>> 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'.
> "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.
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_ > equipment.
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
> communication.
What's the downside for the telco? With the right pricing, I think they could tap a huge market for increased bandwidth.