Optimizing throughput over wireless links and MANETs under delay constraints
Speaker: Spyridon Vassilaras
Date: 29 June 2009 Time: 12:00-14:00
Location: "Stelios Orphanoudakis" Seminar Room, FORTH, Heraklion, Crete.
Host: Maria Papadopouli


Many theoretical results and performance optimization techniques in wireless telecommunications and networks strive to maximize the system throughput without taking into account the delay QoS requirements of transmitted flows. In this talk, we will explore two such cases and describe ways to optimize the system throughput under delay considerations.
The first case is Adaptive Modulation & Coding (AMC) for transmission of streaming media over wireless links. AMC aims at optimizing the trade-off between higher transmission rate and higher BER. When delay tolerant - loss intolerant data are transmitted, erroneously received packets can be retransmitted (using an ARQ protocol) as many times as needed until they get received without non-recoverable errors. In this scenario, traditional AMC algorithms which optimize the useful throughput of the wireless link offer an adequate solution. But for delay intolerant data flows (such as video streaming or VoIP) there is an upper limit to the number of retransmissions. Thus a packet is lost if it has not being correctly received within a certain period of time. In this talk, we use queuing models and Large Deviation techniques in order to derive AMC policies that minimize the overall probability of losing packets in both the no-retransmissions and persistent ARQ cases.
The second case is inspired by the well known work of M. Grossglauser and D. Tse which showed that mobility can drastically increase the capacity of ad hoc wireless networks. By using nodes as data carriers which physically transfer data closer to their intended destination before transmitting them over the air, the network throughput is considerably increased (compared to sending packets over multi-hop routes) at the expense of unbounded delays as the network size grows to infinity. The question addressed in this talk is "how can we design an algorithm that takes advantage of mobility to optimize the network throughput under a maximum allowable delay constraint?". The problem is formulated and solved as a Dynamic Programming problem with some interesting twists to address the intermittent end-to-end connectivity case.


Dr. Spyridon Vassilaras is an Assistant Professor at the Athens Information Technology Center for Research and Graduate Education (AIT). He received the Engineering Diploma in Electrical and Computer Engineering from the National Technical University of Athens in 1995 and both M.S. and Ph.D. degrees in Computer Engineering from Boston University in 1997 and 2001 respectively. Prior to joining AIT in November 2003, Dr. Vassilaras has also worked as a software developer for ABB Industrial Systems in Sweden and as an application developer / IT consultant for small Greek companies. His current research interests include the performance analysis of telecommunication networks using stochastic modelling, queuing theory, Large Deviations theory, advanced simulation techniques (such us variance reduction), linear and non-linear optimization.

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