802.11e多速率多媒體傳輸下的公平性應對機制

Abstract

IEEE 802.11e為針對無線媒體存取層所制定的延伸標準，因應不同的傳輸需求而做出的分級與保障機制。但是它只考慮單一速率的網路環境，當存在著不同速率的傳輸裝置時，在高優先權通道存取種類之下，由於802.11e只達成各速率使用者間的時間公平性，這會無法有效地保障低速率使用者的QoS需求；反觀，在低優先權通道存取種類中，由於802.11e提供給各速率使用者相同的通道存取機會，在低速率使用者傳送長訊框的情形之下，將會霸佔過長的通道時間，除了壓縮高速率使用者的傳送權利之外，也會進而影響整體系統的吞吐量。 因此，本篇文章將針對以下兩個目的，提出了新的改進機制:1.減少低速non-QoS資料佔用時間過長2.加強低速使用者QoS保證。新的改進機制會給予各速率的QoS使用者公平的傳輸量來達到相同的QoS保證；面對non-QoS使用者則給予公平的傳輸時間來增加整體系統的吞吐量，達成同時兼顧系統效能及服務品質的目標。

The IEEE 802.11e Enhanced Distributed Channel Access (EDCA) provides a class-based differentiated quality of service (QoS) to IEEE 802.11 WLANs. But since EDCA mechanism only takes single-rate network environment into account, when it equipped in the multi-rate network it can’t function well. That’s because the 802.11e standard provide time fairness among the high priority access category which may influence the QoS performance of the low speed users. And among the low priority access category, 802.11e provides same probability of channel access to all users. That means when the users are transmitting large frame, the low-rate users will occupy more channel slots then the high-rate users do. This characteristic not only causes the compression to the channel transmission time of the high-rate users but also affect the total system throughput of the network. Regarding of the defect above, we propose a new fairness mechanism for two purposes: 1. Reducing the transmission time of the low speed non-QoS users to enhance the total throughput. 2. Providing same bandwidth guarantee to all QoS requirement stream regardless of the user’s transmission rate. Finally, we simulate the improvement between new fairness mechanism and original EDCA function.