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Understanding PoE Terminology

Power over Ethernet (PoE) is a technology that enables Ethernet cables to carry electrical power to devices such as WiFi access points, IP cameras, VoIP phones, signage, and other networked devices along with data. Before the advent of PoE (circa 2003), outlets and external power adapters were necessary to power these devices. PoE uses two or four twisted pairs in a standard Ethernet cable to supply DC power to PoE-enabled devices. It is transmitted on the data conductors by applying a common-mode DC voltage to each pair. Because twisted-pair Ethernet uses differential signaling for data communication, PoE does not interfere with data transmission.

PoE provides several benefits:

• Time and Cost Savings – Network cables do not require a licensed electrician for installation.

• Flexibility – An electrical outlet is not required, so devices can be located and repositioned as needed.

• Scalability – Power is available on the network and new connections can be added as needed.

• Safety – PoE power delivery is intelligent and has built-in safety features to protect from system failures.

• Reliability – PoE enables a centralized power source, which can be backed-up by an uninterruptible power supply.

The basic PoE  terminology and configurations are described below:

Power Source Equipment (PSE)The device that provides the power, usually a PoE-enabled switch or PoE Injector. The PSE can be either End-Span or Mid-Span
Powered Device (PD)The device powered by the PoE system.
End-SpanEnd-Span power originates from a powered port on a PoE Ethernet switch.
Mid-SpanA mid-span injector is a device that provides PoE power in the middle of a cable run between the network switch and the PD.
CallingPoE uses twisted-pair cabling to connect the PSE and PD. The gauge and material of the cable and interconnecting hardware has an impact on power loss.


The figure below depicts End-Span and Mid-Span PoE configurations. Typically, End-Span equipment is used in new installations (switch upgrades). Mid-Spans are used to add PoE in a system with an existing non-PoE switch.

Download our free PoE Reference Flyer for powered devices including the IEEE 802.3bt specifications and more.

Author Bio –
Product Manager – Wireless

Julio Petrovitch is a product manager at NetAlly, plus a certified CWNA/CWAP/CWDP/CWSP. He’s worked with network design, testing and validation for more than 15 years. Throughout he’s career he has had the opportunity to work with multiple networking technologies, including POTS, DSL, Copper/Fiber Ethernet, Wi-Fi, Bluetooth, and BLE.