Section 11.2: Inline Power

Cisco IP Phones have been added to the switched campus network and are managed by Cisco CallManager servers. A Cisco IP Phone's power can be sourced from inline power over the network data cable or an external AC adapter.

The external AC adapter plugs into an AC wall outlet and supplies 48V DC to the IP Phone. The downside is that the IP Phone fails when a power failure occurs at the wall outlet.

Greater efficiency is obtained when inline power is used. The 48V DC is supplied to an IP Phone over the identical Category 5 cable that is used to enable Ethernet connectivity. The DC power source is actually the Catalyst switch. An AC adapter can be used as a redundant power source. No additional power sources are necessary. The Catalyst switch can be connected to an uninterruptable power supply (UPS) to ensure that it constantly receives and transmits power at times when the ordinary AC source fails. An IP Phone can therefore be used during a power failure. Inline power has the advantage that it is available just to an IP phone. Power is not supplied to a standard PC using the same switch port.

11.2.1: Inline Power Configuration and Verification

A switch has to identify an inline power capable mechanism before it can supply it with power. To do this, the switch port transmits a 340-kHz test tone on the twisted pair Ethernet cable when it initially powers up.

An IP Phone loops the test tone and obtains pairs of its Ethernet connection. When it is connected to an inline power switch port, the switch is able to perceive that its test tone has been looped back. It then presumes that a powered mechanism exists and offers power to it. Inline power is offered on Catalyst 3550-24-PWR, Catalyst 4500 and Catalyst 6500. Inline power is supplied at 48V DC across pairs 2 and 3. The switch power supply has to be gauged correctly in order to supply constant power to an IP Phone on each switch port.

The default configuration is that each switch port tries to determine an inline powered mechanism. Use the following interface configuration command to change to the default configuration:

Switch(config-if)# power inline {auto | never}

Use the following EXEC command to verify the inline power status for a switch port:

Switch# show power inline [ type mod/num]