Until the late 1950s the telephone

Until the late 1950s the telephone system consisted almost entirely of analogue transmission lines.
Trunk lines between exchanges carried multiple voice channels simultaneously using frequency division multiplexing (FDM). This entailed the use of expensive modulators, demodulators and filters for each voice channel, and commercial pressures created a need to develop more cost-effective exchange equipment. The digitisation of an analogue voice channel into a 64 kbps digital channel (designated as digital signalling level zero or simply DS0) using pulse code modulation (PCM) made it possible to use time division multiplexing (TDM) to multiplex a number of voice channels onto a trunk line known as a T-carrier in North America and Japan (the European equivalent used in most of the rest of the world is called an E-carrier). T1 lines were originally four-pair copper wire or coaxial cables, but these have largely been superseded by optical fibre.