[aprssig] Setting of the C bit in the AX.25

Cody Harris qbasicer at gmail.com
Fri Oct 23 12:40:19 CDT 2015


Hello all,

I'd like to get some clarification of how the last byte of the source and
destination fields of the AX.25 packet should be handled.

I'm receiving on my TNC the following packet: (decoded)

VE2PCQ-3>APU25N,VE2REH-3*,WIDE2:>222204zDIGI St-Calixte

Hexdump:

82 A0 AA 64 6A 9C E0 AC 8A 64 A0 86 A2 66 AC 8A 64    ...dj....d...f..d
A4 8A 90 E6 AE 92 88 8A 64 40 E1 03 F0 3E 32 32 32    ........d at ...>222
32 30 34 7A 44 49 47 49 20 53 74 2D 43 61 6C 69 78    204zDIGI St-Calix
74 65 0D                                              te.

The destination packet is;

82 A0 AA 64 6A 9C E0                                  ...dj..

E0 is 1110 0000

I don't see the top bit set in every packet, and the AX.25 spec calls this
"C".  Is this normal?  When I digipeat, should I leave that bit set?

Sometimes I see the bottom bit set on the source field:

VE2RUH-3>APRX26:!4529.52NS07541.12W#Securite Civile ve2bcw at raqi.ca

82 A0 A4 B0 64 6C 60 AC 8A 64 A4 AA 90 67 03 F0 21    ....dl`..d...g..!
34 35 32 39 2E 35 32 4E 53 30 37 35 34 31 2E 31 32    4529.52NS07541.12
57 23 53 65 63 75 72 69 74 65 20 43 69 76 69 6C 65    W#Securite Civile
20 76 65 32 62 63 77 40 72 61 71 69 2E 63 61           ve2bcw at raqi.ca

AC 8A 64 A4 AA 90 67                                  ..d...g

The AX25 spec doesn't seem to indicate this as valid.  Should I preserve or
unset bits that seem invalid?  If they're valid, where can I read about how
to handle these and their meaning in APRS?

Thanks for your clarifications,
Cody VE3QBR
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