Baudot Time-Division Multiplexing combines a number of baseband signals into a single signal. Each baseband signal is allocated a time slot in the combined signal by a multiplexor. A demultiplexor is used to separate the combined signal into its original baseband signals.
Digital signals can tolerate large amounts of noise, interference, and distortion.
Amplifiers strengthen a signal, but they also strengthen the noise. This causes a large loss in quality of the signal. Repeaters will detect the original digital signal and retransmit it removing the noise. This increases the quality of the received signal when travelling very long distances. Development started in 1955.