 The ISDN-CSP Fig. 3.11 The SAP and the CSI An important conclusion from Fig. 3.11 is that, in the case when a communication service is described in terms of these two service components, the user protocol defines only the behaviour of the SAP users (the TEUs in Fig. 3.11). To separate communication service interfaces into SAPs and CSIs is, however, not always relevant. We must consider three main types of communication services and CSPs: Teleservices, as provided by e.g. public switched telephone networks (PSTN), ISDN, GPRS, etc. Packet-switched data communication services, as provided by X.25, CCS and IP networks. We must also separate packet-switched services into connection- oriented (CON) and connection-less (CNL) communication services. Circuit-switched communication services, as provided by transmission, trans- port and connection networks. Figure 3.12 is a high level logical network structure model that describes the dependency between different types of CSPs. It also defines prefixes used for denoting CSPs (TE, CN, PS, CON, CNL, TR and TA). 3.2 The Communication Service Provider Teleservice logical network TE-CSP PS-CSP Circuit-switched connections Packet-switched logical network (CON-CSP logical network CNL-CSP) Transport logical network TR-CSP (may be circuit-switched) Transmission CSP TA-CSP CN-CSP Fig. 3.12 How different types of CSPs are related Teleservice CSPs (TE-CSP) may offer one or several teleservice SAPs (TE- SAP) in a terminal system. There is always a CSI available in each terminal sys- tem, although it may be used for all TE-SAPs in a terminal system. Addresses (SAPI) refer in general not to an individual TE-SAP but to a terminal system. Thus, all TE-SAPs of a terminal system are referred to with the same SAPI value. TE-CSPs can be built on circuit-switched connection CSPs (CN-CSP), on packet- switched CSPs (PS-CSP) or on both types of CSPs. Circuit-switched connectivity builds the infrastructure of teleservice networks and provides services to packet-switched networks. It is provided on 