logical connections in the X.25/frame relay. This logical or virtual connection is established before the communication commences using a signalling protocol SO that the route from the source to the destination is chosen well before time and all the packets, in this case the cells, are routed through the same path in the same order. Therefore the ATM enjoys the flexibility provided to it by the virtual channel concept. At one end there is a fixed path established for the cells to be transferred and thus no need for the intermediary nodes to calculate the route that the cell will take in order to reach the next node. This feature, as will be seen later, would provide good QoS and would also not disturb the arrangement of cells at the destination. At the same time this does not mean that there is a dedicated path present for a particular source-destination combination. The path is established only when a request arises. Each link or the node is shared by a set of virtual channels. A route will take a particular channel identified by its number called the virtual Digital Hierarchies - PDH and SDH Figure 3.27 Virtual channels at node A channel identifier (VCI), as shown in Figure 3.27. Also each link will maintain a table of routes (as shown in Table 3.6) filled by the signalling protocol, which the incoming channel will take. The second sublayer in the ATM layer is the virtual path level. A VPC is a bundle of VCCs that have the same end points. This implies that all of the cells flowing over all of the VCCs in a single VPC are switched along the same route. This concept emerged as a result of a heavy control cost of the network as compared to its overall cost. With this technique the paths sharing the common paths through the network may be grouped together into a single unit called a virtual path and the network management actions can then be applied to a small number of groups of virtual paths rather than a huge number of virtual channels. Some basic advantages of the virtual paths are: Greater reliability and inc