Switching to VoIP

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Chapter 8: VoIP Readiness

radio transmission introduces lag. Moreover, WLAN can’t support nearly the num-

ber of endpoint devices as a wired Ethernet segment that’s comparable in link speed.

If you need wireless phones in the office, using analog cordless phones is still a very

practical solution.

Power over Ethernet

In traditional telephony, the power to run each phone comes from the communica-

tions line that’s attached to the telephone set. On the PSTN, 20 to 50 volts DC are

required to power and signal analog endpoints. Voltage and frequency variances are

what comprise analog signals for legacy protocols such as Ear and Mouth and FXS/

FXO. Digital phone sets often have a similar voltage requirement, though their sig-

nals are carried over a digital bus. Legacy endpoints almost always get their power

from the same line that handles their signaling, though one obvious exception is

cordless phones, which need to be able to draw enough current to charge their hand-

sets’ batteries. Such draw isn’t practical over DC communications wires.

With IP phones, the power situation is a bit more flexible. Most IP phones can be

powered either through an AC/DC power transformer (like a legacy cordless phone),

or through one of two common methods of receiving power through the Ethernet

cable plant. ...