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What is Power over Ethernet (PoE)?

>> What is Power over Ethernet?

1. Introduction

Power-over-Ethernet (PoE) or "Active Ethernet" eliminates the need to run 110/220 VAC power to Wireless Access Points and other devices on a wired LAN. Using Power-over-Ethernet system, installers need to run only a single CAT5 Ethernet cable that carries both power and data to each device. This allows greater flexibility in the locating of AP’s and network devices and significantly decreasing installation costs in many cases.

Power over Ethernet (PoE) technology describes a system to pass electrical power safely, along with data, on Ethernet cabling. The IEEE standard for PoE requires category 5 cable or higher for high power levels, but can operate with category 3 cable for low power levels.

2. How is power supplied?

Power is supplied in common mode over two or more of the differential pairs of wires found in the Ethernet cables and comes from a power supply within a PoE-enabled networking device such as an Ethernet switch or can be injected into a cable run with a midspan power supply.

Power-over-Ethernet begins with a CAT5 "Injector" that inserts a DC Voltage onto the CAT5 cable. The Injector is typically installed in the "wiring closet" near the Ethernet switch or hub.

Some Wireless Access Points and other network accept the injected DC power directly from the CAT5 cable through their RJ45 jack. These devices are considered to be "PoE-Compatible" or "Active Ethernet Compatible".

Devices that are not "PoE Compatible" can be converted to Power-over-Ethernet by way of a DC "Picker" or "Tap". These are sometimes called 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.

Therefore in order to use Power-over-Ethernet (PoE) you need either:

  1. (Injector) + (PoE compatible device)
  2. (Injector) + (non-PoE compatible device) + (Picker)

The following figure shows a wireless LAN access point (non-PoE compatible) which is powered by a PoE splitter (picker).

>> Power over Ethernet Standard

The original IEEE 802.3af-2003 PoE standard provides up to 15.4W of DC power (minimum 44 V DC and 350 mA) to each device. Only 12.95W is assured to be available at the powered device as some power is dissipated in the cable.

The updated IEEE 802.at-2009 PoE standard also known as PoE+ or PoE Plus, provides up to 25.5W of power. Some vendors have announced products that claim to comply with the 802.3at standard and offer up to 51W of power over a single cable by utilizing all four pairs in the Cat5 cable.

Numerous non-standard schemes have been used prior to PoE standardization to provide power over Ethernet cabling. Some are still in active use.

 

>> Applications of Power over Ethernet (PoE)

Uses for Power over Ethernet include:

  • Network routers
  • A mini network switch installed in distant rooms, to support a small cluster of ports from one uplink cable. (These ports on the mini-switch do not themselves provide PoE.)
  • Network webcams
  • Network Intercom/Paging/Public address systems and hallway speaker amplifiers
  • VoIP phones
  • Wall clocks in rooms and hallways, with time set using Network Time Protocol
  • Wireless access points
  • Outdoor roof mounted radios with integrated antennas, 802.11 or 802.16 based wireless CPEs (customer premises equipment) used by wireless ISPs

>> Types of Picker / Taps

Two basic types of Pickers and Taps are available: Passive and Regulated.

A Passive Tap simply 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.

 

>> Voltage and Pinout Standards

Although the IEEE has a PoE standard called IEEE 802.3af, different equipment vendors use different PoE voltages and CAT5 pin configurations to provide the DC power. Therefore it is important to select the appropriate PoE devices for each piece of equipment you plan to power through the CAT5 cable.

The IEEE has standardized on the use of 48 VDC as the Injected PoE voltage. The use of this higher voltage reduces the current flowing through the CAT5 cable and therefore increases the load and increases the CAT5 cable length limitations. Where the maximum cable length has not been a major consideration some vendors have chosen 24 VDC and even 12 VDC as their "injected" voltage.

 

>> Multi-Port Injectors

Several manufacturers offer Multi-Port Injectors including 6 and 12-port models. These models are less versatile since they are only used where many devices are to be powered through the CAT5 cable originating in a single wiring closet or from a single switch. They typically operate in exactly the same manner as their more popular single-port counterparts.

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