The GSM system permits the integration of different voice and data services and the inter-working with existing networks. Services make a network flexible to users, i.e. they can choose one network over another. GSM has three different categories of services, bearer, tele and supplementary services And these are described in the following sections. Figure 3 shows a reference model for the GSM services.
A mobile station (MS) is connected to the GSM Public Land Mobile Network (PLMN) via the Um interface. This network is connected to the Transit network, e.g. Integrated Services Digital Network (ISDN) or the traditional Public Switched Telephone Network (PSTN) though there might also be an additional network, the Source/destination network, before another Terminal (TE) is connected. Bearer services now comprise all services that enable the transparent transmission of data between the interfaces to the network, i.e., S in the case of the MS, and a similar interface for the other terminal. In the original GSM model, bearer services are connection-orientated and circuit or packet switched and these services only need the lower three layers of the ISO/OSI reference model.
Within the mobile station (MS), the mobile terminal (MT) performs all the network specific tasks (such as TDMA, FDMA, coding, etc) and also offers the interface for data transmission (S) to the terminal (TE), which can then be independent of the network. Depending on the capabilities of the TE, more interfaces may be needed, such as R, according to ISDN reference model (Halsall, 1996). Tele services are application specific and may need all seven layers of the ISO/OSI reference model, these services are specified end-to-end, i.e. from one terminal (TE) to another terminal.