[aprssig] Please, standardize UTF-8 for APRS (was: Future Concept for APRS)

[Matti Aarnio]

On Tue, Sep 22, 2009 at 02:20:51PM -0700, Curt, WE7U wrote:
> On Mon, 21 Sep 2009, Robert Bruninga wrote:
>>> One caution - we need to define max
>>> message length in BYTES not characters!
>>
>> I disagree completely.  Designers need to now how big to make
>> displays, and displays display characters, not bytes.  So the
>> message and text length should cleary NOT be in bytes, but in
>> characters.  That was the intent of the original length limits.
>
> Which means a 256 character message could take 256 bytes in ASCII,
> 256, 512, 768, or 1024 bytes in UTF-8 (1 to 4 bytes per char).

Not so simple.   It can take ANY number of bytes in between
256 and 1024.  Characters in US-ASCII subset of UNICODE take one
byte each.  Others take two, three or four.  Standard says that
you MUST use shortest encoding form possible for given codepoint.

Or reversing: A 256 byte frame can contain anything in between 64
and 256 UTF-8 encoded characters.

European (including latin extensions, greek, cyrillic, jiddish) characters
are mostly in the two-byte sequences.  Asiatics are mostly 3 byte sequences,
and only very esoteric ones take 4 bytes.

By the way: APRS specification does not speak of BYTES very much,
everything is always "character".  CCITT/ITU/IETF got the clue of
the dangers this leads to very early on, and speak octets (or bytes)
when meaning really groups of 8 transmitted bits,  and speak of
characters only in cases of data units interpreted as characters
of text.

The 256 octet size limit is very hard ceiling on APRS frame, will
it permit 256 octets of payload, or less?  src+dest+7*via = 9*7
octets (63) for maximum size AX.25 address, plus two bytes of
UI frame tagging, and payload size could be maximum of 191 octets.
Cut away 10 byte message type and destination indication, and 4
bytes for ack sequence: You can have up to 177 bytes of text.

So, is it 256 including AX.25 envelope, or 256 for the AX.25 payload only ?
I can make my systems to send even larger frames, but how many systems
will be able to receive them?

> -- 
> Curt, WE7U.                         <http://www.eskimo.com/~archer>
>    APRS:  Where it's at!                    <http://www.xastir.org>
> The world DOES revolve around me:  I picked the coordinate system!"

Not me.  Reference Origo is here at my desk, but I moved coordinate
system origo 5 000 meters west and south.  Now my estate local grid
coordinates are all positive values.  (Standard Operating Procedure
for most survey teams doing local maps.)

73 de Matti, OH2MQK