TELECOM Digest OnLine - Sorted: Re: DSL Speed

Re: DSL Speed

Robert Bonomi (bonomi@host122.r-bonomi.com)
Sat, 25 Jun 2005 02:33:15 -0000

In article <telecom24.288.13@telecom-digest.org>, Choreboy
<choreboyREMOVE@localnet.com> wrote:

> At the farm, it seems to be the wire that limited my dialups to 46k
> when I got 52k in town.

Yes, and no. The particular _type_of_signaling_used_ over that wire
was limited by that wire to 46k.

> If the wire wouldn't carry more than 46k, it wouldn't matter what
> the telco did at their end.

*NOT* exclusively a 'wire' limitation. Also a limitation of the
signalling technology employed. the distributed capacitance of the
wire was such that it 'blurred' the signal such that reconstruction of
the original waveform =after= the *VOICE*GRADE* analog-to-digital
conversion in the CO switch lost the 'fidelity' required for the
higher data rate.

> I wonder how a DSL signal can carry 1.5M through those mile of wire.

DSL uses a different 'signalling technology' for sending the data down
the wire.

The DSL signal does _not_ go through those 'voice-grade'
analog-to-digital converters that PSTN calls do. the signal is
isolated before that point, and dumped into a totally _different_ kind
of receiver.

DSL _does_ suffer 'performance losses', as the wire length gets
greater. The degree of degradation is considerably worse than with
POTS modems. E.g., at 1,000' from the C.O. you may be able to get
several megabits/sec. at 15000 ft, you'll be lucky to get 256k. At
18,000 ft, even 144kbit/sec is iffy. Beyond 25,000 ft, "forget it"
applies -- an analog POTS modem is higher performance.

>>> I have trouble understanding on the phone, and I often resort to
>>> the phonetic alphabet to be understood. I think the problem may be
>>> more in the typical quality of phones than in bandwidth. You could
>>> have broadcast quality microphones and loudspeakers and it will
>>> still sound like a telephone because of the limited bandwidth.
>>> Since bandwidth is limited, telephone components aren't high
>>> fidelity as it would be a waste to make them so. (I believe the
>>> modern "K" handset is clearer than the older "G" handset.)

> Military AM and SSB are limited to 300-3000 Hz. Shortwave radios can
> be filtered that way for tuning and difficult conditions. Speach
> comes across pretty clearly. If telephone voices are harder to
> understand, I think the problem must be something besides the nominal
> bandwidth of a telephone.

The official specification for a voice-grade POTS call is that same
300-3000Hz passband. Modern digital systems deliver a 'high end' of
4000hz. and often have a lower 'low end' as well.

>>> Does a POTS line from the CO to a house carry multiple voices?

>> Depending on the location, often times yes. Between central offices
>> or within the CO almost always yes. I mean if you live across the
>> street from the CO you probably have dedicated copper pair, but you
>> live some distance you probably are multiplexed over a carrier line.
>> The degree of multiplex determines your bandwidth.

> Would you be able to connect with V90 on a multiplexed line?

Only in *very* rare situations.

> As far as capacity goes, I don't know how fast is the digital stream
> for a voice call,

After digitalization, a standard POTS voice-grade call uses 64000 bits/sec.

> but I'm sure DSL at 2.5Mb/s requires much more of the telco's
> capacity.

"Not Exactly" applies here. The DSL signal rides the wires from the
customer premises _to_ the telco switching facility. *BUT* before it
would get to the telco switching gear, it is separated out,
segeregated, and sent to some *entirely*different* equipment -- called
a DSLAM, if you care. Frequently that DSLAM equipment does *NOT*
belong to the telephone company, but to the company providing DSL
services. the 'upstream' connection out of the DSLAM is a dedicated
data circuit -- possibly rented from the telco, but often _also_
supplied by the company that runs the DSLAM. Regardless, it is not
using up any capacity on the Telco's VOICE network.

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