[ILUG] [ot] broadband satellite access

[Paul Jakma]

On Mon, 16 Dec 2002, kevin lyda wrote:

> latency seems to be an issue.  i've emailed a few users to ask
> about that.

do the basic math. packets have to go up to orbit and back down to
you, which takes time:

if you get your signal from a mesh of low-earth orbit satellites (320 
to 800km altitude apparently):

	(320*1000/(3*10^8))*2 = .0021
	(800*1000/(3*10^8))*2 = .0053

5 milliseconds, not too bad.

However, LEO satellites tend to whizz past. So more likely you'd be 
pointing your dish at a geo-stationary satellite, which sit at about 
35000km above earths surface. if we ignore lateral displacement (ie 
distance ireland is from the spot on earths surface to which sat is 
overhead):

	(35000*1000/(3*10^8))*2 = .2333

so you have quarter of second of latency in your internet connection
alone. And that's just in one direction. Ie if the arrangement is
satellite downstream, modem/something else upstream, then above might
be alright.

However, if you have satellite as both your up and downstream, then 
RTT to /just/ your satellite ISP will be .4666s.

So one-way satellite might just be ok (still about 7 times higher
latency on the return path than ISDN). 2-way would definitely suck
for anything interactive.

> kevin

regards,
-- 
Paul Jakma	paul at clubi.ie	paul at jakma.org	Key ID: 64A2FF6A
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