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UMINF 08.01

Distribured Multiple Access and Service Differentiation Algorithms for Wireless Networks (PhD)

Communicating over a wireless channel poses many unique challenges not found in wired communication because of the special characteristics of th e wireless channel. The capacity in a wireless network is typically scarce as a result of the limited bandwidth and many distinct phenomena, lik e attenuation and interference, that work destructively on the received signals.

The Medium Access Control (MAC) layer is responsible for sharing this limited resource among the users. This allocation problem should be handle d by considering the Quality of Service (QoS) requirements of each user as to maximize the utility. Efficient MAC algorithms are crucial in mini mizing collisions between transmissions and thus achieving high utilization of the channel.

This thesis focuses on conflict resolution and service differentiation algorithms for wireless local area networks, where there is no central co ntrol of the channel and each sender independently contends for access.

In Part { m I}, we study three approaches to improve the IEEE 802.11(e) standards with focus on QoS. In the first approach, utility functions a re considered, that model application preferences, to achieve service differentiation and maximize the aggregated utility. We provide algorithms for two subsidiary problems that arise from the maximization problem, and show that a near--optimal solution is found. In the second approach a collision detection algorithm for multicast transmissions is proposed, that increases the reliability for multicast compared to the protected u nicast traffic. The third approach is an improved MAC algorithm for the QoS standard IEEE 802.11e. The improved algorithm outperforms the standa rd and achieves close to optimal performance for large number of scenarios, which significantly reduces the need of adjusting the contention parameters.

In Part { m II}, we focus on channel bursting protocols that use noise bursts to resolve channel conflicts. These protocols is capable of achie ving very low collision probability. We propose two new bursting protocols, that achieve very high channel utilization, and show that the bursting technique has good fairness properties and provides efficient support for service differentiation. We also show that it is possible to reduce the number of bursts without loosing performance.

In Part { m III}, the optimal backoff distribution that minimizes the collision probability is derived. We then propose a heuristic backoff dis tribution with similar properties that yields high channel utilization. An extension for service differentiation is provided where the sizes of the backoff windows are adjusted.


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Thomas Nilsson

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Entry responsible: Anne-Lie Persson

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