The Algorithm Of Back-Off And Contention Avoidance Of Media Access Control In Wireless Ad Hoc Networks

Wireless ad hoc Network is a communication network which consists of a serie of transceived nodes and it is independent of any network infrastructure. The nodes can exchange information with each other. When locating out the range of each other, they can relize multi-hops communication by the relayed nodes. Not only do the nodes assume the responsibility of terminal, but also play the role of router. Due to the shared trait of wireless channel, multi-hop communication deteriorates channel contention, which leads to the descent of the performance of network.The dissertation focuses on introducing 802.11 DCF protocol and improves the backoff mechanism of DCF in order to alleviate contention. Backoff mechanism is one of the concrete methods of resolving contention in wirless Ad Hoc network. How long can the node idle is determined by contention window, when the node sends data. So, tuning the contention window reasonably can take advantage of system resource efficiently. Not only does the backoff algorithm reduce collision probability, but also avoid the descent of channel efficiency owing to too long backoff time. At the same time, the algorithm can assume that the nodes access the channel fairly. The main research of the dissertation is as follow:1) Based on analizing the detact of IEEE 802.11 binary exponential backoff, the dissertation puts forward a novel adaptively dynamic tuning of the contention window for distributed coordination function in IEEE 802.11 ad hoc networks. The active nodes, transmission rate and contention probability are used to tune contention window, which reduce the collision and improve the network performance.2) Focusing on the backoff mechanism of IEEE 802.11e, the dissertation puts forward a novel dynamic tuning of the contention window for IEEE 802.11e enhanced distributed control function. Based on different traffic categories, the algorithm tunes the corresponding contention window, relizes the fairness and reduces the collisions.3) Firstly, the dissertation defines two concepts of virtual and real channel efficiency and offers counting method. Finally, channel efficiency-based transmission rate Control for congestion avoidance in wireless ad hoc network is proposed. Comparing the real channel efficiency in MAC protocol with the virtual channel efficiency in TCP, we can adaptively control the transmission rate in TCP by using the congestion window and the flow of TCP ACKs, in order to alleviate the load of MAC protocol before a congestion event occurs and to enhance the network performance.