[Asterisk-Users] Help - is voip good for in-house calls?

[Peter Svensson]

On Sat, 14 Aug 2004, Andrew Kohlsmith wrote:

> On Saturday 14 August 2004 18:29, Peter Svensson wrote:
> > Using a BRI will eliminate echos from the pstn connection. Your ip phones
> > should prevent echos from the local phone connections as well. That way
> > you should not cause any noticable echo for the remote party. Being all
> > digital has its advantages. :-)
> 
> *wrong*, unless there is something drastically different between ISDN PRI and 
> ISDN BRI.
> 
> I have *bad* echo on my Bell Canada PRI (via TE405P) -- software echo 
> cancellation helps but it's not perfect.  All the BRI/PRI does is ensure that 
> *your end* is not generating echo, as there is no hybrid there.  There is 
> still a hybrid in your handset, and there is still a hybrid at the other end 
> of the PSTN call.

If you are _all digital_ as I wrote there is no hybrid in your handset. 
I agree that far-end echos can still occur but you are not casuing them. 
The remote party should not be hearing any echos.

Canceling far-end echo is a hard thing to do properly. You can get 
nasty interactions between the echo canceler close to the echo source 
(where the canceling should be done) and your canceler. Also, you have no 
rough estimate of the echo characteristics to start off with so you have 
to train from scratch every time. (I think Asterisk always does this even 
for local lines where it probably could remember the line charateristics).

The only good way to deal with far end echos are by keeping the latencies 
low enough. That way they sound like a pleasent reverb. For an all tdm 
voice path (we are lucky) this is no problem and no echo canceling is 
needed. For short hauls along non-congested links I think you can get low 
enough latencies for VoIP as well.

We have made a few trials with VoIP and have had no annoying echos except 
when going across the atlantic with it and calling certain lines. Still, 
the other party heared no echos.

Peter