6.1 Introduction

The Message Transfer Part (MTP) of CCSS7 provides a comprehensive transport mechanism to carry Level 4 messages through the signalling network. However, the MTP cannot interpret the meaning of the messages being transferred. It is the User Parts (UPs) that define the meaning of the messages that are being transferred for circuit-related connections. The UPs determine the formats of messages and the sequence in which they are sent. The User Parts also interact with the call control function within a node to establish a controlling mechanism for calls.

The ISDN User Part (ISUP)1 defines the messages and procedures for the control of switched services in integrated services digital networks (ISDNs). The ISUP covers both voice (e.g. telephony) and non-voice (e.g. circuit-switched data) applications.

For ISDN calls, there is a basic requirement to use digital transmission links and digital nodes, thus ensuring that there is a digital connection (e.g. 64 kbit/s) from one user to another. The ISUP is designed primarily to control the set-up and release of calls in this all-digital environment and the procedures and formats are optimised accordingly. Supplementary services are also defined using the advanced techniques available in the ISUP. The ISUP was developed from the outset for operation in both national and international networks. Hence, the adoption of ISUP will contribute to the reduction in signalling interworking costs between these networks.

The ISUP was designed for use in narrowband networks. A broadband version of ISUP is described in Chapter 15. To discriminate between the...