# Semi-open queueing networks and robotic mobile fulfillment systems

### Working Groups: Chair Applied Analysis

### Collaborators (MAT): Dr.Â Sonja Otten

### Collaborators (External): Prof.Â Dr.Â Hans Daduna, Dr.Â Ruslan Krenzler, PD Dr.Â Karsten Kruse, Prof.Â Dr.Â Lin Xie

## Description

We consider a semi-open queueing network (SOQN), where one resource from a resource pool is needed to serve a customer. If on arrival of a customer some resource is available, the resource is forwarded to an inner network to complete the customerÂ´s order. If no resource is available, the new customer waits in an external queue until one becomes available (*backordering*). When a resource exits the inner network, it is returned to the resource pool. We develop a new solution approach. In a first step we modify the system such that new arrivals are lost if the resource pool is empty (*lost customers*). We adjust the arrival rate of the modified system such that the throughputs in all nodes of the inner network are pairwise identical to those in the original network. Using queueing theoretical methods, in a second step we reduce this inner network to a two-station system including the resource pool. For this two-station systems, we invert the first step and obtain a standard SOQN which can be solved analytically. We apply our results to storage and delivering systems with robotic mobile fulfilment systems (RMFSs). Instead of sending pickers to the storage area to search for the ordered items and pick them, robots carry shelves with ordered items from the storage area to picking stations. We model the RMFS as an SOQN to determine the minimal number of robots.

## References

[1] S. Otten, R. Krenzler, L. Xie, H. Daduna, K. Kruse. Lost-customers approximation of semi-open queueing networks with backordering - An application to minimise the number of robots in robotic mobile fulfilment systems. *OR Spectrum*, 44(2):603-648, 2022. doi: 10.1007/s00291-021-00662-9.