Mitel MiVOICE BUSINESS, Engineering Manualline

Page: 114 / 381

Engineering Guidelines
100
PLANNING A POWER OVER ETHERNET
INSTALLATION
When planning a power over Ethernet (PoE) installation, the following should be taken into
consideration:
• “Cable Power Loss” on page 100
• “Power Management Features in IEEE 802.3af Compliant Switches” on page 100
• “Phone Power Consumption” on page 101
CABLE POWER LOSS
Some power loss will occur over the Ethernet cable used to connect the phone to the L2 switch
or the mid-span powered hub.
If you are using an IEEE 802.3af compliant L2 switch or mid-span power hub and the power
required by the telephone does not exceed 8 W, the power loss in the cable will be approximately
10% of the power required by the phone.
The IEEE 802.3af standard specifies that the PSE must provide at least 15.4 W and that the
PD cannot draw more than 12.95 W maximum. The difference between these two figures is
intended to allow for cable power losses over 100 m of Category-5 cable when a PSE is powering
a PD that draws that maximum allowable power.
In other words this means that under worst case conditions the cable power loss will be 19%
of the power required by the phone.
The CXi and CXi II total power budget of 100 W takes power losses incurred over cables into
account. There is no need for the installer to manually de-rate the 100 W power budget.
The above guidelines are not applicable if you are using a PoE L2 switch that is not IEEE
802.3af compliant.
POWER MANAGEMENT FEATURES IN IEEE 802.3AF COMPLIANT
SWITCHES
Some innovative vendors of IEEE 802.3af compliant switches, such as Hewlett Packard and
Mitel, provide power management features that can help to manage a situation where a group
of phones might require more total power than the L2 switch can provide. For example:
• Dynamic Power Distribution: If some phones do not require maximum power the switch will
re-distribute the unused power to other phones that may require more power.
• Power Prioritization Per Port: This mechanism allows certain ports or ranges of ports to be
deemed “critical”. Power to phones connected to critical ports will be guaranteed, phones
connected to ports that are not deemed critical may not receive power if the power capacity
of the L2 switch has been exceeded.
For details on specific L2 Switch capability and how to configure port power prioritization refer
to the L2 switch documentation.