Realize TCP Flows Fairness In Wirless LAN With Diffserv

Wireless Local Area Network (WLAN) based on IEEE 802.11 has been appliedwidely due to its mobility and facility. With the highly increasing speed in datatransmission and the development of multimedia applications, supporting Quality ofService (QoS) in WLAN has been increasingly demanded. The basic issues of QoS arehow to ensure the fair allocation of the bandwidth for all wireless users. Hence thefairness issue is one of the key issues to provide QoS in WLAN. In this paper, we willdiscuss the unfairness problem between TCP upstream flows and downstream flowsover infrastructure-mode WLAN.The IEEE 802.11 standard is the most widely used standard in WLAN today. Theactual wireless products use the Distributed Coordination Function (DCF) as the basicaccess method in the MAC layer. It uses a Carrier Sense Multiple Access with CollisionAvoidance (CSMA/CA) protocol, which provides the fair channel access for all accessnodes, but suffers from unfairness problem among TCP flows. Furthermore, the controlmechanism of TCP protocol also impacts on TCP fairness between upstream anddownstream flows. Because of the two reasons, the rate of downstream flows will besmaller than that of upstream flows at the most scenarios. However, in the considerationof the users' network using situation (web page browsing, file downloading, and etc.),most of them depend on the downstream flow more than the upstream flow. Hence, it isvery necessary to maintain the rate of downstream flows and the fairness of TCP flows.There are many solutions to this problem. Most of the solutions try to restrict therate of upstream flows in some degree. All these solutions need to change the existingwireless network framework, however in fact it's quite hard to accomplish. Based onthese solutions and the DiffServ protocol, we do the traffic control on all flows, andrestrict the packets sending from upstream flows in order to make the size of AP's buffer meet the need of all packets and achieve the TCP fairness, The DiffServ is a widelyused mechanism which guarantees the QoS well, so we don't need to change thewireless network. During the experiment, we find this method can achieve TCP fairnesswell. but the DiffServ can't adjust the bandwidth dynamically to the low rate flow.When the low rate flow comes out, a part of bandwidth allocated to this flow can't beused abundantly, meanwhile, the bandwidth for the whole network won't be sufficient.After that, we modified the original DiffServ mechanism, and get a dynamic adaptivearithmetic to solve this problem. According to this arithmetic, when the new flowaccesses in the Wlan, comparing with the classified rule maintained by DiffServ router,fixes on the number of active flows in the Wlan. and allocates the bandwidth to allflows averagely. The routers at the edge of the DiffServ domain meter the rate of allflows periodically. Based on these rates, the dynamic arithmetic can adjust thebandwidth of flows with different rates. When the flows quit from Wlan, the arithmeticcan detect it, then withdrawal the bandwidth and allocate it to other flows. The results ofthe series of simulation indicate that the arithmetic can achieve the TCP fairness well inthe scenario of one upstream flow and N downstream flows or N upstream flows and Ndownstream flows. Further more, the arithmetic offers good performance on throughput,average delay and percent of packet dropped: The throughput of the whole network cankeep around 5Mbps. Most of the time, the values of average delay and percent of packetdropped are lower than the values in the normal Wlan. Moreover, there is no need tomodify the hardware of wireless equipments by using DiffServ dynamic adaptive adjustarithmetic, so that the TCP fairness problem in wireless network could be solved moresimply and economically with this arithmetic.