Michael Coslo wrote:
Dave Oldridge wrote:

Algorithms will only help you by slowing down and using redundancy or
by being frequency agile and thereby clearng off my frequency. And NO
equipment DESIGNED to be a sensitive receiver at the frequencies you
want to use will be immune to even stronger signals picked up by the
open ANTENNA that you're using for a "transmission cable."



Good point, Dave. Any error correction comes with the expense of
time, and of course slows down the transmission of the data. The packet
doesn't jibe, so it is asked for again, and again, and so on. Better be
a heck of a algorithim when the interfering signal is really strong as
to swamp the modem's reciever.

This is only true if you're doing a ARQ type protocol. If you're
expecting interference (and BPL certainly is), a Forward Error
Correction type strategy would be a much better bet. For instance, ECC
memory uses a rate 8/11 code (3 syndrome bits for 8 databits) to correct
any single bit error and detect double bit errors. For BPL, one would
probably choose some form of multiple carrier encoding (since
interferers are likely to be narrow band) with some form of interleaving
(since interferers are bursty), and a fairly robust code.

The whole art and engineering of communication link design comes from
selecting that tradeoff between redundancy and reliability. Do you send
twice as many bits, twice as fast, to get an overall lower data rate
(e.g. a rate 1/2 code).


There are plenty of examples to the contrary of the digital signal
as robust entity. Experiments have been made in which a 5 watt signal in
a car with a mobile antenna will knock out nearby BPL signals. 100 watts
in a car will do even more damage, and a base station yet more. Other
experiments have shown the so called notches being abandoned after the
system was unable to send good packets. Kind of like it was getting
desperate almost.

But this is merely an example of a specific bad implementation of BPL.
There's no physics reason why one couldn't make it work (whether it's
cost effective is another story).


Seeing as how qrp levels into inefficient antennas can cause
problems, I have to wonder what will happen during the next sunspot max.

The signal level received via ionospheric paths are so low, that the BPL
receiver isn't going to have any trouble regardless of how good
propagation is. OTOH, the interference radiated by BPL will spread that
much wider.


The whole BPL affair makes me kind of wonder why shielding was
invented.

And for what? a DSL speed "broadband" digital signal? BPL is a poor
solution to the problems of ten years ago. It is the 8-track of
broadband access.

But it if it provides access to capital markets for the relatively small
number of people working in the BPL industry, so they can get paid their
salaries and bonuses, then it's a good thing for them.

It doesn't actually have to *work* to succesfully employ hundreds of
people and get dozens of people a big bonus. Sure, eventually, it will
fall by the wayside and be abandoned, and all those BPL toilers will go
on to a new technology or job. I'm sure there were folks who made a lot
of money on Betamax cassette design and manufacturing too. (or 8 tracks,
for that matter)

(Oddly, the wayside is about the only place BPL would work.. think of
telemetering data from trains)


- 73 de Mike KB3EIA -