New WLAN MAC structure for supporting multi-rates and fairness

Abstract

WLAN systems based on IEEE 802.11 have been widely deployed since IEEE 802.11b standard, which can support up to 11 Mbps data rate at 2.4GHz ISM band, was published and low-cost equipments were on market. Recently, infrastructure mode WLANs have been deployed rapidly, and applications have been extended to public wireless Internet access in hot spot areas. The PHY extensions of IEEE 802.11 standard support multiple PHY rates. The mandatory DCF access method is based on CSMA/CA with binary exponential backoff. A bi-dimensional Markov chain model for a station and more refined models have been introduced to analyze the performance of IEEE 802.11 DCF. We introduce a new modified analysis model. We take into account a frame retry limit, freezing of backoff counter, and the dependence of backoff procedure on the previous channel status all together. In addition, simplified channel equations are derived under the assumption of no consecutive transmissions at the shared channel. This simplified analysis yields more tractable and considerably exact results. The DCF of the original IEEE 802.11 MAC does not fully reflect an infrastructure mode operation and a multi-rate environment. Since it allows all stations to access fairly regardless of their function and transmission rates, WLAN systems suffer from up-/down-link unfairness and performance anomaly. There have been no generalized analysis on throughput and channel utilization for successful transmissions in multi-rate WLAN systems which consist of stations with different access opportunities. Furthermore, there have been no multi-rate infrastructure mode WLAN oriented analysis. Several schemes have been proposed to enhance the performance of multi-rate WLAN systems. More access opportunities or longer transmission times are allocated to stations with higher PHY rates by adjusting frame sizes or contention window parameters. However, the above-mentioned fairness problem has not been fully considered. Several s...