ISUP: ISDN User Part

The ISUP – ISDN User Part, a key protocol in the SS7 / C7 signaling system, 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) between different switches. ISUP is used for both ISDN and non-ISDN calls. A simple call flow using ISUP signaling is as follows:

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. The destination switch rings the called party line if the line is available and transmits an ISUP address complete message (ACM) to the originating switch to indicate that the remote end of the trunk circuit has been reserved. The STP routes the ACM to the originating switch 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.

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. The STP routes the ANM to the originating switch which verifies that the calling party’s line is connected to the reserved trunk and, if so, initiates billing.

If the calling party hangs up first, the originating switch sends an ISUP release message (REL) to release the trunk circuit between the switches. The STP routes the REL to the destination switch. If the called party hangs up first, or if the line is busy, the destination switch sends a REL to the originating switch indicating the release cause (e.g., normal release or busy). 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 to acknowledge the release of the remote end of the trunk circuit. When the originating switch receives (or generates) the RLC, it terminates the billing cycle and sets the trunk state to idle in preparation for the next call.

The ANSI and ITU-T have slightly different ISUP format. ITU-T ISUP message format:

The routing label is used by the relevant user part to identify particulars to which the message refers. It is also used by the Message Transfer Part (MTP) to route the message towards its destination point.

The allocation of circuit identification codes to individual circuits is determined by the bilateral agreement and/or in accordance with applicable predetermined rules.

The message type code uniquely defines the function and format of each ISDN User Part message. Each message consists of a number of parameters. Message types may be: