City Research Online

Abstract

Multihop ad hoc networks are collection of wireless nodes dynamically forming a temporary network without the use of any pre-existing network infrastructure or centralized administration. Consequently, ad hoc networks and their wireless links are fundamentally different from conventional stationary wireless and wired computer networks. In particular, incorporating the concept of TCP end-to-end congestion control for wireless networks is one of the primary concerns in designing ad hoc networks since TCP was primarily designed and optimized based on the assumptions for wired networks. In this thesis, our interest lies on tackling the TCP instability problem since due to the nature of applications in multi-hop ad hoc networks (e.g. emergency operation and battlefield communication), connection instability or starvation even for a short period of time can have a devastating impact on the Quality of Service and may not be acceptable for the end user. Through a detailed analysis and simulations, it is shown in this thesis that the main causes of TCP instability lie in three factors: overloading the net-work by sending more packets than the capacity of the channel, TCP sensitivity to out of order packets, and the channel access unfairness when multiple TCP connections are sharing the medium using 802.11 MAC protocol. To reduce TCP instability caused by excessive packet contention drops, a novel algorithm has been proposed that aims to reduce packet contention by optimizing the amount of outstanding data in the network. To reduce TCP sensitivity to out of order packets, a new algorithm is proposed with the aim of performing more local recovery rather than end-to-end recovery. Finally, to address the TCP instability caused by channel access unfairness, the 802.11 binary exponential back off algorithm is replaced with a more conservative approach. In addition to addressing the problem of TCP instability, a 3-dimensional Markov model of 802.11 MAC is presented in this thesis to accurately analyze the 802.11 MAC throughput in ad hoc networks.

Publication Type:	Thesis (Doctoral)
Subjects:	Q Science > QA Mathematics T Technology > TA Engineering (General). Civil engineering (General)
Departments:	School of Science & Technology > Engineering School of Science & Technology > School of Science & Technology Doctoral Theses Doctoral Theses

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