Re: Vara Vs. Pactor-III - Maximum Data Speed
First forget about compression mechanisms. The same compression
mechanism will work with all protocols as long as the protocol can
send true binary data (no special codes) The amount of
compression is highly dependent on the entropy of the source
data. In Winlink for example ALL protocols are sent using the
standard FBB B1 compression mechanism (used since the 1980's)
packaged in a B2 envelope that allows multiple files per message.
This is independent of protocol used.
The theoretical maximum data rate of a protocol itself doesn't mean much.....E.g. it is straightforward to develop a high throughput mode (e.g. 256QAM that sends one byte/symbol)l but unless the signal is strong and the propagation has minimal fading and minimal multpath it will not be usable in practice. So the best way to compare modes or protocols is this:
1) Use a HF channel simulator that can model specific S:N across
a 3 KHz bandswidth. The simulator is statistically repeatable so
allows measurements to be duplicated and confirmed INDEPENDENT of
changing HF conditions.
2) The simulator must be able to model the important standard
channels used in HF propagation modeling. E.g. Multipath quiet,
multipath moderate, multipath severe. These are well documented
and used in industry.
3) Run a real ARQ connection using large blocks of typical data
using ARQ sessions of at least 5 minutes duration. over the
standard HF channels above at S:N (3KHz) levels of -5 to +20 dB.
Compute the NET (after all repeats, mode shifts etc) ARQ
throughput. Repeat the measurements using longer times if
necessary until repeatable results (throughput vs channel type @
given S:N) is obtained.
On HF transmission it is rare to see channels much better than 10-15 dB S:N and multipath quiet. More often we have signals of 0 to 5 dB S:N and multipath moderate or worse. Real world performance prediction is to use these more typical conditions to evaluate and compare protocols.
I have a MSEE degree in Computer and Communications have been
working with HF protocols now for over 20 years and have come to
understand that casual over the air comparisons or comparing
protocols based on just their specs alone is not usually
productive and not repeatable.
VARA does look to be a promising protocol and newer high symbol
rate modes like P4 and STANAG /Mil STD 188 variations can achieve
high throughput in challenging conditions. What we should be
doing is pushing for using updated rules (like regulations by
protocol bandwidth and not symbol rate/carrier) which will
encourage more DSP development for faster and more robust HF and
VHF data transmissions.
73, Rick Muething, KN6KB
On 2/23/2018 3:02 PM, Anthony Bombardiere wrote: