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[aprssig] Packet length and effect of TXDelay

Andre v Schaijk aprs at pe1rdw.demon.nl
Thu Dec 23 16:29:24 UTC 2010


Op 23-12-2010 16:35, Ron Stordahl N5IN schreef:
> Can someone estimate the length of a typical compressed position 
> packet such as most mobiles would be sending exclusive of TXDelay and 
> TXTail.  It's the time between these two elements that I need, in 
> order to determine the significance of adjusting TXDelay and TXTail.  
> Typical default values are 300 ms and 50 ms.  Adjustment of the later 
> is usually not offered.
>
> I get reliable operation of our area UIDIGI's with old Motorola Micors 
> at TXD of 100, but I am not sure of the impact upon others users 
> ability to decode, nor am I sure if the reduction in congestion is all 
> that significant.
>
> Ron, AE5E (ex N5IN)
According to the aprs specs the minimum packet length including header 
etc. is 44 bytes add 1 byte for kenwoods you come to 45 bytes from start 
flag to end flag so that would be 0.3 seconds or 300 ms at 1200 baud 
minimum, you can see right away that txd matters a lot, you will find if 
you use a modem with a fast soft dcd or hard dcd rather then using radio 
squelsh it is posible to get even lower even down to 30 ms on modern 
allmode radios but even 50 to 70 ms should be posible on most mobile fm 
rigs providing the recieving radio runs open squelsh

Andre PE1RDW
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