[aprssig] 9600? Faster?

Robert Bruninga bruninga at usna.edu
Thu Jul 7 11:37:03 CDT 2005

>>> needhame1 at plateautel.net 07/07/05 10:04 AM >>>
>>I see no advantages at all to 9600 for APRS 
>>(except to use the 9600 baud satellite)...  Bob
>Well -- are we stuck at 1200?  Or should we 
>try something even  faster than 9600?

Why? I have nothing against 9600 baud or faster
for applications that need it.  It would be fantastic
if we could get the conventinoal packet system
up to 9600 baud or better.

But for APRS, transmitting one packet per minute
using existing PLL radios, it just makes no sense.
The actual data is already EQUAL to the typical
key-up time at 1200 baud.  GOing to 9600 baud
with the same key up time makes almost all of
the TX time spent in just KEY-UP delays and gains
you practically nothing (maybe a factor of 2).
But you take a 5 to 1 hit in power and RF performance.

So where is the advantage?   APRS is a very sparse
bursty system so Key-up delays more or less define
the throughput, not data rate.  So why go higher
in data rate and take a performance hit?

Use 9600 baud for bulk traffic, for backbones
for satellites or anywhere where the TX stays
on long enough to make it worthwhile...

Of course, you could design a system that would
do APRS at 9600 baud very well, but it would
take dedicated FAST T/R radios.  Not the garden
variety that everyone tends to try to use...

Again, nothing wrong with 9600 baud, I am 
all for it.  But it has been studied repeatedly
over the 12 years of APRS and no one has
ever seen any advnatage to it...


Use 9600 for bulk data transfer where you can amortize
the Key-up and PLL delays

Earl Needham, KD5XB, Clovis, New Mexico DM84jk

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