Steve Baldwin <PICLIST@MITVMA.MIT.EDU> wrote:

> > Ideas wanted: PIC-to-PIC comms at speeds greater than 1Mbit/s ?
> >
> > The data is in long bursts (typ. 2-3 minutes each 5 minutes).
>
> Have I read this wrong ?
>
> 1Mbit/s for 2-3minutes ? = 22 megabytes of data ?
> Where's a PIC going to get it from ?

Steve:

It's not all coming from one PIC; presumably, each passenger's PIC
(I'm assuming that this project involves networking train passengers
together) simply FORWARDS 21.999 megabytes while contributing only
about 1K of its own information to the data stream.

One-megabit-per-second data rates shouldn't be too difficult
(although personally, I'd feel a LOT more comfortable using something
faster than a PIC), and the bit-error rates can probably be kept
fairly low, BUT...

A halfway decent binary-symmetric channel will have a bit-error rate
of somewhere between 1 in a million and 1 in ten million.

If you send a 1000-byte packet through such a channel, it's likely
to be received with no errors.  However, forcing that packet (along
with 20,000 others) to be transmitted 20,000 times in a round-robin
sort of scheme practically GUARANTEES that the message will be
horribly corrupted.

One possible solution to this problem would be to tie each passenger
in a car (or on a whole train) to a central hub, then to do the
round-robin thing only between trains.  On the one hand, adding hubs
would increase the hardware costs and system complexity; on the other
hand, the system might have a chance of working.

I expect that Michael has already thought about the error-propogation
problems that the round-robin scheme presents.  Since _I_ can't
think of a way (absent concentrators, hubs, and really good
error-correction coding) to make it work reliably, I can only assume
that:

    a) he knows something I don't, or
    b) I've completely misunderstood his description of the
       networking scheme.

Maybe he can give us a few more details.

-Andy

=== Andrew Warren - fastfwd@ix.netcom.com
=== Fast Forward Engineering - Vista, California
=== http://www.geocities.com/SiliconValley/2499