PoE power supply between switches

PoE allows switches to deliver both data and electrical power over Ethernet cables, with power allocation managed per port and across multiple power sources to ensure devices receive the required energy without exceeding switch limits.

How PoE Works Across Switches

Power over Ethernet (PoE) enables network switches to supply electrical power to connected devices such as IP cameras, VoIP phones, and wireless access points through the same Ethernet cable used for data transmission . Switches that support PoE act as Power Sourcing Equipment (PSE), while the connected devices are Powered Devices (PDs). Each switch has a power budget, which is the total amount of power it can supply across all PoE ports . When multiple switches are involved, each switch independently manages its PoE allocation. The switch detects the PD, negotiates the required power using protocols like CDP (Cisco Discovery Protocol) or LLDP (Link Layer Discovery Protocol), and ensures the PD does not exceed the allocated power . This negotiation allows high-power devices to start in low-power mode and transition to full power once confirmed by the switch.

PoE Standards and Power Levels

• PoE (IEEE 802.3af): Supplies up to 15.4 W per port, with PDs consuming up to 12.95 W .
• PoE+ (IEEE 802.3at): Supplies up to 30 W per port, suitable for PTZ cameras and IP phones .
• PoE++ (IEEE 802.3bt, Type 3 & 4): Supplies 60–100 W per port, supporting high-power devices like multi-radio access points or building automation systems . Switches can use Alternative A (data pairs) or Alternative B (spare pairs) for power delivery, and 4-pair PoE (4PPoE) increases the available power by using all four twisted pairs .

Power Sharing Across Inputs

Some PoE switches or compact switches may receive power from multiple input sources. To manage this, switches implement input power sharing and power limiting:

• Even current sharing: Each power supply contributes equally based on output current measurements. This works best when all channels draw similar power .
• Uneven power sharing: Required when different PDs or feeds have varying power allocations. The switch ensures no PD exceeds its negotiated power limit .
• Power limiting: The switch monitors total power consumption and prevents overloading by adjusting allocations dynamically . This approach ensures that a switch can reliably power multiple devices while maintaining compliance with IEEE standards and avoiding overcurrent conditions.

Practical Considerations

1. Check total power budget: Ensure the sum of all PD requirements does not exceed the switch's PoE capacity.
2. Use PoE+ or PoE++ for high-power devices: Devices like PTZ cameras or multi-radio APs may require higher wattage.
3. Monitor power allocation: Many managed switches provide dashboards to track per-port power usage and available budget.
4. Consider redundancy: Some devices can receive redundant power from both PoE and AC sources, but PoE alone does not provide redundancy .
5. Cabling: Use Cat5e or higher for standard PoE, and Cat6a or Cat7 for higher power delivery or longer distances. By understanding these principles, network designers can efficiently distribute PoE across multiple switches, ensuring all devices receive sufficient power while maintaining network reliability and compliance with IEEE standards.