AT&T MERLIN LEGEND Communications System PBX System Planning Manual page 91

Trunks
Tie Trunks
Direction
3-36 Trunk Options
Use these only if the system has tie trunks, including tie trunks
instructions
emulated through a DS1 facility.
Tie trunks are private lines between your system and the central office or
between two communications systems—for example, between this system and
another system of the same type, or this system and a System 25 or a
System 75.
Tie trunks must be configured to match central office requirements or the
configuration of the system to which they tie. Review tie trunk operation with your
AT&T representative or authorized dealer and make the following decisions
about the way each tie trunk operates:
Direction
Signaling Type
Dial Mode
Dial Tone
Answer Supervision Time
Disconnect Time
Mark each tie trunk configuration on PBX System Form 3d, Outside Trunks
— Tie.
Planning Form Instructions
Transfer the trunk identification from PBX System Form 2c, System
Numbering - Trunk Jacks, to Form 3d. Write the logical ID and trunk
number for each tie trunk connected to the system.
Decide if the tie trunks will send calls one way or two ways (the factory setting).
Two-way is preferable if you anticipate light call traffic; one-way is preferable if
you anticipate heavy call traffic. Two-way in heavy call traffic increases the
chance of users on both ends of a two-way tie trunk trying to use it at the same
time, in which case neither call will go through.
If you choose one-way, also decide if the direction will be out or in. For example,
if users need only to receive information from another location but do not need
to return calls, an incoming tie trunk would be sufficient.
Planning Form Instructions
Under the "Direction" heading on Form 3d, check the direction for each tie
trunk next to its trunk number.
Check the factory-set "Two-Way" column it tie trunk is two-way.
Check either "Outgoing" or "Incoming" for each one-way tie trunk.