Network interface device
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Generically, an NID may also be called a Network Interface Unit (NIU), Telephone Network Interface (TNI), System Network Interface (SNI), 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" or "Intelligent Network Interface Devices" (iNIDs) 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.
A smartjack may provide signal conversion, converting codes and protocols (e.g. framing types) to the type needed by the customer equipment. It may buffer and/or regenerate the signal, to compensate for signal degradation from line transmission, similar to what a repeater does.
Smartjacks also typically provide diagnostic capabilities. A very common capability provided by a smartjack is loopback, such that the signal from the telephone company is transmitted back to the telephone company. This allows the telephone company to test the line from the central office, without the need to have test equipment at the customer site. The telephone company usually has the ability to remotely activate loopback, without even needing personnel at the customer site. When looped back, the customer equipment is disconnected from the line.
Additional smartjack diagnostic capabilities include alarm indication signal, which reports trouble at one end of the line to the far end. This helps the telephone company know if trouble is present in the line, the smartjack, or customer equipment. Indicator lights to show configuration, status, and alarms are also common.
Smartjacks typically derive their operating power from the telephone line, rather than relying on premises electrical power, although this is not a universal rule.
Optical network terminals 
In fiber-to-the-premises systems, the signal is transmitted to the customer premises using fiber optic technologies. Unlike many conventional telephone technologies, this does not provide power for premises equipment, nor is it suitable for direct connection to customer equipment. An ONT (optical network terminal) is used to terminate the fiber optic line, demultiplex the signal into its component parts (voice telephone, television, and Internet), and provide power to customer telephones. As the ONT must derive its power from the customer premises electrical supply, many ONTs have the option for a battery backup, to maintain service in the event of a power outage.
- "Network Interface Device". Verizon Communications, Inc. Retrieved 2009-06-03.
- "Network Interface Device (NID) V9.0". Qwest Communications International. Retrieved 2009-06-03.
- "Functional Criteria for the DS1 Interface Connector" (PDF). BellSouth. June 1993. Retrieved 2009-06-03.[dead link]
- "network interface device". Federal Standard 1037C. U.S. Department of Commerce and National Telecommunications and Information Administration. Retrieved 2009-06-03.
- "Testing, Repairing and Installing Home telephone Wiring – Network Interface Device". Public Service Commission of Wisconsin. Retrieved 2009-06-03.
- "About your Optical Network Terminal (ONT)". Verizon Communications, Inc. Retrieved 2009-06-03.
- "Checking the service at the Optical Network Terminal (ONT)". Verizon Communications, Inc. Retrieved 2009-06-03.
- Point of appearance
- Customer premises wiring
- Customer premises equipment
- Local exchange carrier
- Main distribution frame
- "Network Interface Devices". HomeTech Solutions. Retrieved 2009-06-03. – Photographs of several simple NIDs, including "inside" and "outside" views