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Power over Ethernet
Power over Ethernet or PoE is a cost effective solution that allow both data and power to be provided over an Ethernet cable.
Why using PoE
There are a number of reasons why you would use PoE, The overriding reasons are cost save and easy installation.
PoE Standard
Institute of Electrical Electronics Engineers (IEEE) 802.3af Task Force has ratified the technology into the Ethernet standard.  The IEEE 802.3af standard covers the interoperability of using PoE products and covers the PSE - Endspan or Midspan, the Powered Device (PD) and the cabling requirements. There are two ways to implement PoE: one is to install Power Sourcing Equipment (PSE) called Endspan Equipment (PoE Switch), or by adding Midspan Power Insertion Equipment after the Ethernet switch. 
End Span (PoE Switch)
poe_abc_switch.jpg
Mid Span
Alternatively the power can be added to the cable after the switch by using a PSE called Midspan Equipment, as show below:
 poe_midspan.jpg
The POE Midspan solution uses an intermediate device, usually a powered device, to inject power into the network. The intermediate device is positioned between the switch and the powered device and delivers power over the network using the spare pairs of wires. The intermediate device has multiple channels (typically 8, 16 to 24), and each of the channels has data input and a data plus power RJ-45 output connector
Powered Device Recognition
Before the PSE supplies power, it has to verify that the device connected is a Powered Device (PD).  It does this by checking that the PD has a conforming signature.  All Intellinet PD modules have a built-in signature chip that does that automatically. Only upon successful completion of a valid signature will the PSE apply power to the cable. This ensures that non Powered Devices are not damaged by the PSE, thus making the system backwards compatible.
What is Power Class
This is optional within the IEEE 802.3af specification, that can be used by the PSE to manage the overall power requirements of the system.  Below is a table that shows the various power classes detailed within IEEE 802.3af.
Class Usage Maximum Power
0 Default 15.4W
1 Optional 4.0W
2 Optional 7.0W
3 Optional 15.4W
4 Reserved for future use Treat as CLASS 0
Full Power or Total Power Budget
Full power Guarantee per port like 15.4 watt. It means you can connect  as many as PD devices and the switch will be able to power them at 15.4 watt each port
Total power budget: Can only power as many devices as it has power to spare.  Total Power < 15.4* Number of PoE port
DMC Firewall is developed by Dean Marshall Consultancy Ltd