Specifications

Power
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To determine which standard(s) a particular phone supports refer to Table 33.
Phones can be powered remotely with the following methods:
• If the phone supports the IEEE 802.3af power over Ethernet standard, remote power to the
phone can be supplied by an IEEE 802.3af compliant Ethernet switch. Alternately, if the
phone and the powered Ethernet switch both support LLDP-MED, then the phone can
advertise its power requirements to the IEEE 802.3ab compliant switch.
• If the phone supports the IEEE 802.3af power over Ethernet standard, but the Ethernet
switch does not support the IEEE 802.3af power standard, a midspan IEEE 802.3af power
hub can be used to remotely supply power to the phone over the Ethernet cabling. The
mid-span power hub resides between the Ethernet switch (which in this case does not
support IEEE 802.3af) and the phone.
• Certain older Cisco Ethernet switches are capable of providing power over Ethernet cables
but are not fully IEEE 802.3af compliant. In this instance, a separate 3300 Power Dongle
(Cisco-compliant) can be used to allow the phone to be powered over the Ethernet cable
by the Cisco switch.
• Cisco Discovery Protocol (CDP) is a protocol that allows Cisco switches to learn information
about devices on the LAN. CDP compliant LAN devices, such as IP phones, can advertise
their power requirements to the L2 switch and the L2 switch can then deliver the required
power to the connected device. This exchange of information via CDP is independent of
the 802.3af protocol.
Recommended Phone Powering
Power over Ethernet from a central location is recommended whenever possible, resulting in
the following benefits:
• Reliable and redundant power backup (UPS), especially for emergency 911 operation.
• Lower installation cost (existing cabling can be used).
• International standard.
• Remote reset and power-off capability.
Note: The CXi and CXi II support the IEEE 802.3.af communication standard. Table 36
can be referenced to determine what the phones will advertise as their PoE power
requirements. The CXi uses IEEE 802.3af power advertisements sent from the phones
to determine what the power consumption will be, where as the CXi II actually measures
the current being drawn from the phones.
Note: To ensure proper operation, avoid connecting the IP phone to both local and
remote power sources simultaneously. An IP phone that is locally powered, either
through an AC or an Ethernet power adapter, should have its remote power feed
disabled.