Abstract: Apparatus for testing the performance of a communication line that carries customer data between two modems, including a programmable processor (for example, a digital signal processor) in each modem that either performs modulation or demodulation of customer data or tests the line, and a controller (for example, a programmable host processor) that switches the programmable signal processor selectively between processing customer data and line testing. The communication line carries both customer data and network control commands that include a test command for initiating testing, and each host processor responds to the test command by switching the signal processor to the test mode. There are a plurality of modems forming a data network and a central network control system connected to the network for providing the test command. The test command names two of the modems to participate in the testing and the controller identifies whether a given modem is one of the participants.