[aprssig] Why do we not use APRS 9600?

[Scott Miller]

If you were going to build a whole new system from scratch and set some 
minimum requirements, 9600 baud would be a good idea.

Several years ago I built a batch of magnetic mount trackers for 
ambulances and support vehicles at Burning Man. We had two dozen 9600 
baud trackers on a single channel with a network cycle time of 6 
seconds, and each tracker transmitted every position packet twice for a 
total of 8 packets per second. They were all received by a single 
TM-D710. The transmitters were half-watt modules on the 70cm band and I 
think they were maybe $20 each. They worked well and we could track 
ambulances until they disappeared into the hills about 15 miles away on 
their way to Reno.

Higher power radios tend to not have such a fast TXD though.

Scott
N1VG

On 2/16/2022 2:04 PM, John Gorkos wrote:
>
> The simple answer is "It doesn't really buy us anything, and it 
> decreases the range of the transmission."
> The majority of on-air time for a TNC/Radio combination is taken up 
> waiting for the radio to key up and reach full transmit power.  This 
> is the "txdelay" setting on your TNC, and is generally tuned to your 
> radio.  It defines the time between when the hardware closes the PTT 
> button and begins to actually send modulated audio.  Typical values 
> range from 150-400 milliseconds.  Regardless of the data rate, this 
> txdelay is constant.  The actual data transmission is generally a 
> fraction of the time we spend just waiting for the radio to key up.
>
> So, even though you;'re sending data ~4x faster, you're still waiting 
> for the radio to key up, and the channel is useless while that's 
> happening.  If it takes 250ms to transmit at 1200bps, and you've got 
> 250ms of txdelay, you're tying up the channel for ~500ms.  At 9600bps, 
> you're on air for ~340ms. Also, bandwidth and distance are roughly 
> inverse:  quadruple the data rate, cut your transmission distance by 4x.
>
> In the end, it's generally not worth it.
>
> John Gorkos
> AB0OO
>
> On 2/16/22 13:36, Jonathan Delaney wrote:
>> The discussion this past week has made me wonder on why APRS 9600 is 
>> not used. To my knowledge most if not all APRS mobile/handheld 
>> enabled radios have a TNC capable of 9600 bps however we still set 
>> them to 1200. Is this because most of the digis and igates still use 
>> 1200? What would it take to get everyone to change to 9600?
>>
>> Jon KB3OSP
>>
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