Time-division multiplexingTime-division multiplexing(TDM) is a method of transmitting and receiving independent signals over a common signal path bymeans of synchronized switches at each end of the transmission line so that each signal appears on the line only a fraction of time inan alternating pattern. It is used when thebit rate of the transmission medium exceeds that of the signal to be transmitted. This formof signalmultiplexing was developed intelecommunications fortelegraphy systems in the late 19th century, but found its mostcommon application indigital telephony in the second half of the 20th century.HistoryTechnologyApplication examplesMultiplexed digital transmissionTelecommunications systemsStatistical time-division multiplexingSee alsoReferencesTime-division multiplexing was first developed for applications intelegraphy to route multiple transmissions simultaneously over asingle transmission line. In the 1870s,Ćmile Baudot developed a time-multiplexing system of multipleHughes telegraph machines.In 1953 a 24-channel TDM was placed in commercial operation by RCA Communications to send audio information between RCA'sfacility on Broad Street, New York, their transmiting station at Rocky Point and the receiving station at Riverhead, Long Island, NewYork. The communication was by a microwave system throughout Long Island. The experimental TDM system was developed byRCA Laboratories between 1950 and 1953.[1]In 1962, engineers from Bell Labs developed the first D1 channel banks, which combined 24 digitized voice calls over a four-wirecopper trunk between Bellcentral office analogue switches. A channel bank sliced a 1.544 Mbit/s digital signal into 8,000 separateframes, each composed of 24 contiguous bytes. Each byte represented a single telephone call encoded into a constant bit rate signalof 64 kbit/s. Channel banks used the fixed position (temporal alignment) of one byte in the frame to identify the call it belonged to.[2]Time-division multiplexing is used primarily fordigital signals, but may be applied inanalogmultiplexing in which two or moresignals orbit streams are transferred appearing simultaneously as sub-channels in one communication channel, but are physicallytaking turns on the channel. The time domain is divided into several recurrenttime slotsof fixed length, one for each sub-channel. Asample byte or data block of sub-channel 1 is transmitted during time slot 1, sub-channel 2 during time slot 2, etc. One TDMframeconsists of one time slot per sub-channel plus a synchronization channel and sometimes error correction channel before thesynchronization. After the last sub-channel, error correction, and synchronization, the cycle starts all over again with a new frame,starting with the second sample, byte or data block from sub-channel 1, etc.
