SIMULATION

In mobile ad hoc network (MANET) studies, it is imperative to use highly detailed device models as they provide high-layer protocols with good prediction of underlying wireless communication performance. However, such studies often use abstract models for execution speed and simplicity. This article first shows that physical layer variables, including path loss, shadowing, multipath, and Doppler, have significant effects on the predicted overall networking performance. It then proposes an approach to simulate details of wireless propagation and radio characteristics in networking studies while still maintaining a reasonable simulation execution time. Through runtime performance studies with detailed orthogonal frequency division multiplexing (OFDM) Simulink/MATLAB models and the QualNet network simulator, it is shown that the proposed approach can improve the simulation runtime performance by three to four orders of magnitude without compromising the fidelity of simulation results.

Temporal decomposition of simulation models, which is used in time-parallel simulation, is a promising alternative to classical spatial decomposition. It has been successfully applied to a small number of different models, most prominently for the simulation of computer caches and queuing systems. Unfortunately, widespread use is prevented by the state-match problem, which restricts the application of time-parallel simulation. Instead of a correct solution to the state-match problem, this work proposes the use of approximate solutions to facilitate the temporal decomposition of simulation models and to extend the class of models suitable for time-parallel simulation. However, this introduces an error in the simulation results, which might seriously distort or even invalidate results. Therefore, the error must be closely analyzed, and a method of error control must be provided. In addition to the basic properties of approximate techniques in time-parallel simulation, this work presents two use cases that illustrate the introduced concepts.

The authors discuss an approach to federated network simulations that eases the burdens on the simulation developer in creating space-parallel simulations. Previous approaches have had difficulties that arise from the need for global topology knowledge when forwarding simulated packets between federates. In all but the simplest cases, proper packet-forwarding decisions between federates requires routing tables of size O(mn), where m is the number of nodes modeled in a particular federate, and n is the total number of network nodes in the entire topology. Furthermore, the benefits of the well-known NIx-Vector routing approach cannot be fully achieved without global knowledge of the overall topology. The authors overcome these difficulties by using a topology partitioning methodology that uses ghost nodes. They show experimentally that the memory overhead associated with ghosts is minimal relative to the overall memory footprint of the simulation.

With the wide use of commercial off-the-shelf (COTS) simulation packages and the advent of the High Level Architecture (HLA) standard, it is desirable to build distributed simulations by linking various types of simulation models developed using best-fit COTS packages. While almost all current work on integrating COTS packages and the HLA is based on conservative synchronization, it is worthwhile to investigate the optimistic synchronization approach. The optimistic approach can exploit parallelism and achieve promising performance in situations where causality errors may occur but in fact seldom occur. The authors introduce a rollback controller using a middleware approach to handle the complex rollback procedure on behalf of the simulation model. A new time advance algorithm is proposed that can fully use the benefits of optimistic synchronization. The article also describes a scalability study showing the experimental results for the two synchronization approaches as the number of simulation components increases.

The authors study the adaptation of an optimistic Time Warp kernel to cross-cluster computing on the Grid. Wide-area communication, the primary source of overhead, is offloaded onto dedicated routing processes. This allows the simulation processes to run at full speed and thus significantly decreases the performance gap caused by the wide-area distribution. Further improvements are obtained by employing message aggregation on the wide-area links and using a distributed global virtual time algorithm. The authors achieve many of their objectives for a cellular automaton simulation with lazy cancellation and moderate communication. High communication rates, especially with aggressive cancellation, present a challenge. This is confirmed by the experiments with synthetic loads. Even then, a satisfactory speedup can be achieved, provided that the computational grain of events is large enough.

Computer simulation has been used extensively as an effective tool in the design and evaluation of systems. One should not, however, underestimate the importance of validation—the process of ensuring whether a simulation model is an appropriate representation of the real-world system. Validation of wireless network simulations is difficult due to strong interdependencies among protocols at different layers and uncertainty in the wireless environment. The authors present an approach of coupling direct-execution simulation and traces from real outdoor experiments to validating simple wireless models that are used commonly in simulations of wireless ad hoc networks. This article documents a common testbed that supports direct execution of a set of ad hoc routing protocol implementations in a wireless network simulator. By comparing routing behavior measured in the real experiment with behavior computed by the simulation, the authors validate the models of radio behavior upon which protocol behavior depends.

This article advocates the use of a formal framework for analyzing simulation performance. Simulation performance is characterized based on the three simulation development process boundaries: physical system, simulation model, and simulator implementation. First, the authors formalize simulation event ordering using partially ordered set theory. A simulator implements a simulation event ordering and incurs implementation overheads when enforcing event ordering at runtime. Second, they apply their formalism to extract and formalize the simulation event orderings of both sequential and parallel simulations. Third, they propose the relation stricter and a measure called strictness for comparing and quantifying the degree of event dependency of simulation event orderings, respectively. In contrast to the event parallelism measure, strictness is independent of time.