In your case I'd probably set the fq_codel target at 60ms or so, and interval
pretty high as well (700ms RTTs? really?), and enable ecn in both directions
Well, if their satellite link is via a geosynchronous satellite, it is
something like 22,236 miles above the equator [1]. One RTT that way
will be 4x that distance. However, since they are in Antarctica it will
be a bit farther. The diameter of the Earth is something like 7918
miles (Google). So, if we ass-u-me they are actually at the pole, we
can add 1/2 that (3959 miles) to the 22,236 miles and call that one leg
of a right triangle with the right angle at the center of the Earth, the
other leg then is 3959 miles from the center of the Earth to the pole,
and we can use good old Pythagoras to get the length of the hypotenuse,
which will be the distance from the pole to the satellite.
Distance^2 = 3959^2 + (22236 + 3959)^2
Distance^2 = 15673681 + 686178025
Distance^2 = 701851706
Distance = 26492 miles
4x that is just shy of 105969 miles, which when divided by 186,000 miles
per second yields 570 milliseconds. Of course that assumes one is
talking pole to pole which is unlikely, but it does get us a good chunk
of the way to 700 milliseconds.
I think the initial posting said 512 kbit/s for the link speed, so if we
further assume Ye Olde 64 Byte Ping that will be 64 + 20 + 14 bytes or
784 bits. At 512000 bits/s that would seem to be another 1.5
milliseconds for each time it gets transmitted or 6.125 milliseconds all
told taking us to 576. That assumes 512 Kbit/s is after FEC and such
and that this is all store-and-forward. And probably assumes other
things about the transmission but I've no idea what they would be.
If it is actually something like Iridium then it gets more complicated -
there will be some average number of satellite to satellite hops on the
way to/from the Iridium base station.
And of course there is whatever the RTT's are from the satellite base
station to the actual destination(s). I could see that easily adding
another 100 milliseconds but that is just handwaving.
rick jones
[1] http://en.wikipedia.org/wiki/Geostationary_orbit
There are probably some papers out there on Iriduim performance one
could find via a web search
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