Ad Hoc Network Mac Layer Fairness In Analysis And Improvements

Wireless ad hoc network is a multi-hop and temporily self-rule system composed by a group of mobile nodes such as router with many terminals and mobile communication equipments. It can be used in many environments, for example, military, law executing, and disaster rescuring fields. Ad hoc has increasingly become a very active research filed in recent years.IEEE 802.11 is adopted as a standard of MAC level in Ad Hoc network. At the basis of any access to the channel in IEEE 802.11 is the so-called distributed coordination function (DCF) which adopted BEB algorithem. DCF has an advantage in increasing the network goodput, but it can bring unfair among different nodes. In short, when there are N network nodes competing with each other, the backoff algorithm adopted by IEEE 802.11 tries to adjust the channel access attempt probability of each node such that it approaches the optimal (theoretical) success probability of 1/N, within the shortest possible time interval, thus effectively achieving very high throughput. In the context of wireless LAN where all stations are within radio reach of each other this method achieves its goals perfectly. However, when the network topology is not symmetric, and there are terminals that are hidden from each other such goals are not achieved by the adopted technique, and some stations are at a disadvantage when accessing the channel. This unfairness can be very marked when the traffic load is high.There have been some methods to improve the fairness of MAC, but these methods all need exchange information among nodes or know the topology of network. So in this paper, we bring forward a new backoff algorithm. This algorithm aims to achieve fair channel access without knowledge of the network topology, self adpating to changes in the traffic load as well as topological changes in the network. Through adjust time interval and window size dynamiclly, decrease the issue encounted in IEEE 802. 11. To validate the algorithem, we analyse the implementation of MAC layer in NS-2 simulation system and pay more attention to the implementation of BEB algorithm. We change BEB algorithm with imporoved algorithm, simulatethe two algorithms respectively and implement the simulation work. Based on the simulation results, we analyze and compare two performance parameters of network, fairness index and goodput. The analysis presents that his algorithm improves the fairness of Ad hoc network.