Axis Network video Manual page 74

74 CHAPTER 9 - NETWORk TECHNOLOGIES
According to IEEE 802.3af, a PSE provides a voltage of 48 V DC with a maximum power of
15.4 W per port. Considering that power loss takes place on a twisted pair cable, only 12.95 W
is guaranteed for a PD. The IEEE 802.3af standard specifies various performance categories for
PDs.
PSE such as switches and midspans normally supply a certain amount of power, typically 300 W
to 500 W. On a 48-port switch, that would mean 6 W to 10 W per port if all ports are connected
to devices that use PoE. Unless the PDs support power classification, the full 15.4 W must be
reserved for each port that uses PoE, which means a switch with 300 W can only supply power
on 20 of the 48 ports. However, if all devices let the switch know that they are Class 1 devices,
the 300 W will be enough to supply power to all 48 ports.
Class
0
1
2
3
4
Table 9.1a Power classifications according to IEEE 802.3af.
Most fixed network cameras can receive power via PoE using the IEEE 802.3af standard and are
normally identified as Class 1 or 2 devices.
With IEEE 802.3at pre-standard or PoE+, the power limit will be raised to at least 30 W via two
pairs of wires from a PSE. The final specifications are still to be determined and the standard is
expected to be ratified in mid-2009.
In the meantime, IEEE 802.3at pre-standard (High PoE) midspans and splitters can be used for
devices such as PTZ cameras and PTZ dome cameras with motor control, as well as cameras with
heaters and fans, which require more power than can be delivered by the IEEE 802.3af standard.
Midspans and splitters
Midspans and splitters (also known as active splitters) are equipment that enable an existing
network to support Power over Ethernet.
Minimum power
level at PSE
15.4 W
4.0 W
7.0 W
15.4 W
treat as Class 0
Maximum power
level used by PD
0.44 W - 12.95 W
0.44 W - 3.84 W
3.84 W - 6.49 W
6.49 W - 12.95 W
Usage
default
optional
optional
optional
reserved for future use