TELECOM Digest OnLine - Sorted: Re: How a Telephone Works

Re: How a Telephone Works

Neal McLain (
Thu, 15 Sep 2005 05:34:19 -0600 [TD 24:417] wrote:

> Though I am in the telecommunications field ( software side) I
> am a bit confused about how everything works, though I have a
> high level overview. So I am stating my undestanding, so that
> someone can correct or fill up the gaps.

> 1. Each home suscriber has a twisted copper pair that runs
> from his telephone to a cable containg thousands (why no
> multiplexing here and send it through a single wire??)
> thousands of such pairs; to the local excahnge or the central
> office.

William Warren [TD 24:418] responded:

> There's no multiplexing because it means putting active
> equipment at the end of the wire, and that means the company
> has to build a weather protection enclosure, connect power,
> maintain batteries, and pay for easement(s), maintenance, etc.
> It's more cost-effective to have the pair go back to the CO.,
> at least for most single-family homes.

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 .

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

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

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