Data Communications Networking Devices - 4th Ed.pdf

2.7 ATM _________________________________________________________________________ 261Figure 2.56Relationship between virtual paths and virtual channelsswitches. The virtual channel can represent a single ATM link or a concatenation oftwo or more links, with communications on the channel occurring in cell sequenceorder at a prede®ned quality of service. In comparison, each virtual path representsa group of VCs transported between two points that can ¯ow over one or more ATMlinks. Although VCs are associated with a VP, they are neither unbundled nor processed.Thus, the purpose of a virtual path is to provide a mechanism for bundlingtraf®c routed towards the same destination. This technique enables ATM switchesto examine the VPI ®eld within the cell header to make a decision concerning therelaying of the cell instead of having to examine the entire three-byte addressformed by the VPI and the VCI. When an endpoint is in an idle condition, the VPI®eld is set to all zeros. Although the VCI ®eld will also be set to all zeros to indicatethe idle condition, other non-zero VCI values are reserved for use with a VPI zerovalue to indicate certain prede®ned conditions. For example, the VPI ®eld value ofzero with a VCI ®eld value of 5 is used to transmit a signaling/connection request.Payload Type Identi®er ®eldThe Payload Type Identi®er PTI) ®eld consists of three bits in the fourth byte ofthe cell header. This ®eld is used to identify the type of information carried by thecell. Values 0±3 are reserved to identify various types of user data, 4 and 5 denotemanagement information, while 6 and 7 are reserved for future use.Cell Loss Priority ®eldThe last bit in the fourth byte of the cell header represents the Cell Loss PriorityCLP) ®eld. This bit is set by the AAL Layer and used by the ATM Layerthroughout an ATM network as an indicator of the importance of the cell. If theCLP is set to 1, it indicates the cell can be discarded by a switch experiencingcongestion. If the cell should not be discarded because of the necessity to support apredi®ned quality of service QoS), the AAL Layer will set the CLP bit to 0. TheCLP bit can also be set by the ATM Layer if a connection exceeds the quality ofservice QoS) level agreed to during the initial communications handshakingprocess when setup information is exchanged.