PoE/PoE+ Planning and Implementation Guide 2011-09

1-4
Introduction
Power Through the Cable
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. Only two of these pairs
are used for 10Base-T and 100Base-TX data; 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 802.3af 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. HP 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.