[aprssig] 9600 Baud Packet Network?

Ross Whenmouth ross at topwire.co.nz
Mon Jun 13 05:20:06 EDT 2016


Robert Bruninga wrote;
> 19k2 is there – the modules are $40
Bob, could you please post a link to these modules?

Re: faster than 9k6 without modifying radios
At the penalty of increased sensitivity to noise, modulations more 
complex than K9NG/G3RUH can be fed through the _linear__audio channel_ 
of a pair of 9k6-ready ham transceivers. A 9k6 audio channel provides 5 
kHz bandwidth, more than that of a standard telephone land-line - a 
quick back of the envelope with Shanon's Law indicates that we should be 
able to push bits much faster than 9k6 
Think DSP "TNC" software running in your PC, smartphone, Raspberry pi, 
or in a processor like a PIC32 or STM32, with automatic channel 
equalisation, GMSK/QPSK/n-QAM and Forward Error Correction. Note that 
about twenty years ago you could just go to the computer store and buy a 
consumer modem that would push 33k6 in 3.4 kHz BW down a (then) 40 year 
old telephone line...

Re: Utilising smartphones, tablets, laptops, etc as user interfaces;
Why not use Bluetooth to link your device to the TNC? - it is pretty 
much well universally supported by mobile devices and doesn't chew 
through as much battery power as WiFi
eg http://www.mobilinkd.com/tnc2/

Re: Transferring (larger) files;
Send larger packets so there is less time consumed per unit payload on 
TXdelay, ACK, etc. eg a 1500 byte/12kbit packet takes less than 1.5 
seconds to send at 9k6, and only incurs one TXdelay, where as, if you 
limit yourself to 256 byte AX.25 packets, you need to send 6 of them to 
move 1500 bytes...
Forward Error Correction seems like a good way to reduce packet 
re-transmissions (corruption of a single bit in standard AX.25 results 
in rejection of the packet, requiring a resend) 
New data transfer protocols eg using a "Fountain Code" would allow most 
of the ACK/REJ packets to be eliminated and would be very efficient for 
simultaneously distributing a file to multiple recipients 

Ross Whenmouth
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