Béla Almási

HETEROGENEOUS FINITE-SOURCE RETRIAL QUEUES

In this paper we investigate a single server retrial queue with a finite number of heterogeneous sources of calls. It is assumed when a given source is idle it will generate a primary call after an exponentially distributed time. If the server is free at the time of the request’ s arrival then the call starts to be served. The service time is also exponentially distributed. During the service time the source cannot generate a new primary call. After service the source moves into free state and can generate a new call again. If the server is busy at the time of the arrival of a primary call, then the source starts generating so called repeated calls with exponentially distributed times until it finds the server free. As before, after service the source becomes free and can generate a new primary call again. We assume that the primary calls, repeated attempts and service times are mutually independent. This queueing system and its variants could be used to model magnetic disk memory systems, local area networks with CSMA/CD protocols and collision avoidance local area networks. The novelty of this model is the heterogeneity of the calls, which means that each call is characterized by its own arrival, repeated and service rates. The aim of the paper is to give the usual steady-state performance measures of the system. To do so, an efficient software tool, MOSEL ( Modeling, Specification and Evaluation Language ) developed at the University of Erlangen, Germany, is used to formulate and solve the problem. Several sample numerical results illustrate the power of the tool showing the effect of different parameters on the system measures.

Throughput performance analysis of the multipath communication library MPT

Although the currently used mobile communication equipments (laptops, tablets, phones) usually have many network interfaces, the Internet communication technology uses only a single communication path for a connection. In this paper we would like to introduce a new architecture, which gives an easy-to-use extension for the current TCP/IP protocol stack and offers the possibility of using multiple paths for the communication, without rewriting the applications. The new architecture was implemented in Linux by a software tool named MPT. A laboratory environment was established for the throughput performance evaluation of the MPT based multipath communication. The measurement results show that the throughput capacity of the single paths can be efficiently aggregated by MPT.

A solution for changing the communication interfaces between WiFi and 3G without packet loss

The Internet Protocol (both IPv4 and IPv6) technology uses the IP address of the network interface as the identification of the communication session. The consequence of this idea is that the breakdown of the interface will also stop the communication session, even when other network interface cards would be available for the communication. Many of the currently used end devices (laptops, tablets, mobile phones) have usually more than one network interface cards (e.g. RJ-45, WiFi, 3G), which could be used for redundancy purposes. In this paper we investigate a software tool (named MPT library) which offers a solution in the Network Layer for keeping the communication session in the case of interface breakdown. We performed measurements in different business network environments for changing the WiFi and 3G network interfaces inside a communication session without losing a packet. These results confirm that the MPT multipath communication library can be used efficiently as a solution for the mentioned situation.

A contribution to modeling sensor communication networks by using finite-source queueing systems

In this paper we introduce a finite source retrial queueing model to investigate the performance characteristics of the wireless transmission problem in sensor networks. We divide the sensors into two classes. The first one is the “Emergency” class, which performs the notification of special emergency situations (eg. fire alarms). The second one is the “Normal” class, which measures and transmits environmental data (eg. temperature). For the performance evaluation of the wireless transmission we study and compare two cases: In the first model the RF transmission possibility will be available randomly for the sensor nodes (Non Controlled case) and in the second model the RF transmission requests coming from the emergency class, will access the wireless channel immediately (Controlled case). Our main interest is to give the main steady-state performance measures of the system computed by the help of the MOSEL tool.

The effects of service disciplines on the performance of a nonreliable terminal system

In this paper, we consider a stochastic queueing model for the performance evaluaton of a real-life computer system consisting of n terminals connected with a CPU. A user at terminal i has thinking and processing time depending on the index i. Let us suppose that the operational system is subject to random breakdowns, which may be software or hardware ones, stopping the service both at the terminals and at the CPU. The failure-free operation times of the system and the restoration times are random variables. Busy terminals are also subject to random breakdowns not affecting the system operation. The failure-free operation times and the repair times of a busy terminal i are random variables with distribution function depending on index i. The breakdowns are serviced by a single repairman providing preemptive priority to the systems failure, while the restoration at the terminals are carried out according to the FIFO rule. We assume that each user generates only one job at a time, and he waits at the CPU before he starts thinking again, that is, the terminal is inactive while waiting at the CPU, and it cannot break down. All random variables involved in the model construction are assumed to be exponentially distributed and independent of each other. The aim of this paper is to investigate the effect of different service disciplines, such as FIFO, processor sharing, priority processor sharing, and polling, on the main performance measures, such as utilizations, response times, throughput, and mean queue length.

Comparing two queueing models for nonhomogeneous nonreliable terminal systems

This paper deals with two nonhomogeneous queueing models to describe the performance of finite, multiterminal systems subject to random breakdowns. The difference between the two models is relatively small, but we will see that the differences between some system performance characteristics are very large (100% or more). The other contribution of this paper is to introduce some new terminologies to queueing theory, which are necessary for the second model. All random variables involved here are independent and exponentially distributed. The models described here are generalizations of the homogeneous model treated earlier by different authors. At the end of this paper some numerical results illustrate the problem in question.

Performance Modeling of Finite-Source Cognitive Radio Networks

This paper deals with performance modeling aspects of radio frequencylicensing. The utilization of mobile cellular networks can be increasedby the idea of the cognitive radio. Licensed users Primary Users- PUs and normal users Secondary Users - SUs are considered. Themain idea is, that the SUs are able to access to the available non-licensedradio frequencies. A finite-source retrial queueing model with twonon independent frequency bands considered as service units isproposed for the performance evaluation of the system. A serviceunit with a priority queue and another service unit with an orbitare assigned to the PUs and SUs, respectively. The users are classifiedinto two classes: the PUs have got a licensed frequency, while theSUs have got a frequency band, too but it suffers from the overloading.We assume that during the service of the non-overloaded band thePUs have preemptive priority over SUs. The involved inter-event timesare supposed to be independent and exponentially distributed randomvariables. The novelty of this work lies in the fact that we considerthe effect of retrial phenomenon of SUs in performance modeling ofradio frequency licensing by using a finite-source queueing modelwhich takes the unreliability of radio transmission into accountfor the first time. In the literature, most work studied the performanceof cognitive radio networks under a mixed spectrum environment oflicensed and unlicensed bands where the blocked SUs and the preemptedSUs are forced to leave the system forever when there are no idlechannels in the system. But in practical situation, the blocked SUsand the preempted SUs may do not leave the system forever and tryto continue their services after random amount of time. By the helpof an appropriate continuous time Markov chain using MOSEL MOdelingSpecification and Evaluation Language tool several numerical examplesare provided showing the effects of different input parameters onthe main performance measures of the cognitive radio networks. Ourprimary focus is to determine an optimal number of SUs, where atthe secondary band the gained utilization, that is when switchingto the cognitive radio, has a maximum value.

Comparing the Communication Mechanisms of Health State Monitoring BGM Devices

Monitoring the human’s health state by continuous or high density parameter measurement is a widely used technology in the medical world. This idea is a standard treatment technology in the diabetes control: The “Self Monitoring of Blood Glucose” (SMBG) technology forces the patient to measure the blood glucose value many times in a day, and precisely log the results into the “diabetes diary” (see [1]). The currently used blood glucose meters (BGMs) are able to store and transmit the measurement data to a computer, so opening the ability of the automation of the monitoring. The dierent BGM devices usually use their own protocol in the data transmission process. In this paper we study the communication mechanisms of three popular BGM devices: The “Personal Optimum” (produced by the 77 Electronics Ltd; the most popular BGM device in Hungary); the “AccuCheck Active” (Roche’s BGM device, widely used in Europe); the “OneTouch Select mini” (produced by LifeScan, and it is a popular BGM device in the USA). We discuss the hardware requirements and introduce a software tool “WinDcont”, which can be used to automate the diabetes diary logging for the studied BGMs.

A simulation tool to evaluate the performance of finite-source queueing models

This paper deals with a software tool to evaluate the main characteristics of a nonhomogeneous finite-source queueing model to describe the performance of a multi-terminal system subject to random breakdowns under FIFO, priority processor sharing, and polling service disciplines. The model studied here is actually a closed queueing network network with three nonindependent service stations (CPU, terminals, and repairman), and a finite number of customers (jobs), which have different service rates at the service stations. The aim of this paper is to introduce the FQM (finite-source queueing model) program package, which was developed at the Institute of Mathematics and Informatics of Lajos Kossuth University in Debrecen, Hungary, and to investigate the performance of the above-mentioned finite-source queueing models. At the end we give a sample result to illustrate the problem in question.