Input Versus Output Queueing on a Space-Division Packet Switch

Two simple models of queueing on an
N \times N
space-division packet switch are examined. The switch operates synchronously with fixed-length packets; during each time slot, packets may arrive on any inputs addressed to any outputs. Because packet arrivals to the switch are unscheduled, more than one packet may arrive for the same output during the same time slot, making queueing unavoidable. Mean queue lengths are always greater for queueing on inputs than for queueing on outputs, and the output queues saturate only as the utilization approaches unity. Input queues, on the other hand, saturate at a utilization that depends on
N
, but is approximately
(2 -\sqrt{2}) = 0.586
when
N
is large. If output trunk utilization is the primary consideration, it is possible to slightly increase utilization of the output trunks-upto
(1 - e^{-1}) = 0.632
as
N \rightarrow \infty
-by dropping interfering packets at the end of each time slot, rather than storing them in the input queues. This improvement is possible, however, only when the utilization of the input trunks exceeds a second critical threshold-approximately
ln (1 +\sqrt{2}) = 0.881
for large
N
.