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[aprssig] 9600? Faster?

Scott Miller scott at opentrac.org
Thu Jul 7 16:29:14 UTC 2005


Would there be any improvement from running at 4800 baud?  It's still 4 
times faster than the existing system, and shouldn't push the envelope as 
much as far as the radios are concerned.  That's plenty of extra capacity 
for robust FEC.

Scott
N1VG

----- Original Message ----- 
From: "Gerry Creager" <gerry.creager at tamu.edu>
To: "TAPR APRS Mailing List" <aprssig at lists.tapr.org>
Sent: Thursday, July 07, 2005 9:18 AM
Subject: Re: [aprssig] 9600? Faster?


> 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
> FAX:  979.847.8578 Pager:  979.228.0173
> Office: 903A Eller Bldg, TAMU, College Station, TX 77843
>
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