Nancy D. Griffeth

A feature-interaction benchmark for IN and beyond

It is argued that the goal of the intelligent network (IN) is to accelerate the introduction of new telecommunications features in a multisupplier competitive environment. One major roadblock to fulfilling such requirements is the feature interaction problem-a new feature may interact with existing features in some undesirable ways, resulting in adverse behavior. Feature interaction examples are categorized by their causes, since problems arising from the same cause may have the same solution. These examples and taxonomies are developed to improve understanding of the problems scope and to provide a benchmark for analyzing the coverage of a proposed approach to solving the problem. Informal definitions of the concepts given, feature, service, and feature interaction, are presented. Prototypical examples of feature interactions and their categorization by causes are also presented. Possible approaches to the problem are discussed.<<ETX>>

Global States of a Distributed System

A global state of a distributed transaction system is consistent if no transactions are in progress. A global checkpoint is a transaction which must view a globally consistent system state for correct operation. We present an algorithm for adding global checkpoint transactions to an arbitrary distributed transaction system. The algorithm is nonintrusive in the sense that checkpoint transactions do not interfere with ordinary transactions in progress; however, the checkpoint transactions still produce meaningful results.

Abstraction in recovery management

There are many examples of actions on abstract data types which can be correctly implemented with nonserializable and nonrecoverable schedules of reads and writes. We examine a model of multiple layers of abstraction that explains this phenomenon and suggests an approach to building layered systems with transaction oriented synchronization and roll back. Our model may make it easier to provide the high data integrity of reliable database transaction processing in a broader class of information systems. We concentrate on the recovery aspects here, a technical report [Moss et al 85] has a more complete discussion of concurrency control.

Reasoning about goals to resolve conflicts

A mechanism is presented for negotiating to resolve conflicts that arise when autonomous agents need to agree on a collection of operations in order to achieve a goal. These conflicts arise when the agents have different policies as to which operations are acceptable. The authors mechanism protects the privacy of the agents because they are not required to exchange information freely about their goals. Instead, each agent can infer the goals of the other agents. This characteristic is obtained by incorporating plan recognition in the negotiation process. Their mechanism requires agents to find alternative activities that achieve the goals of all involved agents, rather than allowing them to rely on modification of their own or other agents goals. This approach is reasonable in contexts such as telecommunications systems, where agreement about end goals is usual, and the difficulty is in achieving agreement on the means to achieve the end goals. The negotiation mechanism is illustrated using an example where agents represent subscribers to a telecommunications system.<<ETX>>

Engineering the Virtual Node Layer for Reactive MANET Routing

The VNLayer approach [1] simplifies software development for MANET by providing the developers an abstraction of a network divided into fixed geographical regions, each containing a virtual server for network services. In this paper, we present our study on reactive MANET routing over the VNLayer. During this research, we identified in our initial VNLayer implementation three major limitations that lead to heavy control traffic, long forwarding paths and frequent message collisions in MANET routing. To address the problems, we changed the assumptions made by the VNLayer on the link layer and extended the operations allowed by VNLayer. This results in a VNLayer implementation that can be tuned to optimize the performance of traffic intensive applications (such as routing) while maintaining their simplicity and robustness. Simulation results showed that VNAODV, a VNLayer based routing protocol adapted from AODV [4], delivers more packets, generates less routing traffic and creates more stable routes than AODV in a dense MANET with high node motion rates. This research validated that the VNLayer approach makes software development for MANET easier and improves the performance of MANET protocols.

Probabilistic analysis of a network resource allocation algorithm

Abstract : A distributed algorithm is presented, for allocating a large number of identical resources (such as airline tickets) to requests which can arrive anywhere in a distributed network. Resources, once allocated, are never returned. The algorithm searches sequentially, exhausting certain neighborhoods of the request origin before proceeding to search at greater distances. Choice of search direction is made nondeterministically. Analysis of expected response time is simplified by assuming that the search direction is chosen probabilistically, that messages require constant time, that the network is a tree with all leaves at the same distance from the root, and that requests and resources occur only at leaves. It is shown that the response time is approximated by the number of messages of one that are sent during the execution of the algorithm, and that this number of messages is a nondecreasing function of the interarrival time for requests. Therefore, the worst case occurs when requests come in so far apart that they are processed sequentially. The expected time for the sequential case of the algorithm is analyzed by standard techniques. This time is shown to be bounded by a constant, independent of the size of the network. It follows that the expected response time for the algorithm is bounded in the same way. (Author)

Teaching Cardiac Electrophysiology Modeling to Undergraduate Students: Laboratory Exercises and GPU Programming for the Study of Arrhythmias and Spiral Wave Dynamics

As part of a 3-wk intersession workshop funded by a National Science Foundation Expeditions in Computing award, 15 undergraduate students from the City University of New York(1) collaborated on a study aimed at characterizing the voltage dynamics and arrhythmogenic behavior of cardiac cells for a broad range of physiologically relevant conditions using an in silico model. The primary goal of the workshop was to cultivate student interest in computational modeling and analysis of complex systems by introducing them through lectures and laboratory activities to current research in cardiac modeling and by engaging them in a hands-on research experience. The success of the workshop lay in the exposure of the students to active researchers and experts in their fields, the use of hands-on activities to communicate important concepts, active engagement of the students in research, and explanations of the significance of results as the students generated them. The workshop content addressed how spiral waves of electrical activity are initiated in the heart and how different parameter values affect the dynamics of these reentrant waves. Spiral waves are clinically associated with tachycardia, when the waves remain stable, and with fibrillation, when the waves exhibit breakup. All in silico experiments were conducted by simulating a mathematical model of cardiac cells on graphics processing units instead of the standard central processing units of desktop computers. This approach decreased the run time for each simulation to almost real time, thereby allowing the students to quickly analyze and characterize the simulated arrhythmias. Results from these simulations, as well as some of the background and methodology taught during the workshop, is presented in this article along with the programming code and the explanations of simulation results in an effort to allow other teachers and students to perform their own demonstrations, simulations, and studies.

Testing a Network by Inferring Representative State Machines from Network Traces

This paper describes an innovative approach to network testing based on automatically generating and analyzing state machine models of network behavior. The models are generated by the network test tool AGATE (Automatic Generator of Automata for TEsting), which is also described in this paper. The proposed test approach mimics experimental method, requiring repeated cycles of observing the network, modeling the network, making predictions about network behavior, and evaluating predictions. This paper focusses on the modeling step, in which the test tool AGATE automatically generates representative state machines from observed network traces. The generated state machines closely approximate the behavior of components of the network under test. Faults in the system may be immediately apparent from the state machines, but more importantly the state machines can be used for formal analysis. We propose this as a cost-effective alternative to manually defining a state machine before beginning tests.

Reliable scheduling of database transactions for unreliable systems

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Simulating Fixed Virtual Nodes for Adapting Wireline Protocols to MANET

The Virtual Node Layer (VNLayer) is a programming abstraction for Mobile Ad Hoc Networks (MANETs). It defines simple virtual servers at fixed locations in a network, addressing a central problem for MANETs, which is the absence of fixed infrastructure. Advantages of this abstraction are that persistent state is maintained in each region, even when mobile nodes move or fail, and that simple wireline protocols can be deployed on the infrastructure, thereby taming the difficulties inherent in MANET setting. The major disadvantage is the messaging overhead for maintaining the persistent state.In this paper, we use simulation to determine the magnitude of the messaging overhead and the impact on the performance of the protocol. The overhead of maintaining the servers and the persistent state is small in bytes, but the number of messages required is relatively large. In spite of this, the latency of address allocation is relatively small and almost all mobile nodes have an address for 99 percent of their lifetime. Our ns-2 based simulation package (VNSim) implements the VNLayer using a leader-based state replication strategy to emulate the virtual nodes. VNSim efficiently simulates a virtual node system with up to a few hundred mobile nodes. VNSim can be used to simulate any VNLayer-based application.