IEEE 802.11 傳送速率與後退參數調整機制之實作研究

Abstract

在多重速率的 IEEE 802.11 無線網路中，為了能成功解碼接收端收到的封包，每個傳輸速率需要不同的 訊號干擾雜訊比 (SINR) 門檻。當傳統的適應性速率調整協定啟動時，在傳送失敗的情況下，同時調降傳輸速率及啟動 802.11 DCF 後退機制的加倍競爭視窗 (contention window) 使得傳輸意圖過於保守。另一方面，一旦傳輸成功 802.11 DCF 會重置後退競爭視窗為最小值，鼓勵傳輸行為發生，同時傳統的適應性速率調整協定會增加傳輸速率，但如此會導致傳輸意圖過於積極。由於適應性速率調整機制及後退機制分開考慮，這兩機制不當的作用將會對 802.11 系統效能造成損害。所以我們提出 Enhanced Adaptation of link Rate and Contention window，簡稱 EARC。EARC 是個將後退機制加入考慮的 Closed-loop (receiver-assisted) 適應性速率調整協定。EARC 會根據發射端及接收端附近環境的情況利用 DATA 封包及 ACK 封包攜帶額外 1 byte 的資訊讓發射端根據此資訊對傳輸速率及競爭視窗做調整。本篇論文我們將會進一步實驗 EARC 協定在室內無線環境下的表現，並觀察環境功率強度及接收行為建立出速率選擇參考表 (rate selection reference table)。此表是用來引導接收端選擇最適合的傳輸速率給發射端使用。而我們實驗的結果證明使用速率選擇參考表決定傳輸速率是可行的，另外我們也證明了 EARC 方法在實際的無線環境下比其他適應性速率選擇機制更能維持較高的系統吞吐量 (throughput)。

In multi-rate IEEE 802.11 wireless networks, each link rate is associated with a certain required Signal-to-Interference-and-Noise Ratio (SINR) threshold for successfully decoding received packets. When traditional link adaptation is applied, on transmission failures, both rate reduction and 802.11 DCF binary exponential 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 leads to overly aggressive transmission attempts. This improper interaction of link rate and backoff mechanism that harms the 802.11 system performance, due to separate consideration of those two parameters, inspired us to propose a mechanism entitled Enhanced Adaptation of link Rate and Contention window, abbreviated as EARC. EARC is a closed-loop (receiver-assisted) link rate adaptation protocol that jointly considers the backoff mechanism. With only one extra byte carried by the DATA packet, EARC incurs little controlling overhead despite its receiver-assisted nature. In this thesis, we further implemented the EARC mechanism in a real indoor networking testbed and constructed a rate selection reference (RSR) table derived by constantly monitoring the environmental energy level and reception behavior. The RSR table then guides the receiver to select the best sustainable rate for the transmitter. Empirical results demonstrate that the RSR table is a practical option for making the rate decision, and the proposed EARC approach is effective in maintaining high system throughput, compared to other link adaptation algorithms, in an operational testbed.