IEEE 802.11 WLAN cannot guarantee the QoS of applications, thus admission control has been proposed as an essen-tial solution to enhance the QoS. Packet delay and throughput are commonly employed as assessment criterions to determine whether a new connection can be admitted into the WLAN. Considering the real network condition, the analytical model is presented in this paper, which is aimed to evaluate the packet delay and throughput performance of IEEE 802.11 WLAN in nonsaturated conditions, taking into account diverse transmission rates and diverse traffic flows (i.e. flows with different packet sizes and arrival rates) simultaneously. This model is based on Markov chain and the theoretical predictions are verified by simulation in OPNET 14.5. We also analyze the influences of transmission rate diversity and traffic flow diversity on throughput performance. It is observed that, the presence of even one station with lower transmission rate can cause a considerable degradation in throughput performance of all the stations when they have the same packet size and arrival rate. Higher system throughput can be achieved if lower transmission rate stations transmit packets with smaller size or arrival rate.
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