[aprssig] 9600? Faster?
scott at opentrac.org
Thu Jul 7 11:29:14 CDT 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.
----- 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.
> 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!)
> Gerry Creager -- gerry.creager at tamu.edu
> Texas Mesonet -- AATLT, Texas A&M University Cell: 979.229.5301 Office:
> FAX: 979.847.8578 Pager: 979.228.0173
> Office: 903A Eller Bldg, TAMU, College Station, TX 77843
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