# Signal to noise ratio (SNR)

An estimate of SNR is calculated during two phases of a fax call, when NaturalFax is acting as the receive fax machine. These phases are the initial training check frame (TCF), and the subsequent page or image portion of the facsimile. At the end of each of these phases, a value of SNR is provided to give indication of the line quality during each phase.

The following table illustrates the valid ranges that NaturalFax reports for SNR, as well as what should be considered a good, marginal, or poor value for receive performance during a fax session:

Line quality | SNR in dB
--- | ---
Poor | 15 - 21
Marginal | 22 - 24
Good | 25 - 32

The values in the preceding table should be used as a guide when determining whether receive performance problems are being caused by line quality problems or compatibility issues. For example, if the fax session is training down to lower rates, failing, or aborting fax calls while receiving a fax, and the reported SNR is below 22 dB, the probable cause is poor line quality. However, if the same symptoms occur with an SNR above 25 dB, it is unlikely that line quality is the reason for failure.