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 get access to the special services they subscribed to at home - provided 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 Signalling System Number 7 (SS#7). The Mobile Application Part (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 supplementary services and handover, for subscriber management, for IMEI management, for authentication and identification management, as well as for the user data transport of the Short Message Service. 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 (HLR-VLR), E (MSC-MSC), and G (VLR-VLR) (Figure 3.9). At the subscriber interface, the MAP functions correspond to the functions of Mobility Management (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 Mobility Management are Location Registration with the PLMN and Location Updating to report the current location of an MS, as well as the identification and authentication of subscribers. These actions are closely interrelated. During registration into a GSM network, during the location updating procedure, and also during the setup of a connection, the identity of a mobile subscriber must be determined and verified (authentication).

The mobility management data are the foundation for creating the functions needed for routing and switching of user connections and for the associated services. For example, they are requested for routing an incoming call to the current MSC or for localizing an MS before paging is started. In addition to mobility data management, information about the configuration of supplementary services is requested or changed, e.g. the currently valid target number for unconditional call forwarding in the HLR or VLR registers.

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