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Home News & Views Viewpoints Power over Ethernet

Power over Ethernet

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In this 'Viewpoint' we take a look at Power over Ethernet or PoE which describes any of several standardised or ad-hoc systems which pass electrical power along with data on Ethernet cabling. This allows a single cable to provide both data connection and electrical power to devices such as network hubs or closed-circuit TV cameras. Unlike standards such as Universal Serial Bus (USB) which also power devices over the data cables, PoE allows long cable lengths. Power may be carried on the same conductors as the data, or it may be carried on spare pairs of the cable.

There are several common techniques for transmitting power over Ethernet cabling; two of them have been adopted and standardised by the IEEE 802.3 committee. Power may be transmitted on the unused (spare) conductors of a cable, since only two of the four pairs are needed for the commonly used 10Mbit/s–100Mbit/s physical layers. In the IEEE standards, this is referred to as Alternative B. Power may be transmitted on the data conductors by applying a common-mode voltage to each pair. Because Ethernet uses differential signalling, this does not interfere with data transmission; and the common mode voltage (which may be, e.g., positive for the TX pair and negative for the RX pair) is easily extracted using the centre tap of standard Ethernet magnetics. This is similar to the phantom power technique commonly used for powering audio microphones. In the IEEE standards, this is referred to as Alternative A.

In addition to standardising existing practice for spare-pair and common-mode data pair power transmission, the IEEE PoE standards provide for signalling between the power source equipment (PSE) and powered device (PD). This signalling allows the presence of a conforming device to be detected by the power source, and allows the device and source to negotiate the amount of power required or available. Up to 25 watts is available for a device, depending on the version of the standard in use.

PoE provides both data and power connections in one cable, so equipment doesn't require a separate cable for each need. For equipment that does not already have a power or data connection, PoE can be attractive when the power demand is modest. For example, PoE is useful for IP telephones, wireless access points, cameras with pan tilt and zoom (PTZ), and remote Ethernet switches. PoE can provide long cable runs e.g. 100m (330ft) and deliver 12W of isolated power. PoE-plus provides even more power.

There are competing data and power technologies. The Universal Serial Bus (USB) provides both data and power, but it is designed for short cables with a maximum length of 5m (16ft) and provides less than 2.5W of non-isolated power. It is less expensive than PoE, and works well for low power peripherals such as a computer mouse, a headset/microphone or a serial port. Some peripherals, such as speakers, scanners and printers, need more power than USB can provide. IEEE 1394 (FireWire) is similar to USB but can provide substantially more power (45 W) at a distance of 4.5 m. On the other hand, USB peripherals can operate using very little power; while maintaining an Ethernet connection uses a significant amount of power.

Depending on the application, some of the advantages with PoE over other technologies may be:

  • Inexpensive cabling

  • Fast data rate

  • No batteries required

  • Peer-to-peer network access. Once a device is connected to the network, it is accessible to many users.

At Lanode we have been expanding our PoE offerings, specifically within our Industrial Ethernet Solutions where you'll find numerous PoE device options. The following specific products maybe of interest:

If you want to know more about PoE or what products we have available, please visit www.lanode.com or call us on +44 (0) 1276 677220 in normal business hours.