AT&T MERLIN LEGEND System Reference Manual page 474

Functional Description
Tie-Trunk Signaling
Tie trunks transmit via three different signaling formats, each made up of a
specific mode and a specific type.
The dual in-line packaging (DIP) switches on the 400EM module select the
signaling modes needed for tie-trunk transmission; the signaling type is selected
during system programming.
Signaling Modes
There are two signaling modes:
Simplex mode. Two signaling leads superimposed onto the analog
transmission leads provide a 2-pair wire interface for connecting two local
systems at minimal cost.
E&M mode. This is a standard interface. The E&M signaling leads are
isolated from the transmission leads, requiring a 3-pair wire interface.
In the simplex mode, protective resistance is always included in the circuit. The
E&M mode can be either protected or unprotected from high-voltage transients
or fluctuations. In the protected mode, a resistance is added to the leads to
reduce current peaks. The protected mode is used when there is no network
interface to protect the circuit from outside interference.
The unprotected mode must be used for an E&M type 1 standard interface (see
below) to meet the specified voltage-drop criteria. This mode is used when there
is a network interface.
Signaling Types
Three different signaling types combine with the signaling modes. Together
these create the proper signaling format for each system.
Type 1 standard. This is the factory-set type, which is used to connect two
systems through two intermediate telephone company central offices. The
switches must be set for E&M mode.
Type 1 compatible. This type is used to connect directly without
intermediate telephone company central offices. One system is set to type 1
standard, the other to type 1 compatible. The switches must be set for E&M
m o d e .
Type 5 simplex. This type is used to connect similar systems or systems With
compatible signaling that are located in the same building or on the same
business campus.
The choice of a tie-trunk signaling format to connect two systems depends on
the particular application and the systems being connected, including whether
or not the tie-trunk signals pass through telephone company lines or over
customer-owned cable. Table 4-1 shows how to determine tie-trunk
compatibility between this system and other systems.
Control Unit Operation 4-5