[amsat-bb] Re: ISS Message Board
bruninga at usna.edu
Mon Jul 25 16:26:46 PDT 2011
> If Gordon ... is accurate, then what is
> the purpose of the BBS?
None practically. It is the most inefficient AX.25 packet arrangement
AX.25 connected packet depends on line-by-line ACKS. This works on a
two-way balanced link when both stations hear each other reliably and
without interference. But the Earth to ISS path is anything but balanced.
It is solid ISS to earth, but the other way is probably 10% if that much.
Thus, for each packet delivered, there are 10 RETRIES or more than 20
packets on channel to deliver EACH LINE.
But it is even worse than that. The ISS uses AX25V2L2 which means it does
not even send a RETRY without first sending a "what was your last heard line
number" packet. SO it will not even retry a single line until TWO more
packets have been exchanged. Now multiply that by the 10% channel
performance and it might take 20, 30 or 40 packets to deliver just ONE LINE.
That is why if someone logs onto the BBS, that the pass is more or less
wasted. The logee never gets a successful download due to all the
bazillions of inefficient overhead, and no one else gets to make any
digipeated contacts either.
Just listen to a BBS pass, and its wall to wall packets, but only a line or
two are actually delivered in most cases. The only true use of the BBS is
possible if the ground station is using a few kilowatts of ERP to make sure
that there are no missing ACKS!
Something like that,
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