example, the General Packet Radio Service network concept (GPRS) 1 uses IP- networks for NTC-realization. 2.2 Focus on Telecom Systems and Networks TE-TPC Teleservice Access Protocol A-SAP A-SAP Teleservice Distributed Resource Network Access Transport Network (ATN) control Process Controller A-SAPs (TE-DNCP) TE-UDT TE-SAP N-SAP N-SAP Teleservice Network Protocols Transport Service Distribution Network Protocol N-SAP N-SAP (TE-DNCP) Fig. 2.3 The network transport network supports communication between logical nodes on the network side By introducing the NTN, additional types of SAPs are identified in the model, called N-SAPs (network-SAP) in Fig. 2.3. N-SAPs for NTD and NTC must be kept apart in models, for the same reasons as for A-SAPs. SAPs must be carefully sepa- rated in sysnet models, since this separation preserves the architectural structure. There is, however, not necessarily any difference between some of the SAPs as QoS is concerned. The difference in QoS lies primarily in the type of switching, the transport capacity and the reliability of data transport over SAPs. 2.2.2 Communication Service Provider The logical networks that have been discussed SO far (ATN, ATD, ATC, NTN, NTD, NTC) have been variants of logical systems that provide some type of con- nectivity service, offering information and data flows over some kind of SAP and (possibly) a control interface. The need for a generic definition of such functional systems has been recognized by OSI. It was regarded as an "abstract system" (a black box) and called "service provider" In SM&AML this concept is a very important basic modelling concept. It is called communication service provider (CSP), in order to emphasize that it provides communication services only. CSPs come in many shapes. A CSP is defined primarily by the type of SAPs it provides to its users, the total number of SAPs, and if the connectivity service is GPRS is a new network concept offering mobile terminals high-speed packet data communica- tion. GPRS is presently being standardize