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Clearing up VoIP echo before cancellation

Leaks, loudness and delay are the three main causes of echo on VoIP networks. This tip offers a few suggestions for prevention of these factors, thereby cutting echo off at the source.

Leaks, loudness and delay are the three main causes of echo on VoIP networks. This tip, posted here courtesy of, offers a few suggestions for prevention of these factors, thereby cutting echo off at the source.

There is a common misconception that the only thing you can do to eliminate echo is to use "echo cancellation," a feature commonly configured on voice gateways. To be sure, echo cancellers are the easiest, quickest and generally best solution. But there are many things you can and should do to eliminate or avoid echo, depending on your environment and budget.

To understand the potential ways to solve echo problems, you need to understand the factors that combine to create those problems. Three things have to happen to create an echo that is detectable and annoying to users.

  1. Noise you transmit must be received by you. Depending on where this happens, it's often referred to as leakage -- meaning that the signal is leaked from your transmitting path to your receiving path.

  3. This noise must be loud enough to be detectable and annoying. Of course, this is in the context of normal signal loss that happens over distance.

  5. A noticeable and annoying length of time has to elapse between transmitting and receiving.

All three of these factors must combine to create a problem, so eliminating any one of them will solve the problem. Aside from echo cancellers, which operate by estimating the leaked signal and then removing it, we can do any of three things to eliminate echo.

1. Eliminate the leaks

Unfortunately, this can be difficult to do with one common cause of leakage: two-wire to four-wire conversions. "Two-wire" and "four-wire" refer to different types of traditional telephony circuits. The primary difference between them is that four-wire supports more types of signaling and configurations than two-wire, which is typically just POTS, or "plain old telephone service."

But that's certainly not the only source of leaks. Cheap conference room phones that feed back noise from the speaker into the microphone have long been a problem but are easy to identify and replace. More recently, the designs of many Bluetooth headsets are favoring small size and ergonomics over sound quality, with the result being a poor experience for the other party. This is the case for cellular phones, too -- not just VoIP.

2. Reduce the loudness

The answer here is a fixed loss plan. These are actually required by Part 68 of the FCC rules for devices that connect to the PSTN. TIA/EIA/TSB122-A is the standard you want to use. If you don't have access to their FTP server, you can get the important parts from Cisco's Echo Analysis for Voice over IP guide, in the section labeled "Fixed Loss Plan for IP Voice Gateways."

A loss plan is important for VoIP, but it would probably be a last resort in echo troubleshooting.

3. Reduce the delay

There are actually two delays here that are important. The obvious one is your PING round-trip time between VoIP gateways. The other delay is on the analog tail circuit where your echo canceller works. This is important because echo cancellers can only cancel echoes that occur within a very brief time, such as 32 milliseconds. But this time is just the delay on the analog tail circuit, not the entire voice path. So if your echo canceller isn't working, it may be because you need to do something to reduce the delay on the tail circuit.

Again, echo cancellers are your best bet. In some cases, though, they can't solve the problem because of limitations in the technology, and you'll have to look elsewhere for relief.

About the author:
Tom Lancaster, CCIE# 8829 CNX# 1105, is a consultant with 15 years of experience in the networking industry. He is co-author of several books on networking, most recently,
CCSP: Secure PIX and Secure VPN Study Guide, published by Sybex.

This tip was originally posted on

This was last published in March 2007

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