Working with Power over Ethernet 

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:

 

  • 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

  • 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:

 

Class Usage

Power delivered by port

Power used by PoE Device

0 Default

15.4 W

12.95 W

1 Optional

4 W

3.84 W

2 Optional

7 W

6.49 W

3 Optional

15.4 W

12.95 W

4 Reserved

NA

NA


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.

Featured Video: PoE Basics

Get the basics of Power over Ethernet in our two-minute drill video. Our two-minute drill and other support videos provide brief overviews of key installation topics. These videos are created to arm you with the technical information you need to design dependable IP video systems.

 

Watch the PoE Video
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Tip: Watch Out for Older Cisco Devices

Cisco devices that utilize Cisco Inline Power only supply 7W/port, only work with 10/100 speed switches, and are only compatible with Cisco PoE devices. Cisco devices that support Enhanced PoE only supply 20W and do not replace the IEEE 802.3at (PoE+) standard. Newer devices, such as the Cisco 3750 and 3560, support 802.3at.

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