tion, since ATM (contrary to IP) is specifically designed to provide both good voice quality and data quality. However, ATM is also a rather expensive technique. 3.2.4 Summary of Network Service Interface Notations Networks are built by combining the connectivity services of different kinds of CSPs. For example, a logical network model of an ISDN network would comprise internal logical units that provide all kinds of connectivity services, some circuit- 3 Logical Network Modelling switched, some packet-switched and some not switched at all (as transmission lines and some transport systems). To support this variety of connectivity services, AML offers a set of different SAP symbols. The effect of using these symbols in model- ling is that the models become more expressive. There is also a drawback: it com- plicates modelling since the modelling language becomes more complex. A neutral SAP symbol is therefore also provided. The complete set of symbols for modelling network service interfaces, including symbols for CSIs, is given by Fig. 3.23. Description Symbol Variants Application principles Send-receive SAP, connected CON-SAP CEI is used provider CNL-SAP SAPI is used Receive only CN-SAP Multiple connections Send only SAP, not connected Single connection The same principle for receive only and TA-SAP provider send only for all Transmission SAP types of SAPs TR-SAP Transport SAP TE-SAP Undefined SAP type CSI, connected Uni-directional User< Provider Control service Bi-directional CSI, not connected interface User< Provider CEI connection endpoint Identifier CS circuit switched CN circuit switched connection SAP service access point CNL connection-less SAPI SAP identifier CON connection-oriented Fig. 3.23 Symbols for SAPs and CSI used in CSP and logical network modelling 3.2 The Communication Service Provider The SAP symbols are constructed to show which type of connectivity service is provided by the CSP: A CON-SAP is a SAP that is based on packet switching. The IDUs of a CON- SAP contain fixed or variable length S