Power over Ethernet (PoE)

• PoE delivers DC power and data over standard Ethernet cables - eliminates need for separate power cables to network devices
• Uses spare wire pairs in Cat5e/Cat6 cables OR phantom power over data pairs
• Powered Device (PD) receives power, Power Sourcing Equipment (PSE) provides power
• PSE can be PoE-enabled switch, midspan injector, or PoE adapter

PoE Standards and Power Levels

• Power difference between PSE and PD accounts for cable resistance losses over 100m maximum distance
• Higher wattage standards are backward compatible with lower power devices

Power Delivery Methods

• Alternative A (Data Pairs): Power delivered over pins 1,2,3,6 (same as data transmission)
• Alternative B (Spare Pairs): Power delivered over pins 4,5,7,8 (spare pairs in 10/100 Ethernet)
• 802.3bt uses all four pairs for power delivery to achieve higher wattage
• PSE automatically detects which method PD supports during negotiation phase

PoE Detection and Classification Process

• Detection Phase: PSE applies 2.8-10V to detect valid PD (checks for 25kΩ signature resistor)
• Classification Phase: PSE determines power requirements of PD (Class 0-8)
• Power-Up Phase: PSE ramps voltage to 48V DC
• Operation Phase: Continuous monitoring for PD disconnect or overcurrent
• PSE will shut down port if PD is disconnected to prevent safety hazards

Power Classes

Vocabulary

• PSE (Power Sourcing Equipment): Device that provides PoE power (switch, injector, or midspan)
• PD (Powered Device): Device that receives PoE power (IP phone, AP, camera)
• Endspan: PoE power provided directly from switch port
• Midspan: PoE injector device placed between non-PoE switch and PD
• LLDP-MED: Protocol extension allowing detailed power negotiation between PSE and PD
• CDP: Cisco proprietary protocol that can communicate power requirements

Implementation Considerations

• Calculate total PoE budget - switches have maximum total PoE capacity (e.g., 740W for 48-port switch)
• Cable quality matters - Cat5e minimum for PoE, Cat6/Cat6a recommended for PoE++
• Distance limitation: 100m maximum for reliable power delivery
• Use show power inline to monitor PoE status and consumption on Cisco switches
• Consider UPS sizing - PoE devices increase power requirements during outages

Common Deployment Scenarios

• IP Telephony: Most common PoE application - powers desk phones without AC adapters
• Wireless Networks: Powers APs in ceiling installations where AC power unavailable
• Security Systems: IP cameras, especially outdoor installations
• IoT Devices: Sensors, access control systems, digital signage
• Government/Military applications often require PoE for tactical communications equipment in field deployments

Notes

• Always verify switch PoE budget before deployment - oversubscription will cause port shutdowns
• PoE negotiation happens automatically but can be overridden with manual configuration
• Non-standard PoE can damage equipment - ensure compatibility before connecting
• Use inline PoE testers to troubleshoot power delivery issues
• Consider environmental factors - high temperatures reduce available PoE power
• Budget for 20% overhead when calculating PoE requirements for future expansion
• Legacy devices may require PoE injectors if existing switches lack PoE capability