How does a PoE splitter negotiate power with the source device?

The process of power negotiation between a PoE splitter and the PoE source (typically a PoE-enabled switch or injector) is based on the PoE standard (IEEE 802.3af, 802.3at, or 802.3bt). The PoE negotiation is a method by which the PoE source and the PoE splitter communicate to determine how much power the splitter will be provided to distribute to the connected device.This negotiation process ensures that the PoE source does not overload any device and that the splitter only receives the necessary power for the connected load. The communication happens over the Ethernet cable that carries both data and power.

Detailed Explanation of the PoE Power Negotiation Process:

1. PoE Standards and Power Classes:

--- IEEE 802.3af (PoE): This standard provides 15.4W of power per port (at the source). After losses due to cable resistance and other factors, a typical device receives around 12.95W.

--- IEEE 802.3at (PoE+): This standard provides 25.5W of power per port (at the source), with the device receiving about 22W.

--- IEEE 802.3bt (PoE++ or 4PPoE): This is a high-power standard that provides up to 60W (Type 3) and up to 100W (Type 4) per port. This enables powering more demanding devices like high-performance IP cameras, large access points, or digital signage.

The PoE splitter has to be compatible with the specific PoE standard in use (af, at, or bt). The negotiation process ensures that the appropriate amount of power is delivered.

2. Power Sourcing and Detection:

--- The PoE source (switch or injector) will start by sending a low voltage signal over the Ethernet cable to detect if the connected device (in this case, the PoE splitter) is PoE-capable. This is part of the “Powered Device Detection” phase.

--- The PoE splitter does not initially consume power during this phase. It simply indicates that it is ready to accept power and will only draw power once the negotiation is completed.

3. Power Classification via the “Classification” Process:

--- PoE devices, including PoE splitters, use a mechanism known as classification to communicate to the power source how much power they need.

--- A PoE splitter, after detecting the PoE source, classifies itself by providing a signal on the Ethernet cable's data pairs (in a specific manner depending on the PoE standard). This signal tells the source how much power the device requires.

The PoE source typically supports multiple power classes (for example, Class 0 to Class 4 in 802.3at and 802.3bt). The PoE splitter indicates which class it belongs to based on its power requirements:

--- Class 0: Default, requests maximum power (up to 15.4W for af, 25.5W for at).

--- Class 1-4: These are lower-power classes for devices that only require a specific, smaller amount of power (e.g., cameras or phones that need less than the maximum available).

The splitter itself doesn't necessarily select its class, but the PoE source may dynamically allocate power based on the negotiation response.

4. Power Delivery (PSE to PD):

--- Once the PoE source (PSE - Power Sourcing Equipment) detects the PoE splitter and understands how much power is needed, it will then start delivering power over the same Ethernet cable.

--- The PoE splitter can then distribute this power to the connected non-PoE device (for example, an IP camera, access point, or sensor) via the power output.

--- The power delivered to the splitter is usually negotiated to match the required voltage for the connected device (e.g., 5V, 9V, 12V). This process involves the voltage regulation within the splitter to ensure the connected device gets the right amount of power.

5. Voltage and Current Regulation:

--- The PoE splitter adjusts the voltage (downward conversion) for the device based on what the PoE source has provided. The splitter then regulates the current to provide stable power to the device.

--- For example, a 12V PoE splitter that receives power at 48V will step down the voltage to 12V for the device. It does this by using components such as buck converters or voltage regulators.

6. Safety and Compliance:

--- Both the PoE source and the PoE splitter must comply with the IEEE PoE standards, which define not only the power but also the safety aspects of power transmission (e.g., over-voltage, under-voltage, and short-circuit protection).

--- Power management protocols are in place to prevent the splitter from drawing more power than is available or required. If an overload is detected, the source may shut off the power, or the splitter may disconnect, preventing potential damage.

7. Power Monitoring:

--- Some advanced PoE splitters feature built-in power monitoring to track the amount of power being delivered to the device, ensuring that the device does not overdraw power or exceed safe limits.

--- These systems also may have diagnostic LEDs or other indicators to signal the status of power delivery, which helps with troubleshooting.

Conclusion:

The PoE splitter’s negotiation process primarily involves:

--- Detection: The PoE source detects the splitter and begins the negotiation phase.

--- Classification: The splitter signals its power requirements to the source via the classification process.

--- Power Delivery: The PoE source provides the appropriate power, and the splitter converts it to the required voltage for the device.

--- Voltage Regulation: The splitter steps down and regulates the voltage to match the needs of the connected device.

This negotiation ensures that the PoE splitter receives only the necessary power for its connected load, and it does so in a way that is safe and efficient. For high-power PoE standards like 802.3bt, this process allows for the delivery of up to 100W of power, which can be distributed to demanding devices while maintaining proper data and power management.