Network interface unit
The term network interface unit (NIU) has broad application in the delivery of carrier-based telecommunications services to both residential and commercial customers. In general it is the name for the device that serves as the carrier's point of demarcation, also called the demarc. The customer is responsible for all network equipment and cabling on its side of the NIU, and the service provider is responsible for the NIU and all network and cabling on its side of the NIU. Another commonly used term is network interface device (NID).
In general, the NIU can provide several functions, including (but not limited to):
- Protocol or cabling conversion: If the customer network and the carrier network implement different protocols and/or cabling, conversion must be done by or at the NIU.
- Loop-back testing: The NIU very often has the ability to be placed in a loop-back mode, making it possible to send test signals through the device remotely.
- Signal regeneration: In some cases, the NIU provides for signal regeneration in much the same way a repeater can. If the NIU has a loop-back capability, it is critical for the repeater function to be placed on the carrier side of the loop-back capability. Otherwise, the loop-back function could create failures due to excessively long transmissions distances that would not be present in normal transmission mode.
- Alarm indications: Some NIUs can generate messages into the carrier network reporting on their state, so that the carrier can distinguish between outright failures of the NIU and circumstances where the customer's equipment has been powered down or disconnected from the NIU.
- Network isolation: The NIU often serves as a point of optical or electrical isolation, protecting one network from the negative impacts of failure in the other.
Specific instances of the NIU
There are many examples of devices that would be classified as NIUs. Some of these include:
- T-1 smart jack: Carriers that deliver a T-1 service to their customers (which would include any ISDN PRI service), terminate that service in a device known as a smart jack, which is a form of NIU. The smart jack is powered, and complies with a specific set of standards that includes requirements for loop-back testing capability, DC isolation, signal regeneration, alarm signal indication (AIS), and protocol conversion. The image to the right is an example of a T-1 NIU manufactured by CXR Larus. The graphic above indicates its position in the overall circuit.
- Cable system NIU: If you receive any service from an MSO or other cable television service provider, then somewhere on your property, it could be inside or outside, you will have an NIU. The cable system NIU is usually a grounded but otherwise electrically passive device, and is typically locked. It is the point at which the external coaxial cable plant (owned by the provider) connects to the internal cable plant (owned by the customer). It may contain a splitter that can segment the subscriber drop into multiple internal cables, and this splitter may be powered in some circumstances. An example of this device can be seen to the left.
- POTS NIU: Do you have telephone service from a LEC, and is it delivered over twisted-pair cabling? If so, then poke around and you will find, often suspended on the outside of your building, the NIU (depicted in the image to the right). As with the cable company's NIU, this box is typically unpowered and grounded, and simply serves as a meet point of the inside cabling plant with the carrier's local loop.
- GPON ONT: With more and more carriers providing optical fiber all the way to the customer premises with a variety of passive optical network (PON) technologies, another form of NIU is emerging: one that terminates optical fiber. The device is generically referred to as an optical network terminal (ONT). Unlike the cable system and telephone NIUs, this NIU literally terminates the fiber (in large part because few customers have optical fiber internally). Because it is terminating the optical fiber and converting to traditional in-building wiring (e.g., twisted-pair and coaxial cable), this NIU needs to be powered. On the customer side of this device will often be found a traditional RJ-11 interface to connect to conventional analog telephones, a coaxial interface to deliver traditional television services, and an Ethernet interface to provide any of a variety of data services, most commonly Internet access. The image to the left depicts an example of a Gigabit PON (GPON) ONT. This particular model is from Siemens.
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