Tradeoff between the Cycle Complexity and the Fairness of Ring Networks G. Anastasi • , L. Lenzini • , Y. Ofek * • University of Pisa, Dept. of Information Engineering Via Diotisalvi 2 - 56126 Pisa, Italy anastasi@iet.unipi.it, lenzini@iet.unipi.it * Synchrodyne 2600 Netherland Ave., Riverdale, NY 10463 ofek@synchrodyne.com Abstract In this paper we study performance tradeoffs of fairness algorithms for ring networks with spatial bandwidth reuse, by using two measures: (i) the fairness cycle size as a complexity measure, and (ii) the Max-Min optimal fairness criterion as a throughput measure. The fairness cycle size is determined by the number of communication links involved in every instance of the fairness algorithm (several identical fairness algorithms can be executed concurrently on the same ring). The study compares three fairness algorithms with different cycle sizes: the Global- cycle algorithm (implemented in the Serial Storage Architecture - SSA) in which the cycle size is equal to the number N of links in the ring; the Variable-cycle algorithm in which its cycle size changes between 1 and N links; the One- cycle, where there is a fairness cycle on every link. It is shown how varying the cycle size affects the network performance with respect to the Max-Min optimal fairness criterion. The results show that for non-homogeneous traffic patterns, decreasing the fairness cycle size, while increasing the complexity, can significantly improve the performance with respect to the Max-Min optimal fairness criterion. Keywords: Ring Networks, Fairness, Cycle Complexity, Max-Min Fairness, SANs. 1 Introduction Ring networks enjoy renewed interest as Storage Area Networks (SANs). A SAN [Phi98] is a high speed network connecting hundreds of nodes, which are typically disk drives with a storage capacity of up to 10 Gbytes thus providing a total storage capacity in the order of 10 12 Bytes. The key emerging storage technologies are FC-AL (Fiber Channel - Arbitrated Loop, ANSI Standard X3T11) and SSA (Serial Storage Architecture, ANSI Standard X3T10) [Du98]. SSA is a dual-ring network with spatial bandwidth reuse in which its underlying network is the MetaRing [Cid93, Che93, and Ofe94]. The main motivation for developing ring networks with spatial bandwidth reuse, such as the MetaRing, Orwell [Fal85], ATMR [Ohn89] and others, is to increase the aggregate ring throughput, in each direction, beyond its single link capacity since spatial bandwidth reuse enables concurrent access in each direction of the ring by more than one node. Increasing the aggregate throughput as much as possible is a key