[aprssig] More efficient use of channel capicity through shorter packets
Ron Stordahl *
ron.stordahl at digikey.com
Thu Oct 12 07:48:09 CDT 2006
There has been recent discussion concerning the use of higher baud
rates, shortening packets through shorter TXDelay and a shorter payload
through the use of Mic-E and eliminating extraneous added text.
I would be very much interested in knowing exactly how long (time) APRS
packets are and with that information I could better understand exactly
what could be gained from this approach.
I realize there is both a TXD (at the head of the transmission) and a
TXTail (at the end). Between these is data as required by the
specification. Part of that data is of fixed length and other variable
length. For example as a packet is repeated the path information builds
up (and it does not appear to be insignificant in size). If I
understand the ax.25 spec the path is part of a variable length field.
Then there is the 'payload', which can be large, but could be small if
everyone used Mic-E without extraneous extra text.
Experimentally I have determined that for MFJ1270C's and Motorola
Micor's that between them packets can be reliably decoded with a TXD as
small as 90 milliseconds. This is for relatively strong signals. Below
90 milliseconds reliability drops off rapidly. Since relaying by high
digi's does constitute a major share of the traffic, reducing their TXD
(and TXTail if possible) would seem worthwhile.
With UIDIGI firmware there is no user option to set TXTail, and I do not
actually know what it is hard coded as. Ill take a guess that it's
around 100 milliseconds, but could easily be wrong. Without a way to
control this I have no way to tell experimentally how it's length
effects reliable decoding.
The Motorola Micor's are crystal controlled and work very well. I do
not know if one could run a modern synthesized mobile radio with TDX as
low as 100 milliseconds...my recollection..and it is from many years
ago..was that such radios required a longer TXD for their frequency to
settle down after key down. This may no longer be true.
Or it could be that the TNC itself requires a certain minimum TXD and
I expect someone here who is more current in the technology can explain
the limiting factors in TXD and TXTail and offer numbers which could be
used reliably with current hardware.
Ideally I would like to have a formula into which I could plug TXD,
TXTail as well as the variable length elements in the packet and get the
total channel time for 1200 baud packets. I may be asking for the
nearly impossible here..looking at the AX.25 spec makes my head spin.
But with that it would be possible at least estimate what improvement
could be anticipated by minimizing the elements over which we have some
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