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Time-division multiplexing
Time-division multiplexing (TDM) is a method of transmitting and receiving independent signals over a common signal path by means of synchronized switches at each end of the transmission line so that each signal appears on the line only a fraction of time in an alternating pattern. This form of signal multiplexing was developed in telecommunications for telegraphy systems in the late 19th century, but found its most common application in digital telephony in the second half of the 20th century.
==History==
Time-division multiplexing was first developed for applications in telegraphy to route multiple transmissions simultaneously over a single transmission line. In the 1870s, Émile Baudot developed a time-multiplexing system of multiple Hughes telegraph machines.
In 1953 a 24-channel TDM was placed in commercial operation by RCA Communications to send audio information between RCA's facility on Broad Street, New York, their transmitting station at Rocky Point and the receiving station at Riverhead, Long Island, New York. The communication was by a microwave system throughout Long Island. The experimental TDM system was developed by RCA Laboratories between 1950 and 1953.
In 1962, engineers from Bell Labs developed the first D1 Channel Banks, which combined 24 digitized voice calls over a 4-wire copper trunk between Bell central office analogue switches. A channel bank sliced a 1.544 Mbit/s digital signal into 8,000 separate frames, each composed of 24 contiguous bytes. Each byte represented a single telephone call encoded into a constant bit rate signal of 64 kbit/s. Channel banks used a byte's fixed position (temporal alignment) in the frame to determine which call it belonged to.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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