GSM permits the integration of different voice and data services and the interworking with the existing network. GSM is defined for three different categories of services: bearer, tele, supplementary services. Figure shows the reference model for GSM services. A mobile station (MS) is connected to the GSM public land mobile network (PLMN) via Um interface. GSM-PLMN is the infrastructure needed for GSM network. This network is connected to transit network, e.g., integrated service digital network (ISDN), or traditional public switched telephone network (PSTN). There might be additional network, the source/destination network, before another terminal TE is connected.
Bearer service comprises all services that enable transparent transmission of data between the interfaces to the network. Interfaces like U, S, and R in case of ISDN have not been defined for all networks, so it depends on the specific network which interface is used as a reference for transparent transmission of data. In classical GSM model, bearer services are connection oriented and circuit -or packet- switched. These services only need the lower three layers of the ISO/ISI reference model.
Within the mobile station MS, mobile termination (MT) performs all network specific tasks (TDMA, FDMA, coding etc) and offers an interface for data transmission(S) to the terminal TE which can then be network independent. Tele services are application specific and may thus need all seven layers of ISO/OSI reference model. These services are specific end to end i.e., from one terminal TE to another.
GSM specifies different mechanisms for data transmission, the original GSM allowing data rates up to 9600 bit/s for non-voice services. Bearer services permit transparent and non-transparent, synchronous or asynchronous data transmission. Transparent bearer services only use the functions of the physical layer to transmit data. Data transmission has a constant delay