Performance Comparison of VOQ Selection Policies in Scalable Packet Switches

Virtual output queueing is known to overcome the head of line blocking problem of input queueing. This type of buffering is widely used in ATM networks. In order to deliver desired performance, virtual output queueing requires efficient and effective scheduling algorithm with low operating complexity. For large scale switches this might be difficult to achieve, as algorithm complexity increases together with the size of the switch fabric. It is possible to resolve this problem by using interconnection network architectures with distributed buffers. In this case, each network node is a 2×2 switching element that employs virtual output queueing and dedicated selection policy that operates locally. Thus, large scale switches can be achieved without the expense of complex scheduling algorithms. In this paper, performance characteristics of the longest queue first, oldest cell first and random selection policies in the Banyan like interconnection network are studied. Results show that the longest queue first selection policy outperforms others in terms of packet loss performance, whereas random selection policy achieves low throughput-delay ratio performance.