Power over Ethernet (PoE)

Introduction

Power-over-Ethernet (PoE) is a technique that combines data and power onto a single Ethernet cable. The use of PoE compliant equipment eliminates the need to run 110/220 VAC power to remote network appliance devices in a wired LAN. In most cases the use of PoE can significantly decrease installation costs for remotely located devices. The use of PoE allows greater flexibility in the locating of network devices such as Wireless Access Points, Switches and IP Cameras.

Recommendations

• Select the right cable category: For higher power PoE (802.3bt and beyond), use Cat5e or better (Cat6/Cat6A) to reduce losses and accommodate heat rise.
• Limit cable length and bundling: Keep running within spec (max ~100 m for Ethernet) and avoid tightly bundled cables without derating heat.
• Plan for power budget: Calculate the total power demand of devices, losses in the cable, and margin for reliable operations (especially for 4-pair PoE).
• Use proper injection equipment: Ensure your power-sourcing equipment (PSE) supports the required standard/class and has detection/negotiation for powered devices (PDs).
• Manage thermal issues: Higher PoE portfolios mean more current in cables → increased temperature. Use proper ventilation, spacing, and cable selection.
• Label and document infrastructure: Especially in bundled or complex installs, mark which ports/devices are PoE, their class, power budget, and cable specs.
• Future-proof the design: Even if current devices demand modest power, installing higher-category cabling can save time and cost later.

A Brief History of PoE

The technique of combining data and power onto a single Ethernet cable has been around in one form or another since the early days of networking. The wireless community was one of the first to widely adopt this technique. In a replay of the video tape industry, many companies closely guarded the details of their “PoE” methods in the hope of garnering greater sales. As history has shown, a common standard will actually increase sales. The lack of a unified standard was hindering the growth of PoE capable equipment. Proprietary PoE equipment was costing customers more and in a bid to satisfy customer demands, major manufacturers called on the IEEE 802.3 Ethernet working group to develop a new standard which became 802.3af.

The 802.3af standard describes two types of devices, Power Sourcing Equipment (PSE) and Powered Devices (PD). The PSE provides the power to a PD. 802.3af compliant PSE also uses “smart” techniques so as to not supply power to non-PoE capable equipment thus possibly damaging them. Both the PSE and PD have power supplies and regulators isolated from ground to minimize any shock hazard or the introduction of ground loops that could impact data integrity.

Consumers in most cases, now have a common standard to rely on. They can now buy equipment from multiple vendors that will “in general” interconnect without issues. There is enough flexibility in the 802.3af standard however that there is a small possibility that equipment from different vendors may not work. For example power polarity is not specified. In some cases a simple configuration change of the PSE will solve the problem. In others the only option may be to use a PoE Mid-Span or Injector that is known to work with the PD or network appliance. Another issue is the large amount of legacy PoE equipment that will only operate with specific devices from the same vendor.