  We propose a linear model of the throughput of the IEEE 802.11 Distributed
Coordination Function (DCF) protocol at the data link layer in non-saturated
traffic conditions. We show that the throughput is a linear function of the
packet arrival rate (PAR) $\lambda$ with a slope depending on both the number
of contending stations and the average payload length. We also derive the
interval of validity of the proposed model by showing the presence of a
critical $\lambda$, above which the station begins operating in saturated
traffic conditions.
  The analysis is based on the multi-dimensional Markovian state transition
model proposed by Liaw \textit{et al.} with the aim of describing the behaviour
of the MAC layer in unsaturated traffic conditions. Simulation results closely
match the theoretical derivations, confirming the effectiveness of the proposed
linear model.
