In article <telecom24.276.9@telecom-digest.org>,
Choreboy <choreboyREMOVE@localnet.com> wrote:
> A relative has a farm. His phone service comes in on 700 yards of
> ordinary telephone cable buried along his driveway. Last week he got
> Bellsouth DSL. It comes in on the same conductors as before, but I've
> seen speeds fifty times faster than dialup.
> I thought 56K was the fastest speed possible with conventional
> telephone cable. How can DSL be so much faster with the same old
> cable?
You thought 'wrongly'. <grin>
"56k" is the theoretical maximum speed you can get across a (mostly
analog) POTS service circuit. The limit is not in the wiring, per se,
but in the _equipment_ (the 'switch' in the telco 'central office')
that that signal has to pass through. "Voice" calls, including data
modem, and fax, over POTS PSTN, leave your house as analog signals. at
the telco, the first thing that happens is that they are converted to
a _digital_ data-stream. this conversion is done at a rate of 8000
samples/second., with 8-bits of data 'precision' for each sample.
This means that there is 64,000 bits/second of digital data flowing
through the switch for a voice line. You cannot send more data than
that via _analog_ origin signalling, And, to achieve that 64,000
bit/second, your signalling must exactly match (and be synchronized
with) the intervals used by the analog-to-digital conversion gear in
the C.O. If there is _precisely_one_ analog/digital conversion in the
path, then, with some fancy games on the 'digital' end, you can come
'close' to that 64,000 bit/sec limit, _without_ requiring the exact
synchronization.
The _wire_, itself, is capable of passing a much broader range of
signals. *If* the signal doesn't have to go through the 'voice'
switching equipment, you are not restricted by the limits of _that_
equipment.
This is how DSL works, it bypasses the _voice_ switching gear. It
uses just the 'bare wire' between the telco C.O. and the customer
premises. The special eqipment in the C.O. puts a *different*kind* of
signal on the wires, that the "DSL modem" at the customer premises
understands, and the 'modem' at the customer location does 'something
similar', to communicate back to that special equimpent at the Telco
offices.
Voila! the limitations/restrictions of the telco _voice_ switching
equipment are bypassed, and thus 'not relevant' to this communication.
Robert Bonomi (bonomi@host122.r-bonomi.com)