## 1 Introduction
### Literature Review
#### 1.1.4. Pull Control Mechanisms  
As previously explained, in practice, Kanban is often the first association when pull principle is considered. It was developed from the beginning of TPS [3], and later, when TPS started to spread in western countries and different types of manufacturing industries,other pull mechanisms were developed. Three of them, besides the Kanban, are the focus of this study and they are Conwip, Hybrid Kanban/Conwip and DBR.  
Conwip was developed by [PERSON] et al. [41]. It is a pull mechanism that limits the total amount of work in process as communication cards only exists between the finish goods warehouse and the first operation, whereas in Kanban communication cards flow between every workstation (every operation) upstream in the production process. Conwip is the abbreviation of \"Constant Work in Process\", the name that describes the essence of this pull mechanism [41].  
[PERSON] et al. proposed a new mechanism and called it Hybrid Conwip/Kanban as it combines communication rules both from Conwip and Kanban, thus communication exist both between warehouse and the first station but also between every station upstream in the process. They state that the advantage of the Hybrid Conwip/Kanban is in the processes with more workstations and more variability in the process but also the processes with bottleneck [13].  
DBR is developed by [PERSON] in his Theory of Constraints. The idea is that the signal for the beginning of processing a new item is sent from the bottleneck buffer to the first workstation in the production process [15].  
While some of the studies of pull mechanisms investigate single-stage production processes [42] others investigated multistage processes, mostly with single product production [43; 44; 45; 46; 47]. Some articles describe case studies of implantation of pull control mechanisms [48] describes the development of two different Conwip approaches for bicycle chain manufacturing. They have simulated and compared these two approaches with the existing one and found that one outperforms by 42 percent and another by 50 percent regarding lead time. In [49] a case study of implementation of Hybrid Kanban/Conwip mechanism in high-variety/low-volume production process is presented and an increase of 38 percent in inventory turnover is described.  
In addition, majority of studies deal with discrete production systems, but interesting findings regarding pull and its possibilities for process industry are presented in the research of [PERSON] and [PERSON] who have gained important insights into the importance of pull and flow in process industries and its impact on production service, waste and utilization [50]. Very useful findings were gained in the study by [PERSON] et al. who researched applicability of pull in three different contexts, namely manufacturing, healthcare and construction. They have developed a method for assessment of pull production system according to twenty-three parameters divided in three groups: design, stability, and control [51]. In addition, [PERSON] et al. have investigated Kanban, Conwip and DBR by simulating production process in the textile industry and concluded with the preference of Kanban over Conwip and DBR [52]. [PERSON] and [PERSON], [53], compared Base Stock, Traditional Kanban Control System and Extended Kanban Control System (both dedicated and shared type) by simulation using common total cost measure as a performance measure for comparison. They presented procedures for optimizing multiple product Kanban control systems Numerical results show that the dedicated and shared-extended Kanban control systems outperform the other two systems [53].