AT&T MERLIN LEGEND System Reference Manual page 498

Functional Description
ISDN-PRI
Framing Format
ISDN-PRI is a standard format for ISDN service. Some of the benefits of ISDN-
PRI service are
Speed. Data calls to outside destinations can be established on the same
B channels used for voice calls if the service allows. Modems and dedicated,
conditioned lines/trunks are not needed.
AT&T's INFO2 automatic number identification (ANI) service. Customers
who subscribe to this service can identify the caller on an incoming call on
an ISDN-PRI line/trunk by either telephone number or billing number.
Note: The availability of the caller identification information may be limited by
local-serving (caller's) jurisdiction, availability, or telephone company
equipment.
Dynamic B-channel assignment. An individual B channel can be removed
from service without blocking calls to or from any other B channels.
Improved toll restriction. The ways that toll restriction can be bypassed are
limited on ISDN-PRI lines/trunks.
Reliable indication of far-end disconnect.
Improved SMDR reports. Call timing for SMDR reports is improved since
calls recorded are closer to the actual billed duration.
To identify the DSO 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, 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, 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.
DS1 Facilities 4-29