[aprssig] "station which heard ... directly on the radio"

Georg Lukas georg at op-co.de
Tue May 21 10:06:18 EDT 2013

* Pete Loveall AE5PL Lists <hamlists at ametx.com> [2013-05-21 15:40]:
> Both the TNC2 and AEA formats do not permit -0 as a SSID [...]

It was really hard to find anything resembling a specification for
TNC-2/AEA. The closest thing I got into my hands is APRS101.pdf, where
it is stated on PDF page 94 (chapter 17):

    "In both formats, the SSID may be omitted if it is –0."

I read it as a permission to use both "DO1GL-0" and "DO1GL", even though
most APRS-IS software breaks on the former one, so I would always
implement the latter form.

> While showing all callsign-SSIDs with an asterisk that have the
> digipeated bit set won't generally break anything, it is also not
> compliant with the TNC2 and AEA formats.

This is true, according to APRS101.pdf. However, providing compliant
behavior when converting AX.25 to APRS-IS requires a two-pass solution
to look for the last digi element with the H-bit set and destroys
information on the way (which of the previos digi H-bits were actually
set?), so a lossless conversion back to AX.25 is not possible any more.

A 1:1 mapping is easier to implement and seems to be accepted by the
relevant applications (like, aprs.fi). Therefore I would actually
propose changing the default behavior to map H-bit of digis 1:1 to the
"*" field. Of course, this shall not affect source or dstcall.


Georg DO1GL
APRSdroid - Open Source APRS Client for Android ++ http://aprsdroid.org/m
     ++ https://market.android.com/details?id=org.aprsdroid.app ++
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