When developing networking protocols there exists a need to test the reliability and correctness of them. This can be done using ordinary hardware, but often tests are performed using a simulator. The aim of this thesis was to develop a simulator for Net Insight AB, to be used when testing their DTM protocols. DTM or Dynamic synchronous Transfer Mode is a networking technique based on circuit switching. A simulator that simulates DTM hardware via a hardware interface has been designed and implemented. It runs on Linux and supports simulation of different kinds of errors, for example, fibre breaks and hardware resets. Furthermore, it can be run separately on one computer or distributed on several computers.