a transmission system
is a system that transmits a signal from one place to another. The signal can be an electrical, optical or radio signal.
Some transmission systems contain repeaters, which amplify a signal prior to re-transmission, or regenerators, which attempt to reconstruct and re-shape the coded message before re-transmission.
One of the most widely used transmission system technologies in the Internet and the PSTN is SONET.
See also: Communications satellite, Submarine communications cables