Power Through The Cable - HP ProCurve zl Series Planning And Implementation Manual

Here are some reasons why you might want to do this:
Simplifies installation and saves space - only one set of wires to bring to
■
your appliance.
■ Saves time and money - there is no need to pay for additional electrical
power runs or to delay your installation schedule to make them.
Minimal disruption to the workplace - the appliance can be easily moved,
■
to wherever you can lay a LAN cable.
■ 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 radio
■
frequency (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 and all four are used for
1000Base-T 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 1 and 2 and the pair on pins 3 and 6 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. ProCurve Networking switches, as a PSE, supply PoE power over
the "data pair" or, pins 1 and 2, and the pair on pins 3 and 6. The Powered
Device (PD) must be able to accept power from both options because mid-
span equipment must (according to the specification) supply power over the
"spare pair" or pins 4 and 5, and the pair on pins 7 and 8.
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.
Introduction
Overview
1-3