## 2 Materials and Methods  
Since the focus of this study was to investigate the influence of bottleneck on productivity of production processes controlled by different pull mechanisms, first the pull mechanisms themselves had to be defined. [PERSON] found that Kanban, Conwip and DBR are three the most widely used pull production control mechanisms [14]. Review of the literature revealed the advantages also of Hybrid Kanban/Conwip; thus, these four mechanisms were researched in this study. Kanban in Japanese means \"card\", and in TPS it was used to manage flow of material through the production process. In the production process controlled by Kanban cards, production is triggered by a demand. The operation stations send the signal (Kanban card) to the upstream workstation to replace the part that was just used. [PERSON] and [PERSON] that there is a difference in installing and implementing Kanban and that the Kanban is very often considered as simple, but they state that the idea of Kanban is simple, and implementation is not. That is so because there are prerequisites for implementing Kanban in the process [54]. If production line consisting of two workplaces (two stages of the production process) where the process is controlled by a simple Kanban control mechanism is considered, the process of production control look as follows. In the initial state, in the i-th stage of the production process, the Bi buffer contains k\({}_{1}\) semi-finished products, where each semi-finished product has one Kanban card attached. When the demand from the customer arrives, the request goes to line D and withdraws the finished product from B\({}_{2}\). At this point, there are two possibilities:  
* If there is a finished product in B\({}_{2}\), it is forwarded to the customer after the Kanban k\({}_{2}\) previously attached to it is separated from it. That Kanban is transferred to buffer of Kanban cards, K\({}_{2}\), which is a signal to produce a new piece of semi-finished product in stage 2;
* If there is no finished product in B\({}_{2}\), the request is withdrawn and put on hold until the finished product arrives in B\({}_{2}\). As soon as the finished product arrives, it is delivered to the customer and Kanban goes to K\({}_{2}\);  
When the Kanban card arrives in K\({}_{2}\) it authorizes the start of production of a new product in stage 2. Here, again, two situations can occur:  
* If a semi-finished product is available in the B\({}_{1}\) buffer, Kanban k\({}_{1}\) is immediately removed and k\({}_{2}\) is attached to that product. Then, the product and k\({}_{2}\) go together to workplace 2. Kanban k\({}_{1}\) is transferred to K\({}_{1}\) buffer which authorizes starting production on WP\({}_{1}\);
* If a semi-finished product is not available in the MS\({}_{1}\) buffer, Kanban card of the stage 2, k\({}_{2}\), will wait until new semi-finished product is available in MS\({}_{1}\)[55];  
As in the Kanban system, in Conwip system signal cards control the level of WIP. Only, the signal cards do not go from one workstation to another, that is from one buffer to another, but from the last buffer (warehouse) to the first workstation Thus, Conwip cards control the overall amount of WIP, in contrast with Kanban mechanism controls WIP on every workstation. Figure 1 represents the DBR mechanism, which is similar to Conwip, only the control cards do not travel from the last buffer to the first workstation, but from the bottleneck buffer to the first workstation, thus if the bottleneck is on the last workstation, then the DBR has similar characteristics (similar route) as Conwip. Figure 1: DBR control mechanism. Hybrid is a combination of Kanban and Conwip, thus signal cards follow the same route as in Conwip, which is the global flow of information and also there is a route of the cards as in the Kanban system which is the local flow of information. In order to answer the research questions simulation experimentation was conducted. Simulation is a commonly used research methodology.