BER, FEC, channel access and packet sizes

[Glenn Elmore <glenne@hpsadr2.sr.hp.com>]

Rob Janssen,pe1chl wrote (in part):

> I have been running an experiment with link quality measurement in the
> Dutch network over the past 1.5 year.  This is based on sending UI frames

> Our network is almost completely based on HAPN 4800 bps modem links
> running on 23cm with Kenwood TM-531 and homemade Interlink-TRX I
> transceivers, mostly running 1 W in a yagi.  The average link is about
> 60km between moderate-to-high buildings over a flat countryside.
> The experiment was set up because the feeling was that many links were
> asymmetrical in that the quality in one direction was much better than
> the other.  The classical methods (like using PING) only evaluated the
> aggregate performance in both directions.

Rob,
 This is very interesting to me. As you might imagine, I also have spent
a lot of the last few years watching and analyzing link performance. I've
also noted the shortcoming of using PING for these tests and resorted to
using "J/heard" counts from each end of dedicated backbones in order to
compare directions. This works since there aren't 'stray' packets.

> My experience is that the success rate of the above packets is usually
> in the 90-97% range.  Very good links can achieve 100% over short intervals,
> but never for more than a few hours.  Not-so-perfect setups achieve
> in the 80-90% range.

  The higher speed backbone I've been testing is perhaps something of a
corner case in that while the paths are similar length, they are all
fairly carefully selected and very nearly 100% return on 2Kbyte packets
is possible most of the time.  We definitely do have exceptions though.
Part 15 FH devices which occasionally fire local to one of the hilltops
and an infrequent ship-to-air 100Megawatt radar that paddles by in the
Pacific Ocean within LOS range occasionally disrupt things.

> Guessing from the data accumulated until now, I would think that a 2Kbyte
> packet length would result in an unacceptable loss rate.

scaling things, I would guess that a 100 kbyte packet over our stuff
would probably be too long to be optimum (assuming that buffers etc
could handle it of course). So far, I seem to be limited by limits other
than BER/QRM and 'larger is better' seems to prevail. Even so, due
to the variations seen over even these good paths, I'd not expect that
a constant packet size would be optimum.

> Yes that would certainly be an area where improvements can be made.
> However, before a suitable system can be designed there still has to be
> a lot of research to find what exactly causes the loss of frames on a
> link.

I'll say!

> Many links operate much above the noise, and plausible error causes are:
>
> - interference by other band users (RADAR etc)
>
> - clicks introduced by other local transmitters being keyed
>   (a real problem with the synthesized TM-531)
>
> - collisions between the link ends (the links are halfduplex, both sides
>   can decide to transmit at the same time)
>
> The FEC scheme to be implemented needs to be well thought out to cope with
> the kind of errors that really occur.

I heartily agree. I'm pretty certain that the optimum scheme is going to
depend strongly on the particular hardware which in turn depends strongly
on who is building/supporting it. In an amateur environment this last one
is a strong function of cost.

>
> On local access channels, most errors are caused by collisions, but the
> factor of bit-errors because of insufficient S/N also comes into play.
> A better channel access algorithm can probably do more than FEC, but that
> is just a guess based on a lot of tracing on packet channels.

  Current wisdom among some of the wireless communications professionals
seems to be that expecting requiring significantly better than .1% - 1%
*bit* errors from the physical layer for mobile links is not effective
(I believe that Feher, promotes this strongly).
  While I think amateurs *might* stand a chance of improving on this
number somewhat, I'm  pretty sure that use of proper FEC is vital.
  From what I've seen in this region, I tend to think that physical
layer attention might provide even more return and should precede
channel access methods.  Whether these algorithms or FEC should come
second or third, I'm not sure.

  Rob, Along with Phil I would also encourage you to write up your
findings and publish them somewhere.  I would certainly like to have
them.

very 73

Glenn Elmore n6gn

N6GN's Higher Speed Packet WWW Page
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