and last cells of a frame, so it doesn't need to con- sume any of the cell payload to perform this function. AAL 5 uses a per- frame CRC to detect both transmission and cell-loss errors. The speed- and traffic-shaping requirements of converged networks are increas- ingly challenging ATM, however. The fastest SARs known run at 2.5Gbps and have limited traffic-shaping capabilities, imposing performance bottlenecks at high speeds. The ATM Transmission Path The ATM transmission path includes two elements called the virtual path and the virtual channel (see Figure 7.19). You can think of the virtual channel as an indi- vidual conversation; each voice, video, data, and image transmission has its own unique virtual channel. The number of that channel will change between any two switches, depending on what was assigned at the time the session was negotiated. All similar virtual channels-that is, all those that have the same QoS request- are bundled into a common virtual path. Virtual path 1 might be all real-time voice that has a very low tolerance for delay and loss; virtual path 2 might be for stream- ing media, which requires continuous bandwidth, minimum delay, and no loss; and virtual path 3 might be for non-mission-critical data, SO best-effort service is fine. Transmission Path VC = Virtual Channel VP = Virtual Path Figure 7.19 The relationship of the virtual path, virtual channel, and transmission path Chapter 7 Wide Area Networking ATM is very elastic in terms of its tolerance of any losses and delays and allocation of bandwidth. It provides an easy means for the network operator to administrate QoS. Instead of having to manage each channel individually in order to guarantee the service class requested, the manager can do it on a path basis, thereby easing the network management process. To illustrate how to identify what paths and channels need to be taken within the cell, Figure 7.20 shows the structure of an ATM cell. The header information includes information on the virtual path between switch A and switch 