r the CNL transport link. Since, however, there is no connection setup procedure required, one normally does not talk about "connection identifiers" in CNL transport links. The fact that routers for CNL services have no path through the router for packets when they arrive affects the packet switch (not the transport link), which must perform routing analysis for each individual packet. Today, there exist many well-known packet-switching network concepts, like X.25 (CON and CNL), Frame Relay (CON), ATM (CON), IP (CNL) and MTP (CNL). We will, in the following sections, present three of them, focusing on how transport links and routes are defined, how discrimination is solved, how routing analysis is modelled and how the packet switch works. 3.4.4 Resources in X.25 networks Figure 3.63 shows the logical node structure in an X.25 router. The model is similar to circuit switching access and switching nodes: there exists a distributed network process controller that is separated from the X.25 transport link, from the packet switch and from the routing analyser. Discrimination between user-data flows and process control flows is part of the X.25 transport link function, as it is in ISDN. There is, however, no need for a concentration network, since packet networks depend on other switching networks for connectivity relations between packet switching nodes, and between switching nodes and packet service using terminal systems. In this case, I have assumed that packets are transported over a TR con- nection, which is the common way to interconnect routers in wide area networks. 3 Logical Network Modelling Distributed Network Process Controller Calling Called Facil. for X.25 CON services (X.25-DNCP) Addr. Route Switch Routing CON-PDU Manager controll. Analyser Err & userData contr. X.25 link switch X.25 mess. discrimination X.25- disc. par. packet protocol Discriminator LAPBU CON-IDU LAPB- userData header CON-SDU router TR-connection Fig. 3.63 The switching model for X.25 networks The structure of an X.25 network interface