  To date, most analysis of WLANs has been focused on their operation under
saturation condition. This work is an attempt to understand the fundamental
performance of WLANs under unsaturated condition. In particular, we are
interested in the delay performance when collisions of packets are resolved by
an exponential backoff mechanism. Using a multiple-vacation queueing model, we
derive an explicit expression for packet delay distribution, from which
necessary conditions for finite mean delay and delay jitter are established. It
is found that under some circumstances, mean delay and delay jitter may
approach infinity even when the traffic load is way below the saturation
throughput. Saturation throughput is therefore not a sound measure of WLAN
capacity when the underlying applications are delay sensitive. To bridge the
gap, we define safe-bounded-mean-delay (SBMD) throughput and
safe-bounded-delay-jitter (SBDJ) throughput that reflect the actual network
capacity users can enjoy when they require bounded mean delay and delay jitter,
respectively. The analytical model in this paper is general enough to cover
both single-packet reception (SPR) and multi-packet reception (MPR) WLANs, as
well as carrier-sensing and non-carrier-sensing networks. We show that the SBMD
and SBDJ throughputs scale super-linearly with the MPR capability of a network.
Together with our earlier work that proves super-linear throughput scaling
under saturation condition, our results here complete the demonstration of MPR
as a powerful capacity-enhancement technique for both delay-sensitive and
delay-tolerant applications.
