Cisco 1760 Hardware Installation Guide - Page 104

Appendix E 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. Cisco 1760 Modular Access Router Hardware Installation Guide E-2 78-13342-03