Stijn De Vuyst

Stijn De Vuyst
Ghent University | UGhent · Department of Industrial Systems Engineering and Product Design

assistant professor

About

78
Publications
3,967
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Introduction
Skills and Expertise

Publications

Publications (78)
Article
Full-text available
We analyze the asymptotics of waiting time distributions in the two-class accumulating priority queue with general service times. The accumulating priority queue was suggested by Kleinrock in the 60s—he coined it time-dependent priority—to diversify waiting time objectives of different classes in a paramaterized way. It also avoids the typical star...
Article
Full-text available
Appointment scheduling assigns start times in a session or consultation block to a set of tasks that share a common resource. For a generic repeated appointment scheduling problem, we study the trade-off between waiting for an appointment and waiting at the appointed time. Assuming that being scheduled later during a session implies that one has to...
Article
Anticipating the impact of urgent emergency arrivals on operating room schedules remains methodologically and computationally challenging. This paper investigates a model for surgery scheduling, in which both surgery durations and emergency patient arrivals are stochastic. When an emergency patient arrives he enters the first available room. Given...
Article
Full-text available
A vast amount of literature has appeared on vacation queues. In the well-known number- and time-limited vacation policies, the server goes on vacation if the number of customers, respectively, work (time slots) served since the previous vacation reaches a specified value, or if the system becomes empty, whichever occurs first. However, in practice,...
Article
This paper investigates the problem of surgery sequencing under uncertain surgery durations, with the objective of minimizing the expected maximum waiting time that emergent patients may endure before an operating room becomes available. Given the sets of surgeries assigned to each operating room as well as the distribution of each surgery’s durati...
Article
Full-text available
We consider the discrete-time G/GI/1 queueing system with multiple exhaustive vacations. By a transform approach, we obtain an expression for the probability generating function of the waiting time of customers in such a system. We then show that the results can be used to assess the performance of G/GI/1 queueing systems with server breakdowns as...
Article
We consider the problem of appointment scheduling in the presence of unpunctual patients. To address this problem, many studies combine simulation with some heuristic search method. Such optimization techniques usually involve a huge number of repeated schedule evaluations which makes the simulation run-time an important factor. In this paper, we p...
Article
Full-text available
Tactical capacity planning relies on future estimates of demand for the mid-to long-term. On these forecast horizons there is increased uncertainty that the analysts face. To this purpose, we incorporate macroeconomic variables into microeconomic demand forecasting. Forecast accuracy metrics, which are typically used to assess improvements in predi...
Article
We assess appointment scheduling strategies for outpatient services. To this end, we consider a fixed-length consultation session in which K patients have to be scheduled at predefined appointment times, but who may not be punctual. Their effective arrival time deviates from their appointment time by a stochastic unpunctuality time. We assume gener...
Article
We consider a system where two queues share one server. In case of conflict, the first (second) queue is served with probability β (1−β respectively). We prove strict monotonicity and continuity w.r.t. β of the mean unfinished work in queues 1 and 2. Restrictive assumptions are avoided as much as possible, by only assuming that the total unfinished...
Article
Full-text available
In this paper, we analyze a discrete-time queueing model with two types (classes) of customers and two servers, one for each customer class. Although each server can only process one type of customers, all customers are accommodated in one single queue and served in their order of arrival, irrespective of their types. The numbers of customers arriv...
Article
This paper considers a discrete-time single-server infinite-capacity queue with two classes of packet arrivals, either delay-sensitive (class 1) or delay-tolerant (class 2), and a reservation-based priority scheduling mechanism. The objective is to provide a better quality of service to delay-sensitive packets at the cost of allowing higher delays...
Article
This paper considers a logistics distribution network with multiple suppliers that each replenish a set of retailers having constant demand rates. The underlying optimization problem is the Cyclic Inventory Routing Problem (CIRP), for which a heuristic solution method is developed. Further, horizontal collaboration through a third party Logistics S...
Conference Paper
In this paper, we study the delay characteristics of a discrete-time queue with a reservation-based scheduling mechanism. The objective is to provide a better quality of service to delay-sensitive packets at the cost of allowing higher delays for the best-effort packets. We consider a single-server infinite-capacity queue with general independent p...
Article
Full-text available
Numerous scheduling policies are designed to differentiate quality of service for different applications. Service differentiation can in fact be formulated as a generalized resource allocation optimization towards the minimization of some important system characteristics. For complex scheduling policies, however, optimization can be a demanding tas...
Chapter
Buffer overflow in intermediate network routers is the prime cause of packet loss in wired communication networks. Packet loss is usually quantified by the packet loss ratio , the fraction of packets that are lost in a buffer. While this measure captures part of the loss performance of the buffer, we show that it is insufficient to quantify the eff...
Article
In this paper, we study a discrete-time single-server queueing system where the arriving traffic consists of both delay-sensitive and delay-tolerant streams. We analyze the effects of a reservation-based scheduling discipline on the delay characteristics of both types of traffic. Under this discipline, a placeholder packet is inserted in the queue...
Conference Paper
Today, telecommunication networks host a wide range of heterogeneous services. Some demand strict delay minima, while others only need a best-effort kind of service. To achieve service differentiation, network traffic is partitioned in several classes which is then transmitted according to a flexible and fair scheduling mechanism. Telecommunication...
Article
We study the robustness of performance predictions of discrete-time finite-capacity queues by applying the framework of imprecise probabilities. More concretely , we consider the Geo/Geo/1/L model with probabilities of arrival and departure that are no longer fixed, but are allowed to vary within given intervals. We distinguish between two concepts...
Article
We consider the problem of evaluating and constructing appointment schedules for patients in a health care facility where a single physician treats patients in a consecutive manner, as is common for general practitioners, clinics and for outpatients in hospitals. Specifically, given a fixed-length session during which a physician sees K patients, e...
Conference Paper
In this paper, we present a mechanism for scheduling multi-class traffic in a node of a packet-based communication network. We focus in particular on the output queues used for contention resolution at the output links of a network node. The mechanism, referred to as R-scheduling, is based on the use of in-queue reservations for future arriving dat...
Article
Full-text available
In this paper, we analyse the behaviour of a discrete-time singleserver queueing system with general service times, equipped with the NT-policy. This is a threshold policy designed to reduce the number of service unit activation/deactivation cycles, whilst ensuring an acceptable delay trade-off. Once the server is deactivated, reactivation will be...
Conference Paper
With Generalized Processor Sharing (GPS), packets of different applications are backlogged in different queues and the different queues are served according to predetermined weights. It is well-established that GPS is a viable approach to provide different QoS for different applications. However, since the analysis of systems with GPS is a notoriou...
Conference Paper
In this paper, we study a discrete-time buffer system with a correlated packet arrival process and one unreliable output line. In particular, the output line is governed by a Markovian process, such that the probability that the line is available during a slot depends on the state on the underlying J-state Markov process. Packets arrive to the buff...
Conference Paper
Full-text available
We assess appointment scheduling for outpatients in a hospital. A physician sees K patients during a fixed-length session. Each patient has been given an appointment time during the session in advance. Our evaluation approach aims at obtaining accurate predictions at a very low computational cost for the waiting times of the patients and the idle t...
Article
There is a considerable interest nowadays in improving energy efficiency of wireless telecommunications. The sleep-mode mechanism in WiMAX (IEEE 802.16) and the discontinuous reception (DRX) mechanism of LTE are prime examples of energy saving measures. Recently, Samsung proposed some modifications on the sleep-mode mechanism, scheduled to appear i...
Conference Paper
The first phase in the operation of a Cognitive Radio Network (CRN) is the investigation of the availability of idle channels through channel sensing. In this paper we study the impact of the sensing rate of Secondary Users (SUs) in a CRN. We develop a continuous-time Markov chain (CTMC) to study the effect of the sensing rate σ on the performance...
Article
In this paper we study the effect of session-based arrival streams on the behavior of a discrete-time queueing system with infinite storage capacity and a single output line. These session-based arrival streams are a new and realistic approach for modelling traffic generated by users in a telecommunication network. Users from an infinite population...
Conference Paper
In this paper, a multi-channel cognitive radio network is considered in which both primary users (PUs) and secondary users (SUs) can sense and make use of idle channels, i.e. both kinds of users are cognitive. We study a situation where a PU, while transmitting, can sense idle channels with a better transmission rate than the one currently occupied...
Conference Paper
In this paper, we analyze the behaviour of a discrete-time single-server queueing system with general service times, equipped with the NT-policy. This is a threshold policy designed to reduce the number of service unit activation/deactivation cycles, whilst ensuring an acceptable delay trade-off. Once the server is deactivated, reactivation will be...
Article
Short battery life being a major impediment to a more widespread use of wireless devices, industry standards like WiMAX and 3GPP Long Term Evolution (LTE) embed power saving mechanisms in their protocol stack. Trading in network delay performance for extended battery life, these mechanisms require a careful balance to optimize overall network perfo...
Conference Paper
We consider the problem of evaluating an appointment schedule for outpatients in a hospital. Given a fixed-length session during which a physician sees K patients, each patient has to be given an appointment time during this session in advance. When a patient arrives on its appointment, the consultations of the previous patients are either already...
Conference Paper
Full-text available
In this paper we consider a cognitive radio network in a heterogeneous multi-channel scenario where channels are different in terms of achievable bit rates. Upon arrival licensed users (PUs) will occupy first the channels with higher achievable rates. As a result, secondary users (SUs) will see a set of channels which are different not only in thei...
Conference Paper
Full-text available
In wireless telecommunication networks, there is a strong interest in energy efficiency. Sleep-mode mechanisms, which temporarily switch off mobile devices or base stations, are popular means to attain this goal. We look at the performance of different sleep-mode mechanisms with large-deviations tools. These allow us to look at a general multi-user...
Chapter
In order to differentiate the perceived QoS between traffic classes in heterogeneous packet networks, equipment discriminates incoming packets based on their class, particularly in the way queued packets are scheduled for further transmission. We review a common stochastic modelling framework in which scheduling mechanisms can be evaluated, especia...
Article
This paper studies a single-server non-preemptive priority queue with two traffic classes in order to model Expedited Forwarding Per-Hop Be-havior in the Differentiated Services (DiffServ) architecture. Generally, queue-ing models assume infinite queue capacity but in a DiffServ router the capacity for high priority traffic is typically small to pr...
Conference Paper
Full-text available
In this paper, we analyze the delay incurred by session-based traffic in the output buffer of a file server. Users can start and end sessions during which they are active and download information from the file server. Per time slot, each active user downloads a random but strictly positive number of information packets. Each session lasts for a ran...
Article
The paper considers a discrete-time buffer system with an infinite storage capacity and one single output channel. Users can start and end sessions during which they are active and send packets to the buffer system. In this paper we study a simple model for the resulting session-based arrival process: we assume that each active user generates a ran...
Article
We propose a discrete-time queueing model for the evaluation of the IEEE 802.16e sleep-mode mechanism of Power Saving Class (PSC) I in wireless access networks. Contrary to previous studies, we model the downlink traffic by means of a Discrete Batch Markov Arrival Process (D-BMAP) with N phases, which allows to take traffic correlation into accoun...
Conference Paper
There is a considerable interest nowadays in making wireless telecommunication more energy-efficient. The sleep-mode mechanism in WiMAX (IEEE 802.16e) is one of such energy saving measures. Recently, Samsung proposed some modifications on the sleep-mode mechanism, scheduled to appear in the forthcoming IEEE 802.16m standard, aimed at minimizing the...
Conference Paper
Session-based arrival streams are a new approach for modelling the traffic generated by users in a telecommunication network. In this paper, we analyze the behavior of a discrete-time buffer with one output line, an infinite storage capacity and session-based arrivals. Users from an infinite user population can start and end sessions during which t...
Conference Paper
This paper studies a single-server queue with two traffic classes in order to model Expedited Forwarding Per-Hop Behaviour in the Differentiated Services architecture. Generally, queueing models assume infinite queue capacity but in a DiffServ router the capacity for high priority traffic is often small to prevent this traffic from monopolizing the...
Article
Full-text available
We refine existing performance studies of the WiMAX sleep mode operation to take into account uplink as well as downlink traffic. This as opposed to previous studies which neglected the influence of uplink traffic. We obtain numerically efficient procedures to compute both delay and energy efficiency characteristics. A test scenario with an Individ...
Article
In this contribution, we investigate the performance of the output buer of an 'on- demand' video streaming server. The server maintains a local database of stored video clips and movies which can be streamed to the users upon request. We assume that the stored video is encoded in a scalable way, which means that the data streams contain a base laye...
Article
In this paper, we study the behavior of the transmitter buffer of a system working under a Stop-and-Wait retransmission protocol. The buffer at the transmitter side is modeled as a discrete-time infinite-capacity queue. The numbers of information packets entering the buffer during consecutive slots are assumed to be independent and identically dist...
Article
Full-text available
This paper studies a single-server non-preemptive priority queue with two traffic classes in order to model Expedited Forwarding Per-Hop Behaviour in the Differentiated Services (DiffServ) architecture. Generally, queueing models assume infinite queue capacity but in a DiffServ router the capacity for high priority traffic is typically small to pre...
Article
In this paper, we evaluate the performance of the IEEE 802.16e sleep mode mechanism in wireless access networks. This mechanism reduces the energy consumption of a mobile station (MS) by allowing it to turn off its radio interface (sleep mode) when there is no traffic present at its serving base station (BS). After a sleep period expires, the MS br...
Article
We study the delay performance of a queue with a place reservation mechanism. The objective of this discipline is to provide a better quality of service to arriving packets that are delay sensitive at the cost of allowing higher delays for the best effort packets and was first proposed by Burakowski and Tarasiuk. In our model, we consider a discret...
Conference Paper
Full-text available
In this paper, we consider a discrete-time single-server queueing system with geometric service times and two classes of packet arrivals. Packets of class 1 have higher priority and are assumed to have more stringent delay requirements than packets of class 2. The distinctive feature of our model is a reservation-based scheduling discipline that pr...
Conference Paper
In this paper, we consider a discrete-time queueing model with infinite waiting room and one single output channel. Users from an infinite user population can start and end sessions during which they are active and send packets to a buffer system. Each active user generates a random but strictly positive number of packets per time slot. The queuein...
Article
We analyse a discrete-time queueing model with packet arrivals that are either delay-sensitive (type 1) or delay-tolerant (type 2). The prominent feature of this model is its reservation-based queueing discipline, which reduces the queueing delay perceived by the 1-packets at the cost of allowing higher delays for the 2-packets. A total of N reserv...
Conference Paper
Full-text available
This paper considers a discrete-time queueing model with infinite storage capacity and one single output line. Users can start and end sessions during which they are active and send packets to the queueing system. Each active user generates a random but strictly positive number of packets per time slot: this results in a session-based arrival proce...
Conference Paper
Packet loss in wired communication networks is almost entirely caused by buffer overflow in intermediate network nodes. Although the packet loss ratio at a (bottleneck) buffer is an important performance measure, it is often an insufficient measure to accurately assess the impact of loss on overall network performance. This contribution therefore f...
Article
A queueing model for the buffer between a Web server and the Internet is presented. The generated traffic is modelled as a superposition of sessions started by independent users, where each session corresponds to one file download. This model leads to analytical expressions for the main performance measures (buffer occupancy, delay) of the buffer a...
Conference Paper
We propose and analyse a discrete-time queueing model for the evaluation of the IEEE 802.16e sleep mode operation in wireless access networks. This mechanism reduces the energy consumption of a mobile station (MS) by allowing it to turn off its radio interface (sleep mode) when there is no traffic present at its serving base station (BS). After a s...
Conference Paper
We analyse a discrete-time queueing model with packet arrivals that are either classified as delay-sensitive (type 1) or delay-tolerant (type 2). The queue has a single server and each packet requires a service time of exactly one slot. The prominent feature of this model is its reservation-based queueing discipline, which has the purpose of reduci...
Article
In this contribution we investigate higher-order loss characteristics for M/G/1/N queueing systems. We focus on the lengths of the loss and non-loss periods as well as on the number of arrivals during these periods. For the analysis, we extend the Markovian state of the queueing system with the time and number of admitted arrivals since the instant...
Article
Full-text available
The delay of both high and low priority packets in a discrete-time queue operating under a reservation discipline is studied. The delay distribution is obtained from which closed-form expressions for the mean, variance and the tail distribution are derived. In an example, performance is compared with that of Absolute Priority and FIFO.
Conference Paper
In this contribution, we investigate the performance of the output buer of a video streaming server. We assume that the server en- codes the video stream in a scalable way, i.e. the generated video packets contain a base layer ensuring a minimum of guaranteed quality and a stack of additional enhancement layers progressively improving the qual- ity...
Article
Full-text available
In this paper, we present the analysis of the Stop-and-Wait ARQ (Automatic Repeat reQuest) protocol with the notable complication that the transmission errors occur in a bursty, correlated manner. Fixed-length packets of data are sent from transmitter to receiver over an error-prone channel. The receiver notifies the transmitter whether a packet wa...
Article
We consider a discrete-time gated vacation system. The available buffer space is divided into two subsequent queues separated by a gate and new customers arrive either before or after this gate. Whenever all customers after the gate are served, the server takes a vacation. After each vacation, the gate opens which causes all waiting customers to mo...
Article
In this paper, we study a statistical multiplexer which is modelled as a discrete-time single-server infinite-capacity queueing system. This multiplexer is fed by messages generated by an unbounded population of users. Each message consists of a generally distributed number of fixed-length packets.We assume the packet arrival process to exhibit sim...
Article
Full-text available
We consider a statistical multiplexer which is modeled as a discrete-time single-server queueing system. Messages consisting of a variable number of fixed-length packets arrive to the muliplexer at the rate of one packet per slot (‘train arrivals’), which results in what we call a primary correlation in the packet arrival process. The distribution...
Conference Paper
Full-text available
In this paper, we analyse a discrete-time single-server queue operating under the NT-policy, which aims at clustering the service of customers in order to reduce the number of server activations and deactivations. Starting from an empty queue, the service of arriving customers is postponed until either of two thresholds is reached. Specifically, ex...
Article
In this paper, we present the analysis of the Stop-and-Wait ARQ (Automatic Repeat reQuest) protocol with the notable compli- cation that the transmission errors occur in a bursty, correlated manner. Fixed-length packets of data are sent from transmitter to receiver over an error-prone channel. The receiver notifies the transmitter whether a packet...
Article
Full-text available
We study a statistical multiplexer that is mod- elled as a discrete-time single-server queueing system with innite capacity. At the input of the multiplexer, there is a possibly unbounded population of users that generate 'low-speed' messages. Each such message is composed of a variable number of xed-length packets which consecutively enter the que...
Article
Full-text available
We consider a statistical multiplexer which is modeled as a discrete-time single-server queueing system. Messages consisting of a variable number of fixed-length packets arrive to the multiplexer at the rate of one packet per slot ("train arrivals"), which results in what we call a primary corre-lation in the packet arrival process. The distributio...
Article
Full-text available
In this paper, we study a statistical multiplexer which is modelled as a discrete-time single-server infinite-capacity queueing system. This multiplexer is fed by a user population that generates messages consisting of a generally distributed number of fixed-length packets. We assume the packet arrival process to exhibit the following two types of...
Article
Full-text available
We present an simulation experiment that can be used to asses the impact of using traffic prediction for online control on the performance of communication networks. A server with fixed output link bandwidth and impaired with high priority traffic is considered. The server shares its remaining available rate over K contending low-priority input str...
Article
Full-text available
The goal of cognitive radio networking technologies is to recuperate the unused bandwidth in existing wireless telecommunication infrastructures. We consider a single channel on which licensed users (primary users, PU) can freely transmit packets, while the 'spectrum gaps' in between such transmissions can be used by packets from unlicensed users (...

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