AT&T MERLIN LEGEND Release 2.0 Equipment And Operations Manual page 153

Lines and Trunks
and to identify which frames contain signaling information. The framing
information repeats once every 12 frames; these 12 frames form the 134
superframe. This framing format is used by most DS1 equipment.
ESF Framing Format. The 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) zero code suppression (ZCS). The line-coding formats
guarantee that the "ones-density" requirement is met to achieve clock recovery.
To meet the ones-density requirement, either AMI-ZCS or bipolar 8 zero
substitution (B8ZS) line coding can be chosen.
AMI-ZCS
AMI-ZCS line coding monitors each DS0 channel and prevents strings of eight
or more zeros. Upon detecting an all-zero channel octet, AMI-ZCS line coding
forcibly changes the seventh zero (second least significant bit) to a one. The
factory-set line coding is AMI-ZCS.
With AMI-ZCS line coding, any bit that is overwritten has no noticeable effect on
voice and voice-grade data, However, the AMI-ZCS line-coding format can
cause errors in digital data transmission.
B8ZS
B8ZS line coding inserts eight consecutive zero bits 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 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
DS1 Facilities 3-19