Time-division multiplexing

[1] In 1953, a 24-channel time-division multiplexer 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.

Thus each of 24 voice calls was encoded into two constant-bit-rate streams of 64 kbit/s (one in each direction), and converted back to conventional analog signals by the complementary equipment on the receiving end of the trunk line.

TDM can be further extended into the time-division multiple access (TDMA) scheme, where several stations connected to the same physical medium, for example sharing the same frequency channel, can communicate.

[5] TDM allows transmitting and receiving telephone switches to create channels (tributaries) within a transmission stream.

[7] Plesiochronous digital hierarchy (PDH) was developed as a standard for multiplexing higher order frames.

This solution worked for a while; however PDH suffered from several inherent drawbacks which ultimately resulted in the development of the Synchronous Digital Hierarchy (SDH).

It was developed to allow streams 1.544 Mbit/s and above to be multiplexed, in order to create larger SDH frames known as Synchronous Transport Modules (STM).

[5][6] While SDH is considered to be a transmission protocol (Layer 1 in the OSI Reference Model), it also performs some switching functions, as stated in the third bullet point requirement listed above.

[5][6] Statistical time-division multiplexing (STDM) is an advanced version of TDM in which both the address of the terminal and the data itself are transmitted together for better routing.

On a 10-Mbit line entering a network, STDM can be used to provide 178 terminals with a dedicated 56k connection (178 * 56k = 9.96 Mb).

In pure TDM, the time slots are recurrent in a fixed order and pre-allocated to the channels, rather than scheduled on a packet-by-packet basis.