Nortel DMS-100 Series Maintenance Manual page 91

Extended frame format
The DS-1 extended super frame format (ESF) has 24 frames. The 24
sync-bits are used as follows:
6 bits for framing pattern sequence (FPS)
The fourth framing bit carries a FPS bit. Every fourth framing bit, after
the fourth bit, carries a FPS bit. The FPS and the cyclic redundancy
check (CRC) define an in-frame condition.
12 bits for facility data link (FDL)
The 4-kbit/s facility data link (FDL) bit begins with the first framing bit.
An FDL messaged bit is carried every other frame.
6 bits for cyclic redundancy check (CRC)
The cyclic redundancy check (CRC) bit begins with the second bit and is
carried every fourth bit. In an extended super frame, a block check field
is checked six times. The CRC-6 check detects bits that emulate an FSP
bit. This check determines if an out-of-frame condition is present
The following table describes the superframe alignment pattern.
Signaling for RLCM-EDC 2-7
DMS-100 Family Maintenance Manual ABSK007