Dissertação (mestrado)—Universidade de Brasília, Faculdade de Tecnologia, Departamento de Engenharia Elétrica, 2011. / Submitted by Eduardo Pinheiro Morbeck (eduardomorbeck@gmail.com) on 2011-06-27T21:51:07Z No. of bitstreams: 1 2011_TiagoFlorBento.pdf: 2535462 bytes, checksum: f11027cbef101b83fd7350d1ec16d753 (MD5) / Approved for entry into archive by Raquel Viana(tempestade_b@hotmail.com) on 2011-07-14T01:01:47Z (GMT) No. of bitstreams: 1 2011_TiagoFlorBento.pdf: 2535462 bytes, checksum: f11027cbef101b83fd7350d1ec16d753 (MD5) / Made available in DSpace on 2011-07-14T01:01:47Z (GMT). No. of bitstreams: 1 2011_TiagoFlorBento.pdf: 2535462 bytes, checksum: f11027cbef101b83fd7350d1ec16d753 (MD5) / Este trabalho apresenta a análise das propriedades do tráfego em redes sem fio IEEE 802.11 e examina a eficiência do protocolo da camada de enlace, tendo em vista o desempenho de diversas aplicações. Inicialmente, é apresentado um estudo taxonômico do tráfego de rede sem fio nas camadas de enlace, rede e transporte. O estudo identifica a composiç˜ao do tráfego nas diferentes camadas, o classifica e o modela como um processo autossimilar. Com esse resultado, é feita uma análise crítica do desempenho de aplicações reproduzidas em simulações com o objetivo de verificar duas hipóteses: 1) Determinar a influência do fenômeno de autossimilaridade no desempenho de aplicações. 2) Avaliar o ganho de desempenho o tráfego que pode ser caracterizado como desnecessário. Os resultados comprovam que a propriedade autossimilar do tráfego provoca um efeito negativo no desempenho da rede, e se torna ainda mais grave em situações de uso mais intenso. Também é mostrado que, para melhorar o desempenho de aplicações de tempo real em uma rede sem fio, um ponto inicial de partida é a diminuição dos componentes de tráfego que n˜ao s˜ao estritamente necessários. _________________________________________________________________________________ ABSTRACT / This work presents the analysis of properties of traffic on IEEE 802.11 wireless networks and examines the effectiveness of the link layer protocol for the performance of various applications. Initially, a taxonomic study of wireless network traffic in the network link layer, and transport. The study identifies the composition of traffic on different layers, classifies and models as a self-similar process. With this result, is made a critical analysis of the performance of applications reproduced in simulations to verify two hypotheses: 1) to determine the influence of the phenomenon of self-similarity in the performance of applications. 2) Evaluate the performance gain removing the traffic that can be characterized as unnecessary. The results show that the self-similar traffic causes a negative effect on network performance, and becomes even more acute in situations of intense use. It is also shown that, to improve the performance of real-time applications in a wireless network, an initial point of departure is the reduction of traffic that are not strictly necessary.

Dissertação (mestrado)—Universidade de Brasília, Faculdade de Tecnologia, Departamento de Engenharia Elétrica, 2013. / Submitted by Alaíde Gonçalves dos Santos (alaide@unb.br) on 2014-01-14T14:41:23Z No. of bitstreams: 1 2013_LarissaMarinhoEglemdeOliveira.pdf: 2660793 bytes, checksum: 97d9d784189a1dbc49112666a65d4f32 (MD5) / Approved for entry into archive by Guimaraes Jacqueline(jacqueline.guimaraes@bce.unb.br) on 2014-02-17T15:48:27Z (GMT) No. of bitstreams: 1 2013_LarissaMarinhoEglemdeOliveira.pdf: 2660793 bytes, checksum: 97d9d784189a1dbc49112666a65d4f32 (MD5) / Made available in DSpace on 2014-02-17T15:48:28Z (GMT). No. of bitstreams: 1 2013_LarissaMarinhoEglemdeOliveira.pdf: 2660793 bytes, checksum: 97d9d784189a1dbc49112666a65d4f32 (MD5) / Nessa dissertação, apresentamos o protocolo de controle de acesso ao meio com agregação oportunista de canal (OCA-MAC) para redes ad hoc sem fio. OCA-MAC permite a agregação oportunista de canal em nível de enlace (MAC) por quadro de dados transmitidos. O OCA-MAC é baseado no padrão IEEE 802.11 DCF e necessita de uma interface de rádio secundária para acesso oportunista em um canal alternativo. O OCA-MAC evita o uso de um canal extra de controle para coordenação entre as estações da rede (ou para troca de quadros de controle no canal secundário), ao utilizar a simples ideia de que quem adquirir o direito de acesso ao canal primário também ganha o direito imediato (e implícito) de transmitir um quadro de dados no canal secundário (se disponível nos dois lados do enlace quando da aquisição do canal). Portanto, toda informação de controle necessária para realizar a agregação distribuída de canais é realizada no canal principal, permitindo que o(s) canal(is) secundário(s) sejam utilizados somente para transmissão de quadros de dados. O desempenho do OCA-MAC é avaliado em relação ao padrão IEEE 802.11 DCF MAC por meio de simulações computacionais a eventos discretos com topologias não totalmente conectadas sob tráfego saturado. Os cenários investigados mostram que o espaçamento geográfico e o reuso espacial não levam necessariamente a uma melhora no desempenho das redes IEEE 802.11, e portanto, a agregação oportunista de canais prova-se ser uma forte aliada na melhora da vazão agregada da rede, na sua justiça e no seu atraso ponto-a-ponto. _______________________________________________________________________________________ ABSTRACT / In this dissertation, we present the Opportunistic Channel Aggregation MAC protocol (OCA- MAC) for wireless ad hoc networks. OCA-MAC allows opportunistic MAC-level channel aggregation per frame transmission. To accomplish that, OCA-MAC is built on top of the IEEE 802.11 DCF MAC and requires a secondary radio interface for opportunistic access over alternative channels. OCA-MAC avoids the use of an extra control channel for coordination among nodes (or control frame exchanges over the secondary channel) by using the simple idea that whoever a - quires the floor over the primary channel also gains immediate (and implicit) right to transmit a frame over the secondary channel (if available at both sides of the link at the time of floor acquisition). Thus, all necessary control information for distributed channel aggregation is carried out over the primary channel, allowing the use of the secondary channel(s) for DATA frame trans- missions only. The performance of OCA-MAC is evaluated against the IEEE 802.11 DCF MAC through discrete-event simulations with topologies not fully connected under saturated traffic . The investigated scenarios show that, geographical sparsity and spatial reuse do not necessarily lead to performance improvements of IEEE 802.11 networks, and thus, opportunistic channel aggregation proves to be a strong ally to boost aggregate throughput, overall fairness and end-to-end delay.

Este trabalho apresenta, inicialmente, uma análise comparativa detalhada dos dois padrões, IEEE 802.11a e IEEE802.11b, que foram apresentados recentemente pelo IEEE na área de redes sem fio (wireless). São apresentadas as principais diferenças tecnológicas dos dois padrões, no que se refere, principalmente, à arquitetura, funções de controle, segurança, desempenho e custo de implementação destas duas tecnologias de redes wireless. São avaliados também os aspectos de interoperabilidade, quando estas redes são integradas em redes corporativas fixas, que são baseadas, principalmente, em redes Ethernet, tradicionalmente usadas em redes corporativas. São considerados também, aspectos de custo e flexibilidade de aplicação das duas tecnologias e mostram-se como estas diferenças devem ser levadas em conta em aplicações típicas de um ambiente corporativo. Finalmente, apresenta-se também, como estudo de caso, uma análise focalizada principalmente na integração da tecnologia wireless em aplicações típicas de uma grande empresa local. Consideram-se as vantagens e desvantagens de ambas as tecnologias, como solução para algumas aplicações típicas encontradas nesta empresa, e justifica-se a escolha da solução que foi adotada. Conclui-se com algumas projeções quanto ao futuro da tecnologia wireless no ambiente público e corporativo.

Orientadores: Michel Daoud Yacoub, Omar Carvalho Branquinho / Dissertação (mestrado) - Universidade Estadual de Campinas, Faculdade de Engenharia Eletrica e de Computação / Made available in DSpace on 2018-08-08T18:59:46Z (GMT). No. of bitstreams: 1 Ferreira_DeboraMeyhofer_M.pdf: 1183194 bytes, checksum: 3c93692fbad066caa7e514b9442843d9 (MD5) Previous issue date: 2007 / Resumo: A Ethernet existe há mais de 20 anos e a ¿wireless ethernet¿ (ethernet sem fio), conhecida como o padrão 802.11, há mais de sete anos. Extensões do padrão original, tais como o 802.11g, foram criados para garantir maiores taxas de transmissão. No entanto, existem condições que afetam o seu desempenho. Um desses casos, conhecido como performance anomaly, ou anomalia de desempenho, ocorre quando em uma rede infraestruturada existem estações móveis (STAs) transmitindo com taxas diferentes. Nesse caso, uma STA a uma taxa baixa ocupa o canal por um longo período, degradando o desempenho das demais estações e comprometendo o funcionamento de toda a rede. Esse trabalho constata e quantifica essa anomalia e determina um método de priorizar as STAs com uma taxa de transmissão alta, objetivando a redução do efeito da anomalia. Essa priorização é feita alterando-se a janela de contenção de acordo com a relação sinal ruído da STA. O mecanismo é modelado matematicamente e avaliado através de simulação com o software Network Simulator / Abstract: Ethernet exists more than 20 years and the wireless Ethernet, known as IEEE 802.11, for more than seven years. Extensions of the original standard, as IEEE 802.11g, are been created to guarantee higher transmission rates. However, some conditions there are that affect the performance of the network. One of them, known as Performance Anomaly, occurs when in an infrastructured WLAN stations (STAs) operate at different rates. In such a case, the low rate STA rate captures the channel for a long time, penalizing the higher rate stations, thence degrading the performance of the network. This work substantiates and quantifies such an anomaly and proposes a method to mitigate the effect of it, prioritizating the higher STAs. This mitigation is is carried out using Signal to Noise Ratio as a criterion to control the Contention Window variation. The mechanism is modeled mathematically and evaluated by means of simulation using the software Network Simulator / Mestrado / Telecomunicações e Telemática / Mestre em Engenharia Elétrica

IEEE 802.11 (a.k.a. WiFi) is a popular wireless LAN specification. This thesis documents implementation of this standard within the experimental operating system HelenOS which differs from conventional operating systems with its microkernel-based design. First, the basic concepts of WiFi technology are described. Description of external tools used during development is following. Together with that, the chosen method of integrating the wireless framework into existing system is depicted. In the next chapter we analyse the implementation details of the work. There is thoroughly explained the structure of the driver for selected WiFi device and also the functionality of resulting IEEE 802.11 library. In the end of the thesis there is an evaluation of the features of final work and comparison of enclosed implementation with existing solutions.

The diploma thesis is focused on exploring the possibility of VoIP service in a wireless network of University of Economics, Prague. This thesis describes the basic principles of VoIP and related wireless technologies necessary for its quality and stable operation. Subsequently, different configurations of wireless network and end clients are tested and compared, including its impact on ordinary users, in a laboratory environment with idle and fully utilized frequency band. Finally, a roaming operation with the use of several advanced 802.11 standards is tested in the real environment of the Old building in Žižkov. In conclusion, the ideal settings for all telecommunication devices are recommended in order to maximize the quality of VoIP operation and to minimize the negative impact on ordinary users.

A local call admission control (CAC) algorithm for third generation wireless networks was designed and implemented, which allows for the simulation of network throughput for different spreading factors and various mobility scenarios. A global CAC algorithm is also implemented and used as a benchmark since it is inherently optimized; it yields the best possible performance but has an intensive computational complexity. Optimized local CAC algorithm achieves similar performance as global CAC algorithm at a fraction of the computational cost. Design of a dynamic channel assignment algorithm for IEEE 802.11 wireless systems is also presented. Channels are assigned dynamically depending on the minimal interference generated by the neighboring access points on a reference access point. Analysis of dynamic channel assignment algorithm shows an improvement by a factor of 4 over the default settings of having all access points use the same channel, resulting significantly higher network throughput.

In this paper, we introduce a clean-slate architecture for improving the delivery of data packets in IEEE 802.11 wireless mesh networks. Opposed to the rigid TCP/IP layer architecture which exhibits serious deficiencies in such networks, we propose a unitary layer approach that combines both routing and transport functionalities in a single layer. The new Mesh Transmission Layer (MTL) incorporates cross-interacting routing and transport modules for a reliable data delivery based on the loss probabilities of wireless links. Due to the significant drawbacks of standard TCP over IEEE 802.11, we particularly focus on the transport module, proposing a pure rate-based approach for transmitting data packets according to the current contention in the network. By considering the IEEE 802.11 spatial reuse constraint and employing a novel acknowledgment scheme, the new transport module improves both goodput and fairness in wireless mesh networks. In a comparative performance study, we show that MTL achieves up to 48% more goodput and up to 100% less packet drops than TCP/IP, while maintaining excellent fairness results.

This thesis project’s goal is to enable undergraduate students to gain insight into media access and control protocols based upon carrying out laboratory experiments. The educational goal is to de-mystifying radio and other link and physical layer communication technologies as the students can follow packets from the higher layers down through the physical layer and back up again. The thesis fills the gap between the existing documentation for the Universal Software Radio Peripheral (USRP) resources and the knowledge of undergraduate students. This was necessary because the existing document is targeted at advanced audiences rather than undergraduates. This thesis describes the design and evolution of a workbench for students to experiment with a variety of media access and control protocols, much as Wireshark gives students the ability to watch network and higher layer protocols. Another motivation for this thesis is that an increasing number of communication networks use complex media access and control protocols and existing tools do not allow students to see the details of what is taking place in these protocols, except via simulation. Today an software defined radio and computer are affordable as laboratory equipment for an undergraduate course. Hence the time is ripe for the development of undergraduate laboratory course material using these tools. The thesis is targeted at (1) instructors of undergraduates who might use this work to develop their own lesson plans and course material and (2) students of physical and link layer protocols who want a practical tool for carrying out experiments in these layers. Hopefully by de-mystifying these lower layers and by making the USRP more approachable by undergraduate students we will encourage lots of students to view wireless network technology as being just as approachable as a wired Ethernet. Due to the widespread use of wireless communications technologies, there is a great need by industry for more graduates who can understand communication systems from the physical to the application layer - rather than the current situation where there is a hard boundary between the lower two layers and the upper layers. While there has been a lot of research concerning cross layer optimization, much of this is theoretical and not very approachable by students. A desired outcome of this thesis project is that undergraduate students will be able to understand tradeoffs at all layers of the protocol stack and not be limited to the upper layers. / Detta examensarbete har som mål att göra det möjligt för studenter att få inblick i tillgång till medierna och protokoll som grundar sig på att utföra laboratorieexperiment. Det pedagogiska målet är att de-mystifierande radio och annan länk och fysiska lagret kommunikationsteknik som studenterna kan följa paket från högre skikt ner genom det fysiska lagret och upp igen. Avhandlingen fyller gapet mellan den befintliga dokumentationen för Universal Software Radio Peripheral (usrp) resurser och kunskap om studerande. Detta var nödvändigt eftersom det befintliga dokument riktar sig till avancerade publik snarare än studenter. Denna avhandling beskriver utformningen och utvecklingen av en arbetsbänk för studenter att experimentera med olika tillgång till medierna och protokoll kontroll, mycket som Wireshark ger studenterna möjlighet att titta på nätet och högre skikt protokoll. Ett annat motiv för denna tes är att ett ökande antal kommunikationsnät använda komplicerade tillgång till medierna och protokoll kontroll och befintliga verktyg inte tillåter eleverna att se detaljer om vad som sker i dessa protokoll, utom via simulering. Idag en programvarustyrd radio och dator är överkomliga laboratorieutrustning för en grundutbildningskurs. Därför är tiden mogen för utvecklingen av grundutbildningen laborationer material med hjälp av dessa verktyg. Avhandlingen riktar sig till (1) instruktörer för studenter som kan använda detta arbete för att utveckla sin egen lektionsplanering och kursmaterial och (2) studenter på fysisk och länka protokoll skikt som vill ha ett praktiskt verktyg för att utföra experiment i dessa lager. Förhoppningsvis genom de-mystifierande de undre lagren och genom att göra usrp mer tillgänglig genom att studenter ska vi uppmuntra många elever att visa trådlös nätverksteknik vara lika lättillgänglig som ett ethernet. På grund av den utbredda användningen av trådlös kommunikationsteknik, finns ett stort behov från näringslivet för fler studenter som kan förstå kommunikationssystem från det fysiska till applikationslagret - i stället för den nuvarande situationen där det finns en hård gräns mellan de två lägre skikten och de övre skikten. Samtidigt som det har varit en hel del forskning om cross lager optimering, mycket av detta är teoretisk och inte särskilt tillgänglig av studenter. Ett önskat resultat med detta examensarbete är att studenter ska kunna förstå kompromisser på alla nivåer inom den protokollstack och inte vara begränsade till de övre skikten.

The focus of this research is to investigate the effects of bursts of errors and packet collisions on the performance of the medium access control (MAC) portion of the IEEE 802.11 wireless local area network (LAN) protocol.An important ingredient in rapid expansion of wireless networks is the seamless transition between wired and wireless systems. The IEEE standards group in charge of developing the widely used IEEE 802.3 LAN standard has developed the IEEE 802.11 wireless LAN standard. IEEE 802.11 remains hidden from the upper levels of the network, thus allowing a seamless transition between networks. The foundation protocol for the IEEE 802.11 standard, known as Distributed Foundation Wireless Medium Access Control (DFWMAC), operates at the MAC level of the Data Link Layer. The protocol bases its access control mechanism on a principle called Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA), which is an adaptation of the Carrier Sense Multiple Access with Collision Detection (CSMA/CD) protocol used by IEEE 802.3 standard. The collision avoidance scheme in CSMA/CA allows data packets to be transferred via the wireless medium with lower probability of packet collision. In a slotted multi-access wireless system, performance parameters are affected by the bit error rates on the communication channel. These errors occur as a result of noise introduced by the radio channel or data packet collisions. Collisions occur when two or more stations select the same time slot to transmit their data, thus causing corruption in data packets. In this research, a simulation model coded in Microsoft's Visual Basic programming environment is utilized to investigate the effects of bit errors and packet collisions on performance in CSMA/CA. Performance parameters used in this study include throughput, medium utilization, collisions and station data queue lengths. In the simulation model, error bursts in the communication channel are modeled using a simple Gilbert model with two states, good (G) and bad (B). State G is error free, thus errors can only occur while the model is in state B. Collisions are simulated by two or more stations starting to transmit data packets in the same time slot. Therefore, as the number of stations increases, more and more stations compete for the medium, resulting in an increase in the number of collisions. Collisions are also increased by the amount of traffic that each station introduces into the system. Station load is defined here as the number of data packets per unit time that are released by the higher network protocol layers.The results in Chapter 5 demonstrate that higher network throughput can be achieved when the aggregate load on the network is distributed. For example, 30 stations offering 20 kilobits per second (kbps) of load for a total of 600 kpbs, results in a network throughput of 585 kbps. However, three stations offering 200 kbps of load for a total of 600 kbps offered load, results in a network throughput of 486 kbps. The distributed load is serviced at a 17 percent higher rate. However, once the network becomes saturated at above 40 stations for this model, collisions will more than offset the performance gains produced by the distribution of load.Furthermore, reducing the packet size by 50 percent under an approximately 19.5 percent packet error rate results in a 12 percent gain in throughput. This is primarily due to higher utilization of the network by shorter packets. However, as the packet error rate is reduced, the performance gap between the two packet sizes is reduced. Once the errors are removed completely from the communications channel, the longer packets produce a higher throughput than the shorter packets. / Master of Science