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Chapter 2: Ethernet Line Cards
Power over Ethernet on the AT-SBx81GP24 Line Card
Powered Device
Classes
62
This section applies to the twisted-pair ports on the AT-SBx81GP24 PoE
Line Card. Power over Ethernet (PoE) is a mechanism by which ports
supply power to network devices over the same twisted pair cables that
carry the network traffic. This feature can simplify network installation and
maintenance because it allows you to use the switch as a central power
source for other network devices.
Devices that receive their power over Ethernet cables are called powered
devices (PD), examples of which include wireless access points, IP
telephones, web cams, and even other Ethernet switches. A PD
connected to a port on the switch receives both network traffic and power
over the same twisted-pair cable.
The AT-SBx81GP24 Line Card automatically determines whether a device
connected to a port is a PD. A PD has a signature resistor or signature
capacitor that the line card can detect over the Ethernet cabling. If the
resistor or capacitor is present, the switch assumes that the device is a
PD.
A port connected to a network node that is not a PD (that is, a device that
receives its power from another power source) functions as a regular
Ethernet port, without PoE. The PoE feature remains enabled on the port
but no power is delivered to the device.
The IEEE 802.3af and 802.3at standards define five powered device
classes. The classes are defined by the power requirements of the
powered devices. The classes are shown in Table 20. The AT-
SBx81GP24 Line Card supports all five classes.
Table 20. IEEE802.3af and IEEE802.3at Powered Device Classes
Class
0
1
2
3
4
Maximum
Power Output
Usage
on the PoE
Port
Default
15.4W
Optional
4.0W
Optional
7.0W
Optional
15.4W
Optional
30.0W
PD Power
Range
.044W to
12.95W
0.44W to 3.84W
3.84W to 6.49W
6.49W to
12.95W
12.95W to
25.9W
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