Echo cancellers take a finite time to converge. An echo occurs during this time period.
An echo in the first few seconds of a call is normal behavior as the echo canceller converges. The time of the echo depends on the echo tail length of the Public Switched Telephone Network (PSTN) or the echo cancellation coverage. These are two steps that can be taken to reduce the impact of the echo during this time:
Reduce the echo cancellation coverage. Issue the echo-cancel coverage command on the gateway voice port. The default echo cancellation coverage is 32ms. A value of 32ms means that the echo cancellers keep the original signal in memory for 32ms, and this is compared to the echo signal. It might take the echo cancellers five seconds to converge with an echo cancellation coverage value of 32ms. During this period, the echo cancellers is not able to eliminate echo.
Reducing the echo cancellation coverage also reduces the time taken for the echo cancellers to converge, meaning that the time to converge could be less than two seconds. The echo tail is the time for the echo to travel across the PSTN tail and back. If the echo tail is more than the echo cancellation coverage value, the echo will not be eliminated from the call at all. Typically, there has been a trade-off between the echo cancellation coverage and the echo canceller convergence time.
Note: This command is only available in Cisco IOS versions 12.2(12), 12.2(8)T5, 12.2(11)T, or later.
To address the trade-off between the echo cancellation coverage and the echo canceller convergence time, an echo suppressor has been implemented. This echo suppressor provides the capability to mask the long convergence time from the end users by disconnecting the return speech path while the near end is speaking. When using this echo suppresor , keep these things in mind:
During the time of suppression, if doubletalk occurs, the far-end user is not be heard.
When the echo suppressor is active, the echo canceller continues to converge, assuming no doubletalk is present.
When doubletalk is occurring the echo canceller will not converge. Although not likely, it is possible to have doubletalk during this entire suppressor time, such that the ECAN does not converge in time. A cause of this could be significant background noise on both sides of the call.
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