In a digital transmission network including 1.5 Mbit/s and higher speed systems, it is desirable to signal service outages of the higher speed systems to the multitude of affected channel banks terminating the 1.5 Mbit/s systems in order to avoid unnecessary maintenance activity. To do this, it is proposed that a special signal be transmitted to the channel banks from the multiplex terminating the higher capacity systems. This special signal causes the channel bank alarm lights to flash, and thus can be recognized without special test equipment or modification of the channel banks. While the special signal may be quite complex to operate properly with several generations of channel banks, the circuitry required to generate it fits comfortably on a single circuit board and can be shared by all 1.5 Mbit/s outputs of the multiplex.

A coder-decoder which codes a 600 channel mastergroup into a 44.736 Mbits/s digital is described. It includes a double frequency shift of the mastergroup, eight bit coding with three segment companding, and has format and features necessary for operation in a telephone network.

The margin of a high-speed digital regenerative repeater can be determined easily by adding a misterminated stub at the repeater input. The stub adds an interfering signal which, if large enough, will cause the repeater to make errors. The amplitude of the interfering signal is determined by the termination, so the termination impedance at which errors begin is a good measure of repeater margin. No calibration procedure is required.

The basic code proposed in the referenced parep is identical with a three-zero substitution code proposed previously, and the remaining codes are variants obtained by adding one or more zeros in front of three-zero substitution kernels. The bipolar with six-zero substitution (B6ZS) code is superior in baseline wander performance to codes based on the three-zero substitution kernels, and other differences in performance are small.