Academic literature on the topic 'Contention window'

Internet of Things (IoT) network contains heterogeneous resource-constrained computing devices which has its unique reputation in IoT environments. In spite of its distinctiveness, the network performance deteriorates by the distributed contention of the nodes for the shared wireless medium in IoT. In IoT network, the Medium Access Control (MAC) layer contention impacts the level of congestion at the transport layer. Further, the increasing node contention at the MAC layer increases link layer frame drops resulting in timeouts at the transport layer segments and the performance of TCP degrades. In addition to that, the expiration of maximum retransmission attempts and the high contentions drive the MAC retransmissions and the associated overheads to reduce the link level throughput and the packet delivery ratio. In order to deal with aforementioned problems, the Adaptive Contention Window (ACW) is proposed, which aims to reduce the MAC overhead and retransmissions by determining active queue size at the contending nodes and the energy level of the nodes to improve TCP performance. Further, the MAC contention window is adjusted according to the node’s active queue size and the residual energy and TCP congestion window is dynamically adjusted based on the MAC contention window. Hence, by adjusting the MAC Adaptive Contention Window, the proposed model effectively distributes the access to medium and assures improved network throughput. Finally, the simulation study implemented through ns-2 is compared with an existing methodology such as Cross-Layer Congestion Control and dynamic window adaptation (CC-BADWA); the proposed model enhances the network throughput with the minimal collisions.

This paper proposes the several contention window adjustment schemes in backoff process as well-known backoff algorithm (BA) for improving the performance of wireless local area network (WLAN). In addition, this research introduces a new unsaturated discrete Markov chain model in fixed backoff stages and fixed contention window sizes technique (FBFC). The proposed contention window adjustment schemes are designed by applying the moment generating function concept in random variable and process theorem. Unsaturated throughput parameters are used to compare the performance of all contention window size adjustment techniques based on IEEE802.11b WLAN standards. The comparison results show that Bernoulli and Double adjustment schemes are good contention window size adjustments at light traffic load, and the Even contention window size adjustment operates well at high traffic load condition.

In this paper, we tried to presented the effect of different contention window (CW) size in a realistic road environment with mobility management and different propagation loss model was implemented in ns-3. We focus on the WAVE module of ns-3. Our contribution is to create a realistic two-lane both way highway road environment surrounding by fields, low-rise commercial buildings, street lamps and road signs for vehicular safety messages are Cooperative Awareness Message (CAM) and Decentralized Event Notification Message (DENM). Medium to low traffic density. On the other section, we describe in detail the simulation setup scenario during the implementation. Finally, we present our simulation results that we obtained from the highway scenario.

In order to cope with the rapid development of the Internet of things, IEEE working group has proposed a new wireless network technology, 802.11ah. IEEE 802.11ah operates at sub 1GHz band, and can provide long transmission range, wide coverage, while supporting more than 6,000 of station to be connected. In order to alleviate the high collision probability, 802.11ah MAC protocol involves in the RAW (restricted access window) mechanism combined with grouping mechanism. However, in 802.11ah WLAN (wireless local area network), most of the stations operate at sleep mode, and it is expected that at the start of a BI, the network suffers much higher collision probability than at the latter period of the BI. In order to improve these drawbacks, we propose an adaptive contention window scheme based on which an optimum contention window size is adapted at the start of the BI, and then gradually halved when the transmission completes successfully. Through conducting simulations, we prove that our proposed protocol can enhance the performance of 802.11ah WLAN.

This work was supported by the PhD FCT grant SFRH/BD/38356/2007 and IT grant PEst-OE/EEI/ LA0008/2011. I also acknowledge several projects where I was involved in: COST IC1004, COST 2100, MobileMAN, OPPORTUNISTIC-CR, PROENERGY-WSN, Smart-Clothing, and INSYSM
Nowadays, the user of Wireless Sensor Networks (WSNs) is becoming more and more demanding in terms of choice and diversity of applications. As a consequence, as the diversity of applications continues to grow there is a need to identify and classify the set of detailed characterization parameters that facilitates to sketch up a taxonomy for WSN applications. The proposed taxonomy identi es the services offered by each application makes a tool available to better understand the services and requirements of each application, along with a holistic overview of the WSN proposed application taxonomy. The research also involved the actual development of WSN applications within different research projects, namely in the elds of healthcare (Smart Clothing), civil engineering structure monitoring (INSYSM) and precision agriculture. Different medium access control mechanisms employ different collision avoidance schemes to cope with packet collision and retransmission, trading-off complexity, energy inef ciency and control of packet overhead. In particular, this PhD thesis addresses the study the packet collision probability for a MAC protocol that employs a collision avoidance mechanism with two contention window and consequent proposal of a model for the collision probability. Simulation results validate the model for saturated traf c. For unsaturated traf c and with a small number of nodes, the accuracy of the model is limited by numerical rounding. It is shown that, by using our analytical model, we have been able to obtain performance metrics such as network throughput and average service time for the successful transmissions. In addition, the Enhanced Reliability Decision Algorithm in the physical layer has been proposed. The frame capture effect (FC) feature has been implemented in the IEEE 802.15.4 compliant physical layer of the MiXiM framework. The proposed decision algorithm utilizes the Signal to Noise-plus-Interference ratio (SNIR) and the size of the packet to guarantee the delivery with certain reliability to the MAC layer, of a packet received at the PHY layer. A gain of more than 10 % has been achieved in the delivery ratio. Promising results have also been obtained for the SCP-MAC protocol with the FC effect enabled, for different values of reliability. As one of the main contributions of this thesis, an innovative ef cient multi-channel MAC protocol, based on SCP-MAC, was proposed, the so-called Multi-Channel Scheduled Channel Polling (MC-SCP-MAC) protocol. The in uential range concept, denial channel list (which considers the degradation metric of each slot channel), extra resolution phase algorithm and frame capture effect have been explored to achieve the maximum performance in terms of delivery ratio and energy consumption. It has been shown MC-SCP-MAC outperforms SCP-MAC and MC-LMAC in denser scenarios, with improved throughput fairness. Considering the in uential range concept reduces the redundancy level in the network facilitating to reduce the energy consumption whilst decreasing the latency. The conclusions from this research reveal the importance of an appropriate design for the MAC protocol for the desired WSN application. Depending on the objective or mission of the WSN application, different protocols are required. Therefore, the overall performance of a WSN application certainly depends on the appropriate development and application of the appropriate communication protocols (e.g., MAC, network layer).

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.

Ad hoc networks represent the current de-facto alternative for infrastructure-less environments, due to their self-configuring and resilience characteristics. Ad hoc networks flexibility benefits, such as unrestrained computing, lack of centralisation, and ease of deployment at low costs, are tightly bound with relevant deficiencies such as limited resources and management difficulty. Ad hoc networks witnessed high attention from the research community due to the numerous challenges faced when deploying such a technology in real scenarios. Starting with the nature of the wireless environment, which raises significant transmission issues when compared with the wired counterpart, ad hoc networks require a different approach when addressing the data link problems. Further, the high packet loss due to wireless contention, independent of network congestion, requires a different approach when considering quality of service degradation and unfair channel resources distribution among competing flows. Although these issues have already been considered to some extent by researchers, there is still room to improve quality of service by reducing the effect of packet loss and fairly distributing the medium access among competing nodes. The aim of this thesis is to propose a set of mechanisms to alleviate the effect of packet loss and to improve fairness in ad hoc networks. A transport layer algorithm has been proposed to overcome the effects of hidden node collisions and to reduce the impact of wireless link contention by estimating the four hop delay and pacing packet transmissions accordingly. Furthermore, certain topologies have been identified, in which the standard IEEE 802.11 faces degradation in channel utilisation and unfair bandwidth allocation. Three link layer mechanisms have been proposed to tackle the challenges the IEEE 802.11 faces in the identified scenarios to impose fairness in ad hoc networks through fairly distributing channel resources between competing nodes. These mechanisms are based on monitoring the collision rate and penalising the greedy nodes where no competing nodes can be detected but interference exists, monitoring traffic at source nodes to police access to the channel where only source nodes are within transmission range of each other, and using MAC layer acknowledgements to flag unfair bandwidth allocation in topologies where only the receivers are within transmission range of each other. The proposed mechanisms have been integrated into a framework designed to adapt and to dynamically select which mechanism to adopt, depending on the network topology. It is important to note that the proposed mechanisms and framework are not alternatives to the standard MAC protocol but are an enhancement and are triggered by the failure of the IEEE 802.11 protocol to distribute the channel resources fairly. All the proposed mechanisms have been validated through simulations and the results obtained from the experiments show that the proposed schemes fairly distribute channel resources fairly and outperform the performance of the IEEE 802.11 protocol in terms of channel utilisation as well as fairness.

Cette thèse analyse la possibilité d'utiliser des communications sans fil inter-véhiculaires pour améliorer la sécurité routière. Les performances du nouveau réseau ainsi créé (réseau ad-hoc véhiculaire) sont étudiées analytiquement et par des simulations dans un environnement réaliste. La thèse se concentre surtout sur des scénarios avec une forte densité de véhicules. Dans ce cas, l'accès au support devient un problème essentiel, en principal pour les applications de sécurité routière qui nécessitent une qualité de service élevée pour fonctionner dans un tel contexte. Ce travail montre que la version actuelle du standard IEEE 802.11, proposé comme méthode d'accès dans les réseaux véhiculaires, ne peut pas résoudre ce problème de passage à l'échelle pour supporter correctement les applications de sécurité routière. Plusieurs améliorations possibles sont analysées, liées à l'utilisation optimale de certains paramètres du protocole comme la taille de la fenêtre de contention ou bien le seuil de détection de la porteuse. Des nouveaux mécanismes adaptatifs visant ces paramètres sont proposés et les améliorations ainsi obtenues sont non-négligeables. Finalement, une nouvelle méthode d'accès est définie, en tenant compte des caractéristiques des applications de sécurité routière. Toujours basée sur des techniques CSMA, cette technique donne des résultats largement supérieurs à la version standard actuelle
The equipment of vehicles with wireless communication devices in order to improve road safety is a major component of a future intelligent transportation system. The success and availability of IEEE 802.11-based products make this technology the main competitor for the Medium Access Control (MAC) layer used in vehicle-to-vehicle communication. The IEEE 802.11p amendment has been specially designed in this special context of wireless access in vehicular environments. However, as all the other approaches based on Carrier Sense Multiple Access (CSMA), this protocol presents scalability problems, which leads to poor performance in high density scenarios, quite frequent in the case of a vehicular ad hoc network (VANET). This thesis studies the congestion control problem in the context of safety vehicular communications, with a special focus on the back-off mechanism and the carrier sense function. First of all, a number of important characteristics presented by the safety messages are discovered and understood by the means of an analytical framework. Second, the lessons learned from the analytical study are put into practice with the design of two adaptive mechanisms (one for the contention window and the other one for the carrier sense threshold) that take into account the local vehicular density. These mechanisms remain simple, but highly efficient, while also being straightforward to integrate in IEEE 802.11 devices. Finally, by taking into account the most important properties of a safety VANET, a new CSMA-based MAC protocol is proposed. This new access method, named Safety Range CSMA (SR-CSMA), relies on the idea that collisions can not be avoided in a high density network. However, by increasing the number of simultaneous transmissions between geographically distant nodes, SR-CSMA manages to better protect the immediate neighborhood, the most important area for safety applications

[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
[CAT] 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/10251/64073
TESIS

碩士
長庚大學
資訊工程研究所
92
In wireless local area networks (WLANs), the IEEE 802.11 medium access control (MAC) protocol has been the main element that determines the efficiency of sharing the limited communication bandwidth of the wireless channel. The performance of the IEEE 802.11 protocol could be improved by using an appropriate turning of the backoff algorithm to approach a theoretical throughput limit. However, in real case, a mobile node would not exactly know the information of network and load configurations (e.g., number of active nodes). Hence, in this thesis, we propose a novel and efficient selection of backoff window size mechanism using fuzzy reasoning approach named the fuzzy backoff controller (FBC) to achieve the system throughput limit. This mechanism is developed on the following ideas: by observing the busy degree of medium and the number of neighbors, the proposed FBC could generate a proper backoff window thus reduce the probability of collisions and enhance the throughput. The obtained simulation results show that the proposed FBC not only significantly improves the performance of the IEEE 802.11 protocol but also achieves the fairness of accessing medium between nodes well.

碩士
中興大學
電機工程學系所
95
IEEE WLAN work team established IEEE 802.11 standard in 1997. It defined two different operation types of Wireless LAN - Infrastructure Wireless LAN and Ad-Hoc Wireless LAN. Among these systems, what basic media competitive scheme IEEE 802.11 MAC uses is DCF scheme. DCF can be used in Infrastructure Wireless LAN and Ad-Hoc Wireless LAN simultaneously, and it utilizes Carrier-Sense Multiple Access, Collision Avoidance (can be abbreviated as CSMA/CA), and Random Backoff Scheme to make station to transmit and receive information. Although DCF uses the techniques mentioned above may avoid some collision during the transmission, it sometimes also causes stations waiting for a long time. Every time when a lot of workstations compete for accessing the channel, it is easy to create the same backoff time which may produce collision; however, after collision, repeatedly, it needs to take a long time to get into the competitive cycle. For this reason, how can we create a compatible contention window to reduce the collisions when facing the great amount of stations is a important study. In this paper, we provide an approach of generating a new contention window to reduce the opportunities of collisions happened in the systems. The main idea is aiming at that, under the channel condition of heavy load, stations produces the same backoff time and collides easily. After the collision occurs, we may adopt approaches we assumed to lessen the opportunities of it and to promote the performance of whole transmission.

碩士
國立中正大學
電機工程所
94
In the IEEE 802.11 standard, MAC layer provides the Distributed Coordination Function, in which the Binary Exponential Backoff (BEB) mechanism is used for the contention control. BEB attempts to randomly select the backoff time slots before they can transmit data. Such mechanism efficiently avoids multiple stations to transmit data at the same time, and thus reduce the collision probability. However, the conventional BEB does not perform well when traffic is heavy because under such traffic condition collision occurs frequently, which results in serious delay as well as low. The emphasis of this thesis is on the development of a novel backoff mechanism, the Optimal Backoff (OB) mechanism. Proposed OB can choose an optimal contention window and thus efficiently avoid the collision problem under both light and heavy traffic scenario. Both the analytical and simulation results have confirmed that the proposed OB algorithm can find an optimal contention window according to different traffic load and thus achieve high throughput, low delay as well as low collision probability.

碩士
國立彰化師範大學
資訊工程學系
102
In next generation mobile networks, more and more throughput of network is required under the dense environment. There are some researches discuss how to solve the predicament of throughput. But they usually analyze it under the saturated environment. Most of their contention window size are also not optimal to fetch the maximum throughput. The thesis is the first one to fetch the fixed optimal window size under the unsaturated environment which is closer to the real network. The thesis is based on the Distributed Coordination Function (DCF) in Medium Access Control layer of IEEE 802.11 Wireless LAN to control the contention window size for reducing the collision problem and improving the throughput. A simplified one-dimension Markov Chain with a new idle state is proposed to simulate the unsaturated model. Then the formula of transmission probability under the unsaturated model and the equation which is used to fetch the optimized contention window size are analyzed. Comparing with the related analysis model in the simulation of the throughput and the collision rate. The result shows that the proposed model under unsaturated environment is better than before.

碩士
國立中興大學
電機工程學系所
102
Power line communication (PLC) is one of the most popular topics in recent years. Because it uses existing power line for transmission, it has lower cost while compared with other communications. Since power lines has exist almost everywhere, even in the remote areas. This makes PLC to be one of the ways to solve the ‘last mile’ problem. Since there are more and more people use PLC products in recent years, the PLC congestion problem has also becoming the important research issue. PLC adopts Carrier Sense Multiple Access/ Collision Avoidance (CSMA/CA) to deal with the packet collision problem. There exists “packet collision” in the communications. While there are several data using the same channel for transmission at the same time, they will interfere with each other, which cause data transmission failure and require to retransmit data. Retransmitted data will lead to the serious decline of throughput When the node number in the network environment increases, the probability collision will increase which cause to the low efficiency in transmission. Therefore, one can regard the collision problem as a congestion problem. The HomePlug AV is one of the most famous protocols in PLC. It adopts CSMA/CA to prevent the occurrence of collision. However, although HomePlug AV can keep high throughput in the situation of low node number, it still has the serious throughput decline in the large scale network. That’s because the “contention window” defined in CSMA/CA is not large enough. However,if one increases the contention window setting in the protocol, the network will exhibit lower throughput when the node numbers is few. To tackle the issue, this research proposes an adaptive contention window mechanism to this problem. According to the information of CSMA/CA parameters, obtain the correction factor to adjust the contention window. When the node number is relatively few, it will give a small contention window value. When the node number increases, it automatically increases the contention window value accordingly. Thus, when the node number become large, it still exhibits low collision probability.

Pollution is one of the most pressing issues facing contemporary China and among the most prominent causes for unrest. Much of industry and mining takes place in rural areas, yet we know little about how rural communities affected by severe pollution make sense of it and the diverse form of activism they embrace. This book describes some of these engagements with pollution through three in-depth case studies based on the author’s fieldwork and an analysis of “cancer villages” examined in existing social science accounts. It challenges assumptions that villagers are ignorant about pollution or fully complicit with it and it looks beyond high-profile cases and beyond single strategies. It examines how villagers’ concerns and practices evolve over time and how pollution may become normalised. Through the concept of “resigned activism”, it advocates rethinking conventional approaches to activism to encompass less visible forms of engagement. It offers insights into the complex dynamics of popular contention, environmental movements and their situatedness within local and national political economies. Describing a likely widespread scenario across much of industrialised rural China, this book provides a window onto the staggering human costs of development and the deeply uneven distribution of costs and benefits. It portrays rural environmentalism and its limitations as prisms through which to study key issues surrounding contemporary Chinese culture and society, such as state responsibility, social justice, ambivalence towards development and modernisation and some of the new fault lines of inequality and social conflict which they generate.

A true story of vendetta and intrigue, triumph and tragedy, exile and repatriation in early modern Venice, this book focuses on the marriage between the feudal lord Count Girolamo Della Torre and Giulia Bembo, daughter of a powerful Venetian senator and grand-niece of Cardinal Pietro Bembo. Exiled to Crete for pursuing vendetta to avenge the murder of his father, Girolamo marries Giulia with the aim of enlisting her father as a powerful ally. Thus begins a challenging itinerary that leads from the Mediterranean back to Venice and its mainland territories in the Veneto and the Patria del Friuli. It plays out against a backdrop of the birth of ten children, the Council of Trent, papal and imperial politics, the rise of Girolamo’s brother Michele to the cardinalate, the Ottoman threat, and the golden age of Venetian art. Once a pawn in a marital strategy that failed, Giulia is celebrated after her death with the first independent biography of an ordinary woman published in Italy. The fortunes and misfortunes of the Della Torre bloodline, which survived the end of the Venetian Republic in 1797, are emblematic of a change in feudal culture from clan solidarity to individualism and intrafamily strife, and ultimately redemption. This epic tale opens a precious window into a contentious period in which Venetian republican values clash with the deeply rooted feudal traditions of honour and blood feuds of the mainland.

Black Radio is a window into the most famous radio show you never heard of. The Tom Joyner Morning Show is a quarter-century-old syndicated black morning radio show reaching more than eight million adult, largely working-class listeners. It offers progressive political talk, soul music, humor, advice, philanthropy, and celebrity gossip. But the TJMS is not just an adult “old-school music” radio show: it is an on-air organizer, fusing progressive politics and aesthetics. It focuses on specific political issues affecting and enraging African Americans. Black Radio analyzes the TJMS’s rise in the Clinton era, and its coverage of key events—9/11, Hurricane Katrina, President Obama’s elections and terms, the murders of unarmed black Americans and the rise of Black Lives Matter, and the shocking 2016 Donald Trump electoral triumph. It showcases the varied, contentious, and blackly humorous voices of anchors, guests, and audience members. Finally, it investigates the new synergistic set of cross-medium ties and political connections now affecting print, broadcast, and online politics in anti-racist directions. Despite the dismal present, this new multiracial progressive public sphere has extraordinary potential for shaping future American politics. Black Radio, then, is more than the project of making the invisible visible, bringing to light a major counterpublic phenomenon unjustly ignored for reasons of color, class, generation, and medium. It tunes us in to an alternative understanding of the black public sphere in the digital age. Like the show itself, Black Radio is politically progressive, music-drenched, angry, and blisteringly funny.

Constantly in the news and the subject of much public debate, fracking, as it is known for short, is one of the most promising yet controversial methods of extracting natural gas and oil. Today, 90 percent of natural gas wells use fracking. Though highly effective, the process-which fractures rock with pressurized fluid-has been criticized for polluting land, air, and water, and endangering human health. A timely addition to Oxford's What Everyone Needs to Know series, Hydrofracking tackles this contentious topic, exploring both sides of the debate and providing a clear guide to the science underlying the technique. In concise question-and-answer format, Alex Prud'homme cuts through the maze of opinions and rhetoric to uncover key points, from the economic and political benefits of fracking to the health dangers and negative effects on the environment. Prud'homme offers clear answers to a range of fundamental questions, including: What is fracking fluid? How does it impact water supplies? Who regulates the industry? How much recoverable natural gas exists in the U.S.? What new innovations are on the horizon? Supporters as diverse as President Obama and the conservative billionaire T. Boone Pickens have promoted natural gas as a clean, "21st-century" fuel that will reduce global warming, create jobs, and provide tax revenues, but concerns remain, with environmental activists like Bill McKibben and others leading protests to put an end to fracking as a means of obtaining alternative energy. Prud'homme considers ways to improve methods in the short-term, while also exploring the possibility of transitioning to more sustainable resources-wind, solar, tidal, and perhaps nuclear power-for the long term. Written for general readers, Hydrofracking clearly explains both the complex science of fracking and the equally complex political and economic issues that surround it, giving readers all the information they need to understand what will no doubt remain a contentious issue for years to come.

The New Deal era witnessed a surprising surge in popular engagement with the history and memory of the Civil War era. From the omnipresent book and film Gone with the Wind and the scores of popular theater productions to Aaron Copeland's "A Lincoln Portrait," it was hard to miss America's fascination with the war in the 1930s and 1940s. Nina Silber deftly examines the often conflicting and politically contentious ways in which Americans remembered the Civil War era during the years of the Depression, the New Deal, and World War II. In doing so, she reveals how the debates and events of that earlier period resonated so profoundly with New Deal rhetoric about state power, emerging civil rights activism, labor organizing and trade unionism, and popular culture in wartime. At the heart of this book is an examination of how historical memory offers people a means of understanding and defining themselves in the present. Silber reveals how, during a moment of enormous national turmoil, the events and personages of the Civil War provided a framework for reassessing national identity, class conflict, and racial and ethnic division. The New Deal era may have been the first time Civil War memory loomed so large for the nation as a whole, but, as the present moment suggests, it was hardly the last.

This book sets out to answer a question of global importance: how does inequality persist in our increasingly mobile world? It is a contentious problem. From Barack Obama to Pope Francis, inequality is often referred to as the greatest threat to our democracy, society and economy. Yet in an era some call the ‘age of migration’, opportunity has apparently never been more accessible. Long and short distance transport—from motorbikes to aeroplanes—are available to more people than ever before. What’s more, physical mobility tells only part of the story. Telecommunications have transformed our lives, ushering in an era of translocality, in which the behaviour of people and communities are influenced from hundreds or even thousands of miles apart. Nevertheless, amidst ever more complex flows of people, ideas, and capital, persistent inequality cuts a jarringly static figure. The worst off all too often remain impervious to the winds of economic dynamism, whilst those who were better off in one place remain so in another. This is an age-old story enmeshed in modern complexities. The vast economic successes of India and China have redrawn the map of global poverty in recent decades, contributing to falling inequality between countries even as inequality within countries is on the rise. Scale, in other words, matters and this book sets out to show why. Eschewing the international cross-sectional analysis employed in others on the topic of inequality, in favour of a deep dive approach to its subject, its eight chapters bring together a decade of research across multiple contexts to cast a forensic eye over the many of faces of inequality in a rapidly changing environment. Tracing a “miraculous” decade of development in Cambodia, one of the world’s fastest growing economies since the turn of the millennium, it brings together a broad toolbox of data to make a case for inequality not as an economic phenomenon, but as a ‘total social fact’ in which stories, stigma, obligation, and assets combine to lock social structures in place.