12.1 Introduction

The DSS1 Network Layer (Layer 3) is responsible for establishing, maintaining and clearing connections between users and the narrowband ISDN (e.g. between users and local nodes). The procedures in DSS1 control ( a) circuit-switched connections, ( b) user-to-user signalling connections and ( c) packet-switched connections. The specifications of DSS1 Layer 3 adopt the term 'user' to describe a calling/called party1. This Chapter follows this terminology.

It is Layer 3 of DSS1 that formulates and analyses the messages carried by Layers 1 and 2. In this role, the DSS1 Network Layer interacts with the user and translates the user requirements into messages to be sent to an ISDN local node. At the local node, the DSS1 Network Layer interacts with the call control functions to stimulate actions meeting the user requirements. As outlined in Chapter 3, the DSS1 Layer 3 does not correspond to Layer 3 of the OSI Protocol Reference Model.

DSS1 Layer 3 can be described in terms of its formats and the procedures (defining the logical sequence of events) in meeting user requirements. Section 12.2 describes the principles of the format technique and Section 12.3 gives examples of messages to illustrate the principles. The basic procedures for establishing circuit-switched calls are described in Section 12.4 and the procedures for clearing such calls are described in Section 12.5.

DSS1 Layer 3 includes features additional to those necessary for establishing and clearing calls and these features are outlined in Section 12.6. DSS1 Layer 3 is...