  We consider single-hop topologies with saturated transmitting nodes, using
IEEE~802.11 DCF for medium access. However, unlike the conventional WiFi, we
study systems where one or more of the protocol parameters are different from
the standard, and/or where the propagation delays among the nodes are not
negligible compared to the duration of a backoff slot. We observe that for
several classes of protocol parameters, and for large propagation delays, such
systems exhibit a certain performance anomaly known as short term unfairness,
which may lead to severe performance degradation. The standard fixed point
analysis technique (and its simple extensions) do not predict the system
behavior well in such cases; a mean field model based asymptotic approach also
is not adequate to predict the performance for networks of practical sizes in
such cases. We provide a detailed stochastic model that accurately captures the
system evolution. Since an exact analysis of this model is computationally
intractable, we develop a novel approximate, but accurate, analysis that uses a
parsimonious state representation for computational tractability. Apart from
providing insights into the system behavior, the analytical method is also able
to quantify the extent of short term unfairness in the system, and can
therefore be used for tuning the protocol parameters to achieve desired
throughput and fairness objectives.

Networking and Internet Architecture