is puts an active call on hold. Advice of Charge (AoC) This provides the user with online charge information. Multiparty service This creates the possibility of establishing a multiparty conversation. Closed User Group (CUG) This corresponds to a group of users with limited possibilities of calling (only the people of the group and certain numbers). 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 Operator-determined barring This provides restrictions of different services and call types by the operator. GSM Architecture A GSM network consists of several functional entities, whose functions and interfaces are defined. Figure 1-9 shows the layout of a generic GSM net- work. The GSM network can be divided into three broad parts. The sub- scriber carries the mobile station; the Base Station Subsystem (BSS) controls the radio link with the mobile station; and the Network Subsys- tem, the main part of which is the Mobile services Switching Center (MSC), performs the switching of calls between the mobile and other fixed or mobile network users, as well as the management of mobile services, such as authentication. The Operations and Maintenance Center, which oversees the proper operation and setup of the network, is not shown in the figure. The mobile station and the Base Station Subsystem communicate across the Um interface, also known as the air interface or radio link. The Base Station Subsystem communicates with the network service switching cen- ter across the A interface. The added components of the GSM architecture (Figure 1-10) include the functions of the databases and messaging systems such a Home Location Register (HLR) Visitor Location Register (VLR) Mobile TE Figure 1-9 The GSM architecture. Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The Mc