IEEE 802.11無線網路之TCP傳輸效能評估

Abstract

本篇論文，根據802.11 infrastructure WLAN中downstream TCP兩項重要行為：(1)constant pps(packets per second) throughput與(2)系統內平均的active wireless station(WS)數目<=2﹝active WS的定義為mac transmission queue中有TCP_ACK要回傳給AP的wireless station﹞，因而知道此時系統的utilization很低。我們詳細的整理出限制此架構下TCP效能的causes及effects，並提出了利用802.11e中TXOP概念的一系列enhancement機制。 我們提出的改進機制有以下四種：(1)TXOP (AP-k, WS-1)、(2)TXOP (AP-k, WS-k)、(3)TXOP (AP-k, WS-∞)及(4)TXOP (AP-k, WS-∞)+ACK prioritization and at most one transmission。第一種方法在AP 端使用TXOP，希望提升active WS的數目來提升channel contention level。而結果的確能成功改進infrastructure WLAN下TCP的throughput，因為它有效地同時增加了TCP_DATA下傳及TCP_ACK上傳的速率。TXOP讓AP每次channel access能送出多個封包，因而增加了下傳速率；上傳速率的增加則是因為，AP每次丟出多個封包，而提升了active WS的數目，因此增加了與AP競爭而搶到channel的機率。只是在TXOP較大的情形裡，卻會引發Asymmetry TCP中常見的問題，即forward path speed> reverse path speed，因此我們接著提出的方法(2)、(3)、(4)，都是藉著增加reverse path上ACK回傳的速率，而能更進一步的改善TCP throughput。 我們也針對infrastructure WLAN中online wireless station number為1的scenario，提出了一個analytic model，來描述這一條TCP flow的throughput，此model的確能closely evaluate TCP throughput。另外，我們也針對TXOP (AP-k, WS-1)的enhancement機制，修改Conti[11]paper中的方法，提出了可以準確預估active WS的model，並以之為基礎，來evaluate TCP的throughput。

In this paper we are inspired from two important downstream TCP behaviors in infrastructure WLAN：(1) constant channel pps(packets per second) throughput (2) average active wireless station(WS) number always <=2 (active WS are those who have TCP_ACKs in MAC transmission queue and tend to actively contend for channel access to reply TCP_ACK to AP), and conclude that the channel utilization is low. Therefore, we investigated causes and effects that limit TCP throughput, and propose enhancement mechanisms which adopt the concept of 802.11e TXOP(transmission opportunity) to improve TCP throughput. We propose four measures：(1)TXOP (AP-k, WS-1), (2)TXOP (AP-k, WS-k), (3)TXOP (AP-k, WS-∞), and(4)TXOP (AP-k, WS-∞)+ACK prioritization and at most one transmission. The first method employs TXOP at AP to increase the number of active WS, and indeed improve TCP throughput in infrastructure WLAN. The reason is that the transmission efficiency of downstream and upstream is both increased. Downstream rate rises due to multiple packets transmitted by AP per channel access. Upstream rate increases because the number of active WS gets larger and the probability for WS to grab channel to reply ACK has grown. However, when TXOP value get larger, the common problem of asymmetry TCP has shown：the reverse path throughput cannot catch up with the forward path throughput. Therefore, we propose the second, third, and fourth method to improve TCP throughput by speeding up the reverse path rate. We also propose an analytic model to characterize TCP throughput in infrastructure WLAN when the number of TCP connections equals one. Validated by simulation, we show that our model can precisely evaluate TCP throughput. In addition, we also characterize TCP throughput of the first method by modifying the model of Conti[11]. We closely evaluate the number active station and based on the estimation we can characterize the TCP throughput.