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[aprssig] AX.25 'H' flag

Wes Johnston aprs at kd4rdb.com
Mon Nov 14 12:14:06 UTC 2005


Paccomm (ie TAPR) TNCs treat the H field differently than KPCs do... so
yes, OR...

In my example, I'll use the * to indicate H field...

Some TNCs do this:
digi1*,digi2*,digi3*,digi4

Other TNCs do this:
digi1,digi2,digi3*,digi4


and passing thru a mishmash of digipeaters, you can end up with a packet
like this:
digi1*,digi2,digi3*,digi4

In either of these cases, digi4 should figure out that it's the one
supposed to digi next...  This point drove me nuts when I wrote my kiss
digipeater years ago.  Best to parse packet paths from the END and work
toward the first path element looking for any H flag to be set.  This
also plays well with pre-emptive digipeating.

Wes

scott at opentrac.org wrote:
> Maybe I've been staring at code too long, but I can't make sense of this
> part of the AX.25 spec:
> 
> The H bit is set to "0" on frames going to a repeater. The repeater changes
> the H bit to "1" before it retransmits
> the frame. Stations monitor and repeat frames that meet the following
> conditions:
> 
> a) the frame is addressed to this station in a repeater address subfield;
> b) the H bit in its repeater address subfield is 0; or
> c) all previous H bits are set to one.
> 
> 
> Is that 'or' in B supposed to be there?  Doesn't make sense to me.
> 
> Scott
> N1VG
> 
> 
> 
> 
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