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Past Research Projects


MAC Layer Fairness in IEEE 802.11 based Wireless Mesh Networks

Achieving MAC layer fairness in multi-hop wireless mesh networks is a very challenging issue. In this thesis, the fairness problem in IEEE 802.11 DCF based wireless mesh networks has been studied. IEEE 802.11 based wireless mesh networks shows substantial unfairness due to load variation at different types of nodes such as mesh routers, mesh clients and mesh gateways, hidden-terminal problem, high contention and the concealed information problem. To achieve fairness, a load estimation strategy is proposed to estimate load at mesh routers and mesh clients, and that estimation is used to probabilistically tune the contention window at IEEE 802.11 DCF backoff mechanism such that the nodes with higher load get more share of the channel. Because of the variation of load at mesh clients and mesh routers, and even between mesh routers because of variation of number of active clients, the notion of proportional fairness is used here. The contention model at each node and the load information piggybacking at transmitting packets are used to find out the load at each node in a distributed way, which in turn calculates the required amount of channel share. The actual channel share at each node can be calculated based on information overhearing. To compensate the difference between the required share and actual share, the nodes enter one of the three modes, aggressive, restrictive and normal, indicating how the node should behave when contending for the medium. The contention window (CW) of the node is tuned according to the mode it enters. Thus it minimizes the difference between the required share and actual share and provides fair scheduling among the contending nodes. Simulation results show that the proposed fairness scheme works well without any loss in throughput. The proposed scheme is also implemented in the context of vehicular networks for vehicle to infrastructure communication, where the roadside access points form a wireless mesh backbone. A theoretical analysis based on Markov Chain model is used to verify the results in a generic way, that shows the proposed scheme does not affect total aggregate throughput of the network.



Tuning holdoff exponents for performance optimization in IEEE 802.16 Mesh Distributed Coordinated Scheduler

The IEEE 802.16/WiMAX mesh standard is a promising technology to support next generation wireless broadband metropolitan area networks. The medium access control (MAC) layer of IEEE 802.16 mesh supports both centralized and distributed scheduling mechanisms. The coordinated distributed scheduling mechanism uses a pseudo-random election algorithm to determine the transmission times of the nodes. The holdoff exponent is an important parameter of scheduling and determines the channel contention time of a node. In this paper, we propose a novel distributed search protocol that each node uses to select the holdoff exponent that minimizes the expected delay between its two successive transmissions in the control channel. We analyze the scheduler performance in NS-2 for various kinds of traffic including TCP, UDP and VoIP. Simulation results show that our scheme performs better than the standard algorithm and other comparable schemes in terms of throughput and delay.