15.1 Introduction

The ITU-T signalling standards for broadband services are structured according to the layers outlined in Chapter 14. This chapter focuses on the higher layers for broadband inter-nodal signalling, namely the Broadband ISDN User Part (B-ISUP)1. The specification follows the trend outlined in Chapter 3 for a greater degree of modularity and a more open approach.

Other standards for broadband signalling are also being proposed. The ITU-T standards are used here to describe the principles of evolving towards broadband operation.

15.2 Architecture

The ISUP for narrowband (N-ISUP) takes a step towards the goal of modularity by separating call control from bearer control, using appropriate parameters to provide discrimination. The B-ISUP takes the modular approach to specification much further and separates key functions more comprehensively.

The specification model illustrated in Fig. 15.1 is used to achieve the requisite degree of modularity for basic calls. The model is based on the OSI Application Layer Model (described in Chapter 3). The model separates the actions that a node performs (Nodal Functions) from the communication protocols that occur between peer entities. The Nodal Functions contain procedures for Call Control, Maintenance and Compatibility. The communications protocols are sub-divided into the Single Association Control Function (SACF) and four Application Service Elements (ASEs), namely Call Control ASE, Bearer Control ASE, Maintenance Control ASE and Unrecognised Information ASE.

Figure 15.1: B-ISUP Basic Call Model

The B-ISUP Application Entity (AE) provides all the communication capabilities required by the nodal functions. Each ASE offers a service through the Single...