AT&T MERLIN LEGEND Release 4.0 Feature Reference page 419

Feature Reference
T1 is the factory setting and, when selected for the DS1 facility, allows each of
the 24 channels to be programmed to emulate tie, loop-start, ground-start, or
DID lines or Switched 56 in any combination. Therefore, a single 100D module
can take the place of 24 regular outside trunks.
If common-channel signaling (CCS) is selected, 23 channels are available for
emulation, and the twenty-fourth channel carries trunk supervision signals. (See
"Signaling Mode," later in this section.)
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 one hundred ninety-third bit position of the
1.544-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 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.
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.
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. Pulses 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.
Primary Rate Interface (PRI) and T1
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