A PoE switch incoming line carries both data and DC power over the same Ethernet cable, allowing the switch to supply power to connected devices while maintaining network communication.

How PoE Works on a Switch Port

A PoE switch port, also called the incoming line, transmits data and power simultaneously over twisted-pair Ethernet cabling. The DC voltage, typically 48–57V, is applied through the center taps of LAN transformers, and the current splits evenly across the pairs, ensuring that data transmission is not affected by the power flow . Depending on the standard:

• 802.3af (PoE): Up to 350 mA per conductor pair.
• 802.3at (PoE+): Up to 600 mA per pair.
• 802.3bt (4PPoE): Uses all four pairs to increase power delivery . The switch detects the connected Powered Device (PD) and negotiates the required power using a discovery and classification process. This ensures that the device receives the correct power class without overloading the port .

Configuration and Management

On Cisco-style switches, the incoming line can be configured using commands such as:

• power inline auto – automatically supplies power to a connected PD.
• power inline limit <mW> – sets a maximum power limit per port.
• power inline priority <level> – prioritizes power delivery for critical devices . These commands allow administrators to control power allocation, schedule power delivery, and support legacy devices if needed.

Physical and Electrical Considerations

The Ethernet signals are differential, typically ±1V at 10–40 mA, while the PoE current is superimposed or transmitted on separate pairs depending on the mode (Alternative A, B, or 4PPoE), . LAN transformers and chokes are designed to handle PoE currents without saturating, and the DC voltage is extracted at the PD using bridge rectifiers to accommodate either polarity .

Summary

The PoE switch incoming line is a single Ethernet cable that efficiently delivers both network data and electrical power. Proper configuration ensures safe power delivery, supports multiple devices, and maintains network integrity. Understanding the current, voltage, and port configuration is essential for reliable PoE deployment in enterprise or small office networks .