Research On The MAC Mechanism For Wireless Ad Hoc Networks

The Ad Hoc network is composed of a collection of mobile nodes, which are connected together by their wireless links. In this environment, mobile nodes may function as communication terminals or intermediate nodes (routers). One distinguishing feature is that users, when communicating to each other, may establish a multi-hop path among these wireless links, quite different current existing networks, another distinguishing feature is that the nework topology in Ad Hoc networks may be frequently changed and dynamically adapted. Due to their high flexibility and high survival ability, Ad Hoc networks are very much suitable for the use in battlefield communications, nonetheless, more and more civil communications such as rescue operations and city bus communication are adopting Ad Hoc networks in recent years.It is known that medium access control (MAC) plays an important role on link utilization and fairness issue, Particularly in wireless Ad Hoc networs, MAC mechaims would become more complicated because of hidden-terminal and exposed-terminal occurrence and frequenct changes of network topology. These open problems motivated us to conduct a comprehensive study in this dissertation.Chapter I is an introduction of this disertation, we first describe basic concepts, primary characteristics and typical applications of wireless Ad Hoc networks; then latest progress on routing protocols, security mechaniam, and MAC are reviewed.Chapter II aims to analyze the performance of IEEE 802.11 DCF. Firstly, we introduce a Markov model on IEEE 802.11 DCF backoff algorithm. Then, we use this Markov model to quantitatively analyze the probability distribution of slot selection and saturation througput of the 802.11 DCF. At last, an enchancement scheme -- self-adaptation of contention windows -- is proposed to make this probability distribution properly uniform. The simulation results are presented.In Chapter III, after detailed analyzation of TCP stability in Ad Hoc networks, we present unstable issue of TCP flow, including the long-time route convergence and the unfairness of MAC machanism. Meanwhile, the improved schemes on 802.11 DCF and routing protocol are proposed respectively. Considering the following characteristics of wireless Ad Hoc networks, multi-hop, frequent network topology changing, we define two MAC performance parameters, fairness and network capacity utilization ratio. We quantitatively analyze the fairness of 802.11 DCF, and present a novel algorithm to make the 802.11 DCF fairer by adjusting the contention window. Moreover, a new MAC protocol (FMAC) is proposed to make the contending nodes share the wireless channel in much more fairly and more efficiently.In Chapter IV, we study Quality of Service (QoS) issue from the MAC layer. We propose an enhancement MAC scheme over wireless Ad Hoc networks, called D-PRMA. This scheme aims to reduce the collision duration, avoid the fragment and reassembly of packets, and reserve bandwidth for real-time application. Our simulation results demontstrate that D-PRMA could guarentee the bandwidth and delay of real-time services in wireless Ad Hoc networks.Final Chapter concludes this dissertion and some future works are discussed.