Research On The TCP Performance Enhancement Techniques In The WLAN

In recent years, wireless networks have seen a spectacular development, especially the infrastructure WLAN which based on the IEEE 802.11 protocol suite. The features of easy deployment, low cost, mobility supporting and other merits made the WLAN popularity around almost every corner of the world. In addition, a growing number of mobile equipments have already supported to connect with the WLAN, which further promote the development of the WLAN.Statistical data show that the primary traffic transmitted by the infrastructure WLAN is based on the TCP protocol. However, the traditional infrastructure WLAN cannot offer good service for the TCP traffic. The hostile nature of the wireless channel and the mobile nature of wireless users interact adversely with standard TCP congestion control mechanisms, which makes the WLAN obtain a poor throughput when transmitting the TCP traffic. In addition, the DCF used by the WLAN limits the ability of the AP for accessing the wireless channel, which generates a huge disparity between the throughput of uplink and downlink TCP flows. This phenomenon is more obvious when the WLAN is deployed in locations such as airports and large conventions, which leads to a terrible user experience. In order to elevate the Qo S of the WLAN effectively, it is necessary for us to research the issues that related to the WLAN and TCP congestion control mechanisms.On the basis of the overseas and domestic research, TCP performance enhancement techniques are studied through theoretical analyses and simulations. The author’s major contributions are outlined as follows:1. The main contents of architectures, standardization progress and access protocols of the WLAN have been studied. Meanwhile, the rationale of the TCP congestion control mechanisms and some issues of the TCP protocol when it is applied to the WLAN are discussed.2. The major TCP performance enhancement techniques in the WLAN have been reviewed in detail. The merits and demerits of some representative techniques are analyzed in depth.3. We propose an approach that called adaptive flow control proxy(AFCP) by taking consideration of the characteristics of both the WLAN and TCP congestion control mechanisms. This approach needs no modification to the underlying protocols that used by the WLAN, which makes it easy to be put into effect. Adaptive flow control proxy is implemented at the IP layer of the AP, which acts like a TCP Proxy that can decrease the negative influence caused by the WLAN. In addition, AFCP monitors the traffic flowing through the AP, and uses the traffic statistics to calculate the transmit capacity of the WLAN as well as the RTT of each TCP flow periodically. Taking advantage of the calculated results, AFCP can further figure out the slice windows that each TCP flow should keep accordingly. Then, AFCP uses the calculated slice windows to modify the advertised window field in corresponding TCP ACKs, altering the throughput of each TCP flow. By these means, AFCP can make full use of the transmit ability of the WLAN, and shrink the throughput gap between uplink and downlink TCP flows at the same time, which can promote the user experience availably.4. We use NS-3 simulation tool to simulate the communication scene for verifying the effectiveness of the AFCP. Using the communication scene, three groups of experiments have been implemented. In the first group of experiments, the AP takes no modification. In the second group of experiments, we implement the adaptive priority control approach at the MAC layer of AP. In the third group of experiments, we implement the AFCP at the IP layer of AP. In each group of experiments, we change the number of stations and type of traffic in several sub-experiments for studying the performance variation trend of each approach under different conditions. Every sub-experiment has been done for many times with different seeds of random numbers, which is aiming at obtaining a relatively accurate statistical result. We utilize the total throughput of the WLAN, the average throughput of the downlink TCP flows and the Jain’s fairness index of different TCP flows to evaluate the performance of the WLAN. Through the simulation result, it’s easy to see that AFCP can elevate the performance of the WLAN effectively.