d by ETSI. For an overall description of GPRS, see ETSI recommendation GSM 03.60. 2 Foundations of Sysnet Modelling permanent or ordered by the users when they need it. From a modelling point of view, it is important to differentiate between types of SAPs, partly because they differ in properties, partly because it makes models more expressive. We have already mentioned the distinction between TE-SAPs and CSP-internal SAPs (A- SAP and N-SAP). In Sect. 3.2 classification of and symbols for SAPs (and thereby of CSPs) is defined to distinguish between the following types of SAPs: Teleservice SAP, abbreviated as TE-SAP. This type of SAP defines the interfaces on the boundary of a telecom system where the users of teleservices access the telecom system. Flows over TE-SAPs are flows of information types, as voice, text, facsimile, picture, video and data. Packet-switched SAPs, abbreviated as PS-SAP. These SAPs are used.for data communication. They come in two forms: connection-oriented SAPs, abbrevi- ated as CON-SAP, and connection-less SAPs, abbreviated as CNL-SAP. Circuit-switched SAPs, abbreviated as CS-SAP. These types of SAPs come in three forms: - SAPs of transmission system CSPs, abbreviated to TA-SAP. A transmission CSP transfers strings of bits between fixed points in a network. No switching is performed. - SAPs of transport system CSPs, abbreviated to TR-SAP. A transport CSP con- centrates channels between different fixed points in a network by making use of multiplexers and special types of switches. Transport CSPs constitute the infra- structure for networks that provide teleservices. Switching may occur inside transport CSPs, but not on a per-channel and call basis. SAPs in network terminations of a circuit-oriented telecom system, abbrevi- ated to CN-SAP. These SAPs correspond to the A- and N-SAPs discussed previ- ously. Note that a telecom system as a whole is a CSP, define by the set of TE-SAPs. It includes all functions of an operator network and all terminating functions on the terminal side. It provi