We present a Markov chain analysis for studying the performance of wireless ad hoc networks. The models presented in this dissertation support an arbitrary backoff strategy. We found that the most important parameter affecting the performance of binary exponential backoff is the initial backoff window size. Our experimental results show that the probability of collision can be reduced when the initial backoff window size equals the number of terminals. Thus, the throughput of the system increases and, at the same time, the delay to transmit the frame is reduced. In our second contribution, we present a new analytical model of a Medium Access Control (MAC) layer for wireless ad hoc networks that takes into account frame retry limits for a four-way handshaking mechanism. This model offers flexibility to address some design issues such as the effects of traffic parameters as well as possible improvements for wireless ad hoc networks. It effectively captures important network performance characteristics such as throughput, channel utilization, delay, and average energy. Under this analytical framework, we evaluate the effect of the Request-to-Send (RTS) state on unsuccessful transmission probability and its effect on performance particularly when the hidden terminal problem is dominant, the traffic is heavy, or the data frame length is very large. By using our proposed model, we show that the probability of collision can be reduced when using a Request-to-Send/Clear- to-Send (RTS/CTS) mechanism. Thus, the throughput increases and, at the same time, the delay and the average energy to transmit the frame decrease. In our third contribution, we present a new analytical model of a MAC layer for wireless ad hoc networks that takes into account channel bit errors and frame retry limits for a two-way handshaking mechanism. This model offers flexibility to address design issues such as the effects of traffic parameters and possible improvements for wireless ad hoc networks. We illustrate that an important parameter affecting the performance of binary exponential backoff is the initial backoff window size. We show that for a low bit error rate (BER) the throughput increases and, at the same time, the delay and the average energy to transmit the frame decrease. Results show also that the negative acknowledgment-based (NAK-based) model proves more useful for a high BER. In our fourth contribution, we present a new analytical model of a MAC layer for wireless ad hoc networks that takes into account Quality of Service (QoS) of the MAC layer for a two-way handshaking mechanism. The model includes a high priority traffic class (class 1) and a low priority traffic class (class 2). Extension of the model to more QoS levels is easily accomplished. We illustrate an important parameter affecting the performance of an Arbitration InterFrame Space (AIFS) and small backoff window size limits. They cause the frame to start contending the channel earlier and to complete the backoff sooner. As a result, the probability of sending the frame increases. Under this analytical framework, we evaluate the effect of QoS on successful transmission probability and its effect on performance, particularly when high priority traffic is dominant. / Graduate

Wireless Sensor Networks (WSNs) have enticed a strong attention in the research community due to the broad range of applications and services they support. WSNs are composed of intelligent sensor nodes that have the capabilities to monitor different types of environmental phenomena or critical activities. Sensor nodes operate under stringent requirements of scarce power resources, limited storage capacities, limited processing capabilities, and hostile environmental surroundings. However, conserving sensor nodes’ power resources is the top priority requirement in the design of a WSN as it has a direct impact on its lifetime. The IEEE 802.15.4 standard defines a set of specifications for both the PHY layer and the MAC sub-layer that abide by the distinguished requirements of WSNs. The standard’s MAC protocol employs an intelligent backoff algorithm, called the Binary Exponent Backoff (BEB), that minimizes the drainage of power in these networks. In this thesis we present an in-depth study of the IEEE 802.15.4 MAC protocol to highlight both its strong and weak aspects. We show that we have enticing opportunities to improve the performance of this protocol in the context of WSNs. We propose three new backoff algorithms, namely, the Standby-BEB (SB-BEB), the Adaptive Backoff Algorithm (ABA), and the Priority-Based BEB (PB-BEB), to replace the standard BEB. The main contribution of the thesis is that it develops a new design concept that drives the design of efficient backoff algorithms for the IEEE 802.15.4-based WSNs. The concept dictates that controlling the algorithms parameters probabilistically has a direct impact on enhancing the backoff algorithm’s performance. We provide detailed discrete-time Markov-based models (for AB-BEB and ABA) and extensive simulation studies (for the three algorithms) to prove the superiority of our new algorithms over the standard BEB.

Wireless Sensor Networks (WSNs) have enticed a strong attention in the research community due to the broad range of applications and services they support. WSNs are composed of intelligent sensor nodes that have the capabilities to monitor different types of environmental phenomena or critical activities. Sensor nodes operate under stringent requirements of scarce power resources, limited storage capacities, limited processing capabilities, and hostile environmental surroundings. However, conserving sensor nodes’ power resources is the top priority requirement in the design of a WSN as it has a direct impact on its lifetime. The IEEE 802.15.4 standard defines a set of specifications for both the PHY layer and the MAC sub-layer that abide by the distinguished requirements of WSNs. The standard’s MAC protocol employs an intelligent backoff algorithm, called the Binary Exponent Backoff (BEB), that minimizes the drainage of power in these networks. In this thesis we present an in-depth study of the IEEE 802.15.4 MAC protocol to highlight both its strong and weak aspects. We show that we have enticing opportunities to improve the performance of this protocol in the context of WSNs. We propose three new backoff algorithms, namely, the Standby-BEB (SB-BEB), the Adaptive Backoff Algorithm (ABA), and the Priority-Based BEB (PB-BEB), to replace the standard BEB. The main contribution of the thesis is that it develops a new design concept that drives the design of efficient backoff algorithms for the IEEE 802.15.4-based WSNs. The concept dictates that controlling the algorithms parameters probabilistically has a direct impact on enhancing the backoff algorithm’s performance. We provide detailed discrete-time Markov-based models (for AB-BEB and ABA) and extensive simulation studies (for the three algorithms) to prove the superiority of our new algorithms over the standard BEB.

Wireless Sensor Networks (WSNs) have enticed a strong attention in the research community due to the broad range of applications and services they support. WSNs are composed of intelligent sensor nodes that have the capabilities to monitor different types of environmental phenomena or critical activities. Sensor nodes operate under stringent requirements of scarce power resources, limited storage capacities, limited processing capabilities, and hostile environmental surroundings. However, conserving sensor nodes’ power resources is the top priority requirement in the design of a WSN as it has a direct impact on its lifetime. The IEEE 802.15.4 standard defines a set of specifications for both the PHY layer and the MAC sub-layer that abide by the distinguished requirements of WSNs. The standard’s MAC protocol employs an intelligent backoff algorithm, called the Binary Exponent Backoff (BEB), that minimizes the drainage of power in these networks. In this thesis we present an in-depth study of the IEEE 802.15.4 MAC protocol to highlight both its strong and weak aspects. We show that we have enticing opportunities to improve the performance of this protocol in the context of WSNs. We propose three new backoff algorithms, namely, the Standby-BEB (SB-BEB), the Adaptive Backoff Algorithm (ABA), and the Priority-Based BEB (PB-BEB), to replace the standard BEB. The main contribution of the thesis is that it develops a new design concept that drives the design of efficient backoff algorithms for the IEEE 802.15.4-based WSNs. The concept dictates that controlling the algorithms parameters probabilistically has a direct impact on enhancing the backoff algorithm’s performance. We provide detailed discrete-time Markov-based models (for AB-BEB and ABA) and extensive simulation studies (for the three algorithms) to prove the superiority of our new algorithms over the standard BEB.

The dissertation thesis is focused on analysis, modeling and optimization of the medium access layer of power line communication. In the first part of the paper, the thesis briefly summarizes a basic information of power line communication. In the next sections, the objectives of the dissertation are placed. A simplification of current models of HomePlug and a optimalization of distribution function in back off procedure are one of the main goals of this work. A detailed analysis of HomePlug technology, which is widespread used in the broadband transmission, follows. The work is focused on analyzing of the MAC layer in this part. The discrete simulation models have been implemented in Matlab. The approximation discrete numerical model has been used for analysis of various distribution functions for HomePlug to optimalize backoff algorithm. The new optimalizations are suitable for large networks. The new approach of MAC modelling is introduced in last part of this work. The new model is not exact as Markov models but it is faster and easier for implementation. The analytical model desribes a system using probabilistic functions.

Society depends on computer networks for communication. The networks were built to support and facilitate several important applications such as email, web browsing and instant messaging. Recently, there is a significant interest in leveraging modern local and wide area communication networks for improving reliability and performance in critical infrastructures. Emerging critical infrastructure applications, such as smart grid, require a certain degree of reliability and Quality of Service (QoS). Supporting these applications requires network protocols that enable delay sensitive packet delivery and packet prioritization. However, most of the traditional networks are designed to provide best effort service without any support for QoS. The protocols used in these networks do not support packet prioritization, delay requirements and reliability. In this thesis, we focus on the design and analysis of communication protocols for supporting smart grid applications. In particular, we focus on the Substation Automation Systems (SAS). Substations are nodes in the smart grid infrastructure that help the in transportation of power by connecting the transmission and distribution lines. The SAS applications are con figured to operate with minimal human intervention. The SAS monitors the line loads continuously. If the load values are too high and can lead to damage, the SAS declares those conditions as faults. On fault detection, the SAS must take care of the communication with the relay to open the circuit to prevent any damage. These messages are of high priority and require reliable, delay sensitive delivery. There is a threshold for the delay of these messages, and a slight increase in the delay above the threshold might cause severe damages. Along with such high priority messages, the SAS has a lot of background traffic as well. In spite of the background traffic, the substation network must take care of delivering the priority messages on time. Hence, the network plays a vital role in the operation of the substation. Networks designed for such applications should be analyzed carefully to make sure that the requirements are met properly. We analyzed and compared the performance of the SAS under di erent network topologies. By observing the characteristics of the existing architectures, we came up with new architectures that perform better. We have suggested several modi cations to existing solutions that allow significant improvement in the performance of the existing solutions.

Made available in DSpace on 2014-06-12T15:58:29Z (GMT). No. of bitstreams: 2 arquivo4635_1.pdf: 928763 bytes, checksum: f0830b47cdbb5936c9f93e74336e98d4 (MD5) license.txt: 1748 bytes, checksum: 8a4605be74aa9ea9d79846c1fba20a33 (MD5) Previous issue date: 2003 / O protocolo MAC IEEE 802.11 DFWMAC (Distributed Foundation Wireless Access Control) foi padronizado para uso em redes locais sem fio e tem sido utilizado para testar e simular redes locais sem fio ad hoc multihop. Este protocolo tem apresentado problemas quando trabalhamos com redes ad hoc multihop. Este problema fica evidente quando submetemos tráfego TCP (Transmission Control Protocol) entre duas estações. Por tratar-se de um protocolo de controle de acesso ao meio distribuído, não possuindo um controle central, a decisão de transmissão é feita pelas próprias estações de acordo com o funcionamento do DFWMAC. Ainda pelas suas características de funcionamento distribuído, problemas de terminal escondido e terminal exposto podem ocorrer comprometendo de maneira significativa o tráfego de conexões TCP. Associado aos problemas de terminal escondido e terminal exposto , o algoritmo de Backoff Exponencial Binário (BEB) contribui para que este protocolo não funcione bem em redes ad hoc multihop. O resultado da ação de todos estes problemas é a degradação do throughput do TCP gerando instabilidade e injustiça no acesso ao meio compartilhado. A instabilidade fica evidente quando a variação do throughput é muito alta em intervalos de tempo muito curtos. Isto pode ser visto com apenas uma conexão TCP entre duas estações. Já o problema de injustiça aparece quando submetemos duas conexões TCP simultâneas, sendo que uma consegue transmitir pacotes de dados a uma taxa alta, utilizando toda a largura de banda, enquanto a outra conexão não consegue transmitir nenhum pacote permanecendo com o throughput zero durante o tempo em que as duas conexões estão ativas. Este trabalho propõe uma solução para lidar com estes problemas

Over the past few decades, wireless local area networks (WLANs) have been widely deployed for data communication in indoor environments such as offices, houses, and airports. In order to fairly and efficiently use the unlicensed frequency band that Wi-Fi devices share, the devices follow a set of channel access rules, which is called a wireless medium access control (MAC) protocol. It is known that wireless devices following the 802.11 standard MAC protocol, i.e. the distributed coordination function (DCF), suffer from packet collisions when multiple nodes simultaneously transmit. This significantly degrades the throughput performance. Recently, several studies have reported access techniques to reduce the number of packet collisions and to achieve a collision free WLAN. Although these studies have shown that the number of collisions can be reduced to zero in a simple way, there have been a couple of remaining issues to solve, such as dynamic parameter adjustment and fairness to legacy DCF nodes in terms of channel access opportunity. Recently, In-Band Full Duplex (IBFD) communication has received much attention, because it has significant potential to improve the communication capacity of a radio band. IBFD means that a node can simultaneously transmit one signal and receive another signal in the same band at the same time. In order to maximize the performance of IBFD communication capability and to fairly share access to the wireless medium among distributed devices in WLANs, a number of IBFD MAC protocols have been proposed. However, little attention has been paid to fairness issues between half duplex nodes (i.e. nodes that can either transmit or receive but not both simultaneously in one time-frequency resource block) and IBFD capable nodes in the presence of the hidden node problem.

BFT consensus algorithms can ensure the consistency of distributed systems where nodes may behave arbitrarily due to faults or intentional malicious actions. However, most of the practical BFT consensus algorithms are leader-based. In an adversarial network, leader-based BFT consensus algorithms exhibit vulnerabilities and lack resilience. Byzantine leaders can pose a potential threat to the system; firstly, malicious leaders can actively downgrade the processing speed of handling proposals, thereby diminishing the system’s overall performance. Secondly, they can determine the submission order of received requests, which can be fatal in specific decentralized financial systems. Additionally, external attackers can compromise the system’s stability by conducting DDoS attacks on leader nodes, frequently triggering view changes and potentially causing the system to lose liveness altogether. We present BFT Baxos, a more robust and resilient BFT consensus protocol that equips a BFT random exponential backoff mechanism to ensure each node has the egalitarian right to propose. Employing random exponential backoff as a replacement for leader election eliminates the potential malicious actions of Byzantine leaders and prevents external attackers from conducting targeted DDoS attacks on the leader node within systems. We implemented and evaluated our BFT Baxos prototype. Our results indicate that BFT Baxos exhibits good performance and scalability in low-concurrency scenarios. Additionally, we illustrated the functioning of BFT Baxos even in extremely adverse network conditions by subjecting it to random DDoS attacks. / BFT-konsensusalgoritmer är utformade för att säkerställa konsistensen i distribuerade system där noder kan agera godtyckligt, antingen på grund av fel eller avsiktliga skadliga handlingar. Dock är de flesta praktiska BFT-konsensusalgoritmerna baserade på ledare. I en fientlig nätverksmiljö uppvisar ledar-baserade BFT-konsensusalgoritmer sårbarheter och brist på motståndskraft. Bysantinska ledare kan utgöra en potentiell hot mot systemet; för det första kan skadliga ledare aktivt sänka behandlingshastigheten för hantering av förslag och därigenom minska systemets totala prestanda. För det andra kan de bestämma ordningen för inskickning av mottagna begäranden, vilket kan vara ödesdigert i vissa decentraliserade finansiella system. Dessutom kan externa angripare kompromettera systemets stabilitet genom att genomföra DDoS-attacker mot ledarnoder, vilket ofta utlöser vynändringar och potentiellt orsakar att systemet förlorar livskraft helt och hållet. Vi presenterar BFT Baxos, en mer robust och motståndskraftig BFT-konsensusprotokoll som utrustar en BFT slumpmässig exponentiell backoff-mekanism för att säkerställa att varje nod har rätten att föreslå på ett egalitärt sätt. Genom att använda slumpmässig exponentiell backoff som ett alternativ till ledarval eliminerar det inte bara möjliga skadliga handlingar från bysantinska ledare utan förhindrar även externa angripare från att genomföra riktade DDoS-attacker mot ledarnoden inom system. Vi implementerade och utvärderade vår BFT Baxos-prototyp. Våra resultat visar att BFT Baxos uppvisar god prestanda och skalbarhet i scenarier med låg samtidighet. Dessutom illustrerade vi funktionen av BFT Baxos även under extremt ogynnsamma nätverksförhållanden genom att utsätta den för slumpmässiga DDoS-attacker.

IEEE 802.11/802.16無線網路的MAC機制為儘可能爭取傳輸機會，此機制於壅塞的網狀網路環境中將造成傳輸速率過高及非必要封包碰撞的問題，進而嚴重降低網路傳輸效能。 / 本篇論文的目的旨在改善無線網狀網路環境中，因IEEE 802.11/ 802.16本身的MAC機制所導致的效能低落問題。我們利用賽局理論中零和賽局以強化競爭方式並計算及應用合理傳輸機會(Transmission Opportunities)及傳輸時機(Transmission Timing)，再依此將傳輸速率調降至合理的數值以降低無效封包的傳輸率。同時，此傳輸機會及傳輸時機將作為路徑選擇的重要依據。末了，我們利用網路模擬器NS2 (Network Simulator ver. 2)驗證及評估本論文所提出之方法的效能。 / The MAC mechanism of IEEE 802.11 and 802.16 competes as much transmission opportunities as possible. In a congested wireless mesh network, this greedy competition will result in inappropriate data rates and unintentional packet collision problems, and thus reduce network performance seriously. / The objective of this research is aimed to solve these two problems to enhance the performance of wireless mesh networks. We propose a zero-sum-game based contention-enhancement in MAC mechanism to estimate rational transmission opportunities and transmission timing dynamically. These estimations will then be used in reducing unsuitable packet data rates and selecting better routing paths. At last, we use NS2 (Network Simulator ver.2) to evaluate the system performance of our proposed methods.