[aprssig] D-PRS> D-STAR and APRS Explained

[Rich Garcia]

Yes.. good way of explaining it.

BUT as I learned early on when I first tried D-Star that every time your
radio does key up to send the position report over the gateway other users
will know in a way. Their radio will act just as if someone has "kerchunked"
the repeater, you may not hear the "brapp" of a packet but you will know
someone is out there. Yes you can set up the radio to only send out the GPS
data while you are speaking but how about the users that have little or no
activity on their one and only repeater? If no one is out there to talk to
then you need to remember to key up for a few moments every time you want
the position report to go out.

The origninal "design" of Mic-E was actually a bit better. If built
correctly the MIC-E enabled repeater would re-transmit the packet onto the
APRS channel or directly to the internet but voice users on the repeater
would be unaware/unbothered. Now the APRS user would also need to be sure
that their setup was properly configured to inhibit transmissions during a
voice conversation but hey it was an idea that needed some technical work
just like every new idea that comes to our hobby. I think some regions of
the country did get some good working systems going but it never took off,
so what there are a lot of places in the country that APRS has not taken off
either.

I tried DV for a little bit of time but decided to sell the radio, it was
not my cup of tea. It was a mix of this based on lack of activity, poor
coverage if at all in my area, a single DV repeater but above all when I saw
the way APRS worked over DV and my primary interest is APRS, that was the
final deciding factor.



-----Original Message-----
From: aprssig-bounces at lists.tapr.org
[mailto:aprssig-bounces at lists.tapr.org]On Behalf Of Pete Loveall AE5PL
Lists
Sent: Monday, July 07, 2008 1:34 PM
To: TAPR APRS Mailing List
Subject: [aprssig] D-PRS> D-STAR and APRS Explained


Since there appears to be a prevelant misunderstanding of the D-STAR DV
(digital voice) protocol and how D-PRS works with it, I thought I would take
a different approach and relate it to something that was originally designed
by Bob back in the '90s.  That "something" is Mic-E.  Not the APRS packet
format for Mic-E but how Mic-E was originally presented to work.

The basic concept of Mic-E was to have a single position packet appended to
the end of every voice transmission by use of a standard 1200 bps TNC (with
special circuitry, etc., of course).  If the person was using a voice
repeater with a TNC tied to the repeater receiver, the TNC would see the
position packet and retransmit it on a data frequency or, in later times,
gate it to APRS-IS.  The repeater controller would block the tones from
being retransmitted so other hams on the repeater frequency wouldn't hear
the blap of the AX.25 packet.

This is a one-way use of APRS where the mobile station is tracker, nothing
more.  This is exactly how the Icom D-STAR radios in GPS or GPS-A mode make
use of the digital voice transmission with one big enhancement: instead of
appending a single packet to the end of the voice transmission which is
subject to being dropped due to bit errors, the Icom radios continuously
send the position information while the person is transmitting helping to
ensure at least one position will get through.  This GPS information is
carried with the other control information sharing the voice transmission
but separately encapsulated so the user never sees the other control
information.  The other control information which is carried separately is
information like callsigns, preprogrammed message, etc.

Why wasn't it ever the intent to make Mic-E operation bidirectional (again,
talking about Mic-E functional operation, not the Mic-E format APRS packet)?
Because it would interfere with voice communications on that frequency.
That is the same reason that the "extra bits" accompanying the D-STAR
digital voice transmission do not transform it into a "data" or "APRS
information" channel.  Could Mic-E be bidirectional?  Yes, if the repeater
doesn't block the modem tones.  Does it make sense to do?  No, because
everyone would leave that frequency for one that they can talk on.  Same
goes for D-STAR digital voice repeaters and D-PRS.

Hopefully this will help those who are failing to comprehend the difference
between the serial data information exposed by the Icom D-STAR digital voice
radios and data protocols like AX.25.  And hopefully it will help those who
read the D-PRS information to understand some of the background information
contained in those documents.

73,

Pete Loveall AE5PL
pete at ae5pl dot net




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