[cisco-voip] Echo Cancellation and 7911 Phone Loads

[Wes Sisk]

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_note09186a0080149a1f.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_solutions/EA_ISD.html
> http://www.cisco.com/en/US/partner/tech/tk652/tk698/technologies_tech_note09186a0080149a1f.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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