What causes echo?
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| Patrick Ferriter | |
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Kate Gerwig, Editorial Director
There are two main sources of echo in telephony networks: acoustic echo and line echo. Acoustic echo is generated on any phone (IP or otherwise) when there is feedback from the speaker to the microphone. This is particularly noticeable on many speaker phones. Line echo is very common in the PSTN network, and this most commonly occurs when there is a two-wire to four-wire conversion in the network (for example, where analog is converted into T1 or E1).
How can I combat echo? How do echo cancellers work?
To combat these types of echo, there are echo cancellers. As you can probably imagine, there are acoustic echo cancellers (AECs) and line echo cancellers (LECs). How well the echo is cancelled depends on the quality of the echo canceller. One key parameter in an echo canceller is the tail length. Basically, the way an echo canceller works is that it remembers the waveform sent out and, for a certain period of time, looks for a returning waveform that it can correlate to the original signal (usually arriving later, at lower amplitude, and with more noise). Typically, echo cancellers can be set to 32ms, 64ms, or 128ms tail lengths. If the return signal (echo) arrives too late, the echo canceller won't be able to correlate and cancel it properly. In summary, it is possible to greatly minimize or nearly eliminate echo if proper echo cancellation is in place.
When initiating a VoIP call, I hear no echo or lag, but the person I'm calling does. Do you know why this is happening?
It is possible that your VoIP device has an AEC that is not performing well. Sometimes, a poor-quality headset can also introduce echo that the far end can hear. It is a good idea to report the problem to your VoIP service provider and explain the symptoms. If your VoIP device will allow you to lower the receive gain or volume, this may also help to reduce the level of the echo the called party hears.
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If you're using an uncompressed G.711 codec, over Ethernet, you need approximately 87 kb/s in each direction to carry on a conversation. If you're using a compressed codec such as G.729, you need approximately 24 kb/s in each direction. This is the minimum bandwidth required to carry on a conversation. Note that bandwidth alone does not guarantee good voice quality. If there are dropped packets, random delays or other things of this nature, the voice quality may not be good. You need to have a properly designed network to ensure decent voice quality. Congestion points should be eliminated. If there is going to be traffic congestion, a quality-of-service mechanism that prioritizes voice traffic over other traffic should be used.
Our organization has just implemented a VoIP system, and our users have been complaining of echo, static and amplified background noise during phone usage. The noise can be heard on our end only. Can you please give some insight into what may be causing the problem and how to resolve it?
If this is a premise-based solution, it is a good idea to work with the IP PBX manufacturer or its representative (VAR) to investigate these issues. Most IP PBX manufacturers can obtain call recordings from their systems, and those recordings can detail the problems. You'll want to provide a network diagram to the manufacturer you're working with to help in understanding your network topology. You'll also have to work with your end users to document the calls where the problems occur, so they can be carefully reviewed.
The problems you are experiencing could be caused at any of several points. For example, the echo issue could be related to:
- a problem with the line echo canceller on the IP PBX,
- a problem with the acoustic echo canceller on the destination speaker phone (for internal calls from one phone to another),
- a nonperforming echo canceller in the carrier network (call recording would show a very long echo tail in this case).
If you're using a hosted VoIP solution, work with the provider and the tools it can offer to help solve the problem.
This was first published in August 2006