On Wed, 2014-10-01 at 19:09 -0400, Allen Mulvey wrote:
> I have been seeing a lot of advertisements for Ethernet over mains power
> lines lately, even some claiming gigabit speeds. Has anyone had any
> experience with these?

Yup, unfortunately not good. And the "gigabit" speeds, even in ideal
conditions are NEVER seen. Expect 10-25% of the speed claims.

> Back in the mid 80s I got hooked on X-10 devices. Eventually my whole hou=
se
> was wired with them. I have removed all but one of them and that one will=
 go
> soon. (To be replaced with a PIC and an nRF24L01+.) Initially, I had
> problems with APC battery backup units. I needed to put emi/rfi filters o=
n
> them to prevent them from jamming the X-10 signals. Now, it seems like
> everything has some kind of switching power supply putting all kinds of
> noise on the lines. X-10 has become so unreliable that I have given up on
> them.

Yes, X10 was never very reliable for me, usable, but only marginally.

> X-10 uses a relatively simple signal injected onto the mains. How can an
> Ethernet signal survive all the noise?=20

They use a few techniques. The biggest contributor is they modulate
using OFDM:

http://en.wikipedia.org/wiki/Orthogonal_frequency-division_multiplexing

Basically, you can think of OFDM as dozens/hundreds/thousands of
separate radio modems running on separate frequencies. Each modem
continuously tracks the quality of the channel it uses and the system
adjusts accordingly.

The benefit is interference tends to be wide band (which can be dealt
with by simply overpowering the noise) and narrow band (which can be
dealt with by ignoring those frequencies that are interfered with).

The result is OFDM based technologies dynamically adjust to interference
as it changes, always using as much usable bandwidth as it can to give
you the most throughput possible.

> What keeps you from connecting with
> the house next door?

Usually distance will prevent connections that far. Anybody on the same
pole pig as you can theoretically receive the your signals though (the
signal usually will not pass through power transformers, power companies
have been adding bypasses to enable PLN technologies for their own
uses).

As a result, these devices support encryption. The technique to add a
device to an existing network usually involves holding a button or
something like that for key exchange.

All that said, my experience with them is VERY poor. The reason is the
electrical code in my area requires all outlets in a bedroom to be
protect by AFCIs:

http://en.wikipedia.org/wiki/Orthogonal_frequency-division_multiplexing

The problem with that is these things severely attenuate the bands used
for PLN, making it pretty much impossible for your signals to go from
one breaker to another if one of those breakers is an AFCI. The result
is NONE of the outlets in any of my bedrooms can see another room. All
of the outlets on my second floor share a circuit with some bedroom
outlets, meaning there is no way for PLN to work from the second floor
to anywhere else (which is what I needed).

GFCI protected circuits will suffer the same problem. Luckily in my home
GFCI protection is always at the outlet, not the breaker, so as long as
I'm upstream from a GFCI outlet it works, poorly.

In the end, I was able to get a link from the basement to the garage,
but it was so poor and unreliable I ended up running a cable.

As for my second floor? I drilled holes to the outside and ran a cable
down to the basement. Benefit of course is now my whole home is gigabit
enabled, a speed you'd never get with PLN gear anyways.

I wish coax cable adaptors were more common. I've got a separate coax
run from the basement to nearly every room in my house, would have been
set. Oh well...

TTYL


--=20
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