Time division multiplexing (TDM) is a method of transmitting and receiving independent signals on 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. It is used when the bit rate of the transmission medium exceeds that of the signal to be transmitted. This form of signal multiplexing was developed in telecommunications for telegraphy systems in the late nineteenth century, but found its most common application in digital telephony in the second half of the twentieth century.
Time division multiplexing was first developed for telegraphy applications to simultaneously route multiple transmissions over a single transmission line. In the 1870s, Émile Baudot developed a time multiplexing system of multiple Hughes telegraph machines.
In 1953, RCA Communications commercially launched a 24-channel TDM to send audio information between RCA's facilities in Broad Street, New York, its Rocky Point broadcasting station and Riverhead's Long Island, NY station. The communication was by a microwave system on Long Island. The experimental TDM system was developed by RCA Laboratories between 1950 and 1953.
In 1962, Bell Labs engineers developed the first D1 channel banks, which combined 24 digitized voice calls through a four-wire copper trunk between Bell's main office analog switches. A channel bank cut a digital signal of 1544 Mbit / s into 8,000 separate frames, each composed of 24 contiguous bytes. Each byte represented a single telephone call encoded in a constant bit rate signal of 64 kbit / s. Channel banks used the fixed position (temporary alignment) of a byte in the frame to identify the call to which they belonged.