On backoff mechanisms for wireless Mobile Ad Hoc Networks

Since their emergence within the past decade, which has seen wireless networks being adapted to enable mobility, wireless networks have become increasingly popular in the world of computer research. A Mobile Ad hoc Network (MANET) is a collection of mobile nodes dynamically forming a temporary network without the use of any existing network infrastructure. MANETs have received significant attention in recent years due to their easiness to setup and to their potential applications in many domains. Such networks can be useful in situations where there is not enough time or resource to configure a wired network. Ad hoc networks are also used in military operations where the units are randomly mobile and a central unit cannot be used for synchronization. The shared media used by wireless networks, grant exclusive rights for a node to transmit a packet. Access to this media is controlled by the Media Access Control (MAC) protocol. The Backoff mechanism is a basic part of a MAC protocol. Since only one transmitting node uses the channel at any given time, the MAC protocol must suspend other nodes while the media is busy. In order to decide the length of node suspension, a backoff mechanism is installed in the MAC protocol. The choice of backoff mechanism should consider generating backoff timers which allow adequate time for current transmissions to finish and, at the same time, avoid unneeded idle time that leads to redundant delay in the network. Moreover, the backoff mechanism used should decide the suitable action to be taken in case of repeated failures of a node to attain the media. Further, the mechanism decides the action needed after a successful transmission since this action affects the next time backoff is needed. The Binary exponential Backoff (BEB) is the backoff mechanisms that MANETs have adopted from Ethernet. Similar to Ethernet, MANETs use a shared media. Therefore, the standard MAC protocol used for MANETs uses the standard BEB backoff algorithms. The first part of this work, presented as Chapter 3 of this thesis, studies the effects of changing the backoff behaviour upon a transmission failure or after a successful transmission. The investigation has revealed that using different behaviours directly affects both network throughput and average packet delay. This result indicates that BEB is not the optimal backoff mechanism for MANETs. Up until this research started, no research activity has focused on studying the major parameters of MANETs. These parameters are the speed at which nodes travel inside the network area, the number of nodes in the network and the data size generated per second. These are referred to as mobility speed, network size and traffic load respectively. The investigation has reported that changes made to these parameters values have a major effect on network performance. Existing research on backoff algorithms for MANETs mainly focuses on using external information, as opposed to information available from within the node, to decide the length of backoff timers. Such information includes network traffic load, transmission failures of other nodes and the total number of nodes in the network. In a mobile network, acquiring such information is not feasible at all times. To address this point, the second part of this thesis proposes new backoff algorithms to use with MANETs. These algorithms use internal information only to make their decisions. This part has revealed that it is possible to achieve higher network throughput and less average packet delay under different values of the parameters mentioned above without the use of any external information. This work proposes two new backoff algorithms. The Optimistic Linear-Exponential Backoff, (OLEB), and the Pessimistic Linear-Exponential Backoff (PLEB). In OLEB, the exponential backoff is combined with linear increment behaviour in order to reduce redundant long backoff times, during which the media is available and the node is still on backoff status, by implementing less dramatic increments in the early backoff stages. PLEB is also a combination of exponential and linear increment behaviours. However, the order in which linear and exponential behaviours are used is the reverse of that in OLEB. The two algorithms have been compared with existing work. Results of this research report that PLEB achieves higher network throughput for large numbers of nodes (e.g. 50 nodes and over). Moreover, PLEB achieves higher network throughput with low mobility speed. As for average packet delay, PLEB significantly improves average packet delay for large network sizes especially when combined with high traffic rate and mobility speed. On the other hand, the measurements of network throughput have revealed that for small networks of 10 nodes, OLEB has higher throughput than existing work at high traffic rates. For a medium network size of 50 nodes, OLEB also achieves higher throughput. Finally, at a large network size of 100 nodes, OLEB reaches higher throughput at low mobility speed. Moreover, OLEB produces lower average packet delay than the existing algorithms at low mobility speed for a network size of 50 nodes. Finally, this work has studied the effect of choosing the behaviour changing point between linear and exponential increments in OLEB and PLEB. Results have shown that increasing the number of times in which the linear increment is used increases network throughput. Moreover, using larger linear increments increase network throughput

Mobile Ad-Hoc Networks

Being infrastructure-less and without central administration control, wireless ad-hoc networking is playing a more and more important role in extending the coverage of traditional wireless infrastructure (cellular networks, wireless LAN, etc). This book includes state-of-the-art techniques and solutions for wireless ad-hoc networks. It focuses on the following topics in ad-hoc networks: quality-of-service and video communication, routing protocol and cross-layer design. A few interesting problems about security and delay-tolerant networks are also discussed. This book is targeted to provide network engineers and researchers with design guidelines for large scale wireless ad hoc networks

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

Controlo de acesso ao meio em redes ad hoc móveis IEEE 802.11

Dissertação apresentada para obtenção do Grau de Doutor em Engenharia Electrotécnica, especialidade de Telecomunicações, pela Universidade Nova de Lisboa, Faculdade de Ciências e TecnologiaUma rede ad hoc móvel é constituída por um grupo de nós móveis inter-ligados através de ligações sem fios. O problema inicial abordado nesta dissertação prende-se com a caracterização do sub-nível de controlo de acesso ao meio em redes ad hoc, nomeadamente o controlo realizado na norma de facto IEEE 802.11. A elevada instabilidade na topologia deste tipo de redes e a sua natureza distribuída tornam mais frequente a execução de tarefas de localização de recursos na rede. No entanto, o tráfego utilizado nessas tarefas é do tipo ponto-multiponto, sendo o mecanismo de transmissão diferente do tráfego do tipo ponto-a-ponto. Este facto motivou o desenvolvimento de um modelo capaz de caracterizar o comportamento da norma face aos diferentes tipos de tráfego gerado. O modelo permite concluir que, quando os ritmos de transmissão são iguais para os dois tipos de tráfego, existe um valor de tráfego ponto-multiponto (aprox. 50 %), a partir do qual o débito útil da rede diminui. Na situação mais frequente de operação da norma,em que os ritmos de transmissão são diferentes, conclui-se que o débito útil da rede diminui acentuadamente à medida que é gerado mais tráfego do tipo ponto-multiponto. A segunda parte desta dissertação aborda a localização de recursos em redes ad hoc. É apresentado um algoritmo capaz de criar dinamicamente um conjunto de grupos de nós na rede, tendo em atenção a mobilidade relativa dos nós. Esses grupos de nós são depois utilizados por dois algoritmos de localização de recursos disponíveis na rede. Finalmente,é discutido o impacto da mobilidade e das características do protocolo de controlo de acesso ao meio na caracterização da topologia da rede, avaliando-se a sua influência no desempenho dos algoritmos de localização de recursos.Fundação para a Ciência e Tecnologi

Schemes for multi-hop dissemination of non-safety information in VANETs

Vehicular Ad Hoc Networks (VANETs) are a special case of Mobile Ad Hoc Network (MANET), whose nodes are vehicles and occasional fixed devices with communication capabilities. What makes them special is the limited range of possible movements of the mobile nodes (they can only travel on the existing roads or rails) and their high speed. The potential applications in this new type of network are almost endless. Researchers have typically classified them in four groups: active safety, public service support, improved driving and business/entertainment. The communication patterns that they require are varied, being information dissemination one of them. It is aimed at reaching a group of vehicles in an area that is larger than the reception range of a single node, so that a multi-hop broadcast is necessary. It can take multiple forms depending on the type of message. For example, a warning caused by a sudden brake requires a fast and reliable dissemination, whereas a blocked route announcement is tolerant to delays up to a few seconds and may miss some target without risking safety. The work in this PhD thesis is focused on this last type of use. The objective is to create schemes that would allow for the multi-hop dissemination of messages that do not have hard delay and delivery requirements (typically, any non-safety information). Our goals for this solution are four. First, we want it to be useful in roadways as well as inside cities. Vehicles movements and the occurrence of obstacles to the signal propagation are very different in both scenarios and so we need to adapt it to both. Second, we want it to be independent of infrastructure. The cost of deploying fixed units along every road and street is high and it may take a long time until there is global coverage. Our intention is that this solution can be used regardlessly of the deployment point. In addition, it must avoid the broadcast storm problem by reducing as much as possible the number of generated duplicates. Lastly, the scheme needs to cope with intermittent partitions in the vehicular network. Implementing a store-carry-forward mechanism that allows a message reach disconnected groups of vehicles inside the destination area rises the number of necessary duplicates. In order to achieve the aforementioned goals, we first study how typical infrastructure-less dissemination schemes from the state of the art in MANETs, plus a new specific one, apply to VANETs. According to their results in relation to a series of metrics, we learn that the distance-based scheme is the one that best meets our requirements. We select it to create an optimized scheme for the two existing scenarios-roadways and urban areas. Regarding the adaptation for roadways, we begin by optimizing the scheme so that its forwarding ratio is as close to the minimum as possible, and analyzing its average per-hop delay in a connected network (i.e., there is at least one feasible route between any two nodes in the network). Next, we study how to add a custom store-carry-forward mechanism that, with minimal additions, manages to overcome short-lived network partitions. We validate the addition and the complete scheme under different channel loads and in contrast with a well-known protocol aimed at the same type of traffic, DV-CAST. Our work on the version for urban scenarios parts from the assumption that we need to detect junctions and react accordingly in order to spread the dissemination in new directions and reach as many vehicles as possible. We create two different modifications of the basic distance-based scheme, each using a different method to detect intersections, and test them along with the basic one. This first step leads us to discovering that it is not necessary to detect intersections in order to achieve good results. Then, similarly to the process for the roadway scenario, we work on optimizing the scheme and creating a suitable store-carry-forward mechanism. We follow the same reasoning but this time we consider three different options for subsequent retransmissions. We test each version of the scheme throughly via simulations using real city maps and compare the results to those of the urban counterpart of DV-CAST, named UV-CAST. We use validated simulators as ns-2 and the Veins framework for testing realistically the different stages of our work. The performance of the resulting schemes meet our requirements to a high degree and so we consider that we have fulfilled our goals. In addition, the work done so far opens the door to new lines of research that are either the natural consequence or an application of our achievements.La expresión inglesa Vehicular Ad Hoc Networks (VANETs) nombra a un tipo especial de Mobile Ad Hoc Network (MANET), cuyos nodos son vehículos y, ocasionalmente, dispositivos fijos con capacidad de comunicación. Lo que las hace especiales es el rango limitado de movimientos posibles para los nodos móviles (ya que sólo pueden viajar por las vías existentes) y su alta velocidad. Las aplicaciones potenciales de este nuevo tipo de red son casi infinitas. La comunidad investigadora las ha clasificado típicamente en cuatro grupos: seguridad activa, apoyo a servicios públicos, asistencia a la conducción y negocios/entretenimiento. Los patrones de comunicación que precisan son variados, siendo la diseminación de información uno de ellos. Su objetivo es alcanzar a un grupo de vehículos en un área mayor que el de la cobertura alcanzada por un nodo, de modo que es necesaria una difusión multisalto. Esta puede tomar múltiples formas dependiendo del tipo de mensaje. Por ejemplo, una alarma provocada por un frenazo brusco requiere una diseminación rápida y confiable, mientras que un aviso de calle cortada es tolerante a retardos de hasta algunos segundos y si no alcanza a algún destinatario no supone un riesgo para la seguridad. El trabajo contenido en esta tesis se enfoca en este último caso de uso. La meta es crear esquemas que permitan la diseminación multisalto de mensajes que no tienen requisitos fuertes en cuanto a retardo y entrega (típicamente, cualquier información no relacionada con la seguridad). Nuestros objetivos para esta solución son cuatro. Primero, queremos que sea útil en carretera así como en ciudad. Los movimientos de los vehículos y la existencia de obstáculos para la propagación de la señal son muy diferentes en ambos escenarios y por tanto necesitamos adaptarla a ambos. Segundo, queremos que no dependa de infraestructura. El coste de desplegar unidades fijas a lo largo de cada calle y carretera es alto, y puede llevar un largo tiempo hasta que haya cobertura global. Nuestra intención es que esta solución pueda ser usada en cualquier punto del proceso de despliegue. Además, debe evitar el problema conocido como “tormenta broadcast”, reduciendo en la medida de lo posible el número de duplicados generados. Por último, el esquema necesita hacer frente a particiones intermitentes de la red vehicular. Implementar un mecanismo de los llamados “store-carry-forward” (guardar-llevar-reenviar), que permita a un mensaje llegar a grupos desconectados de vehículos dentro de la zona de destino, aumenta el número de duplicados necesarios. Para conseguir estos objetivos, primero estudiamos cómo esquemas típicos de diseminación sin apoyo de infraestructura, tomados del estado del arte en MANETs, más uno nuevo y específico, se pueden aplicar en VANETs. De acuerdo con los resultados en relación con una serie de métricas, hemos aprendido que el esquema basado en distancia es el que mejor cubre nuestros requisitos. Seleccionamos este para crear un esquema optimizado para los dos tipos de escenarios existentes: carretera (entorno interurbano) y ciudad (entorno urbano). En cuanto a la adaptación al entorno interurbano, comenzamos optimizando el esquema de modo que su tasa de reenvío esté tan cerca del mínimo como sea posible, y analizando su retardo medio por salto en una red conectada (es decir, que hay al menos una ruta posible entre dos nodos cualesquiera de la red). A continuación, estudiamos cómo añadir un mecanismo “store-carry-forward" específico para nuestra solución que, con cambios mínimos, consiga superar particiones de red breves. Validamos este añadido y el esquema completo bajo diferentes cargas de canal y en contraste con un conocido protocolo para este mismo tipo de tráfico, DV-CAST. Nuestro trabajo en la versión para escenarios urbanos parte del supuesto de que necesitamos detectar intersecciones y reaccionar en consecuencia para poder extender la diseminación en nuevas direcciones y alcanzar tantos vehículos como sea posible. Creamos dos modificaciones del esquema basado en distancia, cada una en base a un método distinto para detectar cruces, y las probamos junto con el esquema básico. Este primer paso nos lleva a descubrir que no es necesaria dicha detección para poder conseguir buenos resultados. Después, de forma similar al proceso que seguimos para el escenario de carretera, trabajamos en optimizar el esquema y crear un mecanismo “store-carry-forward" apropiado. Seguimos el mismo razonamiento pero en esta ocasión consideramos tres opciones diferentes para las repetidas retransmisiones. Probamos cada versión del esquema concienzudamente con simulaciones, utilizando mapas reales de ciudades, y comparamos los resultados con los del equivalente urbano de DV-CAST, llamado UV-CAST. Usamos simuladores validados como ns-2 y Veins para probar de forma realista las diferentes etapas de nuestro trabajo. Las prestaciones de los esquemas resultantes cumplen con nuestros requisitos en un alto grado, por lo que consideramos que hemos conseguido alcanzar nuestros objetivos. Además, el trabajo realizado hasta el momento abre la puerta a nuevas líneas de investigación que son, bien consecuencia natural, bien aplicación de nuestros logros.Programa Oficial de Doctorado en Ingeniería TelemáticaPresidente: Juan Carlos Cano Escriba.- Secretario: Florina Almenares Mendoza.- Vocal: José Marta Barcelo Ordina

Advanced Transport Protocols for Wireless and Mobile Ad Hoc Networks

This thesis comprises transport protocols in the following different areas of research: Fast Handover allows mobile IP end-devices to roam between wireless access routers without interruptions while communicating to devices in an infrastructure (e.g., in the Internet). This work optimizes the Fast Handover algorithm and evaluates the performance of the transport protocols UDP and TCP during fast handovers via measurements. The following part of the thesis focuses on vehicular ad hoc networks. The thesis designs and evaluates through simulations a point-to-point transport protocol for vehicular ad hoc networks and an algorithm to facilitate the reliable and efficient distribution of information in a geographically scoped target area. Finally, the thesis evaluates the impact of wireless radio fluctuations on the performance of an Ad Hoc Network. Measurements quantify the wireless radio fluctuations. Based on these results, the thesis develops a simple but realistic radio model that evaluates by means of simulations the impact on the performance of an ad hoc network. As a result, the work provides guidelines for future ad hoc protocol design

Use of Inferential Statistics to Design Effective Communication Protocols for Wireless Sensor Networks

This thesis explores the issues and techniques associated with employing the principles of inferential statistics to design effective Medium Access Control (MAC), routing and duty cycle management strategies for multihop Wireless Sensor Networks (WSNs). The main objective of these protocols are to maximise the throughput of the network, to prolong the lifetime of nodes and to reduce the end-to-end delay of packets over a general network scenario without particular considerations for specific topology configurations, traffic patterns or routing policies. WSNs represent one of the leading-edge technologies that have received substantial research efforts due to their prominent roles in many applications. However, to design effective communication protocols for WSNs is particularly challenging due to the scarce resources of these networks and the requirement for large-scale deployment. The MAC, routing and duty cycle management protocols are amongst the important strategies that are required to ensure correct operations of WSNs. This thesis makes use of the inferential statistics field to design these protocols; inferential statistics was selected as it provides a rich design space with powerful approaches and methods. The MAC protocol proposed in this thesis exploits the statistical characteristics of the Gamma distribution to enable each node to adjust its contention parameters dynamically based on its inference for the channel occupancy. This technique reduces the service time of packets and leverages the throughput by improving the channel utilisation. Reducing the service time minimises the energy consumed in contention to access the channel which in turn prolongs the lifetime of nodes. The proposed duty cycle management scheme uses non-parametric Bayesian inference to enable each node to determine the best times and durations for its sleeping durations without posing overheads on the network. Hence the lifetime of node is prolonged by mitigating the amount of energy wasted in overhearing and idle listening. Prolonging the lifetime of nodes increases the throughput of the network and reduces the end-to-end delay as it allows nodes to route their packets over optimal paths for longer periods. The proposed routing protocol uses one of the state-of-the-art inference techniques dubbed spatial reasoning that enables each node to figure out the spatial relationships between nodes without overwhelming the network with control packets. As a result, the end-to-end delay is reduced while the throughput and lifetime are increased. Besides the proposed protocols, this thesis utilises the analytical aspects of statistics to develop rigorous analytical models that can accurately predict the queuing and medium access delay and energy consumption over multihop networks. Moreover, this thesis provides a broader perspective for design of communication protocols for WSNs by casting the operations of these networks in the domains of the artificial chemistry discipline and the harmony search optimisation algorithm