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

Re: How a Telephone Works
Wed, 14 Sep 2005 16:21:25 EDT

On 14 Sep 2005 07:51:21 -0700 wrote:

> 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.

In about 1957 in Austin I saw a demonstration of a concentrator
intended to be pole mounted. As I recall, it served 100 subscriber
lines and had 10 trunks to the C.O., but that ratio may have been
different or even variable.

It was a tiny crossbar switch, made by a Swiss company, I
believe, and the components (and the whole switching assmbly) looked
like a piece of jewelry.

Of course, a matching control unit was required in the C.O.

I believe it was indeed practical and many were placed in service,
particularly suited to a new addition at some distance from the C.O.

As Lisa says, more sophisticated methods followed ... there are many
"pair gain" systems in use today, some concentrators, some
multiplexing in electronic ways.

Whether to reinforce the cables to the C.O. and extend copper pairs to
customer premises or use some form of pair gain system depends on the
type of customer traffic and is a study in engineering economics.

Wes Leatherock

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