A receiver-initiated MAC protocol with enhancements for multi-hop wireless networks

by 1981- Thind, Balvinder Kaur

Balvinder Kaur Thind Washington State University May 2004 Chair: Muralidhar Medidi Multiple channel access interference is a major cause of throughput degradation in wireless networks because of the shared channel. IEEE 802.11 MAC is a standard for medium access in wireless LANs, but suffers from channel contention and co-channel interference and thus, performs poorly. The focus of this thesis is to design and study a medium access control protocol that mitigates the effect of multiple channel interference. We propose to use a receiver-initiated MAC protocol, instead of the sender-oriented 802.11 MAC. Our protocol is based on carrier sensing and resolves collisions among senders based on a deterministic tree splitting algorithm. By doing collision resolutions, we aim to use time efficiently otherwise wasted in 802.11 MAC for random backoffs, particularly in cases when the channel contention is high. Receiver-initiated based collision resolutions guarantee that, no data packets collide at a receiver because of interference from its own senders: a major cause of hidden node problem. Further, the protocol is enhanced to have multiple subchannels by dividing the common communication channel. All the subchannels serve the purpose for both control as well as data packets. A subchannel assignment scheme is proposed to exploit the parallel transmissions that are possible in multi-channel networks. This should help in reducing the co-channel interference and thereby, improve the throughput. v In order to handle the unfairness issues associated with receiver-initiated protocols, we propose a third enhancement: an adaptive approach of deliberate transitions between receiving and sending modes. These mode transitions force nodes to spend fixed time in each mode, thereby giving a fair chance to become a sender as well as a receiver. We also present simulation results using ns2 simulator, varying several system parameters to evaluate our approach and compare it with standard IEEE 802.11. The simulation results indicate that collision resolution with multiple subchannel access provides throughput enhancement and better packet delays than 802.11 MAC. We also observe that the maximum throughput is achieved when the channel is divided into three or four subchannels irrespective of the size, shape of the network, and traffic load. The results also indicate that we further improve on throughput by doing deliberate transitions and, that the protocol has better fairness for QoS assurance as compared to standard 802.11 MAC. vi