Study On Medium Access Control In Wireless Ad Hoc Networks

For its prospect of application in military and civilian fields, wireless Ad Hoc network has been a focus in current network research. Due to the prevalence of multimedia applications, Quality of Service (QoS) issue has been regarded as a hot spot in Ad Hoc networks. But it is a challenge to support QoS in Ad Hoc networks for its mobile characteristics, distributed multihop, limited bandwidth. Ad Hoc networks'support for QoS involves all protocol layers. Medium Access Control (MAC) layer works on the physical layer and directly controls the transmission and receiving of packets, so it is one of the most important layers for QoS and many researchers focus on MAC layer's support for QoS.This thesis focuses on MAC schemes, including the model building and the backoff functions of MAC layer protocol. The main work is as follows:1) With G/G/1 queuing theory, aiming at the general circumstance of unsaturated wireless networks, AMUWN (Asymmetry Model for Unsaturated Wireless Networks), which is a MAC layer analytic model, is put forward. With AMUWN, the delay, delay jitter, throughput and packets drop probability in the MAC layer of unsaturated one-hop and multi-hop Ad Hoc networks can be deduced. The model is evaluated by NS2 simulation software.2) With AMUWN, the thesis analyzes the delay, delay jitter and packets drop probability in unsaturated one-hop and multi-hop Ad Hoc networks with IEEE 802.11 DCF as their MAC layer protocol. In multi-hop Ad Hoc network, increasing the maximum retry limit can dramatically decrease the packet drop probability, but there are few effects on the delay and jitter. Meanwhile, increasing the minimum contention window of a given node can increase its delay and jitter and the packet drop probability and decrease the packet drop probability of nodes that are out of its transmission range, but their delay and jitter are hardly affected.3) With AMUWN, the thesis analyzes the delay, delay jitter, throughput and packets drop probability in unsaturated one-hop and multi-hop Ad Hoc networks with IEEE 802.11e EDCA as their QoS supported MAC layer protocol. The NS2 is used to evaluate the performance of the proposed model. The model provides an analytic method of studying wireless Ad Hoc networks'support for QoS.4) An adaptive back-off function CTPB (Collision and Transmission Probabilities based Backoff) is proposed. According to the analysis of AMUWN, the delay and jitter and the throughput depend on the minimum contention windows of IEEE 802.11e EDCA and the packet arrival rates. But the minimum contention window of IEEE 802.11e EDCA is static without the consideration of the network load and the busyness of the channel. So, CTPB is proposed. Simulation results show that this function can improve the delay performance of EDCA without impairing the throughput in Ad hoc networks.5) The throughput evaluation of backoff functions. According to the analysis of AMUWN, the collision probabilities of nodes in Ad Hoc network influence the performance of the MAC protocol. For their randomness, the backoff functions are adopted to resolve the collision traditionally. In this thesis, a one-dimensional Markov chain model is used to analyze the saturation throughput in Ad hoc network for generalized exponential, linear and polynomial backoff functions and these backoff functions with maximum retry limit. The analysis provides a basis for the performance optimization of MAC protocol in wireless Ad hoc network.