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
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
I'm having difficulty discerning the maximum data rate for
Pactor-3 and Vara and I was wondering if the experts on the
reflector could clarify?
According to the developer, Vara has a maximum, uncompressed
data rate of 5,629 bps while Pactor-3 has a maximum
uncompressed data rate of 2,722 bps. SCS says the maximum compressed
data rate is 5,200 bps.
If my interpenetration is correct, then it appears that Vara
is twice as fast as Pactor-3 at top speed. If one assumes each
mode uses the same data compression technique, then Vara would
still be twice as fast.
Assuming this is true, it's quite and achievement. The
question is how often Vara can achieve top-speed under average
HF channel conditions. My guess is that the modem uses
techniques to reduce PAPR, but it may be difficult to reach
the required SNR to get near top speed very often.