Digital transmission systems.

摘要

A supervisory channel is provided on a digital transmission system by frequency modulation of the data signal transmitted along a transmission link 10 which includes repeaters 23. The frequency modulation can be carried out using an elastic store 14. Each repeater includes means for demodulating the frequency modulation so that each repeater can be addressed and interrogated. Each repeater can also phase modulate the data signals on the return link to a terminal station so that signals indicative of an interrogated condition can be transmitted to the terminal station.