Smart Buildings Systems for Architects, Owners and Builders

That shortcoming of especially the VoIP technology spurred the effort to
supply power to the telephones over the twisted pairs in the telecommunications
network cable. The result was that in 2003 an IEEE standard was published
allowing low voltage power, 48 VDC, to be transmitted over Category 3 and
Category 5 twisted-pair cable. With the current IEEE 802.af standard, the
maximum that can be delivered to a powered Ethernet device is 15.4 watts. After
counting losses, about 13 watts is the nominal power delivery available.
Additional losses will occur with the use of switch mode power supplies.
The power is transmitted over the pairs of the Category 5 cable in one of
two ways. For 1000BASE-T the power is delivered over the signal pairs. This
method of powering is called “phantom power” (Fig. 2.11). For 10BASE-T
and 100BASE-TX Ethernet the power can be transmitted over the two idle
pairs. Transmitting the power over an idle pair is called “galvanic injection”
(Fig. 2.12).
The device supplying power is called power-sourcing equipment (PSE), and
the device being powered, such as the VoIP telephone or WAP, is called the
powered device (PD). The PSE determines the method (galvanic injection or
phantom power) used to power the PD. The PSE can be a POE-enabled net-
work switch (referred to as an “endspan”) or a device that injects power
between a network switch not POE enabled and the PD (referred to as a “mid-
span”). Midspans allow users without POE-enabled network switches to make
their networks POE enabled without procuring new network switches.

Signal Pair

Signal Pair

48 V Rx

Tx Rx

DC/DC

Converter

Tx

+/−

+/−

Spare Pair

Power Sourcing

Equipment Powered Device

4 5 1 2 3 6 7 8

4 5 1 2 3 6 7 8

Spare Pair

Figure 2.11 POE using the signal cable pairs.

28 Smart Building Systems for Architects, Owners, and Builders