In an XDSL system, a network interface device can connect between a central office and a customer premises. The network interface device can monitor for an indication to enter a test mode. In the test mode, the network interface device can provide a test signal to the central office, which can be used to test central office equipment and transmission lines connecting the network interface device with the central office. The network interface device can also provide the test signal to the customer premises, and the test signal can be used to test customer premises wiring and equipment.