[Asterisk-Users] Best echo-free and trouble-free system?

[Nicolas Bougues]

On Fri, Apr 30, 2004 at 10:28:02AM -0500, Barton Hodges wrote:
> asterisk-users-admin at lists.digium.com wrote:
> > The real problem arises when :
> > - you have some echo induced somewhere (your call goes through a 2  
> > wire line) 
> > - you have some delay induced somewhere (you use VoIP for instance)
> 
> Following the "2-wire to 4-wire causes echo" thought, the following
> should not result in noticable echo, true?
> 
> - Analog phone <-> TDM10B-FXS <-> Asterisk <-> TDM01B-FXO <-> PSTN

Yes, because although echo will exist, delay should be short enough so
that you don't notice. Never tried such a setup myself, though.

Please furthermore note that Asterisk uses "pseudo" TDM. In real telco
world, PCM highways that interconnect trunks and devices switch one
byte every 8000th/sec. OTOH, Zaptel devices switch eight bytes every
1000th/sec. This is due the to PC bus architecture (it would cause way
too much overhead otherwise).

So the delay is actually 8 times longuer (at least) than in the PSTN.

> - VOIP Phone <-> Asterisk <-> VOIP Phone
> - VOIP Phone <-> Asterisk <-> T100P <-> PRI
> 

Nobody's supposed to generate echo on VoIP phones. However, the "PRI"
side will probably connect to a 2-wire PSTN set at the remote end, so
you will get echo from there.

> However, the following could result in noticable echo (as I am
> experiencing):
> 
> - Analog phone <-> ATA <-> Asterisk <-> TDM01B-FXO <-> PSTN
> - VOIP Phone <-> Asterisk <-> PSTN
> 

Definetly. Although Asterisk (zaptel, actually) make a fairly good job
at cancelling it.

> What about the following as described in Raymond McKay's setup (Thank
> you Raymond)
> Does the channel bank provide the needed, and adequate echo
> cancellation?
> 

I don't have any experience with channels banks. Not very common stuff
in Europe.

> 
> Since you state that echo cancellation needs to be performed closest
> to the source, could the Grandstream HandyTone-286 be doing an
> inadequate job of echo cancellation?  If this is the case, does anyone
> have experience with another ATA (Sipura SPA-1000, Cisco ATA-186,
> etc.) that does such a great job of echo cancellation, that the
> "2-wire to 4-wire" situation is not an issue?  Does Grandstream have
> improved echo cancellation scheduled for a future firmware upgrade?
> 

The Sipuras are definetly better than the HandyTones. I've heard that
the forthcoming GS firmwares will enhance echo cancellation
performance, though.

-- 
Nicolas Bougues
Axialys Interactive