Most telecommunication network interfaces are asymmetric. One side implements the network end of the interface, thus called network termination. This is the interface provided by the network provider (a.k.a. carrier) where the customer connects his network equipment. The other side implements the network customers equipments end of the interface. This side is usually called TE (terminal equipment).
Some interface implementations (such as e.g. innovaphone's ISDN interfaces) support both sides. The interface is then said to run in NT-mode or TE-mode.