Some time ago, we discussed the simple side of troubleshooting echo. You can eliminate most echo problems by replacing cheap conference phones and using headphones and endpoints with echo cancellers. But what do you do if that doesn't solve your problem? Unfortunately, the answer may not be easy and you may have to dig into some of the more complex and daunting areas of your system.
Because there are so many numbers associated with networking, and QoS, and the voice Codecs, and they vary so widely from implementation to implementation, it's hard to know whether any given number is "good" or "bad". It's easy for your eyes to glaze over as you scan your system for a problem. But, if you have a serious echo problem and no simple solution seems to work, there are two numbers you want to pay particular attention to:
The first thing you want to look for is an impedance mismatch. Impedance mismatches usually occur between the PSTN and VOIP gateway, or between a PBX and a VOIP gateway. Typical impedance is 600 ohms, and that's the default for many devices. So check the value on your gateway and whatever device it's attached to. It's ok if it's not 600, as long as it's the same on both sides.
If that doesn't solve the problem, then the problem may be related to delay. Your echo canceller works by remembering a signal for a period of time so it can cancel it if necessary. The problem might be that some delay causes the signal to arrive after the timer
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expires and the device (e.g. a gateway) "forgets" the signal. Thus, it passes w/o being cancelled, and you hear an echo. If this is the case, you can adjust the timer to a longer duration.
These two attributes may have different names, depending on your vendor, but you should be able to identify them easily enough if you understand their function.
Thomas Alexander Lancaster IV is a consultant and author with over ten years experience in the networking industry, focused on Internet infrastructure.
This was first published in November 2002