THE COST OF ADAPTIVITY AND VIRTUAL LANES IN A WORMHOLE ROUTER

We examine the cost in router complexity of adaptivity and virtual lan es in wormhole routers, using f-flat adaptive routers (based on a gene ralization of planar-adaptive routing) which include routers with a ra nge of routing freedom. Our studies show that adaptivity is expensive because it requires additional virtual channels and much larger crossb ar switches for both adaptivity and deadlock prevention. Increases of 50 to 100% in channel utilization are required to justify additional d egrees of routing freedom. Three internal router architectures for vir tual lanes are examined and the fully expanded crossbar is found to be most effective because it gives simplest control and minimal internal blocking. Examining router designs with from 1-16 virtual lanes indic ates that 30% improvements in channel utilization are required to just ify each additional virtual lane. These studies combined with publishe d simulation results indicate that only modest numbers of virtual lane s are likely to be cost effective.