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A network interface device is a device that serves as a demarcation point between the carrier's local loop and the customer's on-premises wiring. This is where the data wires end and a customer's home wiring starts. Phone companies are responsible for maintaining the wiring up to the network interface device, while customers are responsible for the wiring after this point. These devices are located outside of houses, generally somewhere near the utility entrance point.
A network interface device is also known as a network interface unit.
Network interface devices are the little gray boxes outside of homes that provide connection to a public switched telephone network. One side of the network is locked with a small tie ring, while the other is left open for the customer to access.
When a connection is not working properly, determining whether it is the customer's or the provider's wiring that is faulty is facilitated through a test jack inside the network interface device. If the test jack works, there is something wrong with the customer's wiring and the customer needs to perform the necessary repairs. If the test jack doesn't work, the line is determined to be faulty and must be repaired by the network service provider.
Most network interface devices include circuit protectors, which protect the customer's wiring, equipment and personnel from transient energy on the line such as from a lightning strike to a telephone pole. A simple network interface device has no intelligence or logic, and lacks capabilities beyond wiring termination, circuit protection and a place to connect test equipment.