Airtime Fairness in a Rate Separation IEEE 802.11b MAC

IEEE 802.11 distributed coordination function (DCF) medium access control (MAC) does not provide airtime fairness for all stations in a multi-rate scenario as it only provides max-min throughput fairness. This gives rise to the rate anomaly problem where the maximum throughput is limited by the slowest transmitting station. In our paper, we propose airtime fairness in a rate separation IEEE 802.11b MAC. Stations are grouped according to their transmission rates for transmitting their packets in different data transmission periods (DTPs) for the different groups of stations. The analytical framework is formulated for N stations, including an access point (AP). The state transition diagram is modeled by a two-dimensional discrete-time Markov chain. One dimension of the Markov chain is for the backoff stage and the second dimension is for the value of the backoff counter. The saturated throughput is approximated by the sum of the product of a weighted ratio of the throughput of the DTP under consideration and the throughput of the DTP minus the period necessary to transmit a packet before the end of the current DTP, the probability of the number of devices in the DTP and the number of DTPs. The DTPs to achieve airtime fairness are formulated as non-linear equations, which are solved using Newton-Raphson method with Jacobian functions. Numerical results of the saturated throughput corresponding to typical parameter values are presented. These results show the advantage of the proposed rate separation IEEE 802.11b MAC with airtime fairness in achieving airtime fairness and high saturated throughput.