[aprssig] what is RELAY, WIDE, TRACE, etc?

[Larry Cerney]

Jason,

Good questions.  I'll try to answer some of them.

"There is a theory which states that if ever anyone discovers exactly what
the Universe is for and why it is here, it will instantly disappear and be
replaced by something even more bizarre and inexplicable. 
 
There is another theory which states that this has already happened."
 
Douglas Adams (1952-2001)

Assuming I've got all that right, first question: what is the 
difference between RELAY and WIDE? As far as I can tell from the 
standpoint of an end user who is transmitting or receiving there is no 
difference: RELAY,RELAY; RELAY,WIDE; WIDE,RELAY; WIDE,WIDE; and WIDE2-2 
would all get me digipeated twice.  I assume what various digipeaters 
do with these may depend on what kind of digi it is and how it's 
configured?
[Larry Cerney] There is no difference between RELAY and WIDE.  That being
said, there is a difference in how they are used.  One would expect there to
be many more RELAY's than WIDE's.  The reason is if a low powered station
beacons its position and can not be heard by a WIDE, the RELAY stations
would repeat it in hopes that a WIDE will hear.  One low powered station
beacon may produce 3 or 4 repeats by RELAYS depending on the density of APRS
stations.  So, using RELAY,WIDE is asking RELAY stations hearing the
beaconing station to repeat the beacon to a WIDE.  The reverse, WIDE,RELAY
is never used.  If a station beacons its position into a WIDE the WIDE would
repeat it and all RELAY stations would repeat it as well.  So one stations
beacon using WIDE,RELAY may very well generate 15 or 20 repeats of the
original beacon.  Not a good thing.  WIDE,WIDE and WIDE2-2 are pretty much
the same thing.  Both limit the number of repeats of a beacon to two.  The
first WIDE that hears the beacon will repeat it and remove the first WIDE.
A second WIDE will than repeat it again and that will be the end of it.

The digi protocol is all about getting the beacon out to be heard, but still
conserving the bandwidth.  It serves no use for anyone to hear 5 or 10
copies of a single beacon.  

What's wrong with some sort of simple hop count/TTL? (Ignoring the fact 
that it probably breaks the current system.)  Wouldn't this allow digi 
operators to easily reign in users who try to use 8 hops, as well as 
shorten the overall packet length and reduce bandwidth?


[Larry Cerney] I'll let someone else answer with the arguments about this
question.  But you ask about digi operators reigning in users.  We are all
users with no central control.  Getting on APRS isn't like signing up with
an ISP.  We all try to operate for the betterment of APRS.  

Would it be better for the digipeater operator and not the user to 
decide how many times a packet should get repeated, (e.g., default 
"digipeater addresses" field is blank)?

[Larry Cerney] See above.  Digipeater operators are users.
How does TRACE fit into the picture, and how does it compare to WIDE?
[Larry Cerney] TRACE is like WIDE with the exception that the digi adds its
callsign and then decrements the TRACE value. I would put out in response to
a TRACE2-2, K0ANI,TRACE2-1.  With a WIDE2-2 a digi would put out WIDE2-1.

Is there some documentation (book, URL, etc) that describes APRS 
digipeating?  Google gets me bits and pieces, but I can't find the 
whole picture anywhere.

[Larry Cerney] Bob Bruninga put out a good primer on Digipeating.  See
http://web.usna.navy.mil/~bruninga/aprs/fix14439.html

[Larry Cerney] Hope this answer some of your questions.

73....
Larry
K0ANI
Denver Igate


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