Jay Martin

Directional virtual carrier sensing for directional antennas in mobile ad hoc networks

This paper presents a new carrier sensing mechanism called DVCS (Directional Virtual Carrier Sensing) for wireless communication using directional antennas. DVCS does not require specific antenna configurations or external devices. Instead it only needs information on AOA (Angle of Arrival) and antenna gain for each signal from the underlying physical device, both of which are commonly used for the adaptation of antenna pattern. DVCS also supports interoperability of directional and omni-directional antennas. In this study, the performance of DVCS for mobile ad hoc networks is evaluated using simulation with a realistic directional antenna model and the full IP protocol stack. The experimental results showed that compared with omni-directional communication, DVCS improved network capacity by a factor of 3 to 4 for a 100 node ad hoc network.

Effects of wireless physical layer modeling in mobile ad hoc networks

In most studies on mobile ad hoc networks (MANET), simulation models are used for the evaluation of devices and protocols. Typically, such simulations focus on the specific higher layer protocols that are being proposed, and tend to ignore details of models at other layers, particularly the interactions with physical layer models. In this paper, we present the set of factors at the physical layer that are relevant to the performance evaluations of higher layer protocols. Such factors include signal reception, path loss, fading, interference and noise computation, and preamble length. We start the discussion with the comparisions of physical layer models in ns-2 and GloMoSim, two commonly used simulators for MANET studies, and then quantify the impact of the preceding factors under typical scenarios used for the performance evaluation of wireless ad hoc routing protocols. Our experimental results show that the factors at the physical layer not only affect the absolute performance of a protocol, but because their impact on different protocols is non-uniform, it can even change the relative ranking among protocols for the same scenari

Compose: an object-oriented environment for parallel discrete-event simulations

Existing environments for parallel discrete-event simulation provide support for either conservative or optimistic algorithms, with very few supporting both. This paper describes a parallel simulation environment that supports the execution of a model using an existing adaptive simulation algorithm where sub-models may be synchronized using conservative or optimistic algorithms, and an object may dynamically change its mode of synchronization. The environment has been designed as a C++ class library and has been implemented on an IBM SP2 multicomputer.

Optimizing parallel execution of detailed wireless network simulation

With parallel and discrete event simulation (PDES) techniques, the runtime performance of detailed wireless network simulation can be improved significantly without compromising fidelity of the simulation results. However, modelling characteristics of wireless communications such as signal propagation and interference may severely hinder the potential speedup yielded by PDES. This paper proposes various optimization techniques to address three major concerns in achieving efficient parallel execution of wireless network simulation: i.e., (1) reducing communication and computation overhead of simulating signal propagation across multiple logical processes; (2) reducing synchronization overhead among logical processes; (3) minimizing event scheduling overhead within individual logical processes. These techniques have been implemented in a parallel version of GloMoSim and QualNet. The experimental results with mobile ad hoc networking scenarios demonstrate that the proposed optimization techniques can improve the performance of parallel wireless network simulation by up to an order of magnitude.

Parallel simulation of a high-speed wormhole routing network

A flexible simulator has been developed to simulate a two-level metropolitan area network which uses wormhole routing. To accurately model the nature of wormhole routing, the simulator performs discretebyte rather than discrete-packet simulation. Despite the increased computational workload that this implies, it has been possible to create a simulator with acceptable performance by writing it in Maisie, a parallel discrete-event simulation language. The simulator provides an accurate model of an actual high-speed, source-routing, wormhole network (the Myrinet) and is the first such simulator. The paper describes the simulator and reports on the performance of parallel implementations of the simulator on a 24-node IBM SP 2 multicomputer. The parallel implementations yielded reasonable speedups. For instance, on 12 nodes, the conservative algorithm yielded a speed-up of about 6 whereas an optimistic algorithm yielded a speed-up of about 4.

Slow memory: the rising cost of optimism

Rapid progress in the design of fast CPU chips has outstripped progress in memory and cache performance. Optimistic algorithms would seem to be more vulnerable to poor memory performance because they require extra memory for state saving and anti-messages. We examine the performance of both optimistic and conservative protocols in controlled experiments to evaluate the effects of memory speed and cache size, using a variety of applications.

Integration of fluid-based analytical model with packet-level simulation for analysis of computer networks

Fluid flow analytical models have been shown to be able to capture the dynamics of TCP flows and can scale well to solving for networks with a large number of flows. However, accurate closed form solutions are not yet available for wireless networks. Traditional packet-level discrete event simulations provide accurate predictions of network behavior, but their solution time can increase significantly with the number of flows being simulated. Integration of fluid flow models with packet-level simulators appears to offer significant benefits. In this paper, we describe an approach to integrate fluid flow models into QualNet, a scalable packet-level simulator. We validate the mixed model with detailed packet-level simulations for the scenarios considered in this paper. The execution time of the mixed model is significantly impacted by the frequency with which the analytical model must be solved in response to changes in the data rate at the interface of the packet-level and analytical models. We present a time averaging approach to mitigate this impact and present the results of the resulting tradeoff between prediction accuracy and model execution time.

A comparative study on the effects of spatial diversity in ad hoc networks using on-demand routing protocols

It is well known that spatial diversity in cellular communication systems is a powerful cost-effective communication technique used to improve the wireless link. Integrating the key benefits of spatial diversity into ad hoc networks is essential to meet the urgent demand for improved performance in scaleable networks. In contrast to cellular systems, mobile ad hoc networks use peer-to-peer packet transmissions to establish link connections to destinations, which require the use of elaborate routing protocols such as AODV and DSR. However, routing overhead compounded with co-channel interference and multipath fading still poses enormous challenges. In this study we propose to improve the networks performance through employing a spatial diversity model, designed to mitigate the effects of fading and reduce delay spread. We show through extensive simulation that using spatial diversity consistently resulted in improved network performance with both of the on-demand protocols, AODV and DSR.