[cisco-voip] Echo Cancellation and 7911 Phone Loads

[Jeff Ruttman]

Thanks again, Wes.
 
I see you mentioned SLIPS, and I had seen that as a possible cause, but
there are zero, so I had ruled that out.  Should have had that in my
little list, though.
 
I've certainly got more to go on now, and I can see this last doc you
referenced.  (I'm not a Partner, so I was "forbidden" some of the other
docs you linked me to.)  Crappy part is the echo is intermittent--of
course--so all I can really do is make a change then wait and see.  I've
sent a number of calls out that circuit to test myself, and I've not
heard any echo.
 
jeff

________________________________

From: Wes Sisk [mailto:wsisk at cisco.com] 
Sent: Wednesday, September 01, 2010 9:54 AM
To: Jeff Ruttman
Cc: Cisco VOIP
Subject: Re: [cisco-voip] Echo Cancellation and 7911 Phone Loads


Hi Jeff,

Inline, ws.

Jeff Ruttman wrote: 

	Thanks Wes.  
	 
	Well I've expanded echo cancellation on the gateway.  (PRI with
MGCP, CCM6.1)  So your first paragraph is saying that most likely echo
is generated too "far away" (too much delay) from my GW to cancel the
tail end echo coming into our system--even with my expanded echo
cancellation in place.  So only the telco might have a chance of
cancelling that echo "closer to" the echo source.

ws: echo is generated too far from the ipphone, like at the the other
endpoint. If you call me and hear your own voice then most likely the
echo is being generated in my phone.  Your gateway is closest to me so
will have the best chance of canceling the echo as the original signal
and echo signal are closest in time in your network at the gateway.
But, in the end, it really is a problem with my customer premise
equipment (CPE). I should be canceling the echo.  The best place to fix
it is in my network.  Next best is in the carrier, next is in your CPE.


	 
	As I understand it, I might also change phone loads (hence my
question about 7911s).  And I might try some gain/attenuation settings
on the GW as well--or so I've read.

ws: Yes, there is tuning on the gateway to get the most out of the
available ecans. If the echo is coming back within the cancellation
coverage then you just need to get levels and ERL in line so the ecans
can do their jobs.


	 
	So in a VOIP system that has echo, I can:
	 
	1. Bug the telco
	2. Expand echo cancellation on the GW
	3. Attenuation/gain statements on GW

There are several parameters on gateways for possible tuning. These
include:
ecan coverage
NLP - non linear processor
ERL - echo return loss - primarily affected by Input Gain and Output
Attenuation
Noise - any noise introduced on the circuit causes the echo signal to
not match the original signal. This prevents echo from being detected
and canceled.  Examples of noise include any slips or errors on T1's and
electrical cables (60Hz, fluorescent lights especially) located too
close to analog (FXS/FXO) connections.  I believe the parameters are
covered pretty well here:
http://www.cisco.com/en/US/customer/tech/tk652/tk698/technologies_tech_n
ote09186a0080149a1f.shtml


	4. Change phone load
	 
	At least so far that's all I've found I can tinker with.  Is
that about it?
	 
	Anyone use attenuation or gain statements on their GWs?  I know
the statements, but I don't know what their function really is, whether
I would use one or the other or both, or what I would set them to.

ws: see the article above.  Use IG and OA to get ERL that works for
ECAN.


	 
	Thanks
	jeff
	 
	 

________________________________

	From: Wes Sisk [mailto:wsisk at cisco.com] 
	Sent: Tuesday, August 31, 2010 2:00 PM
	To: Jeff Ruttman
	Cc: Cisco VOIP
	Subject: Re: [cisco-voip] Echo Cancellation and 7911 Phone Loads
	
	
	Talker echo is usually caused by echo at far end tail circuit.
With VOIP that delay is going to be too long for the talker's phone to
cancel.  It will have to canceled closer to the source of the echo.
	
	Otherwise, AFAIK, all cisco phones (except soft) have local ecan
to prevent speaker audio from going back into mic.
	
	
	http://www.voiptroubleshooter.com/problems/echo.html
	
http://www.cisco.com/en/US/partner/docs/ios/solutions_docs/voip_solution
s/EA_ISD.html
	
http://www.cisco.com/en/US/partner/tech/tk652/tk698/technologies_tech_no
te09186a0080149a1f.shtml
	
	/Wes
	
	Jeff Ruttman wrote: 

		Greetings,
		 
		Can someone confirm or deny my memory?  I recall reading
that phone loads for 7911 phones don't really contain echo cancellation
programming.  In looking now, I can't find much info on echo
cancellation in relation to different phone models/loads in general and
nothing on 7911s.
		 
		We have an office mostly full of 7911s recently
complaining of talker echo.  There are loads out on cisco.com newer than
the ones we're using, and while it's not hard to change phone loads, it
would be pointless to do it if the phone loads don't contain echo
cancellation.
		 
		Any insight?
		 
		Thanks
		jeff
		
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