Adaptive medium access control for VoIP services in IEEE 802.11 WLANs

Abstract- Voice over Internet Protocol (VoIP) is an important service with strict Quality-of-Service (QoS) requirements in Wireless Local Area Networks (WLANs). The popular Distributed Coordination Function (DCF) of IEEE 802.11 Medium Access Control (MAC) protocol adopts a Binary Exponential Back-off (BEB) procedure to reduce the packet collision probability in WLANs. In DCF, the size of contention window is doubled upon a collision regardless of the network loads. This paper presents an adaptive MAC scheme to improve the QoS of VoIP in WLANs. This scheme applies a threshold of the collision rate to switch between two different functions for increasing the size of contention window based on the status of network loads. The performance of this scheme is investigated and compared to the original DCF using the network simulator NS-2. The performance results reveal that the adaptive scheme is able to achieve the higher throughput and medium utilization as well as lower access delay and packet loss probability than the original DCF

Performance modelling and enhancement of wireless communication protocols

In recent years, Wireless Local Area Networks(WLANs) play a key role in the data communications and networking areas, having witnessed significant research and development. WLANs are extremely popular being almost everywhere including business,office and home deployments.In order to deal with the modem Wireless connectivity needs,the Institute of Electrical and Electronics Engineers(IEEE) has developed the 802.11 standard family utilizing mainly radio transmission techniques, whereas the Infrared Data Association (IrDA) addressed the requirement for multipoint connectivity with the development of the Advanced Infrared(Alr) protocol stack. This work studies the collision avoidance procedures of the IEEE 802.11 Distributed Coordination Function (DCF) protocol and suggests certain protocol enhancements aiming at maximising performance. A new, elegant and accurate analysis based on Markov chain modelling is developed for the idealistic assumption of unlimited packet retransmissions as well as for the case of finite packet retry limits. Simple equations are derived for the through put efficiency, the average packet delay, the probability of a packet being discarded when it reaches the maximum retransmission limit, the average time to drop such a packet and the packet inter-arrival time for both basic access and RTS/CTS medium access schemes.The accuracy of the mathematical model is validated by comparing analytical with OPNET simulation results. An extensive and detailed study is carried out on the influence of performance of physical layer, data rate, packet payload size and several backoff parameters for both medium access mechanisms. The previous mathematical model is extended to take into account transmission errors that can occur either independently with fixed Bit Error Rate(BER) or in bursts. The dependency of the protocol performance on BER and other factors related to independent and burst transmission errors is explored. Furthermore, a simple-implement appropriate tuning of the back off algorithm for maximizing IEEE 802-11 protocol performance is proposed depending on the specific communication requirements. The effectiveness of the RTS/CTS scheme in reducing collision duration at high data rates is studied and an all-purpose expression for the optimal use of the RTS/CTS reservation scheme is derived. Moreover, an easy-to-implement backoff algorithm that significantly enhances performance is introduced and an alternative derivation is developed based on elementary conditional probability arguments rather than bi-dimensional Markov chains. Finally, an additional performance improvement scheme is proposed by employing packet bursting in order to reduce overhead costs such as contention time and RTS/CTSex changes. Fairness is explored in short-time and long-time scales for both the legacy DCF and packet bursting cases. AIr protocol employs the RTS/CTS medium reservation scheme to cope with hidden stations and CSMA/CA techniques with linear contention window (CW) adjustment for medium access. A 1-dimensional Markov chain model is constructed instead of the bi-dimensional model in order to obtain simple mathematical equations of the average packet delay.This new approach greatly simplifies previous analyses and can be applied to any CSMA/CA protocol.The derived mathematical model is validated by comparing analytical with simulation results and an extensive Alr packet delay evaluation is carried out by taking into account all the factors and parameters that affect protocol performance. Finally, suitable values for both backoff and protocol parameters are proposed that reduce average packet delay and, thus, maximize performance

Backoff as Performance improvements Algorithms - A Comprehenssive Review

As a significant part of the Media Access Control protocol, the backoff algorithm purpose is to minimize number of collisions if not totally avoid any collision in Mobile Ad Hoc Networks, in the case of contention between nodes to access a channel. Researchers have proposed many algorithms for backoff to enhance the network performance and improve it. This paper aims at exploring the main and most studied backoff algorithms and how do these algorithms lead to an enhancement of the MANETs performance. This paper also compares between the algorithms proposed in the literature and evaluates to what extent they have affected the performance and enhance it

Performance modelling and enhancement of wireless communication protocols

In recent years, Wireless Local Area Networks(WLANs) play a key role in the data communications and networking areas, having witnessed significant research and development. WLANs are extremely popular being almost everywhere including business,office and home deployments.In order to deal with the modem Wireless connectivity needs,the Institute of Electrical and Electronics Engineers(IEEE) has developed the 802.11 standard family utilizing mainly radio transmission techniques, whereas the Infrared Data Association (IrDA) addressed the requirement for multipoint connectivity with the development of the Advanced Infrared(Alr) protocol stack. This work studies the collision avoidance procedures of the IEEE 802.11 Distributed Coordination Function (DCF) protocol and suggests certain protocol enhancements aiming at maximising performance. A new, elegant and accurate analysis based on Markov chain modelling is developed for the idealistic assumption of unlimited packet retransmissions as well as for the case of finite packet retry limits. Simple equations are derived for the through put efficiency, the average packet delay, the probability of a packet being discarded when it reaches the maximum retransmission limit, the average time to drop such a packet and the packet inter-arrival time for both basic access and RTS/CTS medium access schemes.The accuracy of the mathematical model is validated by comparing analytical with OPNET simulation results. An extensive and detailed study is carried out on the influence of performance of physical layer, data rate, packet payload size and several backoff parameters for both medium access mechanisms. The previous mathematical model is extended to take into account transmission errors that can occur either independently with fixed Bit Error Rate(BER) or in bursts. The dependency of the protocol performance on BER and other factors related to independent and burst transmission errors is explored. Furthermore, a simple-implement appropriate tuning of the back off algorithm for maximizing IEEE 802-11 protocol performance is proposed depending on the specific communication requirements. The effectiveness of the RTS/CTS scheme in reducing collision duration at high data rates is studied and an all-purpose expression for the optimal use of the RTS/CTS reservation scheme is derived. Moreover, an easy-to-implement backoff algorithm that significantly enhances performance is introduced and an alternative derivation is developed based on elementary conditional probability arguments rather than bi-dimensional Markov chains. Finally, an additional performance improvement scheme is proposed by employing packet bursting in order to reduce overhead costs such as contention time and RTS/CTSex changes. Fairness is explored in short-time and long-time scales for both the legacy DCF and packet bursting cases. AIr protocol employs the RTS/CTS medium reservation scheme to cope with hidden stations and CSMA/CA techniques with linear contention window (CW) adjustment for medium access. A 1-dimensional Markov chain model is constructed instead of the bi-dimensional model in order to obtain simple mathematical equations of the average packet delay.This new approach greatly simplifies previous analyses and can be applied to any CSMA/CA protocol.The derived mathematical model is validated by comparing analytical with simulation results and an extensive Alr packet delay evaluation is carried out by taking into account all the factors and parameters that affect protocol performance. Finally, suitable values for both backoff and protocol parameters are proposed that reduce average packet delay and, thus, maximize performance.EThOS - Electronic Theses Online ServiceGBUnited Kingdo

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

Design and Evaluation of Efficient Medium Access Control Solutions for Vehicular Environments

[EN] In recent years, advances in wireless technologies and improved sensing and computational capabilities have led to a gradual transition towards Intelligent Transportation Systems (ITS) and related applications. These applications aim at improving road safety, provide smart navigation, and eco-friendly driving. Vehicular Ad hoc Networks (VANETs) provide a communication structure for ITS by equipping cars with advanced sensors and communication devices that enable a direct exchange of information between vehicles. Different types of ITS applications rely on two types of messages: periodic beacons and event-driven messages. Beacons include information such as geographical location, speed, and acceleration, and they are only disseminated to a close neighborhood. Differently from beacons, event-driven messages are only generated when a critical event of general interest occurs, and it is spread within a specific target area for the duration of the event. The reliability of information exchange is one of the main issues for vehicularcommunications since the safety of people on the road is directly related to the effectiveness of these transmissions. A Medium Access Control (MAC) protocol must guarantee reliable beacon broadcasting within deadline bounds to all vehicles in the neighbourhood, thereby providing them timely notifications about unsafe driving conditions or other hazardous events. Moreover, infotainment and comfort applications require reliable unicast transmissions that must be taken into account. However, high node mobility, highly dynamic topology, and lack of a central control unit, are issues that make the design of a reliable MAC protocol for vehicular environments a very difficult and challenging task, especially when efficient broadcasting strategies are required. The IEEE 802.11p MAC protocol, an approved amendment to the IEEE 802.11 standard, is a random access protocol that is unable to provide guaranteed delay bounds with sufficient reliability in vehicular scenarios, especially under high channel usage. This problem is particularly serious when implementing (semi-) automated driving applications such as platooning, where inter-vehicle spacing is drastically reduced, and the control loop that manages and maintains the platoon requires frequent, timely and reliable exchange of status information (beacons). In this thesis novel protocols compatible with the IEEE 802.11 and 802.11p standards are proposed in order to optimally adjust the contention window size for unicast applications in Mobile Ad hoc Networks (MANETs) and VANETs. Experimental tests comparing our proposals to existing solutions show that the former are able to improve the packet delivery ratio and the average end-to-end delay for unicast applications. Concerning efficient message diffusion (broadcast) in VANET environments, we proposed token-based MAC solutions to improve the performance achieved by existing 802.11p driving safety applications in different vehicular environments, including highway, urban, and platooning scenarios. Experimental results show that the proposed solutions clearly outperform 802.11p when delay-bounded beacons and event notifications must be delivered.[ES] Recientemente, los avances en las tecnologías inalámbricas y las mejoras en términos de capacidades de sensorización y computación de los dispositivos electrónicos, han dado lugar a una transición gradual hacia servicios y aplicaciones de los Sistemas Inteligentes de Transporte (ITS). Estas aplicaciones tienen como objetivo mejorar la seguridad vial, proporcionar una navegación inteligente, y promover la conducción eco-eficiente. Las redes vehiculares ad hoc (VANETs) proporcionan una infraestructura de comunicaciones para ITS al equipar los coches con sensores avanzados y dispositivos de comunicación que permiten el intercambio directo de información entre vehículos. Los diferentes tipos de aplicaciones ITS se basan en dos tipos de mensajes: mensajes periódicos conocidos como beacons y mensajes asociados a eventos. Los mensajes periódicos incluyen información relativa a la ubicación geográfica, la velocidad y la aceleración, entre otros, y sólo son distribuidos entre los vehículos vecinos. A diferencia de estos beacons, los mensajes asociados a eventos sólo se generan cuando se produce un evento crítico de interés general, el cual se propaga dentro del área de interés de dicho evento y mientras éste siga activo. La fiabilidad del intercambio de información es uno de los principales problemas para las comunicaciones vehiculares, debido principalmente a que las aplicaciones de seguridad dependen directamente de la eficacia de estas transmisiones. Un protocolo de Control de Acceso al Medio (MAC) debe garantizar la difusión fiable de información a todos los vehículos vecinos dentro de unos límites máximos de retardo, proporcionándoles las notificaciones oportunas respecto a condiciones de conducción inseguras y otros eventos peligrosos. Por otra parte, las aplicaciones de información y entretenimiento, así como las aplicaciones orientadas al confort, también requieren transmisiones fiables extremoa-extremo. Sin embargo, la alta movilidad de los vehículos, la variabilidad de la topología, así como la falta de una unidad central de control, son factores que hacen que el diseño de un protocolo MAC fiable para entornos vehiculares sea una tarea especialmente compleja, especialmente cuando son necesarias estrategias de difusión eficientes. El protocolo MAC IEEE 802.11p, una modificación ya aprobada al estándar IEEE 802.11 original para entornos de comunicación vehiculares, es un protocolo de acceso que no es capaz de garantizar unos límites de retardo con la fiabilidad necesaria para estos entornos, especialmente en escenarios de alta utilización del canal inalámbrico. Este problema es particularmente importante a la hora de implementar aplicaciones de conducción (semi-)automática, como el caso de grupos de vehículos donde la separación entre vehículos se reduce drásticamente, y el sistema de control que gestiona y mantiene el grupo requiere de un intercambio frecuente de información fiable y acotado en retardo. En esta tesis se proponen nuevos protocolos MAC compatibles con los estándares IEEE 802.11 y 802.11p basados en el ajuste del tamaño de la ventana de contención para aplicaciones unicast en rede MANETs y VANETs. Los resultados experimentales obtenidos comparando nuestras propuestas con las soluciones existentes muestran que los protocolos propuestos son capaces de mejorar la tasa de entrega de paquetes y el retardo medio extremo-a-extremo para aplicaciones unicast. En lo que respecta a la difusión eficiente de mensajes broadcast en entornos VANET, se han propuesto soluciones MAC basadas en el uso de tokens que mejoran las prestaciones de aplicaciones de conducción segura basadas en el estándar 802.11p, tanto en autopistas, zonas urbanas, y escenarios con grupos de vehículos. Los resultados experimentales muestran que las soluciones propuestas superan claramente al protocolo 802.11p cuando es necesario entregar mensajes y notificaciones de eventos con restricc[CA] Recentment, els avan en les tecnologies sense fils i les millores en termes de capacitats de sensorització i computació dels dispositius electrònics, han donat lloc a una transició gradual cap a serveis i aplicacions dels sistemes intelligents de transport (ITS). Aquestes aplicacions tenen com a objectiu millorar la seguretat vial, proporcionar una navegació intelligent, i promoure la conducció ecoeficient. Les xarxes vehiculars ad hoc (VANET) proporcionen una infraestructura de comunicacions per a ITS, ja que equipen els cotxes amb sensors avançats i dispositius de comunicació que permeten l'intercanvi directe d'informació entre vehicles. Els diversos tipus d'aplicacions ITS es basen en dos classes de missatges: missatges periòdics coneguts com a beacons i missatges associats a esdeveniments. Els missatges periòdics inclouen informació relativa a la ubicació geogràfica, la velocitat i l'acceleració, entre uns altres, i només són distribuïts entre els vehicles veïns. A diferència d'aquests beacons, els missatges associats a esdeveniments només es generen quan es produeix un esdeveniment crític d'interès general, el qual es propaga dins de l àrea d'interès d'aquest esdeveniment i mentre aquest seguisca actiu. La fiabilitat de l'intercanvi d'informació és un dels principals problemes per a les comunicacions vehicular, principalment perquè les aplicacions de seguretat depenen directament de l'eficàcia d'aquestes transmissions. Un protocol de control d'accés al medi (MAC) ha de garantir la difusió fiable d'informació a tots els vehicles veïns dins d'uns límits màxims de retard, i proporcionar-los les notificacions oportunes respecte a condicions de conducció insegures i altres esdeveniments perillosos. D'altra banda, les aplicacions d'informació i entreteniment, com també les aplicacions orientades al confort, també requereixen transmissions fiables extrema-extrem. No obstant això, l'alta mobilitat dels vehicles, la variabilitat de la topologia, i la falta d'una unitat central de control, són factors que fan que el disseny d'un protocol MAC fiable per a entorns vehiculars siga una tasca especialment complexa, especialment quan són necessàries estratègies de difusió eficients. El protocol MAC IEEE 802.11p, una modificació ja aprovada a l'estàndard IEEE 802.11 original per a entorns de comunicació vehiculars, és un protocol d'accés que no és capa garantir uns límits de retard amb la fiabilitat necessària per a aquests entorns, especialment en escenaris d'alta utilització del canal sense fil. Aquest problema és particularment important a l'hora d'implementar aplicacions de conducció (semi)automàtica, com el cas de grups de vehicles en què la separació entre vehicles es redueix dràsticament, i el sistema de control que gestiona i manté el grup requereix un intercanvi freqüent d'informació fiable i delimitat en retard. En aquesta tesi es proposen nous protocols MAC compatibles amb els estàndards IEEE 802.11 i 802.11p basats en l'ajust de les dimensions de la finestra de contenció per a aplicacions unicast en xarxes MANET i VANET. Els resultats experimentals obtinguts comparant les nostres propostes amb les solucions existents mostren que els protocols proposats són capa de millorar la taxa de lliurament de paquets i el retard mitjà extrem-a-extrem per a aplicacions unicast. Pel que fa a la difusió eficient de missatges broadcast en entorns VANET, s'han proposat solucions MAC basades en l'ús de tokens que milloren les prestacions d'aplicacions de conducció segura basades en l'estàndard 802.11p, tant en autopistes, zones urbanes, i escenaris amb grups de vehicles. Els resultats experimentals mostren que les solucions proposades superen clarament el protocol 802.11p quan cal lliurar missatges i notificacions d'esdeveniments amb restriccions de latència.Balador, A. (2016). Design and Evaluation of Efficient Medium Access Control Solutions for Vehicular Environments [Tesis doctoral no publicada]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/64073TESI