Abstract: A full duplex digital transmission, high-capacity digital, facility loop-back test, diagnostics and maintenance system having a digital transmission facility, transmission medium, and at least one microprocessor-based, full duplex facility loop-back diagnostics interface is located at predetermined end-user locations within the system. The system is capable of initiating a facility loop-back test and predetermined diagnostics for a predetermined digital transmission span and customer premises equipment at the location of the network interface for a specified end-user. The facility loop-back diagnostics interface has a performance monitoring mode of operation and a maintenance facility loop-back mode of operation. The diagnostics interface can be activated from a remote location and is compatible with and supports a particular protocol, specifically LAP-B protocol.