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

京都大学 / 新制・課程博士 / 博士(情報学) / 甲第24267号 / 情博第811号 / 新制||情||136(附属図書館) / 京都大学大学院情報学研究科通信情報システム専攻 / (主査)教授 原田 博司, 教授 梅野 健, 准教授 山本 高至 / 学位規則第4条第1項該当 / Doctor of Informatics / Kyoto University / DGAM

With the explosive deployment of wireless LAN technology in the past few years and increasing demand on multimedia applications, the efficient utilization of the precious wireless radio link resources and support of Quality of Service (QoS) in WLANs has become a prominent research issue. In this thesis, an adaptive p-persistent based I 802.11 MAC scheme in WLANs has been proposed. The proposed scheme can maximize the total channel throughput, and also provide service differentiation among multiple traffic classes. This is achieved by updating the transmission probabilities for the stations that compete for transmissions in a WLAN, adaptively based on the real time network measurements. Extensive simulation experiments in ns-2 demonstrate that the proposed scheme is capable of achieving the system throughput bound and the target throughout ratios among different traffic stations in a dynamic WLAN environment. Also, the low computational complexity makes the proposed scheme a suitable choice for real-time implementation.

No description available.

An adaptive MAC protocol is developed and analyzed that offers a "best case" scenario by allowing the MAC to control medium parameters thereby fully exploiting the channel of an ad hoc wireless LAN. This new, opportunistic medium access control protocol is called CATER (Code Adapts To Enhance Reliability) and is based on the proposed MAC standard for wireless local area networks (WLAN)-IEEE 802.11 [IEE96]. As currently proposed, IEEE 802.11 uses a fixed pseudo-noise (PN) code for spreading the information signal, implying a fixed process gain at the receiver. When the channel degrades, IEEE 802.11 offers only retransmissions at the MAC layer to combat a corrupt medium. However, CATER allows communicating stations to reconfigure their transceivers to use a longer PN code after a prescribed number of failed retransmissions. This longer code increases the process gain of the receiver and reduces the error rate. After the two stations are reconfigured, the source station sends the frame in question. Immediately after that frame is acknowledged, the source station may send additional frames during the reconfigured period. Simulation and emulation are used to demonstrate and validate the adaptive protocol's capabilities. Results show that this new protocol offers substantial improvement in system throughput when the channel degrades to a point that reliable transmission of frames is not feasible in a standard IEEE 802.11 WLAN. Specifically, CATER continues to function, permitting up to 14 percent normalized aggregate throughput at times when IEEE 802.11 permits no frames to pass through the WLAN. In addition, throughput experiences only a small decrease due to protocol overhead during periods when stations experience a good channel with few bit errors. Moreover, CATER does not adversely affect the predominate transport layer protocol (i.e., TCP), and provides equitable service to all stations within the network. / Ph. D.

Internetworking third generation (3G) technologies with wireless LAN (WLAN) technologies such as Universal Mobile Telecommunication Systems (UMTS) and IEEE 802.11, respectively, is an emerging trend in the wireless domain. Its development was aimed at increasing the UMTS network'­s capacity and optimizing performance. The increase in the number of wireless users requires an increase in the number of smaller WLAN cells in order to maintain an acceptable level of QoS. Deploying smaller cells in areas of higher mobility (e.g., campuses, subway stations, city blocks, malls, etc.) results in the user only spending a short period of time in each cell, which significantly increases the rate of handoff. If the user does not spend sufficient time in the discovered WLAN's coverage area, the application cannot benefit from the higher data rates. Therefore, the data interruption and performance degradation associated with the handoff cannot be compensated for. This counters the initial objective for integrating heterogeneous technologies, thus only handoffs that are followed by a sufficient visit to the discovered WLAN should be triggered. The conventional RF-based handoff decision method does not have the necessary means for making an accurate decision in the type of environments described above. Therefore, a location-aided handoff decision algorithm was developed to prevent the triggering of handoffs that result from short visits to discovered WLAN's ­ coverage area. The algorithm includes a location-based evaluation that runs on the network side and utilizes a user's location, speed, and direction as well as handoff-delay values to compute the minimum required visit duration and the user'­s trajectory. A WLAN coverage database is queried to determine whether the trajectory's end point falls within the boundaries of the discovered WLAN's coverage area. If so, the mobile node is notified by the UMTS network to trigger the handoff. Otherwise, the location-based evaluation reiterates until the estimated trajectory falls within the boundaries of the discovered WLAN'­s coverage area, or the user exits the coverage area. By taking into consideration more then merely RF-measurements, the proposed algorithm is able to predict whether the user'­s visit to the WLAN will exceed the minimum requirements and make the decision accordingly. This allows the algorithm to prevent the performance degradation and cost associated with unbeneficial/unnecessary handoffs. / Master of Science

近年來興起一個前瞻性無線技術，稱之為無線網狀網路（Wireless Mesh Networks；WMNs） 以所費低廉方式提供無線網路最後一哩存取Internet，同時具備ad hoc網路全部優點。例如自我組織（self-organization）、自我組態（self-configuration）等。而802.11協議已經納入802.11s草案。雖然802.11的實體層支持多元速率，大多數研究為了簡化多假設在單一速率的情況下。但事實上，802.11可以支援的Automatic Rate Fallback (ARF)多元速率演算法；換句話說，由於信號雜訊比和資料錯誤率的不同，資料傳輸速率將自動調整。在這裡，我們假定在WMNs上使用 802.11協定，並且考慮路徑容量，延遲，流量公平及多元速率多跳接的環境。為了設法指出和改善這方面的議題，我們提出了經由改進802.11競爭視窗和加權公平調度機制的跨階層設計。透過一系列的模擬指出問題並找出合適的解決方案。結果顯示，如果增加低速率連結的優先權和考慮流量公平問題，容量及延遲將得到改善。 / A new promising wireless technology has emerged recently, called wireless mesh networks (WMNs). WMNs are an inexpensive way to provide wireless last-mile broadband Internet access and have all the advantages of ad hoc networks, such as self-organization, self-configuration. IEEE 802.11 MAC protocol has been adopted in 802.11s draft. Although IEEE 802.11 physical layer supports multiple rates, most researches assume single rate environment for simplicity. However, in reality, 802.11 adopts automatic rate fallback (ARF) multi-rate algorithm. In other words, the data rate will be automatically adjusted due to its signal-to-noise ratio, or error rate. Here, we assume the fitness of IEEE 802.11 over WMNs, and considering path capacity, delay, flow fairness, in multi-hop multi-rate environments. They all are affected by data rates on the links along the path. In order to address and improve the above issues, we propose a cross layer scheme which is modified by the contention window of IEEE 802.11 DCF MAC and weighted fairness scheduling mechanism. We point out the problem and find out the suitable solution via a series of scenarios simulations. The results show that if increasing the priority of the low data link and taking care about flow fairness problem, the capacity and delay will be improved.

Made available in DSpace on 2016-04-04T18:31:21Z (GMT). No. of bitstreams: 1 Fernando Lino.pdf: 1954006 bytes, checksum: a697088d7a7477d92d2183541ef1e1c6 (MD5) Previous issue date: 2008-02-08 / The literature describes the fast flat fading propagation model in mobile environment as a Rayleigh distribution with different standard deviations which considers only one parameter to describe the channel fading. In the Weibull distribution there are two parameters to be modeled: shape factor and scale factor. The Rayleigh distribution is a particular case of the Weibull distribution where the shape factor has a firm value equals to two. This work has the objective to evaluate, through practical measurements, both parameters in the Weibull distribution at an indoor environment. The results will show the real value of the shape factor parameter and the scale factor as well, indicating that the Weibull distribution is more flexible to describe a fast flat fading channel at an indoor propagation. / A literatura descreve a propaga??o fast flat fading em sistemas m?veis como o modelo de distribui??o Rayleigh com diferentes desvios padr?es, o qual considera apenas um par?metro para descrever o canal de desvanecimento. Na distribui??o de Weibull existem dois par?metros a serem modelados: fator de forma e fator de escala. A distribui??o Rayleigh ? um caso particular da distribui??o de Weibull onde o fator de forma tem um valor igual a dois. Este trabalho tem o objetivo de avaliar, atrav?s de medidas pr?ticas, ambos os par?metros da distribui??o de Weibull em um ambiente indoor. Os resultados ir?o mostrar os reais valores dos par?metros de forma e de escala, indicando que a distribui??o de Weibull ? mais flex?vel para descrever um canal flat fast fading em uma propaga??o indoor.