Cisco 1751 - Manual (Page 88)

Appendix E Installing the Echo Canceler Expansion Modules on Cisco Interface Cards
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Cisco 1751 Router Hardware Installation Guide
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Echo
Echo occurs when the speech energy being generated and transmitted down the
signal path is coupled into the receive path from the far end. This causes a speaker
to hear the sound of his or her own voice, delayed by the total echo path delay
time.
In a traditional voice network, voice can reflect back, but it usually goes unnoticed
because the delay is so low. In a Voice over IP (VoIP) network, echo is more
noticeable because packetization and compression contribute to delay.
Echo cancellation technology is a functional component of a voice gateway that
is used to reduce the effects of echo. An echo canceler monitors a caller’s speech.
If that caller’s speech echoes, the echo canceler generates and transmits a signal
that is sent back to the caller to cancel out the echo. The amount of time that it
takes the echo canceler to locate the echo and to generate its opposite signal is
called convergence time (typically, a few seconds). During convergence, the caller
hears echo, which should gradually decrease in amplitude to zero when
convergence is complete.
For additional information about echo and echo cancellation, refer to Echo
Analysis for Voice over IP .
Echo Canceler Expansion Modules
The echo canceler expansion modules are user-installable, plug-in modules. The
expansion modules provide a hardware-based echo cancellation feature that
simultaneously offloads the work of the DSPs on the Signal Processing
Mini-Module (SPMM) on the motherboard. By offloading this processing,
hardware-based echo cancellation results in more robust echo cancellation.
The echo canceler expansion modules enable high performance hardware-based
echo cancellation for up to 64 voice calls.