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

Re: How a Telephone Works

Paul Coxwell (
Fri, 16 Sep 2005 21:07:37 +0100

> 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.

We do indeed. The pair-gain units employed here are commonly known as
DACS (Digital Access Carrier System).

The small settlement in which I live is about 5 miles from the central
office as the wire runs. In recent years many of the houses have been
changed from holiday homes to permanent residences, and as a result
there has been a huge increase in the demand for lines, far in excess
of the spare pairs in the cables which run down the road to the
nearest cabinet distribution point in a village about a mile away. The
result is a proliferation of DACS units atop poles to keep up with the

The problem we have now that ADSL service has just become available is
that it can't be implemented via DACS, so when somebody orders
broadband service it might be necessary to rearrange connections and
put non-ADSL lines onto a DACS to free up a pair for the ADSL

> I believe that the pilot tone the modem sends is defined to tell the
> echo cancellers to go away. Of course, now that phone calls are
> typically digitized at the originating CO and turned back to analog at
> the callee's CO, whether it's across the street or half way around the
> world, how much echo cancelling do we need?

The CCITT standards for data comms employed in Europe specified a
separate guard tone years ago which was to be applied by the answering
modem, and intended -- I believe -- to turn off the echo cancellation
and any other line conditioning.

The legacy of this can be seen in the Hayes command set under the AT&G
option, which provides options of 550 or 1800Hz for the guard tone.


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