[asterisk-users] Moving from DSL to T1

[Kevin Keane]

-----Original Message-----
From: asterisk-users-bounces at lists.digium.com [mailto:asterisk-users-bounces at lists.digium.com] On Behalf Of Hans Witvliet
Sent: Monday, September 13, 2010 12:13 PM
To: asterisk-users at lists.digium.com
Subject: Re: [asterisk-users] Moving from DSL to T1

On Mon, 2010-09-13 at 00:32 -0700, Kevin Keane wrote:

> 
> > Latency also is the reason VoIP does not work at all over satellite 
> > connections even though they tend to have plenty of bandwidth.
> Please define "does not work at all over satellite" ???
> Sure, it is not studio HIFI quality, but is th same quality as you get from official commercial telco providers.
> We still have voip over S-band and X-band satelites running NOW between NL and afghanistan. All the people are more than satisfied.
> 
> **********
> Should have been more specific. I was talking about Internet over satellite in the USA. I believe those are geostationary TV satellites. I am not familiar with S-band and X-band, but assume they are in lower orbit. That would explain how it can work for you.
> 

No these are also geo-stationary (same altitude, so same delay), commercial and military satelites, 

**************
In that case, my guess is that they have a dedicated channel for the voice, maybe even some kind of clocking mechanism. Some T-1 lines here in the USA also have that (one more reason why T-1 works better than DSL/Cable for VoIP). The consumer internet satellite services just mix all kind of Internet traffic, so one packet may have a very low latency while the next one may have a much higher latency, or get lost altogether.

Another thing about the consumer satellites is that they are probably optimized for TCP rather than UDP. For TCP, they are using huge retransmission window sizes. That allows large chunks of data to arrive without waiting for confirmation, and the satellite can organize the data into a stream. With UDP, each packet basically stands on its own. Just a guess about another area where these two could be different.