Chapter 13. GSM and ANSI-41 Mobile Application Part (MAP)

In fixed-line networks, the subscriber's location is static and specified according to the numbering scheme used in the network.

In cellular telephony systems, the subscriber's location can change drastically without the system being aware—for example, the subscriber might switch his cell phone off just before boarding a plane, and then switch it back on in a new country. For incoming calls to mobile subscribers, there is no direct relationship between the subscriber's location and the cell phone number. Because the location and other information must be derived real-time before a call can be delivered to a cell phone, such mobile terminating calls require the performance of a large amount of initial noncircuit-related signaling.

In contrast, mobile-originated calls (outgoing calls) place far less initial signaling overhead because the radio system to which the subscriber is connected knows the subscriber's location. Furthermore, because a subscriber is on the move, the base transceiver system (BTS), the base station controller (BSC), and even the mobile switching centre (MSC) can change. These changes require a lot of noncircuit-related signaling, particularly if the subscriber is currently engaged in a call—the subscriber should not be aware that such handovers between cellular network equipment takes place.

Retrieving the subscriber's profile is also a straightforward task for fixed-line networks because it resides at the subscriber's local exchange. In cellular networks, the ultimate exchange (MSC) to which the mobile subscriber is connected changes because the subscriber is mobile, and it would be completely unmanageable to place the subscriber's profile (which might change) at every MSC throughout the world.

It is primarily for these reasons that cellular networks contain two databases, known as the Home Location Register (HLR) and the Visitor Location Register (VLR), in addition to the cellular-specific switch known as the MSC. For a description of the nodes used in a Global System for Mobile communications (GSM) network, see Chapter 12, "Cellular Networks."

Mobile application part (MAP) is the protocol that is used to allow the GSM network nodes within the Network Switching Subsystem (NSS) to communicate with each other to provide services, such as roaming capability, text messaging (SMS), and subscriber authentication. MAP provides an application layer on which to build the services that support a GSM network. This application layer provides a standardized set of operations. MAP is transported and encapsulated with the SS7 protocols MTP, SCCP, and TCAP.

This chapter specifies the MAP operations (or messages) that are used in GSM Phase 2. A small number of operations have been added to support General Packet Radio Service (GPRS) and 3^rd Generation (3G) Universal Mobile Telecommunications System (UMTS), but they are beyond the scope of this book.

See Appendix F, "GSM and ANSI MAP Operations," for a list of the MAP operations used in GSM.