Power over Ethernet (PoE)

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Power over Ethernet (PoE) is a means of supplying electrical power from a switch port over ordinary data cabling to a connected powered device (PD).

• powering through a switch is more efficient than using a wall-socket
• allows network management software to control the devices and apply schemes
  • making unused devices go into sleep states and power capping
• defined in two IEEE standards (now combined in 802.3-2018):
  • 802.3af
    • power is supplied as 350mA@48V and limited to 15.4 W output
    • support PDs that require up to 13 W
      • power dissipates over the length of the cable
  • 802.3at (PoE+)
    • supplies power at 30 W, with maximum current of 600 mA
    • supports PDs that require up to 25 W
  • 802.3bt (PoE++)
    • supplies power at 60 W (Type 3) or 90 W (Type 4)
    • up to 51 W and 71 W usable power, respectively
• PoE switches are referred to as an endspan (or endpoint) power sourcing equipment (PSE)
• Cisco Switch commands:
  • power inline auto max 15000 enables a port for PoE and sets max output of 15,000 mW (15 W)
• When a device is connected to a PoW switch:
  • switch goes through a detection phase to determine whether the device is PoE enabled
  • if not, it does not supply power over the port
  • if it does, switch determines the device’s power consumption and sets the voltage level appropriately

Info

If a switch does not support PoE, a device called a power injector or midspan can be used.