This thesis focuses on the issues in transport protocol performance in wireless ad-hoc networks.From the review of existing research we identify the following three main factors contributing to the poor performance of transport protocols such as TCP. These are: the inability of transport protocol to detect/identify and respond to network events in wireless ad-hoc networks; the inability of TCP window-based flow control mechanism to avoid overloading the wireless channel; and the frequent route failures due to mobility, causing interruptions in transmission. / We focus on 2 mechanisms to improve the performance of transport protocols. Recognising the many benefits of multipath routing in wireless ad-hoc networks, we propose a multipath transport design framework, incorporating a number of transport protocol components that are suitable for wireless ad-hoc networks, with different degrees of cross-interference. We first consider the use of independent multiple paths, and show that the throughput performance of multiple paths is no more than 55-60%, due to a spatial reuse of the wireless channel at the source. We then show that the overall improvement resulting from the use of independent multiple paths is further limited by the scarcity of independent multiple paths between a given pair of source and destination nodes. We conclude that independent multipaths are worth consideration only in sufficiently dense and large networks. Then we propose metrics to characterise node-disjoint multipaths with different degrees of cross-interference, and investigate correlation of these metrics with the throughput performance of multipath. / Our simulation results show that the throughput performance of multiple paths is determined by both the number of cross-interference links between paths and the local density of cross path links. These observations can be used in selecting multiple paths for good throughput performance of multipath transport. / The second mechanism we focus on is the rate-based flow control over a multi-hop path. Recognising the lack of suitable bandwidth estimation techniques for multi-hop wireless networks, we propose a new bandwidth estimation technique (termed Saturation Throughput Estimate or STE) suitable for wireless ad-hoc networks. It is based on a nearly linear relationship between channel occupancy (a local measurement of the channel activity level) and throughput available to a node. It also accounts for the traffic forwarded by the neighbouring nodes on behalf of the node in question. We show that our estimation technique is capable of providing good estimates of bandwidth available to a node in small to medium size ad-hoc networks, over a broad range of traffic loads. We also develop and verify an analytical model of IEEE 802.11 suitable for studying 802.11 performance under limited load conditions. Our analytical studies confirm the key assumption of a nearly linear relationship between channel occupancy and throughput underlying the proposed STE bandwidth estimation technique, and provide useful insights into selecting a suitable operating point to achieve high channel utilisation and avoid congestion in the wireless channel. Subsequently, the proposed STE bandwidth estimation technique is fine-tuned to meet the requirements of rate-based flow control over a multi-hop path. Finally, we present and develop a design of a feedback rate-based flow control framework to evaluate the merit of our STE technique and other rate estimation techniques proposed in the subject literature. We show that ATP (Ad-Hoc Transport Protocol) rate estimation technique tends to saturate the network, resulting in long queuing delay. In contrast, the rate-based flow control scheme using our STE rate estimate as rate feedback is effective in regulating and maintaining traffic just below the congestion point, thus achieving high network utilisation in static and low mobility networks. / Thesis (PhDTelecommunications)--University of South Australia, 2006.
| Identifer | oai:union.ndltd.org:ADTP/267231 |
| Creators | Liaw, Yong Shyang. |
| Source Sets | Australiasian Digital Theses Program |
| Language | English |
| Detected Language | English |
| Rights | copyright under review |
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