Adaptive crankback schemes for hierarchical QoS routing in ATM networks

To reduce routing information and achieve scalability in large ATM networks, ATM Private Network-to-Network Interface (PNNI) adopts hierarchical routing. For efficient routing, a large ATM network is decomposed into subnetworks called Peer Groups (PG), which advertise aggregated routing information only. Because of lack of precision in the aggregated information, a call set up message on a chosen route can be rejected. When an ATM node discovers that a call set up message cannot proceed due to insufficient resources, a back-tracking procedure called 'crankback' is initiated against call blocking. Although crankback reduces blocking probability, it also causes additional overhead, such as longer setup delay. Consequently, this study proposes two approaches to reduce crankback overhead. The first approach adds extra information to the setup message, referred to as Crankback Information Stack (CIS), to reduce crankback overhead. Meanwhile, the other approach, referred to as Cost Threshold (CT), uses the aggregated path cost and the cost information of previously rejected call setup message to determine whether call setup should be tried on the next alternate path. The simulation results herein demonstrate that both of the proposed approaches significantly reduce crankback overhead, while the combination of the CIS and CT approach achieves further improvement.