The audio power originating from the IP network may be too high, which causes a reflection to return from the Public Switched Telephone Network (PSTN). Delay in processing in the packet network causes this reflection to be perceived as an echo.
Note: The sound of an echo can vary depending on a number of factors. If the secondary signal is delayed considerably and has very little loss, then it would likely be perceived as a second discrete signal or echo. At the other end of the spectrum, a secondary signal could be minimally delayed with more loss. In this case it might be perceived more like Tunnel voice or it may not be perceived at all.
The key issues are identifying which end hears the echo and whose voice is heard. When a talker hears his own voice echoed, it is known as talker echo. This is the usual case. When a listener hears the speaker's voice twice, it is known as listener echo, which is uncommon. If the talker hears his own voice echoed, the echo is generated at the far end.
In this case, the IP phone user hears his own voice echoed. Therefore, the echo is sourced from the PSTN.
To resolve this issue, choose from one of these options:
Reduce the level of the signal on output from the gateway to the PSTN by using a positive output attenuation value.
To reduce the level of the echo, use a negative gain on input to the gateway from the PSTN.
Note: Increasing and decreasing signal levels affects the volume of the main signal as well as the echo. Ensure that you increase and decrease levels in small increments.
For specific procedures, refer to these examples:
For Cisco IOS gateway configuration, perform these steps:
Issue these commands under the voice port on a Cisco IOS gateway not controlled by Cisco CallManager (digital example shown):
voice-port 0/0:23 echo-cancel enable input gain value output attenuation value
!--- Where the value is an integer from -6 to 14, the default is 0.
Change the values, place the test calls and adjust as needed. For more information, refer to Configuring Voice Ports.
For Cisco CallManager controlled, Cisco IOS gateway configuration, perform these steps:
Issue the input gain and output attenuation commands under the Cisco CallManager gateway configuration screen for Cisco CallManager controlled Cisco IOS gateway using Media Gateway Control Protocol (MGCP). This is equivalent to those added directly on Cisco IOS gateways not controlled by Cisco CallManager.
Change the values, place the test calls and adjust as needed.
Note: These changes would apply for Cisco IOS gateway, like a 3725 with NM-HDV (Network Modules-High-Density Voice).
For more information on Cisco CallManager configuration of gateway ports, refer to the Gateway Configuration Settings section of Gateway Configuration.
For Cisco CallManager controlled, non-Cisco IOS gateway configuration, perform these steps:
On the Cisco CallManager gateway configuration screen, set these equivalent CallManager parameters:
Audio signal adjustment into the IP network reduces the impact of echo from the PSTN by reducing the signal (and therefore the echo) inbound through the gateway.
Audio signal adjustment from the IP network reduces the echo signal from the PSTN by reducing the primary signal outbound through the gateway.
Change the values, place the test calls and adjust as needed. Note: These changes would apply for non-Cisco IOS gateway, like a 6608 or DT24+.
For more information on echo problems on Cisco CallManager controlled, non-IOS gateways, refer to these documents: