Weighted Fairness in Buffered Crossbar Scheduling

The crossbar is the most popular packet switch architecture. By adding small buffers at the crosspoints, important advantages can be obtained: (1) Crossbar scheduling is simplified. (2) High throughput is achievable. (3) Weighted scheduling becomes feasible. In this paper we study the fairness properties of a buffered crossbar with weighted fair schedulers. We show by means of simulation that, under heavy demand, the system will allocate throughput in a weighted max-min fair manner. We study the impact of the size of the crosspoint buffers in approximating the weighted max-min fair rates and we find that a small amount of buffering per crosspoint (3-8 cells) suffices for the maximum percentage discrepancy, to fall below 5% for switches.