What is PoE
Power over Ethernet or PoE is a cost effective solution that allow both data and power to be provided over an (or same) ethernet cable to an endpoint.
Why use PoE
There are a number of reasons why you would use PoE, but in most cases the overriding reason is cost and easy of install.
For an example:
It can cost between $100 and $200 to run an ethernet cable to a wireless access point. But it can costs a further $400 to run an electrical conduit to power that access point.
The other alternative is to install Power Sourcing Equipment (PSE) called Endspan Equipment into the ethernet switch, or by adding Midspan Power Insertion Equipment after the ethernet switch.
This will combine the data and power at the source to be sent through the ethernet cable. The power from the cable can then be extracted at the endpoint with a Powered Device (PD) to operate the access point.
This solution removes the need to run expensive electrical conduit and will cost considerably less than $400.
What is the 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. It is endorsed by all network players that power IP-connected devices, such as voice-over-IP (VoIP) phones, wireless LAN (WLAN) access points (AP) and advanced security devices, over the existing Ethernet cabling infrastructure.
How is the power put onto the ethernet cable
The power can be added to the ethernet cable if the switch has a built-in power source in the form of PSE is called Endspan Equipment, as shown below:
Alternatively the power can be added to the cable after the switch by using a PSE called Midspan Equipment, as show below:
PoE Injector below
Under normal operation the PSE supplies a nominal d.c. voltage of 48V, this is limited to a maximum of 15.4 Watts (at present time).
How is the power extracted from the cable
To extract power from the cable the connected device must be PoE enabled.
How does the PSE know that the connected device must be PoE enabled
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 |
What is Power over LAN
Power over LAN or PoL is (bassically) another term used for Power over Ethernet or PoE.
Example below of another mid-span
More yet to consider
Businesses invest huge amounts of funds in their existing IP infrastructure, which includes high speed switching, security capabilities and guaranteed QoS features that are critical to VoIP implementation.
As deployment of IP telephony systems become more widespread, businesses also need consider methods of delivering power to their IP phones.
The approval of the IEEE 802.3af Power over Ethernet (PoE) standard effectively removed a significant barrier to the widespread adoption of IP telephony, in that it greatly reduced the challenge of providing power to VoIP devices. 803.2af enables electric power to be delivered along the same cables that transport Ethernet data, allowing PoE-enabled switches and Midspans to deliver power over standard Ethernet cabling to IP phones, Access Points, Routers, Switches and more.
PoE's advantage over the other protocols is the ubiquity of Ethernet, which encourages the development of products to support the standard—not only in the Enterprise market where it was born, but also in a broadening range of applications in industrial environments.
Increasingly, in new industrial system implementations and in system upgrades, the benefits of an all-Ethernet network are winning over specialized local networks and winning bids/projects.
Ethernet data transmission requires two of the four twisted pairs (most often) that are available in a standard CAT5 Ethernet cable. The CAT5 cable is the most widely used Ethernet connectivity medium on the market.
PoE also utilizes two twisted pairs, and the IEEE 802.3af standard allows either the unused pair or the data pair for power transmission.
A midspan power source injects power into the cables in current enterprise Ethernet deployments. The MPS (mid-span) sits between the central switch and the target PDs. In the PDs, a picker or splitter extracts the power current from the wires in the cable, while allowing the data signal to proceed as normal.
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