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[aprssig] APRS low-power-local ALT input channel

Henk de Groot henk.de.groot at hetnet.nl
Tue Sep 28 18:44:52 UTC 2004


At 19:21 27-9-2004 -0400, nc8q wrote:
>  When I did BBS<>BBS forwarding on a non-user channel. I used
>(256/n)-1 where 'n' is the number of BBSs and nodes sharing the channel.
>If there are only 2 BBSs/nodes they can each use 255 as identical keyup
>collisions would be taken care of by the numbered "I" frames & SLOTTIME.
>3 stations = 127 PERSIST, 4 stations = 84, 5 stations use 63, ...

Looks good to me if the amount of stations includes yourself. I assume the 
2 BBSses talk to eachother in this case, the transmission will nicely 
switch between the two in that case.

Note that your equations do not match what you states before; (256/n)-1. In 
that case 4 would yield (256/4)-1 = 63. I assume "n" in this formula is the 
number of other stations.

>However, IMHO, APRS stations have such little data to send
>and such infrequent transmissions,
>that a PERSISTance of 1 should be adequate.

You mean a setting of the PERSIST parameter to 255 I guess so the 
probability of transmission P is always "1" regardless of the random 
number. Be carefull however, the TH-D7 and TMD-700, which default to a 
persist value of 128 (P = 0.5), have the PERSIST parameter upside down; 
setting PERSIST to 255 will block transmission, setting it to 0 will give a 
probability P = 1.

>However, this may not work with APRS's "UI" frames.  I am not familiar

The PERSIST and SLOTTIME values are channel access parameters, they apply 
to any transmission to access the shared medium, including UI frame 
transmission. Some stations may however still use the depricated DWAIT 
method. Other HDLC parameters like FRACK and RESP do not apply, since they 
only make sense for connection-oriented links. APRS is a connection-less 
protocol, none of the connection-oriented link parameters make any 
difference to the stations behaviour on the RF channel.

Kind regards,

Henk.






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