## 2 An Overview of Related Research
These costs are still relevant because a user must evaluate costs of delivery;
* DRP systems cannot recalculate forecast if demand is changed in real-time (stochastic demand). Inventory management of pharmaceutical products represents a ubiquitous topic in theory and practice. Consequently, there are numerous overview papers of high quality dealing with the subject. For example, [10; 34; 35] and many others. However, despite various studies, inventory control is still a daily burning issue for pharmaceutical companies. The implementation of different optimization models is often impossible in real-world settings. A possible solution implies a conceptual model of a system that enables an overview of relevant facts for a mathematical model and actions that should be recognized as the identified solution [10]. The aim of this paper is not the comparison of and with different approaches to pharmaceutical inventory control, and thus we will not further analyse related papers. This paper represents a real-life inventory control problem in a private pharmaceutical distribution company. The authors studied the problem within two years and noticed specific practical assumptions and constraints of the supply chain in the company. Another important request, defined by the company, was a software solution that is affordable, but dynamic, flexible, and relatively easy to implement and use. To the best of the authors' knowledge, there is no research in the literature that has developed an inventory control solution appropriate to the identified characteristics of the problem, specific practical assumptions, constraints, and the company's requests. This comprises:  
* a centralized multi-echelon multi-product inventory control system with transparency of information within the distribution network, but without the necessary software information systems;
* nine regional pharmaceutical markets should be managed by a centralized inventory control model in PSC;
* the products should be distributed directly from the manufacturer to the customers because the distribution company does not store items in the main branch;
* a different FOQ or LFL order quantity should be considered for each product;
* a lead time (LT) is variable (approximately five months) and different for each product;
* an inventory plan is based on a monthly sales forecast for three months after the lead time period expiration and realized stochastic demand;
* shortages are allowed but not backlogged;
* two objective functions should be considered, the maximization of the difference between the planned average inventory level and the realized average inventory level and the minimization of the stock-out situations;
* the software solution should be affordable, but dynamic, flexible, and relatively easy to implement and use. Additionally, we propose the conceptual model, which reflects the specificities of the system that are necessary for the mathematical model's development. The conceptual model is in accordance with the modern control theory; consequently, the system is based on feedback control. The modern control theory concept and its implementation are described further in the paper, as is a discrete-time system control used for the system dynamics modelling.