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 19th century, this is us time slot found its most common application in digital telephony in the second half of the 20th century. 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 four-wire copper trunk between Bell central office analogue switches. A channel bank sliced a 1. 8,000 separate frames, each composed of 24 contiguous bytes.
Time-division multiplexing is used primarily for digital signals, but may be applied in analog multiplexing in which two or more signals or bit streams are transferred appearing simultaneously as sub-channels in one communication channel, but are physically taking turns on the channel. The time domain is divided into several recurrent time slots of fixed length, one for each sub-channel.
A sample byte or data block of sub-channel 1 is transmitted during time slot 1, sub-channel 2 during time slot 2, etc. One TDM frame consists of one time slot per sub-channel plus a synchronization channel and sometimes error correction channel before the synchronization.
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. Each voice time slot in the TDM frame is called a channel.
Multiplexing more than 24 or 30 digital voice channels is called higher order multiplexing. Higher order multiplexing is accomplished by multiplexing the standard TDM frames. For example, a European 120 channel TDM frame is formed by multiplexing four standard 30 channel TDM frames. PDH created larger numbers of channels by multiplexing the standard Europeans 30 channel TDM frames.