Sound mobility models
Simulation has become an indispensable tool in the construction and evaluation of mobile systems. By using mobility models that describe constituent movement, one can explore large systems, producing repeatable results for comparison between alternatives. Unfortunately, the vast majority of mobility models---including all those in which nodal speed and distance or destination are chosen independently---suffer from decay; average speed decreases until converging to some long-term average. Such decay provides an unsound basis for simulation studies that collect results averaged over time, complicating the experimental process.This paper shows via analysis that such decay is inevitable in a wide variety of mobility models, including the most common in use today. We derive a general framework for describing this decay, and apply it to a number of practical cases. Furthermore, this framework allows us to transform any given mobility model into a stationary one: choose initial speeds from the steady-state distribution, and subsequent speeds from the original. This transformation provides sound models for simulation, eliminating variations in average nodal speed.

title: Modeling and Analysis of TCP Enhancement over Heterogeneous Links
abstract: In this paper we focus on one type of TCP enhancement commonly used to improve performance for connections over heterogeneous physical layer, namely, TCP connection splitting or split TCP, where the end-to-end TCP connection is split in segments so that each segment runs over a homogeneous environment and is optimized separately. This paper presents a simple model capturing some of the features of this scheme where split segments are essentially "coupled" TCP connections. We use simulation to validate our models and use our model to analyze situations where split TCP is preferred to the end-to-end scheme, and where the performance gain of splitting vanishes by investigating factors including initial window size, file size, number of on-going connections and loss.

title: On the Many-to-One Transport Capacity of a Dense Wireless Sensor Network and the Compressibility of Its Data
abstract: In this paper we investigate the capability of large-scale sensor networks to measure and transport a two-dimensional field. We consider a data-gathering wireless sensor network in which densely deployed sensors take periodic samples of the sensed field, and then scalar quantize, encode and transmit them to a single receiver/central controller where snapshot images of the sensed field are reconstructed. The quality of the reconstructed field is limited by the ability of the encoder to compress the data to a rate less than the single-receiver transport capacity of the network. Subject to a constraint on the quality of the reconstructed field, we are interested in how fast data can be collected (or equivalently how closely in time these snapshots can be taken) due to the limitation just mentioned. As the sensor density increases to infinity, more sensors send data to the central controller. However, the data is more correlated, and the encoder can do more compression. The question is: Can the encoder compress sufficiently to meet the limit imposed by the transport capacity? Alternatively, how long does it take to transport one snapshot? We show that as the density increases to infinity, the total number of bits required to attain a given quality also increases to infinity under any compression scheme. At the same time, the single-receiver transport capacity of the network remains constant as the density increases. We therefore conclude that for the given scenario, even though the correlation between sensor data increases as the density increases, any data compression scheme is insufficient to transport the required amount of data for the given quality. Equivalently, the amount of time it takes to transport one snapshot goes to infinity.

title: An Efficient and Robust Computational Framework for Studying Lifetime and Information Capacity in Sensor Networks
abstract: In this paper we investigate the expected lifetime and information capacity, defined as the maximum amount of data (bits) transferred before the first sensor node death due to energy depletion, of a data-gathering wireless sensor network. We develop a fluid-flow based computational framework that extends the existing approach, which requires precise knowledge of the layout/deployment of the network, i.e., exact sensor positions. Our method, on the other hand, views a specific network deployment as a particular instance (sample path) from an underlying distribution of sensor node layouts and sensor data rates. To compute the expected information capacity under this distribution-based viewpoint, we model parameters such as the node density, the energy density and the sensed data rate as continuous spatial functions. This continuous-space flow model is then discretized into grids and solved using a linear programming approach. Numerical studies show that this model produces very accurate results, compared to averaging over results from random instances of deployment, with significantly less computation. Moreover, we develop a robust version of the linear program, which generates robust solutions that apply not just to a specific deployment, but also to topologies that are appropriately perturbed versions. This is especially important for a network designer studying the fundamental lifetime limit of a family of network layouts, since the lifetime of specific network deployment instances may differ appreciably. As an example of this model's use, we determine the optimal node distribution for a linear network and study the properties of optimal routing that maximizes the lifetime of the network.

title: AMRoute: Ad Hoc Multicast Routing Protocol
abstract: The Ad hoc Multicast Routing protocol (AMRoute) presents a novel approach for robust IP Multicast in mobile ad hoc networks by exploiting user-multicast trees and dynamic logical cores. It creates a bidirectional, shared tree for data distribution using only group senders and receivers as tree nodes. Unicast tunnels are used as tree links to connect neighbors on the user-multicast tree. Thus, AMRoute does not need to be supported by network nodes that are not interested/capable of multicast, and group state cost is incurred only by group senders and receivers. Also, the use of tunnels as tree links implies that tree structure does not need to change even in case of a dynamic network topology, which reduces the signaling traffic and packet loss. Thus AMRoute does not need to track network dynamics; the underlying unicast protocol is solely responsible for this function. AMRoute does not require a specific unicast routing protocol; therefore, it can operate seamlessly over separate domains with different unicast protocols. Certain tree nodes are designated by AMRoute as logical cores, and are responsible for initiating and managing the signaling component of AMRoute, such as detection of group members and tree setup. Logical cores differ significantly from those in CBT and PIM-SM, since they are not a central point for data distribution and can migrate dynamically among member nodes. Simulation results (using ns-2) demonstrate that AMRoute signaling traffic remains at relatively low level for typical group sizes. The results also indicate that group members receive a high proportion of data multicast by senders, even in the case of a highly dynamic network.

title: Distributed computing
abstract: A distributed computing system consists of a collection autonomous computers connected by a communication network. The motivation for such a distributed system includes resource and load sharing, reduced costs in computation and communication, higher performance and throughput, and increased reliability and availability.

title: On the retention of female computer science students

title: Applying Design Patterns in Distributing a Genetic Algorithm Application
abstract: -This paper describes an experiment that employs an incremental approach to distribute a genetic algorithm application. The incremental approach allows the application to be initially developed for a standalone system, and then extended to run on multiple systems concurrently. For the extension, design patterns are employed wherever appropriate. The use of design patterns in the experiment is described, and the outcome of the experiment, including quantitative measurements, is presented.

title: A High Population, Fault Tolerant Parallel Raytracer
abstract: We present hierarchical master-slave architecture for performing parallel raytracing algorithms that can support a large population of participating clients and at the same time maintain fault tolerance at the application level. Our design allows for scalability with minimal data redundancy and maximizes the utilization of each client involved in the raytracing process. Our results show that this three-layer system can survive any type or number of client failures, and any non-concurrent server failures, while maintaining a near linear increase in performance with the addition of each new processing client.