AT&T MERLIN LEGEND Release 4.0 Feature Reference page 414

Feature Reference
PRI must include a D-channel, but may include fewer than 23 B-channels. The
remaining channels cannot be used for any other purpose.
NOTE:
The MERLIN LEGEND Communications System does not support multiple PRI
trunks sharing one D-channel (as allowed with Non-Facility Associated
Signaling).
Up to three DS1 carrier trunks (maximum of two in one carrier), and therefore
three PRIs, can be connected to the system through separate 100D modules,
each of which occupies a slot in the system carrier. In terms of system capacity,
each DS1 channel counts as a trunk endpoint, so the maximum number of
B-channels supported by the system is 69. Their signaling is provided over 3
separate D-channels, using up 72 of the system's 80-line capacity.
The following benefits are provided by PRI service:
Speed. Data calls to outside destinations can be established on the
same B-channels used for voice calls if the service allows; ACCUNET
Switched Data Service must be subscribed to. Modems and dedicated,
conditioned lines/trunks are not needed. By supporting high-speed
digital data transmission, PRI provides the capability for video
conferencing and Group IV (G4) fax.
AT&T's INFO2 Station Identification/Automatic Number Identification
(SID-ANI) Service. Customers who subscribe to this service can identify
the incoming caller on a 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.
Dynamic B-Channel Assignment. An individual B-channel can be
removed from service without blocking ISDN calls to or from any station.
Improved Toll Restriction. The ways that toll restriction can be
bypassed are limited on PRI lines/trunks. Specifically, three types of toll
abuse are eliminated with PRI service:
Since dialing is in the form of out-of-band messages that must be
generated by the system, a user cannot use a touch-tone generating
device, such as a pocket dialer, to send dialed digits directly through
the system to the trunk.
Without PRI service, toll restriction can be deceived by dialing digits
on a loop-start trunk before the far-end switch applies dial tone. These
initial digits may indicate a local call to the system's toll restriction
checking while the subsequent digits, those actually recognized by
the far-end switch, may produce a toll call. This is not possible with
PRI service, because every digit screened and passed on by the
414
Primary Rate Interface (PRI) and T1