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ISUP

ISUP (p. 2)

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ISDN User Part

The ISDN User Part (ISUP) defines the protocol and procedures used to set-up, manage, and release trunk circuits that carry voice and data calls over the public switched telephone network (PSTN). ISUP is used for both ISDN and non-ISDN calls. Calls that originate and terminate at the same switch do not use ISUP signaling.

Basic ISUP Call Control

Figure 8 depicts the ISUP signaling associated with a basic call.

  1. When a call is placed to an out-of-switch number, the originating SSP transmits an ISUP initial address message (IAM) to reserve an idle trunk circuit from the originating switch to the destination switch (1a). The IAM includes the originating point code, destination point code, circuit identification code (circuit "5" in Fig. 8), dialed digits and, optionally, the calling party number and name. In the example below, the IAM is routed via the home STP of the originating switch to the destination switch (1b). Note that the same signaling link(s) are used for the duration of the call unless a link failure condition forces a switch to use an alternate signaling link.

    Figure 8. Basic ISUP Signaling

    Figure 8. Basic ISUP Signaling

    1. The destination switch examines the dialed number, determines that it serves the called party, and that the line is available for ringing. The destination switch rings the called party line and transmits an ISUP address complete message (ACM) to the originating switch (2a) (via its home STP) to indicate that the remote end of the trunk circuit has been reserved. The STP routes the ACM to the originating switch (2b) which rings the calling party's line and connects it to the trunk to complete the voice circuit from the calling party to the called party.

      In the example shown above, the originating and destination switches are directly connected with trunks. If the originating and destination switches are not directly connected with trunks, the originating switch transmits an IAM to reserve a trunk circuit to an intermediate switch. The intermediate switch sends an ACM to acknowledge the circuit reservation request and then transmits an IAM to reserve a trunk circuit to another switch. This processes continues until all trunks required to complete the voice circuit from the originating switch to the destination switch are reserved.

    2. When the called party picks up the phone, the destination switch terminates the ringing tone and transmits an ISUP answer message (ANM) to the originating switch via its home STP (3a). The STP routes the ANM to the originating switch (3b) which verifies that the calling party's line is connected to the reserved trunk and, if so, initiates billing.

    3. If the calling party hangs-up first, the originating switch sends an ISUP release message (REL) to release the trunk circuit between the switches (4a). The STP routes the REL to the destination switch (4b). If the called party hangs up first, or if the line is busy, the destination switch sends an REL to the originating switch indicating the release cause (e.g., normal release or busy).

    4. Upon receiving the REL, the destination switch disconnects the trunk from the called party's line, sets the trunk state to idle, and transmits an ISUP release complete message (RLC) to the originating switch (5a) to acknowledge the release of the remote end of the trunk circuit. When the originating switch receives (or generates) the RLC (5b), it terminates the billing cycle and sets the trunk state to idle in preparation for the next call.

    ISUP messages may also be transmitted during the connection phase of the call (i.e., between the ISUP Answer (ANM) and Release (REL) messages.

    ISUP Message Format

    ISUP information is carried in the Signaling Information Field (SIF) of an MSU. The SIF contains the routing label followed by a 14-bit (ANSI) or 12-bit (ITU) circuit identification code (CIC). The CIC indicates the trunk circuit reserved by the originating switch to carry the call. The CIC is followed by the message type field (e.g., IAM, ACM, ANM, REL, RLC) which defines the contents of the remainder of the message (Fig. 9).

    Figure 9. ISUP Message Format

    Figure 9. ISUP Message Format

    Each ISUP message contains a mandatory fixed part containing mandatory fixed-length parameters. Sometimes the mandatory fixed part is comprised only of the message type field. The mandatory fixed part may be followed by the mandatory variable part and/or the optional part. The mandatory variable part contains mandatory variable-length parameters. The optional part contains optional parameters which are identified by a one-octet parameter code followed by a length indicator ("octets to follow") field. Optional parameters may occur in any order. If optional parameters are included, the end of the optional parameters is indicated by an octet containing all zeros.

    Initial Address Message

    An Initial Address Message (IAM) is sent in the "forward" direction by each switch needed to complete the circuit between the calling party and called party until the circuit connects to the destination switch. An IAM contains the called party number in the mandatory variable part and may contain the calling party name and number in the optional part.

    Figure 10. ANSI and ITU-T Initial Address Message (IAM) Format

    Figure 10. ANSI and ITU-T Initial Address Message (IAM) Format

     

     

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