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[aprssig] APRS Digipeter Design Questions

AE5PL Lists HamLists at ametx.com
Fri Apr 29 16:48:32 UTC 2005


> -----Original Message-----
> From: Bill Vodall
> Posted At: Friday, April 29, 2005 11:22 AM
> Subject: Re: [aprssig] APRS Digipeter Design Questions
> 
> This is a minor issue now.   Lots of "dupes" are showing up 
> on the APRS-IS
> where one packet has a trailing space and one doesn't.

Technically not a dupe (hence your quotes).  The packet without the
trailing space is a mangled packet (the I-Field has been modified either
by a TNC or by software).  javAPRSSrvr will filter out the mangled
packet IF it is received by the server _after_ the packet which has the
trailing space.

The only characters precluded from being in an APRS packet are cr and
lf.  This is because the non-AX.25 networks, such as APRS-IS, use these
characters as line delimiters.  There are also exclusions in the spec
for | and ~ (if I remember correctly) because those characters may be
used by TNC's which are in converse mode.

It is important to note at this point what a digipeater is.  A
digipeater is a AX.25 level (link level) digital repeater.  As such, it
should do no interpretation of what is in the I-Field.  Computing a CRC
or check sum on the I-Field is not interpreting the data, just verifying
it for duplicate checking.  An APRS digipeater which computes CRC for
dupe checking must also be aware of the destination callsign (unproto),
as pointed out earlier, since APRS uses that callsign as part of the
I-Field.  Even in this case, however, the digipeater is not
_interpreting_ the data, just using it for dupe check calculations.

73,

Pete Loveall AE5PL
mailto:pete at ae5pl.net 




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