PoE (Power over Ethernet)

PoE (Power over Ethernet) – definition

Power over Ethernet (PoE) is a family of IEEE Ethernet standards that delivers DC electrical power and Ethernet data through the same twisted-pair cable. In mobile robotics, PoE is used to connect networked payloads such as LiDAR sensors, depth cameras, IP cameras, GNSS receivers, Ethernet switches, and embedded compute devices without installing a separate power cable for each device.

A PoE system consists of Power Sourcing Equipment (PSE), such as a PoE switch or injector, and a Powered Device (PD), such as a camera or sensor. The PSE detects a compatible PD before supplying power. This detection process prevents a compliant PSE from applying PoE voltage to a non-PoE Ethernet device. IEEE 802.3 defines the electrical detection, classification, power negotiation, and cable-pair requirements.

For an unmanned ground vehicle (UGV), PoE reduces cable count around the sensor mast, payload enclosure, or external compute module. It does not replace the rover’s main battery distribution system. A PoE switch, injector, DC-DC converter, and connected PDs must still be sized for the vehicle battery voltage, peak current, connector ratings, and thermal conditions.

IEEE PoE standards and power classes

IEEE PoE standards differ mainly in the available power and the number of cable pairs used. The power available at the PD is lower than the power delivered by the PSE because of cable resistance and connector losses. System designers should size a PoE budget using PD input power rather than the nominal PSE rating.

IEEE standard	PoE type	Maximum PSE power	Maximum PD power	Pairs used
IEEE 802.3af-2003	Type 1	15.4 W	12.95 W	2 pairs
IEEE 802.3at-2009	Type 2	30 W	25.5 W	2 pairs
IEEE 802.3bt-2018	Type 3	60 W	51 W	4 pairs
IEEE 802.3bt-2018	Type 4	90 W	71.3 W	4 pairs

The values in the table are defined by the IEEE 802.3 PoE amendments and assume compliant equipment and cabling. Actual available power can be lower when a switch applies its own port limits, total power budget, cable derating, or thermal protection.

Source: IEEE Std 802.3-2022, which incorporates the PoE provisions introduced by IEEE 802.3af-2003, IEEE 802.3at-2009, and IEEE 802.3bt-2018.

PoE architecture on a mobile robot

A UGV normally has a battery-powered DC bus and one or more voltage converters. PoE is added downstream of this bus. A typical installation uses a regulated DC-DC converter to power a PoE switch or injector, followed by Ethernet cables routed to PD sensors.

The electrical power budget should include continuous load, startup load, cable loss, and switch consumption. A simplified calculation is:

P_required = Σ(PD power) + P_switch + P_cable_margin

For example, a design must not assume that a 30 W Type 2 PSE can always provide 30 W to a camera. IEEE 802.3at specifies up to 25.5 W at the PD under the applicable standard conditions. The difference accounts for losses in the channel between PSE and PD.

Before selecting components, engineers should verify the following properties:

PoE type supported by both the PSE and each PD.
Total switch power budget across all active ports.
Input-voltage range of the injector or PoE switch.
Ethernet speed requirement, such as 100BASE-TX or 1000BASE-T.
Shielding, connector sealing, bend radius, and vibration resistance of the cable assembly.
Maximum cable length and voltage drop under the expected payload current.

PoE in ROS 2 sensor integration

PoE is not a ROS 2 transport protocol and does not define ROS messages, topics, or Quality of Service settings. It provides physical-layer power while Ethernet carries IP traffic used by sensor drivers, ROS 2 nodes, DDS middleware, web interfaces, and time-synchronisation services.

A PoE-connected sensor commonly appears as an Ethernet device with a vendor SDK or a ROS 2 driver. The driver may publish data such as sensor_msgs/msg/Image, sensor_msgs/msg/PointCloud2, sensor_msgs/msg/Imu, or, in the case of a GNSS receiver, sensor_msgs/msg/NavSatFix. Message definitions are maintained in the ROS 2 common interface packages.

Source: ROS 2 sensor_msgs documentation and ROS 2 Humble documentation.

For a networked sensor, the ROS 2 host must have IP connectivity to the sensor. When the host uses more than one network interface, interface addresses, DDS discovery scope, multicast availability, firewall rules, and network namespaces can affect node discovery. A PoE switch should therefore be treated as part of the robot network architecture, not only as a power accessory.

Use with Leo Rover and Raph Rover

Leo Rover uses a Raspberry Pi-based compute unit and supports ROS 2 Humble or newer distributions. PoE can be used when an external Ethernet sensor requires both network connectivity and power, but PoE is not an autonomous-navigation feature by itself. The rover still requires sensor drivers, robot transforms, state estimation, mapping or localisation, and a navigation stack appropriate to the application.

On Leo Rover, a compact PoE injector or managed/unmanaged PoE switch can be installed as part of an external payload integration. The installation must account for the rover’s available battery capacity, payload mass, cable routing, and IP55 enclosure constraints. Engineers should consult the current platform documentation before modifying power distribution or attaching electrical accessories.

Raph Rover can be a more suitable platform when a payload includes several Ethernet sensors, a larger PoE switch, external compute hardware, or higher battery demand. Its larger payload capability does not remove the need for a measured power budget, fused supply paths, and validation under expected terrain vibration and temperature conditions.

Limitations and integration risks

PoE simplifies wiring, but it introduces dependencies between power and network availability. If a switch loses power, every PD connected to it can become unavailable simultaneously. This may affect perception, localisation, recording, and remote access nodes.

Common integration risks include insufficient switch budget, incompatible passive PoE equipment, voltage drop in long cables, unsealed RJ45 connections, electromagnetic interference, and power cycling during sensor startup. Passive PoE must not be confused with IEEE-compliant PoE. Passive injectors may apply voltage without IEEE detection and can damage equipment if used incorrectly.

Normative references and standards

The primary normative reference for Ethernet PoE is IEEE Std 802.3. The PoE provisions introduced by IEEE 802.3af, IEEE 802.3at, and IEEE 802.3bt are incorporated into later revisions of IEEE 802.3.

PoE (Power over Ethernet)