Abstract: A performance monitoring system is provided for in-service monitoring of digital carrier facilities of the type having a chain of facilities serially linked together. Each facility has at least one component which processes a digital signal received from an upstream facility, and which has bit error detection circuitry for producing a logic level output in response to detection of an error in the digital signal. A logic circuit is associated with each component in a facility and is responsive to the logic level output of the bit error detection circuitry with which the logic circuit is associated for storing a representation of the number of errors detected thereby. A control center is provided for in-service monitoring of the performance of the components in the facilities. A connection circuit is provided for selectively connecting the logic means to the control center.