FDM. Combining analog circuits by assigning each a different Carrier Frequency and merging them into a single communication signal.
Frequency division multiplexing (FDM) is a process that allows each signal to travel on its own unique frequency, the individual signals are then modulated by Data type and are combined to form a single signal.
Frequency division Multiplexing was first employed in the 1930s in telephone systems to combine multiple conversations onto one single line. FDM has taken a backseat to Time Division Multiplexing (TDM) with the emergence of Digital telephone systems.