Feasibility of Using Passive Monitoring Techniques in Mesh Networks for the Support of Routing

In recent years, Wireless Mesh Networks (WMNs) have emerged as a promising solution to provide low cost access networks that extend Internet access and other networking services. Mesh routers form the backbone connectivity through cooperative routing in an often unstable wireless medium. Therefore, the techniques used to monitor and manage the performance of the wireless network are expected to play a significant role in providing the necessary performance metrics to help optimize the link performance in WMNs. This thesis initially presents an assessment of the correlation between passive monitoring and active probing techniques used for link performance measurement in single radio WMNs. The study reveals that by combining multiple performance metrics obtained by using passive monitoring, a high correlation with active probing can be achieved. The thesis then addresses the problem of the system performance degradation associated with simultaneous activation of multiple radios within a mesh node in a multi-radio environment. The experiments results suggest that the finite computing resource seems to be the limiting factor in the performance of a multi-radio mesh network. Having studied this characteristic of multi-radio networks, a similar approach as used in single radio mesh network analysis was taken to investigate the feasibility of passive monitoring in a multi-radio environment. The accuracy of the passive monitoring technique was compared with that of the active probing technique and the conclusion reached is that passive monitoring is a viable alternative to active probing technique in multi-radio mesh networks

Improving Performance for CSMA/CA Based Wireless Networks

Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) based wireless networks are becoming increasingly ubiquitous. With the aim of supporting rich multimedia applications such as high-definition television (HDTV, 20Mbps) and DVD (9.8Mbps), one of the technology trends is towards increasingly higher bandwidth. Some recent IEEE 802.11n proposals seek to provide PHY rates of up to 600 Mbps. In addition to increasing bandwidth, there is also strong interest in extending the coverage of CSMA/CA based wireless networks. One solution is to relay traffic via multiple intermediate stations if the sender and the receiver are far apart. The so called “mesh” networks based on this relay-based approach, if properly designed, may feature both “high speed” and “large coverage” at the same time. This thesis focusses on MAC layer performance enhancements in CSMA/CA based networks in this context. Firstly, we observe that higher PHY rates do not necessarily translate into corresponding increases in MAC layer throughput due to the overhead of the CSMA/CA based MAC/PHY layers. To mitigate the overhead, we propose a novel MAC scheme whereby transported information is partially acknowledged and retransmitted. Theoretical analysis and extensive simulations show that the proposed MAC approach can achieve high efficiency (low MAC overhead) for a wide range of channel variations and realistic traffic types. Secondly, we investigate the close interaction between the MAC layer and the buffer above it to improve performance for real world traffic such as TCP. Surprisingly, the issue of buffer sizing in 802.11 wireless networks has received little attention in the literature yet it poses fundamentally new challenges compared to buffer sizing in wired networks. We propose a new adaptive buffer sizing approach for 802.11e WLANs that maintains a high level of link utilisation, while minimising queueing delay. Thirdly, we highlight that gross unfairness can exist between competing flows in multihop mesh networks even if we assume that orthogonal channels are used in neighbouring hops. That is, even without inter-channel interference and hidden terminals, multi-hop mesh networks which aim to offer a both “high speed” and “large coverage” are not achieved. We propose the use of 802.11e’s TXOP mechanism to restore/enfore fairness. The proposed approach is implementable using off-the-shelf devices and fully decentralised (requires no message passing)

IEEE802.11 アドホックネットワークにおける通信制御に関する研究

東京情報大学201

A Game Theory based Contention Window Adjustment for IEEE 802.11 under Heavy Load

The 802.11 families are considered as the most applicable set of standards for Wireless Local Area Networks (WLANs) where nodes make access to the wireless media using random access techniques. In such networks, each node adjusts its contention window to the minimum size irrespective of the number of competing nodes, so in saturated mode and excessive number of nodes available, the network performance is reduced due to severe collision probability. A cooperative game is being proposed to adjust the users’ contention windows in improving the network throughput, delay and packet drop ratio under heavy traffic load circumstances. The system’s performance evaluated by simulations indicate some superiorities of the proposed method over 802.11-DCF (Distribute Coordinate Function)

Control-theoretic approaches for efficient transmission on IEEE 802.11e wireless networks

With the increasing use of multimedia applications on the wireless network, the functionalities of the IEEE 802.11 WLAN was extended to allow traffic differentiation so that priority traffic gets quicker service time depending on their Quality of Service (QoS) requirements. The extended functionalities contained in the IEEE Medium Access Control (MAC) and Physical Layer (PHY) Specifications, i.e. the IEEE 802.11e specifications, are recommended values for channel access parameters along traffic lines and the channel access parameters are: the Minimum Contention Window CWmin, Maximum Contention Window CWmax, Arbitration inter-frame space number, (AIFSN) and the Transmission Opportunity (TXOP). These default Enhanced Distributed Channel Access (EDCA) contention values used by each traffic type in accessing the wireless medium are only recommended values which could be adjusted or changed based on the condition of number of associated nodes on the network. In particular, we focus on the Contention Window (CW) parameter and it has been shown that when the number of nodes on the network is small, a smaller value of CWmin should be used for channel access in order to avoid underutilization of channel time and when the number of associated nodes is large, a larger value of CWmin should be used in order to avoid large collisions and retransmissions on the network. Fortunately, allowance was made for these default values to be adjusted or changed but the challenge has been in designing an algorithm that constantly and automatically tunes the CWmin value so that the Access Point (AP) gives out the right CWmin value to be used on the WLAN and this value should be derived based on the level of activity experienced on the network or predefined QoS constraints while considering the dynamic nature of the WLAN. In this thesis, we propose the use of feedback based control and we design a controller for wireless medium access. The controller will give an output which will be the EDCA CWmin value to be used by contending stations/nodes in accessing the medium and this value will be based on current WLAN conditions. We propose the use of feedback control due to its established mathematical concepts particularly for single-input-single-output systems and multi-variable systems which are scenarios that apply to the WLAN

Wireless remote patient monitoring on general hospital wards.

A novel approach which has potential to improve quality of patient care on general hospital wards is proposed. Patient care is a labour-intensive task that requires high input of human resources. A Remote Patient Monitoring (RPM) system is proposed which can go some way towards improving patient monitoring on general hospital wards. In this system vital signs are gathered from patients and sent to a control unit for centralized monitoring. The RPM system can complement the role of nurses in monitoring patients’ vital signs. They will be able to focus on holistic needs of patients thereby providing better personal care. Wireless network technologies, ZigBee and Wi-Fi, are utilized for transmission of vital signs in the proposed RPM system. They provide flexibility and mobility to patients. A prototype system for RPM is designed and simulated. The results illustrated the capability, suitability and limitation of the chosen technology

Experimenting with commodity 802.11 hardware: overview and future directions

The huge adoption of 802.11 technologies has triggered a vast amount of experimentally-driven research works. These works range from performance analysis to protocol enhancements, including the proposal of novel applications and services. Due to the affordability of the technology, this experimental research is typically based on commercial off-the-shelf (COTS) devices, and, given the rate at which 802.11 releases new standards (which are adopted into new, affordable devices), the field is likely to continue to produce results. In this paper, we review and categorise the most prevalent works carried out with 802.11 COTS devices over the past 15 years, to present a timely snapshot of the areas that have attracted the most attention so far, through a taxonomy that distinguishes between performance studies, enhancements, services, and methodology. In this way, we provide a quick overview of the results achieved by the research community that enables prospective authors to identify potential areas of new research, some of which are discussed after the presentation of the survey.This work has been partly supported by the European Community through the CROWD project (FP7-ICT-318115) and by the Madrid Regional Government through the TIGRE5-CM program (S2013/ICE-2919).Publicad