Airtime Fairness in a Rate Separation IEEE 802.11b MAC David Tung Chong Wong, Anh Tuan Hoang and Chen Khong Tham Institute for Infocomm Research, Agency for Science, Technology and Research (A*STAR) Fusionopolis Way, #21-01 Connexis, Singapore 138632 {wongtc, athoang, cktham}@i2r.a-star.edu.sg Abstract – 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 Keywords – analytical formulation, airtime fairness, saturated throughput, data transmission periods, beacon period, rate separation IEEE 802.11b MAC I INTRODUCTION IEEE 802.11 distributed coordination function (DCF) medium access control (MAC) does not provide airtime fairness for all stations in a multi-rate scenario This is because it is designed to provide max-min throughput fairness This design philosophy of IEEE 802.11 MAC causes the rate anomaly problem [1], where the maximum throughput is limited by the slowest transmitting station Every station is given equal channel access probability in IEEE 802.11 MAC Thus, in a multi-rate scenario, faster stations are penalized as they need to wait longer for slower stations to transmit their packets If the packets are all equal, slower stations will take up a larger portion of the airtime for transmitting their packets as compared to that of faster stations Reference [2] proposed a scheme for airtime fairness in IEEE 802.11b MAC by controlling the minimum contention window of each station All stations compete for channel access with airtime fairness A rate separation IEEE 802.11b MAC is proposed to achieve higher throughput in [3] Stations are grouped according to their transmission rates for transmitting their packets in different data transmission periods (DTPs) for the different groups of stations Only stations belonging to the DTP can transmit in it In our paper, airtime fairness in a rate separation IEEE 802.11b MAC is our focus The goal is to find the different DTPs such that airtime fairness per station for different classes is achieved, given a fixed superframe time The contributions of this paper are as follows Firstly, an analytical framework for the saturated throughput is formulated for the rate separation IEEE 802.11b MAC 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 Secondly, the DTPs to achieve airtime fairness are formulated as non-linear equations, which are solved using Newton-Raphson method with Jacobian functions Thirdly, the advantage of the rate separation IEEE 802.11b MAC with airtime fairness is shown to achieve airtime fairness and high saturated throughput in the numerical results section The rest of the paper is organized as follows Section II describes the superframe of a rate separation IEEE 802.11b MAC, while Section III describes our CSMA/CA MAC scheme used in the DTPs In Section IV, we present an analytical model for transmissions in the DTPs Numerical results are presented in Section V Both analytical and simulation results are presented Finally, concluding remarks are made in Section VI II SUPERFRAME FORMAT OF RATE SEPARATION IEEE 802.11b MAC The superframe format is shown in Fig Each superframe consists of eight parts: four beacon periods (BPs) and four data transmission periods (DTPs) All beacon periods are assumed to be of equal size In each beacon, the AP coordinates and informs which stations, using the same data rate, can transmit in the following DTP after the beacon In the DTP, the stations that can transmit in the DTP using the same data rate will cooperate by attempting to transmit only in the DTP that is designated to them in the beacon There is a period before the end of each DTP that does not allow a packet transmission to be successful as it is insufficiently long to transmit the packet, short inter-frame space (SIFS) time and ACK frame This is for basic access {R1 = 11 Mbps} BP DTP 1 {R2 = 5.5 Mbps} BP DTP 2 {R3 = Mbps} BP DTP 3 {R4 = Mbps} BP DTP 4 Superframe of a rate separation IEEE 802.11b MAC BP – beacon period DTP – data transmission period Ri, i=1,2,3,4 – data rate i Figure Superframe format of rate separation IEEE 802.11b MAC protocol For request-to-send/clear-to-send (RTS/CTS) access, additional RTS frame, CTS frame and two SIFS times have to be added DTP i is for stations with data rate i, Ri, i = 1, 2, 3, 4, where R1=11 Mbps, R2=5.5 Mbps, R3=2 Mbps and R4=1 Mbps We assume that station associations have been completed III MAC PROTOCOL Our CSMA/CA MAC protocol works in the DTPs as follows: • If the channel is idle for more than a DCF inter-frame space time (DIFS), a station can transmit immediately • If the channel is busy, the station will generate a random backoff period This random backoff period is uniformly selected from zero to the current contention window size The backoff counter will decrement by one if the channel is idle for each time slot and will freeze if the channel is sensed busy or if the its DTP is not active The backoff counter is re-activated to count down when the channel is sensed idle for more than a DIFS time or when its DTP is active At the initial backoff stage, the current contention window size is set at the minimum contention window size • If the backoff counter reaches zero, the station will attempt to transmit its frame if the remaining time to the end of its active DTP is greater than or equal to the period mentioned earlier If it is successful, the destination device will send an acknowledgement after a SIFS and the current contention window size is reset to the minimum contention window size If it is not successful, it will increase the current contention window size by doubling it and adding one in the next backoff stage and a new random backoff period is selected as before • If the backoff counter reaches zero, the device will not attempt to transmit its frame if the remaining time to the end of its current active DTP is less than the period mentioned earlier Instead, it will increase the current contention window size by doubling it and adding one in the next backoff stage and a new random backoff period is selected as before and the count down of the backoff counter will start after a DIFS after the next BP If the maximum retry limit is reached, the packet will be dropped and the next packet will start its backoff process with a minimum contention window size after a DIFS from the beginning of its next active DTP • If a busy period ends within the period mentioned earlier, all the backoff counters will freeze until after a DIFS from the beginning of its next active DTP • This process repeats itself until the frame is successfully transmitted or until the maximum retry limit is reached If the frame is still not successfully transmitted, then it is dropped • If a station does not receive an acknowledgement within an acknowledgement timeout period after a frame is transmitted, it will continue to attempt to re-transmit the frame according to the backoff algorithm For each of the next backoff stage, the maximum contention window size is doubled that of the previous maximum contention window size and plus one Then, the backoff counter value is uniformly chosen from zero to the maximum contention window size This is equivalent to doubling the previous maximum contention window size and choosing the backoff counter value uniformly from zero to the maximum contention window size minus one IV ANALYTICAL MODEL We assume there are a generic I number of data rates, denoted by Ri, where i=1,…,I, and R1>R2>…>RI Their corresponding coverage distances are denoted by Di, D1