Voice Port Echo

On a Cisco 2851 router / Call Manager Express, we have our T1 Voice line set as follows:

In Gain is Set to 0 dB Out Attenuation is Set to 0 dB Echo Cancellation is enabled Echo Cancellation NLP mute is disabled Echo Cancellation NLP threshold is -21 dB Echo Cancel Coverage is set to 64 ms Echo Cancel worst case ERL is set to 6 dB

We are getting a lot of echo on calls to external analog phones.

We have tried different settings on the ERL, Echo Cancel Coverage, Input Gain and Output Attenuation. So far nothing works to diminish the echo. Cisco IOS version is 12.3(11).

Please advise. Thank you.

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Read up on how echo is formed. Its either feedback from the handset, where gain adjustments help minimise the 'applitude' of the echo - and the echo problem is therefore at how the remote end is setup (ie echo on one end ?) - the router can only do so much, but maybe you tweaking the end where the echo is ? instead of remote end ?

The real killer is analog to digital points in the network. Like where you may have an E&M card to PABX ?

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Jay, E&M cards are 4-wire devices, i.e., they use a separate pair for tx audio and another pair for rcv audio with no hybrid involved.

TexasMirty, I'm not sure about the Ciscos, but in PBX's and channel banks a 2db loss and 3db on some chl-banks would be recommended values for a Rcv-side loss-insertion.

I know you said you've played around with those, but maybe by changing too many parameters in a short time you lost track of what was tried?

Assuming the analog phones are called parties directly connected to a a Telco C.O., see the following:

Aside from acoustic echo which is probably not your issue (the handset audio coupled back into mic element) the biggest offender is what's hitting you--the 2-wire analog set. It uses a device called a hybrid. Everytime you convert from 4-wire to two-wire, you need a "hybrid" to accomplish that transition. (handset is 4-wire; telco 'drop' line is

2-wire. It's usually the impedance mismatches or challenges accentuated by the hybrid devices that contribute and ultimately cause the echo. The end-office E.O or C.O. has to take what arrives as a "4-wire" (a separate path each way in other words) circuit [between other Telco facilities i.e., that's how call gets there] and with the line-side interface convert it to a 2=wire 'drop' to run to the house or business. So, viewing it from the PSTN 'cloud' heading toward that line-side drop, there's two hybrids. One in the E.O. and another in the phone set. The whole point of this was not to place blame on the Telco ckts, but rather to offer an understanding. Furthermore, if you happen to place a call to an enterprise PBX that has digital trunks (local loops) and then on top of that have digital or 4-wire phones on the desk--then that will sound good; no echo.

Try inserting a 2 db loss and see what happens.

I can't believe I actually spent all this time doing that....must be feeling really generous :)


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This is your problem.

You need 6 dB of separation.

Try this:

Output-gain -3 dB Input Attenuation 3dB Echo Cancel Coverage 64


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