On 06/08/13 17:12, Michael E. Fox -
It’s even worse
than that, especially in the real world. A 20% loss
rate will cause enough retransmits that a cascade
failure will occur. In other words, for those 20% that
must be retransmitted, 20% of those will need to be
retransmitted again, and so on. If the link is more
than a single user occasionally checking his BBS for
short messages, the link becomes worthless pretty
quickly. For example, if there are a 3-4 systems on the
frequency, even at only 128 byte packets, the channel
becomes hopelessly clogged in very short order.
10^-4 example shows 10% loss of just 128 byte
packets. Again, way too high for anything more than a
single user talking to a single station. So, 20% loss
is NOT manageable for AX.25. Not even 10%.
Around here in order to test anything I have to buy more radios and
computers, there's no-one else to talk to. That said, I'm
interested in getting more radios :)
When we first
deployed our BBS network, we performed real testing
with 9600 baud TNCs (no FEC). We were getting
somewhere in the range of 5% to 10% packet loss at
9600 baud and 0% packet loss at 1200 baud. (This was
not a lab test. This was between real sites using
real antennas.) Two different TNC brands were used
and, yes, deviation was verified to be per
manufacturer’s specs. As a result, even with the
higher baud rate, the effective throughput was lower
on 9600 than on 1200 baud.
Interesting, thanks for the info. This was using Connected Mode
AX.25 or TCP/IP over Disconnected Mode?
To think that
one could go higher in speed and not make things even
worse, is just not facing reality. In *real*
environments, with more than just one user talking to
one other station at a time, the BER must be much
higher (10^-5 to 10^-6) in order for the channel to
not rapidly degrade due to cascading retransmits. For
any *real* environment, FEC is going to be
Agreed. John Ronan, EI7IG, and I have been experimenting with Delay
Tolerant Networks over ham radio links. There is convergence layer
support for Connected Mode AX.25 (implemented by yours truly) in the
DTN2 reference implementation and a Nack Oriented Reliable Multicast
(NORM) protocol convergence layer too, both of which seem useful.
We've never tried NORM over UDP/IP/UI-Frames on AX.25 yet, but
John's been dabbling with it on D-STAR DD. NORM's packet level
erasure codes work well, but with some modest link layer FEC too I
should think it would work very well.
level for measuring receiver sensitivity in digital
radios is 10^-5 BER. Anyone who is familiar with P25
or DMR testing will be familiar with modulation
fidelity vs. BER. Those systems have error
correction. The modulation fidelity (measure of how
accurately the symbols are being received) can degrade
quite severely while still maintaining a 0% BER.
Without forward error correction, these systems would
be unusable in any real environment.
Sure. This is why i thought quoting the sensitivity for a BER of
10^-3 was a bit odd. I assume that post-correction residual BER
figures for modems with FEC would be used in the sensitivity figures
when presented in the specs.
I’d love to
deploy the UDR56 on our BBSs that share a common
forwarding frequency. But without FEC, there’s just
no way. The result would be predictably terrible.
I hope that any new modems developed for the UDR56k are prototyped
in gnuradio and/or implemented in a userspace soundmodem so that I
can experiment with them. I don't currently have a car and I'm not
too tempted to buy 2 UDR56k units to run at home, but I was
thinking of getting one in the hope that all my old AX.25 kit would
interoperate with it and that I could use my USRP or a soundmodem to
try out any fancy new waveforms.
I wonder what the minimum tx power level achievable with the UDR56k