Analysis of the finite-source multiclass priority queue with an unreliable server and setup time

In this article, we study a queueing system serving multiple classes of customers. Each class has a finite-calling population. The customers are served according to the preemptive-resume priority policy. We assume general distributions for the service times. For each priority class, we derive the steady-state system size distributions at departure/arrival and arbitrary time epochs. We introduce the residual augmented process completion times conditioned on the number of customers in the system to obtain the system time distribution. We then extend the model by assuming that the server is subject to operation-independent failures upon which a repair process with random duration starts immediately. We also demonstrate how setup times, which may be required before resuming interrupted service or picking up a new customer, can be incorporated in the model

Strategies for a centralized single product multiclass M/G/1 make-to-stock queue

Make-to-stock queues are typically investigated in the M/M/1 settings. For centralized single-item systems with backlogs, the multilevel rationing (MR) policy is established as optimal and the strict priority (SP) policy is a practical compromise, balancing cost and ease of implementation. However, the optimal policy is unknown when service time is general, i.e., for M/G/1 queues. Dynamic programming, the tool commonly used to investigate the MR policy in make-to-stock queues, is less practical when service time is general. In this paper we focus on customer composition: the proportion of customers of each class to the total number of customers in the queue. We do so because the number of customers in M/G/1 queues is invariant for any nonidling and nonanticipating policy. To characterize customer composition, we consider a series of two-priority M/G/1 queues where the first service time in each busy period is different from standard service times, i.e., this first service time is exceptional. We characterize the required exceptional first service times and the exact solution of such queues. From our results, we derive the optimal cost and control for the MR and SP policies for M/G/1 make-to-stock queues

A first passage time problem for spectrally positive Lévy processes and its application to a dynamic priority queue

We study a first passage time problem for a class of spectrally positive Lévy processes. By considering the special case where the Lévy process is a compound Poisson process with negative drift, we obtain the Laplace–Stieltjes transform of the steady-state waiting time distribution of low-priority customers in a two-class M/GI/1 queue operating under a dynamic non-preemptive priority discipline. This allows us to observe how the waiting time of customers is affected as the policy parameter varies

The dynamic nearest neighbor policy for the multi-vehicle pick-up and delivery problem

In this paper, a dynamic nearest neighbor (DNN) policy is proposed for operating a fleet of vehicles to serve customers, who place calls in a Euclidean service area according to a Poisson process. Each vehicle serves one customer at a time, who has a distinct origin and destination independently and uniformly distributed within the service area. The new DNN policy is a refined version of the nearest neighbor (NN) policy that is well known to perform sub-optimally when the frequency of customer requests is high. The DNN policy maintains geographically closest customer-to-vehicle assignments, due to its ability to divert/re-assign vehicles that may be already en-route to pick up other customers, when another vehicle becomes available or a new customer call arrives. Two other pertinent issues addressed include: the pro-active deployment of the vehicles by anticipating in which regions of the service area future calls are more likely to arise; and, imposition of limits to avoid prohibitively long customer wait times. The paper also presents accurate approximations for all the policies compared. Extensive simulations, some of which are included herein, clearly show the DNN policy to be tangibly superior to the first-comefirst-served (FCFS) and NN policies

Coping with production time variability via dynamic lead-time quotation

In this paper, we propose two dynamic lead-time quotation policies in an M/GI/1 type make-to-stock queueing system serving lead-time sensitive customers with a single type of product. Incorporating non-exponential service times in an exact method for make-to-stock queues is usually deemed difficult. Our analysis of the proposed policies is exact and requires the numerical inversion of the Laplace transform of the sojourn time of an order to be placed. The first policy assures that the long-run probability of delivering the product within the quoted lead-time is the same for all backlogged customers. The second policy is a refinement of the first which improves the profitability if customers are oversensitive to even short delays in delivery. Numerical results show that both policies perform close to the optimal policy that was characterized only for exponential service times. The new insight gained is that the worsening impact of the production time variability, which is felt significantly in systems accepting all customers by quoting zero lead times, decreases when dynamic lead-time quotation policies are employed

The impact of disruption characteristics on the performance of a server

In this paper, we study a queueing system serving N customers with an unreliable server subject to disruptions even when idle. Times between server interruptions, service times, and times between customer arrivals are assumed to follow exponential distributions. The main contribution of the paper is to use general distributions for the length of server interruption periods/down times. Our numerical analysis reveals the importance of incorporating the down time distribution into the model, since their impact on customer service levels could be counterintuitive. For instance, while higher down time variability increases the mean queue length, for other service levels, can prove to be improving system performance. We also show how the process completion time approach from the literature can be extended to analyze the queueing system if the unreliable server fails only when it is serving a customer

Scheduling policies for a repair shop problem

In this paper, we analyze a repair shop serving several fleets of machines that fail from time to time. To reduce downtime costs, a continuous-review spare machine inventory is kept for each fleet. A spare machine, if available on stock, is installed instantaneously in place of a broken machine. When a repaired machine is returned from the repair shop, it is placed in inventory for future use if the fleet has the required number of machines operating. Since the repair shop is shared by different fleets, choosing which type of broken machine to repair is crucial to minimize downtime and holding costs. The optimal policy of this problem is difficult to characterize, and, therefore, is only formulated as a Markov Decision Process to numerically compute the optimal cost and base-stock level for each spare machine inventory. As an alternative, we propose the dynamic Myopic(R) policy, which is easy to implement, yielding costs very close to the optimal. Most of the time it outperforms the static first-come-first-served, and preemptive-resume priority policies. Additionally, via our numerical study, we demonstrate that repair shop pooling is better than reserving a repair shop for each fleet

Edeb'in tercümanı, 'ilm'in gücü: meşâ‘irü’ş-şu‘arâ mukaddimesinde şiirin meşruiyeti

Cataloged from PDF version of article.Thesis (M.S.): Bilkent University, Department of Turkish Literature, İhsan Doğramacı Bilkent University, 2016.Includes bibliographical references (leaves 78-83).Focusing on the introduction (mukaddime) of Meşâirü'ş-Şu'arâ, a biographical dictionary written by Aşık Çelebi, this thesis deals with the text‘s description and discussion of the legitimacy of poetry with reference to the conception of adab. Firstly, in order to interpret the theoretical framework given for poetry in the introduction, I summarized the historical contexts and the usage areas of adab. Among these contexts, I determined that the introduction puts forward the understanding of adab as a set of literary skills needed for religious scholarship, which was prevalent in 'the post-Sunni revival'. Interestingly, Âşık‘s introduction to his tezkire, whose form and content were determined according to this specific meaning of adab, differs from its Iranian and Ottoman predecessors in that it often refers to the 'ulema, recounting chronologically from the eponymous founders of Hanafi and Shafii schools to Ottoman Shaykh al-Islams. Additionally, I propose that by defining the role of poetry as an auxiliary tool to comprehend Quran, the introduction reflects a scholar-centered vision. Related to this, I try to interpret the relationship between poetry and Quran through the Quranic studies, which starting with al-Zamakhshari take rhetorics as a basis for approaching the holy text. By doing so, I aim to show how the key rhetorical concepts fesahat (clarity of expression) and belagat (eloquence) were able to connect the 'literary' and the 'religious'. Apart from promoting the harmony between şi'r (poetry) and şer'(Islamic law), one of the main intentions of the introduction is to demonstrate that poetry has not only a passive but also an active part in the practice of Islamic law. That the introduction has Ebussuud Efendi in its center as a model of scholar-litterateur type exemplifies this active role. By attributing Ebussuud‘s superiority over other Islamic scholars to his eloquence, the introduction reinforces its main message: On condition that poetry conveys adab, it provides a thorough (and thereby supreme) knowledge of religion. Finally, I offer an autobiographical reading in an attempt to explain the scholarcentered point of view reflected in the introduction. To support my arguments, I refer to Meşa'irü‟ş-Şuara as well as his poetry collection (Divan), especially the point in the introduction in which the author, Âşık Çelebi gets involved in the text as a pupil of Ebussuud to show that his master‘s eloquence seen in his fatwa was partly thanks to him. Moreover, by examining these autobiographical fragments which provide elements to establish the author‘s identity, I aim to show how the content of the introduction becomes in a way the author‘s 'natural legacy'.by Ahmet Barış Ekiz.M.S

Modeling the supplier uncertainty with phase-type distributions in inventory problems

Ankara : The Department of Industrial Engineering and the Institute of Engineering and Sciences of Bilkent Univ., 1996.Thesis (Master's) -- Bilkent University, 1996.Includes bibliographical references leaves 109-111.This study considers a stochastic inventory nnodel where the supply availability is subject to random fluctuations. The periods in which the supplier is available (ON) or unavailable (OFF) are modeled as a semi-Markov process. During ON periods the {q,r) policy is applied. During OFF periods, the amount enough to bring the inventory position to q + r is ordered as soon as the supplier becomes available again. The regenerative cycles are identifled by observing the inventory position and using the renewal reward theorem the average cost per time objective function is derived. In our study, a K-stage Phase-Type distribution for ON periods and a general distribution for OFF periods are assumed. In our study, the problem is theoretically solved for Kstage Phase-Type distributions; additionally numerical computations are made for 2-stage Phase-Type distributions. For large q values the structure of the objective function is investigated.Balcıoğlu, Ahmet BarışM.S

Fizikalno-kemijska svojstva i aktivnost kompozitnih jestivih gelova od želatine obogaćene lizozimom i polifenolima iz zelenog čaja

Research background. The use of gel-based systems as a novel method for the delivery of natural antimicrobial, antioxidant and bioactive compounds is a developing innovative solution for the food industry. This research aims to develop multifunctional active edible gels based on gelatine and its composites with improved mechanical properties. Experimental approach. Antilisterial and bioactive composite gels showing different physical and active properties from classical gelatine gel were developed by loading lysozyme and green tea extract into gelatine/starch and gelatine/wax composite gels. Mechanical properties, swelling profiles, colour, release profiles, and antimicrobial and bioactive properties of the gels were characterised. Results and conclusions. Gelatine/wax gels showed 1.3- to 2.1-fold higher firmness and cutting strength than gelatine and gelatine/starch composite gels that had similar firmness and cutting strengths. Work to shear of both composite gels was 1.4- to 1.9-fold higher than that of gelatine gel. The gelatine/starch gel showed the highest water absorption capacity. Green tea extract reduced soluble lysozyme in all gels, but composite gels contained higher amount of soluble lysozyme than gelatine gel. All the gels with lysozyme inhibited Listeria innocua growth in the broth media, while green tea extract showed antilisterial activity only in gelatine/wax gels. Gels with green tea extract showed antioxidant, antidiabetic (α-glucosidase and α-amylase inhibition), antihypertensive (angiotensin-converting enzyme inhibition) and antiproliferative activities (on Caco-2 human colon carcinoma cells). However, gelatine and gelatine/wax gels showed the highest antioxidant and antidiabetic activity. The gelatine/wax gels prevented phenolic browning, while green tea extract in other gels showed moderate or extensive browning. Novelty and scientific contribution. This work clearly showed the possibility of improving mechanical properties and modifying water absorption and controlled release profiles of gelatine gels using gelatine/starch and gelatine/wax composites. The novel composite gels reduced browning of incorporated polyphenols and showed antilisterial and bioactive properties.Pozadina istraživanja. Gelovi se sve više primjenjuju u prehrambenoj industriji kao inovativno rješenje za obogaćivanje hrane prirodnim antimikrobnim, antioksidacijskim i bioaktivnim spojevima. Svrha je ovoga rada bila razviti aktivne višenamjenske jestive gelove od želatine i njezinih kompozita s poboljšanim mehaničkim svojstvima. Eksperimentalni pristup. U radu je razvijen kompozitni gel s antilisterijskim i bioaktivnim sastojcima, čija su se fizikalna svojstva i aktivnost razlikovali od gela izrađenog od obične želatine. Gel je proizveden dodavanjem lizozima i ekstrakta zelenog čaja kompozitnim gelovima od želatine i škroba ili želatine i voska. Opisani su: mehanička svojstva, bubrenje, boja, način oslobađanja aktivnih sastojaka te antimikrobna i bioaktivna svojstva gelova. Rezultati i zaključci. Kompozitni gelovi od želatine i voska bili su od 1,3 do 2,1 puta čvršći i imali veće vrijednosti snage rezanja od same želatine i kompozitnog gela od želatine i škroba, koji su imali međusobno slična svojstva. Smično naprezanje oba kompozitna gela bilo je od 1,4 do 1,9 puta veće od one želatinoznog gela. Gel od želatine i škroba najbolje je upijao vodu. Ekstrakt zelenog čaja smanjio je topljivost lizozima u gelu, ali su kompozitni gelovi imali više topljivog lizozima od želatinoznog gela. Svi gelovi s lizozimom inhibirali su rast bakterije Listeria innocua u podlozi, dok je ekstrakt zelenog čaja imao antilisterijska svojstva samo u gelu od želatine i voska. Gelovi koji su sadržavali ekstrakt zelenog čaja imali su antioksidacijski, antidijabetički (inhibicija α-glukozidaze i α-amilaze), antihipertenzivni (inhibicija angiotenzin konvertirajućeg enzima) i antiproliferativni učinak (na Caco-2 stanice karcinoma debelog crijeva). Međutim, želatinozni te kompozitni gel od želatine i voska pokazali su najveću antioksidacijsku i antidijabetičku aktivnost. U gelovima od želatine i voska nakon dodatka ekstrakta zelenog čaja nije došlo do posmeđivanja uslijed oksidacije fenolnih spojeva, dok je ekstrakt zelenog čaja u ostalim gelovima prouzročio blago do intenzivno posmeđivanje. Novina i znanstveni doprinos. U ovom je radu prikazana mogućnost poboljšanja mehaničkih svojstava te modificiranja apsorpcije vode i kontroliranog otpuštanja aktivnih tvari iz gelova od želatine i škroba ili želatine i voska. U novim kompozitnim gelovima smanjilo se posmeđivanje dodanih polifenola, a gelovi su imali antilisterijska i bioaktivna svojstva