[aprssig] Poor Object Performance for Events
bruninga at usna.edu
Thu Nov 25 23:06:11 CST 2004
This email describes the differences in OBJECT performance
during an event between software that uses the original decaying
APRS algoirthm and some follow-on clones that use a simplistic
fixed rate beaconing . (A 50% reliability channel is assumed):
Original APRS Decay method:
Latency averages 15 sec, average packet rate 6 to 10 per hour.
Simplistic Fixed beacon rate method:
Latency averages 20 minutes at a packet rate of 6 per hour
Latency averages 12 minutes at a packet rate of 10 per hour
Latency averages 4 minutes at a packet rate of 30 per hour
1) For the Decay Method, there are *no* user settings. It
always works, it always gives very short latency under
all applications, both immediate and routine. AND it also
averages fewer packets in the long term too.
2) Fixed rate systems depend on intelligent User settings
depending on application, number of users, type of net
and channel loading. And the disasterous results of
poor settings and user errors can do an order of magnitude's
worth of damage to communications reliability.
3) ANY attempt to reduce latency in the fixed method has an
exactly opposite degrading effect on channel load.
There is just not one single advantage to Fixed Rate beaconing
systems on APRS other than programmer simplicity.
As a 24/7 or Internet User, you may never have experienced the
poor performance of these fixed rate systems in the field, but
an average latency of 12 to 20 minutes to see a new, or moved
object is unsatisfactory for any real-time event. Any attempt to
reduce this poor latency by increasing the OBJECT beacon
rate has disasterous effects on the reliability of the channel.
Remember, APRS was designed for tactical-real-time evnts
for the rapid dissimination of usable data about everything
at an event. A latency of even a few minutes is not acceptible.
For the purpose of analysis, a fixed 10 minute Object Beacon
rate was assumed and a channel collision rate of 50%. To
account for the variable rate of the decay method, a total
of 16 objects was assumed with 1/4th not moving during a
4 hour event, 1/4th moving twice, 1/4th moving 4 times and
1/4th moving 8 times.
You should think twice or carefully plan how you will use your
software in the field. If you need to keep track of lots of
objects (Troops at a camporee, or assets at a marathon) you may
need to consider other software just for the objects or suffer
an order of magnitude worse channel performance.
Fixed rate beaconing which does not distinguish new relevant
info from old stale info has no place on a viable real-time
de WB4APR, Bob
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