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

Re: How a Telephone Works
14 Sep 2005 07:51:21 -0700 wrote:

> 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.

Since the beginning of telephone service there were various forms of
"multiplexing". First, people only had one wire instead of a pair,
the earth ground was used as the other. Early on people simply shared
the wire pair as party lines. In the 1960s they used "concentrators"
in which a whole neighborhood shared a group of common trunks to the
C.O. Today more sophisticated methods are used.

> 4. Now if a suscriber dials a number, the DTMF tones are resceived at
> the CO which has a directory (databse ???) look up. It finds that this
> number is at antother exchange and sends a SS7 signal to that . From
> there how is the trunk reserved ????

Plenty of people still use pulse. The switch is always scanning lines
for off hook and detects dial pulses the same way. I'm pretty sure
today's switches could handle 20 pulses per second if anyone had a
fast dial.

> 5. Also how is the incoming call from a modem and telephone
> distinguished at the CO. Or does the modem also dial DTMF signals???

The central office makes no distinction between voice calls and plain
dial up computer calls. The whole point of a modem is to "modulate"
-- to convert digital pulses into analog sound signals that can be
transmitted over the telephone line. Now internally at the central
office those analog sound signals are converted into digital pulses
for high capacity transmission through the telephone system network.
At the other end those digital pulses are converted back to analog
sound waves for a person to listen to or a modem to demodulate (modem=
MODulate/DEmodulate). Older modems used to have an acoustic
connection, not electrical, by placing the telephone handset into
suction cups. (Such an arrangement, while slow, would still work
today if you don't mind a 300 baud connection.)

One can wonder if the conversion of computer pusles to analog sound
waves and then back into computer pulses is inefficient. It is.
That's why today we have broadband communications like DSL. In those
cases digital signals are directly sent out in an efficient form, and
as such, more bandwidth can be handled. Keep in mind that the basic
principal of dial-up modems is about 50 years old now and they're
gradually fading from the scene.

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