The P32xi/mxi systems have an imaginary line drawn through them which divides them into two parts: interfaces to the outside world (these are the Trunk side interfaces) and any interface that is considered local to the system (the Line side interfaces).
One area of confusion is that T1 circuits can be either Trunk or Line interfaces. T1's are Trunks whenever they interface to the PSTN or a private external network. T1's are (almost always) considered to be Line interfaces whenever they are connected to devices (such as routers and PBX's) located in the same physical building (or local facility) as the P32xi/mxi system.
FXO/DPT circuits are also (almost always!) considered Trunk interfaces since they are typically connected to the local Central Office (PSTN). FXO circuits can be configured as Off-Premise eXtensions (OPX) where they would be considered to be Line side circuits, but this is extremely rare. In DPT (DID) mode the circuit is also considered to be a Line side circuit since it is connecting to a local PBX. For more description of DID circuits, see the section on DID Signaling.
FXS/DPO circuits are also (almost always!) considered Line interfaces since they are typically connected to the local Central Office (PSTN). In DPO (DID) mode the circuit is considered to be a Trunk side circuit since it is connecting directly to a central office.
Note that either Lines and Trunks can be provisioned from either the Line or Trunk Provisioning screens. This is handy for experienced StarCAD users.
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