ying is performed between global links, and in a terminal system, between a global and a local link. The key to isolating control data flows from user- data flows, and thereby protocol elements from relaying units, is that there exists some unit in the protocol stack that performs the corresponding discrimination. In the OSI model this is (or should be) the OSI link layer. The role of the network layer entity in OSI RM is (among other things) to con- trol relaying by performing routing analysis. The purpose of this analysis is: given the destination address and the link on the incoming side, determine a suitable out- going link. OSI RM places routing analysis also as a function in the network layer. This seems quite harmless if one considers the need for routing analysis for circuit- switched and connection-oriented connections, but does not work at all for connec- tion-less networks. CNL networks do not have a separate network layer. Still, a CNL switching node (a router) must perform routing analysis for every single packet. The solution to the problem is similar to the relaying problem: let routing analysis be performed by a unit that is separated from other functions of the net- work layer, and not necessarily bound to any specific layer. We will call a unit that performs routing analysis routing analyser. 3.4.2 Resources in Circuit Switched Networks As an example, Fig. 3.39 shows how the ISDN-CSP provides teleservices that are based on circuit switching of 64-kbps channels. Most of the concepts and names in this model are found in Sects. 2.2.1 and 3.3.1. The model shows the logical node structure in the DC-plane of two physical nodes: a terminal system and a combined access and switching node in the operator network. 3 Logical Network Modelling Terminal System (TS) The ISDN-CSP for circuit-switched teleservices (CS-TE) Teleservice Distributed Network Process Controller TE-TPC for circuit-switched services (TE-DNPC) SAPI=0 SAPI=0 SAPI=0 {SWC} {SWC} SAPI 63 Switch Switch Routing controller controller analyser 