[aprssig] 9600? Faster?

[Gerry Creager]

Back in the dark ages, we did some tests in the Houston, TX area with 
G3RUH modems at 9600 and 19200 baud.  We found that using conventional 
voice radios with full narrow-band filters in place, optimized for voice 
operations, caused problems.

Not wanting to restart the firestorm about eliminating the 
preemphasis/deemphasis networks for TNC connections, and hence direct 
connections to discriminators for receive data, our findings were that 
both data rates tested performed much better with emphasis network 
removal.  Period.

I suspect all of Tim's comments are on-track.

gerry

Tim Cailloux wrote:
> Earl Needham wrote:
> 
>>         Well -- are we stuck at 1200?  Or should we try something even 
>> faster than 9600?
> 
> 
> I work on mobile data for a career, and 19200bps is about the limit of 
> what you can do with one receiver and still get decent range in a land 
> mobile environment, with a FEC optimized for mobile data.  Faster speeds 
> require some sort of diversity receive.  It comes down to a decision 
> between capacity and coverage, and it is always a trade-off.  9600bps 
> buys you more users than 1200bps, but the radios have to be set up to do 
> it to gain any advantage.
> 
> I think someone on the list posted that the D700s run a 250ms TX on 
> delay (or some large number N, where N >>> time to transmit the data at 
> 1200bps).  Why run 9600bps when the speed increase on a per-transmission 
> basis is negligible compared to the overall packet transmission time?  
> Running faster data speeds will require some optimization of the 
> radio-modem interface and careful selection of the radios used in the 
> environment.  Even among the land mobile voice radios, the fastest a lot 
> of radios could reasonably go is less than 9600bps.  If amateur 
> transceivers are designed around voice and 1200/2400bps packet, I 
> wouldn't say it's reasonable to expect all that many to do 9600bps out 
> of the box, regardless of the presence of a 9600bps pin on the accessory 
> jack of the radio.
> 
> On a smaller scale, I certainly think that users can build a 9600bps or 
> faster setup (and I enjoy seeing it done), but large scale the chances 
> of higher speed seem limited.
> 
> (I only wish I could retrofit my company's 43.2Kbps stuff for APRS and 
> amateur data!)
> 
> tim
> 

-- 
Gerry Creager -- gerry.creager at tamu.edu
Texas Mesonet -- AATLT, Texas A&M University	
Cell: 979.229.5301 Office: 979.458.4020
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