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Re: Satellite latency

  • From: Michael Painter
  • Date: Wed Feb 27 15:22:01 2002


----- Original Message -----
From: "Leo Bicknell" <bicknell@ufp.org>
To: "Steven M. Bellovin" <smb@research.att.com>
Cc: "Tim Devries" <zsolutions@cogeco.ca>; <nanog@merit.edu>
Sent: Tuesday, February 26, 2002 5:27 PM
Subject: Re: Satellite latency

>>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.<<


Not wanting to get picky about ~20,000 km., but the maximum -usable- slant
path is ~41,000 km.

--Michael




>
> 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@ufp.org - CCIE 3440
>         PGP keys at http://www.ufp.org/~bicknell/
> Read TMBG List - tmbg-list-request@tmbg.org, www.tmbg.org
>





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