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2014  
Journal articles:  

A New Finitesource Queueing Model for Mobile Cellular Networks Applying Spectrum Renting
Asia Pacific Journal of Operational Research (APJOR),
31(2):1440004:119
2014
Keywords: Mobile cellular networks; Retrial queues; Finitesource; Impatience; Spectrum renting; Discount factor Abstract: This paper proposes a retrial queueing model with the finite number of sources to evaluate the performance of spectrum renting in mobile cellular networks. The model incorporates necessary ingredients such as the finite number of subscribers, their impatience and a queue for the outbound service. To consider the specific feature of spectrum renting and the current mobile cellular technology, a variable number of servers that are switched on and off in groups is introduced. We present a novel way to take into account the renting fee, which can be used to finetune the operation of the spectrum renting procedure. Numerical results show that it is still profitable to initiate a spectrum renting request at high loads, even if no discount is offered by the frequency bands? owners. Note: (c) World Scientific Publishing Co. & Operational Research Society of Singapore 

2013  
PhD theses:  
EnergyEfficient and Timely Event Reporting Using Wireless Sensor Networks
University of Passau,
2013
Keywords: Wireless sensor network; Energy efficiency; Communication protocol; Mathematical modeling; Continuoustime Markov chain; Finitesource retrial queue Abstract: This thesis investigates the suitability of stateoftheart protocols for largescale and longterm environmental event monitoring using wireless sensor networks based on the application scenario of early forest fire detection. By suitable combination of energyefficient protocol mechanisms a novel communication protocol, referred to as crosslayer messagemerging protocol (XLMMP), is developed. Qualitative and quantitative protocol analyses are carried out to confirm that XLMMP is particularly suitable for this application area. The quantitative analysis is mainly based on finitesource retrial queues with multiple unreliable servers. While this queueing model is widely applicable in various research areas even beyond communication networks, this thesis is the first to determine the distribution of the response time in this model. The model evaluation is mainly carried out using Markovian analysis and the method of phases. The obtained quantitative results show that XLMMP is a feasible basis to design scalable wireless sensor networks that (1) may comprise hundreds of thousands of tiny sensor nodes with reduced node complexity, (2) are suitable to monitor an area of tens of square kilometers, (3) achieve a lifetime of several years. The deduced quantifiable relationships between key network parameterse.g., node size, node density, size of the monitored area, aspired lifetime, and the maximum endtoend communication delayenable applicationspecific optimization of the protocol. 

2012  
Conference and workshop papers:  
Combined Contention and TDMABased Communication in Wireless Sensor Networks
Proc. of the 8th EuroNF Conf. on Next Generation Internet (NGI2012)
Publisher: IEEE,
2012
Keywords: Wireless sensor networks; Clustering; Contention; TDMA; Energy; Delay; Queueing theory; Discretetime Markov model Abstract: Wireless sensor networks usually consist of a large number of very small, energyconstrained sensor nodes. The nodes capture information from their immediate environment to send it to a destination node (sink) in a timely manner. This work proposes a combined contention and TDMAbased approach for load balancing and a discretetime Markov model that allows to study the tradeoff between energy consumption and transfer delay in clustered wireless sensor networks. Our investigations show that the length of the TDMA frame needs to be configured close to the minimum that is capable of transferring the offered load for optimizing energy efficiency and minimizing the delay. 

2011  
Journal articles:  
TargetOriented SelfStructuring in Classifying Cellular Automata
Journal of Cellular Automata (JCA),
6(1):323
2011
ISSN: 15575969 (Print); 15575977 (Online)
Keywords: Onedimensional cellular automata; Classification problem; Emergent computation; Formal measures Abstract: Some cellular automata are able to solve classification problems on their initial configuration by building globally visible structures. However, no formal measures exist yet for describing or detecting this behavior in general. The lack of such formal methods often leads to quite observerdependent discussions of emergent computation. In this paper, we propose the measures of target orientation and selfstructuring that allow to formally evaluate a cellular automaton?s ability to solve a classification problem by emergent computation. By the means of these measures, globally emerging patterns can be recognized and their contribution to the solution of the classification problem can be judged in an observerindependent way. 

Conference and workshop papers:  
Modeling Wireless Sensor Networks Using FiniteSource Retrial Queues with Unreliable Orbit
Proc. of the Workshop on Performance Evaluation of Computer and Communication Systems (PERFORM 2010)
Volume 6821
of Lecture Notes in Computer Science (LNCS)
, page 7386.
Publisher: SpringerVerlag,
2011
Keywords: Performance evaluation; Unreliable finitesource retrial queue; Wireless sensor network; Energy efficiency; Selforganization Abstract: Motivated by the need for performance models suitable for modeling and evaluation of wireless sensor networks, we introduce a retrial queueing system with a finite number of homogeneous sources, unreliable servers, orbital search, and unreliable orbit. All random variables involved in model construction are assumed to be independent and exponentially distributed. Providing a generalized stochastic Petri net model of the system, steadystate analysis of the underlying continuoustime Markov chain is performed and steadystate performance measures are computed by the help of the MOSEL2 tool. The main novelty of this investigation is the introduction of an unreliable orbit and its application to wireless sensor networks. Numerical examples are derived to show the influence of sleep/awake time ratio, message dropping, and message blocking on the senor nodes' performance. Note: The original publication is available at www.springerlink.com 

2010  
Journal articles:  
Modeling of SelfOrganizing Systems: An Overview
Electronic Communications of the EASST,
27:112
2010
ISSN: 18632122
Keywords: SelfOrganization; Modeling; Systems Abstract: This paper gives a systematic overview on modeling formalisms suitable for modeling selforganizing systems. We distinguish between microlevel modeling and macrolevel modeling. On the micro level, the behavior of each entity and the interaction between different object must be described by the model. Macrolevel modeling abstracts from the individual entities and only looks at the behavior of the system variables of interest. The differentiations between discrete and continuous time and between discrete and continuous state space lead to different descriptions of the model. 

Conference and workshop papers:  
AdHoc information spread between mobile devices: a case study in analytical modeling of controlled selforganization in IT systems
Proc. of 23rd Int'l Conf. on Architecture of Computing Systems (ARCS 2010)
Volume 5974
of Lecture Notes in Computer Science (LNCS)
, page 101112.
Publisher: SpringerVerlag,
2010
ISBN: 9783642119491
Abstract: We present an example of the use of analytical models to predict global properties of largescale information technology systems from the parameters of simple local interactions. The example is intended as a first step towards using complex systems modeling methods to control selforganization in organic systems. It is motivated by a concrete application scenario of information distribution in emergency situations, but is relevant to other domains such as malware spread or social interactions. Specifically, we show how the spread of information through adhoc interactions between mobile devices depends on simple local interaction rules and parameters such as user mobility and physical interaction range. We show how three qualitatively different regimes of information 'infection rate' can be analytically derived and validate our model in extensive simulations. Note: The original publication is available at www.springerlink.com (2010) 

FiniteSource Retrial Queues with Applications
Proc. of the 8th Int'l Conf. on Applied Informatics
Volume 2
, page 275285.
2010
Keywords: Retrial queueing systems; Finite number of sources; Orbital search; Performance tool; Performance measures; Hypoexponential distribution; Generalized exponential distribution Abstract: We consider a singleserver retrial queueing system with a finite number of homogeneous sources generating requests after a random time. The service of an arriving customer finding the server idle starts immediately; otherwise the request enters into an orbit from where the calls are repeated after a random time. Upon completion of a service, with a certain probability the server searches for an orbiting customer. We assume the search time to be negligible. While modeling generation, retrial, and service times by their first two moments, we are interested in steadystate performance measures. The novelty of the investigation is the discussion of the second moment's effect on the performance of finitesource retrial queues with orbital search. The MOSEL2 tool is used to formulate and solve the problem resulting in various illustrative numerical examples showing the effect of the moments and the search probability on the mean response time of the requests. 

In Memory of Dr. Gunter Bolch
Proc. of the 15th Int'l GI/ITG Conf. on Measurement, Modelling and Evaluation of Computing Systems and Dependability and Fault Tolerance (MMB & DFT 2010)
Volume 5987
of Lecture Notes in Computer Science (LNCS)
, page 37.
Publisher: SpringerVerlag,
2010
ISBN: 9783642121036
Abstract: The MMB Special Interest Group mourns for a highly valued member. Gunter Bolch died on May 29, 2008 in Erlangen, after a serious illness. He was very open minded, a beautifully moderate person, and a friend in its truest meaning. With MMB, he shared his academic passion of performance modelling, in general, and queueing networks, in particular. Note: The original publication is available at www.springerlink.com (2010) 

2009  
Journal articles:  
Finitesource M/M/S retrial queue with search for balking and impatient customers from the orbit
Computer Networks (ComNet),
53(8):12641273
2009
Keywords: Performance modeling; Finitesource retrial queues; Orbital search; Balking customers; Impatient customers; MOSEL2 Abstract: The present paper deals with a generalization of the homogeneous multiserver finitesource retrial queue with search for customers in the orbit. The novelty of the investigation is the introduction of balking and impatience for requests who arrive at the service facility with a limited capacity and FIFO queue. Arriving customers may balk, i.e., they either join the queue or go to the orbit. Moreover, the requests are impatient and abandon the buffer after a random time and enter the orbit, too. In case of an empty buffer, each server searches for a customer in the orbit after finishing service. All random variables involved in the model construction are supposed to be exponentially distributed and independent of each other. The primary aim of this analysis is to show the effect of balking, impatience, and buffer size on the steadystate performance measures. Concentrating on the mean response time, several numerical examples are investigated by the help of the MOSEL2 tool used for creating the model and calculating the stationary characteristics. Note: Performance of Computer Networks: Special Issue in Memory of Dr. Gunter Bolch. 