【作者】 虞万荣；

【导师】 周兴铭；

【作者基本信息】 国防科学技术大学 ， 计算机科学与技术， 2006， 博士

【摘要】 近年来,随着研究的深入和相关硬件技术的发展与成熟,无线自组网技术在实际应用中得到越来越多的部署。无线自组网是由无线通信节点通过分布式协议连接而成的系统,具有独立性、多跳性、分布性和移动性等特点。介质访问控制协议(Media Access Control,MAC)是无线自组网协议栈的重要组成部分,决定了节点如何通过共享的无线空间信道发送和接收报文。MAC协议能否高效的利用有限的无线信道资源对无线自组网的性能有决定性的影响。基于随机竞争的MAC协议开销低,适合在无线自组网环境下单个节点上实现且实现开销不高,因而成为无线自组网MAC协议研究中最主流的技术。作为基于随机竞争的无线自组网MAC协议最典型和最成功的代表,IEEE 802.11协议在研究和实际应用中使用最为广泛。但是,IEEE 802.11通过RTS/CTS控制报文预约信道的通信方式牺牲了邻近节点可能的并发传输机会,导致网络的吞吐量不高。无线自组网共享信道的接入方式决定了节点在通信过程中不可避免的面临流内竞争乃至流间竞争,而IEEE 802.11协议处理这些MAC竞争不够完善是导致其在无线自组网多跳环境下性能不够理想的主要原因。围绕无线自组网MAC协议性能优化这个关键问题,本文深入分析了影响MAC协议性能的本质原因,针对现有工作存在的对并发传输支持不够理想、对流内竞争和流间竞争等MAC层竞争处理不够完善等不足之处,提出了相应的解决方法。本文主要工作包括:1.MAC层并发传输机制研究。在无线自组网中,在给定区域内同时进行的传输越多,则整个系统的吞吐量就越高。因而,在确保彼此不破坏对方传输的前提下,在一定的空间范围内调度尽可能多的传输是提高无线自组网MAC协议吞吐量的有效途径之一。在无线自组网中,无线通信节点在接收报文时可以容忍一定程度的干扰存在,这就是无线通信所特有的捕获效应。利用无线通信的这一内在特征,本文基于现有符合IEEE 802.11规范的硬件,提出一种用于无线自组网的并发传输MAC协议——CTMAC。为了实现并发传输,CTMAC协议在控制报文和数据报文间插入附加控制时隙,并让节点在成功交换控制报文后进行相应的等待,直到附加控制时隙结束才开始数据报文的发送。配合附加控制时隙机制的引入,CTMAC协议的控制报文只是有条件的使邻居节点静默,以便邻居节点有更多的机会交换控制报文并调度传输。为了确保并发调度的传输可以互不干扰的成功进行,本文对RTS/CTS控制报文机制进行扩展,并在控制报文中引入必要的冲突避免信息作为并发调度的依据。为了更充分的利用无线自组网内潜在的并发机会,本文提出一种新颖的ACK报文串行化机制,有效隔离了不同传输的DATA报文和ACK报文,在提高协议并发度的同时极大的简化了协议的设计。由于附加控制时隙的长度对协议的性能有重要的影响,本文提出一种附加控制时隙自适应调整技术,节点可以通过报文级的调整对周围的情况做出及时的反应,优化自己的行为。模拟结果表明,CTMAC协议突破了传统的基于CSMA/CA机制的MAC协议对并发传输的限制,在网络中存在并发可能时明显的提高了系统的吞吐量。2.多跳通信环境下流内竞争问题研究。无线自组网分布、自组的特性决定了其中的流量以多跳流量为主。多跳通信路径上的相邻节点不可避免的面临流内竞争问题,制约了MAC协议的性能。本文在深入分析多跳通信流本质特征的基础上,着眼于流内竞争问题的解决,提出主动等待MAC协议——VWMAC。VWMAC协议采用一种自我克制策略,节点在成功发送DATA报文后根据所发送DATA报文的长度和与多跳路径上邻近节点的位置关系主动的等待一段时间,并在等待期间切换到睡眠状态以节约宝贵的电池能量。本文得出了节点主动等待时长计算的经验公式,并通过模拟实验进行正确性验证。模拟结果显示,VWMAC协议较好的解决了流内竞争问题,与已有协议相比,VWMAC提高了多跳通信流的端到端吞吐量,降低了端到端延迟并改善了节点的能源使用效率。3.MAC层竞争动态处理机制研究。在MAC层,无线自组网节点同时面临流内竞争和流间竞争等多种竞争。对竞争和冲突的处理是否及时和主动在很大程度上决定了MAC协议的性能。针对传统的二进制指数回退机制存在的问题,本文提出一种动态等待MAC协议——DWMAC,使节点根据自己所面临竞争的激烈程度动态的调整自己参与无线信道竞争的行为。DWMAC协议下,节点在成功发送DATA报文后将主动进行等待,等待时长由节点动态的根据其面临的竞争程度和相邻节点所声明的信道占用时间决定。在动态等待期间,节点放弃对信道的主动接入,但可以被动的响应CTS报文并接收数据。位于多条多跳通信路径上的交叉节点需要更多的机会占用信道进行报文转发。对此,本文提出一种交叉节点标识机制,根据交叉节点需要转发的报文数量来确定该节点的动态等待时长。利用网络内通信流量在时间上的内在联系,本文根据节点周围竞争的历史信息对动态等待时长的计算进行平滑优化,进一步提高了协议的性能。模拟结果表明,DWMAC协议能够更加高效的处理MAC层竞争和冲突,主动和及时的调整节点的竞争行为,提高了无线自组网吞吐量和报文延迟方面的性能。更多还原

【Abstract】 In recent years, with the in-depth research of related technologies and the improvement in hardware, there are more and more deployed wireless ad hoc networks in practice. A wireless ad hoc network is a network temporarily and spontaneously established by wireless communication nodes through distributed protocols, with the characteristics of infrastructureless, multihop, distributed and mobility. As one important part of the wireless ad hoc network protocol suite, the media access control (MAC) protocol determines how one wireless communication node transmits and receives packets through wireless channel. Thus, the MAC protocol largely determines the network performance which can be measured in terms of throughput, transmission delay and energy consumption, etc.Focus on how to improve the wireless ad hoc MAC protocol, this thesis first deeply analyses the essential factors which determine the performance of a MAC protocol. Then, this dissertation proposes and evaluates new solutions for concurrent transmission, intra-flow contention and inter-flow contention.The major contributions of this thesis include:1. Research on concurrent transmission mechanism. In a given area of the wireless ad hoc network, more scheduled transmissions means higher throughout. So, without violating each other, it is a promising approach to improve the throughput of a wireless ad hoc network MAC protocol by scheduling as more as possible transmissions in a given space. In wireless communication, a packet can be received successfully even if there exist other overlapping or interfering packets. This phenomenon is the so called capture effect. Based on the capture effect, this dissertation proposes a concurrent transmission MAC protocol (CTMAC), which can work on existing IEEE 802.11 hardware.CTMAC inserts additional control gap between the transmission of control packets (RTS/CTS/ATS) and DATA packet. After the successful exchange of control packets, a node waits until the finish of current additional control gap. Additionally, the control packets of CTMAC will not always silence the neighboring nodes. Thus, the additional control gap allows a series of RTS/CTS exchanges to take place between the nodes in the vicinity of the transmitting or receiving node to schedule possible multiple, concurrent data transmissions. To safeguard the concurrent data transmissions, collision avoidance information is included in the control packets and used by the neighboring nodes to determine whether they should begin their transmissions. Also, to isolate the possible interference between DATA packets and ACK packets, a novel ACK sequence mechanism is proposed. Because the size of additional control gap has a significant impact on the performance of the CTMAC, this thesis proposes a packet-level adaptive mechanism to adjust the length of additional control gap according to the number of concurrent transmissions in the vicinity. Simulation results show that a significant gain in throughput can be obtained by the CTMAC protocol compared with the existing work including the IEEE 802.11 MAC protocol.2. Research of intra-flow contention problem under multihop scenarios. Due to the characteristics of infrastructureless and multihop, most of the traffics in a wireless ad hoc network are multihop. To fulfill the forwarding of the received packets, the neighboring nodes on a multihop route will contend with each other inevitably. This is the so called intra-flow contention problem, which limits the performance of the MAC protocol. By analyzing the essential characteristics of a multihop traffic, this dissertation proposed a novel voluntary waiting MAC protocol (VWMAC) to solve this problem. Through voluntary waiting by wireless hosts according to the length of data packet transmitted and the distance between neighboring nodes on the multihop path, VWMAC uses a surprisingly simple strategy to achieve significant performance enhancement. This thesis gains the equation to compute the length of voluntary waiting time and validate its correctness through simulation. In addition, voluntary waiting of participating nodes present opportunity for these nodes to transit to sleep state to conserve scarce energy. Our simulation results show that VWMAC outperforms IEEE 802.11 and existing approaches in terms of throughput, transmission delay and energy efficiency.3. Research of mechanisms for dealing with the MAC layer contentions. At the MAC layer, a wireless communication node faces various kinds of contentions which include intra-flow contention and inter-flow contention. The MAC layer contentions and the resulting collisions have significant impact on the performance of a MAC protocol. Dealing with these problems quickly and actively is necessary for a desirable wireless ad hoc MAC protocol. In this thesis, a novel MAC protocol, dynamic waiting MAC (DWMAC), is proposed to manage the contending actions of a wireless node dynamically according to the contention level.In DWMAC, a node waits voluntarily after the successful transmission of a DATA packet. The duration of waiting is determined jointly by the contention level in the vicinity and the length of the contended transmissions. In the state of dynamic waiting, a node loses the right of transmitting, even if it has packet waiting in it’s transmit queue. However, it can receive passively from neighboring nodes when RTS packet is received.The cross node which is on the routes of multiple multihop traffics needs more consideration, because it requires more chances to forward the received packets. In this dissertation, a marking mechanism is proposed to assign the cross node higher priority during the contention for the shared channel. Because the flows in the wireless ad hoc network have some intrinsic relationship, DWMAC uses an ARMA Filter method to utilize the history information and improve the performance further. Compared with existing solutions, DWMAC deals with the MAC layer contentions more effectively. The simulation results show that DWMAC protocol is able to adjust the action of a wireless communication node timely and actively, resulting significant improvements in throughput and packet delay.更多还原