Performance analysis and enhancement of MAC protocol for IEEE 802.11 WLANs

An ad hoc network is a basic WLAN. In the IEEE 802.11 WLANs, CSMA/CA is a widely used MAC layer protocol and considerable work has been done on the performance evaluation of this protocol. But most research is confined to saturation performance of single-hop ad hoc networks. In this work, we employ a linear feedback model to evaluate the performance for CSMA/CA according to the Poisson distributed traffic in both single-hop and multi-hop ad hoc networks. To the best of our knowledge, this is the first attempt to evaluate the performance for CSMA/CA protocol analytically under a general assumption about the traffic. This work also gives quantitative results, showing the impact of RTS/CTS. Although RTS/CTS do add the overhead to the system, they become essential when either the hidden terminal problem is dominant, or the traffic is heavy, or the packet length is very large. They have also shown that the performance degrades dramatically in multi-hop ad hoc networks when the number of competing nodes increases, which implies that the scalability is still a major problem in ad hoc networks. The IEEE 802.11e working group aims to enhance the current 802.11 MAC to support integrated data and voice (or video) communications. Till now a draft of the IEEE 802.11e EDCF has been proposed. In the literature, there is very limited analysis about EDCF since it is a new protocol, and most related work are only confined to the simulation or the saturation state. In this work, we evaluate the performance of EDCF by dividing the traffic into two groups, namely real-time packets and non real-time packets, and use an analytical model to quantify the performance of both IFS priority and CW priority in the EDCF. In our analysis, we assume that traffic arrival is a Poisson process. Since there are multiple stations contending for one channel in the system, the queue model for EDCF cannot be a regular M/G/1 model. We redefine the service time and the waiting time so that we can still use M/G/1 model in the analysis. To validate the accuracy of our analytical results, we have done extensive simulations and we observe that EDCF does provide service differentiation between different traffic categories. But due to the inherent contention characteristic of CSMA/CA MAC protocol, the delay cannot be guaranteed, even for the highest priority traffic. A lot more work need to be done for the MAC protocol to effectively support service differentiation in the future. To enhance the performance of 802.11, we proposed optimal contention window scheme. The results show that the optimal contention window scheme greatly outperforms the 802.11 exponential contention window scheme. We also proposed UEDCF, a priority scheme to provide further priority to real-time packets. It has been shown that UEDCF can improve the performance of real-time packets without degrading the overall performance. In multi-hop ad hoc networks, hidden terminal problem might still be present, even after employing RTS/CTS. We proposed a new MAC protocol called CAA, which mainly focuses on the correct transmission of RTS/CTS messages.{09}It is observed that this protocol can greatly improve the performance.