Introduction

Power over Ethernet (PoE) is a convenient and secure way to provide power to devices such as network cameras, IP telephones and wireless device points. By injecting power into the network cable, the requirement for local power at the location of a device is removed. And this is done without any degradation of network performance. There are a couple of clear benefits associated with the use of PoE:

• Reduced cabling, only one cable is required
• Easy installation for locations which are hard to reach, as no local power is needed
• No need for certified electricians
• A centrally provided power provides an easy way to add UPS functionality
• Improved security, no risk for power interruptions due to “borrowed” power outlets
• Lower system costs since new power outlets do not need to be installed

A PoE system contains of two basic components:

Power Supply Equipment (PSE)
This is a unit, which inject power into the network cable. There are two types of PSE available:

o Midspan. This is a standalone injector which just adds power to a network cable. It is used when adding PoE functionality to an existing network. Normally available in 1, 6, 12 or 24-port versions.

o Endspan. This refers to network switches that have built-in PoE functionality in addition to its switching functionality.

Powered Device (PD)

This is the device being powered via the network. It can either have built-in functionality for PoE, such as the AXIS 221 network camera, or use an external splitter such as the Power over LAN Active Splitter offered by Axis. The external splitter is a small unit that splits network and power into two separate functions/cables, and is used to provide PoE functionality to products without built-in support for PoE.

Example below of a PoE based system:
 

How does it work? A Basic description:

All networked products have two things in common; they require a network connection and a power supply. Networking is normally achieved by providing network sockets at the location of the networked product connecting to the facilities network infrastructure. The power supply normally means using the nearest power socket without considering what other devices are connected to this group or knowing where the fuse for this group is located. If controlled power is needed, e.g using UPS, it normally means providing separate power cabling from one or two locations within the facility.

Using PoE combines these two functions into one cable, the network cable.

The standard of PoE:

The functionality of PoE is defined in the IEEE 802.3af standard, and it describes the power requirements as well as detecting and removing devices.

The standard covers two different means to carry the power. The first one is built around using the spare wires (4,5,7 & 8) in a Cat 5 or 5e cable, while the other one uses the data wires (1,2,3 & 6) and so called Phantom feeding which means that power and data will share the same physical wires. The benefit of using Phantom feeding is that functionally is present when power is and the other way around, so a bad connection or damaged network wire will not cause the product to be powered up without being correctly networked which may be the case when different wires are used. The standard defines that all PDs must comply with both methods of providing power.

The standard provides 48VDC and a maximum of 15.4W per port. Due to expected cable losses, a PD may require no more than 12.95W.

The operating distance for a PoE connection is the same as for a normal network connection using Cat 5 or 5e cabling, i.e. up to 100m (328 ft).

To improve the performance of a PSE, the unit will classify connected devices based on its power requirement. Classifying a PD is optional, if the device does not support this, it will automatically be classified as Class 0.
 

Connecting a device:

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