Research On Busy Tone Based Mac Protocol In Ad Hoc Network

Ad hoc network, also known as MANET, is a kind of network which consists of a set of nodes with wireless device for transmitting and receiving and can be quickly set up. As it has the features such as multi-hop, dynamic network topology, distributed control, and does not rely on base stations, it has a broad application prospect and has been subject to academic and industrial attention.Media access control protocol is used to coordinate nodes to access the shared channel and reduces conflicts, so it is a key technology to ad hoc network. Existing MAC protocols like IEEE 802.11 is widely used in ad hoc network, however, 802.11 will bring about the unfairness of channel access, hidden terminal and exposed terminal problems. Many studies have tried to improve the performance of ad hoc network, whereas, these MAC protocols usually consider only an ideal network environment, namely, the receiver's interference range is equal to its transmission range, thus can not solve the large interference area problem.As busy tone can be successfully received in the carrier sensing range, it will effectively solve hidden terminal and exposed terminal problems in the large interference area once applied to the MAC protocol, which has been a current research focus. In this paper, we research the MAC protocol from the following aspects: Firstly, by analysising the shortcomings of the 802.11 binary exponential backoff algorithm, we propose an improved backoff algorithm. By enlarging the value of initial contention window, the algorithm could reduce probability of packet collision. Once a node has occupied channel successfully for several times, it will take the initiative to backoff to ensure equitable use of channel resource for all the nodes. Secondly, by analysising the large interference area problem, a variable frequency of dual busy tone based MAC protocol is proposed, which codes node's id using frequency and duration of busy tone. Other nodes which intending to send packets obtain transmitter-receiver's id, then evaluate whether they are within the receiver's interference range, thus determine whether to send packets or not. Analysis and simulation show that, the proposed protocol can prevent the hidden terminal in the large interference area and also alleviate the exposed terminal problem. Finally, we make an analysis of the interference accumulation phenomenon in ad hoc network and propose a Hexagon model based MAC protocol. The protocol adjusts the transmission power of receiver's busy tone according to conflicts of data packet. When receiver fails to receive data packet, it will enhance the busy tone transmission power to enlarge interference control radius, so as to receive data packet successfully. Analysis and simulation show that, the protocol can improve the probability of the receiver to receive packet correctly in the case of multiple interference source, and better alleviate the hidden terminal problem in the large interference area.