9A656-04 and 9A600 Configuration Guide4-9Routing Overview4.3.3 CrankbackCrankback occurs when a requested connection exceeds the services currently available forthe desired path. When this occurs, the first switch in the desired path will attempt to re-routethe requested connection. refer to Figure 4-3.Figure 4-3 Crankback ExampleIn this example, End User 1 requests a CBR connection with a PCR of 20,000 cells per second(CPS) with End User 2. Switch 1 attempts to route this request through Switch 2. Switch 2,however, refuses the connection because it cannot meet the QoS requirements of the CBRconnection. Switch 1 then re-routes the connection through Switches 3 and 4 to establish theconnection. The SmartSwitch 9500 will attempt to re-route a connection request three times.In the event that no switch can meet the QoS requirements, the connection will be refused.PNNI attempts to limit the amount of crankback on the network by using Generic CallAdmission Control (GCAC), as described in Section 4.3.4.Peer Group 1Switch1Switch3Switch4Switch2Rejected ConnectionRequestRe-routed ConnectionAfter CrankbackEnd User 2End User 1Signifies Physical Connection Between DevicesSignifies Attempted Connection RequestSignifies Re-routed Connection
