Power over Ethernet (PoE) is a revolutionary technology that allows wireless LAN Access Points, Security network cameras, IP telephones and other IP-based devices to receive power in parallel to data over the existing Ethernet infrastructure without the need to make any modifications. PoE integrates power and data on the same cable. It keeps the structured cabling safe and does not interfere with existing network operation.
There are increasing numbers of PoE devices being deployed in industry these days. The more common uses of PoE are for communication devices such as VoIP phones and wireless LAN access points. There are many devices now taking advantage of PoE technology such as:
- VoIP phones
- Ethernet hubs
- Wireless access points
- Digital clocks
- Security cameras
- Plus others…
The IEEE 802.3af standard defines two major types of devices, the Powered Device (the actual device receiving the power) and the Power Sourcing Equipment (the device providing power to the PD).
An IP phone is an example of a PD, and a PoE capable switch is a PSE device. The 802.3af standard defines the process of detecting PD, and how power is maintained to this device.
Powered Device (PD)
An 802.3af compliant powered device is able to accept power. Powered Devices that do not support receiving power on either the data or spare pairs are not compliant, nor are devices that require power on all four pairs. Therefore, some vendor-specific PoE technologies may not work with a switch that is providing 802.3af power to the PD.
PDs are qualified into classes according to their power needs. Some PDs need low power to operate, while some may need the full 15.4 watts provided by the PSE. The class assignments are as follows:
- Class 0: requires 0.44 to 12.95 watts
- Class 1: requires 0.44 to 3.84 watts
- Class 2: requires 3.84 to 6.49 watts
- Class 3: requires 6.49 to 12.95 watts
Class 0 is the default state for a PSE when providing power to the PD. Unless this optional classification is supported, the PSE will assume the device is class 0, and provide 15.4W to the PD.
The PSE identifies the class of the connected PD by supplying voltage to the PD on the positive power pair, and measures the attenuation of the voltage on the negative power pair. The level of the returned signal classifies the PD.
Power Sourcing Equipment (PSE)
The PSE device has three main functions:
- Detect a PoE capable PD
- Supplying power to the PD
- Monitoring and terminating the supplied power
There are two types of PSE: endpoint PSE and midspan PSE. An endpoint PSE is a Power over Ethernet capable port on a switch that is directly connected to the Ethernet cable supplying power to the PD. Power is delivered by the endpoint PSE using either the active data pairs (Pins 1,2 and 3,6) or the spare pairs (4,5 and 7,8). These two power delivery methods are known as alternative A and B. Alternative A uses the active data pairs, and the alternative B uses the spare pairs.
Benefits of PoE include:
Cost Saving – It saves time and money by avoiding the need for separate Data and Power infrastructures.
Simplicity – Power is completely integrated with Ethernet infrastructure, so access to rough places for power deployment, such as ceilings, becomes simpler.
Mobility – PDs can be easily moved without need for AC power outlets. Wherever there is an Ethernet connection, a PD can be easily deployed. Because of PoE the deployment of wireless access points and network cameras has increased because power, which is very rarely available above the ceiling, can now be offered over the Ethernet cable.
Reliability – The central ups installed increases reliability.
Safety – No mains AC voltages required only safe direct-current 48V levels.
Control – SNMP management capabilities provide control and monitor of the PoE device and the powered devices.
Security – shutting down unused PDs when no one is at office ensures business security.