rs, to Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. Introduction to GSM Chapter 1 exchange information via an underlying data network. The communica- tions subsystem can be divided into two sublayers: An application-dependent sublayer providing functions that are application-dependent but network-independent A network-dependent sublayer providing functions that are dependent on the underlying data network but are application-independent Ensuring the transmission of voice or data of a given quality over the radio link is only part of the function of a cellular mobile network. A GSM mobile can seamlessly roam nationally and internationally, which requires that registration, authentication, call routing, and location-updating func- tions exist and are standardized in GSM networks. In addition, the fact that the geographical area covered by the network is divided into cells necessi- tates the implementation of a handover mechanism. The Network Subsys- tem performs these functions using the Mobile Application Part (MAP) built on top of the Signaling System Number 7 protocol, as shown in Figure 1-19. Layer Functionality In the GSM architecture, the layered model integrates and links the peer- to-peer communications between two different systems. If we look across the platform, the underlying layers satisfy the services of the upper-layer Figure 1-19 SS7 and GSM SS7 Signaling working together. (G)MSC Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. Introduction to GSM Introduction to GSM protocols. For example, at Layer 3, the Service Access Point (SAP) between Layer 3 and 2 addresses the services being served. Service Access Point Identifiers (SAPIs) describe the services t