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Posted by alt on October 18, 2007, 11:31 pm
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>> We have a customer in a remote area, relatively near the equator
>> (their ISP is too). They don't have much voice coverage (no land
>> lines and scant cell) but they have a satellite-based DSL Internet
>> link. I would like to try to solve their problem with a VoIP ATA
>> from Linksys, because its configuration parameters can be tweaked at
>> will.
>
>> I have the obvious concerns: how much will the delay affect voice
>> quality? Is there some TCP/UDP window that should be adjusted? Any
>> question I haven't addressed?
>
> Did you ever use one of those old satelite links for a
> long-distance/overseas call? I suspect that VoIP over a
> satellite-based DSL link wouldn't even be that good. I have vague
> memories of speaking with my grandfather over one of those a few times
> and it did not lend itself to a normal flowing dialog.
>
> Assuming we are talking about a geosync satellite here, that is at
> least one hop of ~46000 miles each way, which translates to a one-way
> latency of ~250 milliseconds. (Handwaving math...) And then add-in
> whatever happens once their voice data hits the regular pots (?)
> network, and whatever other delays there might be on the IP side at
> the ISP or whatnot.
>
> So, your customer would say "boo!" and it would be a full half-second
> before you could say "eek!" No tweaking of TCP windows (UDP has no
> window) or other stuff could change that. While at first blush half a
> second might not sound like a big deal, there was a very compelling
> reason the telcos/whatever put all those trans-oceanic links in :)
>
> rick jones
Hi Rick:
I've set up a lot of VoIP-over-Satellite systems. Yes, the latency is a
bit of an issue, but for the most part you can have some well flowing
conversations. And as long as you don't have any packet loss or large
packet jitter, the sound will be very clear and clean because it is
all-digital. I routinely see 300ms-350ms of latency and the customers
don't seem to mind.
That being said, if you can get terrestrial, get it, for all the reasons
you've laid out above.
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