Early telephone networks were entirely analogue networks end-to-end and carrying only one call per line. However, by mid-20th century the backbone trunks had gradually been converted to Frequency Division Multiplexed and later to Pulse Coded Modulated (PCM) digital Time Division Multiplexed coaxial cable carrier frequency lines, or microwave radio links, and more recently to optical Wavelength Division Multiplexed lines. A simultaneous development naturally took place for the exchanges, which developed from electromechanical devices to analogue electronic switches and then on to fully digital operation.
In spite of the progress of IP based telephony, the overwhelming majority of telephone trunks are still today PCM based digital channels organized into hierarchies. These hierarchies fall into two major groups; the European or CEPT hierarchy and the North American hierarchy. The standard CEPT E1 link consists of 30 voice and 2 signaling channels, each 64 kbps, giving a total bandwidth of 2.048 Mbps whereas the North American T1 requires a bandwidth of 1.544 Mbps for a frame consisting of 24 channels, each 64 kbps, plus one framing bit. The basic PCM channel samples the analogue voice signal 8000 times per second, producing 8000 8-bit words per second. Whereas the CEPT basic channel utilizes all 8 sample bits for voice, the North American channel utilizes one bit for signaling (“robbed bit” signaling). Two different algorithms are used for sampling, A law and mu law.
Large organizations use E1 or T1 subscriber loops to connect their PBX (Private Branch Exchanges) to a PSTN central office.
The line interface is either two coaxial lines or two balanced pairs, one for each direction.