Sharif Digital Repository

Vehicular ad hoc networks (VANETs) can prepare scalable and cost effective solutions for applications such as traffic monitoring and vehicle safety. However, VANETs often experience congestion quicker than wired networks, leading to high end to end delay. The optimization method was proposed to improve the end to end delay of Ad-hoc On-demand Distance Vector (AODV) routing protocol in VANETs. It was shown that Last-In-First-Out with FrontDrop (LIFO-FD) queuing system for AODV routing protocol performs better in high traffic periods in term of end to end delay. The simulation studies were performed on the impact of queuing systems on end to end delay of AODV routing protocol in VANETs. The... 

In this thesis, we consider various Make-to-Order systems as a version of integrated production-inventory systems. Most Make-to-order systems can be sufficiently modeled by a queueing system with inventory in which completing each service in the queueing system requires an on-hand inventory. Unlike most researches in this field, we present an analytical approach. In all systems we embed a stochastic process with joint state space that can entirely describe the state of system. Then steady state distributions are derived. Those distributions are applied to compute performance measures and long-run average cost function which can be used for optimization. In our Make-to-Order models, demands... 

In this thesis, we study a queuing system with one server in which customers differ in both sensitivity to delay and willingness to pay. In this system, the server’s goal is to design a mechanism in order to maximize her revenue. It is worth to mention that nonlinear delay cost is implemented.Making use of the achievable region approach, we show that a well-designed menu of probabilistic admission control along with priority pricing contracts, may force customers to reveal their true valuations and at the same time induce customers that are more sensitive to delay to opt for higher priorities. Thus, the probabilistic admission control allows the server to identify the customers that are... 

The purpose of this study is to model people's behavior on both sides of the kidney market. The healthy person decides to donate a live kidney (for a certain amount) on the supply side. On the demand side, the patient chooses between the two existing kidney queues, the living kidney and the cadaveric kidney, based on the individual's income and belief from the waiting time in each of the two queues. The sick person faces two critical risks in life: risk of death and kidney failure. To model these two risks, it is assumed that each person has a level of health that consists of two deterministic and stochastic parts. In this way, with a deterministic rate, a person's health level will decrease... 

Social media can be helpful to people in vast areas, especially in queuing systems, in the real world there is remarkably unknown information like service time, average waiting time or average queue length to customers, thus people are likely to share their experiences to posterior customers who want to use the same service. This research examines the impact of user-generated reviews, on the performances of a Markovian queuing system, we assumed that customers are not aware of the service rate, therefore they will assist next customers by writing reviews about their service time they had experienced. We intend to examine the impact of information on queueing performance such as system... 

As the number of patients waiting for receiving kidney transplantation increases all over the world, the average waiting time for a patient in queue rises too, and consequently, the rate of death goes beyond the accepted level. Also, unbalanced numbers of donors and patients with same blood type make the problem bolder. The aim of this research is to design a queuing model in which kidney could be transplanted from deceased donors to patients with different but compatible blood type (Cross-Transplantation) in order to make the average waiting time for patients with different blood type closer to each other. In other words, the transplant system is supposed to make a decision whether to... 

In this research we will develop an algorithm to find the optimal robust solution via simulation optimization by using an artificial neural network metamodel. Following Taghuchi, in design phase of the algorithm, we will discriminate between decision or control variables and environmental or noise variables. To arrive of the best new solution in every iteration, the algorithm will use a symmetrical probabilistic distribution about the optimum point of the previous iteration. In comparison with the existing methods, our algorithm displays an improvement in results when applied to such problems as single channel queueing system problem and economic order quantity problem  

The thesis has considered a two-echelon supply chain which consists of one central warehouse and multiple retailers. Assume all retailers and the warehouse follow a continuous review inventory model. The costumer’s demand comes to retailers in a Poisson process. The retailers replenish their stocks from the warehouse in an (S-1, S) inventory control policy and the warehouse replenishes its stocks in an (r, Q) inventory control policy from an external supplier with an infinite capacity. It may be more representative to model stockouts as lost sales when the retailers are in a competitive market or in monopoly markets all demand will be backorder. Therefore, if the market situation goes... 

Queuing systems and their optimization have always been an important concern of every society. It has been a long time that people understand that with quantifying the queues they are standing on, they can save great amount of money and time. Because of that fact, they have been studying these systems, and reached to the interesting conclusions. These researches have shown that there exist a couple of equilibriums in every queuing system, such as: speed-price and speed-quality equilibriums.Knowing these equilibriums along with modeling them will help us optimize queues. The importance of this optimization is its incredible effect on cost reduction in economical and social queues. In many... 

In this contemporary era, distributed computing is a vital solution for many applications. The traditional computing scheme based on allocating one complete computation to only one server is not efficient anymore. Delay has a very important role in computation’s performance especially for the computation at the edge of the network. To alleviate the effect of big data and complex algorithms, distributed computing is a very promising solution. In this structure, all computations are divided into some small tasks or coded, based on the system's status and will be executed in parallel. In fully parallelized distributed computing, a task is broken into N smaller tasks and whenever all N subtasks... 

To reduce customer waiting times in various service conditions, admission control and service speedup can be implemented during times of congestion. In a healthcare setting, this may result in a patient being redirected to an alternative treatment facility. Additionally, waiting for patients can have negative impacts on their health and controlling population density can be costly. Implementing these measures may also take too much time or deviate from the ideal treatment. In this thesis, we will evaluate multi-server queuing systems by considering both speed and admission control simultaneously. We will use dynamic programming to determine the optimal control characteristics and calculate... 

The assumption of infinite lifetime is common in most of the inventory models. However, the perishability of products is a major problem of some industry sectors in which by disregarding the finite lifetime of their products the resulting model may give inaccurate results. Although, significant studies have been carried out for inventory control of perishable products, yet the literature needs more studies to develop or optimize the inventory policies. In this thesis, the inventory control of perishable products is considered. For the case of stochastic demand, it was well known that there is no inventory control policy in which both the order size and the order interval are constant with an... 

In this thesis, we study the effect of cognition and cooperation on two important MAC layer performance metrics, i.e. maximum stable throughput and delay. To this end, we consider a broadband secondary user which interferes with N narrowband primary users. In our study we focus on four transmission protocols as well as two transmission policies, i.e. parallel and non-parallel transmissions. In the cooperative protocols, the broadband transmitter relays the packets of the primary users which have not correctly decoded at the primary receiver. In non-parallel transmission case, the packets of the broadband user are transmitted in the total available bandwidth and in parallel transmission case,...