Explaining ISUP MESSAGE Format

ISUP Message Format

The User Data portion of the MTP3 Signaling Information Field contains the ISUP message, identified by a Service Indicator of 5 in the MTP3 SIO field. Each ISUP message follows a standard format that includes the following information:

• CIC— The Circuit Identification Code for the circuit to which the message is related.
• Message Type— The ISUP Message Type for the message (for example, an IAM, ACM, and so on).
• Mandatory Fixed Part— Required message parameters that are of fixed length.
• Mandatory Variable Part— Required message parameters that are of variable length. Each variable parameter has the following form:- Length of Parameter- Parameter Contents

Because the parameter is not a fixed length, a field is included to specify the actual length.

• Optional Part— Optional fields that can be included in the message, but are not mandatory. Each optional parameter has the following form:- Parameter Name- Length of Parameter- Parameter Contents

Figure 8-10 shows the ISUP message structure, as described here. This message structure provides a great deal of flexibility for constructing new messages. Each message type defines the mandatory parameters that are necessary for constructing a message. The mandatory fixed variables do not contain length information because the ISUP standards specify them to be a fixed length. Because the mandatory variable parameters are of variable lengths, pointers immediately follow the mandatory fixed part to point to the beginning of each variable parameter. The pointer value is simply the number of octets from the pointer field to the variable parameter length field.

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 ISUP Message Format

In addition to the mandatory fields, each message can include optional fields. The last of the pointer fields is a pointer to the optional part. Optional fields allow information to be included or omitted as needed on a per-message basis. The optional fields differ based on variables such as the call type or the supplementary services involved. For example, the Calling Party Number (CgPN) field is an optional parameter of the IAM, but is usually included to provide such services as Caller ID and Call Screening.

A single message can include many optional parameters. The optional part pointer field only points to the first parameter. Because the message might or might not include the parameters, and because the parameters can appear in any order, the first octet includes the name of each parameter in order to identify it. The parameter length follows the name to indicate how many octets the parameter contents include. When the parameter name is coded as zero, it signals the end of the optional parameters. During parsing of an incoming ISUP message, optional parameters are processed until the end of optional parameters marker is reached. If the message does not have any optional parameters, the pointer to the optional part is coded to zero.

Basic Call Message Formats in ISUP

Here, we examine the six messages shown in the basic call setup because they comprise the core message set for basic call setup and release, and are therefore used frequently. There are slight variations in the messages used based on the individual network. For example, Europe uses the SAM frequently and the COT message more rarely. In North America, SAM is not used at all, but COT is used more often. This section considers the following messages:

• Initial Address Message (IAM)
• Subsequent Address Message (SAM–ITU Networks only)
• Continuity Message (COT)
• Address Complete Message (ACM)
• Answer Message (ANM)
• Release Message (REL)
• Release Complete Message (RLC)

The following sections show only the mandatory fields for each message. Keep in mind that many optional parameters can also be included. In each of the figures, the fixed mandatory fields with sub-fields have been expanded to show what they are. For the sake of brevity in the figures, the variable subfields have not been expanded. All of the ISUP Message formats and parameters are documented in ITU-T Q.763. ANSI T1.113 documents the North American version of the messages.