Despite recent advances in wireless local area network (WLAN) technologies, today's WLANs still cannot offer the same data rates as their wired counterparts. The throughput problem is further aggravated in multi-hop wireless environments due to collisions and interference caused by multi-hop routing. Because all current IEEE 802.11 physical (PHY) standards divide the available frequency into several orthogonal channels, which can be used simultaneously within a neighborhood, increasing capacity by exploiting multiple channels becomes particularly appealing. To improve the capacity of wireless ad hoc networks by exploiting multiple available channels, I propose three principles that facilitate the design of efficient distributed channel assignment protocols. Distributed channel assignment problems have been proven to be <i>NP</i>-complete and, thus, computationally intractable. Though being a subject of many years of research, distributed channel assignment remains a challenging problem. There exist only a few heuristic solutions, none of which is efficient, especially for the mobile ad hoc environment. However, protocols that implement the proposed design principles are shown to require fewer channels and exhibit significantly lower communication, computation, and storage complexity, compared with existing approaches. As examples, I present two such protocols that build on standard reactive and proactive routing protocols. In addition, I prove the correctness of the algorithms and derive an upper bound on the number of channels required to both resolve collisions and mitigate interference. A new multi-channel medium access control (MC-MAC) protocol is also proposed for multi-hop wireless ad hoc networks. MC-MAC is compatible with the IEEE 802.11 medium access control (MAC) standard and imposes the minimum system requirements among all existing multi-channel MAC protocols. In addition, simulation results show that even with only a single half-duplex transceiver, MC-MAC, by exploiting multiple channels, can offer up to a factor of four improvement in throughput over the IEEE 802.11 MAC protocol. The reduction in delay is even more significant. Therefore, the MC-MAC protocol and the accompanying distributed channel assignment protocols constitute an effective solution to the aforementioned performance problem in a multi-hop wireless network. Finally, I generalize the cross-layer design principle to more general networking functions and present a network architecture to motivate and facilitate cross-layer designs in wireless networks. A literature survey is provided to validate the proposed cross-layer design architecture. Current cross-layer design research can be categorized into two classes: joint-layer design using optimization techniques, and adaptive techniques based on system-profile and/or QoS requirements. Joint-layer design based on optimization techniques can achieve optimal performance, but at the expense of complexity. Adaptive schemes may achieve relatively good performance with less complexity. Nevertheless, without careful design and a holistic view of the network architecture, adaptive schemes may actually cause more damage than benefit. / Ph. D.

Cognitive radio (CR) is seen as one of the enabling technologies for realizing a new regulatory spectrum management paradigm, viz. opportunistic spectrum sharing (OSS). In the OSS paradigm, unlicensed users (a.k.a. secondary users) opportunistically operate in fallow licensed spectrum on a non-interference basis to licensed users (a.k.a. incumbent or primary users). Incumbent users have absolute priority in licensed bands, and secondary users must vacate the channel where incumbent user signals are detected. A CR network is composed of secondary users equipped with CRs and it can coexist with incumbent users in licensed bands under the OSS paradigm. The coexistence between incumbent users and secondary users is referred to as incumbent coexistence, and the coexistence between CR networks of the same type is referred to as self-coexistence. In this dissertation, we address three coexistence-related problems at the medium access control (MAC) layer in CR networks: (1) the rendezvous (control channel) establishment problem, (2) the channel assignment problem in an ad hoc CR network, and (3) the spectrum sharing problem between infrastructure-based CR networks, i.e., the 802.22 wireless regional area networks (WRANs). Existing MAC layer protocols in conventional wireless networks fail to adequately address the key issues concerning incumbent and self coexistence that emerge in CR networks. To solve the rendezvous establishment problem, we present a systematic approach, based on quorum systems, for designing channel hopping protocols that ensure a pair of CRs to "rendezvous" within an upper-bounded time over a common channel that is free of incumbent user signals. In a single radio interface, ad hoc CR network, we propose a distributed channel assignment scheme that assigns channels at the granularity of "segments" for minimizing the channel switching overhead. By taking into account the coexistence requirements, we propose an inter-network spectrum sharing protocol that enables the sharing of vacant TV white space among coexisting WRANs. Our analytical and simulation results show that these proposed schemes can effectively address the aforementioned MAC layer coexistence problems in CR networks. / Ph. D.

An important aspect of wireless networks is their dynamic behavior. The conventional protocol stack is inflexible as various protocol layers communicate in a strict manner. In such a case the layers are designed to operate under the worst conditions as opposed to adapting to changing conditions. This leads to inefficient use of spectrum and energy. Adaptation represents the ability of network protocols and applications to observe and respond to channel conditions. Traditional simulation methodologies independently model the physical and higher layers. When multiple layer simulations are required, an abstraction of one layer is inserted into the other to provide the multiple layer simulation. However, recent advances in wireless communication technologies, such as adaptive modulation and adaptive antenna algorithms, demand a cross layer perspective to this problem in order to provide a sufficient level of fidelity. However, a full simulation of both layers often results in excessively burdensome simulation run-times. The benefits and possible parametric characterization issues arising due to the cross-layer integration of lower physical and higher network layers are investigated in this thesis. The primary objective of investigating cross-layer simulation techniques is to increase the fidelity of cross-layer network simulations while minimizing the simulation runtime penalties. As a study of cross-layer system design a medium access control (MAC) scheme is studied for a MANET wherein the nodes are equipped with smart antennas. Traditional MAC protocols assume the use of omnidirectional antennas. Nodes with directional antennas are capable of transmitting in certain directions only and significantly reduce the chances of collision and increase the effective network capacity. MANETs using omni-directional antennas severely limit system performance as the entire space around a node up to its radio range is seen as a single logical channel. In this research a MAC protocol is studied that exploits space division multiple access at the physical layer. This is a strong example where physical and MAC design must be carried out simultaneously for adequate system performance. Power control is a very important in the design of cellular CDMA systems which suffer from the near-far problem. Finally, the interaction between successive interference cancellation (SIC) receivers at the physical layer and power control, which is a layer 2 radio resource management issue, is studied. Traffic for future wireless networks is expected to be a mix of real-time traffic such as voice, multimedia teleconferencing, and games and data traffic such as web browsing, messaging, etc. All these applications will require very diverse quality of service guarantees. A power control algorithm is studied, which drives the average received powers to those required, based on the QoS requirements of the individual users for a cellular CDMA system using SIC receivers. / Master of Science

A new class of Wireless Sensor Networks (WSNs) is emerging within the Internet of Things (IoT) that features extremely high node density, low data rates per node, and high network dependability. Applications such as industrial IoT, factory automation, vehicular networks, aviation, spacecraft and others will soon feature hundreds of low power, low data rate (1-15 kbps) wireless sensor nodes within a limited spatial environment. Existing Medium Access Control (MAC) layer protocols, namely IEEE 802.15.4, may not be suitable for highly dense, low rate networks. A new MAC protocol has been proposed that supports a Receiver-Assigned Code Division Multiple Access (RA-CDMA) physical (PHY) layer multiple access technique, which may enable higher network scalability while maintaining performance and contributing additional robustness. This thesis presents a comparison of the contention mechanisms of IEEE 802.15.4 non- beacon enabled mode and RA-CDMA along with a Matlab simulation framework used for end-to-end simulations of the protocols. Simulations suggest that IEEE 802.15.4 networks begin to break down in terms of throughput, latency, and delivery ratio at a relatively low overall traffic rate compared to RA-CDMA networks. Results show that networks using the proposed RA-CDMA multiple access can support node densities on the order of two to three times higher than IEEE 802.15.4 within the same bandwidth. Furthermore, features of a new MAC layer protocol are proposed that is optimized for RA-CDMA, which could further improve network performance over IEEE 802.15.4. The protocol's simple and lightweight design eliminates significant overhead compared to other protocols while meeting performance requirements, and could further enable the deployment of RA-CDMA WSNs. / Master of Science

Machine-to-Machine (M2M) communications is an emerging communication paradigm that provides ubiquitous connectivity between devices along with an ability to communicate autonomously without human intervention. M2M communications acts as an enabling technology for the practical realization of Internet-of-Things (IoT). However, M2M communications differs from conventional Human-to-Human (H2H) communications due to its unique features such as massive number of connected devices, small data transmissions, little or no mobility, requirements of high energy efficiency and reliability, etc. These features create various challenges for existing communication networks which are primarily optimized for H2H communications. Therefore, novel solutions are required to meet the key requirements of M2M communications. In addition, enhancements are required at different layers of the protocol stack to support co-existence of M2M devices and H2H users. The main objective of this research is to investigate the challenges of M2M communications in two broad types of M2M networks; capillary M2M and cellular M2M networks. The primary focus is on developing novel solutions, algorithms, and protocol enhancements for successfully enabling M2M communications. Since cognitive radio technology is very promising for M2M communications, special emphasis is on capillary M2M networks with cognitive radio based Physical layer. Besides, the focus is also on exploring new frontiers in M2M communications. This thesis covers different aspects of M2M communications. Considering the motivation for cognitive M2M and service requirements of M2M devices, two cognitive MAC protocols have been proposed. The first protocol is centralized in nature and utilizes a specialized frame structure for co-existence with the primary network as well as handling different Quality-of-Service (QoS) requirements of M2M devices. The second protocol is a distributed cognitive MAC protocol, which is specially designed to provide high energy efficiency and reliability for M2M devices operating in challenging wireless environments. Both protocols explicitly account for the peculiarities of cognitive radio environments. The protocols have been evaluated using analytical modeling and simulation studies. Recently IETF has standardized a specially designed routing protocol for capillary M2M networks, known as RPL (Routing for Low Power and Lossy Networks). RPL is emerging as the de facto routing protocol for many M2M applications including the smart grid. On the other hand, the application of cognitive radio for smart grid communication is under active investigation in the research community. Hence, it is important to investigate the applicability and adaptation of RPL in cognitive radio environments. In this regard, an enhanced RPL based routing protocol has been proposed for cognitive radio enabled smart grid networks. The enhanced protocol provides novel modifications to RPL for protecting the primary users along with meeting the utility requirements of the secondary network. An important challenge in LTE-based cellular networks with M2M communications is the uplink radio resource management as available resources are shared between M2M devices and H2H users, having different and often conflicting QoS requirements. Apart from this, energy efficiency requirements become critically important. Further, the specific constraints of Single Carrier Frequency Division Multiple Access (SC-FDMA) complicate the resource allocation problem. In this respect, an energy efficient resource allocation algorithm for the uplink of LTE networks with M2M/H2H co-existence under statistical QoS guarantees has been developed, that is based on canonical duality theory. The proposed algorithm outperforms classical algorithms in terms of energy efficiency while satisfying the QoS requirements of M2M devices and H2H users. A new frontier in M2M communications is the nano-M2M communications, which is envisioned to create the Internet-of-Nano-Things (IoNT). Molecular communication (MC) is a promising communication technique for nano-M2M communications. In literature, no model for error performance of MC exists. Therefore, an error performance model has been developed that explicitly accounts for noise and interference effects. Since relaying and network coding based solutions are gaining popularity for nano-M2M networks, the error performance of a network coded molecular nano-M2M network has been evaluated as well. Finally, the thesis is concluded based on the overall picture of the research conducted. In addition, some directions for future work are included as well.

Thesis (PhD)-- Stellenbosch University, 2013. / ENGLISH ABSTRACT: The hidden terminal problem, coupled with high node mobility apparent in vehicular networks, present challenges to e cient communication between vehicles at the Medium Access Control (MAC) layer. Both of these challenges are fundamentally problems of lack of awareness, and manifest most prominently in the broadcasting of safety messages in infrastructure-free vehicle-to-vehicle communications. The design of existing contention-free and contention-based MAC approaches generally assumes that nodes that are in range of one another can take steps to coordinate communications at the MAC layer to overcome the hidden terminal problem and node mobility. Unicasting with the existing MAC standard, IEEE 802.11p, implicitly assumes an awareness range of twice the transmission range (a 1-hop awareness range) at most, since handshaking is used. For broadcasting, the assumption implies an awareness range that is at most equal to the transmission range, since only carrier sensing is used. Existing alternative contention-free approaches make the same assumption, with some protocols explicitly using a 1-hop awareness range to avoid packet collisions. This dissertation challenges the convention of assuming that a 1-hop awareness range is su cient for networks with high mobility, such as VANETs. In this dissertation, the impact of awareness range and management of the awareness information on MAC performance is researched. The impact of the number of slots that is required to support the awareness range is also evaluated. Three contention-free MAC protocols are introduced to support the research. The rst is an improved version of an existing MAC method, which is used to demonstrate the e ects on performance of changes to awareness management. The second MAC uses three competing processes to manage awareness information. The second MAC is designed for a con gurable awareness range and con gurable number of slots, and is used to evaluate the e ects of awareness range and number of slots on MAC performance. The third MAC is random access based and is used to evaluate the impact on performance of removing awareness completely. An analytical model is developed to support the simulated results. The simulation results demonstrate that awareness range, awareness information management, and number of slots used are key design parameters that signi cantly impact on MAC performance. The results further show that optimal awareness-related design parameters exist for given scenarios. Finally, the proposed contention-free and random access MAC methods are simulated and performance compared with IEEE 802.11p. All three outperform the contentionbased standard IEEE 802.11p. / AFRIKAANSE OPSOMMING: Die versteekte-nodus-probleem, gekoppel met die hoë vlakke van nodusbeweging teenwoordig in voertuignetwerke, bied uitdagings vir doeltre ende kommunikasie tussen voertuie in die medium-toegangbeheer- (MAC) vlak. Beide van hierdie probleme spruit uit beperkte bewustheid, en manifesteer veral in die uitsaai van veiligheidsboodskappe in infrastruktuurvrye voertuig-na-voertuig-kommunikasie. Die ontwerp van bestaande wedywerende en nie-wedywerende MAC benaderings neem aan dat nodusse wat binne bereik van mekaar is, stappe kan neem om kommunikasie op die MAC-vlak te koördineer, ten einde probleme met versteekte nodusse en mobiliteit te oorkom. Vir punt-tot-puntkommunikasie met IEEE 802.11p, impliseer dié aanname 'n bewustheidstrekking van hoogstens twee keer die radiobereik (1-hop bewustheidstrekking), aangesien bladskud gebruik word. In die geval van uitsaai, impliseer die aanname 'n bewustheidstrekking hoogstens gelyk is aan die radiobereik, aangesien slegs draeropsporing gebruik word. Nie-wedywerende metodes maak dieselfde aanname, met sommiges wat eksplisiet 1-hop-bewustheidstrekking gebruik om pakkieverliese te voorkom. Hierdie verhandeling wys dat hierdie aanname nie geld vir netwerke met hoë mobiliteit nie, soos wat die geval is vir VANET. In hierdie verhandeling word die impak van bewustheidstrekking en bestuur van die bewustheidsinligting in die MAC-vlak ondersoek. Die impak van die aantal tydgleuwe wat nodig is om die bewustheidstrekking te ondersteun word ook ondersoek. Drie nie-wedywerende metodes word bekendgestel om die navorsing te ondersteun. Die eerste is 'n verbeterde weergawe van 'n bestaande MAC, wat gebruik word om die e ekte van bewustheidsbestuur op MAC-werkverrigting te beoordeel. Die tweede MAC is ontwerp om veranderbare bewustheidstrekking en hoeveelheid tydgleuwe te ondersteun, en word gebruik om die e ekte van bewustheidstrekking en hoeveelheid tydgleuwe op MAC werkverrigting aan te beoordeel. Die derde MAC is ewetoeganklik (onbewus van omliggende nodusse) en word gebruik om die impak van die verwydering van bewustheid op werkverrigting te ondersoek. 'n Analitiese model is ontwikkel om die simulasieresultate te ondersteun. Die simulasieresultate dui aan dat bewustheidstrekking, bestuur van bewustheidsinligting, en hoeveelheid tydsgleuwe sleutel-ontwerpsveranderlikes is wat 'n beduidende impak het op MAC werkverrigting. Die resultate wys verder dat optimale ontwerpsveranderlikes, in terme van bewustheid, bestaan vir gegewe scenario's. Laastens, word die nie-wedywerende en ewetoeganklike MAC-metodes wat gesimuleer word se werkverrigting vergelyk met IEEE 802.11p. Al drie MAC metodes vaar beter as die wedywerende standaard, IEEE 802.11p.

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.

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.

O protocolo Token Ring sem o (WTRP) é um protocolo distribuído de controle de acesso ao meio que provê qualidade de serviço em termos de uso de largura de banda e latência limitada. WTRP consiste de nós (estações) que formam topologicamente um anel. Contudo, quando o número de nós em um anel aumenta a latência aumenta e o tempo de reuso de token por parte de um nó em anel também aumenta. Neste trabalho, apresentamos uma versão extendida de WTRP com foco em reduzir a latência, tempo de reuso de token e permitir encaminhamento de dados entre anéis sem aumentar signicativamente o consumo de energia. Para provar o conceito que propomos, implementamos e testamos a nossa versão de WTRP usando simulador de rede - NS. / Wireless Token Ring Protocol (WTRP) is a distributed Medium Access Control protocol that provides quality of service in terms of reserved bandwidth and limited latency]. It consists of nodes or stations structured in ring topology. However, when the number of nodes in a ring increases latency and time of a node reuse token increases. In this work, we present an extended version WTRP that focus on reducing latency, time of token reuse and data forwarding among the rings in a MANet , without suggestive increasing of energy consumption. We have implemented and tested our version of WTRP in network simulator - NS.

I avhandlingen beskrivs ett hypotetiskt system som kan användas av mobila användare, bland andra taxichaufförer, som exempelvis vill följa en viktig fotbollsmatch. Flera faktorer ställer till problem: Ibland står bilen still och föraren har inget annat att tänka på än matchen. Ibland kör denne runt med en kund som inte vill bli störd av matchen. Dessutom kan det vara svårt att titta på rörliga bilder och köra bil samtidigt. I och med att bilen körs runt har man också olika bra anslutning till Internet vid olika tillfällen – det kan variera mellan inget alls, en dålig GSM/GPRS förbindelse (8 kbps) och en snabb WLAN anslutning (100 Mbps). I avhandlingen presenteras en tre-lagers modell som kan användas för att beskriva den här typen av applikationers kvalitet. Modellen delas in i tre lager: nätverk, applikation och användare/använding. Det sistnämnda lagret ligger utanför det tekniska systemet och definieras av att det är där de verkliga informationsutbytet sker. På applikationsnivån samlas data in, packas och packas upp i samband med nätverkstransport och visas sedan för användaren. Det är också här som eventuell interaktion sker med användaren. Nätverkslagret är ansvarigt för ändamålsenlig transport av data. De tre lagren är ömsesidigt beroende av varandra. Dålig prestanda på ettlager påverkar de andra lagren och tvärtom. Tre studier har genomförts av hur problem på nätverkslagret i form av begränsad bandbredd och hög fördöjning påverkar användarna. Låg bandbredd ger låg videokvalitet vilket inte uppskattas av användarnamnen genom att skifta till animeringar som fungerar med lägre bandbredd kan man ändå få användarna nöjda. Om användarna måste välja mellandålig videokvalitet och animeringar väljer de som ser sig som fotbollskunniga det förstnämnda och de som ser sig som okunniga men dock fotbollsfans väljer det sistnämnda. Men i en annan studie där användarna spelade bluffstopp mot varandra över ett datanätverk fick vi ett annat resultat. Där var det negativt med högre videokvalitet (bilder per sekund). En förklaring kan vara att användarna distraherades mer av högre bildfrekvens. I den tredje studien studerades vad som händer i Pong om man läggerin fördröjningar i spelet. Sedan tidigare visste man att det blir svårare attspela med fördröjningar – särskilt om man inte märker dem. Vi ställde ossfrågan om man kan kompensera för dem genom att informera användarna om dem. Det visade sig att användare som får information med i vårtfall en prediktiv visning lättare anpassar sin mentala insats till uppgiftens svårighetsgrad. Det är alltså inte bara möjligt utan ibland också önskvärt att utnyttja en lägre bandbredd från användarens perspektiv. Med det sagt finns det ändå i långt fler situationer där det är bättre med bättre nätverksprestanda. Pongspelet var roligare med lägre delay. Videon uppfattades som bättre medhögre bandbredd i den förstnämnda studien. Multicast, där ett paket skickas till flera användare i stället för att de skafå varsin, identiska paket, är ett viktigt verktyg för att få bättre prestanda i videoapplikationer. Tyvärr är det inbyggda stödet för multicast i den viktiga IEEE 802.11 standardfamiljen för trådlösa nätverk mycket outvecklat. Ettstort problem är att det inte går att veta om ett paket har kommit fram eller om det har försvunnit i en, mycket trolig, krock. Vi har vidareutvecklat och anpassat en föga känd krockdetektionsmekanism från 80-talet för använding i IEEE 802.11 nätverk. Den anpassade algoritmen kallar vi EMCD vilket är en förkortning för ‘‘Early Multicast Collision Detection’’ eller tidig krockupptäckt för multicast. Vi har presenterat en nysannolikhetsbaserad modell för att beräkna algoritmens prestanda undermaximal belastning. Modellen som har verifierats genom simuleringar kanäven användas för att beräkna optimala parametrar för algoritmen. Algoritmen har visats kraftigt reducera risken för oupptäckta kollisioner och reducerar den tid som går åt för dem. EMCD-algoritmen inspirerade till att utveckla en ytterligare algoritm som inte bara kan upptäcka utan också undvika kollisioner: PREMA som står för ‘‘Prioritized Repeated Eliminations Multiple Access’’ eller prioriterad kanal-åtkomst med upprepade eliminationer. Det finns två viktiga skillnader mellanhur de fungerar. I EMCD bygger kollisionsdetektionen på rektangelfördelade slumptal och en enda upptäcktsomgång. I PREMA används i stället geometriskt fördelade slumptal och upprepade omgångar. Effekten blir att man med stor säkerhet får en enda vinnare. även för PREMA presenteras en sannolikhetskalkylsbaserad prestandaanalys för maxlastfallet vilken stöds av simuleringar. Samma formler kan användas för att approximativt skatta prestanda i EY-NPMA som är en närliggande algoritm. Den var tänkt att använda i Hiperlan/1; en standard som aldrig fick något kommersiellt genombrott. Använder man den modell som vi presenterar i avhandlingens sista studiekan man med ganska god noggrannhet beräkna optimala parametrar för EY-NPMA med en beräkningsinsats O(mY S) mot O(mES×mY S) för tidigare kända algoritmer. / Imagine a taxi driver wanting to watch a football game while working. Events in the game cannot be predetermined, the driver's available attentional resources vary and network connections change from non-existing to excellent, so it will be necessary to develop a viewing application that can adapt to circumstances. This thesis presents a system model and sketches a framework for design and run time adaptations. The model has three layers: user/usage, application and network. Quality of service metrics are proposed for each layer. A particular emphasis is placed on the difference between the user/usage layer and the application layer. Satisfaction at the former means a job well done, a match played to your liking etc. Satisfaction at the latter means good picture quality, nice colours etc. The thesis continues by identifying and describing elements required to build the system used by the taxi driver. Three studies are presented where either bandwidth or delay are varied at the network level. Video is better the higher the bandwidth; animations can be used as a complement. They are shown to be better than low quality video but worse than high quality video for watching a football game. Better video in the form of higher frame rates turned out to be worse for playing a card game over the Internet. A possible explanation is the distraction experienced when the image is updated constantly. Another result of our studies is that users can adapt their mental effort to the actual load when given feedback on the network delay affecting a computer game. The results mentioned above show that it is possible to compensate for poor network performance. For the user, improved network performance is generally more satisfactory. Early multicast collision detection is a method for improved multicast performance in high load IEEE 802.11 networks. Prioritised repeated eliminations multiple access is a method for multicast and other traffic which can be used alone or in an IEEE 802.11 network. Probabilistic performance analysis and simulations show that both protocols drastically reduce the time spent in collisions and improve throughput compared to IEEE 802.11. Some of the formulae are applied to EY-NPMA as well; they are used to estimate performance and to estimate optimal operating parameters more efficiently than with previously known methods.