Abstract: A test system and method are disclosed for testing a communications protocol software implementation or combination of communications protocol software implementations. The test system has a memory for storing a communications protocol software implementation of a particular layer in a hierarchy of protocols. The test system has a processor for catching data outputted from the communications protocol software implementation to upper and lower layer communications protocol software. The processor is also for interpreting each frame contained in a test case file. In response to interpreting an input data frame, the processor feeds input data contained in the input data frame to the communications protocol software implementation. In response to interpreting an output data frame, the processor compares received data outputted from the communications protocol software implementation with expected output data interpreted from the output data frame, and generates a message depending on this comparison.