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The appliance can be easily moved, to wherever you can lay a LAN cable

- minimal disruption to the workplace.

Safer - no AC voltages need to be added for additional network devices.

As well as the data transfer to and from the appliance, you can use SNMP

network management infrastructure to monitor and control the

appliances.

Appliances can be shut down or reset remotely - no need for a reset button

or power switch.

When implementing wireless LAN systems it simplifies the RF survey task,

as the access point can easily be moved and wired in.

Power Through the Cable

A standard CAT5 Ethernet cable has four twisted pairs, but only two of these

pairs are used for 10Base-T and 100Base-TX data. The specification allows

two options for using these cables for power:

The spare pairs are used. The pair on pins 4 and 5 are connected

together and form the positive supply, and the pair on pins 7 and 8 are

connected and form the negative supply.

The data pairs are used. Since Ethernet pairs are transformer coupled

at each end, it is possible to apply DC power to the center tap of the

isolation transformer without upsetting the data transfer. In this mode of

operation the pair on pins 3 and 6 and the pair on pins 1 and 2 can be of

either polarity.

The standard does not allow both pairs (spare and data) to be used - a choice

must be made. The Power Sourcing Equipment (PSE) applies power to either

set of wires. Hewlett-Packard has chosen to supply PoE power over the data

pair. The Powered Device (PD) must be able to accept power from both

options.

An obvious requirement of the specification is to prevent damage to existing

Ethernet equipment. A discovery process, run from the PSE, examines the

Ethernet cables, looking for devices that comply with the specification. It does

this by applying a small current-limited voltage to the cable and checks for the

presence of a 25k ohm resistor in the remote device. Only if the resistor is

present, will the full wattage be applied, but this is still current-limited to

prevent damage to cables and equipment in fault conditions.

The Powered Device must continue to draw a minimum current. If it does not

(for example, when the device is unplugged) then the PSE removes the power

and the discovery process begins again.

Introduction

Overview

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