Power Over Ethernet (PoE) at the High Site

Power Over Ethernet (PoE) is a game-changer in the network infrastructure, particularly for WISPs. It simplifies the deployment of wireless access points, IP cameras, and other network devices by allowing both power and data to be carried over a single Ethernet cable. This not only reduces installation complexity but also enhances flexibility in device placement. This blog post will dive deep into everything PoE, covering passive systems, the IEEE 802.3af/at standards, voltage considerations, and the rationale behind choosing specific PoE systems for WISP operations.

Understanding PoE: Passive vs. IEEE 802.3 Standards

Passive PoE

Passive PoE is a simpler and often more cost-effective solution compared to its IEEE 802.3 counterparts. It supplies power over Ethernet cables without the negotiation between the power source equipment (PSE) and powered devices (PD). However, it lacks the sophisticated power management and safety features of active PoE systems. Passive PoE typically operates on lower voltages, like 24V, and is widely used in WISP networks for its compatibility with a variety of devices.

IEEE 802.3af/at Standards

The IEEE 802.3af (PoE) and 802.3at (PoE+) standards are active PoE systems that include power negotiation between the PSE and PD, allowing for a more flexible and safer power supply. These standards typically operate at higher voltages, up to 48V, and include features like overload protection, underpowering, and safe disconnection. The IEEE standards are ideal for environments requiring high power outputs and advanced safety features, such as office buildings or industrial setups.

Voltage Considerations: 24V vs. 48V Systems

Choosing between 24V and 48V systems involves understanding the trade-offs between voltage and amperage. Lower voltage systems (e.g., 24V) require higher amperage to deliver the same amount of power as higher voltage systems (e.g., 48V), which can lead to increased cable load and potential voltage drop over long distances. However, 24V systems are often favored in WISP networks for their compatibility with a wide range of outdoor wireless equipment and the ability to be directly connected to DC power systems, simplifying the power infrastructure.

Cable Wiring and Power Distribution

In both passive and active PoE systems, power is typically carried over the spare (unused) or data-carrying wires within a Cat5e, Cat6, or higher category Ethernet cable. For example, in a 10/100Mbps network (using two pairs for data), power can be transmitted over the unused pairs. In Gigabit Ethernet (using all four pairs for data), power is distributed across the data-carrying pairs using sophisticated techniques that prevent interference with data transmission.

• Positive and Negative Wires: The IEEE 802.3af/at standards define two types of power delivery. Mode A delivers power on the same wires used for data (1,2 and 3,6 pairs), while Mode B delivers power on the spare pairs (4,5 and 7,8 pairs). In passive PoE systems, polarity (positive and negative) must be carefully observed to prevent damage to equipment.
• Maximum Distances: PoE can effectively transmit power up to 100 meters (328 feet), which is the maximum distance for Ethernet data transmission. However, voltage drop considerations may necessitate voltage adjustments or the use of PoE extenders for longer runs.

Why WISPs Prefer 24V Systems or Passive PoE

For WISPs, the choice often leans towards 24V passive PoE systems for several reasons:

• Compatibility: Many outdoor wireless devices designed for WISP deployments, especially those from Ubiquiti and MikroTik, are optimized for 24V passive PoE.
• Simplicity and Cost-Efficiency: Passive PoE systems are simpler and more cost-effective, making them ideal for deployments where advanced power management features are not required.
• Direct DC Connection: Passive PoE systems can be directly powered from battery-backed DC power sources, offering resilience in power outage situations without the need for AC-DC power conversion, thereby improving system efficiency and reliability.

PoE technology offers a streamlined solution for powering network devices, with options ranging from simple, cost-effective passive PoE to sophisticated IEEE 802.3af/at standards. For WISPs, the choice between 24V passive systems and other PoE options hinges on factors like device compatibility, installation complexity, and power infrastructure. By carefully selecting the appropriate PoE system, WISPs can ensure efficient, reliable, and flexible network deployments, minimizing installation costs and maximizing network uptime.