In telecommunications, a Network Interface Device (NID) is a device that serves as the demarcation point between the carrier's local loop and the customer's premises wiring.
Generically, an NID may also be called a Network Interface Unit (NIU), Telephone Network Interface (TNI), or telephone network box. 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.
The simplest NIDs are essentially just a specialized set of wiring terminals. These will typically take the form of a small, weather-proof box, mounted on the outside of the building. The telephone line from the telephone company will enter the NID and be connected to one side. The customer connects their wiring to the other side. A single NID enclosure may contain termination for a single line or multiple lines.
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.
Most NIDs also include "circuit protectors", which are surge protectors for a telephone line. They protect customer wiring, equipment, and personnel from any transient energy on the line, such as from a lightning strike to a telephone pole.
Simple NIDs contain no "intelligence" or "logic"; they are "dumb" devices. They have no capabilities beyond wiring termination, circuit protection, and providing a place to connect test equipment.
Several types of NIDs provide more than just a terminal for the connection of wiring. Such NIDs are colloquially called "smartjacks" as an indication of their built-in "intelligence", as opposed to a simple NID, which is just a wiring device. Smartjacks are typically used for more complicated types of telecommunications service, such as T1 lines. Plain old telephone service lines generally cannot be equipped with smartjacks.
Despite the name, most smartjacks are much more than a simple telephone jack. One common form for a smartjack is a circuit board with a face plate on one edge, mounted in an enclosure.
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