Satellite latency

Wed Feb 27 03:27:54 UTC 2002

In a message written on Tue, Feb 26, 2002 at 09:07:14PM -0500, Steven M. Bellovin wrote:
> Geosynchronous orbit is about 36,000 km from the center of the earth.  
> Round-trip to the satellite is ~72,000 km; the speed of light is 
> 300,000 km/sec.  That works out to 240 milliseconds at the minimum for 
> one-way packet delivery.

Remember that a geosynchronous satellte must orbit the equator.
Let's say for the sake of argument it's over mexico, you're in New
York, and the downlink station is in San Diego.  The 36,000 is the
distance straight "down" to mexico, It's probably more like 50,000
to New York, and 45,000 to San Diego.  And if you're in New York,
and your mail server is in New York, but the downlink was to San
Diego, you've got another 4,000 across country.  Now you're up
closer to 100,000km.

Add to this some inefficient encoding done on satellites, and most
(consumer) systems using a broadcast medium that can buffer packets
and you see why people report 1 second RTT's with services like
StarBand.

It's better than nothing, but it's a rough primary connection.

-- 
       Leo Bicknell - bicknell at ufp.org - CCIE 3440
        PGP keys at http://www.ufp.org/~bicknell/
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