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[aprssig] A few clarifications for a club APRS presentation, Please

Steve Noskowicz noskosteve at yahoo.com
Sat May 24 06:02:23 UTC 2008


  Whomever wants to answer...

I'm giving an APRS overview at the local club and want to test my understanding
of a few things and get some clarifications.  The AX.25 specs go to the bit
level and it isn't a good tool to provide an understanding of the higher
layer(s).  I don't need a ultra detailed, in depth explanation of all the
specifics.  I just need simplified fundamentals of the higher layers of the
protocol, in the areas I am asking about.

  I have, and will use, Bob's PowerPoint.  

PLEASE keep answers short.  If I don't understand or want more, I'll ask for
it.  Its an overview.

I'm sure there are many exceptions or additional important details, but I'm
just looking for the "first approximation" or the "high level" of what's going
on in APRS.

I also want to give some comparisons of "standard Packet" and APRS and I know
very little about standard Packet, but have semi-indirectly absorbed some
concepts over time. 

SO... Correct me if any of this is grossly wrong.

Regular Packet:
1 - Must address specific stations.
2 – Must also specify specific digis.
3 – Must specify the complete path (*all* station/digi names) to reach a
distant station.
4 – Must "connect" (verify with a two way exchange) with a station to transfer
traffic.

APRS:
1 – Transmits (generally) to any APRS station  (but can address specific
stations  - including EMAIL and WHO-IS & WHO-15).  An address gets you to
anyone (everyone) with that address.

2 – Uses what can be thought of as generic digi names, called aliases.  This
allows a packet to go through any digi since 'all' digis have the same name
(call sign), right?  
Going a little deeper, we can actually specify several station names (aliases)
to allow for Fill (WIDE1-1) and Wide area (WIDE2-2) digis.
This is sort of a  "Flex-path", since the Fill sends to the Wide – but the Wide
also has a Fill alias, in case you get to him first.

2A – Read this whole section before answering parts...  Is it specific to APRS
software that allows me to receive anyone's Beacon?  After all, it isn't
addressed to me.  OR is the concept of a Beacon also in Packet (like a
broadcast bulletin addressed to anyone)?  
I know I can listen to a packet cluster station without being connected – all
"I" have to do is decode the packets and the cluster doesn't need to know I am
listening.  [[It seems that I could also do this to a standard Packet system,
given capable software]]   My D700 has a place to put which of these general
purpose messages I will receive, Right?  This appears to be the "Unprotocol"
parameter...and... the "APxxx" appears to be what allows me to decode and
display any other "APxxx" group packet.  I think this can be interpreted as
putting MY RADIO in the "AP" Group and, thus, allows me to receive all other
"AP" packets.    Does AP stand for anything?

Reading the D700 Special Comm Manual says that there are "group Codes" as well
as some "Others", such as "CQ", "QST", and "Beacon", but I suspect that these
are handled the same and therefore are just other Groups.  All Group members
can receive messages to the group.  
It this a good interpretation?

One confusion is that (on the D700) there are also a "Group Code" and "BNL
(bulletin) Code" parameters.   Is the Unproto group sort of an APRS Super Group
and the others sub groups, within APRS? 

Unnumbered Information (UI) frames and unproto seem to refer to the same thing.
  I believe standard Packet packets are numbered since a message may require
more than one packet.   Is this correct?  Since I suspect APRS is all single
packets  we use Unnumbered frames, because they are more flexible (have fewer
restrictions), so to speak...?

Does "Unnumbered" actually mean "unconnected".



3 – The actual "Path" in APRS is "determined by" the network (the old saw:
don't need knowledge of the network).  That is, digis aren't needed, but the
Digis re-send and others pick up and re-send (to the hop limit), hopefully, up
to the iGate.  

An iGate may or may not be a digi, Right?

4 – APRS doesn't "connect".  It just transmits and crosses its fingers (lets
ignore a directed message ACK.)

I think that's it.

One more.   If my packet makes it to two iGates, the APRS-IS sorts out dups?





-- 
73, Steve, K9DCI


      




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