I purchased a few AIR-PWRINJ4= thinking they were 802.3at compliant, but they are indicated as "pre-compliance" in documentation. I am using them with 3700 units so they work as expected, but I got curious.
When connecting a compliant 802.3at PD, it will not negotiate Class 4 power. The AIR-PWRINJ4= contains the MAX5922 which is an 802.3af chip, but does support Class 4 power per the datasheet.
Class 3 to Class 4 Threshold ICLASS_3-4 31 33 35 m
From what I understand about 802.3af, there is no mechanism to communicate from the PSE to the PD which power Classes are supported (e.g. the PSE must support Class 0-3). The mechanism 802.3at uses to indicate Class 4 availability is a second handshake to enable Type 2, which is not present in 802.3af (or the MAX5922). Additionally, the standard specifically calls out "non-compliant" scenario I think we are in.
Class 4 can only be used by IEEE 802.3at (Type 2) devices, requiring valid Class 2 and Mark 2 currents for the power up stages. An 802.3af device presenting a class 4 current is considered non-compliant and, instead, will be treated as a Class 0 device. https://en.wikipedia.org/wiki/Power_over_Ethernet emphasis added
The Aironet 3700 series differentiates between 802.3af (up to Class 3) and this pre-compliant AIR-PWRINJ4= (and other compliant?) 802.3at (up to Class 4), but from what I can tell based on the MAX5922, it does not (have to) use the "second handshake" procedure. What trick is it using to determine if Class 4 is available when using the AIR-PWRINJ4= (non-compliant PSE which supports Class 4)?
%CDP_PD-4-POWER_OK: Full power - HIGH_POWER inline power source
