When a system suffers from an intractable echo, it is often very difficult to diagnose the origins and possible solutions to the echo problem. WANPIPE^® has a utility that allows the echo from a unit impulse to be examined and analyzed.

To debug the Echo problem during the phone call:

1. Install WANPIPE version beta1w-2.3.4 or later, use the following command when installing:

./Setup install --echo-debug

The Setup script will patch Zaptel driver, so it will have to be recompiled/reinstalled after WANPIPE installation is complete. Software Echo canceller should be enabled on the span, where the test will be done (or on all spans).

2. Make the phone call and check to see if there is an echo.

Find a number that shows a consistent echo for this test. The test should be done from the side where the echo is heard. Ideally, mute the phone on the remote side during the test. Note the Zap channel number on which the test call is done, for example channel 12. The rest of this document assumes the test call is active on Zap channel 12.

3. Run the following command:

#>wanpipemon -zap -c ses -zapchan 12

A test impulse is sent on zap channel 12. The Finite Impulse Response (FIR) is saved to the file: /etc/wanpipe/span_1_chan_12_before_ec.spike as a set of linear samples taken 8000 times per second.

4. Copy the data from /etc/wanpipe/span_1_chan_12_before_ec.spike into a spreadsheet.

Create a line chart of the data.  The result should look similar to this picture:

 

The graph shows the response to a unit impulse (FIR).  Note that the start of the FIR is  at 55 taps (7ms) after the impulse, and the echo was attenuated by 80 taps (10 ms). If the echo canceller had been configured for 128 taps, the echo  would have been properly cancelled. If the Echo Canceller had been set to 64 taps, much of the echo would have survived the echo cancellation, and there would have been audible echo on this call.