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.