Network Interface Device NID
NID is a common term that describes the interface at which an ISDN BRI circuit connects to the customer's premises in North American implementations. The NID separates the service provider's portion of the ISDN circuit from the customer's ISDN equipment and inside wiring (ISW). This device is a result of the deregulation of the phone company after the divestiture of 1984.
A typical NID is a grey box that sits on the side of the customer's house or office, and it is split into two halves. The customer's side is usually accessible by the customer, but the service provider's side is normally locked with a special type of bolt. A special tool is required to access that side of the NID and service technicians normally carry it with them. If an ISDN technician is ever sent out to your premises, the first thing they will do is try to test from the NID back to the CO. If they test successfully and you can't access the network, chances are there is something wrong with your equipment or ISW.
Continue reading here: ISDN Specifications
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