It is well known that TCP does not fully utilize the available bandwidth in fast long-distance networks. To solve this scalability problem, several high speed transport protocols have been proposed. They include HighSpeed TCP (HS-TCP), Scalable TCP (S-TCP), Binary increase control TCP (BIC-TCP), and H-TCP. These protocols increase (decrease) their window size more aggressively (slowly) compared to standard TCP (STD-TCP). This paper aims at evaluating and comparing these high speed transport protocols through computer simulations. We select six metrics that are important for high speed protocols; scalability, buffer requirement, TCP friendliness, TCP compatibility, RTT fairness, and responsiveness. Simulation scenarios are carefully designed to investigate the performance of these protocols in terms of the metrics. Results clarify that each high speed protocol successfully solves the problem of STD-TCP. In terms of the buffer requirement, S-TCP and BIC-TCP have better performance. For TCP friendliness and compatibility, HS-TCP and H-TCP offer better performance. For RTT fairness, BIC-TCP and H-TCP are superior. For responsiveness, HS-TCP and H-TCP are preferred. However, H-TCP achieves a high degree of fairness at the expense of the link utilization. Thus, we understand that all the proposed high speed transport protocols have their own shortcomings. Thus, much more research is needed on high speed transport protocols.

In this paper we propose a novel spatial fading simulator to evaluate the performance of an array antenna and show its spatial stochastic characteristics by computer simulation based on parameters verified by experimental data. We introduce a cavity-excited circular array (CECA) as a fading simulator that can simulate realistic mobile communication environments. To evaluate the antenna array, two stochastic characteristics are necessary. The first one is the fading phenomenon and the second is the angular spread (AS) of the incident wave. The computer simulation results with respect to fading and AS show that CECA works well as a spatial fading simulator for performance evaluation of an antenna array. We first present the basic structure, features and design methodology of CECA, and then show computer simulation results of the spatial stochastic characteristics. The results convince us that CECA is useful to evaluate performance of antenna arrays.

Random Early Detection (RED), an active queue management scheme, has been recommended by the Internet Engineering Task Force (IETF) for the next generation routers. RED suffers from a number of performance problems, such as low throughput, large delay/jitter, and induces instability in networks. Many of the previous attempts to improve the performance of RED have been based on optimizing the values of the RED parameters. However, results have shown that such optimizations resulted in limited improvement in the performance. In this paper, we propose Double Slope RED (DSRED), a new active queue management scheme to improve the performance of RED. The proposed scheme is based on dynamically changing the slope of the packet drop probability curve as a function of the level of congestion in the buffer. Results show that our proposed scheme results in better performance than original RED.

In this letter, new results on the BER performance of multitone DS-CDMA systems for transmissions over Nakagami-m fading channels with exponentially decaying multipath intensity profile are presented. The results show that, in viewpoint of the BER performance, there is a critical relation between the number of resolvable paths and the effect of the rate of average power decay.

We can model embedded systems as hybrid systems. Moreover, they are distributed and real-time systems. Therefore, it is important to specify and verify randomness and soft real-time properties. For the purpose of system verification, we formally define probabilistic linear hybrid automaton and its symbolic reachability analysis method. It can describe uncertainties and soft real-time characteristics.

In this letter, we propose a novel approach for use in the analytical modeling of the overall performance of a Hybrid ARQ (type I and II) together with arbitrary channel model, based on Hidden Markov Model (HMM). Using the combined HMM model developed for involved ARQ protocols with the finite state channel model, such critical performance measure as throughput and delay can be derived in closed form. Analytical results are derived for Stop-and-Wait as well as Go-back-N type together with the type I and type II Hybrid ARQ scheme adopted. We compare the analytical results along with the simulation results in order to check the correctness our model, and show the efficiency of our approach by applying it to realistic environments such as the CDMA IS-95 system with its derived equations.

Data accessed by many sites are replicated in distributed environments for performance and availability. In this paper, replication schemes are examined in parallel image convolution processing. This paper presents a system architecture that we have developed with CORBA (Common Object Request Broker Architecture) for the processing. Employing CORBA enables us to make use of a cluster of workstations, each of which has a different level of computing power. The paper also describes a parallel and distributed image convolution processing model using replicas stored in a network of workstations, and reports some experimental results showing that our analytical model can agree with practical situations.

We investigate the effects of the performance of sensor networks on network availability and in turn evaluate the impact of protocols and network configuration on these effects. The typical wireless sensor network of the future consists of a large number of micro-sized sensors that are equipped with batteries of limited capacity. In such a network, energy consumption is one of the most important issues. Several representative protocols that are applied in ring and linear network configurations are analyzed, and explicit formulae for network availability are derived for each of them. Numerical values derived by using these formulae yielded the surprising result that backup routes do not always improve network availability. This is because the loads imposed by the backup routes on network segments that do not include dead sensor nodes reduce sensor-node lifetimes in these segments.

Discrete-time queueing models have been studied for many years because of their direct applicability in the performance evaluation of digital communication system and networks, where buffers are used to temporarily store information packets which cannot be transmitted instantaneously. In this paper, we investigate the behavior of a discrete-time multiserver buffer system with infinite buffer size. Packets arrive in the system according to a two-state correlated arrival process. The service times of the packets are assumed to be independent and identically distributed according to a geometric distribution. We present an analytical technique, based on the use of generating functions, for the analysis of the system. Explicit expressions are obtained for the mean values, the variances and the tail distributions of the system contents and the packet delay. The influence of the various model parameters on the behavior of the system is shown by means of some numerical examples.

In this paper, we consider a location management scheme using Limited Pointer forwarding from Commonly visited sites (LPC) strategy for Personal Communication Services (PCS) networks. The Commonly Visited Site (CVS) is defined as a site in which a mobile user is found with high probability. A feature of the strategy is that it skips updating location information of the mobile user, provided that the mobile user moves within its CVSs. Such a strategy is expected to significantly reduce the location update cost. We evaluate the location management cost of the LPC scheme by employing a Continuous-Time Markov Chain (CTMC) model. We show that the LPC scheme can reduce the location management cost of a highly mobile user who is found in its CVS with high probability.

The multiplication of large spare matrices is a basic operation in many scientific and engineering applications. There exist some high-performance library routines for this operation. They are often optimized based on the target architecture. For a parallel environment, it is essential to partition the entire operation into well balanced tasks and assign them to individual processing elements. Most of the existing techniques partition the given matrices based on some kind of workload estimation. For irregular sparse matrices on PC clusters, however, the workloads may not be well estimated in advance. Any approach other than run-time dynamic partitioning may degrade performance. In this paper, we apply our super-programming approach to parallel large matrix multiplication on PC clusters. In our approach, tasks are partitioned into super-instructions that are dynamically assigned to member computer nodes. Thus, the load balancing logic is separated from the computing logic; the former is taken over by the runtime environment. Our super-programming approach facilitates ease of program development and targets high efficiency in dynamic load balancing. Workloads can be balanced effectively and the optimization overhead is small. The results prove the viability of our approach.

For an additive white Gaussian noise (AWGN) channel, a performance evaluation of parallel concatenated turbo trellis-coded modulation (turbo TCM) using bit-interleavers is reported. By obtaining weight distribution, the performance is evaluated by using a union bound method. Comparison between the result of evaluated performance and simulation results is shown, and the usefulness of the evaluated performance is shown. An optimum code and an optimum mapping are sought. The result of the optimum code with the optimum mapping is a new interleaver size N dependency which is proportional to N-3. It is better than the interleaver size dependency for Benedetto code with the natural mapping which is proportional to N-1. The reasons why these dependencies can happen are also discussed.

We study the user reneging behavior in a hybrid Multimedia-on-Demand (MoD) system, which provides both interactive and batch services. Reneging happens when users become reluctant to wait for a batch service and quit the service. They will either join the interactive service or quit the system. In this letter, the analytical model for the MoD system with user reneging behavior is provided. Numerical results show that the reneging behavior has a significant impact on system performance. Therefore, careful consideration on reneging behavior should be made for designing a MoD system.

Classical studies of Asynchronous Transfer Mode (ATM) switching elements and in particular the buffer behavior of the Shared Buffer Memory (SBM), assume that all read and write operations of cells to, respectively from, the SBM are executed simultaneously. However, in a real switching element, the inlets (outlets) are scanned sequentially for arriving (departing) cells during the so-called input (output) cycle. Furthermore, the input and output cycles are intermingled, each read operation being followed by a write operation. This is referred to as the Timeslot Interchange Mechanism (TIM). In this paper, we present the analysis of a queueing model that includes the TIM. We model the cell arrival processes on the inlets of the switching element as independent Bernoulli arrival processes. Moreover, we assume that cells are routed from the inlets to the outlets of the switching element according to an independent and uniform process, i.e., the destinations of consecutive cell arrivals on any given inlet are independent and for a given cell all destinations are equiprobable. Under these assumptions, we will derive expressions for the probability generating functions of the queue length in an individual routing group (a logical queue that contains all cells scheduled for the same destination), the (total) queue length in the SBM, and the cell waiting time. From these results, expressions for the mean values and the tail distributions of these quantities are calculated, and the influence of the TIM on the buffer behavior is studied through comparison with a model where all read and write operations occur simultaneously.

A number of on-line and off-line algorithms for load balancing on multiple paths for MPLS (MultiProtocol Label Switching) traffic engineering have now been proposed, in which it is always assumed that sets of LSPs (Label Switched Path) have already been established between node pairs. While how to choose these paths is an important issue in traffic engineering, it has not been well studied yet. In this paper, we attempt to fill in this gap. As the shortest paths are always preferred in routing problems, we evaluate several k shortest path algorithms from the viewpoint of bandwidth use efficiency and the number of the found paths. Extensive simulations have been performed in different kinds of topologies to factor out effects of network characteristics on these algorithms' path calculation performances. It is found out that the performances of the evaluated algorithms are limited in some cases and the design of new algorithms for the path calculation problem is worth studying in the future.

This paper discusses performance issues for a sensor network. It describes the unique features of the sensor network and discusses studies on its protocols. Performance measures for the sensor network are investigated and studies related to them are surveyed. As an example of performance measures, this paper analyzes a sensor network's availability, which is the probability that all the sensor nodes are working without any of them having run out of energy. An explicit formula for the sensor network availability is derived, and the optimal placement of sensor nodes is investigated.

To speed up on-line analytical processing (OLAP), data warehouse, which is usually derived from operational databases, is introduced. When the operational databases happen to change, the data warehouse gets stale. To maintain the freshness of data warehouse, operational database changes need to be frequently and concurrently propagated into the data warehouse. However, if several update transactions are allowed to execute concurrently without an appropriate concurrency control, data inconsistency between data warehouse and operational databases could arise due to incorrect propagation of changes on the operational databases into the data warehouse. In this paper, we propose a new concurrency control scheme, which could execute a number of update transactions in a consistent way. Whenever an update transaction tries to update a data that is being used by OLAP transactions, our scheme allows the update transaction to create a new version of the data. To investigate the applicable areas of our scheme, its performance is evaluated by means of simulation approach. Our experimental results show that the proposed scheme enables OLAP transactions to continuously read a very fresh data without wasting a lot of time to find out an appropriate version of the data from the version pool.

Analysis of concurrent systems, such as computer/communication networks and manufacturing systems, usually employs formal discrete event models. The analysis then includes model validation, property verification, and performance evaluation of such models. The DEVS (Discrete Event Systems Specification) formalism is a well-known formal modeling framework which supports specification of discrete event models in a hierarchical, modular manner. While validation and verification using formal models may not resort to discrete event simulation, accurate performance evaluation must employ discrete event simulation of formal models. Since formal models, such as DEVS models, explicitly represent communication semantics between component models, their simulation cost is much higher than using simulation languages with informal models. This paper proposes a method for simulation speedup in performance evaluation of concurrent systems using DEVS models. The method is viewed as a compiled simulation technique which eliminates run-time interpretation of communication paths between component models. The elimination has been done by a behavior-preserved transformation method, called model composition, which is based on the closed under coupling property in DEVS theory. Experimental results show that the simulation speed of transformed DEVS models is about 14 times faster than original ones.

Recently, layer-4 (L4) switches have been widely used as load balancing front-end routers for Web server clusters. The typical L4 switch attempts to balance load among the servers by estimating load using the load metrics measured in the front-end and/or the servers. However, insufficient load metrics, measurement overhead, and feedback delay often cause misestimate of server load. This may incur significant dynamic load imbalance among the servers particularly when the variation of requested content is high. In this paper, we propose a new content sniffer based load distribution strategy. By sniffing the requests being forwarded to the servers and by extracting load metrics from them, the L4 switch with our strategy more timely and accurately estimates server load without the help of back-end servers. Thus it can properly react to dynamic load imbalance among the servers under various workloads. Our experimental results demonstrate substantial performance improvements over other load balancing strategies used in the typical L4 switch.

This paper discusses formal modeling and performance evaluation for a type of dynamic change of workflow, called Migrate. Workflow means the flow of work and is designed to process similar instances, called cases. Companies need to continuously refine their current workflow in order to adapt them to various requirements. The change of a current workflow is called dynamic change of the workflow. Before changing a workflow, there exist cases in the workflow. If these cases are ignored or fall into deadlock, the changed workflow would become inconsistent. Since Ellis et al. proposed three change types, Flush, Abort, and Synthetic Cut-Over that keep consistency of workflows in 1995, various change types have been proposed, in which there is a promising change type called Migrate that is proposed by Sadiq et al. Sadiq et al. proposed the concept of Migrate, but did not give a formal model of Migrate. Meanwhile, we have proposed a measure, called change time, in order to evaluate dynamic change of workflows, and used this measure to evaluate the performance on change time for Ellis et al. 's three change types. However, the performance evaluation on change time for Migrate has not been done. In this paper, we first give a Petri-nets-based model of Migrate. Then we present a method of computing change time based on the net model. Finally, we apply the method to 270 examples and show the comparison results between Migrate and Ellis et al. 's three change types.