[aprssig] Interesting Findings - 300 Baud AX.25 on VHF-FM APRS
Stephen H. Smith
wa8lmf2 at aol.com
Sat Apr 2 16:11:37 CDT 2016
On 4/2/2016 4:22 PM, Ross Whenmouth via aprssig wrote:
> AX.25 does not use forward error correction - typically, the corruption of a
> single bit results in the complete loss of a packet (though some packet
> software will try toggling every bit in a bad packet to see if it can get the
> CRC to match). FX.25 has been proposed as a backwards compatible method of
> adding forward error correction to AX.25 -
> https://en.wikipedia.org/wiki/FX.25_Forward_Error_Correction (some of the links
> at the bottom of that page are broken but archive.org is your friend)
> If you are prepared to break backwards compatibility, then you could get even
> better weak signal performance by wrapping a convolutional coder + interleaver
> around a Reed-Solomon coded packet eg http://www.ka9q.net/ao40/ (using a better
> modem than AFSK helps too).
Of course all this cool stuff would work better --IF-- you are a hardcore
software developer. I view myself as a systems integrator working with
existing devices; I am not a programmer/developer though I do work a lot with
PCs and existing off-the-shelf software.
We have the little issue of what IBM once called (when they failed to displace
the standard PC/AT hardware architecture with their newer PS/2 "Microchannel"
designs in the mid 1980s) the "Tyranny Of The Installed Base".
Every few months, someone on this list proposes radically better coding/error
correction schemes/modulation schemes/faster bit rates, do away with AFSK, etc.
But they fail to take into account that the existing infrastructure of
thousands and thousands of digipeaters, igates and APRS-embedded radios would
have to be replaced to do it.
Perhaps in another decade or so, when virtually all radios are
software/DSP-based, the introduction of new transmission
formats/protocols/modulation formats will be acceptable when they can be
implemented by a minor software download or firmware flash.
Though even here there are problems. The disruptions from implementing "easy
software upgrades" can be massively disruptive in large organizations. Just
over a decade ago, I was involved in the rollout of one of the first
large-scale P25 digital public safety radio networks. Motorola was issuing
software updates to both the trunked repeaters and the fleet of 2000 mobiles on
nearly a weekly basis. The most important tool the field techs carried to the
mountain top radio sites was 3 or 4 flash drives on a lanyard around their
necks containing multiple versions of the radio firmware. The county and city
governments involved were getting really really upset because nearly every cop
was wasting an average of half-a-day every week waiting in line at the radio
shop for the latest flash to his mobile radio and hand-held!
My experiments were to determine what performance improvements could be
achieved, within the confines of the existing AX.25 AFSK standards, by merely
using HF-style 300 baud transmission rather than the customary 1200 baud over
difficult RF paths on VHF.
Stephen H. Smith wa8lmf (at) aol.com
EchoLink: Node # 14400 [Think bottom of the 2-meter band]
Home Page: http://wa8lmf.net
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