The services offered at the User-Network Interface (UNI) of GSM are patterned after the services offered by the Integrated Services Digital Network (ISDN) [7] for fixed terminals tied to telephone lines. GSM services are therefore divided just like ISDN services into three categories: bearer services, teleservices, and supplementary services. A bearer service offers the basic technical capability for the transmission of binary data; i.e. it offers the data transfer between end terminals at reference points R or S of the Reference Model (Figures 4.1 and 9.1). Such bearer services are made use of by the teleservices for the transfer of data with higher-level protocols.

Figure 4.1: Bearer and teleservices

Notice that bearer and teleservices both require special measures not only at the air interface, but also inside the PLMN, which must offer a matching fixed-net infrastructure and special Interworking Functions (IWFs). Especially at the level of bearer services, the IWF must provide a mapping of GSM PLMN services within their respective service characteristics onto corresponding bearer services and characteristics of the other networks, such as PSTN and ISDN. Teleservices are end-to-end services, for which there is usually no translation in the IWF. But they do use bearer services, which again need IWF functions.

Bearer and teleservices are carried under the umbrella term telecommunication services. The simultaneous use of two telecommunication services is precluded, except for the case of Short Message Services (SMS), which can at least be received during the use of another telecommunication service. Supplementary services are complementing the control and modification of extended services and are only usable in connection with a telecommunication service.

For telecommunication services, the GSM standard includes agreements of target times for their market introduction. This fact is especially important, since GSM is an international standard which aims at worldwide compatibility of mobile stations and networks. Accordingly, only a minimum of services has been defined, which must be offered by the operators at various time phases. For this purpose, the services are divided into the categories essential (E) and additional (A). Group E must be implemented at the given date by all network operators, whereas the decision about the time of introduction of Group A services is left to the operators. Table 4.1 gives a rough overview over the implementation and introduction phases. The most important services to be implemented and introduced with their respective introduction dates are described briefly in the following.

Table 4.1: Phases of implementation and introduction






Basic operation consisting of telephone services and some appropriate supplementary services



Extended operation with telephone services, first nonspeech services (e.g. BS26) and an extended range of supplementary services



Enhanced service range with even more telecommunication and supplementary services

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