以動態競爭視窗調整機制改善無線區域網路IEEE 802.11e規格之效能

Abstract

In recent years, we have witnessed an amazingly rapid evolution in wireless local area networks (WLANs). To fulfill the ever-increasing demand for quality-of-service (QoS) in multimedia applications, a new supplement to the existing legacy 802.11 medium access control (MAC) layer, namely 802.11e, is defined. The supplement introduces a new MAC access method named hybrid coordination function (HCF) to support QoS requirements. The HCF included two access medium methods: contention-based EDCA (Enhanced Distribution Channel Access) and contention-free HCCA (HCF Controlled Channel Access). In EDCA, each station has four queues to buffer packets for the corresponding ACs (Access Category). A set of EDCA parameters is assigned to each AC to differentiate the channel access priority. In order to provide relatively more medium access opportunity for higher priority ACs, the default CW sizes (in 802.11e) for higher priority ACs are set much smaller than those for lower priority ACs. However, as the number of contending stations in high priority ACs increases, small CW size causes significant increase in the frequency of collisions, which results in drastically throughput degradation. Setting large CW values can solve this problem, but it also induces longer access delay and decreases throughput when the network has little traffic. To address this problem, we proposed an adaptive contention algorithm, in which access point (AP) chooses the value of CW for each ACs based on the number of contending stations in every ACs and collision cost, then according the transmission requirement of AC to allocate throughput ratio to different ACs.