In special cases, tie trunks are used to join one PBX directly to another PBX. This may occur if multiple PBXs serve a
Tandem Tie Trunk Operation
In the case cited above, a station user on one PBX used the tie trunk to reach a station user on a second PBX. This is basic tie trunk operation, and it is available on any LEGEND system.
A more advanced form of tie trunk operation is known as tandem trunking. This involves using a tie trunk from one PBX to reach a trunk or other network facility. not just a station user, at a second PBX. Using two or more facilities connected in series is called tandem operation.
In a tandem trunking operation a user on one PBX can access a facility that is not available on the local PBX, but is available on a remote PBX. For instance, consider the case of a company with offices in New York, Chicago, and Los Angeles. The New York office is linked with the Chicago office by a tie trunk, and the Chicago office is linked with the Los Angeles office by a tie trunk. A station user in New York needs to talk to an outside supplier in the Los Angeles area.
If tandem operation is supported at all of the PBXs, then the station user in New York can access the tie trunk to Chicago and, from Chicago, access the tie trunk to Los Angeles. When the PBX at Los Angeles has been accessed, the station user in New York can use a local, Los Angeles trunk to call the supplier. (There will be some transmission degradation in this type of connection unless a digital network is used.) Of course, these tie trunks can still be used to access other station users at the remote PBXs, just like ordinary tie trunks.
\fBTandem Tie Trunk Operation\fR
Sta. B (New York) uses Tie Trunk to access a local Los Angeles Trunk