Definitions of terms used in this article:
When the PSE detects a connected PD, it will send a short voltage pulse to determine how much power the PD needs. Depending on how much current is drawn during this pulse, the PSE will classify the PD into one of classes 0-4 (or 0-3 for type 1 PSEs and PDs).
- Power Sourcing Equipment (PSE) - any device that is capable of providing power over ethernet
- Powered Device (PD) - any device capable of receiving power over ethernet from a PSE
- Type 1 PSE or PD - an 802.3af compliant PSE or PD
- Type 2 PSE or PD - an 802.3at (PoE+) compliant PSE or PD
If a type 2 PSE classifies the connected PD into class 4 (PoE+ power), a second handshake is required. This is both to allow the PD to know that full 802.3at power is available, as well as to allow the PSE to confirm that a type 2 PD is actually connected to the port for safety reasons. Prior to this second handshake taking place, the PD and PSE will restrict power to 802.3af limits.
This second handshake can take one of two forms: a hardware-based Two-Event Classification or a software-based LLDP classification. Per the 802.3at standard, PSEs must support only one of these mechanisms. However, PDs must support both classification methods.
In the hardware-based two-event classification, the initial voltage pulse classification event is repeated. Once the PD passes the second classification event as a class 4 PD, it will be provided full class 4 802.3at power. This method was initially intended to be used by mid-span PoE injectors, as they generally will not support LLDP required for software-based classification.
For the software-based LLDP classification, LLDP must be enabled on both the PSE and PD, with the 802.3at TLVs enabled. This will allow the PSE and PD to negotiate power requirements via software, rather than the second classification pulse used in the hardware based method. This also allows power requirements to be dynamically negotiated, allowing a PSE's power budget to be better managed.