ISUP Message Format

This section describes the general format of ISUP messages (Fig. 11.2-2). The circuit identification code (CIC) has 14 bits, and is located in octets 1 and 2. In a

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Figure 11.2-1 ISUP parameters, (a): mandatory, fixed-length (MF) parameters, (b): mandatory, variable-length (MV) parameters, (c): optional (OP) parameters.

network, an ISUP trunk is uniquely identified by the combination of DPC and OPC (located in RL—see Fig. 7.3-2), and CIC. In TUP signaling, the four low-order bits of CIC are also used as the signaling link selector (9.1.1). In ISUP signaling, SLS and CIC are separate parameters. This allows the rotation of the contents of SLS at the signaling points along the message route (see example 3 of Section 8.8.5).

The message type code (octet 3) identifies a particular ISUP message. CIC and the message type code are mandatory parameters (value fields only).

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Isup Messages
Figure 11.2-2 General ISUP message format. (From Rec. Q.763. Courtesy of ITU-T.)

Octets 4 through n hold the message parameters. In this example, A through F are mandatory fixed (MF) parameters. In a particular ISUP message type, these parameters always appear—in a predetermined order—starting at octet 4. Since their lengths are fixed, the locations of the MF parameters in the message are also fixed, and known by the software entity that reads the message. Therefore, "name" and "length" fields are not needed.

The mandatory variable (MV) part of the message-parameter field begins with a block of one-octet pointers to the locations of the "length" fields of the MV parameters (in this example, M through P). In messages of a particular type, this block always starts at the same location, and the pointers appear in a predetermined order. The software entity that reads the message can thus determine the locations of any MV parameter. "Name" fields are therefore not necessary.

The octet following the pointer of the last MV parameter holds the pointer to the first octet of the optional part of the message-parameter field, where the OP parameters X through Z are located. Optional parameters can appear in any combination, and in any order, and therefore appear with their "name," "length," and "value" fields.

When a message does not include optional parameters, the pointer to the optional part of the message is coded 0000 0000. When a message includes optional parameters, an octet coded 0000 0000 follows the value field of the last parameter.

The message structure described above is also used for messages of SCCP. 11.2.3 Call-control Messages

The most important ISUP call-control messages are outlined below. Most messages are the counterparts of TUP messages, but generally include more parameters.

Initial Address Message (1AM). As in No.6 and TUP signaling, the IAM is the first message in a call set-up. It includes the called number, and other parameters that are of importance for the set-up. In most countries, and in the international network, ISUP address signaling is en-bloc (the IAM includes the complete called number). However, CCITT has also specified a subsequent address message (SAM), for overlap address signaling. The IAM then holds the initial digits of the called address, and the remaining digits are transferred in one or more SAMs.

Continuity Message (COT). This message is sent forward during the call setup, if the outgoing exchange has made a continuity test on an outgoing trunk. It indicates whether the test has been successful.

Address Complete Message (ACM). This is a backward message. When sent by the terminating local exchange, it indicates that the exchange is ringing the called subscriber, or has received a Q.931 ALERT message from a terminal on the called digital subscriber line (DSL). When sent by an intermediate exchange, it indicates that the exchange has seized an outgoing trunk that does not have ISUP signaling.

Call Progress Message (CPG). This is a backward message, sent by a terminating exchange that already has sent an ACM message, when it needs to report the occurrence of an "event" in the call set-up.

Answer Message (ANM). This is a backward message, sent when the called party answers.

Release Message (REL). This is a general message that requests the immediate release of a connection. It can be sent forward or backward, and includes—like the Q.931 DISC (disconnect) message—a "cause" parameter that indicates the reason for the release, and the originator of the release. In other interexchange signaling systems, the release of a connection is normally initiated by the calling subscriber. In ISUP signaling, the calling and called ISDN users can initiate a release.

A REL message is also sent by an intermediate exchange, or by the terminating local exchange, when the set-up of a call is not possible.

Release Complete Message (RLC). This is a forward or backward message, sent by an exchange in response to a received REL message for a trunk. It indicates that the exchange has completed the release of the trunk at its end. The RLC message has the same function as the "release guard" message of TUP signaling.

Suspend Message (SUS). This is a backward message that is similar to the "clear-back" message of TUP. It requests to suspend the call, but to leave the connection intact.

Resume Message (RES). This message requests to resume a suspended call.

Forward Transfer Message (FOT). An outgoing operator, who is assisting a subscriber or ISDN user in a call set-up, can request the help of an incoming operator, by sending a (forward) FOT message (see Section 4.3.5).

Information Request Message (INR). This is a request for additional call-related information, usually sent from the terminating exchange to the originating exchange of a connection (end-to-end signaling).

Information Message (INF). This is a response to an INR message, and containing the requested information.

Table 11.2-1

Message type codes of ISUP call-control messages.

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