Patent ID: 6178025
Filing Date: 2001-01-23
Classification: H04B

Abstract:
A method of monitoring the transmission of an optical signal to determine the existence of a loss of signal condition in a communications system in which the optical signal is modulated at a bit rate with a digitally coded data stream; the method comprising the steps of:(a) monitoring the optical signal at a monitoring location of the system to obtain a monitor signal representative of the optical signal;(b) detecting the presence of features of the monitor signal occurring at a detection frequency lower than the bit rate and corresponding to a periodicity of recurrence of a feature of the digitally coded data stream; and(c) determining whether a loss of signal condition exists in dependence upon the results of said detecting step;wherein the detecting step comprises an auto-correlation process providing a detection value representative of a degree of correlation between the monitor signal and at least one delayed monitor signal corresponding to the monitor signal delayed by a respective integral multiple of a delay period; said delay period corresponding to the inverse of the detection frequency.