Asterisk Echo Cancellation: FXO and FXS lines

The equipment in the central office (regardless of who the manufacturer happens to be) is balanced and supplies -48 volts that is fed through the outside plant to your location. The Digium Wildcard X100p card (and equivalents) do not care whether the tip and ring are reversed. If the cards really cared, they would totally fail and not just create an echo condition. (Polarity can be an issue with some other equipment, and with some types of central office trunks, but not the stuff we're talking about here.)

Echo is frequently produced by external 2-wire imbalances or imperfections on the PSTN line, and not by the Wildcard X100P (or whatever) itself. You can create an example of imbalance by placing a resistor from ground to either tip or ring (not both). Some real world examples are:

• wet or damaged PSTN cable
• bridge-taps (something done by the telco, seldom seen any more)
• cheap analog phones attached to the local PSTN line
• some expensive analog phones on the local PSTN line
• use of lengthy untwisted inside-wire within your home/business
• poor cabling techniques (inside-wire near ballast or other AC induction)
• as well as many other problems external to the Asterisk cards

Note: The x100p only support the US line impedance (600 ohm resisitive). Some European countries use a different line impedance. This will cause a lot of echo on the line.
The TDM400P has a line driver with programmable impedance.

Note 2: The zapata.conf file is very sensitive about where the echocancel=yes directive goes. For it to be recognised, make sure that your channel => 1 (etc) definition is _after_ it in the configuration file.

In many cases, the imperfections may be bad enough that you might even hear AC hum, noise, crosstalk, or other degradation by listening very carefully with an ordinary analog phone. (If you can hear any imperfections, the problem is rather bad.)

The echo cancel function is attempting to compensate for those imperfections and imbalances. One can either muck with the echo canceling software or go find the "real" source of the problem. (Incidently, that's why some people are having echo problems and others don't.)

Given how many of our offices/labs look in terms of cabling, etc, one rather simple step to help find the source of problems is to simply run shielded twisted pair cable between the Asterisk interface and the telco demarc (ensuring the shield is actually grounded), remove all other cabling and phones from the line, and verify the Asterisk box is running on clean AC power with an appropriate ground. (If you find the echo is gone or better, start adding those items back one at a time to see which item(s) is causing the problem. Then post it to the Asterisk user's list so the rest of us can learn from it.)

Most of the telco's have technicians with the equipment necessary to help find the problem if the problem really is their outside plant. However, getting to that person can be a real challenge.

Rich Adamsson, November 2003

See Also

• I Can Hear Myself (or can't hear myself): The lowdown on call sidetone
• Hunting the Echo: What to do if you have echo problems in Asterisk
• Echo Cancellation on the Wildcard X100P
• Zap Channel Module Configuration: Audio Quality Tuning Options
• Asterisk Echo Avoidance
• Echo Analysis for Voice over IP
• On-board echo cancellation on all Rhino Equipment analog cards

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