Modelação de redes sem fios para comunicações multimédia

Mestrado em Engenharia Electrónica e TelecomunicaçõesAs técnicas de codificação, transporte e transmissão de sinais digitais multimédia têm evoluído significativamente na última década. Têm sido publicados uma grande quantidade de estudos e normas relativos às questões acima referidas. Essencialmente, esta dissertação discute as normas da camada de transporte e apresenta resultados relativos à qualidade duma comunicação multimédia multiplexada em pacotes. Ao nível da camada de rede, o protocolo IP (Internet Protocol) tem servido não apenas para endereçamento mas também para a interligação de redes de telecomunicações, com tecnologias heterogéneas nas camadas inferiores (ligação e física). Acima da camada de rede, a informação fonte comprimida é encapsulada e transportada pelos protocolos UDP (User Datagram Protocol) e RTP (Real Time Transport Protocol). O objectivo destes protocolos de transporte/sessão é identificar a existência de erros nos dados fonte e associar informação de sincronismo e sequência. Esta informação permite o receptor minimizar os efeitos dos erros de comunicação e sincronizar diferentes conteúdos de media. Esta dissertação apresenta as técnicas de encapsulamento usadas num sistema de comunicação áudio-visual sobre um canal de onda curta HF (High Frequency). Estas técnicas foram estudadas no âmbito do projecto NAVIO (Navy Video), apoiado pela Marinha de Guerra Portuguesa. Para além disso, são apresentados alguns resultados relacionados com as perdas de pacotes que permitem avaliar a qualidade dos sinais reconstruídos na recepção. O principal objectivo do projecto é investigar e implementar um serviço de videoconferência, terra-navio. O sistema de comunicação sobre HF apresentado nesta dissertação utiliza apenas 4 kbps, dos quais 2 kbps são usados para a codificação do áudio e os outros 2 kbps para a codificação de alguns parâmetros que representam a face humana. Esta dissertação apresenta também um estudo semelhante para a rede IEEE 802.11g em resultado do trabalho realizado no projecto TRIVial (Transmissão Rádio Interactiva de Video Local) apoiado pela Philips-RCS (Remote Control System) e pela Agência de Inovação. Como resultado do conhecimento obtido, do estudo de ambas as redes em separado, é feita a análise da possibilidade de implementar um sistema constituído por duas redes locais sem fios ligadas por um canal HF. Esta análise permite prever a perda de pacotes duma comunicação multimédia caracterizada segundo o seguinte cenário: um terminal inserido numa rede local sem fios em terra a comunicar com um terminal inserido numa rede sem fios existente num navio, permanecendo ambas ligadas por um canal HF.In the last decade, the coding techniques and transmission of digital multimedia has evolved significantly. Here have been published an enormous amount of papers, reports and standards concerned with the issues mentioned above. Generally speaking, this thesis discusses some standards related to the transport layer and presents some results related to the quality of a packetized multimedia communication. At the network layer, the IP (Internet Protocol) has served, not only for routing, but also for the interconnection of diverse telecommunications networks with heterogeneous technologies at lower layers (link and physical). Above the network layer, the compressed source data is encapsulated and transported making use of UDP (User Datagram Protocol) and RTP (Real Time Transport Protocol) protocol structures. The main goal of these transport/session protocols are to signal any communications errors and to include synchronization information as well as inserting packet sequence numbers. This thesis presents the encapsulation techniques used in an audio-visual communication system over a HF (High Frequency) channel. These techniques were studied in the NAVIO (Navy Video) project framework, a project supported by the Portuguese Navy. Furthermore, this thesis presents some results related to packet loss which allow us to evaluate the quality of the reconstructed signals at the receiver. The main goal of NAVIO project is to research and develop a video conference service running on earth and war vessel stations. The communication system over HF spends 4 kbps, in which 2 kbps are used for audio encoders and the remaining 2 kbps for encoding some face animation parameters. This thesis also presents a similar detailed study for IEEE 802.11g wireless network in consequence of a research work carried out in the TRIVial (Transmissão Rádio Interactiva de Video Local) project supported by Philips- RCS (Remote Control System) and Agência de Inovação. As a result of the obtained knowledge, from both networks study, we analyse the possibility of implementing an interconnected platform thereby, two wireless local networks are connected by a HF link. This analysis allow us to estimate the multimedia packet loss under the following scenario: a terminal as part of a wireless local network in earth, connected to a terminal as part of another wireless local network in a war vessel where both networks are interconnected by a HF link

Packet Loss in Terrestrial Wireless and Hybrid Networks

The presence of both a geostationary satellite link and a terrestrial local wireless link on the same path of a given network connection is becoming increasingly common, thanks to the popularity of the IEEE 802.11 protocol. The most common situation where a hybrid network comes into play is having a Wi-Fi link at the network edge and the satellite link somewhere in the network core. Example of scenarios where this can happen are ships or airplanes where Internet connection on board is provided through a Wi-Fi access point and a satellite link with a geostationary satellite; a small office located in remote or isolated area without cabled Internet access; a rescue team using a mobile ad hoc Wi-Fi network connected to the Internet or to a command centre through a mobile gateway using a satellite link. The serialisation of terrestrial and satellite wireless links is problematic from the point of view of a number of applications, be they based on video streaming, interactive audio or TCP. The reason is the combination of high latency, caused by the geostationary satellite link, and frequent, correlated packet losses caused by the local wireless terrestrial link. In fact, GEO satellites are placed in equatorial orbit at 36,000 km altitude, which takes the radio signal about 250 ms to travel up and down. Satellite systems exhibit low packet loss most of the time, with typical project constraints of 10−8 bit error rate 99% of the time, which translates into a packet error rate of 10−4, except for a few days a year. Wi-Fi links, on the other hand, have quite different characteristics. While the delay introduced by the MAC level is in the order of the milliseconds, and is consequently too small to affect most applications, its packet loss characteristics are generally far from negligible. In fact, multipath fading, interference and collisions affect most environments, causing correlated packet losses: this means that often more than one packet at a time is lost for a single fading even

Interaktive latenzkritische Anwendungen in mobilen Ad-hoc Netzen

In this thesis we discuss the challenges that latency-sensitive interactive applications face in mobile ad-hoc networks. By using multi-player games as an example, we argue that the traditional client-server architecture is unsuitable for this new environment. We consequently create a novel communication architecture as well as quality of service mechanisms that can support the network requirements of such applications in mobile environments. By using a number of distributed zone servers that are selected and managed dynamically by our server selection algorithm, we provide a scalable approach that offers the necessary redundancy. Furthermore, we propose additional quality of service mechanisms to reduce latency and packet loss for interactive applications. We evaluate our approach through network simulation and realistic mobile gaming scenarios. The performance of our evaluation is checked against real-world measurements.In dieser Arbeit werden die Probleme und Herausforderungen von latenz-kritischen interactiven Computeranwendungen in mobilen Ad-hoc Netzen untersucht. Am Beispiel von Mehrbenutzercomputerspielen zeigen wir, dass traditionelle Client-Server Architekturen für diese neuen Umgebungen ungeeignet sind. Im Rahmen dieser Arbeit wird daher eine neue Kommunikationsarchitektur sowie verschiedene Mechanismen zur Erhöhung der Dienstgüte vorgeschlagen. Mit Hilfe von Zonenserver, die durch den Serverauswahlalgorithmus ausgesucht und verwaltet werden zeigen wir einen Ansatz auf, der sowohl bezüglich der Netzgröße skalierbar ist als auch die notwendige Redundanz bereitstellt. Wir zeigen die Funktionalität und die Leistung unseres Ansatzes mit Hilfe von Netzsimulationen bei denen realistische Szenarien für mobiles Spielen simuliert werden. Der hierbei benutze Netzsimulator wurde dafür auf Basis von eigenen Messungen verbessert und für das jeweilige Szenario passend eingestellt