IEEE 802.3bt Features

PoE on and devices is IEEE 802.3bt standard compliant. The IEEE 802.3bt standard has increased the amount of power that can be transmitted over the Ethernet cable by using all four wire pairs of the cable.

IEEE 802.3bt is backward compatible with 802.3af and 802.3at.

Note: and devices support IEEE 802.3bt specifications.
Note: RUCKUS ICX 8100-24P, ICX 8100-48P, ICX 8100-C08PF, ICX 8100-24P-X, ICX 8100-48P-X, and ICX 8100-C08PF-X devices support IEEE 802.3bt type 2 class 4 specification.

The IEEE 802.3bt standard contains a number of the following features:

  • Type 3 and Type 4 PSE or PD: PoE++ is the latest PoE that supports the 802.3bt standard. The IEEE 802.3bt standard introduced two new types. The Type 3 supports 60W power for the PSE over two or four twisted pairs in a copper cable to supply 51W for the PD. Type 4 supports up to 90W for the PSE and 71.3W for the PD over four twisted pairs.

  • Classes: The new standard also introduced additional classes from Class 5 to Class 8. 802.3bt-compliant switches can deliver up to 60W on each port under Type 3 and up to 90W under Type 4.

  • PoE over 4-pair twisted cables: The IEEE 802.3bt standard increases the maximum PoE power by utilizing all four pairs in a 4-pair twisted cable such as Cat5 or Cat6 Ethernet cables. Type 3 utilizes both 2-pair and 4-pair twisted cables. Type 4 uses only 4-pair cables. The current 802.3at- and 802.3af-compliant switches use only two out of the four twisted pairs provided in a network cable.

  • Short Maintain Power Signature (MPS): The minimum amount of current that the PD must draw from the PSE, so that the PD does not get disconnected from the PSE, is called the Maintain Power Signature. The minimum standby power has been reduced from the current standard of 200mW to 20mW. This low standby power level enables the IoT applications to be powered with PoE.

  • Autoclass: Autoclass allows the PSE to measure the actual power consumption of the connected PD during power-on. Based on this reference power measurement, the PSE can power multiple PDs on the unit. For example, a PSE can allocate an additional bulb using the leftover power if it knows that the connected PD consumes less power than its class power.

    802.3bt proposes the PD to support Autoclass, in which case, there will not be any cable loss calculation at the and . The power reserved for the PD is based on Autoclass information. If Autoclass is not supported, cable loss is calculated based on the real cable length. and devices are capable of cable length detection. When an LLDP request for power comes from a PD, the and calculates cable loss for the requested power considering that the cable length is already detected.

  • Power Demotion: Power Demotion allows the PSE to supply lower power to the PD when the PD demands excess power from a PSE and the PSE cannot meet this power demand. The PD can operate using this limited available power. If a PD ignores this power demotion and tries to operate at its higher power, the PD makes the PSE unavailable.

  • Mandatory Classification: This is a hardware-based classification where Type 2 PSEs are not required to support full hardware classification. They can use LLDP to provide full power to PDs. But, it is mandatory for Type 3 and Type 4 PSEs ( and ) to fully support hardware classification.

  • Connection Check: The IEEE 802.3bt standard defines two topologies: single-signature PD and dual-signature PD. Connection check determines if the connected PD is a single-signature PD or a dual-signature PD. This is applicable only for Type 3 and Type 4 PSEs that use twisted 4-pair Ethernet cables.

    A dual-signature PD can support two different loads with different power classes. For example, in a CCTV camera built with a dual-signature PD, one pair of the cable can be connected to the camera and the other pair can be connected to the heater.

Parent topic: Power over Ethernet Overview