Network-powered broadband communications systems have several defining characteristics that taken together differentiate them from other types of premises wiring systems:
They provide any combination of voice, audio, video, data and interactive services through a network interface unit. At any given time the customer can subscribe to all, some, one or none of the services even while the system remains in place. One example of a network-powered broadband communications system is that of a hybrid fiber-coaxial cable used for either video/audio conferencing or interactive multimedia entertainment systems.
For these systems, the utility provides the requisite data, voice etc. signals over the incoming line as well as electrical power to supply the equipment. The information signal and electrical power can coexist in the same conductors, even coming from opposite ends, without harming either.
A land-line telephone, for example, contains active semiconductor devices that require electrical power for bias. Often a wall-mounted telephone has an ac cord with a transformer-equipped plug that connects to a branch-circuit receptacle. This powers the alphanumeric readout and certain capabilities of the phone including battery recharging for a cordless receiver, but in the event of an electrical outage the phone still functions because it is powered by the network.
The utility-owned line terminates at a network interface device, usually contained in a weatherproof gray plastic enclosure that mounts on the outside wall of the facility. Typically, the enclosure has two hinged doors. One of these opens with a simple screwdriver, for customer access. It takes a specialized tool to open the other door so only utility technicians can get inside. On the customer side is a standard RJ-11 telephone jack. The idea is before calling the utility to report an outage, the customer should connect a known good phone here, listen for a dial tone and thereby ascertain whether the fault lies within the building or in the utility service.
Typical cabling consists of coaxial cable with both broadband signal and power on the center conductor, composite metallic cable with a coaxial member for the broadband signal and a twisted pair for network-supplied power, or composite optical fiber cable with a pair of conductors for electrical power. Larger systems frequently contain amplifiers that require network power. These may sit at intervals along the utility. Checking at various points along the line for electrical power, using a simple multimeter, is an effective way to test for line integrity and locate any faults, without recourse to a field-strength meter.
National Electrical Code (NEC) requirements for network-powered broadband communications systems are in Article 830. There are mandates pertaining to power limitations, outside overhead clearances, ground cover for underground installations, cable types for indoor locations (plenum, riser, general use, etc.), primary protection and grounding requirements.
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