connection Fig. 3.53 A CN connection comprises (semipermanent) channels in transport links and (on- demand) paths through circuit switches 3.4 Modelling Telecom Resource Systems A CN connection is a connectivity relation between teleservice resources (like encoders) in different physical nodes in a network. A CN connection may be used between teleservice resources in terminal systems alone or between teleservice resources in terminal systems and teleservice resources (as protocol converters) in network internal physical nodes. A CN connection is created by using channels in transport networks and paths through CS switches, that interconnect channel end- points. The basic relaying resource in a teleservice network is the circuit switch (CS switch). Figure 3.54 shows the ISDN circuit-switching access and switching node from Fig. 3.39 in a slightly simplified way. Switching CS access and switching node nodes Network TE-DNCP SAPI=0 SAPI=0 {SWC} {SWC} Terminal System Signalling link Switch Switch SAPI 63 Routing controller controller analyser Discrimination Transmission Physical access medium core network links 64K CS Fig. 3.54 The ISDN access and circuit-switching node model The CS switch does not terminate any protocols. The switch interconnects chan- nel endpoints (normally 64 kbps) on demand (controlled by a switch controller of the teleservice network process control system). The CS switch connects to trans- port links that are all of the same capacity, normally 2 Mbps. The transport links can be divided into two groups: core network links that interconnect 64K CS- switches, and access links that connect terminal systems to a 64K CS switch, nor- mally over a switched-access network (ATDN in the model). An ISDN circuit-switching network handles discrimination between user data and process control messages in its own way, since ISDN (contrary to CON net- works) has separate transport systems for user data and process control messages, both on the access side (LAPD) and on the network side (CCS). Discrimination on t