Research On Key Technologies Of Congestion Control In Mobile Ad Hoc Networks

Mobile Ad Hoc network is a multi-hop, temporary autonomy system which is constituted by a group of mobile hosts with wireless receive-send equipment. All nodes in the network are equality in status. Each mobile terminal has two functions of router and host. It is widely applied to special military field and civil field thanks to its features of no-infrastructure and self-organization. But its features of node mobility, media wireless, control mechanism of MAC, route protocol in network layer, path asymmetry are likely to induce poor performance of Ad Hoc network such as high bit error rate (BER), network partition, route change frequently, et al. These may result in packet loss. The TCP performance will be affected badly under the adoption of the TCP congestion control mechanism that applied in wired network, which treats a packet loss as an indication of network congestion, and then invokes congestion avoidance or slow start. This dissertation researches the Ad Hoc network operation mechanism of data link layer, network layer, transport layer, and explores congestion control strategy from them respectively. The main research content is as follows:(1)Analyzes the mechanism of MAC layer congestion, and proposes an adaptive auto rate-congestion control (AAR-CC) protocol. The AAR-CC improves the AAR protocol with POCC algorithm and realizes the adaptive rate congestion control mechanism. It chooses the source node's data sending rate according to the state of link congestion. The AAR-CC protocol just adds one byte for CI (Congestion Indicator) and makes the MAC frame only a small change, so it is a scalable low-overhead algorithm. The AAR-CC can enhance the utilization of channel quality by back-to-back data transmission, and it can effectively alleviate congestion level and improve end-to-end network saturation throughput.(2)Analyzes the backoff mechanism of data link layer, improves IEEE 802.11 Distributed Coordination Function (DCF), and proposes a real-time traffic backoff adaptive congestion control (RBA-CC) protocol. The RBA-CC protocol has two important improvements. Firstly, it applies Quick-exchange and Fast-forward mechanisms to multi-hop Ad Hoc network. This alleviates MAC layer inter-stream contention and intra-stream contention, and improves the transmission efficiency of real-time traffic. Secondly, it gets congestion information (CI) from queue instantaneous state of node, and then uses ACK frame piggybacking to transfer the CI to the preceding node, realizes adaptive backoff congestion control mechanism. The simulation results show that the RBA-CC protocol can decrease the delay of real-time traffic, remarkably increase the network end-to-end saturation throughput. (3) Analyzes the route protocols in network layer, presents a concept of local congestion table, improves Dynamic Source Routing (DSR) protocol and Ad Hoc on Demand Distance Vector (AODV) protocol on the bases of bypass, local congestion table, hello message systems, and then proposes two routing protocols of Congestion Adaptive Dynamic Source Routing (CA-DSR) and Congestion Adaptive Ad Hoc on Demand Distance Vector (CA-AODV). In the two protocols agreement, a primary node can be aware of network congestion immediately by using local congestion table. When congestion happens, the previous node on the primary route tries to find a bypass path on-demand to go around the congestion node. When data flow gets to the node with a bypass, it will be distributed in primary path and the bypass proportionally. As a result, it can realize the congestion adaptive function. They update their local congestion table by hello message. The simulation results show the two protocols can obtain less average delay from end to end, higher data delivery ratio, less normalized overhead.(4)On the basis of analysis of present improvement schemes of Ad Hoc network, this dissertation proposes an improving TCP congestion control algorithm for mobile Ad Hoc networks through employing end-to-end identification (IADTCP). A new method of slow-start strategy is proposed to resolve the problem that the congestion window grows unsmooth. The IADTCP uses two metrics IDD(Inter Delay Difference, the delay difference between consecutive forward packets) and STT (Short Term Throughput) to perform multi-metric joint identification for congestion detection, and uses PLR (Packet Loss Ratio) and POR (Packet Out-of-Order Delivery Ratio) to identify the network states of CHANNEL ERR and ROUTE CHANGE. Then the sender side will take advisable measures in light of network state information carried by back ACK package. The simulation results have validated the feasibility and efficiency of the proposal.