# Training check frame (TCF)

The training check frame is the phase of a fax call when the sending fax transmits a sequence of zeros to the receiving fax machine at the highest common data rate negotiated during prior phases. This check determines whether the line quality is adequate to pass information at the desired rate. If the line quality is good, the receiving fax machine receives this sequence of zeros without error. If line quality is not good, bit errors will occur during reception, and the total amount of zeros is not received.

The duration of the zero sequence sent by the transmitting fax is 1.5 seconds. The T.30 protocol states that to successfully receive this training data, 1.5 seconds +/- 10% of these zero symbols should be received. If successful, the receiving fax sends back a confirm signal (CFR) to the transmitting fax. If the standard is not met, the receiving fax sends back a fail to train (FTT) signal, indicating to the transmitting fax that a lower data rate should be tried.

NaturalFax reports the results of training information. This information can be used to determine the quality of the training event, and therefore the quality of the line. This metric can be used to determine whether a receive problem is the result of poor line quality, insufficient training, or a compatibility issue. The rx_training_zeros metric reports the length of zeros received in tens of milliseconds. A good value for the TCF is greater than 1.35 seconds. If values less than this are reported, the receiver is not able to demodulate the incoming signal appropriately. This results in bit errors.