Re: Vara Vs. Pactor-III - Maximum Data Speed


Tony
 

Rick:

I wasn't sure if data compression was independent of the protocol so thanks for clearing that up. It would be wonderful to have a path simulator capable of handling ARQ modes. But as Andy mentioned, the only ones I've worked with are software simulators which do not have that capability.

I suspect latency might be a problem with a software-based simulators, especially with Pactor modems that have fast Tx/Rx switching. I believe you've tested Pactor modems with a hardware simulator? Wonder if you could elaborate on that.

In the meantime, most of us can only run on-air comparisons taking the results with a BIG grain of salt.

Thanks Rick.

Tony -K2MO


On 2/23/2018 3:59 PM, Rick Muething wrote:

Tony,

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:

 All:

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.

Tony -K2MO



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