ISUP message format

ISUP: ISDN User Part

Posted on |

ISUP: ISDN User Part


Protocol Description

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:


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 teardown:

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.


Protocol Structure

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

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 the 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
Blocking acknowledgement
Call progress
Circuit group blocking
Circuit group blocking acknowledgment
Circuit group query
Circuit group query response
Circuit group reset
Circuit group reset acknowledgment
Circuit group unblocking
Circuit group unblocking acknowledgment
Charge information
Continuity check request
Facility accepted
Facility reject
Forward transfer
Identification request
Identification response
Information request
Initial address
Loopback acknowledgment
Network resource management
Release complete
Reset circuit
Subsequent address
Unblocking acknowledgement
Unequipped CIC
User Part available
User Part test
User-to-user information


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.
ISUP is defined by ITU-T Q.763 documents.
Books you may interested
Books on ISDNISUP: ISDN User Part 13Books on ISDNISUP: ISDN User Part 14Books on ISDNISUP: ISDN User Part 15Books on ISDNISUP: ISDN User Part 16Books on ISDNISUP: ISDN User Part 17Books on ISDNISUP: ISDN User Part 18
Books on BroadbandISUP: ISDN User Part 19Books on ISDN SS7ISUP: ISDN User Part 20Books on ISDN and CCS7ISUP: ISDN User Part 21Books on ISDNISUP: ISDN User Part 22Books on Broadband NetworksISUP: ISDN User Part 23Books on TelecomISUP: ISDN User Part 24

Leave a Reply

Your email address will not be published. Required fields are marked *

This site uses Akismet to reduce spam. Learn how your comment data is processed.