Cisco PoE Explained – What is Power over Ethernet?

All devices need electricity for them to operate. What if devices are installed farther from the power outlet like an access point that needs to be installed on an upper level to provide good quality Wi-Fi signal and coverage? One best solution is by using Power over Ethernet (PoE), typically on a networking device like a network switch.

Power over Ethernet (PoE) is a technology that transmits both electrical power and network data over an ethernet cable. With PoE, each Ethernet interface of LAN switches can supply power to devices like VoIP phones, IP cameras or security cameras, and wireless access points (AP).


The PoE device like LAN switches that are supplying power is called Power Sourcing Equipment (PSE). The power that is supplying is in Direct Current (DC) form. The device like IP phones or access points that are being powered is called Powered Device (PD).


How Does Power over Ethernet Work?

Some devices are not capable of being powered through ethernet ports, which might destroy the device if being plugged into a PSE (PoE Switch). PSE must also ensure that the power level supplied to PD is enough and will not destroy it. To meet those requirements, PoE has an IEEE standardized mechanism called autonegotiation.

Autonegotiation initiates a handshake procedure that establishes how much power the PD or connected device requires. The handshake needs to be established while PD is off, as PD needs the power to boot and initialize. Using autonegotiation, PSE (PoE Switch) avoids powering up devices that are not capable of receiving power over ethernet ports. Thus, it avoids damaging the ethernet port or the device itself.


PoE (Power over Ethernet) Standards

During autonegotiation, the PD is signalling the PSE of how much wattage of power it requires. The below standards are the power in watts that the PSE will supply to PD based on its requirement:

1. PoE –IEEE 802.3af standard that supplies up to 15 watts of DC power from PSE and 12.95 watts from PD due to losses on an ethernet cable. It uses two pairs of wires like CAT3 or CAT5 cables as a medium.

2. PoE+ – IEEE 802.3at standard that supplies power up to 30 watts of DC power from PSE and 25.5 watts from PD due to losses on an ethernet cable. It is also using two pairs of wires like CAT5 or higher as a medium.

3. UPoE (Universal PoE) – IEEE 802.3bt standard that supplies power up to 60 watts of DC power from PSE and 51 watts from PD due to losses on an ethernet cable. It uses four pairs of wire as a medium.

4. UPoE+ (Universal PoE +) – IEEE 802.3bt standard that supplies power up to 100 watts of DC power from PSE and 71.3 watts from PD due to losses on an ethernet cable. It is also using four pairs of ethernet cabling as a medium.


There are classes under the PoE standards. Under 802.3af, there are four classes under it that have different power in watts. On 802.3at, there is only one class, and lastly, 802.3bt has four classes under it.


PoE Implementation

Implementing PoE on the LAN network connection requires an effort for planning and designing. Powered devices, power requirements, switch ports, switch power supplies, and PoE standards should be checked before implementing PoE on the LAN network. Below are the ways we can implement PoE on our network using network switches.

1. Endspan – is a PoE switch and sometimes called “endpoint”. The ethernet port of the switch can supply both power and data to devices that support PoE like PD.

Power over Ethernet

2. Midspan – if there is an existing non-PoE switch on the network and needs to power up a device that requires PoE, then a PoE device needs to be put in between the non-PoE switch and a PD. The PoE device will connect to the non-PoE switch and will supply power to PD. A commonly known midspan is a PoE injector.
Power over Ethernet - Midspan

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