AT&T MERLIN LEGEND Release 2.0 Equipment And Operations Manual page 152

Lines and Trunks
Primary Rate Interface (PRI)
The Primary Rate Interface (PRI) is a standard access arrangement that can be
used to connect the system to a network providing voice and digital data
services through a 4ESS™ Generic 16, a 5ESS Generic 6, and a 5ESS serving
the FTS2000 network.
A PRI line consists of 24 channels, sometimes referred to as DS0 channels,
each with a capacity of 64 kbps. Each channel can be designated as either a
B-channel (bearer channel) or a D-channel (data channel). DS1, then, refers to
the twenty-four 64-kbps channels plus framing and signaling bits multiplexed
together to form a 1,544-Mbps signal.
A B-channel is used to carry end-to-end user information, such as the voice or
data content of a call, between the system and the far-end switch. Each B-
channel provides access to one or more network services. Release 1.0 of the
system supports access to only one network service per B-channel. For
Release 2.0, Call-by-Call Service Selection allows multiple network services over
the same B-channels. The D-channel conveys signaling required to set up,
control, and clear calls made over all of the B-channels.
The most common configuration of a PRI consists of 23 B-channels and 1 D-
channel, although other combinations are possible. Each PRI must include a
D-channel, but may include fewer than 23 B-channels. The remaining channels
cannot be used for any other purpose.
Up to three PRIs can be connected to the system through separate 100D circuit
modules, each of which may occupy a slot in the system carrier. In terms of
system capacity, each PRI line counts as a trunk endpoint, so the maximum
number of B-channels supported by the system is 69. Their signaling is
provided over three separate D-channels.
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 1.544-
Mbps DS1 signal.
Frames repeat at a rate of 8000 per second, with each frame repeating DS0
channels 1 through 24 sequentially.
Two methods of framing can be used by a 100D module (T1 service): D4 or
Extended Super Frame (ESF). The framing method chosen must match the
framing at the far end, and must be programmed to the format selected when
service was ordered.
To identify the DS0 channels, the DS1 signal is segmented
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
3-18 DS1 Facilities