Research On MAC Mechanisms And Schemes For Supporting QoS In 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 network 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 communication, 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 networks, MAC mechanism would become more complicated because of hidden-terminal and exposed-terminal occurrence and frequent changes of network topology. These open problems motivated us to conduct a comprehensive study in this dissertation.Chapterâ… is an introduction of this dissertation, we first describe basic concepts, primary characteristics and typical applications of wireless Ad Hoc networks; then latest progress on routing protocols, security mechanism, and MAC are reviewed.Chapterâ…¡aims to analyze the performance of IEEE 802.11e EDCA (The Enhanced Distributed Channel Access). EDCA mechanism in IEEE 802.11e MAC protocol is proposed to support prioritized QoS on the basis of the Distributed Coordination Function (DCF) in IEEE 802.11. Firstly, We first build a new Markov chain model for the EDCA. The model takes into account both the idle state that represents that there are no packets to be transmitted and differentiation based on different AIFS-value for different traffic. We can use the signal transfer function of the generalized state transition diagram to derive a probability distribution of the MAC layer service time and analyze the performance of EDCA from a non-saturated channel to a saturated medium by using presented M/G/1/K queuing model. We observed a good match between the analytical model and simulation which conforms the veracity of proposed model. Simulation and theoretical results show that despite providing prioritized QoS, the EDCA still can not support strict QoS for real-time application. On the other hand, it is inevitable that there is unfairness of channel access to prioritized traffic due to EDCA. Then, we present a novel admission control scheme for multi-priority wireless ad hoc networks. Based on bandwidth allocation scheme, we calculate the bandwidth prediction function and two upper bounds that guarantee the quality (QoS) of admitted real-time flows. Proposed scheme protects the throughput of active flows and prevents QoS degradation due to flow's number increases. Best effort traffic doesn't hurt any existing real-time flows while it fills the bandwidth that is not used by real-time flows. Simulation results show proposed admission control is feasible. At last, this chapter assesses the effectiveness of the IEEE802.11e access mechanism in multi-hop wireless networks by quantitative research and simulation results. Simulation and theoretical results show that there is a great restriction in multi-hop wireless networks using 802.11e protocol despite providing prioritized QoS. It is very necessary to investigate the scheme which can resolve hidden nodes problem to improve performance of 802.11e EDCA in multi-hop wireless networks.In Chapterâ…¢, we investigate adaptive p-persistent MAC scheme for dynamic optimization of the channel utilization based on the QoS differentiation. Firstly, we propose an adaptive p-persistent MAC scheme, named QDA-MAC (QoS differentiation based adaptive MAC scheme), for WLAN to maximize the channel utilization and provide the service differentiation among different traffic stations. Specifically, different from the previous works, the proposed scheme does not need to estimate the number of active stations for each priority class and still achieves the channel utilization close to its optimal value by exploiting a new parameter, persistent factor, whose optimal value can dynamically follow the change of the load based on a simple estimation of the network status. At the same time, the transmission probability of each priority class can be updated by optimal persistent factor. Simulation and numerical results show that QDA-MAC can achieve much higher channel utilization and shorter delay than standard IEEE 802.11 DCF and IEEE 802.11e EDCA in all different network environments. Secondly, we derived an AIFS-based multi-class model to study the saturation channel throughput on the basis of the operation mechanism of IEEE 802.11e EDCA supporting service differentiation. The proposed model is calculated numerically and validated against simulation results, we observed a good match between the analytical model and simulation. In particular, we compared the proposed model with Xiao's Markov chain model under same configures. Based on the proposed model, we research quasi-optimal condition to avoid the computational complexity but still maintain channel throughput close to its optimal value. Finally, we propose an adaptive p-persistent MAC scheme, named DPS (Dynamic Parameter-tuning Scheme), to assign appropriate different transmission probabilities of different classes. Through DPS, it is feasible to provide service differentiation and achieve targeted throughput ratio among different classes, at the same time maximize the total channel throughput. Simulation and numerical results show that DPS can effectively achieve the performance goal under a variety of network conditions, and achieve higher channel throughput than standard IEEE 802.11e EDCA in all different environments.In Chapterâ…£, we comprehensively summarize and deeply analyze the researches of recent years on the MAC mechanisms that support resource reservation in wireless ad hoc networks. Based on the analysis, this chapter discusses the problems that must be resolved to support QoS in wireless ad hoc networks, and points out the direction to the research in the future. And then, based on the theory of block designs, this paper gives a new MAC scheme, Balanced Incomplete Block Design based MAC scheme (BIBD-MAC), which can reserve bandwidth for real-time traffics. The proposed scheme guarantees conflict-free transmission slots for each node in each frame and maximizes the minimum throughput. To improve channel utilization, enhanced BIBD-MAC (E-BIBMAC) is proposed to resolve possible collisions in assigned slots and utilize non-assigned slot by carrier sense. Numerical results show that the proposed schemes can outperform existing schemes in achieving a higher minimum throughput and lower access delay.Final Chapter concludes this dissertation and some future works are discussed.