| Abstract |
The last few years, wireless networks based on the IEEE 802.11 protocol, experience a tremendous growth. Their success can be attributed to utilization of unlicensed spectrum, interoperability granted by adherence to a standard, low cost, ease of installation and above all provision of untethered access to Internet resources. Nowadays, the prominent deployment scenario for 802.11 wireless networks is the so-called hot-spot scenario which provides access to nomadic users in both public (hotels, airports, etc.) and private (offices, conferences, etc.) environments. Within this scenario, mechanisms that support service differentiation, guarantee fairness and improve performance, are a prerequisite for supporting the modern demanding applications. Provision of service differentiation in wireless networks is hindered by several factors such as, location dependent channel errors, uplink-downlink unfairness and performance reduction due to the multirate physical layer. Nevertheless, a plurality of such mechanisms has been proposed at various levels in the protocol hierarchy. Few of the proposed solutions, however, provide a mechanism that can be implemented in a feasible and effective way without requiring alternation of the 802.11 medium access control (MAC) layer. In this thesis we propose a dynamic Class Based Weighted Fair Queuing (CBWFQ) scheme, properly adapted to the wireless network particularities, that differentiates service on a per node basis, improves fairness and enhances utilization of the network resources. The proposed mechanism is based on a definition of fairness inherently different from the fairness perspective of the 802.11 MAC layer. Fairness is expressed in terms of channel occupation time shares; a way that suits best the multirate physical layer of 802.11. However, a direct implementation of this fairness scheme would require significant changes at the MAC layer of 802.11. In order to avoid this requirement, we propose an approach that is based on dynamically adjusting the weights of a CBWFQ scheme and enables the implementation of our mechanism solely at the network layer. The mechanism is comprised of two algorithms that utilize cross-layer information to periodically adjust the weights of a CBWFQ scheme in a way that collectively deals with significant issues of WLANs, such as location dependent channel errors, incontrollable uplink traffic and unfairness due to multiple transmission rates. The proposed scheme was implemented on an experimental test bed and the conducted experiments confirmed its resultfulness in real life scenarios.
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Διαφοροποίηση Υπηρεσίας σε Ασύρματα Δίκτυα με χρήση ενός Δυναμικού CBWFQ Σχήματος
