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RE: Satellite latency

  • From: Jeb R. Linton
  • Date: Thu Feb 28 11:04:40 2002

Very true, though in Ku band, the figure-8s should lie entirely within a
couple of decibels of peak for the beamwidth of your dish. 

Likewise, the DBS satellites in question (GE-4 I think, but donít quote
me) uses spot beam antennae, so realistically there's a great deal of
what could be serviceable surface area in North America which they may
or may not choose to light up.

I also use "DirecWay" as my main internet link, working from home every
day. When the system isn't buffering, the ping time is about 700ms...
But ping time is a bit funny. During higher load, pings are apparently
buffered more than, say, web cache traffic, so I can see several-second
ping times even when general traffic is flowing well and SSH sessions
are fairly responsive.

Speaking of responsive, this system uses a Web proxy system with
software loaded on the client machine that allows other PCs to take
advantage of it through Windows "Internet Connection Sharing" of the USB
interface; the central proxy pulls down your web page as multiple TCP
sessions and sends it to you as a single, separate session; no end to
end ACKs. Ends up working pretty well.

The only apps where it is really noticable, as others mentioned and as
we would all expect, are SSH/Telnet (which takes some getting used to
can be done), and interactive sessions like Terminal Services,
whiteboarding, shared desktops, gaming etc... These are effectively
unusable.

- JL

-----Original Message-----
From: owner-nanog@merit.edu [mailto:owner-nanog@merit.edu] On Behalf Of
Rowland, Alan D
Sent: Wednesday, February 27, 2002 3:59 PM
To: nanog@merit.edu
Subject: RE: Satellite latency



Not to mention that -5 degree or so look angle (negative elevation) from
the poles being hard to acquire. ;) But with enough elevation and a big
enough dish...

Also about that satellite drift...The earth wobbles during rotation
meaning geo stats do small figure 8s overhead. IIRC there is some
computer software out there (from my days of TVRO in Europe) that given
a latitude/longitude and sat placement would return a time where the sat
was in the middle of that 8 so you could best aim the dish. Or maybe I
just had too much Heineken at the bar, can't remember which.

Just my 2Ę. The delete key is your friend.

-Al

-----Original Message-----
From: Michael Painter [mailto:tvhawaii@shaka.com]
Sent: Wednesday, February 27, 2002 12:22 PM
To: nanog@merit.edu
Subject: Re: Satellite latency




----- 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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