TELECOM Digest OnLine - Sorted: Re: DSL Speed

Re: DSL Speed

Robert Bonomi (
Sun, 26 Jun 2005 02:05:23 -0000

In article <>,
Choreboy <> wrote:

> Robert Bonomi wrote:

>> In article <>, Choreboy
>> <> 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.

> Is DSL modulated into some sort of analog signal? It's hard to imagine
> carrying hig-frequency digital pulses on copper telephone lines.

Get thee to a _library_. they have entire books on the subject.

>> 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.

> The farm appears to be 35,000 feet from the central office. My browser
> often shows downloads faster than 1.5 Mb/s (150kB/s).

That which "appears" to be the situation is often not the reality.

There may be a 'remote node' outlying from where you "think" the central
office is. The DSLAM equipment can be located there.

>>>>> 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.

> Some modern phones sound very good. It depends on who's calling.

Some phones are made cheaper than others. <grin>

>>>>> 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.

> Is that between telco facilities?

Or even between the telco and _customer_ facilities that use 'digital
entrance' to the telco.

>>> 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.

> If the telco owns the DSLAM, won't their investment cost depend on
> capacity? If they contract for the DSLAM service, won't they be
> charged according to traffic?

If the telco itself is offering/providing DSL service, then it is
virtually certain that they own the DSLAM equipment. If a third-party
provider is doing the DSL provisioning, then the incumbent telco may,
or _may_not_ have any involvement with the DSL equipment.

As to 'how things are priced/charged-for', that's a whole 'nuther kettle of
fish. Some arrangements are 'flat rate', where you pay a fixed price for
the capacity that is available to you. Pthers are so-called 'burstable'
rates, where you pay based on how much traffic you send.

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