ISDN User Part continued [Under SS7]
ISUP provide two types of services: Basic and Supplementary. Basic service is nothing but Call setup and Tear down procedures. Supplementary services deals messages related to call path already established e.g. Call waiting, Call forward (during call) etc. Supplementary services are not defined separately but parameters within certain messages helps in implementing them e.g. "service activation" parameter in "Facility" message.
In addition, ISUP is flexible to provide end-to-end signaling. Let us say a network operator comes with a beautiful feature which require originating and destination exchanges to exchange certain operator specific (proprietary) information. This could be done in number of ways e.g. with "Pass Along Message" or "User-to-user information" parameter in IAM and other messages. Note that the information travel same path as the IAM message path.
Alternative for end-to-end signaling is to use SCCP to transfer the information directly to destination exchange through STP network ! In practice though ISUP method is preferred.
ISUP use MTP services for its message transfer; ISUP message is filled into SIF of MTP message - refer SS7 (MTP) - 3, diagram MTP - SU. ISUP message consist of Circuit Identification Code (CIC) followed by Message type and related parameters. Parameters are coded as "Mandatory fixed parameters" followed by "Mandatory variable parameters" and then "Optional parameters" at the end (similar to SCCP message format with addition of CIC before Message type).
Circuit Identification Code (CIC) indicate the circuit that is being setup or released. It is 2 octet long and is to be agreed among telephone companies.
This complete the brief on ISUP.
References: Q.700 (SS7), Q.761 (ISUP description), Q.762 (ISUP message functions), Q.763 (ISUP message formats), Q.764 (ISUP signaling procedures), Q.730 (ISUP supplementary services), SS7 book by Russell.
© Copyright Samir Amberkar 2010
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