Instruction manual

Features
Framing Format
To identify the DS0 channels, the DS1 signal is segmented into blocks of 193
bits called frames. A frame consists of 24 eight-bit words (one for each
channel) plus a framing bit at the beginning of each frame (24 words x 8
bits = 192 bits). Thus, a framing bit appears in every 193rd bit position of the
1544-Mbps DS1 signal.
Frames repeat at a rate of 8000 per second, with each frame repeating DS0
channels 1 through 24 sequentially.
The following two methods of framing can be used by a 100D module, but the
framing method 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 perform synchronization, 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. The 24 framing
bits include a cyclic redundancy check (CRC) for the entire ESF and a
facility data link for maintenance. The ESF can detect more errors than
D4 framing can; however, ESF is not used universally by DS1 equipment.
Line Coding
The DS1 signal consists of a continuous 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 1s 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, but the line
coding chosen must match the line coding at the far end.
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 (the second least significant bit)
to a one. The factory-set line coding is ZCS.
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.
354 Primary Rate Interface (PRI)