LINK ADAPTATION IN WIRELESS NETWORKS: A CROSS-LAYER APPROACH

Conventional Link Adaptation Techniques in wireless networks aim to overcome harsh link conditions caused by physical environmental properties, by adaptively regulating modulation, coding and other signal and protocol specific parameters. These techniques are essential for the overall performance of the networks, especially for environments where the ambient noise level is high or the noise level changes rapidly. Link adaptation techniques answer the questions of What to change? and When to change? in order to improve the present layer performance. Once these decisions are made, other layers are expected to function perfectly with the new communication channel conditions. In our work, we have shown that this assumption does not always hold; and provide two mechanisms that lessen the negative outcomes caused by these decisions. Our first solution, MORAL, is a MAC layer link adaptation technique which utilizes the physical transmission information in order to create differentiation between wireless users with different communication capabilities. MORAL passively collects information from its neighbors and re-aligns the MAC layer parameters according to the observed conditions. MORAL improves the fairness and total throughput of the system through distributing the mutually shared network assets to the wireless users in a fairer manner, according to their capabilities. Our second solution, Data Rate and Fragmentation Aware Ad-hoc Routing protocol, is a network layer link adaptation technique which utilizes the physical transmission information in order to differentiate the wireless links according to their communication capabilities. The proposed mechanism takes the physical transmission parameters into account during the path creation process and produces energy-efficient network paths. The research demonstrated in this dissertation contributes to our understanding of link adaptation techniques and broadens the scope of such techniques beyond simple, one-step physical parameter adjustments. We have designed and implemented two cross-layer mechanisms that utilize the physical layer information to better adapt to the varying channel conditions caused by physical link adaptation mechanisms. These mechanisms has shown that even though the Link Adaptation concept starts at the physical layer, its effects are by no means restricted to this layer; and the wireless networks can benefit considerably by expanding the scope of this concept throughout the entire network stack

Wireless audio networking modifying the IEEE 802.11 standard to handle multi-channel real-time wireless audio networks

This thesis was submitted for the degree of Doctor of Philosophy and awarded by Brunel UniversityAudio networking is a rapidly increasing field which introduces new exiting possibilities for the professional audio industry. When well established, it will drastically change the way live sound systems will be designed, built and used. Today's networks have enough bandwidth that enables them to transfer hundreds of high quality audio channels, replacing analogue cables and intricate installations of conventional analogue audio systems. Currently there are many systems in the market that distribute audio over networks for live music and studio applications, but this technology is not yet widespread. The reasons that audio networks are not as popular as it was expected are mainly the lack of interoperability between different vendors and still, the need of a wired network infrastructure. Therefore, the development of a wireless digital audio networking system based on the existing widespread wireless technology is a major research challenge. However, the ΙΕΕΕ 802.11 standard, which is the primary wireless networking technology today, appears to be unable to handle this type of application despite the large bandwidth available. Apart from the well-known drawbacks of interference and security, encountered in all wireless data transmission systems, the way that ΙΕΕΕ 802.11 arbitrates the wireless channel access causes significantly high collision rate, low throughput and long overall delay. The aim of this research was to identify the causes that impede this technology to support real time wireless audio networks and to propose possible solutions. Initially the standard was tested thoroughly using a data traffic model which emulates a multi-channel real time audio environment. Broadcasting was found to be the optimal communication method, in order to satisfy the intolerance of live audio, when it comes to delay. The results were analysed and the drawback was identified in the hereditary weakness of the IEEE 802.11 standard to manage broadcasting, from multiple sources in the same network. To resolve this, a series of modifications was proposed for the Medium Access Control algorithm of the standard. First, the extended use of the "CTS-to-Self" control message was introduced in order to act as a protection mechanism in broadcasting, similar to the RTC/CTS protection mechanism, already used in unicast transmission. Then, an alternative "random backoff" method was proposed taking into account the characteristics of live audio wireless networks. For this method a novel "Exclusive Backoff Number Allocation" (EBNA) algorithm was designed aiming to minimize collisions. The results showed that significant improvement in throughput can be achieved using the above modifications but further improvement was needed, when it comes to delay, in order to reach the internationally accepted standards for real time audio delivery. Thus, a traffic adaptive version of the EBNA algorithm was designed. This algorithm monitors the traffic in the network, calculates the probability of collision and accordingly switches between classic IEEE 802.11 MAC and EBNA which is applied only between active stations, rather than to all stations in the network. All amendments were designed to operate as an alternative mode of the existing technology rather as an independent proprietary system. For this reason interoperability with classic IEEE 802.11 was also tested and analysed at the last part of this research. The results showed that the IEEE 802.11 standard, suitably modified, is able to support multiple broadcasting transmission and therefore it can be the platform upon which, the future wireless audio networks will be developed

A modified IEEE 802.11 MAC for optimizing broadcasting in wireless audio networks

The use of network infrastructures to replace conventional professional audio systems is a rapidly increasing field which is expected to play an important role within the professional audio industry. Currently, the market is dominated by numerous proprietary protocols which does not allowing interoperability and does not promote the evolution on this sector. Recent standardization actions are intending to resolve this issue excluding however the use of wireless networks. Existing wireless networking technologies are considered unsuitable for supporting real-time audio networks, not because of lack of bandwidth but due to their inefficient congestion control mechanisms in broadcasting. In this paper, we propose an amendment of the IEEE 802.11 MAC that improves the performance of the standard when is used for real-time audio data delivery. The proposed amendment is based on two innovative ideas. First, it provides a protection mechanism for broadcasting and second, replaces the classic congestion control mechanism, based in random backoff, with an alternative traffic adaptive algorithm, designed to minimize collisions. The proposed MAC is able to operate as an alternative mode, allowing regular Wi-Fi networks to coexist and interoperate efficiently with audio networks, with the last ones being able to be deployed over existing wireless network infrastructures

MAC regenerative analysis of wireless Ad-Hoc networks

Dissertação apresentada na Faculdade de Ciências e Tecnologia da Universidade Nova de Lisboa para obtenção do grau de Mestre em Engenharia Electrotécnica e de ComputadoresThe IEEE 802.11 is a fast growing technology all over the world. This growth is essentially due to the increasing number of users in the network. Despite the increasing number of users, not all of them need the same quality of service. Thus, service differentiation is an important aspect that shall be considered in mathematical models that describe network performance. Moreover, users typically communicate using point-to-point connections(unicast transmission scheme) and point-to-multipoint connections (broadcast transmission scheme). The co-existence of unicast and broadcast traffic impacts the network performance and its importance cannot be neglected in the network performance evaluation. This motivates the work presented in this thesis, which characterizes the network accounting for these important parameters. This thesis formulates a model to describe the behavior of the medium access control used in IEEE 802.11-based networks. This is the first step to develop a model that considers both different groups of users configured with different medium access control parameters and the co-existence of two different transmission schemes (unicast and broadcast). The model also assumes a finite number of retransmissions for unicast packets and it is confirmed that several models already proposed in other works are especial cases of the proposed model. Finally, a theoretical validation of the model is done as well as some simulations to assess its accuracy and, some realistic network features are discussed

Interference Mitigation in Frequency Hopping Ad Hoc Networks

Radio systems today exhibit a degree of flexibility that was unheard of only a few years ago. Software-defined radio architectures have emerged that are able to service large swathes of spectrum, covering up to several GHz in the UHF bands. This dissertation investigates interference mitigation techniques in frequency hopping ad hoc networks that are capable of exploiting the frequency agility of software-defined radio platforms

Centralized Rate Allocation and Control in 802.11-based Wireless Mesh Networks

Wireless Mesh Networks (WMNs) built with commodity 802.11 radios are a cost-effective means of providing last mile broadband Internet access. Their multihop architecture allows for rapid deployment and organic growth of these networks. 802.11 radios are an important building block in WMNs. These low cost radios are readily available, and can be used globally in license-exempt frequency bands. However, the 802.11 Distributed Coordination Function (DCF) medium access mechanism does not scale well in large multihop networks. This produces suboptimal behavior in many transport protocols, including TCP, the dominant transport protocol in the Internet. In particular, cross-layer interaction between DCF and TCP results in flow level unfairness, including starvation, with backlogged traffic sources. Solutions found in the literature propose distributed source rate control algorithms to alleviate this problem. However, this requires MAC-layer or transport-layer changes on all mesh routers. This is often infeasible in practical deployments. In wireline networks, router-assisted rate control techniques have been proposed for use alongside end-to-end mechanisms. We evaluate the feasibility of establishing similar centralized control via gateway mesh routers in WMNs. We find that commonly used router-assisted flow control schemes designed for wired networks fail in WMNs. This is because they assume that: (1) links can be scheduled independently, and (2) router queue buildups are sufficient for detecting congestion. These abstractions do not hold in a wireless network, rendering wired scheduling algorithms such as Fair Queueing (and its variants) and Active Queue Management (AQM) techniques ineffective as a gateway-enforceable solution in a WMN. We show that only non-work-conserving rate-based scheduling can effectively enforce rate allocation via a single centralized traffic-aggregation point. In this context we propose, design, and evaluate a framework of centralized, measurement-based, feedback-driven mechanisms that can enforce a rate allocation policy objective for adaptive traffic streams in a WMN. In this dissertation we focus on fair rate allocation requirements. Our approach does not require any changes to individual mesh routers. Further, it uses existing data traffic as capacity probes, thus incurring a zero control traffic overhead. We propose two mechanisms based on this approach: aggregate rate control (ARC) and per-flow rate control (PFRC). ARC limits the aggregate capacity of a network to the sum of fair rates for a given set of flows. We show that the resulting rate allocation achieved by DCF is approximately max-min fair. PFRC allows us to exercise finer-grained control over the rate allocation process. We show how it can be used to achieve weighted flow rate fairness. We evaluate the performance of these mechanisms using simulations as well as implementation on a multihop wireless testbed. Our comparative analysis show that our mechanisms improve fairness indices by a factor of 2 to 3 when compared with networks without any rate limiting, and are approximately equivalent to results achieved with distributed source rate limiting mechanisms that require software modifications on all mesh routers