Abstract: The reliability of a high-speed digital data traffic channel is determined by establishing end-to-end synchronization by use of confirming error detecting data transmitted on a side channel from an originating end, comparing the traffic data and the error detecting data and from time to time re-confirming the validity of the synchronization.

Abstract: An addressed transmission line test system having bidirectional input and output lines with through connections in a quiescent state which is capable of recognizing properly addressed signals at either direction input for breaking the through connections and for allowing responses to commands sent from either direction for controlling the application of signals to the other direction.

Abstract: There are disclosed instruments and methods for facilitating the measurement of characteristics of only a selected portion of a transmission channel. In my copending application Ser. No. 661,106, an apparatus located at the remote end of a channel of interest amplifies the received disturbance component relative to the received test tone, and forwards the resulting processed signal over a second channel to a test site. In this manner, the disturbance signal received by the test instrument represents the disturbance signal received at the remote end of the channel of interest, the disturbance introduced by the second channel being insignificant relative to the amplified disturbance component. In accordance with the present invention, an apparatus at the test site attenuates the received disturbance relative to the test tone by the same factor that it is amplified at the remote end of the first channel.