Five PoE myths

Power over Ethernet (PoE) is invaluable for powering devices such as surveillance cameras, VoIP phones, and wireless access points over the same UTP cable used for data. In the last decade, this technology has matured and gone from a hodgepodge of home-brew and proprietary methods to the safe, reliable IEEE 802.3af PoE standard and the 802.3at PoE+ standard. But many misconceptions from the early days of PoE still linger. Here are the top five misconceptions about today’s PoE:

1. Power over Ethernet is the same as Ethernet over powerline. These concepts are often confused because they are the inverse of each other—power over Ethernet uses existing data lines to send power; Ethernet over powerline uses existing electrical wiring to send Ethernet.

2. Power over Ethernet requires special wiring. Because power over Ethernet operates on CAT5, CAT5e, or CAT6 cable with RJ-45 connectors, there’s no need to modify or upgrade your existing cable infrastructure to use PoE.

3. Power over Ethernet requires electrical expertise. Although early home-brew PoE required electrical expertise and a lot of calculating, today’s 802.3af/at standards-based PoE requires no special electrical expertise. IEEE 802.3af and 802.3at PoE can be installed without worry about whether a device is getting the wrong amount of power or—worse—getting power when it shouldn’t be. This is because power over Ethernet power source equipment (PSE) communicates with powered devices (PD) to determine power requirements.

A 802.3af or 802.3at PSE doesn’t add power to the data line until the PD indicates that it’s compatible. The PD may have an optional power class that indicates its power requirements to the PSE, enabling the PSE to budget its power load. A PSE also advertises its maximum power to the PD, which is not allowed to draw more than its allocated power.

4. All PoE is the same. The most common PoE today is standards-based IEEE 802.3af and 802.3at PoE—802.2af provides power up to 15 watts per port and 802.3at provides up to 25 watts per port to support higher-powered devices. 802.3at is backwards compatible with 802.3af.

But in addition to standards-based power over Ethernet, there are other methods for delivering power over data lines, including legacy proprietary PoE, high-wattage PoE, and passive PoE. Different kinds of power over Ethernet are not interchangeable, and you may damage a device by connecting it to the wrong kind of PSE.

5. You need to buy all new network components to add Power over Ethernet to an existing network. There’s no need to buy new PoE switches—just add midspan power injectors such as Black Box PoE Gigabit Injectors to existing switches. The power over Ethernet Gigabit Injector family is available in 802.3af and 802.3at versions in sizes ranging from 16-port rackmount models all the way down to our 802.3at PoE Gigabit Injector (LPJ001A-T), which enables you to add 802.3at PoE to just one port.

You can even power non-PoE devices by using a PoE splitter such as our Gigabit PoE+ Splitter (LPS2001), which separates the PoE power on the data line to provide DC power to a device’s power jack.

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