The VNLayer approach 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, 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.

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.

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.

This work has been motivated by the need to test interoperability of systems carrying voice calls over the IP network. The voice over IP (VoIP) systems must be integrated and interoperate with the existing public switched telephone network (PSTN) before they are widely adopted. Standards have been developed to address this problem, but unfortunately different standards bodies and commercial consortiums have defined different standards. Furthermore, the prevailing VoIP standard such as H.323 is incomplete, complex, and presents the implementers with "vendors latitudes". As a result, there is no guarantee that the integrated VoIP systems would interoperate properly even if the implementations are all H.323-compliant. Thus interoperability testing has become indispensable. We want to test all the system interoperations by exercising all the required patterns of "interoperating behaviors". On the other hand, test execution in real environment is expensive, and we want to minimize the number of tests while maintaining the coverage. We present a general method for automatic generation of test cases, which cover all the required system interoperations and contain a minimal number of tests. We study data structures and efficient test generation algorithms, which take time proportional to the total test case size. Finally, we report experimental results on VoIP systems.

Voice over IP (VoIP) systems have to be integrated and interoperate with the existing telephone network before they become widely adopted. However, different standards have been proposed and the prevailing standard (H.323) is incomplete, complex, and provides the implementers with "vendors latitudes". As a result, there is no guarantee that even H.323-compliant systems would interoperate properly. Interoperability testing has become indispensable. We present techniques for automatic generation of a minimal set of test cases, covering all the required system interoperations. Our algorithms have been implemented and incorporated in a software tool-ITIS. We report experimental results on VoIP systems interoperability testing, using ITIS.

The international telecommunications system is the world's largest distributed computing system, offering a wide range of services and service features.Telecommunications service providers are eager to offer new services on this infrastructure, but the development of these new services is hampered by interactions of the service features among the different services.Undetected and undesirable feature interactions cause confusion and dissatisfaction among the users of services and add delay and expense to the development and deployment of new services.This panel reviews the progress that has been made in anticipating and controlling undesirable feature interactions in telecommunications services.This paper sets the stage by discussing the impact of the feature interaction problem on the different phases of the software development lifecycle.

The problem of placing a number t of identical resources at nodes of a tree so as to minimize the total expected cost of servicing a set of t requests arriving randomly at nodes is considered. The cost of servicing a particular set of requests is the total distance in the tree between each request and its assigned resource. Distance is measured by the number of edges along the unique path from the request to the resource. Optimal placements can be found in time O(mt), where m is the number of edges in the tree. Allowing resources to be split into fractional-sized pieces which can be placed separately neither reduces the cost of an optimal placement nor provides an obvious way to find optimal placements significantly faster. Simple, natural “fair” placements whose cost differs from optimality by at most the number of edges in the tree are described. For any fixed tree T, the cost of these placements grows as O(t), where the constant implicit in the “O” notation depends on the size and shape of T. In the case of balanced trees with k leaves, that constant is at most 2kφ. The placement problem becomes somewhat simpler for a complete (rooted) d-ary tree with a symmetric probability density function for request arrivals, and in that case slightly stronger results are possible. For example, an optimal placement can be found in time O(min{ℓ, logdt} + t), where ℓ is the height of the tree, and the placement is symmetric and fair.

The problem of managing telephone network resources for private virtual networks is discussed. Implementing a real-time resource counter on a distributed system is investigated. A correctness condition is developed for a real-time resource counter, and several approaches to implementing it are described. The various approaches are evaluated on a model that accounts for load-dependent message-processing delays, using both analytic and simulation techniques. The results of the performance studies also suggest heuristics for configuring a resource counter on a distributed system. A very simple approach, in which requests that cannot be satisfied locally are randomly forwarded to other processors, works well over a wide range of loads and system sizes. Unless a very large number of processors is required to handle the requests for resources or heavy overloads are expected, this may be the best algorithm.>

The authors address the topic of how to meet a real-time constraint as the load on a distributed system increases, without increasing the capacity of the individual processors. The application studied is a real-time resource counter with a probabilistic correctness criterion, and is motivated by the problem of implementing resource management in the telephone network. They introduce a model that combines distributed computing, real-time constraints, probabilistic correctness, and large system size; to the authors' knowledge, no previous work addresses this combination.>

Abstract. There are many examples of actions on abstract data types which can be correctly implemented with nonserralizable and nonrecoverable schedules of reads and writes We examme a model of multiple lay-ers of abstraction that explains this phenomenon and suggests an approach to burldmg layered systems with transaction oriented synchromzatron and roll back Our model may make rt easier to provide the high data m-tegrrty of reliable database transaction processmg m a broader class of mformatron systems We concentrate on the recovery aspects here, a technical report [Moss et al 851 has a more complete drscussron of concurrency control 1

The performance modeling described in this paper compares several protocols which ensure that a database can be recovered to a consistent state after a transaction failure or system crash. The contributions of the paper include a collection of simple analytic models, based on Markov processes, for these protocols and some surprising results on the relative performance of the protocols. We consider only two-stage transactions (all reads before writes) and ignore effects of serializing transactions. The most interesting performance result presented is that, for systems obeying the assumptions of this paper, the "pessimistic" policy of holding write locks to commit point is considerably less efficient than the "optimistic" policy which allows reading of uncommitted data, but risks cascading aborts. A multiversion policy introduced in [2] was also studied and found always to be nearly as good as the optimistic policy and sometimes much better.

Article Free Access Share on Reliable scheduling of database transactions for unreliable systems Authors: Marc H. Graham Georgia Institute of Technology, Atlanta, Georgia Georgia Institute of Technology, Atlanta, GeorgiaView Profile , Nancy Griffeth Georgia Institute of Technology, Atlanta, Georgia Georgia Institute of Technology, Atlanta, GeorgiaView Profile , Barbara Smith-Thomas University of North Carolina at Greensboro, Greensboro, North Carolina University of North Carolina at Greensboro, Greensboro, North CarolinaView Profile Authors Info & Claims PODS '84: Proceedings of the 3rd ACM SIGACT-SIGMOD symposium on Principles of database systemsApril 1984Pages 300–310https://doi.org/10.1145/588011.588055Published:02 April 1984Publication History 6citation199DownloadsMetricsTotal Citations6Total Downloads199Last 12 Months18Last 6 weeks0 Get Citation AlertsNew Citation Alert added!This alert has been successfully added and will be sent to:You will be notified whenever a record that you have chosen has been cited.To manage your alert preferences, click on the button below.Manage my AlertsNew Citation Alert!Please log in to your account Save to BinderSave to BinderCreate a New BinderNameCancelCreateExport CitationPublisher SiteeReaderPDF

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.

The use of "rules of inference" in database systems with access paths--e.g., CODASYL and IMS databases--is proposed to allow nonprocedural querying of the database systems. The kinds of access paths for which these rules of inference are required are isolated. It is shown that the rules of inference required for a CODASYL or an IMS database depend on the configurations of the edges in a diagram of the database.