Power over Ethernet

The basic principle of PoE (Power over Ethernet) is a technology that enables electrical power to pass over Ethernet cabling at distances up to 100 m. It is designed to provide end devices with sufficient power to operate with no need for a local power source.

This technology complements standard Ethernet communication; it does not affect the transmission of data throughout a network.

Benefits of PoE

The benefits of PoE are numerous: the ability to transmit data and power over Ethernet cabling at significant distances creates many possibilities for network designers.

Simple low-cost installations

The amount of equipment required for installations can be greatly reduced. End devices no longer need power supplies, extensive power cabling or AC outlets. Reducing the equipment required also reduces the installation time and complexity and, ultimately, removes many potential points of failure in a system.

Flexibility

The reduction of equipment required allows for the flexible deployment of networks. End devices can be relocated easily and temporary installs require much less time to implement.

Efficient network design

With a PoE system, there is no longer a requirement for AC/DC outlets or power supplies to be positioned in close proximity to the PD. With the shackles of power availability removed, the optimum placement of end devices can be considered.

IEEE 802.3at-2009 (PoE+)

IEEE 802.3at-2009 Power over Ethernet Plus (PoE+) is a gradual improvement of the IEEE 802.3af-2003 standard. As end device technology has evolved, so has the requirement for more power. For example, Wi-Fi access points are now supporting the latest power-hungry 802.11n protocol, IP cameras now feature pan, tilt and zoom functionality (PTZ), some are heated for outdoor installations and VoIP phones now support video. All of these products require more power to operate and hence PoE+ was developed to accommodate market demands.

PoE+ can supply 34.2 W of power at the PSE; the maximum power available at the PD is 25.5 W due to losses in the PoE+ system. Power feeding is achieved by using all four pairs of an Ethernet cable, compared to two pairs used by the older standard, and hence delivers twice the power. It is backward compatible with IEEE 802.3af devices — the PSE will simply classify the PD (using the resistive power discovery method) and supply the appropriate amount of power to avoid damaging the PD.

Table 1: PoE vs PoE+ parameters. For a larger image, click here.

Conclusion

Products and applications

There is a growing demand for industrial-grade PoE equipment in many different industry sectors such as factory automation, intelligent traffic control systems (ITS), security automation, transport and building management systems (BMS). Networks in these industries need to utilise rugged hardware to withstand operation in environments that are uncontrolled — they need to be reliable to ensure mission-critical operations are always functional and available.

Managed and unmanaged switches

Managed switches provide advanced network features and remote management capabilities, and so they are extremely useful when combined with PoE because the PoE power budget can be managed from a central location, typically via a web-based graphical user interface.

Unmanaged switches are completely plug and play — they have no configuration interface or options. Their relative simplicity means that they are cost-effective devices and hence extremely low-cost networks can be assembled.

PoE has become an extremely useful technology for industrial applications. Today its predominant market driving force is the requirement for connecting IP cameras and wireless access points; however, more and more end devices are appearing in the market that can take advantage of PoE, such as sensors, detectors, security access devices, displays and HMIs.