PoE/PoE+ Planning and Implementation Guide 2011-09

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Introduction

Overview

The technology is bound to make a big impact in the world of embedded

computing. In the realm of embedded computers, where the systems are

increasingly connected to LANs and the internet, the advantages of providing

power and data through a single cable should be obvious. Consider a typical

application: a system for a multi-level car parking garage that includes security

cameras, information signs, call-for-help telephones and vehicle sensors. Such

a system is distributed over a significant area, where main power is not easily

available. A single link to a PoE Ethernet Switch makes implementing this

system less expensive and faster than using a non-PoE switch.

Since the original introduction of PoE, the IEEE has initiated a new project

called 802.3at which is commonly referred to as PoE+. This project enhances

PoE in a couple of very important ways. First, it provides up to 30W of power

to a Powered Device (PD), 25.5 watts to the device and 4.5 for line loss, and

allows this power to also run on cabling designed for 1000BASE-T. Secondly,

it provides a new mechanism for communicating power capability and

requirements using the 802.1ab Link Layer Discovery Protocol (LLDP). This

protocol addition allows PoE+ switches to deliver power more efficiently and

thereby provide power to more devices for a given power supply capacity. The

new standard is going to be a superset of the 802.3af because it provides all

the same functionality, and more. The table below shows the capabilities of

802.3af versus 802.3at.

In order for 802.3at to provide higher power, Class D (Cat5e) or better cables

are required. 802.3at also increases the minimum output voltage of the Power

Source Equipment (PSE) from 44 volts to 50 volts. For this reason, you may

note that PoE+ devices use a 54 volt power supply.

Note The detection and classification functions ensure that if two PoE sources are

attached together, power will not be improperly applied.

Classification Discovery

Power to PD Physical Logical

<25.5W 802.3at 802.3at

<12.95W 802.3af

802.3at