Media for Re: How a Telephone Works

Re: How a Telephone Works

William Warren [TD 24:418] responded:

Furthermore, the telco has to apply DC bias voltage and AC ring voltage across nthe loop.

The typical bias voltage for loop-start lines in North America is: Tip = ground Ring = -48 volts (approximately)

This voltage causes a direct current to flow in the loop. Originally this current was needed to operate the old carbon "transmitters" (microphones); more recent electronic telephones use other types of microphones (e.g. electret), but DC is still required to operate the electronic circuitry. The loop current is also used for signaling functions such as on-hook/off-hook status, call supervision, and rotary dial pulses.

The minimum loop current for proper operation is about 23 ma. [1] The maximum permissible loop current is 120 ma., but currents far below that value can cause problems with some terminal equipment. Mike Sandman Enterprises has a comprehensive article about all this at

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The typical ring voltage is 90 volts at 20 Hz applied across ring and tip, but variations in voltage and frequency exist.

Even ground-start PBX trunks carry DC loop current and AC ring voltage. Although loop current isn't needed to operate the PBX (which presumably has its own source of power), it's still needed for signaling.

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It is indeed possible to multiplex many POTS lines onto some other medium; e.g., copper pairs, coax, fiber, or microwave. But no matter what medium is used, there still has to be some sort of equipment at the far end to convert the multiplexed signals into individual POTS lines. This equipment has to apply DC bias voltage and AC ring voltage on each line, and deal with the signaling functions associated with loop current.

Here in the USA, we call these systems "pair gain." There are many types of pair-gain equipment in use, but the most common in current use is "Digital Loop Carrier" (DLC); I assume you have something similar in the UK. The simplest DLC uses a T1 carrying 24 voice channels, often on two copper pairs, at 1.544 Mbps. The European equivalent is the E1 which (as I understand it) carries 30 voice channels at 2.048 Mbps.

[1] John L. Pike et al. Understanding Telephone Electronics. Dallas: Texas Instruments, 1983. Table 1-6, p. 1-35.

Neal McLain