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Basic Cable Uses

It is important to know the fundamental differences between power, telephone, and LAN links. Power cables carry low-frequency signals (typically 50 or 60Hz) and are designed to minimize power loss. Standard telephone cables do not carry much power but use up to 4kHz of bandwidth. LAN cables carry high-bandwidth, low-power signals (the most widely adopted LAN protocols use bandwidths of 4MHz and higher) and are designed to allow correct decoding of signals that are transmitted over the cabling. The industry references for this type of cable are low-voltage, telco, or data communications cable, as opposed to the normal or high-voltage electrical power cables.

The actual bandwidth required by the media access standard being installed (Ethernet, Fiber Channel, and so on) determines the minimum parameters for selecting, installing, testing, and operating twisted-pair links for networking—not the raw throughput bit-rate. The existing LAN standards that carry raw throughput data rates of between 10Mbps and 1000Mbps across two or four pairs of copper conductors typically use bandwidths between 10MHz and 100MHz. Higher speed implementations may require components and workmanship that place their costs about equal to a fiber optic cable system. Although fiber has its own set of issues, it can also avoid some distance limitations, various forms of electrical interference, and other problems associated with higher speeds on copper.

Recent efforts to define ways to deliver DC power (Power over Ethernet [PoE]) across a point-to-point network cable link for operating an end station such as a wireless access point have added complexity to an already difficult topic. The cabling standards working groups are cooperating with the IEEE to discover what effect this will have in the workplace.

The primary concerns for PoE fall into two areas:

  • Ensure that the power is supplied in an electrically safe way, both for the users and for the equipment.
  • Monitor the installation to ensure that pushing current through closely grouped cables (loose, in cable trays, or in conduit) does not cause the temperature to rise above the working range of the cable plant (60°C or 140°F) and create a fire hazard. See TIA-TSB 184 for more details.

A single 48-port Ethernet switch with all ports supplying 15 watts would need a big UPS and would deliver a combined 720 watts to the cable plant. The cable, the switch, and the UPS will become warmer. In addition to fire hazard concerns about the cable raceways as the amount of PoE-delivered power increases, it may be necessary to install extra power circuits and air conditioning in the wiring closet.

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