Abstract: An improved network interface unit (NIU) for remotely monitoring and testing the performance of DS1 telephone circuits, installed on the network side of an interface between customer premises equipment (CPE) and equipment provided by the network provider. The inventive NIU is used to non-intrusively collect and transmit full-time performance monitoring data to the network provider. The inventive NIU provides continuous and non-intrusive performance monitoring of DS1 transmission systems. With the inventive NIU installed at the interface between the customer's CPE and the LECs' equipment, network service providers are alerted to potential problems before they adversely affect the service provided by the network providers to their customers. The inventive NIU enables a network service provider to quickly and non-intrusively determine whether a problem exists in the equipment provided by the network provider or in the equipment on the customer's premises.