Performance improvements to the AODV routing protocol and multiple hop wireless routes : a thesis presented in partial fulfilment of the requirements for the degree of Masters of Engineering in Computer Systems Engineering at Massey University, Palmerston North, New Zealand

This research focused on improving the performance of the Ad-hoc On-demand Distance Vector (AODV) routing protocol over multiple hop routes. The two specific areas that this research addressed were the dramatic decrease in throughput over multiple hop IEEE 802.11 wireless routes and the problems caused by the use of hello messages by AODV implementations to detect broken routes. To help ensure that this research was suitable for real world scenarios, only off-the-shelf software and hardware was used for both the implementations and the tests. This thesis firstly presents an overview of IEEE 802.11 based wireless networking and the AODV protocol, along with wireless networking and networking in general within the Linux operating system. The thesis then presents the problems caused by hello messages and shows how the IEEE 802.11 wireless standard contributes to the dramatic decrease in throughput over multiple hop routes. To overcome the hello message problems, an AODV implementation was developed which used existing mechanisms on the data link layer, specifically the transmit retry limit, rather then hello messages to detect broken links. To address the multiple hop route throughput problem, the use of two and four IEEE 802.11 based wireless network interfaces per node were investigated, rather than using just a single wireless interface per node. These proposed solutions, and the AODV implementation that was developed as part of this research, were then tested in the areas of functionality and throughput performance improvements. The thesis concludes by presenting the performance improvements resulting from using multiple interfaces per node and the non hello message based AODV implementation along with outlining possible future research in this area

Random Access Game and Medium Access Control Design

Motivated partially by a control-theoretic viewpoint, we propose a game-theoretic model, called random access game, for contention control. We characterize Nash equilibria of random access games, study their dynamics, and propose distributed algorithms (strategy evolutions) to achieve Nash equilibria. This provides a general analytical framework that is capable of modeling a large class of system-wide quality-of-service (QoS) models via the specification of per-node utility functions, in which system-wide fairness or service differentiation can be achieved in a distributed manner as long as each node executes a contention resolution algorithm that is designed to achieve the Nash equilibrium. We thus propose a novel medium access method derived from carrier sense multiple access/collision avoidance (CSMA/CA) according to distributed strategy update mechanism achieving the Nash equilibrium of random access game. We present a concrete medium access method that adapts to a continuous contention measure called conditional collision probability, stabilizes the network into a steady state that achieves optimal throughput with targeted fairness (or service differentiation), and can decouple contention control from handling failed transmissions. In addition to guiding medium access control design, the random access game model also provides an analytical framework to understand equilibrium and dynamic properties of different medium access protocols

Insights into the Design of Congestion Control Protocols for Multi-Hop Wireless Mesh Networks

The widespread deployment of multi-hop wireless mesh networks will depend on the performance seen by the user. Unfortunately, the most predominant transport protocol, TCP, performs poorly over such networks, even leading to starvation in some topologies. In this work, we characterize the root causes of starvation in 802.11 scheduled multi-hop wireless networks via simulations. We analyze the performance of three categories of transport protocols. (1) end-to-end protocols that require implicit feedback (TCP SACK), (2) Explicit feedback based protocols (XCP and VCP) and (3) Open-loop protocol (UDP). We ask and answer the following questions in relation to these protocols: (a) Why does starvation occur in different topologies? Is it intrinsic to TCP or, in general, to feedback-based protocols? or does it also occur in the case of open-loop transfers such as CBR over UDP? (a) What is the role of application behavior on transport layer performance in multi-hop wireless mesh networks? (b) Is sharing congestion in the wireless neighborhood essential for avoiding starvation? (c) For explicit feedback based transport protocols, such as XCP and VCP, what performance can be expected when their capacity estimate is inaccurate? Based on the insights derived from the above analysis, we design a rate-based protocol called VRate that uses the two ECN bits for conveying load feedback information. VRate achieves near optimal rates when configured with the correct capacity estimate