In telecommunications, an extended superframe (ESF) is a T1 framing standard, sometimes called D5 framing because it was first used in the D5 channel bank, invented in the 1980s. It is preferred to its predecessor, superframe, because it includes a cyclic redundancy check (CRC) and 4000 bit/s channel capacity for a data link channel (used to pass out-of-band data between equipment.) It requires less frequent synchronization than the earlier superframe or D-4 format, and provides on-line, real-time testing of circuit capability and operating condition.
In ESF, a superframe is 24 frames long, and the 193rd bit of each frame is used in the following manner:
- Frames 4, 8, 12, 16, 20, and 24 are used to send the framing pattern, 001011
- Frames 1, 3, 5, 7, 9, 11, 13, 15, 17, 19, 21 and 23 are used for the data link (totalling half of all framing bits, or 4000 bits per second)
- Frames 2, 6, 10, 14, 18, and 22 are used to pass the CRC total for each previous super frame.
Note: Less-frequent synchronization frees overhead bits for use in testing and monitoring.
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