Network interface device

Three NIU's for T-1 circuits are pictured here in two shelves to the right of a 66 block

In telecommunications, the term network interface device (NID) is a device that serves as the demarcation point between the carrier's local loop and the customer's network. The terms network interface unit (NIU) and smartjack are interchangeable with network interface device.

Meanings of the term

According to the ATIS Telecom Glossary 2000 (An updated version of the Federal Standard 1037C standard) a NIU is a device that “performs interface functions, such as code conversion, protocol conversion, and buffering, required for communications to and from a network.”^[1] A network interface, also commonly referred to as a smart jack, is a device that serves as the demarcation point between the end user's inside wiring and local access carriers' facilities.^[2] According to the TR-73569 standard put forth by Bellsouth^[3], an NIU must have these functions:

• Conversion: Convert protocols and codes such as framing types to information usable by the channel service unit (CSU).
• Logical Loop back: Allow a logical loop to be placed in the front of the jack, to allow for remote testing. This allows for the line to be tested without an on site tech being required to put up a hard loop at the back of the jack. When the device is allowing strait-away traffic is is in “normal” state, while when in loop back mode, it is referred to as being in maintenance state.^[3]
• Signal Regeneration: The device will amplify the incoming signal as a repeater does. The regeneration must take place in front of the loop back according to this standard, because the loopback causes the non-repeated distance to effectively double, and will cause a loss of signal that would not be there when the device is in normal operation.
• Alarm indication signal (AIS): All smartjacks that comply to the TR-73569 standard need to have an AIS generator inside the device. This allows the local exchange carrier (LEC) to know that the jack is still functioning properly, and that the consumer just removed his CSU from the line.
• DC Isolation of the LEC and the end-user. The NIU is a powered device, and requires two surge protectors to comply with the TR-73569 standard: one on the send pairs, and one on the receive pairs. From there, the wire pairs enter the DC isolators.

Common practice

The NIU belongs to the public switched telephone network (PSTN) provider. Together with the CSU it represents the PSTN demarcation point.^{[citation needed]} The CSU is the first piece of customer-premises equipment.^{[citation needed]} If the NIU fails, the PSTN provider is responsible for repairs. If the CSU fails, the customer is responsible for repairs if it is owned by the customer, or the PSTN provider is responsible for repairs if the CSU is leased from the PSTN.^{[citation needed]}

For T1's in the United States, it is common to have a local loop made up of single or double pair of wire (a T1 span) from the telco. This generally carries an HDSL signal and in the case of a double pair, often carries voltage to power the NIU instead of requiring local AC connection. While common, this may vary between implementations even at the same ILEC/CLEC. The output of this device usually provides a typical four wire T1 connection that can be connected to a CSU/DSU or a router equipped with the right interface. Most NIU's come in locable enclosures capable of two or more line-card style units; the chassis usually has internal lugs for the telco connection, an edge connector for the actual NIU, and have an RJ48 or similar modular jack for the connection to the CPE. Some NIU's, frequently called smartjacks, will automatically loop the T1 connection at the modular jack if the customer removes the cable, making it easier to differentiate between faults. The PSTN provider can generally connect to the NIU over the wire directly and perform tests regardless of the presence or type of CPE connected to the device.

References

See also

• Main distribution frame
• Point of appearance
• Telephone network box