Research On Congestion Detection And Control For Ad Hoc Networks

With the development of wireless communication technology and the prevalence of portable devices, the demands of wireless networks with low cost and easy deployment characters keep increasing. Ad Hoc network is thought to be one of the most competitive candidates for the next generation mobile networks, because of its the characteristic of self-organization. Ad Hoc network is a network formed without any central administration which consists of mobile nodes that use a wireless interface to send packet data. The nodes in Ad Hoc networks can serve as routers and hosts, and forward packets for the other nodes. Nodes in Ad Hoc networks can self-organize and realize distributed management. They are widely used in military and civil communication fields such as urgent search, temporary conference and wireless home network.The research hotpots of Ad Hoc networks include: routing protocol, MAC protocol, congestion control, cross layer design, security of network, QoS guarantee, power control and management, transport layer protocol, and so on.Congestion is one of the most important restrictions of Ad Hoc networks. Nodes transmit and receive packets for theirselves and forward packets for others, some of them may become the junctions of several flows, and these nodes are very likely to be congested. Many packets may be accumulated at congested nodes and discarded finally. Congestion may lead to long delay, high overhead and low throughput in wireless Ad Hoc networks. According to analyses of congestion control methods, this paper researches on several aspects about congestion problem in Ad Hoc networks as follows.Firstly, congestion detection method for Ad Hoc networks. According to the characteristics of congestion in Ad Hoc networks, a congestion detection method for Ad Hoc networks, based on Congestion Trend Degree, is proposed. This method combines queue occupancy with input rate and output rate in MAC layer, and it indicates the rate at which the queue goes to full. This method can measure congestion level accurately and timely, and what's more it can predict the congestion tendency, it can provide accurate information for congestion control methods.Secondely, routing protocol for mitigating congestion in Ad Hoc networks. Routing protocols designed for Ad Hoc networks usually take the shortest path as the routing metric, and always neglect the influence caused by congestion in Ad Hoc networks. A routing protocol for mitigating congestion in Ad Hoc networks is presented in this thesis, called AH-RBP(Ad Hoc Routing Based on Potential). AH-RBP is a multipath routing protocol. The nodes are assigned transmission potential values by congestion detection, and packets route along the shorter path with lower congestion level based on the transmission potential. AH-RBP can mitigate the congestion influence, improve the throughput and packet delivery ratio.Thirdly, Auto rate MAC protocol. Auto rate MAC protocols can improve the channel utilization. However, When congestion occurs, if nodes transmit too much packets at a high rate in a good channel, congestion levels will badly increase. an auto rate MAC protocol for Ad Hoc networks based on congestion detection, called ARCD(Auto Rate based on Congestion Detection) is proposed. ARCD protocol selects transmission rate according to the channel condition, and determines the number of back-to-back packet according to the channel condition and congestion detection. Nodes transmit more back-to-back packets at higher rate when the channel is good and congestion level is low, and transmit less back-to-back packets at lower rate when the channel is bad and congestion level is high. ARCD protocol can improve the channel utilization, decrease the control overhead for channel contention, and mitigate congestion level.