移动Ad Hoc网络的跨层优化拥塞控制

基于网络协议层框架充分分析了TCP应用到Ad Hoc网络导致性能下降的原因,包括:物理层中易损耗的无线信道, MAC层中的过度竞争和不公平接入,网络层中节点移动引起的频繁路由失效,传输层中TCP采用的不合适机制,包括基于窗口的传输、基于数据包丢失的拥塞指示,拥塞窗口的慢启动和AIMD、对ACK自定时的依赖;提出了全新的适合Ad Hoc网络特性的跨层优化拥塞控制协议CCOC(cross-layer optimal congestion control).CCOC运用了跨层设计框架来改进MAC层的接入信道公平性、检测虚假链路失效、减少路由失效次数、加快路由切换后的重启动、运用SACK实现可靠传输和实施非线性优化论指导设计的自适应优化策略;描述了CCOC的协议体系结构;实施了详细的NS仿真实验,仿真结果表明,在几乎所有的仿真场景和移动环境下,CCOC比TCP和ATCP在许多重要性能指标,如吞吐量和公平性方面都有了明显的改进.

Cross-Layer Optimal Congestion Control Scheme in Mobile Ad Hoc Networks

This paper identifies all factors impairing TCP performance based on network protocol hierarchy: Lossy wireless channel at the physical layer; excessive contention and unfair access at the MAC layer; frequent routing changes due to node mobility at the network layer; the fundamentally inappropriate mechanisms of TCP at the transport layer, including window-based congestion control, loss-based congestion detection, slow-start and AIMD (additive increase/multiplicative-decrease) of congestion window; reliance on ACK-clocked characteristics. Then, it designs a novel cross-layer optimal congestion control (CCOC) protocol tailored toward the characteristics of ad hoc networks. Cross-Layer design framework is applied in CCOC to improve fair access at MAC layer, to detect false link failure, to reduce the number of route failures, to quick-start during route changes, to transmit reliably based on SACK, and to implement the adaptive optimization strategy guided by the nonlinear optimization theory. Then, this paper outlines the protocol framework of CCOC. Finally, the extensive packet-level simulations is implemented in NS2 environment. The simulation results show that CCOC significantly outperforms TCP and ATCP in many important performances such as throughput and fairness, under a variety of scenario and mobility conditions.