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Industrial Ethernet Book Issue 69 / 36
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PoE Plus technology finds new applications

PoE technology allows data and electric power to be simultaneously supplied to remote devices over an Ethernet network. The technology has been around for some years, so what are the latest advances? This article about PoE Plus (PoE+) by Alvis Chen, identifies the key additional advantages offered by PoE Plus, and explores the significant potential of the technology in scenarios that could not previously take full advantage of PoE because of the lack of power provided.

THE INDUSTRY-WIDE Power over Ethernet (PoE) standard was officially ratified in 2003 with the IEEE 802.3af standard. Since then, the use of PoE technology has rapidly grown. It allows data and electric power to be simultaneously supplied to remote devices over an Ethernet network, which provides a number of well-known advantages. PoE networking requires power sourcing equipment (PSE), such as a switch or hub, to provide power to various devices. The IEEE 802.3af standard designates 15.4W as the maximum continuous power output allowed per power sourcing device.

Devices that are powered by the sourcing equipment in a PoE network may include IP cameras, IP phones, and wireless access points. The maximum power usage of powered devices (PD) is 12.95W according to IEEE 802.3af.

PoE can be installed using either a PoE midspan system or via PoE switches. An example would be a PoE midspan system where an existing Ethernet switch is connected to an intermediary power source to inject power into the Ethernet cables that connect to various powered devices. In this setup, a PoE midspan hub serves as the PSE.

The benefit of using a midspan solution is the ability to quickly upgrade an existing Ethernet network for PoE applications. However, newer PoE switches offer a more integrated solution by incorporating the midspan hub's PSE function.

PoE uses standard CAT-5 Ethernet cables as specified by IEEE 802.3af. Although CAT-5 cables comprise four twisted pairs, only two of these pairs are used for 10BaseT and 100BaseT data transmission. There are two ways to use CAT-5 cables to simultaneously transmit data and power (see Figures 1 and 2). IEEE specifies that all PSEs and PDs must be compatible with both methods, but only one may be used at a time.

Fig. 1: Using CAT-5 cable to simultaneously transmit data and power. Power through the 'Spare Pair' is one method

Fig. 2: Using CAT-5 cable to simultaneously transmit data and power. Power through the 'Signal Pair' is the second method.

Ever more system operators took advantage of PoE's capabilities and benefits. However, IEEE 802.3af has limitations that make it unsuitable for the huge numbers of applications that demand more power. There are many Ethernet-connected devices that simply need more power. These include:

• PTZ / outdoor cameras - Low-power IP cameras are well-served by existing 802.3af PoE technology. However, cameras with Pan/Tilt/Zoom (PTZ) functionality are increasingly popular, and these require more power to operate their motors. Cameras that are ruggedised for operations outdoors, especially in harsh conditions, also need more power in order to use their heating units. 802.3af PoE technology simply isn't capable of providing enough power for these devices.

• LED display boards - Ethernet-connected LED display boards display up-to-date messages from the network. These displays use bright LEDs, which demand substantially more power than the 15.4W available over an 802.3af PoE line.

• Long distance LAN devices - Basic wireless access points can accept 15.4W of power. However, high-performance wireless devices that provide wider coverage use high gain antennas and multiple RFs, all of which demand much more power. These high-powered wireless devices are particularly common in widely distributed, remote applications. This is precisely the type of application where power supplies and cables are a major cost - yet the power supplied by 802.3af-compliant PSEs is too weak to offer a viable alternative to conventional power sourcing.

Fig. 3: PoE vs PoE+ This table provides a simple but useful comparison between IEEE 802.3af and IEEE 802.3at performance with applications.

In response, the IEEE 802.3at standard, also known as PoE Plus (or PoE+), was ratified to further expand the scope and capabilities of standardised PoE devices. PoE complying with IEEE 802.3at can supply power to devices in locations where it was previously difficult, if not impossible, to deliver power. Examples include remote network switches, security cameras and outdoor WLAN access points.

Transforming applications

In September 2009, the IEEE ratified 802.3at, which dramatically improved PoE's ability to meet the requirements of more demanding applications. With PoE Plus, it is possible to use just an Ethernet cable to deliver power to more power-hungry devices. PoE Plus therefore offers the following key advantages over conventional PoE:

• More powerful - IEEE 802.3at technology increases the power delivered to PDs to 34.2W - nearly double the power delivered by 'af' PoE technology. It is, therefore, now possible to use the Ethernet cable to deliver power to devices such as outdoor cameras, LED display boards, WiMax devices, and even some embedded computers or terminal computers. This capability alone unlocks the possibility of using PoE in applications that were previously too demanding.

• More efficient - PoE Plus is not only more powerful, it is also more efficient. IEEE 802.3at defines a new method of communication and identification between PSEs and PDs. In 802.3af, classification on the hardware layer (Layer 1) was optional. In 802.3at, this identification is mandatory. Further, 802.3at adds an additional data-layer (Layer 2) classification mechanism, known as the Link Layer Discovery Protocol (LLDP). LLDP allows the PSE to repeatedly request status information from the PD and dynamically allocate power in response. Therefore, PDs will use power more efficiently and only request maximum power when it is needed. This feature of IEEE 802.3at is especially suited to those applications that must be careful with their power consumption, such as remote systems powered by batteries or solar PV cells. However, less power consumption will also translate into an improved bottom line no matter the application, simply by reducing the power bill.

PoE Plus brings entire categories of applications previously off-limits for PoE into play. Outdoor applications that make heavy use of ruggedised power-consuming devices can now make use of PoE. Applications with limited power resources, or those where a separate powering bus is to be avoided, can now exploit advanced power allocation mechanisms of PoE Plus to maximise their efficiency.

More about PoE Plus

IEEE 802.3at's 34.2W at the PSE, 25.5W at the PD (with currents to 600mA), and voltages of 50-57V (42.5-57V at the PD) is ample to power most modern higher-powered 'edge of network' devices, but the race is on to see just who can squeeze the most power out of a PoE cable. Some manufacturers are already claiming 60W; others say they can push this even higher. The limiting factor is not the PSE, but the CAT5 cable, which - even at 34.4W - may need temperature de-rating to avoid overheating.

The PSE is the start of PoE Plus networks, as it delivers power to the PDs. Unmanaged switches supply power through four IEEE 802.3af/at compliant PoE/Ethernet combo ports. These may feature 10/100M, full/half-duplex, and MDI/MDI-X auto-sensing. Such a specification provides an economic PoE Plus system foundation. It also allows future-proofing of an existing PoE system to support more PDs. A suitable PoE managed switch allows advanced management and protocol support, including SNMP security, multicast filtering, quality of service and automatic warning.

Another benefit of PoE Plus is that it allows two PD interfaces on a single RJ45 connector, with one operating in Mode A and the other in Mode B. This allows higher installation flexibility.

Much of the focus today is about maximising the advantages of PoE flexibility, which often means use of industrially hardened PSEs that meet the performance requirements of the PDs - as distinct from the power requirements.

James Hunt

PoE Plus in deployment

Figure 4 shows an Intelligent Traffic System (ITS). Intelligent traffic management and video surveillance has emerged as a popular program for many local municipalities. Traffic cameras, sensors, intelligent lights and embedded computers are often deployed along motorways and other roads in order to monitor and improve traffic safety. Such cameras are exposed to the elements and are often deployed at remote locations. They must have a tolerance to a wide operating temperature range in order to work reliably in such an operating environment.

Fig. 4: An Intelligent Traffic System (ITS). Intelligent traffic management and video surveillance has emerged as a popular program for many local municipalities.

Safety considerations relating to PoE

The 48V DC power design makes industrial PoE switches compliant with Underwriters Laboratories (UL) Safety Extra-LowVoltage (SELV) classification to provide users with a safe working environment.

As an additional safety precaution in the PoE network, users can set the power limitation for each port on the industrial PoE switch. Without this function, the PD will immediately request overload current from the PoE switch if its power supply circuit fails (e.g. the power circuit in a PoE camera shorts out because of excessive humidity). This will shut down the Ethernet switch and all related network communication. As a result, power limit configuration can protect PoE switches from providing too much power, even when requested by the powered device.

An industrial PoE switch will not only safeguard against excessive power output, but also send alarm messages to the network administrator.

James Hunt

Robust Gigabit Ethernet infrastructures with PoE Plus support form the ideal backbone for ITS networks. Long-distance fibre gives the network enough range to cover the entire traffic grid. Meanwhile, PoE Plus technology allows devices to be deployed anywhere Ethernet cables go, even if a power supply is not available. Using this technology also completely eliminates the extra cost of power supplies and power cables.

A complete intelligent traffic system integrates cameras, lights, embedded computers, and sensors all of which can be powered with PoE Plus. The extra power supplied allows municipal authorities to deploy cameras, even where conditions are severe enough to require highly ruggedised self-heating camera units. Additionally, PTZ cameras can be deployed in high-traffic locations. Figure 5 shows the railway equivalent. Typical train networks include the Ethernet train backbone, ground-totrain communications and onboard IP video surveillance.

PoE Plus also offers enough power to supply local intelligence and front-end control in the form of embedded computers. With this flexibility, systems can perform advanced automated actions, such as triggering a camera's zoom function when a car runs a red light. Finally, with LLDP communications maximising the power efficiency of all the PDs in this network, the entire system will consume less electricity.

Fig. 5: An intelligent railway system. Typical train networks include the Ethernet train backbone, ground-to-train communications and onboard IP video surveillance.

Foundation stone

A PoE Plus network begins with the power source equipment, which is responsible for delivering power to the powered devices. Managed and unmanaged Ethernet switches add PoE Plus capability. For example, Moxa's EDS-206A-4PoE switch supplies power through four IEEE 802.3af/at compliant PoE/Ethernet combo ports with 10/100M, full/halfduplex, and MDI/MDI-X auto-sensing to provide an economical foundation for a PoE Plus system, or future-proof an existing PoE system to support more powered devices.

Managed switches can add advanced management and protocol support, such as multicast filtering, quality of service, automatic warning, and SNMP security.

In conclusion

PoE creates systems that are more efficient, reliable, safe, and easy to maintain. Now that PoE Plus is available, PoE is a viable option for even more industrial applications than before.

Alvis Chen is Product Manager at Moxa Inc.

Source: Industrial Ethernet Book Issue 69 / 36
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