Network termination 1

Network termination 1 (NT1): In Integrated Services Digital Networks (ISDN), a functional grouping of customer-premises equipment that includes functions that may be regarded as belonging to OSI Layer 1, i.e., functions associated with ISDN electrical and physical terminations on the user premises.

Note: The NT1 forms a boundary to the network and may be controlled by the provider of the ISDN services.

Source: from Federal Standard 1037C