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Abstracts of Selected Ph.D. Theses in the Area of Mobile Computing Awarded in 2002

Modeling of Networks Composed of Satellite Constellations

Riadh Dhaou

Institut National des TÚlÚcommunication & University of Paris 6
Paris, France
8 November 2002

When evaluating the performance of satellite communication systems, we are confronted with difficulties of modeling and of simulation of these systems. The need to take into account their specificity, in simulation tools, is quickly felt. We tried to bring some kind of answers to the problems, which are raised, by developing a new LEO Satellite Simulation Tool for IP and ATM traffic. This tool was developed as a part of the freely distributed NS simulator. It enables advanced studies in the scope of satellite network modeling and simulation. We describe the design of this tool, and detail how we modeled some critical parameters and system attributes, including link handovers between ground terminals and satellites. We next illustrate some fundamental delay performance results of LEO systems, obtained by studying simulation models of a system based on the proposed Skybridge constellation. We obtain lower bound on the delay performance that can be achieved in this constellation. We describe the use of this tool for different levels of studies and the possible reutilization of both its code and SDL specification. Finally we describe validating scenarios for HTTP and file transfer traffic on some LEO satellites constellations.

Furthermore, we study the influence of handovers on the quality of service in ATM LEO satellite networks, to provide continuous service of communication to a nomadic user passing alternatively between beams of the wide coverage satellite networks. We contribute to define transfer strategies. These strategies commonly called handovers take importance as well as routing strategies in current networks. In general, data flow interruption, in course of transfer, is much more undesirable than new access refusal. This makes it possible to preserve the system efficiency. We are interested in two points of view, to be satisfied at the same time: the user point of view, and the operator point of view. This new criterion, compromised between each one of these two parts respective waiting, must be integrated in modeling. We proposed to extend the fixed channel allocation mechanisms towards adaptive dynamic channel allocation mechanisms. Considering multiservice network architecture, we evaluate its performances by taking into account the interactions between various allocation mechanisms (one by flow nature). We propose thereafter to consider, for all kind of traffic, only one adaptive mechanism based on the TCRA (a time based channel reservation mechanism), adapted to our study. We highlight the importance to provide each accepted traffic flow a minimal quality of service. Finally, an approximate analytical model validates our simulations. These solutions will be implemented in a method treating of resources allocation problems for satellites with Satellite Fixed Cell (SFC) coverage concept.

The ACM Special Interest Group on Mobility of Systems, Users, Data and Computing