What Is PoE Negotiation?

How Does PoE Negotiation Work?

Defined by IEEE 802.3 PoE standards, PoE negotiation (also called a “handshake”) ensures that devices are PoE compliant and receive only the amount of power they need to operate. This maximizes efficiency and prevents harmful power from being applied to non-PoE devices.

PoE negotiation happens in three sequential stages:

1. ディスカバリー: A PSE determines if a PoE-compliant PD is connected and if it’s a single- or dual-signature device.
2. Classification: A PSE determines if PoE power is required and what class of PoE must be delivered.
3. Operation: The PSE delivers sufficient power for the PD to become fully functional. The PD can operate only when the discovery and classification stages are successfully completed.

First Stage of Negotiation: ディスカバリー

PSE ports remain unpowered when no device is connected. The discovery stage begins once the PSE detects that something is plugged in. During discovery, which takes less than half a second, the PSE applies a small amount of voltage to verify the presence of a resistor within the PD. The PSE sends the detection voltage over two pairs for 802.3af/at and over four pairs for 802.3bt, and it performs measurements at various levels (at least 1 V apart) to reliably detect a PD.

IEEE standards require that PoE-compliant devices present a resistance between 23.75kW and 26.25kW on each pair set in order to be considered a valid PD. If the PSE detects anything other than the specified resistance, it immediately stops power. It’s important to note that the power applied during the discovery stage ranges between 2.7 V and 10.1 V and cannot exceed a current of 5 mA (.005 W), which is low enough to prevent any damage to non-PoE devices.

During discovery, IEEE 802.bt Type 3 and Type 4 PSE also detect if a PD is a single- or dual-signature device. A single-signature PD maintains the same power signature across both pair sets (pairs 1,2 and 3,6 and pairs 4,5 and 7,8). A dual-signature device can have an independent power signature of up to 35.5 W on each pair set. While most PDs are single-signature devices due to cost, a dual-signature device can distribute diverse power levels to internal components, such as a camera that needs one power level for camera operation and another for pan/tilt/zoom functionality. If a dual-signature PD is detected during the discovery stage, the PSE performs classification on both sets of pairs.

^{A dual-signature device can have an independent power signature on pair set 1,2 and 3,6}

Next Stage: PoE Classification

Once a PSE detects a valid PD, it must determine how much power to deliver. IEEE 802.3 PoE standards assign classes to PoE systems, ranging from 0 (lowest) to 8 (highest). For PoE to function, the class of the PSE must be equal to or higher than the class of the PD.

Classes are based on the power output from the PSE and the amount of power available at the PD, since some energy is lost over the link. Type 1 and Type 2 PoE encompass Class 0 through Class 4, Type 3 includes Class 5 and Class 6, and Type 4 includes Class 7 and Class 8.

PoE Classes

During the classification stage of negotiation, the PSE determines the class of the PD in one or multiple events, depending on PoE type:

• • Type 1 PSE (single-event classification): Sends a single voltage pulse and measures the current draw of the PD to determine if it is Class 0, Class 1, Class 2, or Class 3.
• • Type 2 PSE (two-event classification): Sends a second voltage pulse if the current draw of the PD indicates that it is Class 4 to verify the need for higher power.
• • Type 3 and Type 4 PSE (five-event classification): Sends out three additional pulses to determine if the device is Class 5, Class 6, Class 7, or Class 8.

For Type 2 and higher PoE that use multiple classification events, the first event occurs at the hardware level while subsequent events can occur at either the hardware or software level. IEEE standards require Type 2 and higher PDs to support classification at both the hardware and software levels for the second event. While PSEs can support either hardware or software classification, most PoE-enabled switches do both.

• • For hardware classification, the PSE reads the current value from the PD at the physical layer.
• • For software classification, the PSE and PD negotiate power requirements using Link Layer Discovery Protocol (LLDP), an Ethernet protocol network that devices use to advertise their capabilities.

The first event during negotiation always occurs at the hardware level. If during the first event, the PSE determines that the PD is Class 4 or higher during, it supplies only Class 3 power. For the second event, the PSE then negotiates power requirements with the PD using LLDP. One of the key benefits of software negotiation, is the ability for a PD to request a specific amount of power that may be lower than its class designation. This allows  the PSE to allocate leftover power to other devices for more efficient power budgeting.

The IEEE 802.3bt standard for Type 3 and Type 4 PoE also introduced optional auto-classification functionality, which allows the PSE to determine the maximum power drawn by the PD. The PSE measures the PD’s power consumption over time to determine and set the maximum power output. Type 3 and Type 4 PSEs identify PDs with auto-class functionality during the discovery stage.

Final Stage: Operation

Once a PSE has determined that a PoE-compliant PD is connected and how much power is required, it shifts into operation mode and delivers sufficient power to the PD. During operation, the PD generates a maintain power signature (MPS) signal to ensure ongoing functionality. The PSE continuously monitors the PD for this MPS signal and shuts off power if the signal is lost. This ensures that power is not wasted if the device is unplugged and also keeps you from getting a possibly unpleasant surprise if you touch an unplugged cable. The PSE then restarts the negotiation process.

The Right PoE Tester Matters!

When testing PoE, look for a tester that considers the PoE negotiation process for all IEEE 802.3 standards. For example, the Fluke Networks LinkIQ Cable+Wi-Fi+Network Tester shows which pairs of a four-pair network cable carry power and indicates if the switch can negotiate single-signature or dual-signature power. This information can be useful for troubleshooting. For example, if an 802.3bt PD is not properly functioning, it may be because it is a dual-signature device but your switch only supports single-signature devices. In that case, you may need to upgrade your switch or connect the device to a different switch that supports dual-signature devices.

^{Switch port PoE result shows pairs used, power and class available, and results of the PoE test under load.}

The LinkIQ tester also shows the negotiated power class (0-8) at both the hardware and software levels to ensure that a device can connect to the network and enable dynamic allocation by the PoE switch. To test PoE, it then places a load on the line while drawing the negotiated power. If the voltage drops lower than the minimum required by the PD (which could be caused by a cabling problem or an oversubscribed PSE), the power isn't actually available, and the test fails.

学習を続ける

• • LinkIQ Duo：ケーブル、PoE、Wi-Fi、ネットワーク・テストを 1 つに
• • Power Over Ethernet (PoE)
• • What You Need to Know to Choose a PoE Tester
• • PoE Load Testing: PoE システムの高度なトラブルシューティング