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Working with Power over Ethernet
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The maximum distance of a Power over Ethernet (PoE) device from the switch or power injector is 100 meters, the same limitation as standard network cable runs. When choosing a switch for your installation, there are two PoE specifications:
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IEEE 802.3af defines 15.4W of DC power to a device
of which only 12.95W is assured. Most fixed cameras
and domes are 802.3af specified
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IEEE 802.3at PoE+ defines 30W/port, which is
backwards compatible with 802.3af. This is used for
PTZ cameras that consume more power
Most switches communicate with PoE devices via a Link Layer Data Protocol-Media Endpoint Discovery (LLDP-MED), which allows automated power management. In some instances, you may need to enable LLDP-MED on a per-port basis or manually specify the amount of power to be supplied to each switch port. There are four classes that specify power usage by devices:
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Class Usage
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Power delivered by port
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Power used by PoE Device
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0 Default
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15.4 W
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12.95 W
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1 Optional
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4 W
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3.84 W
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2 Optional
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7 W
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6.49 W
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3 Optional
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15.4 W
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12.95 W
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4 Reserved
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NA
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NA
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If you have a PoE device that does not specify its class, the switch may default to class 0 and provide the maximum power (15W). This can cause a switch to run out of power resources when fully populated.
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