Perfectionnement des mécanismes d’accès au médium du standard IEEE802.11 pour la gestion de la qualité de service

Abstract

This thesis focuses on the study of the IEEE 802.11b DCF protocol, specifically analyzing the problem of frame loss due to channel noise, which has a direct impact on IEEE 802.11 network performance. To address this problem, we propose a new analytical model based on a three-dimensional Markov chain, which integrates two key mechanisms: packet fragmentation (PFM) and fragment retransmission (DFR/RCRM) without channel reservation renewal. Our model has several assumptions that make it more realistic: we assume that the backoff counter is blocked when the channel is busy, and retransmissions for control frames (RTS and CTS) and data frames are limited according to the SSRC and SLRC thresholds, respectively. In addition, we make a fundamental distinction between the reasons for the loss of various types of frames during the transmission process. Control frames (RTS and CTS) can be lost due to both collisions and channel noise, while data and acknowledgment frames are affected only by noise. The results of our analysis demonstrate that implementing the DFR/RCRM mechanism significantly improves saturation throughput, especially in a highly noisy environment. We also demonstrated that fragment length is a key factor directly affecting protocol performance: long fragments are more efficient in a low-noise environment, while short fragments, combined with repeated retransmissions, perform better in a very noisy environment. In conclusion, our contribution provides a more realistic model and highlights the importance of properly combining fragmentation and optimized fragment retransmission to enhance the robustness and performance of the IEEE 802.11b DCF protocol against channel disturbances.