Frequency Division Multiplexing, Central Office Trunks, Analog Private Lines, Tie Lines and Direct Inward Dial

Analog comes from the word analogous, which means similar to. In telephone transmission, the signal being transmitted-voice, video, or image-is analogous to the original signal. The only difference between a signal being transmitted and the real thing is that the electrically transmitted signal is at a higher frequency than regular voice.

In correct English usage, analog is meaningless as a word by itself. But in telecommunications, analog means telephone transmission and/or switching that is not digital.

Standard telephone signals have historically been analog transmissions that vary continuously, representing the relative loudness in the speech they transmit. Analog signals can take on any value between rather wide limits.

Frequency Division Multiplexing (FDM)

Within a few decades after the telephone was commercialized, the number of local loops and interoffice trunks increased rapidly. The skies above cities were filled with poles, cross arms, and countless wires. Eventually there was no more room for additional wire.

The invention of the vacuum tube, plus amplifiers and oscillators, enabled telco engineers to develop frequency division multiplexing.

Multiplexing reduced the need for wires. The FDM (Frequency Division Multiplexing) technique assigns each voice channel to a different 4 kHz (4,000 Hertz) frequency band. When added together, 24 voice channels total 96 kHz (96,000 Hertz), well within the capacity of a copper twisted pair.

A solid copper loop can carry a much wider bandwidth signal than the 4 kHz required for a single voice...