Re: UDR56k-4 Sensitivity & BER

Darren Long <darren.long@...>

On 06/08/13 17:12, Michael E. Fox - N6MEF wrote:
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.


Even your 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 essential. 

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.

The standard 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 is?


Darren, G0HWW

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