The ISDN User Part (ISUP), a key protocol in the SS7/C7 signalling 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:

Call set up: 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.

Call connection: 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.

Call tear down: 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 an 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.

Protocol Structure

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

Routing label - 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.

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

Message type code - 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:

• Address complete
• Answer
• Blocking
• Blocking acknowledgement
• Call progress
• Circuit group blocking
• Circuit group blocking acknowledgement
• Circuit group query @
• Circuit group query response @
• Circuit group reset
• Circuit group reset acknowledgement
• Circuit group unblocking
• Circuit group unblocking acknowledgement
• Charge information @
• Confusion
• Connect
• Continuity
• Continuity check request
• Facility @
• Facility accepted
• Facility reject
• Forward transfer
• Identification request
• Identification response
• Information @
• Information request @
• Initial address
• Loop back acknowledgement
• Network resource management
• Overload @
• Pass-along @
• Release
• Release complete
• Reset circuit
• Resume
• Segmentation
• Subsequent address
• Suspend
• Unblocking
• Unblocking acknowledgement
• Unequipped CIC @
• User Part available
• User Part test
• User-to-user information

Parameters - Each parameter has a name which is coded as a single octet. The length of a parameter may be fixed or variable, and a length indicator for each parameter may be included.

Related Terms: SS7, ASP, BICC, BISUP, DUP, MTP, SCCP, TCAP, TUP

Sponsor Source: ISUP is defined by ITU-T Q.763 documents.

Reference Links: http://www.itu.int/rec/: Q.763: SS7 ISDN User Part