<p>Much effort today in the Internet research community isaimed at providing network services for applications that werenot under consideration when the Internet was originallydesigned. Nowadays the network has to support real-timecommunication services that allow clients to transportinformation with expectations on network performance in termsof loss rate, maximum end-to-end delay, and maximum delayjitter. Today there exist two quality of service (QoS)architecture for the Internet: The integrated services, whichis usually referred to as intserv, and the differentiatedservices referred to as diffserv. Although the intserv clearlydefines the quality levels for each of its three serviceclasses, the limited scalability of this QoS architecture is acontinuous topic for discussion among the researchers. Theanalysis of the tradeoffs of the two QoS architecturesmotivated us to design a new QoS architecture which will takethe strength of the existing approaches and will combine themin a simpler, efficient and more scalable manner.</p><p>In this LicentiateThesis we introduce a guaranteed servicefor the Internet, which definition is similar to the one inintserv: The guaranteed service (GS) is a network servicerecommended for applications with firm requirements on qualityof end-to-end communication. The service should provide zeropacket loss in routers and tightly bound the end-to-end delay.The capacity for a GS connection should be explicitly reservedin every router along a path of a connection. However, incontrary to intserv the necessary quality level will beprovided without per-flow scheduling in the core routers, whichis the major drawback of the intserv architecture. We use thediffserv principle of dealing with aggregates in the corenetwork since this approach is proven to be scalable andefficient.</p><p>The thesis considers two major building blocks of the newarchitecture: The packet scheduling and the signaling protocol.We have developed a special scheduling algorithm. Our formaland experimental analysis of its delay properties shows thatthe maximum end-to-end delay is acceptable for real-timecommunication. Moreover, our scheme provides a fair service tothe traffic of other service classes. In order to achieve thedesired QoS level, a sufficient amount of capacity should bereserved for the GS connections in all intermediate routersend-to-end. We have developed a both simple and robustsignaling protocol. The realization of our protocol shows thatrouters are able to process up to 700,000 signaling messagesper second without overloading the processor.</p>

Much effort today in the Internet research community isaimed at providing network services for applications that werenot under consideration when the Internet was originallydesigned. Nowadays the network has to support real-timecommunication services that allow clients to transportinformation with expectations on network performance in termsof loss rate, maximum end-to-end delay, and maximum delayjitter. Today there exist two quality of service (QoS)architecture for the Internet: The integrated services, whichis usually referred to as intserv, and the differentiatedservices referred to as diffserv. Although the intserv clearlydefines the quality levels for each of its three serviceclasses, the limited scalability of this QoS architecture is acontinuous topic for discussion among the researchers. Theanalysis of the tradeoffs of the two QoS architecturesmotivated us to design a new QoS architecture which will takethe strength of the existing approaches and will combine themin a simpler, efficient and more scalable manner. In this LicentiateThesis we introduce a guaranteed servicefor the Internet, which definition is similar to the one inintserv: The guaranteed service (GS) is a network servicerecommended for applications with firm requirements on qualityof end-to-end communication. The service should provide zeropacket loss in routers and tightly bound the end-to-end delay.The capacity for a GS connection should be explicitly reservedin every router along a path of a connection. However, incontrary to intserv the necessary quality level will beprovided without per-flow scheduling in the core routers, whichis the major drawback of the intserv architecture. We use thediffserv principle of dealing with aggregates in the corenetwork since this approach is proven to be scalable andefficient. The thesis considers two major building blocks of the newarchitecture: The packet scheduling and the signaling protocol.We have developed a special scheduling algorithm. Our formaland experimental analysis of its delay properties shows thatthe maximum end-to-end delay is acceptable for real-timecommunication. Moreover, our scheme provides a fair service tothe traffic of other service classes. In order to achieve thedesired QoS level, a sufficient amount of capacity should bereserved for the GS connections in all intermediate routersend-to-end. We have developed a both simple and robustsignaling protocol. The realization of our protocol shows thatrouters are able to process up to 700,000 signaling messagesper second without overloading the processor. / NR 20140805

Existing multi-echelon inventory optimization models and formulas were studied to get an understanding of how safety stock levels are determined. Because of the restrictive distribution assumptions of the existing safety stock formula, which are not necessarily realistic in practice, a method to analyze the performance of this formula in a more realistic setting was desired. A SimPy simulation model was designed and implemented for a simple two-stage supply chain as a way to test the performance of the safety stock formula. This implementation produced results which led to the conclusion that the safety stock formula tends to underestimate the level of safety stock needed to provide a certain service level when predicted standard deviation of demand is underestimated and the assumptions of normally distributed demand and normally distributed lead times are not fulfilled.

Many real-world supply chains can be characterised as large and complex multi-echelon systems since they consist of several stages incorporating assembly and distribution processes. A challenge facing such systems is the efficient management of inventory when demand is uncertain, operating costs and customer service requirements are high. This requires specifying the inventory levels at different stages that minimise the total cost and meet target customer service levels. In order to address this problem, researchers proposed the Stochastic-Service Model and the Guaranteed-Service Model (GSM) approaches. These two approaches differ in terms of assumptions with regard to how to address demand variations and service times. This thesis develops several contributions to the GSM based multi-echelon inventory optimisation problem. First of all, we conduct a comprehensive literature review which gives a synthesis of the various GSM work developed so far. Then, we study the impact of some specific assumptions of the GSM such as bounded demand, guaranteed-service times and common review periods. Our numerical analysis shows that the bounded demand assumption may cause a deviation on customer service levels while the guaranteed-service times and common review periods assumptions may result in an increase on the total cost. In real-world supply chains the impact of these assumptions might be significant. Based on the findings presented while investigating the impact of the common review periods assumption, we develop an extension of the GSM that enables to simultaneously optimise the review periods (reorder intervals) and safety stock levels (order-up-to levels) in general acyclic multi-echelon systems. We formulate this problem as a nonlinear integer programming model. Then, we propose a sequential optimisation procedure that enables to obtain near optimal solutions with reasonable computational time. Finally, we focus on the issue of customer service level deviation in the GSM and propose two approaches in order to mitigate this deviation. The numerical study shows that the first approach outperforms the second one in terms of computational time while the second approach provides more accurate solutions in terms of cost. We also present some related issues in decentralised supply chain settings.

De nombreuses chaînes logistiques peuvent être caractérisées comme de larges systèmes multi-échelons, car ils se composent souvent de plusieurs étages qui intègrent des activités d'assemblage et de distribution. L’un des enjeux majeurs associé au management de ces systèmes multi-échelons est la gestion efficace de stocks surtout dans des environnements où la demande est incertaine, les coûts de stocks sont importants et les exigences en termes de niveau de service client sont élevées. Cela nécessite en particulier de spécifier les niveaux de stocks aux différents étages afin de minimiser le coût total du système global et de satisfaire les niveaux cibles de service client. Pour faire face à ce problème, deux approches existent dans la littérature; il s’agit du Modèle de Service Stochastique (SSM) et le Modèle de Service Garanti (GSM). Ces deux approches diffèrent en termes d'hypothèses utilisées concernant la façon de gérer les variations de la demande et les temps de service. Cette thèse amène plusieurs contributions au problème d'optimisation de stocks multi-échelons basé sur le GSM. Tout d'abord, nous menons une revue de la littérature internationale qui donne une synthèse des différents travaux réalisés à ce jour. Ensuite, nous étudions l'impact de certaines hypothèses spécifiques du GSM comme la demande bornée, les temps de service garanti et les périodes d’approvisionnement communes. Notre analyse numérique montre que l'hypothèse de demande bornée peut causer une déviation sur les niveaux de service client tandis que les hypothèses de temps de service garanti et de périodes d’approvisionnement communes peuvent entraîner une augmentation du coût total. En pratique, l’impact de ces hypothèses peut être important. En se basant sur les résultats présentés lors de l'analyse de l’hypothèse des périodes d'approvisionnement communes, nous développons une extension du GSM qui permet d'optimiser simultanément les périodes d’approvisionnement (les intervalles de réapprovisionnement) et les niveaux de stocks de sécurité (les niveaux de recomplétement) dans les systèmes multi-échelons acycliques généraux. Nous formulons ce problème comme un modèle de programmation non-linaire en nombres entiers. Ensuite, nous proposons une procédure d'optimisation séquentielle qui permet d'obtenir des solutions proches de l’optimal avec un temps de calcul raisonnable. Enfin, nous nous concentrons sur le problème de déviation de niveau de service client dans le GSM et nous proposons deux approches afin d'atténuer cette déviation. L'étude numérique montre que la première approche est plus performante que la deuxième en termes de temps de calcul tandis que la deuxième approche offre des meilleures solutions en termes de coût. Nous présentons également des problèmes similaires dans les chaînes logistiques décentralisées. / Many real-world supply chains can be characterised as large and complex multi-echelon systems since they consist of several stages incorporating assembly and distribution processes. A challenge facing such systems is the efficient management of inventory when demand is uncertain, operating costs and customer service requirements are high. This requires specifying the inventory levels at different stages that minimise the total cost and meet target customer service levels. In order to address this problem, researchers proposed the Stochastic-Service Model and the Guaranteed-Service Model (GSM) approaches. These two approaches differ in terms of assumptions with regard to how to address demand variations and service times. This thesis develops several contributions to the GSM based multi-echelon inventory optimisation problem. First of all, we conduct a comprehensive literature review which gives a synthesis of the various GSM work developed so far. Then, we study the impact of some specific assumptions of the GSM such as bounded demand, guaranteed-service times and common review periods. Our numerical analysis shows that the bounded demand assumption may cause a deviation on customer service levels while the guaranteed-service times and common review periods assumptions may result in an increase on the total cost. In real-world supply chains the impact of these assumptions might be significant. Based on the findings presented while investigating the impact of the common review periods assumption, we develop an extension of the GSM that enables to simultaneously optimise the review periods (reorder intervals) and safety stock levels (order-up-to levels) in general acyclic multi-echelon systems. We formulate this problem as a nonlinear integer programming model. Then, we propose a sequential optimisation procedure that enables to obtain near optimal solutions with reasonable computational time. Finally, we focus on the issue of customer service level deviation in the GSM and propose two approaches in order to mitigate this deviation. The numerical study shows that the first approach outperforms the second one in terms of computational time while the second approach provides more accurate solutions in terms of cost. We also present some related issues in decentralised supply chain settings.