A Quick Study of Power over Ethernet

Power over Ethernet (PoE) is a technology that lets network cables carry electrical power, which eliminates the need to run VAC power to Wireless Access Points and other devices on a wired LAN. A single CAT5 Ethernet cable carries both power and data to each device, permitting superior flexibility in the locating of access points (AP) and network devices; and it appreciably reduces installation costs.

PoE begins with a CAT5 “Injector” that introduces a DC Voltage onto the CAT5 cable. The Injector is typically installed in the wiring closet near the Ethernet switch or hub. PoE injectors are compact and cost effective.

 

A PoE injector allows an installer to connect IEEE 802.3af-compliant products to a non-PoE LAN switch port. The injector can be used to connect a wireless access point, IP phone, network camera or any IEEE 802.3af powered device to a switch. AC power lines are not required for the wireless access point, network camera or IP phone. Simply connect the PoE Injector to the LAN switch port, and use the existing Cat5 cabling to deliver DC power as well as transfer data. The injector leaves the network infrastructure totally unaltered.

 

Devices that are not PoE compatible can be converted to PoE by way of a DC “Picker” or “Tap,” otherwise known as Active Ethernet “Splitters”. This device picks off the DC Voltage that has been injected into the CAT5 cable by the Injector and makes it available to the equipment through the regular DC power jack. There are two basic types of Pickers and Taps—Passive and Regulated.

 

RocketLinx® ES7105 PoE Splitter

A Passive Tap takes the voltage from the CAT5 cable and directs it to the equipment for direct connection. Therefore, if 48 VDC is injected by the Injector, then 48 VDC will be produced at the output of the Passive Tap.

A Regulated Tap takes the voltage on the CAT5 cable and converts it to another voltage. Several standard regulated voltages are available: 12VDC, 6 VDC, 5 VDC. This allows a wide variety of non-PoE equipment to be powered through the CAT5 cable.

 

To have Power-over-Ethernet, an injector and a PoE compatible device will be needed; or an injector and a non-PoE compatible device, plus an active Ethernet splitter will be required.

When a digital security camera is installed, it requires two connections. The first connection needed is a network connection to communicate between the camera and the display. Then, there is the power connection, in order to deliver the electrical power to operate the camera. However, if the camera is PoE enabled, the camera will receive its power through the network cable.

WiFi and Bluetooth APs and RFID readers are normally PoE compatible, which allows them to be away from AC outlets, and they can be relocated as needed.

Why use Power over Ethernet? By reducing the time to install a separate power cable, costs are reduced, as a certified electrician would not be needed. Devices, such as IP cameras can be moved easily, since they are not tied to a fixed electrical outlet. By having power on the network, installation and distribution of network connections is straightforward and efficient.

PoE is designed to protect network equipment from overload and under-power, since PoE power comes from a common compatible source—not a collection of distributed wall adapters. An uninterruptible power supply can be used to support the system. IEEE 802.3af PoE is designed to ensure reliable operation in any configuration that would be possible with regular Ethernet.  All the user has to do is wire up the network as normal, and the equipment will take care of power delivery. CAT5e, CAT6, etc. and RJ45 connectors are used for both regular and PoE-enabled local area networks.

The 802.3af PoE standard is okay for network devices that use up to 13 watts of electrical power, but there are devices that have adopted PoE but require more than 15 watts. There is now a new IEEE standard for additional power. It’s called PoE+ or 802.3at. The prime difference between the 802.3af (PoE) and 802.3at (PoE+) standards is the maximum amount of power they provide over the CAT5 cabling. The maximum amount of power for the 802.3af (PoE) standard is 15.4 watts, while the maximum for the 802.3at (PoE+) standard is 25.5 watts.

PoE+ provides a much needed boost to the existing 15 watt 802.3af standard. It provides up to 25 watts, enough for many devices that currently demand dedicated AC power runs. While the existing 802.3af standard has made it possible to power VoIP phones, wireless APs, even some cameras over standard Ethernet cabling it can’t meet the demands of some high-end devices, such as cameras with pan/tilt/zoom capabilities, door controllers and POS terminals.

In addition, APs that support the upcoming 802.11n standard will likely require the power of 802.3at, although single radio 802.11n APs should be able to work with 802.3af. While PoE devices are becoming more efficient and better at conserving power, new wireless access points, cameras, and certain types of security devices all continue to push the limits of the original PoE standard. PoE+ provides double the power of PoE, but it can’t be used with CAT3.

POE Power Class	1	2	3
PSE Power available	4.0W	7.0W	15.4W
Max device power	3.84W	6.49W	12.95W

Power levels available PoE+
Class	Usage	Classification current [mA]	Power range [Watt]	Class description
0	Default	0–4	0.44–12.94	Classification unimplemented
1	Optional	9–12	0.44–3.84	Very Low power
2	Optional	17–20	3.84–6.49	Low power
3	Optional	26–30	6.49–12.95	Mid power
4	Valid for 802.3at (Type 2) devices, not allowed for 802.3af devices	36–44	12.95–25.50	High power

All of this power demand is done using Ethernet Layer 2 LLDP messages sent between the Ethernet Switch and the endpoint device. This allows the switch to tell an endpoint that it is operating on backup power. Power priority is established by allowing the switch to tell an endpoint what priority it has over other endpoints. If there is a power shortage, low priority devices will lose power before high priority devices and critical devices will stay on as long as possible, even if it means shutting off everything else. The switch also tells the endpoint how much power is available and what it is allowed to draw. The endpoint uses the same variable to ask for more (or less) power, depending on what is needed.

So where do we go from here? Some engineers are now working on providing PoE+ level power on all four pairs of wire simultaneously. This would increase the total wattage up to about 50 watts. To draw more power, a few devices will be using two separate PoE+ Ethernet ports. 

For additional information:

1. https://www.veracityglobal.com/media/27197/vwp-002-poe-explained.pdf
2. https://www.cisco.com/c/en/us/products/collateral/switches/catalyst-4500-series-switches/white_paper_c11-670993.html
3. https://www.streakwave.com/powerdsine/Understanding_802_3at_PowerDsine.pdf