  Analytical models for IEEE 802.11-based WLANs are invariably based on
approximations, such as the well-known \textit{decoupling approximation}
proposed by Bianchi for modeling single cell WLANs consisting of saturated
nodes. In this paper, we provide a new approach to model the situation when the
nodes are not saturated. We study a State Dependent Attempt Rate (SDAR)
approximation to model $M$ queues (one queue per node) served by the CSMA/CA
protocol as standardized in the IEEE 802.11 DCF MAC protocol. The approximation
is that, when $n$ of the $M$ queues are non-empty, the transmission attempt
probability of the $n$ non-empty nodes is given by the long-term transmission
attempt probability of $n$ "saturated" nodes as provided by Bianchi's model.
The SDAR approximation reduces a single cell WLAN with non-saturated nodes to a
"coupled queue system". When packets arrive to the $M$ queues according to
independent Poisson processes, we provide a Markov model for the coupled queue
system with SDAR service. \textit{The main contribution of this paper is to
provide an analysis of the coupled queue process by studying a lower
dimensional process, and by introducing a certain conditional independence
approximation}. We show that the SDAR model of contention provides an accurate
model for the DCF MAC protocol in single cells, and report the simulation
speed-ups thus obtained by our \textit{model-based simulation}.
