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Analytical models for IEEE 802.11-based WLANs are invariably based onapproximations, such as the well-known \textit{decoupling approximation}proposed by Bianchi for modeling single cell WLANs consisting of saturatednodes. In this paper, we provide a new approach to model the situation when thenodes are not saturated. We study a State Dependent Attempt Rate (SDAR)approximation to model $M$ queues (one queue per node) served by the CSMA/CAprotocol as standardized in the IEEE 802.11 DCF MAC protocol. The approximationis that, when $n$ of the $M$ queues are non-empty, the transmission attemptprobability of the $n$ non-empty nodes is given by the long-term transmissionattempt 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 toindependent Poisson processes, we provide a Markov model for the coupled queuesystem with SDAR service. \textit{The main contribution of this paper is toprovide an analysis of the coupled queue process by studying a lowerdimensional process, and by introducing a certain conditional independenceapproximation}. We show that the SDAR model of contention provides an accuratemodel for the DCF MAC protocol in single cells, and report the simulationspeed-ups thus obtained by our \textit{model-based simulation}.

State Dependent Attempt Rate Modeling of Single Cell IEEE~802.11 WLANs with Homogeneous Nodes and Poisson Packet Arrivals