2.3.2.2. It must not function when a signal, of which the characteristics (level and frequency) are such that the representative point is below the graph in Figure 8/Q.415, is applied to the same point. This test may be replaced by the following one to check the protection against unwanted signals (impulsive noise): - The sending part of the group terminal equipment is connected to its receiving part by a closed-circuit loop at the group distribution frame, this loop introducing a slight gain (e.g. 3 dB) if possible. The standardized click generator (see Figure 7/Q.414) is applied to each speech channel successively at the point where this channel is connected to the switching equipment, and a check is made to ensure that no wrong signals are retransmitted at the receiving end to the switching equipment by the channel signalling equipment concerned or by those of the other channels in the group. 6.7.2.2 Closed-circuit loop tests: response time When the transmission-reception loop of the terminal equipment is effected at the group distribution frame or at an equivalent point, a check is made to ensure that less than 30 ms elapse between the moment when the change of condition is applied to the transmitter associated with each channel and the moment when it appears at the output of the corresponding receiver. 6.7.2.3 End-to-end tests When the terminal channel-translating equipments are normally connected to the extremities of the link, an end-to-end operating test is carried out. The level of the line-signalling frequencies transmitted and received for each channel are likewise measured, to provide a reference, at the group terminal distribution frames or at equivalent points. ANNEX A (to Signalling System R2 Specifications) (see Recommendations Q.400 and Q.441) Provision of a forward-transfer signalling facility A.1 General The System R2 does not provide a forward-transfer line signal. However for certain relations it may be decided by bilateral or multilateral agreement to introduce the forward-transfer signalling facility into System R2. One possible procedure that has been adopted for use within Europe, is to use the PYY in-band signal of System No. 4. This solution is only economical in regions where the facility is needed for a small proportion of the calls. For international working the method as described below may be followed. Note - The method given in this Annex may also be adopted in national networks where the forward-transfer facility is considered necessary for trunk offering and recalling operators. However, care must be taken to see that the transmission limits applying to the forward-transfer signal specified are observed. A.2 Method recommended for introducing the forward-transfer signalling facility into System R2 Forward-transfer signalling will be provided by means of spe- cial equipment which uses in-band signalling and which is switched only on to those connections which may require this facility. The amount of special equipment necessary can, accordingly, be reduced to a minimum and adapted, in a flexible manner, to actual needs. The in-band signal constituting the forward-transfer signal is sent end-to-end between the outgoing and incoming international exchanges. When the special equipment receives the forward-transfer signal, it performs the necessary operations at the incoming exchange. A.2.1 Access to the special equipment in an incoming inter- national exchange In an incoming international exchange access to the special equipment for forward-transfer signalling can be determined by the use of the following indicators: 1) Special marking of incoming routes on which forward-transfer signalling is used. 2) Language digit indicating semi-automatic traffic. 3) Calls for code 11 or code 12 operator. 4) Special interregister signalling sequence in which the incoming exchange sends signal A-5, send calling party's category the forward-transfer signalling facility is required the outgoing R2 register will respond to this by sending the signal II-10. This signal indicates an operator-initiated call on which special equipment for forward-transfer signalling is needed. The use of these indicators will depend on the amount of traffic for which forward-transfer signalling is employed. In some cases one or two of the indicators will be utilized. In others, combinations of all will be used to reduce to a minimum the amount of special equipment required. A.2.2 In-band forward-transfer signalling In System R2 the in-band forward-transfer signal is the same as that used in System No. 4. For the definition of this signal see Recommendation Q.120, S 1.12. The signal is the signal PYY defined in Recommendation Q.121, S 2.3. The forward-transfer signal is sent in accordance with Recommendations Q.122 and Q.124. The signal receiver and the splitting arrangements to be incorporated in the special equipment at the incoming international exchange are in accordance with Recommendations Q.123 and Q.124. Provided it creates no difficulty for incoming national net- work signalling, no splitting need be effected at the receiving end and the caller will then hear the entire signal PYY. MONTAGE: PAGE 144 = PAGE BLANCHE