vice (OS layer 3). The two lowest layers in ATM may represent an SDH network. SDH defines another five layers in its internal layer structure (see ITU-T G.803): physical media (which actually is not a layer, but a degenerated form of CSP), regenera- tion section, multiplex section, higher order path and lower order path. From an SDH user point of view, these layers provide a transport service (more or less corresponding to OSI layer 1). 3 Logical Network Modelling Note also that you should not use generic prefixes in your models. These pre- fixes only act as classifying references. The prefixes you shall use are those you define for your particular CSP, as PDH for that concept, IP, ISDN, ATM and GPRS. Make sure that when you annotate a unit in your model with a particular prefix, that unit represents a function that is a unique part of the CSP that is referred to by the prefix. In Sect. 3.4 we also identified three composed type of units for resource system modelling: link, transport network and concentration network. All these units can be constructed from the primitive units. Every logical structure level in a network has its own set of composed units. The type of unit and its transfer characteristics are defined by the type of SAP, the direction of SAPs and the prefix, They are also defined by annotations that define the number of SAPs, number of channels, trans- fer capacity and (if relevant) type of switching, according to Sect. 3.4. Figure 3.84 shows the characteristics of some types of composed units that were discussed in that section. Transmission link (TA-link): A CSP that has two SAPs of the same type for user data. SAP for OM may exist as well. Model: TA-UDP C Bitrate in bits/second (bps) TAU I Transport network (TR network): A CSP that offers many transport links for user data. A transport link has two SAPs of the same type. Can be switched or non-switched. SAPs for OM are not shown. Model: 2*(M-3) M Number of links See Figs. 3.50 and 3.52 N Number of channels per link C Bitrate per channel Concentra