controller of calls within its area of jurisdiction. To route a call to a mobile station, the path through the network links to the MSC in the MSC area where the subscriber is currently located. The mobile location can be uniquely identified because the MS is registered in a VLR, which is associated with an MSC. Public Land Mobile Network (PLMN) A Public Land Mobile Network (PLMN) is the area served by one network operator, as shown in Figure 1-18. One country can have several PLMNs, based on its size. The links between a GSM/PLMN network and other PSTN, ISDN, or PLMNs will be at the level of national or international 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 Figure 1-18 The PLMN. E1 or better PSPDN transit. All incoming calls for a GSM/PLMN will be routed to the Gateway MSC. A Gateway MSC works as an incoming transit exchange for the GSM/PLMN. All mobile-terminated (MT) calls will be routed to the Gate- way MSC. Call connections between PLMNs or fixed networks must be routed through certain designated MSCs. OSI Model-How GSM Signaling Functions in the OSI Model The Open Standards Interface (OSI) is a guideline of how systems commu- nicate transparently. SS7 is used for signaling between the outside world and the GSM architectures. Moreover, SS7 is used to communicate between the MSC and the HLR. To satisfy other functions in GSM architecture, the model is applied for other services from the mobile station outward. In real- ity, the model works at the bottom three layers of the OSI model for the bulk of the transmissions that take place in call setup and teardown, registration and authentication, and SO on. Thus, Layers 3, 2, and 1 of the OSI model are most applicable. OSI defines a communications subsystem consisting of functions that enable distributed application processes, resident on compute