ISDN User Part
According to ITU-T Q.761 section 2.4.1 "ISUP interworking" ISUP'92 is backwards compatible with ISUP Blue Book and Q.767 for basic call procedures and supplementary services except for some procedures (eg. number portability). [http://www.itu.int/rec/T-REC-Q.761/en ITU-T Recommendation Q.761] ] Additionally the compatibility features introduced in this version ensure forward compatibility with newer versions.
Message types
An ISUP message contains a fixed header containing the circuit identification code and the ISUP message type, followed by a fixed-length part and optional variable-length part that are dependent on the type of message being sent. ISUP messages can be sent using the services of the Message Transfer Part, or, less often, the Signalling Connection Control Part. These messages are transmitted in various stages of call setup and teardown. The most common messages are: [harv|Russell|2002|p=362-419.]
*Initial Address Message (IAM) — First message sent to inform the partner switch, that a call has to be established on the CIC contained in the message. Contains the called and calling number, type of service (speech or data) and many more optional parameters.
*Subsequent Address Message (SAM) — In case the IAM did not contain the full called number, one or more SAMs may follow containing additional digits.
*Address Complete Message (ACM) — Message returned from the terminating switch when the subscriber is reached and the phone starts ringing.
*Answer Message (ANM) — Sent when the subscriber picks up the phone. Normally charging starts at this moment.
*Release (REL) — Sent to clear the call when a subscriber goes on hook.
*Release complete (RLC) — Acknowledgement of the release – the timeslot is idle afterwards and can be used again. This is also sent (without a preceding Release message) if the terminating switch determines that the call cannot be completed. The terminating switch also sends a Cause Value to explain the reason for the failure, "e.g.", "User busy".
Sample call flow
This is a very basic call flow involving only two telecom switches which exchange the ISUP messages. The subscriber interfaces are not covered here and are only listed for a better understanding.
A subscriber telco switch A telco switch B B subscriber Off hook Dial digits ---> -- IAM --> -Ringing -> <-- ACM -- Off hook <-- ANM -- ----------------------- Conversation ----------------------- On hook -- REL --> On hook <-- RLC --
Detailed call flows are provided in [http://www.itu.int/rec/T-REC-Q.784.1/en/ ITU-T Recommendation Q.784.1] .
Message format
The "Signalling Information Field (SIF)" for all ISUP Message Signal Units (MSU) contain the following components: [ [http://www.itu.int/rec/T-REC-Q.763-199912-I/en/ ITU-T Recommendation Q.763 (12/1999)] , Section 1, "General coding principles", pp 3-4.]
* Routing Label
* Circuit Identification Code
* Message Type
* Mandatory Fixed Part
* Mandatory Variable Part
* Optional Part
The "Routing Label" indicates the Point Codes of the originating and destination nodes in the network; it also includes the Signalling Link Selection field that is used to select between the multiple routes an MSU could take between two nodes.
The "Circuit Identification Code" is used to specify which trunk between two switches is used to carry a particular call. Note that some versions of ANSI ISUP permit a CIC with 14 significant bits instead of the 12 that are shown. [harv|Russell|2002|p=321.]
The "Message Type" indicates the ISUP message type. The presence and form of the remaining 3 components are determined by this message type.The "Mandatory fixed part", when present, contains the mandatory, fixed-length parameters associated with the message type.The "Mandatory variable part", when present, contains the mandatory, variable-length parameters associated with the message type.The "Optional part", when present, contains the optional parameters permitted to be included in the message type.
When sent using the services of the Signalling Connection Control Part, ISUP messages passed to SCCP in the User Data parameter (NSDU) consist of only the last 4 components ("Message Type, Mandatory fixed part, Mandatory variable part, Optional part""). The routing label and circuit identification code are not included in the user data passed to SCCP. [ [http://www.itu.int/rec/T-REC-Q.763-199912-I/en/ ITU-T Recommendation Q.763 (12/1999)] , Section 1, "General coding principles", p 3.]
Notes
References
* cite book
last=Russell
first=Travis
title=Signaling System #7
year=2002
edition=4th Edition
publisher=McGraw-Hill
location=New York
isbn=978-0071387729
* [http://www.itu.int/rec/T-REC-Q.784.1/en/ ITU-T Recommendation Q.784.1 (07/96)] , "ISUP basic call test specification: Validation and compatibility for ISUP'92 and Q.767 protocols".