Quality of Service Support in High-Speed, Wormhole Routing Networks

0 50 100 Interarrival Delay Jitter Offered Load Percentage Figure 15: A three-dimensional plot of the Delay Jitter histogram at the destination for four VBR connections for diierent background non-QoS (datagram) loads with preemptive priority to QoS Virtual Channels. Note the log scale for the delay jitter in byte times at the destination. we have presented and evaluated several diierent approaches to this problem. Dedicating a subnet to carry QoS traac exclusively is an approach that when combined with pacing and call admission control can support QoS traac. Imposing a synchronous framework on top of the asynchronous high-speed network (via a Hyper Token Ring, FDDI etc.) provides support for guaranteed bandwidth and delay applications. Both approaches can beneet from switch priorities in order to reduce interference of QoS and non-QoS traac at the host interface. The use of virtual channels (one set dedicated to QoS traac and one set to non-QoS traac) along with a priority mechanism (especially the preemptive priority mechanism) was also shown to be an eeective approach to supporting QoS traac. It becomes even more attractive when combined with deadlock-free shortest path routing (as considered in this paper). QoS multicasting must be performed via storing and forwarding and replication through the host interface, in the absence of switch level hardware support for multicasting. This tends to increase latency because of reassembly at each host interface. However, proper choice of worm size can ensure that latencies are within predeened constraints.