AT&T MERLIN LEGEND Release 2.0 Equipment And Operations Manual page 46

Introduction
Key mode has the flexibility to provide trunk access according to user
needs. For example, tie trunks can be terminated on the telephones of
only those users who need them.
The loop-start trunks traditionally associated with Key mode operation
can cost less than the trunks used in the other modes.
Key systems are best suited to smaller businesses.
To take advantage of the features and functionality of the system, all
users should have multibutton telephones when the system is operating
in Key mode.
If the number of trunks connected to the system is larger than the
number of buttons available on the DLC, Hybrid/PBX mode, which offers
the QCC, may be more functional.
To make more efficient use of outside trunks by grouping them into pools
for shared use, or to use ARS, the system must be programmed to
operate for Hybrid/PBX operation.
Hybrid/PBX Mode
A private branch exchange (PBX) originally was a large switchboard installed at
a customer's office that functioned like a small, self-contained telephone
company. The switchboard was manually operated, and the system operators
physically connected calls by plugging cords into the board's jacks. Today's
PBX is a processor in the communication system control unit programmed to
connect both inside and outside calls on a single button. In Hybrid/PBX mode,
this button is called a System Access button, and is labeled SA.
Although there is no longer a person handling cords, the communications
system operating as a PBX still requires the user to request an outside trunk. A
user simply dials a dial-out code (usually a 9) and the telephone number on an
SA button, and the system routes the call to an available outside trunk.
Thus, the major distinction of Hybrid/PBX mode, is that both inside and outside
calls can be made on the same button.
The Hybrid/PBX mode of operation accommodates the following kinds of
outside trunks:
Loop-start trunks, including basic lines, WATS, and FX
Ground-start trunks, including basic lines, WATS, and FX
DS1 facilities
Tie trunks and emulated tie trunks on T1 facilities
DID trunks
Programming the system for Hybrid/PBX mode automatically arranges the
outside trunks in functional groups, or pools, within the control unit. (See Figure
1-5. ) The system can have up to 11 separate trunk pools. The number of pools
programmed depends on both the kinds of trunks and the special needs of the
users.
1-18 Modes of Operation