Network interface device

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.

Accessing the test jack disconnects the customer premises wiring from the public switched telephone network and allows the customer to plug a "known good" telephone into the jack to isolate trouble.

They protect customer wiring, equipment, and personnel from any transient energy on the line, such as from a lightning strike to a utility pole.

They have no capabilities beyond wiring termination, circuit protection, and providing a place to connect test equipment.

One common form for a smartjack is a printed circuit board with a face plate on one edge, mounted in an enclosure.

The telephone company usually has the ability to remotely activate loopback, without even needing personnel at the customer site.

This helps the telephone company know if trouble is present in the line, the smartjack, or customer equipment.

In fiber-to-the-premises systems, the signal is transmitted to the customer premises using optical fiber technologies.

Two simple NIDs, carrying six lines each, on the outside of a building
A German copper phone line termination box called Abschlusspunkt LinienTechnik (APL, " Demarcation point ")
Three smartjacks for T-1 circuits, in two shelves; a 66 block is on the left
An optical network terminal mounted to the outside of a building, with the cover open