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The Infrared Data Association (IrDA) advanced infrared (AIr) protocol for indoor optical wireless LAN is examined. AIr medium access control (MAC) layer utilizes a carrier sense multiple access with collision avoidance (CSMA/CA) scheme to coordinate medium access. To deal with hidden stations, AIr employs a request to send/clear to send (RTS/CTS) reservation scheme and a long collision avoidance slot (CAS) duration that includes the beginning of the receiver's CTS packet. AIr employs linear adjustment of the contention window (CW) size to minimize delays emerging from the long CAS duration. This paper develops an analytical model for the collision avoidance scheme of the AIr protocol that computes throughput performance assuming error free transmissions and a fixed number of stations. The model is validated by comparing analysis with simulation results. By differentiating the throughput equation, the optimum CW size that maximises throughput as a function of the number of the transmitting stations is derived. In the case of the AIr protocol where a collision lasts exactly one CAS, different conclusions result for maximum throughput as compared with the corresponding conclusions for the similar IEEE 802.11 protocol. By employing the analytical model, throughput performance for various parameter values is evaluated. The proposed linear CW adjustment is very effective in minimizing delays emerging from empty slots and collisions during the contention period. Linear CW adjustment combined with the long CAS duration provides an effective protection from collisions caused by hidden stations and offers an attractive collision avoidance scheme.