AT&T MERLIN LEGEND Release 3.1 System Manager's Manual page 54

Incoming Trunks
In more complex tie trunk configurations, a person can tie into another
system and use a trunk that does not exist on his or her own system. For
example, in a company with locations in New York, Chicago, and Los
Angeles (with tie trunks between New York and Chicago, and Chicago
and Los Angeles), users in New York can access a Los Angeles trunk
and make a local call as if they were in Los Angeles themselves.
Direct Inward Dialing (DID) Trunks (Incoming Calls Only). Provide fast
access to specific individuals; incoming DID calls can be routed directly
to an extension or calling group without system operator assistance.
Digital Facilities. MERLIN LEGEND supports two different types of
digital facilities: Digital Signal 1 (DS1) and, in Release 4.0 and later,
National Integrated Services Digital Network 1 Basic Rate Interface
(abbreviated NI-1 BRI ) facilities. T1, PRI, and BRI are the system's
interfaces to these Integrated Services Digital Network (ISDN) facilities,
which provide end-to-end digital connectivity and switched connections
to other networks. All three interfaces allow high-speed data transfer.
National Integrated Services Digital Network 1 Basic Rate
Interface (Incoming and Outgoing Calls). One NI-1 BRI facility carries
the equivalent of three "lines." Two are called B-channels and provide
voice and data communications services. A third D-channel controls
signaling and maintains operations on the B-channels.
Digital Signal 1 Facility Programmed for Either T1 or Primary Rate
Interface Operation (Incoming and Outgoing Calls). One Digital
Signal 1 (DS1) facility provides the equivalent of 24 lines. In Release
4.0 and later, the DS1 facility can be programmed to operate in one of
three ways:
¨
T1 Voice Operation. A "line" is called a channel and can be
programmed through the system, without the services of a
telephone company installer, to emulate a ground-start, loop-start,
tie, or DID trunk. This type of T1 operation also gives you access to
special services, like Megacom 800 service for incoming, toll-free
service for voice calls. Only this type of T1 operation is available in
releases prior to Release 4.0.
NOTE:
While you can quickly and easily change the type of line that a
channel imitates, you must coordinate the change with the CO so
that both ends of the connection are set up for the same type of
line/trunk.
2–13
About the System