[Asterisk-Users] Re: 128 kbs satelite link

[Cees de Groot]

Senad Jordanovic <asterisk-users at lists.digium.com> said:
>But, apparently the "C band" is suitable for VOIP.
>
It doesn't depend on a band (dunnow what the 'C band' is, but bear with
me). A satellite in geostationary orbit hangs above the equator, at
about 35780 km. So in the ideal case, if you communicate from a station
directly below the satellite to a station nearby, your signal is going
to travel 71560 km. The further the stations are away from the
satellite's position, the longer this gets, of course.

The speed of light in a vacuum is 299,792.458 km/s. Your signal will
never travel faster than that. So the round-trip to a satellite is going
to cost 239ms, excluding switching time. 239ms is a noticeable delay, no
satellite band (bar one that would use satellites in low orbit, like
Iridium) is going to change that. 

On the positive side, as long as you use UDP to transfer voice data you
won't be bitten by another big problem of satellite links: the capacity
of the link (defined as the amount of data 'inside' the link). With a
medium-bandwidth (say 128kbit/s) link and a quarter of a second RTT you
would (with TCP) have 32kbit of unacknowledged data outstanding at any
one time, and that's a mimimum amount (that's 21 packets...).

Probably the best thing to do is test and see whether the delay is
bearable. Fiddle with the IP options field to see whether you can get
the packets through with a minimal delay - every intermediate router
that helps here is going to shave off a bit of time from the end-to-end
RTT, who knows...

-- 
Cees de Groot               http://www.tric.nl     <cg at tric.nl>
tric, the new way           helpdesk/ticketing software, VoIP/CTI, 
                            web applications, custom development