Power-over-Ethernet (PoE)

  • The Power-over-Ethernet (PoE) subsystem manages power supplied to devices using standard Ethernet data cables. A Power Sourcing Equipment (PSE) supplies DC power as well as Ethernet connectivity to a Powered Device (PD) using a standard Ethernet cable. The maximum current depends on the PD Requested Class.

  • A PoE subsystem contains two parts : a PSE and PD. A Power Sourcing Equipment (PSE) is a device that provides power through a standard Ethernet cable. A PoE capable switch functions as PSE. All Aruba PoE switches are considered as PSEs. A PD is a device powered by a PSE. Examples of PD are VoIP phones, Wireless APs, and IP cameras.

  • When a PD or any network cable is connected to a PSE port, the PSE applies a detection voltage and measures the resistance value of the PD. If resistance is within IEEE 802.3 standard values (23 - 26k ohm), the connected device is treated as PD and classification begins. For legacy devices to be detected, you must enable prestandard detection on the switch.

  • PDs are divided into different types and classes based on PD power requirements. The power supplied by the PSE is higher than the power PD draws to accommodate for the line losses that can result with the use of the standard maximum length cable(100m).

    • Type 1: PSE can supply minimum of 15.4W, and PD can draw a maximum of 13W.

    • Type 2: PSE can supply minimum of 30W, and PD can draw a maximum of 25.5W.

    • Type 3: PSE can supply minimum of 60W, and PD can draw a maximum of 51W.

    • Type 4: PSE can supply minimum of 90W, and PD can draw a maximum of 71W.

  • Classes of PD:

    • Class 0: Type1 PD, it can draw a maximum of 13W.

    • Class 1: Type1 PD, it can draw a maximum of 3.84W.

    • Class 2: Type1 PD, it can draw a maximum of 6.49W.

    • Class 3: Type1 PD, it can draw a maximum of 13W.

    • Class 4: Type2 PD, it can draw a maximum of 25.5W.

    • Class 5: Type3 PD, it can draw a maximum of 40W.

    • Class 6: Type3 PD, it can draw a maximum of 51W.

    • Class 7: Type4 PD, it can draw a maximum of 62W.

    • Class 8: Type4 PD, it can draw a maximum of 71.3W.

  • IEEE 802.3bt introduced 4-Pair PoE as a means of supplying higher power to PDs that need more than the current 25.5W supplied by IEEE 802.3at. To increase the available power without damaging the Ethernet cable, the standard introduced the ability to use all four pairs within the Ethernet cable instead of the two pairs used by previous standards (802.3at, 802.3af).

  • Supported protocols:

    • Compatibility with IEEE 802.3af, 802.3at, 802.3bt and prestandard.

    • Long first class event supported on Type 3-4 PSE.

    • Support for Single Signature (SS) Type 1-4 and Dual Signature (DS) Type 3-4 PDs.

    • Multi-Event classification permits mutual ID of SS Class 0-8 and DS Class 1-5.

    • Support LLDP Data Link Layer (DLL) Type 1-2 extension 12-octet TLV and Type 3-4 extension 29-octet TLV.

    • Default PSE assigned class delivers the maximum PSE capable power at initial power up based on PD requested class.

  • Always-on PoE is a feature that provides the ability for a switch to continue to provide power across user initiated reboots through software. Always-on PoE is enabled by default and no additional configuration is needed.

NOTE:

PDs only remain powered, no data transfer or PoE power negotiation can occur until the switch has completely booted up and in normal operation. PD faults occurring prior to full switch boot up will result in PoE power removal and restart the detection process only after switch returns to normal operation.