Extended superframe


In telecommunications, extended superframe is a T1 framing standard. ESF is 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 and 4000 bit/s channel capacity for a data link channel It requires less frequent synchronization than the earlier superframe format, and provides on-line, real-time of circuit capability and operating condition.

Structure

An extended superframe is 24 frames long, and the framing bit of each frame is used in the following manner:
The CRC is computed using the polynomial over all 24×193 = 4632 bits of the previous superframe, but with its framing bits forced to 1 for the purpose of CRC computation. The purpose of this small CRC is not to take any immediate action, but to keep statistics on the performance of the link.
Like the predecessor superframe, every sixth frame's least-significant data bit can be used for robbed-bit signaling of call supervision state. However, there are four such bits per channel per extended superframe, rather than the two bits provided per superframe.
Unlike the superframe, it is possible to avoid robbed-bit signalling and send call supervision over the data link instead.