Interface between Vendor and Supplier

Interface is a device that serves as the demarcation point between the carrier's local loop and the customer's premises wiring. Outdoor telephone NIDs also provide the subscriber with access to the station wiring and serve as a convenient test point for verification of loop integrity and of the subscriber’s inside wiring.

Generically, an NID may also be called a Network Interface Unit (NIU), Telephone Network Interface (TNI), System Network Interface (SNI), or telephone network box. Australia's National Broadband Network uses the term Network Termination Device or NTD. A smartjack is a type of NID with capabilities beyond simple electrical connection, such as diagnostics. An Optical Network Terminal (ONT) is a type of NID used with fiber-to-the-premises applications.

In its role as the demarcation point, the NID separates the telephone company's equipment from the customer's wiring and equipment. The telephone company owns the NID itself, and all wiring up to it. Anything past the NID is the customer's responsibility. To facilitate this, there is typically a test jack inside the NID. Accessing the test jack disconnects the customer premises wiring from the network and allows the customer to plug a "known good" telephone into the jack to isolate trouble. If the telephone works at the test jack, the problem is the customer's wiring, and the customer is responsible for repair. If the telephone does not work, the line is faulty and the telephone company is responsible for repair.