s. The Gateway Mobile services Switching Center (GMSC) is used in the PLMN. A gateway is a node interconnecting two networks. The GMSC is the interface between the mobile cellular network and the PSTN. It is in charge of routing calls from the fixed network towards a GSM user. The GMSC is often implemented in the same machines as the MSC. A PLMN may have many MSCs, but it has only one gateway access to the wireline network to accommodate the network operator. The gateway then is the high-speed trunking machine connected via E1 or Synchronous Digital Hierarchy (SDH) to the outside world. The Registers Completing the NSS The Home Location Register (HLR) and Visitor Location Register (VLR), together with the MSC, provide the call routing and roaming capabilities of GSM, called the Network Switching Systems (NSS). The HLR contains all the administrative information of each subscriber registered in the corre- sponding GSM network, along with the current location of the mobile. The current location of the mobile is in the form of a Mobile Station Roaming Number (MSRN), which is a regular ISDN number used to route a call to the MSC where the mobile is currently located. One HLR exists logically per GSM network, although it may be implemented as a distributed data- base. Figure 1-16 shows the HLR. The VLR contains selected administrative information from the HLR, which is necessary for call control (CC) and provision of the subscribed 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 1-16 The HLR. Interface services, for each mobile currently located in the geographical area con- trolled by the VLR. Although each functional entity can be implemented as an independent unit, most manufacturers of switching equipment imple- ment one VLR together with one MSC (Figure 1-17) SO that the geographi- cal area cont