[H[70C[0m8 [0;4mmagazines, computer data communications journals, handbooks, and sometimes just plain hearsay.[K [KI have tried to correlate as much as possible all the little bits and pieces into a coherent picture which makes sense, but there is no easy way to be sure of all the little details. So think of this article as if it is a historical novel - generally accurate and, regardless of whether it is completely true or not, fascinating. With this out of the way, let's go on. You, as a customer, are generally referred to as the "subscriber." Your[0m [0;4mtelephone connects to the Central Office through a two-wire cable which may be miles long, and which may have a resistance on the order of hundreds or even thousands of Ohms. This cable is essentially a balanced line with a[0m [0;4mcharacteristic impedance of around 900 Ohms, but this varies greatly with[0m [0;4mdifferent cables, different weather conditions, and different calls. This is why it is so hard to keep a hybrid phone-patch balanced. The main power in the central office comes from 48 volt storage batteries which are constantly kept trickle-charged. This battery is connected to your line through a subscriber relay and a balanced audio transformer. The relay is sensitive enough to detect even quite small currents through your line.[0m [K[75C[H[70C9 [0;4mThe buttons which stick up out of your telephone case when you lift the handse activate the hook switch. The name probably dates back to the days when the handset (or even earlier, the earpiece) hung on the side of the phone from a hook. In any case, when your phone is hung up it is said to be on the hook, and when you lift the handset to make a call it is said to go off the hook.[0m [0;4mWith the phone on hook, the line is connected only to the bell (called the[0m [0;4mringer). Because the bell circuit has a capacitor in it, no DC current can[0m [0;4mflow through the phone. As a result, the subscriber relay back in the central office will be de-energized, indicating to the central office (let's abbreviate that as CO from now on) that your phone is hung up.[K [KSince there is no current through your line or phone, there is no voltage drop anywhere, and so if you measure the voltage across the phone line at your phone you will see the entire 48 volts (or even more if the CO batteries are well charged).[K [KThe positive (grounded) lead is called the tip and the negative lead is called the ring; these names correspond to the tip and ring of a three-circuit phone[0m [0;4mplug. Now suppose you want to place a call; You pick up the handset and th phone goes off the hook. This completes the DC circuit through the dial,[0m [0;4mmicrophone, and the hybrid network which is basically a complicated transformer circuit. [75C[H[69C[0m10 [K[0;4mAt this point current starts to flow from the battery through your line and[0m [0;4mphone, and the subscriber relay back at the CO pulls in. The line voltage[0m [0;4macross your phone now drops to just a few volts because the line is loaded down by the low resistance of the phone. The CO now searches for some idle dialing circuits, and when it finds them, connects a dial tone back to your phone. When you hear this, you start dialing.[K [KSo lets talk about rotary dial, the type of phone which you turn with your[0m [0;4mfinger (we will talk about Touchtone dials later). When you dial a number, the dial acts as a short circuit until you release the dial and let the built-in spring return it back to the resting position. As it is returning, it starts[0m [0;4mto open and close the circuit in sequence to indicate the number you dialed.[0m [0;4mIf you dial a 1, it opens the circuit once; if you dial a 9 it opens the[0m [74D[0;4mircuit nine times. As the dial is returning it cause the subscriber relay to open and close in step. This enables the CO to recognize the number you want. When you finish dialing, the dial becomes just a plain short circuit which[0m [0;4mpasses current through the microphone and the hybrid network. Since the mik is a carbon unit, it needs this current to work. When the CO receives he[0m [0;4mcomplete number, it starts to process your call. If you dialed another[0m [0;4msubscriber in the same area, it may connect you directly to that subscriber's[0m [0;4mline. Calls to phones a little further away may have to be routed through [75C[H[70C[0m1 [0;4manother CO, while long distance calls may go through one or more long distance switching centers (called tandems) and possibly many other CO's before arriving at the destination. At the completion of this process, you may get either a ringing signal, indicating that the phone at the other end is ringing, one of[0m [0;4mseveral types of busy signals, or possibly just silence, if something goes[0m [0;4mwrong somewhere.[K [KWhen you talk to the person at the other end, the cable carries audio in both directions at the same time. Your carbon microphone varies the current in your circuit, and this current variation is detected by a balanced transformer in[0m [0;4mthe CO. At the same time, audio coming back to your phone goes through the[0m [0;4mhybrid network to your earphone. In phone company lingo they like to call the mike a transmitter, and the earphone is called the receiver.[0m [K[0;4mYou may be interested in the makeup of the various tones you may hear on your[0m [0;4mtelephone; these tones are important to people such as computer communications designers who have to build equipment which will recognize dial or other[0m [0;4msignaling tones:[K [K Dial tone in older exchanges may still be a combination of 120 and 600 Hz, but the newer exchanges use a combination of 350 and 440 Hz. There is[0m [0;4m often a slight change in the DC line voltage at the beginning of dial [75C[H[70C[0m2 [0;4m tone, and this may also be detected.[K [K Busy signal is a combination of 480 and 620 Hz which alternates for 1/2 second on and 1/2 second off (i.e., 60 interruptions per minute) when the party you are calling is busy.[K [0m [0;4m The same busy signal may be used for other conditions such as busy interoffice or long distance circuits, but would then be interrupted[0m [0;4m either 30 times a minute or 120 times per minute. This is a standard[0m [0;4m agreed on by an international telecommunications organization called CCITT (and I don't offhand remember the French words it stands for), but[0m [0;4m occasionally other frequencies up to 2 kHz are used. A siren-like sound[0m [0;4m varying between 200 and 400 Hz is often used for other error conditions. The ringing tone, which you hear coming back to you when the phone rings on the other end of the connection, is nowadays mostly a combination of[0m [0;4m 440 and 480 Hz, but there is great variation between CO's. Very often a higher frequency such as 500 Hz is interrupted at 20 Hz, and other tones are used as well. The tone is usually on for 2 seconds and off for 4[69Dseconds.[K [K The ringing current, actually used to ring the bell in a telephone, is an [75C[H[70C[0m3[66D[0;4mAC voltage since it has to activate a ringer which has a capacitor in series with it. Different companies use different ringing currents, but[72Dthe most common is 90 volts at 20 Hz. Since a typical phone may be[0m [71D[0;4mthousands of feet away from the CO, the thin wires used may have a fairly[73Dhigh line resistance. Hence only a relatively small current can be applied to the bell, certainly not enough to ring something like a[66Ddoorbell. This problem is solved by making the bell resonant mechanically[74Dat the ringing frequency so that even a fairly small amount of power is[70Dnough to start the striker moving hard enough to produce a loud sound.[71DThis is the reason why a low-frequency AC is used. Although this raises[0m [74D[0;4msome problems in generating a 20 Hz signal at a high enough voltage, it[71Dhas the advantage that a bell will respond to a ringing current only if[0m [72D[0;4mthe frequency is quite close to the bell's naturally resonant frequency If you build two bells, one resonant at 20 Hz and the other resonant at 30[74DHz, and connect them together to the same line, you can ring just one bell[74Dat a time by connecting a ringing current of the right frequency to the[71Dline; this has some useful applications in ringing just one phone on a[0m [72D[0;4mparty line.[K [KNow let's look at some of the components of the phone itself. We will consider the most common new phone, a model 500 C/D manufactured by Western Electric and used by Bell System affiliated phone companies. This is the standard desk[0m [75C[H[70C4 [0;4mphone, having modern rounded lines and usually having a G1 or G3 handset. It was developed about 1950 and replaced the older 300-series phones which had the older F1 handset and had sharper corners and edges. There was an in between phone, where they took an old 300-series phone and put a new case on it which[0m [0;4mresembled the 500-style case, but had a straight up and down back - the back of the case came straight down right behind the handset cradle, whereas the true 500-style telephone has what looks like a set sticking out behind the cradle).[0m [K[0;4mIf you are still in doubt as to which phone you have, the bell loudness control is a wheel on the 500-type phone and a lever on the 300-type. If you live in the boondocks, you may still have the 200-type phone (sometimes called the[0m [0;4movalbase) or maybe even the desk-stand type that looked like a candlestick,[0m [0;4mwith the microphone mounted on the top and the earpiece hanging on the side[0m [0;4mfrom a hook.[K [KNeither of these phones had a built in bell, and so you probably have a bell box attached to your wall. If you have a phone with a handle on the side which you crack to call the operator, the following does not apply to your phone![68Ds discuss the bell circuit, which consists of a two-coil ringer and a[0m [0;4m0.5 uF capacitor. On Western Electric phones the capacitor is mounted inside[0m [0;4mthe network assembly, which also has a large number of screws on top which act [75C[H[70C[0m5 [0;4mas connection points for almost everything inside the phone. I have never[0m [0;4mbeen able to find out why the ringer has two coils of unequal resistance, but[0m [0;4mit apparently has something to do with determining which subscriber on a party line makes which call. In most phones, the yellow and the green wires are[0m [0;4mconnected together at the wall terminal block so that the bell is connected[0m [0;4mdirectly across the telephone line; disconnecting the yellow lead would turn[0m [0;4moff the bell (although sometimes the connection is made internally by[0m [0;4mconnecting the black lead from the ringer directly to the L1 terminal, in which case the yellow lead is disconnected.[K [KYou may wonder why a yellow lead is needed at all when only two wires are[0m [0;4mnormally used anyway. It is true that only two wires enter the house from the outside; one of these is the tip and the other is the ring. In a non-party line the ringing current as well as all talk voltages are applied between the tip and the ring, and it doesn't actually matter which of the phone leads goes to the tip and which to the ring if you have a rotary dial phone. If you have a Touchtone dial, then you have to observe polarity so that the transistor[0m [0;4mcircuit in the dial works, in which case you have to make sure that the green lead goes to the tip and the red lead goes to the ring. [KThe yellow lead is commonly used for party lines. On a two-party line ringing current from the CO is applied not between the two lines, but between one line [75C[H[70C[0m6[70D[0;4mnd ground. In that case the yellow lead goes to ground while the other side of the ringer (the red lead) is connected to either the tip or the ring,[0m [0;4mdepending on the party. In this way, it is possible to ring only one party's[0m [0;4mbell at a time.[K [KThe remaining connections inside the telephone are varistors; the phone[0m [0;4mcompanies must be the world's biggest users of these devices, which are variable resistors whose resistance drops as the voltage across them rises.[0m [0;4mTheir function in the phone set is to short out parts of the set if the applied voltage gets too high. [KThe hook switch actually has three sets of contacts, two normally open (open,[0m [0;4mthat is, when the hand set is on hook) which completes the DC circuit when you pick up the handset, and a normally closed contact which is wired directly[0m [0;4macross the earphone. This contact's function is to short the earphone during[0m [77D[0;4mhe time that the DC circuit is being opened or closed through the phone - this prevents you from being blasted by a loud click in the earphone.[0m [K[0;4mThe dial has two contacts. One of these is the pulsing contact, which is normally closed and only opens during dialing on the return path of the dial after you let go of it. The second contact (the off-normal contact), shorts[0m [0;4mthe earphone as soon as you start turning the dial, and releases the short only [75C[H[70C[0m7[70D[0;4mfter the dial returns back to the normal position. In this way you do not[0m [0;4mhear the clicking of the dial in the phone as you dial. Finally, the phone has the hybrid network which consists of a four-winding transformer and whole[0m [0;4mcollection of resistors, capacitors, and varistors. The main function of the network is to attenuate your own voice to lower its volume in your earphone. [KThe simplest phone you could build would be just a series circuit consisting of a dial, a mike, and an earphone. But the signals coming back from the other party so much weaker than your own signals, that than earphone sensitive enough to reproduce clearly and loudly the voice of the other person would then blast your eardrums with the sound of your own voice. The function of the network is to partially cancel out the signal produced by the local mike, while permitting all of the received signal to go to the earphone. This technique is similar to the use of the hybrid phone patch with a VOX circuit, where you want the voice of the party on the telephone to go to your transmitter, but want to keep the receiver signal out the transmitter.[K [KIn addition to the parts needed for the hybrid, the network also contains a few other components (such as the RC network across the dial pulsing contacts) and screwtype connection points for the entire phone.[K [KA Touchtone phone is similar to the dial phone described above, except that the [75C[H[70C[0m8 [0;4mrotary dial is replaced by a Touchtone dial. In addition to its transistorized tone generator, the standard Touchtone pad has the same switch contacts to mute[75Dearphone, except that instead of completely shorting the earphone, as the rotary dial does, the Touchtone dial switches in a resistor which only[0m [0;4mpartially mutes the phone.[K It is fairly common knowledge as to what frequencies are used for Touchtone[0m [0;4msignalling, but a it never hurts to reiterate information. Each digit is[0m [0;4mcomposed of one frequency from the low group and one frequency from the high[0m [0;4mgroup; for instance, the digit 6 is generated by producing a low tone of 770 Hz (Hertz) and a high tone of 1477 Hz at the same time. The American Touchtone[0m [0;4mpads generate both of these tones with the same transistor, while European pads (yes, there are some) use two transistors, one for reach tone. In addition to[0m [75D[0;4mfirst three high tones, a fourth tone of 1633 Hz has been decided on for[0m [0;4mgenerating four more combinations. These are not presently in use, although[0m [0;4mthe standard phone Touchtone pad can easily be modified to produce this tone, since the required tap on the inductor used to generate the the tone is already present and only an additional switch contact is needed to use it.[0m [K[0;4mWhat is not generally known is that the United States Air Force uses a different set of Touchtone frequencies, in the range of 1020 to 1980 Hz. Since many of th[7Cs available for purchase in stores come from Department of[0m [75C[H[70C9 [0;4mDefense surplus sales, it will be interesting when these phones become[0m [0;4mavailable.[K [KAnother Touchtone dial presently used by amateurs is made up from a thin elastomeric switch pad made by the Chomerics Corporation (77 Dragon Court, Woburn, Mass. 01801) and a thick-film hybrid IC made by Microsystems International (800 Dorchester Boulevard, Montreal, Quebec). The pad is th Chomerics ER-20071, which measures about 2 1/4 inch wide by 3 inches high, and only about 3/16 inch thick (Chomerics also makes a smaller model ER21289, but it is very difficult to use and also apparently unreliable). Microsystems[0m [0;4mInternational makes several very similar ICs in the ME8900 series, which us different amounts of power and generate different amounts of audio. Some of[0m [0;4mthese also contain protection diodes to avoid problems if you use the wrong[0m [0;4mpolarity on the IC, and there are so many models to choose from that you should get the technical data from the manufacturer before ordering one. There are a number of United States distributors, including Newark Electronics, Milgray and Arrow Electronics in New York.[K [KOne of the problems with any current IC oscillator is that the frequency changes if rf gets near it. Many hams are having a hard time mounting such IC pads on their 2 meter handie-Talkies. A solution seems in sight as Mostek, a[0m [0;4mlarge IC company, is coming out with an IC Touchtone generator which has a[0m [75C[H[69C20 [0;4mcheap 3.58 MHz external crystal as reference, and then produces the tone frequencies by dividing the 3.58 MHz down with flip flops to get the required tone frequencies. This approach not only promises to be more reliable in the presence of rf, but should also be cheaper since it would not need the custom (and expensive) laser trimming of components that the Microsystems[0m [0;4mInternational IC needs to adjust the frequencies within tolerance.[0m [K[0;4mAt the other end of the telephone circuit, in the CO, various circuits are used to decode the digit you dial into the appropriate signals needed to perform the actual connection. In dial systems, this decoding is done by relay circuits, such as steppers. This circuitry is designed for dialing at the rate of 10[0m [0;4mpulses per second, with a duty cycle of about 60% open, 40% closed. The[0m [0;4mminimum time between digits is about 600 milliseconds, although a slightly[0m [0;4mgreater time between digits is safer since it avoids errors.[K [KIn practice, many COs will accept dialing at substantially slower or faster[0m [0;4mrates, and often you will see a dial that has been speeded up by changing the mechanical governor to operate almost twice as fast; it depends on the type of CO equipment.[K [KTouchtone decoding is usually done by filter circuits which separate out the[0m [0;4mTouchtone tones by filters and then use a transistor circuit to operate a[0m [75C[H[70C1 [0;4mrelay. A common decoder is the 247B, which is designed for use in small dial switchboard systems of the type that would be installed on the premises of a[0m [0;4mbusiness for local communication between extensions. It consists of a limiter amplifier, seven filters and relay drivers (one for each of the seven tones[0m [0;4mcommonly used) and some timing and checking circuitry. Each of the seven relays has multiple contacts, which are then connected in various[0m [0;4mseries/parallel combinations to provide a grounding of one of ten output contacts, when a digit is received. The standard 247B does not recognize the * and digits, but can be modified easily enough if you have the unit diagram.[0m [K[0;4mThe 247B decoder is not very selective, and can easily be triggered by voice unless some additional timing circuits are connected at the output to require that the relay closure exceed some minimum time interval before it is accepted. Slightly more complicated decoders which have the time delays built in are the A3-type and the C-type Touchtone Receivers. both of these are used in customer-owned automatic switchboards when a caller from the outside (via the telephone company) wants to be able to dial directly into the private[0m [0;4mswitchboard to call a specific extension.[K [0m [0;4mThe C-type unit is similar to the 247B in that it has ten outputs one for each digit. The A3-type does not have output relays, but instead has seven voltage outputs, one for each of the seven basic tones, for activating external 48-volt [75C[H[70C[0m2[66D[0;4ms. The A-3 unit is ideal for activating a Touchtone encoder, which can[0m [0;4mthen be used to regenerate the Touchtone digits if the original input is noisy. This might be very useful in a repeater autopatch, for cleaning up Touchtone[0m [0;4mdigits before they are sent into the telephone system.[K [KIn addition to the above, there are probably other types of units specially designed for use in the CO, but information on these is not readily available. It is also fairly easy to build a Touchtone decoder from scratch. Though the[0m [0;4mstandard telephone company decoders all use filter circuits, it is much easier (though perhaps not as reliable) to use NE567 phase-locked-loop integrated circuits.[K [KAn interesting sidelight to Touchtone operation is that it greatly speeds up[0m [0;4mthe process of placing a call. With a Touchtone dial it is possible to dial a call perhaps 3 or 5 times faster than with a rotary dial. Since the CO equipment which receives and decodes the number is only needed on your line[0m [0;4mduring the dialing time, this means that this equipment can be switched off your line sooner and can therefore handle more calls. In fact, the entire Touchtone system was invented so that CO operation would be streamlined and less equipment would be needed for handling calls. It is ironic that the[0m [0;4mcustomer should be charged extra for a service which not only costs the[0m [0;4mtelephone company nothing, but even saves it money.[K [75C[H[70C[0m3 [K[0;4mAnother practice which may or may not cost the company money is the connection[0m [0;4mof privately-owned extension phones. You have probably seen these sold by mail order houses and local stores. The telephone companies claim that connecting these phones to their lines robs them of revenue and also may cause damage to their equipment. There are others, of course, who hold the opinion that the easy availability of extensions only causes people to make more calls since[0m [0;4mthey are more convenient, and that the companies really benefit from such use. The question of damage to equipment is also not easily answered, since most of the extension phones are directly compatible, and in many cases the same type as the telephone company itself uses. Be that as it may, this may be a good time to discuss such use. [KPrior to an FCC decision to telephone company interconnection in the Carterfone case in 1968, all telephone companies claimed that the connection of any[62Dto their lines was illegal. This was a slight misstatement as no specific laws against such use were on the books. Instead, each local[0m [0;4mtelephone company had to file a tariff with the public service commission in that state, and one of the provisions of that tariff was that no connection of any external equipment was allowed. By its approval of that tariff, the public service commission gave a sort of implicit legal status to the prohibition. [K[75C[H[70C[0m4 [0;4mIn the Carterfone case, however, the FCC ruled that the connection of outside equipment had to be allowed. The phone companies then relaxed their tariff[0m [0;4mwording such that connection of outside equipment was allowed if this[0m [0;4mconnection was through a connecting arrangement provided by the telephone[0m [0;4mcompany for the purpose of protecting its equipment from damage. Although this result has been challenged in several states, that seems to be the present[0m [0;4mstatus. The strange thing is that some telephone companies allow[0m [0;4minterconnection of customer equipment without any hassle whatsoever, while[0m [0;4mothers really make things difficult for the customer.[K _______________________________________________________________________________ [K ==Phrack Inc.== Volume Two, Issue 24, File 10 of 13[K [K ()()()()()()()()()()()()()()()()()()()[0m [0;4m () ()[0m [0;4m () Network Progression ()[K () ()[K () by Dedicated Link ()[K () ()[0m [0;4m () January 1989 () [75C[H[70C[0m5 [0;4m () ()[K ()()()()()()()()()()()()()()()()()()()[0m [K[K[0;4mThis file provides a general overview of how networks have progressed from[0m [0;4mphone lines to T1 lines.[K [KThere are numerous reasons to share networking facilities. The concept of networking is to optimize all the aspects of voice and data transmission, and to utilize all the amounts of space in the transmission lines.[0m [0;4mNot long ago companies used AT&T's switching facilities for all local calls. This means use of the Centrex, which is the switching of local calls by AT&T (which is much more expensive than using your own switching facilities). Then the larger organizations started to put in PBXes (Private Branch Exchange) to enable them to switch local calls (class 5 ESS) without having anything to do with AT&T. The process of using a PBX (or a Computerized Branch Exchange CBX) is much more efficient if the phone traffic is high. This is the beginning of a Local Area Network (LAN). Once an organization has it's own LAN it can lease the extra transmission space to another company, because they are paying for it anyway. Another method of bypassing AT&T's service is to use a foreign exchange (FX) line. Which is a long distance dedicated point-to-point private [75C[H[70C[0m6 [0;4mline, which is paid for on a flat rate basis. A FX line can be purchased from AT&T or many other vendors. These private lines (PL) are used with voice and data transmissions. Data transmission must have a higher grade quality than voice because any minor break in the transmission can cause major, expensive errors in data information being processed.[K [KOne of the most optimum ways of transmitting data is a T1 line which transmits data at 1.544 megabits per second. Microwave, Satellite, and Fiber Optic[0m [0;4msystems are being used for data transmission. These methods multiplex several lines into one to create greater capacity of the transmission. A multiplexed line has 24 channels that can be divided into the appropriate space needed to utilize each transmission (i.e. a simple voice transmission which has about[0m [0;4m300-3000 Hz uses a small portion of the multiplexed line). There are two types of multiplexing; time-division and frequency. Time-division multiplexing[0m [0;4mdivides the channels into separate time slots. Frequency-division multiplexing separates the different channels with the use of different bandwidths.[0m [0;4mTypically, data is transmitted through digital systems rather than analog.[0m [0;4mHowever, all the state-of-the-art equipment is now digital.[K [KWhen the data is being processed from the computer to another computer there[0m [0;4mmust be a standard protocol for communicating the interexchange within the network. The protocol is the set of rules that the computer says are necessary [75C[H[70C[0m5 [0;4m () ()[K ()()()()()()()()()()()()()()()()()()()[K [K[KThis file provides a general overview of how networks have progressed from phone lines to T1 lines. [KThere are numerous reasons to share networking facilities. The concept of networking is to optimize all the aspects of voice and data transmission, and[0m [0;4mto utilize all the amounts of space in the transmission lines.[0m [K[0;4mNot long ago companies used AT&T's switching facilities for all local calls. This means use of the Centrex, which is the switching of local calls by AT&T[0m [0;4m(which is much more expensive than using your own switching facilities). Then the larger organizations started to put in PBXes (Private Branch Exchange) to[0m [0;4menable them to switch local calls (class 5 ESS) without having anything to do with AT&T. The process of using a PBX (or a Computerized Branch Exchange CBX) is much more efficient if the phone traffic is high. This is the beginning of a Local Area Network (LAN). Once an organization has it's own LAN it can lease the extra transmission space to another company, because they are paying for it anyway. Another method of bypassing AT&T's service is to use a foreign[0m [0;4mexchange (FX) line. Which is a long distance dedicated point-to-point private[0m [75C