Doctor of Philosophy
School of Electrical, Computer and Telecommunications Engineering
Ware, Christopher Graham, An investigation of capture effects in IEEE 802.11 networks, Doctor of Philosophy thesis, School of Electrical, Computer and Telecommunications Engineering, University of Wollongong, 2001. http://ro.uow.edu.au/theses/1941
Local area wireless networking is quickly becoming a preferred technology for many networking applications. This is driven by the introduction of the IEEE 802.11 and ETSI HiperLAN standards, combined with the promise of explicit quality of service mechanisms designed to support exciting new services over wireless media. These developments have the potential to fundamentally alter the way a user interacts with the network, as well as opening a raft of potential new applications able to exploit the inherent benefits of wireless media.
Incorporating both the wireless local area and mobile ad hoc network paradigm, this thesis presents a comprehensive investigation of the impact of capture effects on the fairness properties of the IEEE 802.11 wireless Medium Access Control ( MAC ) protocol in topologies involving hidden terminals. Through empirical investigation, a strong relationship between the relative received signal power of contending hidden connections and the fairness behaviour of the network is identified. A signal power difference of greater than 5dB between competing connections was observed to result in a channel capture state for the stronger connection. This behaviour has a significant impact on the ability of the MAC to provide fair service to all contending nodes, and in extreme circumstances can result in extremely poor performance for the weaker hidden nodes in the network. The signal strength dependent capture behaviour identified in this thesis has been presented within the IEEE 802.1 Working Group, having significant influence on the design of the Hybrid Co-ordination Function centralised QoS MAC.
Analytical investigation of the impact of common spreading code interference confirms the empirical observation, with a received signal power difference of greater than 2dB found to be sufficient to result in the observed bias. Simulation tools such as ns-2 play a significant role in the development of new wireless protocols and services. The ability of current receiver models to accurately match empirical data is investigated. A new model based on the physical operation of an IEEE 802.11 interface is introduced in response to inadequacies identified in current receiver models. This model, termed Message Retraining, is shown to provide a significant improvement over current receiver models in terms of the ability to match the modem capture characteristics of an IEEE 802.11 network interface.
Finally, techniques designed to prevent unfair behaviour resulting from relative signal power dependent capture effects are presented. These techniques are able to operate within a distributed or centralised MAC . An algorithm is developed employing the relative observed signal power to determine a probability variable for each identified hidden neighbour. This variable is then employed by one of three separate techniques designed to provide additional transmission opportunities for hidden nodes at a relative disadvantage. Each scheme is shown to be able to significantly improve the fairness characteristics for hidden connections, preventing stronger hidden hosts from dominating the radio resource. However, the three schemes are differentiated on the basis of the impact on aggregate throughput, implementation complexity, and flexibility.