Supervision of digital transmission systems

A digital transmission system having a pair of terminals, one or more repeaters and separate undirectional paths, comprising means for sending a loop-back signal to the repeater from one of the terminals for establishing a loop-back condition in the repeater, means for sending to the repeater a digital multi-bit test signal from the other terminal, the mix of the bits in the test signal being controlled, loop-back circuitry in the repeater sensitive to the test signal to change the loop-back condition in the repeater, and means at one of the terminals to determine the test signal level which caused the change.

A method of supervising or controlling a digital transmission system having at least one repeater in the transmission path, comprising sending to the repeater via the path a bit stream divided into identical words of zero disparity, each word being n bit long, filtering at least one of the fundamental and harmonic frequencies, deriving from the filtered signal in the repeater a first d.c. signal whose level is dependent on the mix of the bits, comparing the d.c. signal with a second d.c. signal in the repeater, and varying the mix of the bits during the comparison to establish the level of the first and second d.c. signals relative to one another by operating a switch in the repeater.

A repeater for a digital transmission system, the repeater having two digital regenerators for respectively separate unidirectional paths of the system, loop-back circuitry sensitive to receipt of a loop-back signal from a terminal of the system to establish a loop-back condition between the unidirectional paths of the system, and a first comparator arrangement which compares a reference signal level with another signal level derived from and representative of a succession of incoming multi-bit test signals with different Fourier transforms and in response thereto changes the loop-back condition when the compared signal levels bear a predetermined relationship.