AT&T MERLIN LEGEND System Reference Manual page 499

Functional Description
Line Coding
ZCS
B8ZS
4-30 DS1 Facilities
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 can be chosen.
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.
The factory-set line coding is ZCS.
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.
B8ZS line coding encodes an all-zero channel octet into a unique binary
sequence with a "bipolar violation" in bit positions 4 and 7. Normally for bipolar
transmission, ones are encoded alternately as a positive then negative, or
negative then positive, pulse. if two positive or two negative pulses are received
in succession, a bipolar violation occurs.
Ordinarily, bipolar violations are caused by noise hits to the signal. However,
B8ZS line coding allows the 8-bit strings to be detected at the receiving end and
converted back into the original sequence.
B8ZS line coding is preferred over ZCS because it does not cause errors in data
transmission.
B8ZS violations are passed by the ESF T1 Channel Service Unit (CSU) but not
by other CSUs. The CSU is a hardware component needed when two endpoints
are located in different buildings or when the distance between the two
endpoints makes office or line repeaters necessary. The CSU is located on the
customer's premises and is used to connect the system to DS1 network facilities
(see "Channel Service Unit" in Chapter 5, "Hardware"). The CSU has three
functions:
It terminates an outside DS1 facility on the 100D module.
It ensures that the signals entering the public network comply with the
requirements of the DS1 facility as specified by the FCC.
It includes maintenance, diagnostic, and testing capabilities.