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Re: Satellite latency

  • From: Jim Mercer
  • Date: Wed Feb 27 09:19:36 2002

On Tue, Feb 26, 2002 at 09:07:14PM -0500, Steven M. Bellovin wrote:
> In message <014d01c1bf3b$44bfca00$ea9a8d18@evilinc>, "Tim Devries" writes:
> >I think this question may have been asked before, but what is the minimum
> >latency and delay I can expect from a satellite connection?  What kind of
> >delay have others seen in a working situation?  What factors should be
> >considered in end to end connectivity architecture when utilizing a
> >satellite link?
> 
> 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.

in my experience, the "normal" latency is somewhere around 600-800 ms
(ping times).

however, you should also be aware of issues related to TCP window sizes.

due to the windowing mechanism between the sending system (say a web server
in a farm connected with multiple OC192 connections) and the receiving
system (say a PC connected to a broadband infrastructure like @home), an
individual TCP session (like SMTP) will reach a limit of throughput which
means that even with a DS3 satelite connection, an individual TCP session won't
see more than something like 600Kbps throughput (i forget the actual number)
if there is a satelite connection somewhere in the middle.

one time a client was complaining that they couldn't do 2Mbit ftp's on their
E1 satelite connection.  they were under the impression that the link itself
was being throttled.

in order to demonstrate that the link itself could do 2mbit, i set up 10 or
20 concurrent ssh scp's, and then showed that the interface was doing an
aggregate of 2mbit (well, a bit shy of that).

if you adjust the window size on the sending and receiving systems, you can
improve this, but this solution is impractical, as you would need to get
everyone on the internet (or at least all of the webservers and websurfers
you are servicing) to make adjustments to their local TCP stack.

there are 3rd party solutions which can improve the throughput, but even
with those, there are still speed of light issues which will cause individual
throughput limitations.

-- 
[ Jim Mercer        jim@reptiles.org         +1 416 410-5633 ]
[          I want to live forever, or die trying.            ]




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