manager also selects an X.25 link (if there are several links in the route) and an idle VCN on the outgoing link. The X.25-DNCP then orders the switch controller to establish a path through the packet switch between the VCN in the incoming link to the VCN on the out- going link. When the paths through all intermediate routers are established, the user can start sending data. The sending node creates CON-PDUs that comprise userData (a fragment of a CON-SDU), the VCN and some parameters for error handling, flow control and fragmentation. Different fragmentation schemes may be used over different links, in which case, the X25-UDT may re-fragment incoming packets. Correct incoming packets are then sent to the packet switch as CON-IDUs, com- prising two information elements: the VCN and the CON-SDU fragment. In the interface between the discriminator and the packet switch, the VCN is a channel endpoint identifier. The router's control store tells which outgoing link to send the packet on, and on which VCN. The packet to the outgoing link looks the same as on the incoming link, only the VCN value is changed. An X.25-router connects to many terminal systems and other X.25 routers, or just to other routers. An X.25 route can comprise more than one X.25 link, depend- ing on expected traffic volume. The standard defines a maximum of 4096 virtual circuits per X.25 transport link (actually, only 4095 since the standard reserves one virtual circuit for future use). If we assume that the X.25 network is based on (sem- ipermanent) TR connections, an X.25 route endpoint can be mapped on a single TR-SAP. Figure 3.64 identifies the route concept for X.25 networks. The model shows, at the top, four X.25 packet switches, fully interconnected by X.25 routes. The expanded route consists of two X.25 links. Each X.25 links is based on a single TR connection. The packet-switching logical network is built on top of other (often cir- cuit-switched) logical networks. To build networks in this way implies (if we con- sider all logical networ