Patent ID: 7103042

Claim:
A node unit provided in a digital signal transmission system which includes a plurality of node units for transmitting a digital transmission signal in frame form, each frame of said digital transmission signal including at least a frame header indicating the end of the signal frame, and said plurality of node units exchanging state notice information about autonomous operation with one another, said node unit comprising: state notice information protecting means for taking in said state notice information when said state notice information coincides with predetermined n, where n is a natural number, stages of frames consecutively; frame synchronization protecting means for protecting the frame synchronization of said digital transmission signal until errors are sensed consecutively over predetermined m, where m is a natural number, stages of frames in said frame header; and information taking-in means for, if the expression m>2n holds when the frame synchronization protecting means has sensed errors consecutively over m stages of frames in said frame header, regarding said state notice information included in a k-th stage, provided that k meets the expression m<k≦(Int(m/n)+1)×n, where Int(m/n) is an integer obtained by discarding decimals of m/n, frame as valid information using the frame including the frame header where an error first occurred as a reference and taking in the valid information.