Darren Long <darren.long@...>
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