Echo in VOIP – Why echo gets generated & What you can do about it

Echo or listening to your own voice after a minor delay, has always been there in any telephone system be it analog/digital or VOIP.

But in PSTN (analog), the echo is heard after a very minor delay (less than 20 milliseconds) and hence the echo is indistinguishable from the normal voice. So, there is a perception in our minds that there is no echo. This is true for local calls & calls to nearby locations. For STD/ISD calls and calls to cell phones, the delay might be slightly more, and you might be able to notice the echo if the phone company have not implemented any echo cancellation techniques.

Echo is generated mostly at the far end of the circuit due to some reasons like:

  • Analog circuits use the same pair of wires to send and receive the voice signals. If the circuits are not balanced electrically, there might be an echo generated at the far end, which might be audible at the near end.
  • The telephone/ handset connected on the other end might reflect voice signals, generating echo at your side. This mainly happens with inexpensive phones, that are not properly tested. This can happen for both analog and VOIP phones.
  • If the delay between the two ends (local end and far end) is high (more than 20 ms), an echo can be generated. This can happen for long distance calls, as the signals have to travel a longer distance.
  • In the VOIP world, there is a certain latency (or delay) introduced when the voice packets are traveling from IP phones to the server over the IP network/ WAN network. This can also be a cause for echo, especially if the latency in the IP network is high.

How to deal with Echo in VOIP systems?

Echo cancellation algorithms can be implemented either in software or hardware.

The VOIP software (PBX Soft switch) will most probably having an option to implement certain echo cancellation algorithms, and most VOIP softwares have multiple such options. The user can select which ever echo cancellation algorithm works for their environment. In this method, the CPU processor cycles of the host server/ computer are used for echo suppression and hence there might be additional strain on them. In large installations, this strain could be considerable. So, this method is not recommended unless the below mentioned hardware echo cancellation devices are not available.

There are PRI Cards/ Analog Trunk Cards available in VOIP systems to connect the VOIP server to PRI lines & analog trunk lines respectively. These cards usually plug in to the PCI/PCI Express slots of the server and connect the VOIP system to the trunk lines. Most of such VOIP Cards come with two options – With hardware Echo Cancellation modules and Without hardware Echo cancellation modules. These modules are basically additional chipsets that offload echo cancellation function from the server CPU. Buying and installing trunk cards with echo cancellation modules is the most effective way to contain echo on VOIP Systems. The echo cancellation modules can also be ordered and installed separately (on the trunk card), after purchasing the trunk cards, as well.

The cost of PRI/Analog Trunk cards with echo cancellation modules might be almost twice when compared to the cards without echo cancellation modules. But still, the investment is worth it.

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