Order Tray | Contact Us | Home | SIG Lists

[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:

Other TNCs do this:

and passing thru a mishmash of digipeaters, you can end up with a packet
like this:

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.


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
> _______________________________________________
> aprssig mailing list
> aprssig at lists.tapr.org
> https://lists.tapr.org/cgi-bin/mailman/listinfo/aprssig

More information about the aprssig mailing list