GSM - Architecture, Protocols and Services, Third Edition

Chapter 6. Roaming and handover

Mobile application part interfaces

The main benefit for the mobile subscribers that the international standardization of GSM has brought is that they can move freely not only within their home networks but also in international GSM networks and that at the same time they can even gain access to the special services they subscribed to at home provided that there are agreements between the operators. The functions needed for this free roaming are called roaming or mobility functions. They rely mostly on the GSM-specific extension of the SS#7. The MAP procedures relevant for roaming are first the location registration/update, IMSI attach/detach, requesting subscriber data for call setup and paging. In addition, the MAP contains functions and procedures for the control of SS and handover, for subscriber management, for IMEI management, for authentication and identification management, as well as for the user data transport of the SMS. MAP entities for roaming services reside in the MSC, HLR and VLR. The corresponding MAP interfaces are defined as B (MSC-VLR), C (MSC-HLR), D (HLRVLR), E (MSC-MSC) and G (VLR-VLR) (Figure 3.6). At the subscriber interface, the MAP functions correspond to the functions of MM, i.e. the MM messages and procedures of the Um interface are translated into the MAP protocols in the MSC.

The most important functions of GSM MM are location registration with the PLMN and location updating to report the current location of a MS, as well ...