[aprssig] Why do we not use APRS 9600?

John Gorkos jgorkos at gmail.com
Wed Feb 16 17:04:15 EST 2022

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

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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