AT&T MERLIN LEGEND Installation & Maintenance Manual page 75

System Description
Tie-Trunk Networks
The following tie-trunk networks can be used with the system.
Nontandem
The nontandem network is used primarily to connect station lines
at both ends; it does not connect to another tie trunk (see
Figure 9).
Tandem
The tandem network has a station on one system connected to a
station on a third system via a tie-trunk connection in a second
system. The tandem network can also be used to connect to an
external facility in the third system, but the grade of service will
be fair to poor if one or both tie trunks are analog (see Figure 9).
Intertandem
The intertandem network has a tandem tie trunk connected to tie
trunks at both ends. It is not recommended for analog tie trunks
(see Figure 9).
1-44 Networking Capabilities