11 research outputs found
Multi-threshold Control of the BMAP/SM/1/K Queue with Group Services
We consider a finite capacity queue in which arrivals occur according to a batch Markovian arrival process (BMAP). The customers are served in groups of varying sizes. The services are governed by a controlled semi-Markovian process according to a multithreshold strategy. We perform the steady-state analysis of this model by computing (a) the queue length distributions at departure and arbitrary epochs, (b) the Laplace-Stieltjes transform of the sojourn time distribution of an admitted customer, and (c) some selected system performance measures. An optimization problem of interest is presented and some numerical examples are illustrated
Wavelength converter sharing in asynchronous optical packet/burst switching: An exact blocking analysis for markovian arrivals
Cataloged from PDF version of article.In this paper, we study the blocking probabilities
in a wavelength division multiplexing-based asynchronous
bufferless optical packet/burst switch equipped with a bank of
tuneable wavelength converters dedicated to each output fiber
line. Wavelength converter sharing, also referred to as partial
wavelength conversion, corresponds to the case of a number
of converters shared amongst a larger number of wavelength
channels. In this study, we present a probabilistic framework for
exactly calculating the packet blocking probabilities for optical
packet/burst switching systems utilizing wavelength converter
sharing. In our model, packet arrivals at the optical switch are
first assumed to be Poisson and later generalized to the more
general Markovian arrival process to cope with very general
traffic patterns whereas packet lengths are assumed to be exponentially
distributed. As opposed to the existing literature based
on approximations and/or simulations, we formulate the problem
as one of finding the steady-state solution of a continuous-time
Markov chain with a block tridiagonal infinitesimal generator. To
find such solutions, we propose a numerically efficient and stable
algorithm based on block tridiagonal LU factorizations. We show
that exact blocking probabilities can be efficiently calculated
even for very large systems and rare blocking probabilities, e.g.,
systems with 256 wavelengths per fiber and blocking probabilities
in the order of 10â40. Relying on the stability and speed of the
proposed algorithm, we also provide a means of provisioning
wavelength channels and converters in optical packet/burst
switching systems
Some aspects of traffic control and performance evaluation of ATM networks
The emerging high-speed Asynchronous Transfer Mode (ATM) networks are expected to integrate through statistical multiplexing large numbers of traffic sources having a broad range of statistical characteristics and different Quality of Service (QOS) requirements. To achieve high utilisation of network resources while maintaining the QOS, efficient traffic management strategies have to be developed. This thesis considers the problem of traffic control for ATM networks. The thesis studies the application of neural networks to various ATM traffic control issues such as feedback congestion control, traffic characterization, bandwidth estimation, and Call Admission Control (CAC). A novel adaptive congestion control approach based on a neural network that uses reinforcement learning is developed. It is shown that the neural controller is very effective in providing general QOS control. A Finite Impulse Response (FIR) neural network is proposed to adaptively predict the traffic arrival process by learning the relationship between the past and future traffic variations. On the basis of this prediction, a feedback flow control scheme at input access nodes of the network is presented. Simulation results demonstrate significant performance improvement over conventional control mechanisms. In addition, an accurate yet computationally efficient approach to effective bandwidth estimation for multiplexed connections is investigated. In this method, a feed forward neural network is employed to model the nonlinear relationship between the effective bandwidth and the traffic situations and a QOS measure. Applications of this approach to admission control, bandwidth allocation and dynamic routing are also discussed. A detailed investigation has indicated that CAC schemes based on effective bandwidth approximation can be very conservative and prevent optimal use of network resources. A modified effective bandwidth CAC approach is therefore proposed to overcome the drawback of conventional methods. Considering statistical multiplexing between traffic sources, we directly calculate the effective bandwidth of the aggregate traffic which is modelled by a two-state Markov modulated Poisson process via matching four important statistics. We use the theory of large deviations to provide a unified description of effective bandwidths for various traffic sources and the associated ATM multiplexer queueing performance approximations, illustrating their strengths and limitations. In addition, a more accurate estimation method for ATM QOS parameters based on the Bahadur-Rao theorem is proposed, which is a refinement of the original effective bandwidth approximation and can lead to higher link utilisation
Design, Modeling, and Analysis for MAC Protocols in Ultra-wideband Networks
Ultra-wideband (UWB) is an appealing transmission technology for
short-range, bandwidth demanded wireless communications. With the
data rate of several hundred megabits per second, UWB demonstrates
great potential in supporting multimedia streams such as
high-definition television (HDTV), voice over Internet Protocol
(VoIP), and console gaming in office or home networks, known as the
wireless personal area network (WPAN). While vast research effort
has been made on the physical layer issues of UWB, the corresponding
medium access control (MAC) protocols that exploit UWB technology
have not been well developed.
Given an extremely wide bandwidth of UWB, a fundamental problem on
how to manage multiple users to efficiently utilize the bandwidth is
a MAC design issue. Without explicitly considering the physical
properties of UWB, existing MAC protocols are not optimized for
UWB-based networks. In addition, the limited processing capability
of UWB devices poses challenges to the design of low-complexity MAC
protocols. In this thesis, we comprehensively investigate the MAC
protocols for UWB networks. The objective is to link the physical
characteristics of UWB with the MAC protocols to fully exploit its
advantage. We consider two themes: centralized and distributed UWB
networks.
For centralized networks, the most critical issue surrounding the
MAC protocol is the resource allocation with fairness and quality of
service (QoS) provisioning. We address this issue by breaking down
into two scenarios: homogeneous and heterogeneous network
configurations. In the homogeneous case, users have the same
bandwidth requirement, and the objective of resource allocation is
to maximize the network throughput. In the heterogeneous case, users
have different bandwidth requirements, and the objective of resource
allocation is to provide differentiated services. For both design
objectives, the optimal scheduling problem is NP-hard. Our
contributions lie in the development of low-complexity scheduling
algorithms that fully exploit the characteristics of UWB.
For distributed networks, the MAC becomes node-based problems,
rather than link-based problems as in centralized networks. Each
node either contends for channel access or reserves transmission
opportunity through negotiation. We investigate two representative
protocols that have been adopted in the WiMedia specification for
future UWB-based WPANs. One is a contention-based protocol called
prioritized channel access (PCA), which employs the same mechanisms
as the enhanced distributed channel access (EDCA) in IEEE 802.11e
for providing differentiated services. The other is a
reservation-based protocol called distributed reservation protocol
(DRP), which allows time slots to be reserved in a distributed
manner. Our goal is to identify the capabilities of these two
protocols in supporting multimedia applications for UWB networks. To
achieve this, we develop analytical models and conduct detailed
analysis for respective protocols. The proposed analytical models
have several merits. They are accurate and provide close-form
expressions with low computational effort. Through a cross-layer
approach, our analytical models can capture the near-realistic
protocol behaviors, thus useful insights into the protocol can be
obtained to improve or fine-tune the protocol operations. The
proposed models can also be readily extended to incorporate more
sophisticated considerations, which should benefit future UWB
network design
Performance Evaluation of Stochastic Multi-Echelon Inventory Systems: A Survey
Globalization, product proliferation, and fast product innovation have significantly increased
the complexities of supply chains in many industries. One of the most important advancements
of supply chain management in recent years is the development of models and methodologies
for controlling inventory in general supply networks under uncertainty and their widefspread
applications to industry. These developments are based on three generic methods: the queueing-inventory method, the lead-time demand method and the flow-unit method. In this paper,
we compare and contrast these methods by discussing their strengths and weaknesses, their
differences and connections, and showing how to apply them systematically to characterize
and evaluate various supply networks with different supply processes, inventory policies, and
demand processes. Our objective is to forge links among research strands on different methods
and various network topologies so as to develop unified methodologies.Masdar Institute of Science and TechnologyNational Science Foundation (U.S.) (NSF Contract CMMI-0758069)National Science Foundation (U.S.) (Career Award CMMI-0747779)Bayer Business ServicesSAP A
Stochastig modeling with continuous feedback markov fluid queues
Cataloged from PDF version of article.Markov fluid queues (MFQ) are systems in which a continuous-time Markov chain
determines the net rate into (or out of ) a buffer. We deal with continuous feedback
MFQs (CFMFQ) for which the infinitesimal generator of the background process
and the drifts in each state are allowed to depend on the buffer level through continuous
functions. Explicit solutions of CFMFQs for a few special cases has been
reported, but usually numerical methods are preferred.
A numerically stable solution method based on ordered Schur decomposition
is already known for multi-regime MFQs (MRMFQ). We propose a framework for
approximating CFMFQs by MRMFQs via discretizing the buffer space. The parameters
of the CFMFQ are approximated by piecewise constant functions. Then, the
solution is obtained by block-tridiagonal LU decomposition for the related MRMFQ.
Moreover, we describe a numerical method that enables us to solve large scale systems
efficiently.
We model basically two different stochastic systems with CFMFQs. The first is
the workload-bounded MAP/PH/1 queue, to which the arrivals occur according to
a workload-dependent MAP (Markovian Arrival Process), and the arriving job size
distribution is phase-type. The jobs that would cause the buffer to overflow are rejected
partially or completely. Also, the service speed is allowed to depend on the
buffer level. As the second application, we model the horizon-based delayed reservation
mechanism in Optical Burst Switching networks with or without fiber delay
lines. We allow multiple traffic classes and the effect of offset-based and FDL-based
differentiation among traffic classes in terms of burst blocking is investigated.
Lastly, we propose a distributed algorithm for air-time fairness in multi-rate
WLANs that overcomes the performance anomaly in IEEE 802.11 WLANs. We also
give a stochastic model of the proposed model, and provide a novel and elaborate proof for its effectiveness. We also present an extensive simulation study.Yazıcı, Mehmet AkifPh.D
Allocation des ressources et des solutions pour économiser de l'énergie dans les réseaux optiques d'accÚs
In this thesis, general overview about PON systems is presented and existing PON mechanisms and classification are investigated. After, a novel dynamic bandwidth allocation algorithm for EPON is introduced. This proposed algorithm is named as âHalf Cycling Dynamic Bandwidth Allocation-hcDBAâ by the inspiration of its half cycling processing mode. Later, an improvement of hcDBA algorithm with early prediction mechanism is presented. As a result statement of the study, hcDBA algorithm performs better than existing mechanism in terms of packet loss ratio and access delays. Beside, simulation traffic behavior of EPONâs upstream channel has been investigated in order to support the decision of selecting suitable traffic generator in further studies. Energy conversation is one of the hot topics in telecommunication networks. Access networks constitute remarkable portion of the total energy consumption in telecommunication networks. ITU-T and IEEE organizations published recommendation for energy conversation in PONs. While, total energy consumption of ONUs is more than other equipment in fix access network the standards and most of the researches focused on saving energy at ONU side. In this thesis I focused on an energy efficiency method based on energy conversation on OLT side. The proposed method save energy by dynamically moving OLT cards to deep sleep mode according to the incoming and outgoing traffic loadsDans ce travail de thĂšse, un aperçu gĂ©nĂ©ral sur les systĂšmes PON est prĂ©sentĂ© et sont Ă©tudiĂ©s les mĂ©canismes et classification PON existants. AprĂšs, nous introduisons notre premiĂšre contribution qui est un algorithme d'allocation dynamique de bande passante pour EPON. Cet algorithme proposĂ© est dĂ©signĂ© comme «hcDBA». Par la suite, une amĂ©lioration de l'algorithme de hcDBA avec mĂ©canisme de prĂ©diction prĂ©coce est prĂ©sentĂ©e. Notre simulation montre bien que notre algorithme hcDBA est performant comparĂ© aux mĂ©canismes existants en termes de taux de perte de paquets et de dĂ©lai dâaccĂšs. Dans notre seconde contribution, nous sommes intĂ©ressĂ©s au problĂšme de consommation dâĂ©nergie qui est un sujet dâactualitĂ© dans les rĂ©seaux de tĂ©lĂ©communication. Les Ă©tudes montrent aujourdâhui que les rĂ©seaux d'accĂšs constituent une partie remarquable de la consommation totale d'Ă©nergie dans les rĂ©seaux de tĂ©lĂ©communication. Les organisations ITU-T et IEEE ont publiĂ© la recommandation pour la conversation de l'Ă©nergie pour les rĂ©seaux PONs. Bien que, la consommation totale d'Ă©nergie des nĆuds ONU est plus importantes que d'autres Ă©quipements dans le rĂ©seau d'accĂšs fixe, les normes et la plupart des travaux de recherches ont portĂ© sur les Ă©conomies d'Ă©nergie du cĂŽtĂ© de ONU. Dans cette thĂšse, nous sommes concentrĂ©s sur une mĂ©thode d'efficacitĂ© Ă©nergĂ©tique basĂ©e sur la conservation de l'Ă©nergie du cĂŽtĂ© de lâOLT. La mĂ©thode proposĂ©e permet dâĂ©conomiser de l'Ă©nergie en dĂ©plaçant dynamiquement des cartes dâOLT en mode de sommeil profond en fonction des charges de trafic entrant et sortan
A Finite Capacity Queue with Nonrenewal Input and Exponential Dynamic Group Services
In this article we consider a finite capacity queuing model in which jobs (or customers) arrive according to a nonrenewal process. The jobs are processed by a single server in groups of varying size, between a predetermined threshold value and the buffer size. A dynamic probability rule is associated with the service mechanism. The services are assumed to be exponential whose parameter may depend on the group size. The steady-state analysis of this queuing model is performed using Markov chain theory. It is shown that the idle period of the server and the stationary waiting time of an admitted job are of phase type and that the departure process can be modeled using a versatile Markovian point process. Efficient algorithms for computing various performance measures such as throughput, mean number served, job overflow probability, server idle probability, the stationary mean waiting time, and the stationary mean idle time of the server, useful in qualitative and quantitative interpretations are developed. Some illustrative numerical examples are discussed
Current Topics on Risk Analysis: ICRA6 and RISK2015 Conference
Peer ReviewedPostprint (published version
Current Topics on Risk Analysis: ICRA6 and RISK2015 Conference
ArtĂculos presentados en la International Conference on Risk Analysis ICRA 6/RISK
2015, celebrada en Barcelona del 26 al 29 de mayo de 2015.Peer ReviewedPostprint (published version