IIUC Notes – Powering Cisco Phones

Feel free to correct anything that is wrong or incomplete.

  • Power over Ethernet (PoE)
    • Can provide power to a Cisco phone, access point, security camera, etc., through the network cabling, eliminating the need to plug the phone into the wall for power.
    • Generic term for providing power on the Ethernet cable
    • Provides centralized power that can be put on a UPS
    • Allows devices to be located away from power outlets
    • Removes cabling clutter at the user's desk
    • Can be provided through PoE-enabled switches, power panels or inline couplers (power injectors)
    • Oversubscription is common
      • If every device on a switch asks for full power, the switch may not be able to handle the load.
    • Of course, devices can be powered with a power brick at the desk
  • 802.3af
    • IEEE standard for PoE from 2003
    • Defines power classes so different devices can ask for different power levels
      • Class 0:  15.4W allocated
        • Used for el cheapo devices that just want power
      • Class 1:  4.0W
      • Class 2:  7.0W
      • Class 3:  15.4W
    • Uses all 4 pairs of wire, so works on gig links
    • Power procedure
    1. Small DC current is applied to the line
    2. If an 802.3af device is attached, it runs the current through a resistor
    3. The resistance is detected by the switch which can determine the class of power
    4. Power is applied to the device
  • Cisco Inline Power
    • Cisco's version of PoE created in 2000 (before 802.3af)
    • Each device tells the switch what its power needs are
    • Power procedure
    1. PoE device connected to the switch
    2. Switch sends Fast Link Pulse (FLP)
    3. If FLP is received back, 6.3W of power are applied
    4. Device boots off of 6.3W and tells the switch what its real power requirements are via CDP