IEEE 802.11 多重速率無線網路之加強型參數整合調整策略

Abstract

在IEEE 802.11的無線網路中提供了實體層多重傳輸速率的選擇，為了能成功解讀接收到的封包，針對不同的傳輸速率有不同的訊號干擾雜訊比(Signal-to-Interference-and-Noise Ratio (SINR))的要求。在802.11的分散式協調功能(DCF)中，二進制指數後退機制 (binary exponential backoff mechanism)在封包傳送失敗時，會將競爭視窗值(contention window)提升二倍，試圖以此減低環境中傳輸行為的擁塞情況；而當封包傳送成功時，又會將競爭視窗(contention window)重新設定回最低的cwmin值，試圖讓無線網路中的傳輸行為變得更為積極。然而傳統的速率調適機制，在降低傳輸速率的同時也將競爭視窗(contention window)做提升二倍的調整；亦或是在提升傳輸速率的同時又將競爭視窗(contention window)重新設回最低值cwmin，而將傳輸速率及競爭視窗(contention window)分別獨立做調整，可能會導致傳送行為變得過於保守或是變得太過於積極，這樣將傳輸速率及競爭視窗(contention window)二個參數分別考慮而做調整會造成不適合的傳送行為而損害無線網路中的吞吐量(throughput)。因此，我們提出了IEEE 802.11多重速率無線網路之加強型參數整合調整策略(Enhanced Adaptation of Link Rate and Contention Window for IEEE 802.11 Multi-rate Wireless Networks，以下簡稱為EARC),EARC的調整策略是將傳輸速率及競爭視窗(contention window)二參數一同納入考慮，再做適度地調整。EARC是閉迴式(closed-loop)的速率調適機制，首先在接收端我們經由觀測環境的能量狀況及接收行為設計建立了速率選擇參考表(rate selection reference (RSR) table)，接收端藉由RSR table決定最適合的傳輸速率並以ACK封包夾帶此訊息讓傳送端得以用最適合的傳輸速率傳送封包。若接收端所預估的傳輸速率與目前的傳輸速率相同，接收端進而比較與傳送端之間的平均觀測能量差，同樣以ACK封包夾帶此訊息作為傳送端調整競爭視窗(contention window)的參考。EARC藉由同時考慮傳輸速率及競爭視窗(contention window)二參數值進而再整合做參數調整，能有效提升系統的吞吐量(throughput)，在模擬結果中也驗證了此理論。

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. On transmission failures, the 802.11 DCF performs a binary exponential backoff mechanism to discourage channel access attempts, hoping to reduce congestion. 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 leads 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 thesis, we propose to jointly adapt the rate and backoff parameters. Specifically, an Enhanced Adaptation of link Rate and Contention window, abbreviated as EARC, is devised. 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. Moreover, since SINR information commonly utilized by receiver-assisted protocols is not precisely supported in real devices, we introduce a rate selection reference (RSR) table empirically 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. Simulation results demonstrate 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.