TELECOM Digest OnLine - Sorted: Re: DSL Speed

Re: DSL Speed

W Howard (
Wed, 29 Jun 2005 05:52:27 UTC

In article <>,
<> wrote:

> Choreboy <> wrote:

>>> Each RT is connected to a host CO, and from the point of view of the
>>> customer, it's indistinguishable from the CO. POTS lines served
>>> from the RT are switched at the CO; the RT simply relays signals
>>> back and forth between the customer and the CO. Numbers are part of
>>> the same NPA-NXX blocks as the host CO.

>> Is it indistinguishable if the customer has a V.90 modem? I think
>> I've read that an RT won't allow 56k dialups.

> Some RTs allow V.90 modems; some don't. Apparently, yours does
> (assuming that's the reason the farm can be so far from the CO and
> still get DSL). A good tutorial on this issue is at
> .

Many RTs do not maintain signal phase when multiplexing many signals
onto one pair. Doesn't much matter for voice, but high-speed modems
use phase modulation so it matters a lot for them.

>>> [ more snippage ]

>>> LOAD COILS. The frequency-dependent attenuation characteristics of
>>> the loop (as described above) also affect voice band frequencies
>>> (300-3000 Hz), resulting in rolloff of the higher frequencies of voice
>>> signals. To solve this problem, telcos have traditionally installed
>>> "load coils" at 6000-foot intervals on long (typically >18K feet)
>>> loops. A load coil is a small inductor installed across the
>>> conductors to cancel the affects of interconductor capacitance.
>>> Although load coils reduce high-frequency rolloff within the voice
>>> band, they cause severe attenuation above 4000 Hz. See
>>> .

If the load coil is installed between the conductors, it would cause
low-frequency rolloff, not decrease high-frequency rolloff. This
could flatten the total frequency response curve but also requires
boosting the whole signal to make up for all those rolloffs.

>> At DSL frequencies I would have thought coil impedance would be too
>> high to matter. I don't quite grasp it.

> Any other reader want to tackle this question?

I betcha they aren't plain inductors. For use at those rather low
frequencies, they are probably wound on some iron core, and were
designed to do the right thing for voiceband frequencies. For a long
time, it didn't matter how they behaved at 500 kHz because telco did
not intend to have 500 kHz signals on the wires.

For the curious, current flowing through an iron-core inductor tries
to induce little current loops in the iron. Higher frequencies induce
physically smaller loops. So the iron core is sliced or powdered to
make sure no piece of it is small enough to hold a current loop of the
frequencies of interest -- those current loops are lost energy for the
signal. If the cores were designed to be OK for 4 kHz, they could
absorb most of the energy at 40 or 400 kHz (and convert it to heat).

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