 to the teleservice distributed network process controller (TE-DNPC) that controls that CN switch. There will, therefore, exist a unit in the TE-DNPC that manages this information, the route manager. This unit might also manage the channels of all routes of concern, i.e. keep track of which channels are presently in use for some connections and which are idle. 3.4 Modelling Telecom Resource Systems TE-DNPC Route Manager Transport links CS switch controller controller 4 Mbps 2 Mbps Fig. 3.57 A teleservice network model, visualizing interconnections between switches as routes Figure 3.58 provides the symbols for the last two resources that are used in cir- cuit-switched networks to realize CN connections: the CS switch and the concen- tration network. The CS switch relay symbol Type: Instance: CS switch CS switch Controller Controller 2048*32 M Number of transport links N Number of channels per link C Bitrate per channel The concentration network CSP symbol controller M,M1,M2, Number of access links N,N1,N2, Number of channels per access link C Bitrate per channel Fig. 3.58 Symbols for CS switch and concentration network 3 Logical Network Modelling A switched concentration network is a CSP that is constructed by more primi- tive resource units. Figure 3.59 shows an example of an ISDN-ATD. controller ISDN- Access and TE-TS switching node 100 000 CS switch controller TE-TS 300 0 X2 9 000 X 32 Fig. 3.59 An access transport network interconnects terminal systems with the switches The ISDN-ATD connects a number of 64-kbps channels to the 64K CS switch. On the terminal side, there are two types of transport SAPs: one which includes two 64-kbps channels (the most common type of access, intended for the residen- tial user), and one that includes thirty-two 64-kbps channels (an access type for business communication). We will, in the following, analyse the internal structure of that ATD, see Fig. 3.60. This is also an exercise in using the basic resource units to construct a more complex resource unit. Access Switching TE-T