Technical Note: Power-over-Ethernet on BES products
NN70000-003 issue 1.00 Copyright © 2008 Nortel Networks - 5 -
1. Introduction
Power over Ethernet is a technology enabling efficient and simplified wiring of various Ethernet-based devices
like IP Phones, WLAN Access Points, printers, etc… It also brings the value of a centralized the power source
for network critical devices and enables efficient installation of power protection mechanisms.
The Nortel Business Ethernet Switch (BES) products include a wide range of PoE-capable Ethernet switches.
The purpose of this document is to provide information about the PoE capabilities of the various BES products
such as to facilitate the deployment and engineering of PoE solutions.
Different BES models have different PoE power budgets and capabilities. This document includes one section
for each sub-family of BES models.
2. Definition and Concepts
Guard Band
Some PoE capable switch includes a "Guard Band" which protects against an additional Powered Device (PD)
from causing the Ethernet switch to cycle a Powered Device (PD) on and off when the new connected device
brings the total power demand above total PoE power budget.
If the current power draw across all PoE devices exceeds the total power budget minus the Guard Band, then
no new devices are allowed to be power up.
PoE Watts and Current and Voltage
Per 802.3af standard, each port on a PSE device must be capable of delivering up to 350 mA of current within a
voltage range of 44 to 57V volts. For the minimum 44V, this yields 15.4 watts of power, which is the normal
documented max for a PoE Port, but this is the power at the Ethernet Switch port. The Powered Device cannot
draw more than 12.95 watts max – the difference accounts for the excepted power loss over 100m of twisted-
pair cable.
The 802.3af standard allows the PoE Voltage to vary from 44V to 57V. Since the voltage is a property of the
Ethernet Switch and not the Powered Device (PD), and the current draw is a property of the PD, PoE power
management is typically based off of current draw at a nominal 44V. This means that on a typical PoE
installation, where the voltage is the nominal 48V, the measured power draw on the devices, will be higher than
one would expect. For example a 15.4W max power device (based on 44V nominal) will draw 16.8W at 48V at
the Power Sourcing Equipment (PSE).