Robert Bonomi wrote:
> In article <firstname.lastname@example.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
> 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
If 56K is the theoretical limit usually given, does this mean only
seven bits are useful to the customer? It seems as if modems
negotiate speeds in increments of 4% or so. I wonder why that is.
> 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_
On dialup, the farm couldn't negotiate modem speeds quite as fast as I
could in town. I assumed the limitation was in the wire. That's why
I was amazed to see that DSL seems to use the wire in the same way as
dialup. Was I wrong to think the reason dialup data rates were slower
at the farm was that the wire to the CO is longer?
> 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
What's the downside for the telco? With the right pricing, I think
they could tap a huge market for increased bandwidth.