AT&T MERLIN LEGEND Key System Planning page 71

Lines
Line Code
Two methods of framing can be used by a 100D module, but the framing
chosen must match the framing at the far end:
D4 Framing Format The system is factory set for the most common framing
format, D4 framing. A D4 frame consists of 24 eight-bit time slots and one
framing bit. To synchronize the 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. The ESF frame format is required for ISDN-PRI and
international data transmission.
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 are
represented as the absence of a pulse. The line-coding formats guarantee that
the ones-density requirement is met to achieve clock recovery.
To meet the ones-density requirement, either alternate mark inversion zero code
suppression (ZCS) or bipolar 8 zero substitution (B8ZS) line coding is used. The
factory-set line coding is ZCS.
AMI-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.
B8ZS. 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 on 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 will be passed by the ESF T1 Channel Service
Unit, but not by other CSUs.
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Line Options