présentée par

Wireless networks have experienced drastic evolutions in the last few years. While many aspects related to “single cell” wireless networks were already resolved, wireless multihop networks still suffer poor performances and limited capacities. This thesis aims at providing acceptable quality of service in terms of throughput and loss ratio to end users in various wireless multihop environments. Today, access control in multihop wireless networks is mainly achieved through the IEEE 802.11 protocol at the MAC layer. At the transport layer TCP, the widely used protocol, adds an end-to-end control mechanism. Essentially, the final goal of many if not all these techniques is to enhance either the global system or a particular connection throughput. However, due to the numerous challenges introduced by the wireless multihop, these two control levels, even when used together, show numerous limitations. These new challenges originate basically from the fact that the IEEE 802.11 and TCP protocols at the time of their conception were not targeting the specific constraints added by the wireless multihop environments. In fact, many aspects such as the interference, the hidden nodes, the contentions and the lossy link properties that appeared in the wireless multihop environments made the existing MAC and transport layer control mechanisms become inefficient hence inappropriate. In this thesis, we propose three control mechanisms for wireless multihop networks aiming to improve the achieved goodputs seen by a user. We first start by presenting an analytical model to evaluate the hidden terminal effects in an IEEE 802.11 network. This model is then exploited to tune MAC layer control parameters and we show that when this is done in conjunction with the TCP control, throughput enhancement can be achieved. Second, since the wireless medium gives broadcast for free, we exploit this feature to design HbH our second control technique. Considering that not all data exchange entail a TCP connection, with HbH we enforce the MAC layer control by using the broadcast nature of the wireless medium. When using HbH, every node after the transmission of a packet continues to listen on the channel to make sure that its neighbor also transmitted the packet before the former decides to discard it. HbH can be even used to detect and react to node failures in multihop wireless networks due to mobility for instance. Third, we enlarge the spectrum of our research to look at multihop Cognitive Radio