al channel link. This is a means of unidirectional transport of ATM cells between a point where a VCI value is assigned and the point where that value is translated or terminated. Virtual channel identifier (VCI). This is a unique numerical tag that identifies a particular VC link for a given VPC. Virtual channel connection (VCC). This is the concatenation of VC links that extends between two points where ATM service users access the ATM layer. VCCs are provided for the purpose of user-user, user-network or network-network information transfer. Cell sequence integrity is preserved for cells belonging to the same VCC. Virtual path (VP). This is a generic term used to describe the unidirectional transport of ATM cells belonging to the virtual channels that are associated by a common unique identifier value. Virtual path link. This is a group of VC links identified by a common value of VPI, between a point where a VPI value is assigned and the point where that value is translated or terminated. Virtual path identifier (VPI). This is a unique numerical tag that identifies a particular VP link. Virtual path connection (VPC). This is the concatenation of VP links that extends between two points where the VCI values are assigned and the point where that value is translated or terminated. VCCs are provided for the purpose of user-user, user-network or network-network information transfer. The definitions above give a clear picture of the range/extent of each link or path. Thus with these terminologies in mind it is safe to move to the switching of VCCs and VPCs. VP switches terminate VP links. As shown in Figure 3.32, a VP switch translates incoming VPIs to the corresponding outgoing VPIs according to the destination of the VPC, whereas VCI values remain unchanged. VC switches terminate Digital Hierarchies - PDH and SDH VP/VC Switch VP Switch C1 VC2 VC3 VC4VC5 C4 VC5 VC1VC2 C6 VC7 VC VC Figure 3.32 (a) A VP switch; (b) aVP/VC switch VC links and necessarily VP links. A VC switch must therefore switch virtual paths and vi