Telephone companies use ................ to combine voice channels into successively larger groups for efficient transmission.
Telephone companies use FDM
Frequency-division multiplexing. In telecommunications, frequency-division multiplexing (FDM) is a technique by which the total bandwidth available in a communication medium is divided into a series of non-overlapping frequency bands, each of which is used to carry a separate signal.
Frequency Division Multiplexing (FDM) is a networking technique in which multiple data signals are combined for simultaneous transmission via a shared communication medium. FDM uses a carrier signal at a discrete frequency for each data stream and then combines many modulated signals.
Fused deposition modeling (FDM) is an additive manufacturing (AM) technology commonly used for modeling, prototyping, and production applications as trademarked by Stratasys Inc.. It is one of the techniques used for 3D printing.