IEEE 802.11 多重速率無線網路之整合型參數調整策略

Abstract

IEEE 802.11 在實體層提供多重速率傳輸的能力與機制。不同傳輸速率有各 自訊號與雜訊干擾值的需求。若在固定雜訊能量的情況下，節點對於傳輸速率的 調整會與傳輸周圍附近的節點干擾有關。而802.11 DCF亦提供指數退回演算機 制 (binary exponential backoff)來降低節點使用通道的機會，解決通道壅塞 的問題。傳統的傳輸速率調整機制為，當節點傳輸封包發生失敗時，會同時調降 傳輸速率且讓目前的壅塞視窗 (cwp)呈指數成長，但這樣的調整會讓節點的傳送 過於保守，降低通道使用效能；然而，當節點傳送封包成功時，節點會調升傳送 速率且重設目前的壅塞視窗，但這樣的調整卻會讓節點的傳送過於積極，造成不 必要的碰撞，影響系統的吞吐量。這樣的問題，主要是由於802.11 沒有將傳輸 速率與壅塞視窗，這兩參數的調整一起考慮。 本篇論文由於觀察到上述的現象，所以我們提出了新的速率調整機制名為 ARC (Joint Adaptation of Link Rate and Contention Window)。藉由預測目前通 道最佳的壅塞視窗的大小 (optCW)，若cwp > optCW (cwp < optCW)，在節點傳輸 成功時 (失敗)，ARC會調降 (增大)目前壅塞視窗的值，但仍維持相同的傳送速 率 R；否則，才會去調升 (調降)目前的傳送速率R值得大小。ARC的特點就是， 它讓節點可顧及傳送速率的穩定，避免不必要的傳輸速率變動的問題。模擬結果 顯現，ARC表現比傳統速率調整機制來的出色。最後我們亦提出馬可夫鏈的數學 模型來驗證ARC模擬結果。

IEEE 802.11 wireless network supports multiple link rates at the physical layer. Each link rate is associated with a certain required Signal-to-Interference-and-Noise Ratio (SINR) threshold for successfully decoding received packets. Suppose constant noise and no power adjustment exists, apparently SINR is solely affected by the accumulated interference power level I. The method of selecting an appropriate link rate for transmitting/retransmitting packets is generally known as the link adaptation mechanism. Traditional link adaptation approaches try to reduce the transmit rate (hence lower SINR threshold is required) on transmission failures (potentially due to the increased denominator I of SINR), whereas upgrade the transmit rate (hence higher SINR threshold is required) on successful transmissions (potentially due to the decreased denominator I of SINR). The accumulated interference power level I in some sense indicates the medium congestion status. In 802.11, on transmission failures, the DCF performs a binary exponential backoff mechanism to discourage channel access attempts. When traditional link adaptation is applied, both rate reduction and binary backoff represent double penalties for this wireless link, which may cause overly conservative transmission attempts. On the other hand, once transmission succeeds, 802.11 DCF resets the backoff contention window to the minimum value to encourage channel access attempts. At the same time, traditional link adaptation may also decide to increase the data rate, which may lead to overly aggressive transmission attempts. We observe this improper interaction of link rate and backoff mechanism that harms the 802.11 system performance, due to separate consideration of those two parameters. In this paper, rather than independently dealing with the two parameters, we propose to perform link adaptations by firstly considering if a proper backoff window has been reached. Specifically, if the medium congestion level I can be reduced by imposing a larger backoff window on transmissions, then there may be no need to decrease the link rate, given SINR can be sustained. Conversely, if there is extra interference that may be tolerated in I, a smaller backoff window can be used to encourage more transmission activities while keeping the required SINR. In particular, a joint Adaptation of link Rate and backoff Contention window, abbreviated as ARC, is devised. Our ARC protocol first estimates the optimal contention window (optCW) based on Cali's approximation methods. On transmission successes (failures), the current contention window size cw_p should be compared with optCW. If cw_p > optCW (cw_p < optCW), then cw_p is decreased (increased) to perform more aggressive (conservative) transmission attempts while leaving the link rate R unchanged. Otherwise, R is upgraded (reduced) to the next higher (lower) rate. One nice property of ARC is the ability to intelligently maintain link stability, avoiding unnecessary rate fluctuations. Simulation results show that the proposed ARC protocol outperforms several traditional link adaptation mechanisms. We also propose an analytic Markov chain model on ARC operations for performance validation.