# Using SNR and TCF together

Using both SNR and TCF modem metrics, quick diagnostics of poor or failing receiver performance can be made. The following table shows how these two parameters can be used:

Indication | SNR | Training zeros | Possible NFX receiver performance
--- | --- | --- | ---
Good line quality | >25 dB | > 1.35 seconds | No problems encountered.
Good line quality Non-compliant TCF | >25 dB | 1.0 to 1.35 seconds | Possible FTT due to non-compliant sending fax. Possible line errors in non-ECM mode, and retransmits in ECM mode.
Marginal line quality Compliant TCF | 22 to 25 dB | 1.0 to 1.35 seconds | Possible FTT due to line conditions. Possible line errors in non-ECM mode, and retransmits in ECM mode.
Poor line quality Compliant TCF | <22 dB | < 1.0 seconds | Probable FTT due to poor line conditions. Possible line errors in non-ECM mode, and retransmits in ECM mode.