FDL - Telecommunications
Facilities Data Link. An ESF term. ESF extends the superframe from 12 to 24 consecutive and repetitive frames of information. The framing overhead of 8 kbps in previous T-1 versions was used exclusively for purposes of synchronization. ESF takes advantage of newer channel banks and multiplexors which can accomplish this process of synchronization using only 2 kbps of the framing bit of only every fourth frame being used for this purpose. As a result, 6kbps is freed up for other purposes. This allows 2kbps to be used for continuous error checking using a CRC-6 (Cyclic Redundancy Check-6), and 4kbps to be used for FDL which supports the communication of various network information in the form of an in-service monitoring and diagnostics.