Re: UDR56k-4 Sensitivity & BER

Bryan Hoyer <bhhoyer@...>

Hi Darren,

You are quite right. 10-3 is basically where AX25 falls apart as 1 error in 1000 prevents a 128 byte packet from ever getting through. So in a sense it is the limit of a usable channel for AX.25.

Those numbers are both preliminary and conservative. We will publish Eb/No curves after final characterization.

On the other hand  even a little FEC would make this a usable channel.

We will be releasing our plans for future protocols at DCC in September.

Bryan K7UDR

On Aug 6, 2013, at 6:12 AM, Darren Long <darren.long@...> wrote:


Hi all.

I was just reading the datasheet for the UDR56k-4 and noticed the receiver sensitivity figures:

Receiver Sensitivity, BER 10-3
•    4k8 -113 dBm
•    9k6 -110 dBm
•    56k -100 dBm

That BER of 10^-3 is pretty severe. According to my calcs in octave:

octave:10> ber = 10^-3
ber =  0.0010000
octave:11> ploss = (1-(1-ber)^(128*8))
ploss =  0.64103
octave:12> ploss = (1-(1-ber)^(256*8))
ploss =  0.87114
octave:13> ploss = (1-(1-ber)^(1500*8))
ploss =  0.99999

That's a 64% packet loss rate for 128 byte frames, 87% for 256 byte frames and almost 100% for a typical TCP/IP MTU.   Not a particularly good reference for planning link budgets, I wouldn't have thought.

A BER of 10^-4 looks to be a more useful baseline for a sensitivity figure:

octave:19> ber =  0.00010000
ber =  1.0000e-04
octave:20> ploss = (1-(1-ber)^(128*8))
ploss =  0.097336
octave:21> ploss = (1-(1-ber)^(256*8))
ploss =  0.18520
octave:22> ploss = (1-(1-ber)^(1500*8))
ploss =  0.69882

A ~20% loss rate would be manageable for say a reliable AX.25 connection mode link, but would kill TCP performance over an unreliable link.  NORM would cope OK, I've regularly seen it do well with >50% loss rates. 

I suppose the BER/(Eb/N0) curves for the modems in question would show how much margin one needed to get up to a BER of 10^e-4.  Would it be more useful to have the sensitivity figures quoted for a better BER?

What do you think?


Darren Long, G0HWW

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