This licentiate thesis work investigates two medium access control (MAC) methods, when used in traffic safety applications over vehicular ad hoc networks (VANETs). The MAC methods are carrier sense multiple access (CSMA), as specified by the leading standard for VANETs IEEE 802.11p, and self-organizing time-division multiple access (STDMA) as used by the leading standard for transponders on ships. All vehicles in traffic safety applications periodically broadcast cooperative awareness messages (CAMs). The CAM based data traffic implies requirements on a predictable, fair and scalable medium access mechanism. The investigated performance measures are channel access delay, number of consecutive packet drops and the distance between concurrently transmitting nodes. Performance is evaluated by computer simulations of a highway scenario in which all vehicles broadcast CAMs with different update rates and packet lengths. The obtained results show that nodes in a CSMA system can experience unbounded channel access delays and further that there is a significant difference between the best case and worst case channel access delay that a node could experience. In addition, with CSMA there is a very high probability that several concurrently transmitting nodes are located close to each other. This occurs when nodes start their listening periods at the same time or when nodes choose the same backoff value, which results in nodes starting to transmit at the same time instant. The CSMA algorithm is therefore both unpredictable and unfair besides the fact that it scales badly for broadcasted CAMs. STDMA, on the other hand, will always grant channel access for all packets before a predetermined time, regardless of the number of competing nodes. Therefore, the STDMA algorithm is predictable and fair. STDMA, using parameter settings that have been adapted to the vehicular environment, is shown to outperform CSMA when considering the performance measure distance between concurrently transmitting nodes. In CSMA the distance between concurrent transmissions is random, whereas STDMA uses the side information from the CAMs to properly schedule concurrent transmissions in space. The price paid for the superior performance of STDMA is the required network synchronization through a global navigation satellite system, e.g., GPS. That aside since STDMA was shown to be scalable, predictable and fair; it is an excellent candidate for use in VANETs when complex communication requirements from traffic safety applications should be met.

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This thesis is concerned the technology with technologies of transfer through wireless devices. It is about possibilities of optimization of transfer in wireless net, especially of the reduction of interference influence. The first part is about theory and the standards development of wireless technology. There are methods and principles used in wireless technologies. The second part is a practical measuring and testing of an internal wireless net, with the emphasis being on the source of interference influence. Measuring was focused on interference between access points and was not about meteorological influence. It continues with theory and testing of interference of the microwave oven in our wireless net. Then, it is a practical demonstration of interference in an external environment. There are practical examples of interference between different providers’ nets on the same area and solutions to prevent or to cancel this interference. In next part of work the wireless net is parsed and described in a given territory. Here is described the way how to precede the reciprocal interference and eventually how this mutual disturbance suppress. Consequently, our net is optimized for customer growth and for today’s options of newer and more powerful components for wireless transfer. Finally there are two laboratory tasks. The first one is about practical measuring and testing of wireless net’s parameters and the second one is about simulation of wireless networks in the Opnet program.

Beim Entwurf großer Netzwerke der Automation ermöglichen Methoden der Leistungsbewertung den Test und die Dimensionierung eines Netzwerkes bereits vor der Inbetriebnahme und erlauben damit einen effizienten, qualitätsorientierten Entwurf. Es ist jedoch sehr wissensintensiv und aufwendig, entsprechende Analyse- oder Simulations-Modelle aufzustellen und die Ergebnisse auszuwerten, weshalb die Methoden in der Praxis der Automation selten verwendet werden. Viel vertrauter sind dem Entwerfer hingegen die speziellen Software-Tools, mit denen in der Automation Netzwerke entworfen werden. Auf Basis der Datenbanken dieser Tools werden in dieser Arbeit verschieden Verfahren zur automatischen Systemmodellierung, Verkehrsmodellierung, Leistungsbewertung und Fehlerdiagnose zu einem Control Network Performance Engineering kombiniert, welches die Qualitätsbewertung und -beratung nahtlos und ohne Mehraufwand in den Netzwerkentwurf integriert. (Die Dissertation wurde veröffentlicht im Jörg Vogt Verlag, Voglerstr. 20, 01277 Dresden, Internet: http://www.vogtverlag.de/, email: info@vogtverlag.de, ISBN 978-3-938860-10-6) / During the design of large automation networks, performance analysis methods can be used for testing and dimensioning the network before implementation and are essential for an efficient and reliable design process. However, setting up the necessary analytical or simulative models is time-consuming, requires in-depth knowledge, and is therefore often not applicable in practice. The network designers are much more used to the design tools used to develop automation networks. Based on these tools' databases various methods for automated system and traffic modeling, performance analysis and diagnoses are combined in the control network performance engineering that seamlessly integrates quality analysis and consulting into network design without requiring additional effort. (This manuscript is also available - in the form of a book - from Jörg Vogt Verlag, Voglerstr. 20, 01277 Dresden, Germany world-wide web address: http://www.vogtverlag.de/, electronic-mail address: info@vogtverlag.de, ISBN 978-3-938860-10-6 )

Denna rapport behandlar prestandaproblem i den nyligen standardiserade Mesh kommunikationsstandarden (IEEE 802.11s). I denna rapport, undersöker och förbättra vi ett förhållande som resulterar i reduktion av genomströmningen i en kedja av noder topologi. IEEE802.11s är mycket lovande med många fördelar för både IoT-systemen och trådlösa nätverk i båda hemmet och arbete.Vi arbetar med frågan om orättvisa när CSMA/CA tillämpas, vilket orsakar genomströmningsreduktion på grund av paketförluster och indikerar svältning. Vi analyserar konsekvenserna av Collision Avoidance (CA) mekanism och föreslår en ersättning för CA som är både rättvist och samtidigt kan upprätthålla undvikande av kollisioner. Vi implementera detta i en simulator och resultatet visar på betydligt högre end-to-end-genomströmning än standard CSMA/CA och inga paketförluster på grund av buffertspill. / This thesis rapport deals with the performance issues of the newly standardized Wireless mesh protocol (IEEE 802.11s). In this thesis, we work on improving the conditions that results in throughput degradation in a chain of nodes topology. The mesh standard is very promising with many advantages for both IoT systems and home wireless networks.We work on the issue of unfairness when CSMA/CA is applied, which causes throughput degradation due to packet loss and indicates starvation. We analyze the implication of the Collision Avoidance (CA) mechanism and propose a replacement for the CA that is both fair and able to maintain collision avoidance. We implement this in a simulator and the result shows significantly higher end-to-end throughput compared to the original CSMA/CA and no packet loss due to buffer overflow.