Christopher G. Ware

Unfairness and capture behaviour in 802.11 adhoc networks

We address issues with the performance of IEEE 802.11, when used in the adhoc mode, in the presence of hidden terminals. We present results illustrating the strong dependence of channel capture behavior on the SNR observed on contending hidden connections. Experimental work has illustrated that in a hidden terminal scenario, the connection having the strongest SNR is able to capture the channel, despite the use of the RTS-CTS-DATA-ACK 4-way handshake designed to alleviate this problem. Our results indicate that the near-far SNR problem may have a significant effect on the performance of an adhoc 802.11 network.

Simulation of capture behaviour in IEEE 802.11 radio modems

We investigate the performance of common capture models in terms of the fairness properties they reflect across contenting hidden connections. We propose a new capture model, message retraining, as a means of providing an accurate description of experimental data. Using two fairness indices we undertake a quantitative study of the accuracy with which each capture model is able to reflect experimental data. Standard capture models are shown to be unable to accurately reflect the fairness properties of empirical data. The message retraining capture model is shown to provide a good estimate of actual system performance in varying signal strength conditions.

TCMCA: a source-based distributed topology control algorithm for mission critical applications in mobile ad-hoc networks

Topology control in a wireless ad-hoc network allows better spatial reuse of the wireless channel and network resources. The existing topology control algorithms tend to optimise network power usage by keeping the topology connected but do not take the network application requirements into account. Mission critical applications which require explicit end-to-end bandwidth and delay guarantees may not find enough resources in the network with the existing network topology. We have devised a topology control algorithm for mission critical applications (TCMCA) in wireless ad-hoc networks, which adapts the network topology to improve the available resources for a set of mission critical applications (high priority services) in a network. TCMCA is a source based algorithm where topology control decisions are made on distributed network knowledge. The performance of TCMCA is evaluated for a static wireless network and compared against algorithms such as connect, LINT and full power transmissions. We demonstrate that TCMCA shows better support for mission critical services for varying number of mission critical applications in the network.

Provision of data (IP) services in third generation mobile networks

Third-generation mobile networks will enable services which are regularly available through fixed networks (WWW, Internet) to be offered into the mobile domain. The impact IP data services may have on the base station is, at this time, unclear. Coupled with this is a particularly poor performance characteristic achieved with TCP over (lossy) wireless links. Starting with results from IMT-2000 proposals, this paper makes a preliminary investigation into these issues, indicating that placement of a router in the base station controller is feasible, in terms of both capacity and additional system complexity. Results also indicate that the relationship between goodput and base station capacity (number of users) shows a significant reduction in efficient use of routing capacity, even though the radio resource may be fully occupied.