Speedup and buffer division in input/output queuing ATM switches

The capacity of a switch is built out of two factors: space parallelism and speedup. A switch has space parallelism if more than one input port can transmit simultaneously. Speedup is the ratio of the switch´s internal link speed over the incoming link speed. An input-queuing switch uses only the first factor (space parallelism), and a share-medium or a share-memory output queuing switch uses only the second factor (speedup). However, to build a large switch, both factors are normally used. A large switch´s capacity can be built with less space parallelism (the space factor), but more speedup (the time factor), or vise versa. Buffers are needed at both the input and the output ports. In this paper, we show how to divide the buffers between the input and the output queues and how the optimal division is affected by the (space, time) combinations.