| Abstract |
Wireless local area networks (WLANs) will continue to be a major growth factor for communication networks in the up-coming years. The 802.11 standard sees increasing public deployment and, given this fact, it is important to ensure that different users gain fair access to the network resources. Nevertheless, the nature of the wireless medium cannot guarantee reliable data transfer, neither fair share of the resources. The resource sharing model of the underlying 802.11 MAC protocol requires stations to continuously contend to capture the channel, in order to transmit. In infrastructure networks the unfairness problem occurs between the uplink and the downlink traffic. In multihop wireless networks, the unfairness problem is more intense due to location dependent contention, which can result in different stations obtaining a different estimate of the level of congestion. Under these circumstances, some flows can manage to increase their throughput, while others might starve. In the present thesis we argue that we can improve fairness in a wireless network, by conveying more accurate congestion information to the end systems, which should consider the traffic load within the corresponding contention area. This information, along with an appropriate packet marking algorithm, can enhance resource sharing and improve TCP's fairness, maintaining the TCP's end-to-end behavior and semantics. Our approach is based on two key ideas; first, it uses Explicit Congestion Notification (ECN), as the common end-to-end signaling mechanism, for conveying congestion information across the wireless links. Second, marking is performed using a Load-Based Marking (LBM) algorithm, where the marking probability is a function of the aggregate utilization, measured appropriately for each network topology. For the case of infrastructure networks, where an access point acts as the gateway for the traffic flowing in both the uplink and the downlink direction, we consider the aggregate traffic in both directions. For the case of multihop wireless networks, the level of congestion around a station is given by the sum of the receiving rates within the station's collision domain. The proposed approach was evaluated through NS-2 simulations. It demonstrated slightly higher fairness in the case of infrastructure networks, and noticeably higher fairness in multihop networks. On the other hand, in both network topologies, the achieved utilization remained the same, as with drop-tail queuing. Finally, the proposed approach significantly reduced the end-to-end delay and delay jitter.
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Έλεγχος συμφόρησης σε ασύρματα δίκτυα τεχνολογίας IEEE** 802.11, με χρήση του Explicit** Congestion** Notification** και ενός μηχανισμού μαρκαρίσματος βασισμένου στην εκτίμηση φόρτου
