AT&T MERLIN LEGEND Installation & Maintenance Manual page 88

System Description
signals, the receiving equipment uses the framing information
to identify the start of each frame and to identify which frames
contain signaling information. The framing information repeats
once every 12 frames; these 12 frames form the D4
superframe. This framing format is used by most DS1
equipment.
ESF Framing Format. The extended superframe (ESF)
format extends the 12-frame D4 superframe to a 24-frame
superframe, hence its name. The 24 framing bits include a
cyclic redundancy check (CRC) for the entire extended
superframe and a facility data link for maintenance. The ESF
can detect more errors than D4 framing; however, ESF is not
used universally by DS1 equipment.
Line Coding
The DS1 signal consists of a continuous bit stream of ones and
zeros, encoded into bipolar pulses for transmission. Only the
ones create a pulse; the zeros represent the absence of a pulse.
The pulses of the ones alternate between positive and negative.
This type of line coding is called bipolar or alternate mark
inversion (AMI). The line-coding formats guarantee that the
"ones-density" requirement is met to achieve clock recovery.
To meet the ones-density requirement, either zero code
suppression (ZCS) or bipolar 8 zero substitution (B8ZS) line
coding is used. The factory-set line coding is ZCS.
ZCS
ZCS line coding monitors each DS0 channel and prevents
strings of eight or more zeros. Upon detecting eight consecutive
zeros in a channel octet, ZCS line coding forcibly changes the
seventh zero (second least significant bit) to a one.
With ZCS line coding, any bit that is overwritten has no noticeable
effect on voice and voice-grade data. However, the ZCS line-
coding format can cause errors in digital data transmission.
DS1 Capabilities 1-57