The TNC HID function and APRS                   20 July 2004
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APRS is being QRM'ed by a proliferation of HID packets which
have no purpose on APRS.  In some cases there are more HID
packets per station than actual APRS packets!  We must fix this.

WHAT ARE HID PACKETS?

All TNC's have a command called HID which enables a special
"ID" packet once every 10 minutes if the TNC is used in
repeater service.  But this is for use on conventional packet
channels for Digipeaters, and Nodes to identify their
presence.  In APRS, this HID function has been replaced by
the much more valuable APRS POSITION packet which not only
identifes the digi, but also, its type, position, elevation
and range.  Thus the HID is supposed to be OFF and remain off.

HOW DID THIS PROBLEM OCCUR?

The original APRSdos included the HID OFF commmand in all
TNC initialization files.  But over the years, other
software has become more prevalent and apparently overlooked
the requirement to suppress the HID packets.  Thus, it is
now incumbent on every user to manually set his HID to OFF
to prevent his station from adding unneeded QRM to the channel.

WHY ARE HID PACKETS SO BAD?

Under benign conditions, a home station normally sends out
only two packets every 30 minutfes, one Position and one Status
packet.  If HID is left turned on in a home station, then this
adds THREE more packets every 30 minutes resulting in 3 out of
every 5 packets per station being nothing but unwanted QRM!
This is just not part of APRS and must be turned off.

It is even worse for DIGIpeaters!  This is because HID packets
are once every 10 minutes AND they go to the full path length
of the TNC's UNPROTO commands.  Although in APRS we DO want to
see at least a local Position ID packet once every 10 minutes,
this process has been specifically engineered to place a minimum
load on the network.  A properly configured APRS digi uses the
four LTexts, four LPaths and four BLT's to specially schedule
these Position ID packets so that it only sends a local (direct)
POSITION ID once every 10 minutes, then a one-HOP POSITION
ID every 30 minutes, and then a 2 hop regional POSITION ID
once every hour.  In an area where digis typically hear four
other digis, this results in 6 local, 8 one-hop and 12 two-hop
packets every hour.  But if HID is on, and the UNPROTO path
is set to 3 hops, then the resulting number of packets is
THREE TIMES GREATER at over 78 HID packets per hour per digi!
For these digipeaters 67% of their packets are HID QRM and
of no value on APRS.

SOLUTION:

Home stations: Set HID off, and add HID OFF to your init files
Authors:       Set HID OFF in all TNC initialization files.
WEB Masters:   Be sure all your "advice" has HID OFF.
Digis:         Set HID off and set LT's, LTP's and BLT's properly
               according to DIGIS.txt.

This is a big problem, but it is easy to fix.
Lets fix it.

de WB4APR, Bob

