What is PoE

Power over Ethernet has become increasingly popular with boaters over the years due to its reliability and allowance of flexible installations. The technology enables power to be sent via Network cables. This enables PD (Powered Devices) to operate with just the network cable connected, rather than a separate cable for power.

PoE works with no special cables or connectors, it utilises the 8 wires (4 pairs) already found in a standard Cat5e cable. They can be used to send both power and data over the same pair, or separated between power pairs and data pairs depending on the connection method and speed.

As the new OneNet standard supports PoE we are likely to see more installations using it. So let’s find out what it’s all about…

Methods of transmitting power

There are 3 standard methods for transmitting power, which are ‘Alternative A’, ‘Alternative B’ and 4PPoE. These are standardized in IEEE 802.3.

Alternative A and B are utilised for 10BASE-T, 100BASE-T and 100BASE-TX where only two of the four pairs in a cable are being used for data transfer. 4PPoE is used for higher speed connections of 1000BASE-T, 2.5GBASE-T, 5GBASE-T and 10GBASE-T because all four pairs are being used for data transfer.

Alternative A (common-mode) uses the same wires for data and power in both 10 and 100Mbit connections. This is achievable using a technique very similar to Phantom Power. (More on this later…)

Alternative B (spare-pair) makes use of the spare two pairs not being used for data transfer, by transmitting power over the two un-used pairs.

4PPoE (4 pair) utilises all 8 wires (4 pairs) within a category cable for data transfer, allowing for much higher speeds. This also allows for all 4 pairs to have power added to them, which enables much higher power devices to utilise PoE.

Geek mode

Phantom Power is common in the audio equipment industry, where DC is sent via microphone cables to operate the internal electronic circuitry within the microphone. The term ‘phantom’ is because the power cannot be seen because there is no power cable. Whilst Common-Mode (A) doesn’t use this exact method, it uses something very similar.

The way this works is the power is applied to each pair of the data wires using a common voltage. As differential signaling is used with twisted-pair Ethernet, it means that the power won’t interfere with the data. The common voltage is taken off / extracted via the Centre Tap, leaving only the differential voltages. This voltage is usually between 44 and 57 volts, typically at 48-50V.

For example; If 50V is applied, then the power signal (let’s say bit 0) is 50V, whilst the data signal (bit 1) is now 55V. When this 50V common voltage is removed at the other end, left is bit 0 @ 0V and bit 1 @ 5V. This 0V and 5V is your data, which is then sent to the differential receiver for data.

Where does OneNet come into this?

OneNet supports PoE as the standard is based upon IEEE 802.3 Ethernet Local Area Network. Specifically, it will allow up to 25.5W of power to be transmitted via Ethernet, following IEEE 802.3at PoE+. This offers a much greater power capacity.

It is worth highlighting here that whilst there are older Category 3 (Cat3) cables etc. being used for PoE, the OneNet standard details that a minimum of Cat5e cable is to be used. Everything referenced to Ethernet cables and wiring in OneNet is from Cat5e or higher. Cat6 and Cat6e cable for example is also supported for higher speeds.

Cables that pre-date Cat5 such as Cat4 are untwisted pairs of wires, which are more susceptible to interference than twisted pair wires.

PoE offers a number of benefits;

– Network Expansion

Having power available on the network already means that expansion of the network is simple with the use of PoE Injectors, and switches. The maximum length of an Ethernet network is not unlimited, but it is far greater than that of NMEA 2000 when Ethernet switches are used.

– Flexible Installations

A device that requires power from a battery or wall socket is limited on the installation location. Having devices that are powered via a network connection means there is no longer a restriction on installation location or distance from a socket/battery. Alongside this, when installing in tight areas such as bulkheads, two cables will always be more difficult to fit than one. Halving the cable number creates double the space to work with.

– Time + Cost

As there is no requirement for socket or battery connection with power cables, the time of installation is reduced alongside the cost of installation thanks to having no separate power cabling. A lower installation time means a lower cost when looked at from a rate/hr perspective.

– Reliable and Safe

Any power delivered over Ethernet is safe as PoE technology is intelligent, with an interfacing process between the PSE (Power Sourcing Equipment) and the PD (Powered Device) which enables the device to be detected by the source, and determine how much power is needed to avoid over-loading and wasting power. This is a process called ‘signature detection’ where the PSE will supply a lower voltage to detect a 25KOhm resistance before providing a higher voltage once the PSE has determined the device is a PoE PD. This is a characteristic of IEEE 802.3 PoE Devices.

On larger installations with a higher number of devices, power management is important to ensure all devices are reliably powered. The power budget allocation is done using the negotiation between the PD and the PSE, allowing only the required amount of power to be allocated.


NMEA Talk Banner