Information Technology Reference
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tap off the power, so they don't need a separate electrical connection. You can also
get PoE adapters that connect between your existing Ethernet hubs and the device
that needs power. They inject the voltage that is needed to remotely power the
DTE.
All you need to know about PoE from a wiring perspective is that the voltage
is a nominal 4Vv open-circuit, that most PoE ports can supply about 350 mA, and
that the voltage is considered Class 1 (nonhazardous) by the NEC . 2 You also need
to know that a miswire on the power wires could cause a direct short and overheat
or damage some PoE supplies, but that is not supposed to happen. At the very least,
the device doing the supplying will get very angry at you.
Plan for the Future
Wireless technology has been evolving much faster than ordinary LAN wiring.
Many of the new technologies are complex to use and involve early versions of
equipment and wireless protocols. New wireless standards are coming out almost
monthly. How can you keep ahead of this?
Stay Compatible
One of the major considerations in making your network available for future inno-
vations is to make sure that your wireless component vendors adhere strictly to the
standards. We cannot predict what twists and turns technology will take over many
years, but fortunately, all the IEEE 802.11 innovations have so-far remained back-
ward-compatible. This means that if you bought 11 Mbps hardware, it will still
work within a faster 54 or 108 Mbps system. Consequently, the same should be true
of faster networks to come.
Standards compatibility is the key here. It is unfortunately true that final stan-
dards are often not able to support early implementations. This was certainly true
with early twisted-pair Ethernet technology, and the same is true with wireless. As
a matter of fact, early nonstandards-based hardware is often discontinued overnight
when a noncompatible standard is released.
As these new standards emerge, some manufacturers rush the latest innovation
to market, often before the new standard has been approved. The motivation to
2 Do keep in mind that T1 span power may run as high as 165 volts or so open-circuit, and even at
very low current, it is enough to give you a very good shock. If you run that type of span power over
your cables, recommended practice requires you to use special red-topped bridge clips or protective
boots to keep anyone from inadvertently touching the circuit terminals. We don't want you to be part
of the circuit. Your components can't handle the voltage.
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