brary.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 management layer is a relay function at the BTS to the BSC. The RR pro- tocols are responsible for the allocation and reallocation of traffic channels between the MS and the BTS. These services include controlling the initial access to the system, paging for mobile-terminated calls, handover of calls between cell sites, power control, and call termination. The RR protocols provide the procedures for the use, allocation, reallocation, and release of the GSM channels. The BSC still has some radio resource management in place for the frequency coordination, frequency allocation, and the man- agement of the overall network layer for the Layer 2 interfaces. From the BSC, the relay is using SS7 protocols SO the MTP 1-3 is used as the underlying architecture and the BSS mobile application part or the Direct Application Part is used to communicate from the BSC to the MSC. MSC Protocols At the MSC, the information is mapped across the A interface to the MTP Layers 1 through 3 to the MSC from the BSC. Here the equivalent set of radio resources is called the BSS Mobile Application Part. The BSS Mobile Application Part/Direct Termination Application Part (MAP/DTAP) and the mobility management and connection management are at the upper layers of Layer 3 protocols. This completes the relay process. Through the control-signaling network, the MSCs interact to locate and connect to users throughout the network. Location registers are included in the MSC databases to assist in the role of determining how and whether connec- tions are to be made to roaming users. Each user of a GSM MS is assigned a Home Location Register (HLR) that is used to contain the user's location and subscribed services. A separate register, the Visitor Location Register (VLR), is used to track the location of a user. As the users roam out of the area covered by the HLR, the MS not