Signal In and Signal Out

The Signal In and Signal Out metrics are useful for testing a circuit to see whether it is introducing echo. Cisco System's echo testing procedures use these metrics to measure the effects of altering the signal strengths when tuning echo levels.

Signal In is the audio signal traveling in the direction of the IP network, measured as it enters the ECAN from the tail circuit. It contains echo that needs to be canceled. Signal Out is the audio signal coming out of the ECAN and going into the tail circuit, from the IP network to the PSTN. The Signal Out stream contains an estimation of the amount of echo in the audio stream. Both metrics are measured by the gateway on the PSTN side of the gateway’s ECAN.

ERL is used along with Signal In and Signal Out to tune the echo canceller. Remember that ERL must be at least 15 dB to distinguish the echo portion of the signal from the voice itself. To enhance this difference and enable cancellation, the voice gateway ports perform “input gain” and “output attenuation” on the signals. Input gain is performed at Signal In, before the gateway’s ECAN sees the echo, and output attenuation is performed at Signal Out, after the gateway’s ECAN has seen and cached the original signal. Thus, these metrics provide reference points for adjusting the overall strength of the signals to enable echo cancellation. They are used in echo troubleshooting to adjust the signal gain or attenuation performed by the gateway so that:

Signal Out - Signal In > 6 dB