POE (Power Over Ethernet) Technology Overview

This document provides POE Summary & Overview, including the following topics that all together create POE:

  • Standards
  • Signature
  • Classification
  • Power
  • Cables
  • Limitations
  • Filtering
  • Protection


  • PoE technology is a way of sending (DC)
    power over CAT5/6 Ethernet cable


  • Benefits of PoE
  • Distance – Place devices up to 100m away
  • Low Cost Data cable – CAT-5/6/7 (802.3af CAT3)
  • Low Cost Install – By IT staff, Mains Electricians not needed
  • Safety – Low voltage<60V (SELV) Classed safe to touch
  • High Power – Up to ~180W



General Terms

  • PSE – Power Sourcing Equipment
  • PD – Powered Device
  • A Midspan PSE is a unit which adds PoE capability to legacy switches
  • An Endspan PSE is a open power supply which is included in modern ‘PoE capable’ switches



POE – Basic Idea


Without POE

POE Standards

POE : IEEE 802.3af,2003 : 15.4W

POE+ : IEEE 802.3at, 2009 : 30W

POE “Ultra” IEEE 802.3bt : 60W

PoH : PoHDBaseT : 95W


PoE Original Standard (802.3af-2003)

  • Approved in 2003 for PDs up to 12.95W (350mA) at 48V DC

– PoE requires handshake before applying power
– Power Sourcing Equipment (PSE) transmits power over
2- pairs (4 of 8 wires) on Cat3/5/5e/6 cable

– Connect/disconnect protocol for applying/disconnecting power

  • Detect device that needs power
  • Determines power needed
  • When to turn on power
  • Detect device disconnect (AC and DC disconnect)
  • When to remove power

Physical layer mechanism for PSE’s to characterize power demands of individual PD’s and thus manage power delivered per port.


How it works?

Power Sourcing Equipment (PSE) puts a DC voltage of typically 48V onto a CAT-5 cable.

E.g. Up to 15.4W of power is put onto the cable but only ~13W max is available to the IP device.

A Powered Device (PD) module extracts the 48V and converts it to lower voltage to power the end device.

Silver Telecom’s IEEE802.3af PD modules fit inside the IP device enclosure and can output any voltage between 2.5V to 14V, and 17.5V to 25V.


Functions of PD module

Power disconnect

  • A user swapping a connection from a powered to non-powered device means the 48V connection could damage it.
  • 3af specifies the PSE disconnect power when it stops receiving a power signature from the PD. E.g. current dropping below 5mA causes the connection to be terminated, power will be cut around 250ms later.

How is isolation achieved?

  • IEEE 802.3af – 2003 requires Powered Devices to be isolated between input and output (but the isolation performance is specified in EN60950).
  • Silvertel can do this using a 1500V (Impulse test) isolation transformer (DC/DC converter) and clearances under the transformer and on the PCB on the PD module.


PSE and PD interconnection

  • PSE must see valid signature before ouputting e.g. 48V
  • The PD module provides this (25k Signature resistance)
  • PD module ensures the polarity of 48V feed is correct
  • PD module can signal to the PSE how much power
    the end equipment requires (Classification current).
  • DC/DC on the PD module converts 48V to a lower
    voltage to power the network equipment.
  • The system must conform to the requirements
    in e.g. IEEE Std 802.3af – 2003.

 Classification Overview

Power capacity limits

  • Power is lost in the cables between PSE and PD
  • Example given for 802.3af – 2.45W/7V drop over 100m
  • This defines the input power to the PD.
  • Power is also lost in:
    • The bridge rectifiers (Volt drop of ~ 0.7V per diode)
    • Filtering components (low)
    • The DC/DC converter (~85% efficient)
  • All losses must be taken into consideration during design process
  • Use of CAT6 -cable resistance/100m=7W will decrease losses


Minimum Voltages


  • Compliant PSEs must provide 44V
  • However, Silvertel Minimum PD module input Voltage at start up>= 42V to ensure correct power up of PD module
  • Once running – PD module input voltage can drop to 36V


  • Compliant PSEs must provide 50V
  • PSE output voltage minimum is directly dependent on the PD module load – i.e. more than 50V may be needed