tion network: A CSP that has many SAPs of different types for user data. Can be switched or non-switched. SAPs for OM are not shown. Model: See Figs. 3.59-3.61 N.. Number of channels per link C Bitrate per channel Fig. 3.84 Some composed units in resource systems 3.5 Logical Network Structures There a three types of abstract units: The (logical) connection, the association and the route. Associations and connections are created in a similar way. The dif- ference lies in which type of unit creates it. Connections are created by CON- CSPs. Associations are created by logical nodes on top of any kind of CSP, although they are mostly applied only when the CSP used is a CNL-CSP. Associa- tions are discussed further in Sect. 5.3.4. Connections and routes are used to simplify logical models in cases when the way they are realized is of no interest for the particular model. Both connections and routes represent the service aspect of some other logical network than the one that is modelled. The abstractions hide SAPs, UDTs and UDPs in those logical net- works. Figure 3.85 exemplifies the use of the (logical) connection abstraction in the case of a CSP that provides a teleservice based on AAL1 and ATM for trans- port. TE-UDP AAL1U AAL1U AAL1-UDP AAL1- {VPI,VCI} {VPI,VCI} The ATM-CSP The ATM-connection: TE-UDP AAL1U AAL1U AAL1-UDP AAL1- AAL1- ATM, {VPI,VCI} The AAL1-connection: TE-UDP Fig. 3.85 The logical connection abstraction 3 Logical Network Modelling At the top in Fig. 3.85 is the three-level structure of the CSP. If we assume that the teleservice is voice transfer, AAL1 adapts the cell-oriented, asynchronous service of ATM to a service that emulates the service of a suitable CN connec- tion. The characteristics of that connection are that it has a minimum variable time delay, and offers an AAL1-SDU size that suits the TE-UDTs. The actual SDU size depends on the type of voice encoding that is applied on the TE level. The dotted-line notation of the UDTs indicates that they are temporarily instanti- ated in the connect