Basic Characteristics of FIFO Packet Switches

In this paper we compare characteristics of different FIFO packet switch types. In accordance with our expectations, the best results are obtained by using the output-queued packet switches. The main drawback of these switches is the need for a fast switching fabric. This can be avoided by using the input-queued packet switches, whose performance depends upon individual packet sizes. Characteristics of the input-queued packet switches are similar to those of the output-queued packet switches; if fixed-size packets are transferred, the consequences of the output-contention problem are relatively small. Transfer of the variable-size packets in the input-queued switches is more complicated. Practically, a 100% throughput can be achieved if an asynchronous switching fabric is applied. Packet delays are in this case comparable to those in the output-queued packet switches. The drawback of such switches is the inability to differentiate among multiple service classes. On the other hand, by using a synchronous switching fabric, a 100% switch throughput of cannot be achieved. Another disadvantage of the synchronous packet switches are relatively high packet delays. However, usage of such switches enables a differentiation between multiple service classes, that is one of the conditions for enabling quality of service.