h circuit established between nodes has a certain identification number and there is no global identification that could be used on all links through the network. Instead, one of the free circuit identifica- tions on each intermediate link is allocated for each virtual circuit being established and the connection tables of switching nodes in the network are updated to contain all established circuits, as shown in Figure 6.6. VCIs have only local significance on a specific network link and, there- fore, virtual circuit identifiers are changed as a frame traverses the virtual path through the network. Introduction to Telecommunications Network Engineering Link 1 Frame or cell Link 3 switch Link 2 VCI = Virtual circuit identifier Link 1 routing table Link 2 routing table Link 3 routing table Link- Link- Link- Figure 6.6 Switching of frames on virtual circuits. When a frame is received from a certain link, the frame switch simply reads the VCI and combines the incoming link number to determine the corresponding outgoing link and VCI. The new VCI is then written into the frame header and the frame is queued for forwarding on the appropriate link. The order of frames is preserved and routing them is very fast because the routing process does not require analysis of a global address. In Figure 6.6 a frame switch has connection tables for each incoming data link. Let us assume that a frame with identification 3 is received from link 3. The switch looks up the link 3 routing table and finds out that this frame should be transmitted to link 2, SO identification 3 is replaced by 1. This process is fast because it does not require any network layer rout- ing with a global address. Instead switching is done in the data link layer. The VCI is also very short and the utilization of data capacity in this kind of network is more efficient than if the global address were included in each frame or packet. 6.2.5 Polling In local data networks, such as shared media LANs described in Section 6.5, all devices are usually connected to s