Mark Lloyd wrote:
On 11/15/19 12:47 PM, Ken Blake wrote:

[snip]

Asymmetric broadband makes little
sense nowadays.

I don't know why it's asymmetric. Can you or some else here shed some
light on it?

Could it have something to do with cable being originally designed for
one-to-many communication?


It's spectral planning.

The "bucket" system of dialup modems, is very flexible. If
a cable had a loading coil, and say as a symptom, a single
frequency was "impeded", the system of buckets on dialup
modems allows the other frequencies to carry a message.
If there is a "frequency notch" in the cable, dialup modems
can work around that. (The dialup modem also had a command,
after a session finished, so you could check the bucket amplitudes
and see a bucket that was impeded.)

It's the same with ADSL. It has a lot more frequency buckets
than a dialup modem. And it seems the later standards allow
different partitions between upstream and downstream buckets.

In principle, they could also knock holes ("not emit")
in frequencies used by ham radio operators. In the same
way that HomePlug claims to notch certain bands, so
that ham radio operators can still use the lower frequencies.

The articles on ADSL and VDSL and VDSL2 in Wikipedia, will
have more information.

ADSL was designed to co-exist with POTS, and that's why the
no-truck-roll version (nobody comes to the house to install it),
provides filter plugs so the phone signal will be "clean". The
phone was never designed for out-of-band signals, which is
why a filter helps if ADSL is on the line too. The ADSL
itself, ignores 8KHz down to DC, as not a band it will
be using. And there is a frequency gap between the passband
for voice and the lowest part of ADSL, to make filter design
less demanding and allow the filters to be cheaper.

When I got ADSL2 here, someone came to the house and installed
a "whole-house filter". It turned out to be a plastic assembly,
about 1.5" x 3", to filter the 8KHz range for the phones. The
phones go on one plug hole, the ADSL on the other. The module
could easily have had screw terminals, and allowed a home
installation by the user, but it's not like the phone company
to miss an opportunity to pretend they're providing a service.

Paul