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[aprssig] Careless AX.25 encoding in APRS messages

Randy Love rlove31 at gmail.com
Fri Oct 2 12:43:57 UTC 2009


>On Fri, Oct 2, 2009 at 6:57 AM, Matti Aarnio <oh2mqk at sral.fi> wrote:
> To protect the innocent users, I show TNC2 dump order of SSID
> bytes on SOURCE and on DESTINATION bytes (which is reverse of
> what is really in the AX.25 packet):
>
>  USER>APN391 SSID-bytes: 60,60
>  USER>APND0Z SSID-bytes: e0,60
>  USER>APOT21 SSID-bytes: e0,e0
>  USER>APOTC1 SSID-bytes: f2,e0
>  USER>APX195 SSID-bytes: 60,60
>  USER>APZMDR SSID-bytes: 60,60
>  USER>APZMER SSID-bytes: 70,60
>  USER>BEACON SSID-bytes: 60,60
>  USER>UIDIGI SSID-bytes: 61,80
>  USER>VP1W43 SSID-bytes: f2,60
>  USER>VP1W68 SSID-bytes: f2,60
>  USER>VP1W75 SSID-bytes: f2,60
>  USER>VPQP17 SSID-bytes: e0,60
>

Matti,

The VP* destinations that you are seeing are the latitude portion of
the Mic-E encoding scheme. There are several APRS devices that use
this method to compress the position information. The biggest groups
that use this are all Kenwood APRS-ready radios ( TH-D7,
TM-D700/TM-D710) as well as all TinyTrak 2 or newer TT trackers. ( I
don't recall if the original TinyTrak did Mic-E, it probably did ).
So, in order to determine who is not encoding high bits correctly for
the Mic-E posits, a little more research of the raw packets will be
needed.

However, you have pretty much covered it in your final statement:

>if all MDR and OT users in Helsinki and Tampere areas all the sudden would not
>get their positions relayed to APRSIS or digipeated at all, that would not
>be a very positive thing either.

If these improper by AX.25 spec packets were rejected, the worldwide
APRS network, not just your part of Finland, would come to a
screeching halt. That would break the system, so backward
compatibility would have to be maintained. For the most part, it has
been my observation that APRS and regular AX.25 connected packet
networks are kept separate, therefore the concerns about these SSID
high bits being set incorrectly is not a major concern. Since APRS is
an unconnected mode using UI frame packets, aren't these command flags
ignored anyway?

73,
Randy
WF5X



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