The error free second, Error Free Second (EFS), normally specifies the transmission characteristics of T-lines and is determined for a fixed period (hours, days, weeks). This measurement method gives better results about the bit error behavior than the bit error rate( BER) measurement.
The EFS value actually refers to a 1-second interval in which no erroneous data blocks are detected. It is given as a percentage. In addition to the error-free second, there is also the specification Error Second( ES), in which the number of errors within one second is measured.