Power Over Ethernet (PoE) and Networking Considerations

Overview of PoE

• Power Over Ethernet (PoE): Allows power to be delivered over the same cable as data.
  • Simplifies installation of devices like telephones, access points, and cameras.

Sources of PoE

• Endspan: Power provided by the PoE-enabled switch.
• Midspan: Power injected by a separate device between switch and PoE device.

PoE Standards

• Original PoE: Provides 15.4 watts of DC power, 350 milliamp current.
  • Suitable for small devices (e.g., simple telephones, small access points).
• PoE+: Provides 25.5 watts of DC power, 600 milliamp current.
  • Used for larger telephones, cameras, and other devices.
• PoE++: Provides up to 51 watts (Type 3) or 71.3 watts (Type 4) with 960 milliamp max current.
  • Supports larger devices (e.g., laptops, pan/tilt/zoom cameras).
  • Compatible with higher Ethernet standards (2.5, 5, 10 Gbit connections).

Compatibility Considerations

• Ensure compatibility between PoE switches and devices.
  • A PoE+ switch cannot power a PoE++ device.
• Check switch specifications for PoE support across interfaces.
  • Some switches provide PoE on all ports, others on specific ports.
• Verify the maximum PoE power capacity of the switch.
  • Ensure total device power draw is within switch capacity.

Transceivers and Connections

• Transceivers: Ensure correct transceiver type for connections.
  • Match transceiver wavelength with fiber specifications (e.g., 850 nm, 1310 nm).
  • Incorrect transceivers lead to signal loss and network issues.
• Verify transceiver and fiber compatibility to avoid errors.

Signal and Power Budget Calculations

• Calculate signal transmission and loss over connections:
  • Transmitted Power: Measured in dBm (decibels per milliwatt).
  • Account for signal loss over distance and through connectors/splices.
• Sensitivity Level: Minimum signal level a device can interpret.
  • Ensure received power exceeds sensitivity level for proper signal reception.

Example Calculation

• Sensitivity Example: SFP transceiver at 850 nm with sensitivity -17 dBm.
  • Calculate power budget; ensure received power is -17 dBm or higher for good signal.
  • If received power is -20 dBm, signal is insufficient.