RESEARCH

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My research interests include mathematical modeling and performance evaluation of communication systems and computer networks. My current research, conducted as part of my Ph.D. thesis, concentrates on combinatorial optimization problems in content distribution networks.

PUBLICATIONS

•  Content Distribution for Seamless Transmission (extended abstract) (pdf), In SIGMETRICS Performance Evaluation Review 32(2): 31-32 (2004). (Presented at ACM SIGMETRICS MAMA '04 Workshop.), Joint work with: E. G. Coffman, Jr., Dan Rubenstein, Bruce Shepherd, and Angelos Stavrou.

•  Content Distribution for Seamless Downloading (extended abstract) (pdf), Presented at INFOCOM 2005 Student Workshop, Miami, FL, March 2005, Joint work with: E. G. Coffman, Jr.

•  Optimal Seamless Self-Assembly of Files in Linear Networks, (pdf), In Optimization Letters 1(2): 119-128 (2007), Joint work with: E. G. Coffman, Jr.

•  Seamless Self-Assembly of Files in Cache Networks at Minimum Achievable Storage Cost, (pdf), Submitted for publication, Joint work with: E. G. Coffman, Jr.

CURRENT RESEARCH

Content Distribution for Seamless Downloading

We introduced a new paradigm in information transmission for networks of cooperative cache servers, called seamless downloading. A file F is sited at the root of a tree network of nodes, each able to cache part or all of F. Each node may also have a client wanting to download F. Links between neighbors in the tree have given transmission delays associated with them, but there is no transmission delay from a client to the node where it is located. The paradigm of seamless downloading requires that requesting nodes start receiving F immediately, receive it continuously until F is fully assembled, and be implemented by means of a protocol which is unaware of network structure beyond links to immediate neighbors. Thus, seamless downloading can be viewed as a way to implement a maximum quality of service (QoS) guarantee to all clients of a network.

Nodes where clients are located are specified as part of the problem instance, all client requests are made simultaneously, and file-unit transmissions are in the root-to-leaf direction. We formulated a discrete-time model and showed how to assign segments of F to the node caches in such a way that seamless downloading is realized and, simultaneously, the total cache size achieves a lower bound determined solely by the network link delays. One interesting question that we are pursuing is the relation between streaming and seamlessness: From the allocations in the set of mimimum total cache solutions, which allocation also minimizes the maximum delay until streaming, in the usual sequential sense, can begin at each client?

Another variant of the problem we studied leaves the subset of requesting nodes unspecified, and the solution provides seamlessness for every subset of client nodes. Multiple-file problems, trees where file units can also go upstream, structures more general than trees, and non-simultaneous request times are among the many directions for future research in seamless downloading that are worth pursuing.

 

 

PAST RESEARCH

Simulation study of search algorithms in peer to peer networks (pdf file)

This simulation study investigates the performance of different search algorithms in decentralized and unstructured peer to peer networks. The results show that flooding is extremely unscalable because it creates a lot of duplicate messages. Other algorithms which are more scalable than flooding are described, simulated and evaluated.

MIMO Systems - Joint work with Assaf Shacham (pdf file)

Communication in a slow flat Rayleigh fading channel with AWGN is not reliable as the channel frequently enters into deep fades, (i.e., the channel attenuation is large). Antenna diversity techniques are well known techniques for combating fading. In the first part of this report, we concentrate on antenna diversity techniques, giving both analytical as well as simulation results for the performance of the different techniques. The second part of our report concentrates on a recent important extension of antenna diversity, and more specifically the Multiple-Input-Multiple-Output (MIMO) wireless systems idea.

MIMO systems can be defined simply as having multiple transmitting and receiving antennas, and one of their key feature is the ability to turn multipath propagation, traditionally a pitfall in wireless transmission, into a benefit for the user. We look at the benefits derived by using MIMO, and in particular we look at a specific implementation, V-BLAST. Again here, we include extensive MATLAB simulation results that give the performance of V-BLAST. Finally, we compare and contrast the benefits gained from using antenna diversity or MIMO.

Survey on Caching (pdf file)

This survey aims to give a small introduction to the fascinating field of caching and to how it is applied to both computer systems and the World Wide Web. A number of analytical models of both computer and web caching are presented from the literature, and different performance evaluation methods are described.

Product-form solutions for Stochastic Petri Networks (pdf file)

This report serves as a small introduction to Stochastic Petri Nets(SPNs). It offers a brief comparison for two analysis approaches of SPNs that lead to Product Form Solution (PFS) results. The ultimate goal of this project (and a future research goal) is to try and identify biological examples that would conform to the described PFS framework.

 

 

 

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