Research On Media Access Control For Mobile Ad Hoc Networks

The wireless mobile Ad Hoc network is a dynamic mobile communication network consisting of a number of terminals linked by the wireless channel without any fixed communication infrastructure. Wireless mobile Ad Hoc network is considered as an important part of the personal communication system in the future due to its advantages of flexibility and rapid networking capability. The key elements of mobile Ad Hoc network include Media Access Control (MAC), routing, multicasting, power control, QoS and security, etc. This thesis focuses mainly on the investigation of MAC protocols and their fairness, efficiency and related performance.In the design of the wireless mobile Ad Hoc networks, access fairness and throughput are important metrics to evaluate a MAC protocol. By investigating the backoff strategy to eliminate the conflicts in wireless MAC access, this thesis firstly presents a backoff fairness algorithm based on queuing theory. In this algorithm, the backoff time is the same to all nodes (including the new nodes). Before sending the RTS packet, one node should sense the channel. If the channel is idle, the backoff delayer would be activated. If the channel is busy, the node would keep sensing the channel until the channel is idle to activate the backoff delayer. During the delay period, the node keeps sensing the channel and does not send the RTS packet. If the state of the channel switches to busy mode during the delay, the backoff delayer would be paused and the node would keep sensing the channel. The remaining time in the delayer is the new backoff delay in the next competition. Only when the channel is idle and the backoff delay is 0, the node can send the RTS packet. After a successful interaction, the backoff delayer is reset. Our investigation shows that the proposed strategy has excellent access fairness.Secondly, a fairness medium access protocol for mobile Ad Hoc networks with a strategy based on counting the defeats of node competition (invalid RTS) is proposed. In the MAC protocols based on the RTS/CTS handshake mechanism, when the networks load is heavy, one node would send the RTS packet many times before access the channel to send the data packets. Before the node accesses the channel successfully, the number of times the node sent the RTS packet in some extent reflects how busy the network is. This strategy is proposed to assign dynamically the initial value of the backoff counter, hence realizing fair medium access control in mobile Ad Hoc network. The investigation shows that, the proposed strategy has excellent access fairness and can increase the network throughput, as well as guarantees the system quality of service (QoS), due to the fact that it reflects effectively the characteristic of the source nodes.Then, an improved FAMA (Fair Floor Acquisition Multiple Access) protocol is proposed by the author. FAMA-NCS (FAMA Non-persistent Carrier Sensing) protocol has two characteristics, one is to eliminate the packet conflicts by extending the length of the RTS and CTS control packet and to resolve the hidden terminal problem, the other is to send several packets continuously after one successful interaction of the RTS-CTS, thus resulting enhanced network throughput. The investigation of RTS/CTS handshake mechanism and FAMA protocol show that, the proposed strategy which can adjust the packet train of FAMA protocol dynamically has excellent access fairness and satisfactory network throughput.Next, a delayed backoff window adjustment algorithm is proposed based on the active states of the neighboring nodes. In general, the conflicts is caused by the neighboring nodes. The more nodes who want to send data in a period of time, the more conflicts which will occur when the nodes communicate to each other. The active states of the neighboring nodes in some extent reflect how busy the network is. The more active the neighboring nodes are, the fiercer the channel competition is, and the busier the network will be. Contrarily, the network would be more idle. The CW (Competition Window) could be adaptively adjusted based on the number of the active neighboring nodes, thus the throughput performance of the network is significantly increased.Finally, this thesis presents a new medium access protocol Based on the Active States of the Neighboring Nodes (BASNN) for Mobile Ad Hoc Networks. In the shared channel of the wireless Ad Hoc network, the MAC access protocol with delayed backoff window scheme is commonly adopted. Probability backoff mechanism is used in BASNN. If the source node has a packet to send, and the active node number is n in the T cycle, the source node sends a RTS (request-to-send) packet with probability Min{α/(n+1),1} or backoff with probability 1-Min{α /(n+1 ),1}. Our analysis and simulation results show that the throughput performance can be significantly increased.