First-Fit Scheduling for Multi-Stage Packet Switching Networks

We first propose a First-Fit Scheduling algorithm for a single stage interconnection network. We then classify multi-stage interconnection network in space, such as the 3-stage Clos network, when each spatial stage may allow time domain switching. We examine how scheduling could be performed for these stages for various combinations of space-time switching. The First-Fit algorithm is then extended to a three stage Clos network with scheduling in the first and third stage. Similar to other scheduling algorithms, full loading is achievable, while randomization of input selection is replaced by the use of arrival timing. This distributed algorithm applies to both cases of fixed length versus variable length packets. We simulated the performance for both cases. A case is made that for multi-gigabit per second link speed, variable length packets should be used to simplify scheduling and avoid fragmentation, with acceptable increase in delay.