Enhanced Collision Resolution for the IEEE 802.11 Distributed Coordination Function

The IEEE 802.11 standard relies on the Distributed Coordination Function (DCF) as the fundamental medium access control method. DCF uses the Binary Exponential Backoff (BEB) algorithm to regulate channel access. The backoff period determined by BEB depends on a contention window (CW) whose size is doubled if a station suffers a collision and reset to its minimum value after a successful transmission. BEB doubles the CW size upon collision to reduce the collision probability in retransmission. However, this CW increase reduces channel access time because stations will spend more time sensing the channel rather than accessing it. Although resetting the CW to its minimum value increases channel access, it negatively affects fairness because it favours successfully transmitting stations over stations suffering from collisions. Moreover, resetting CW leads to increasing the collision probability and therefore increases the number of collisions. % Quality control editor: Please ensure that the intended meaning has been maintained in the edits of the previous sentence. Since increasing channel access time and reducing the probability of collisions are important factors to improve the DCF performance, and they conflict with each other, improving one will have an adverse effect on the other and consequently will harm the DCF performance. We propose an algorithm, \gls{ECRA}, that solves collisions once they occur without instantly increasing the CW size. Our algorithm reduces the collision probability without affecting channel access time. We also propose an accurate analytical model that allows comparing the theoretical saturation and maximum throughputs of our algorithm with those of benchmark algorithms. Our model uses a collision probability that is dependent on the station transmission history and thus provides a precise estimation of the probability that a station transmits in a random timeslot, which results in a more accurate throughput analysis. We present extensive simulations for fixed and mobile scenarios. The results show that on average, our algorithm outperformed BEB in terms of throughput and fairness. Compared to other benchmark algorithms, our algorithm improved, on average, throughput and delay performance

Reverse back-off mechanism for safety vehicular ad hoc networks

International audienceVehicular ad hoc networks can play an important role in enhancing transportation efficiency and improving road safety. Therefore, direct vehicle-to-vehicle communications are considered as one of the main building blocks of a future Intelligent Transportation System. The success and availability of IEEE 802.11 radios made this technology the most probable choice for the medium access control layer in vehicular networks. However, IEEE 802.11 was originally designed in a wireless local area network context and it is not optimised for a dynamic, ad hoc vehicular scenario. In this paper, we investigate the compatibility of the IEEE 802.11 medium access control protocol with the requirements of safety vehicular applications. As the protocols in this family are well-known for their scalability problems, we are especially interested in high density scenarios, quite frequent on today’s roads. Using an analytical framework, we study the performance of the back-off mechanism and the role of the contention window on the control channel of a vehicular network. Based on these findings, we propose a reverse back-off mechanism, specifically designed with road safety applications in mind. Extensive simulations are carried out to prove the efficiency of the proposed enhancement scheme and to better understand the characteristics of vehicular communications

Medium Access Control Protocols for Ad-Hoc Wireless Networks: A Survey

Studies of ad hoc wireless networks are a relatively new field gaining more popularity for various new applications. In these networks, the Medium Access Control (MAC) protocols are responsible for coordinating the access from active nodes. These protocols are of significant importance since the wireless communication channel is inherently prone to errors and unique problems such as the hidden-terminal problem, the exposed-terminal problem, and signal fading effects. Although a lot of research has been conducted on MAC protocols, the various issues involved have mostly been presented in isolation of each other. We therefore make an attempt to present a comprehensive survey of major schemes, integrating various related issues and challenges with a view to providing a big-picture outlook to this vast area. We present a classification of MAC protocols and their brief description, based on their operating principles and underlying features. In conclusion, we present a brief summary of key ideas and a general direction for future work

Impact of Medium Access Control Strategies on the Effectiveness of Advanced Cooperative Hybrid ARQ Techniques

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Mitigating the Impact of Physical Layer Capture and ACK Interference in Wireless 802.11 Networks

Ph.DDOCTOR OF PHILOSOPH

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

Fairness issues in multihop wireless ad hoc networks

Ph.DDOCTOR OF PHILOSOPH

A Quality of Service-Aware Secured Communication Scheme for Internet of Things-Based Networks

The Internet of Things (IoT) is an emerging technology that aims to enable the interconnection of a large number of smart devices and heterogeneous networks. Ad hoc networks play an important role in the designing of IoT-enabled platforms due to their efficient, flexible, low-cost and dynamic infrastructures. These networks utilize the available resources efficiently to maintain the Quality of Service (QoS) in a multi-hop communication. However, in a multi-hop communication, the relay nodes can be malicious, thus requiring a secured and reliable data transmission. In this paper, we propose a QoS-aware secured communication scheme for IoT-based networks (QoS-IoT). In QoS-IoT, a Sybil attack detection mechanism is used for the identification of Sybil nodes and their forged identities in multi-hop communication. After Sybil nodes detection, an optimal contention window (CW) is selected for QoS provisioning, that is, to achieve per-flow fairness and efficient utilization of the available bandwidth. In a multi-hop communication, the medium access control (MAC) layer protocols do not perform well in terms of fairness and throughput, especially when the nodes generate a large amount of data. It is because the MAC layer has no capability of providing QoS to prioritized or forwarding flows. We evaluate the performance of QoS-IoT in terms of Sybil attack detection, fairness, throughput and buffer utilization. The simulation results show that the proposed scheme outperforms the existing schemes and significantly enhances the performance of the network with a large volume of data. Moreover, the proposed scheme is resilient against Sybil attack