It is important to understand the functions of both the PD and PSE before designing the PoDL system. The PSE has 4 main functions according to the IEEE standards. From the IEEE 802.3bu standard, the main functions of the PSE include:

1. To search the link segment for a PD
2. To supply power to a detected PD through the link segment
3. To monitor the power applied to a link segment
4. To remove the full operating voltage when no longer required, or when a short-circuit or other fault is detected

Conversely, the PD, is simply the portion of a device that draws power from the link or the portion that participates in the detection or classification algorithms. The PD system presented in this document includes:

1. The coupling and decoupling network (CDN) which decouples power from the power and data path
2. The VID block required for the PD to be detected as PoDL-compliant to a PSE
3. SCCP communication necessary for the optional classification function for power class negotiation
4. Power protection and regulated and isolated 24-V class-12 power supply to the device being powered

Figure 3-1 shows these system blocks.