Fast mode means no interleaved mode interlacing. In fact, there is a delay, but it is very small. This feature is useful for real-time traffic, but introduces certain requirements for line quality. Using this feature is not recommended for normal Internet access. In this case, using the default Interleave Delay is recommended. It is more stable than Fast mode.
For Interleave mode, delay configuration is possible. The greater the delay, the more likely it is to transmit the data without error. This is due to the fact that in the data that is buffered during this delay, the correction bits are added in accordance with the Reed-Solomon algorithm. Accordingly, the larger the data block processed by this algorithm, the greater the likelihood of error correction at the receiving side. With the help of delay, you can optimize the ratio of the quality of transmitted data / delay on different lines of quality.
This configuration is available in the ADSL profile and is applied to the port.