EFFECT OF SERVICE DIFFERENTIATION ON QUALITY OF SERVICE (QOS) IN IEEE 802.11E ENHANCED DISTRIBUTED CHANNEL ACCESS (EDCA)
With recent advances in wireless technology, users of wireless network now expect quality of service and performance comparable to what is available in fixed networks. The IEEE 802.11e Medium Access Control (MAC) is an emerging supplement to the IEEE 802.11 Wireless Local Area Network (WLAN) standard that supports Quality-of-Service (QoS) requirements of both data and real-time applications. The IEEE802.11e MAC is based on both centrally-controlled and contention-based channel accesses. One of the most important functions in this MAC is the contention-based channel access mechanism called enhanced distributed coordination function (EDCF), which provides a priority scheme by differentiating the arbitration inter-frame spacing (AIFS), transmission opportunity (TXOP), and contention window parameters (CWmin and CWmax), for each access category. In this research work the EDCA priority scheme was evaluated to ascertain the effect of differentiating frames with different priorities on QoS using a MatLab computer simulation model. This model was used to compute the optimal performance, maximum sustainable throughput, loss rate and service delay distribution for each priority class under saturation load. Insight obtained frrom the analysis shows that the acquisition of the radio channel by the higher priority traffic is much more aggressive than for the lower priority traffic, causing the packets in the lower priority queue to be starved. Considering the performance of different access parameters, arbitration inter-frame space number (AIFSN) has more influence on the QoS performance of IEEE 802.11e EDCA Protocol, than the CW size. It was also observed that small CW values generate higher packet drops and collision rate probability. As a consequence, the EDCA mechanism suffers significantly. It is recommended that small values of AIFS should be cautiously used in order not to starve the lower priority traffics while CW size has to be tuned dynamically in response to varying load. Finally, larger CW size is advised to reduce the chances of collision.
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