Improving the Performance of Wireless LANs

This book quantifies the key factors of WLAN performance and describes methods for improvement. It provides theoretical background and empirical results for the optimum planning and deployment of indoor WLAN systems, explaining the fundamentals while supplying guidelines for design, modeling, and performance evaluation. It discusses environmental effects on WLAN systems, protocol redesign for routing and MAC, and traffic distribution; examines emerging and future network technologies; and includes radio propagation and site measurements, simulations for various network design scenarios, numerous illustrations, practical examples, and learning aids

WLAN Channel Selection Without Communication

In this paper we consider how a group of wireless access-points can self-configure their channel choice so as to avoid interference between one another and thereby maximise network capacity. We make the observation that communication between access points is not necessary, although it is a feature of almost all published channel allocation algorithms. We argue that this observation is of key practical importance as, except in special circumstances, interfering WLANs need not all lie in the same administrative domain and/or may be beyond wireless communication distance (although within interference distance). We demonstrate the feasibility of the communicationfree paradigm via a new class of decentralized algorithms that are simple, robust and provably correct for arbitrary interference graphs. The algorithm requires only standard hardware and we demonstrate its effectiveness via experimental measurements

Cross-layer design and optimization of medium access control protocols for wlans

This thesis provides a contribution to the field of Medium Access Control (MAC) layer protocol design for wireless networks by proposing and evaluating mechanisms that enhance different aspects of the network performance. These enhancements are achieved through the exchange of information between different layers of the traditional protocol stack, a concept known as Cross-Layer (CL) design. The main thesis contributions are divided into two parts. The first part of the thesis introduces a novel MAC layer protocol named Distributed Queuing Collision Avoidance (DQCA). DQCA behaves as a reservation scheme that ensures collision-free data transmissions at the majority of the time and switches automatically to an Aloha-like random access mechanism when the traffic load is low. DQCA can be enriched by more advanced scheduling algorithms based on a CL dialogue between the MAC and other protocol layers, to provide higher throughput and Quality of Service (QoS) guarantees. The second part of the thesis explores a different challenge in MAC layer design, related to the ability of multiple antenna systems to offer point-to-multipoint communications. Some modifications to the recently approved IEEE 802.11n standard are proposed in order to handle simultaneous multiuser downlink transmissions. A number of multiuser MAC schemes that handle channel access and scheduling issues and provide mechanisms for feedback acquisition have been presented and evaluated. The obtained performance enhancements have been demonstrated with the help of both theoretical analysis and simulation obtained results

Measuring Transmission Opportunities in 802.11 Links

We propose a powerful MAC/PHY cross-layer approach to measuring IEEE 802.11 transmission opportunities in WLAN networks on a per-link basis. Our estimator can operate at a single station and it is able to: 1) classify losses caused by noise, collisions, and hidden nodes; and 2) distinguish between these losses and the unfairness caused by both exposed nodes and channel capture. Our estimator provides quantitative measures of the different causes of lost transmission opportunities, requiring only local measures at the 802.11 transmitter and no modification to the 802.11 protocol or in other stations. Our approach is suited to implementation on commodity hardware, and we demonstrate our prototype implementation via experimental assessments. We finally show how our estimator can help the WLAN station to improve its local performance

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

IEEE 802.11 parameters adaptation for Performance enhancement in high density Wireless networks

Tribunal : Ramón Agüero, Álvaro Martín, Federico LarrocaNowadays, it is common to find wireless networks that are based on the IEEE 802.11 standard deployed in an unplanned and unmanaged manner. Moreover, because of the low hardware cost and, trying to obtain optimal coverage and performance, a large number of devices are usually installed in reduced spaces generating high-density deployments. This kind of networks experiment a myriad of problems (e.g., interference, medium access control, etc.) related with the shared nature of the transmission medium. In recent years, different physical-layer- and link-layer-adaptation mechanisms have been proposed to palliate those problems, however, their feedback-loop-based behaviour in a highly complex RF medium makes their performance hard to assess. In this work, we study the problems of high-density networks, experimentally evaluate some existing solutions and propose a new adaptation mechanism, PRCS, that tackles some common weakness of those solutions. PRCS control the transmit power, the data rate, and the carrier sense threshold of APs of a wireless network so as to mitigate the effects of interference in high-density deployments without causing unfairness between links. In simulation-based experiments, PRCS outperforms similar existing mechanisms in various scenarios and in a particular scenario, where most mechanisms fail, duplicates global network throughput.En la actualidad, es muy común encontrar redes inalámbricas basadas en el estándar IEEE 802.11 desplegadas de manera no planificada ni gestionada. Además, debido al bajo costo de los dispositivos y con la intención de obtener una cobertura y rendimiento óptimos, un gran número de dispositivos son instalados en espacios reducidos, generado despliegues de alta densidad. Este tipo de redes experimentan una gran variedad de problemas (por ej., interferencia, control de acceso al medio, etc.) relacionados con el hecho de que utilizan un medio de transmisión compartido. En los últimos años, diferentes mecanismos de adaptación de parámetros de la capa física y de enlace han sido propuestos con el objetivo de mitigar estos problemas. Estas soluciones adaptan parámetros tales como la potencia de transmisión o la tasa de transmisión. En este trabajo, estudiamos los problemas de las redes inalámbricas de alta densidad, evaluamos mediante experimentos algunas de las soluciones existentes y proponemos un nuevo mecanismo de adaptación, PRCS, que aborda algunas de las debilidades de estas soluciones. PRCS controla la potencia de transmisión, la tasa de transmisión y el umbral del mecanismo de sensado de portadora de los puntos de acceso de una red inalámbrica. El objetivo de este mecanismo es mitigar los efectos de la interferencia en despliegues de alta densidad sin causar asimetrías entre los enlaces. En experimentos basados en simulaciones, mostramos que PRCS supera a los mecanismos existentes en varios escenarios y, en un escenario en particular donde la mayoría de los mecanismos fallan, duplica el rendimiento global de la red