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[aprssig] which digi

Kai Gunter Brandt kai.brandt at hjemme.no
Thu Feb 26 21:30:46 UTC 2009


Scott Miller skrev:
>> If the digipeater callsign are LD5BE you can connect to this changing 
>> your UNPROTO to LD5BE
> 
> So it's just an UNPROTO packet and not an APRS message?

yes.

>> Send a "empty" packet and the TNC responds with a string.
>> Put this string into a calculator and send the result back.
>> If password is ok then you can change everything but the callsign.
> 
> So it's a challenge-response authentication scheme of some sort.  Are 
> the details of the scheme public?

Yes you store a password in the TNC the first time. Based on this you 
get a string and you have to calculate this. Not a very advanced method 
but it's probably enough security. It's a java applet.

> 
>> Upgrading firmware over the air sounds spooky ;o) is there a fallback if 
>>   it fails?
> 
> For the SR1, the firmware is sent in numbered blocks, one packet at a 
> time.  As each block is received (and the packet checksum passes) it 
> gets written to the appropriate place in temporary (in this case, 
> external flash) memory.  Once every block has been sent, a 'write' 
> command provides the start/end address and a master checksum.  If the 
> checksum for the received firmware passes, it stops what it's doing, 
> jumps to the bootloader, and writes everything to internal flash and resets.
> 
> You can take as long as you want to send all of the blocks, and it'll 
> just set aside that 64k block of flash and not use it for anything else 
> while it's keeping firmware there.

ahh interesting and probably very stable. without a system like this it 
would probably be more safe going to to do the upgrade manualy.

> 
> For a proper on-air format, there should be a way for the sending 
> station to request a list of missing blocks.  So you'd sent each block 
> once, expecting no acknowledgment, and then see which ones were missed. 
>   You'd send all of the missing blocks and then query again, until all 
> were received and the checksums matched.
> 
> Scott
> N1VG
> 
> 

use torrent ;o)

The old checksum system used in computer magazines could probably be 
used but i'm not sure how complicated this is. I remember that this 
system could tell me that there was an error in line xx position yy.

But i see what you want and this seems like a better and faster method 
flashing over RF without all the ACK etc.

Kai Gunter
LA3QMA



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