Save Time and Money with Power over Ethernet


Remember when you used to have a landline phone? Remember how easy it was to use? The beauty was in its simplicity: a twisted pair of copper wire carried both power and data to the handset. Today, Ethernet dominates network infrastructure, but it lacks an often overlooked benefit of its older analog cousin: power.

In 2003, the IEEE ratified the 802.3af standard, commonly known as power over Ethernet (PoE), to address this deficiency. Since then, PoE has changed the way enterprise network administrators design and deploy their corporate networks – one cable to carry both power and data to end devices. Limitations in the standard have previously prohibited its use in most measurement and automation applications; however, with the new higher-power IEEE 802.3at (PoE+) standard set to be ratified in the second half of 2009, engineers and scientists stand to save substantial time and money with this technology.

The Ethernet Migration

The measurement and control industry continues to standardize on PC-based technology, and Ethernet is no exception. The benefits are obvious: lower installation costs, better device interoperability, communication with the enterprise network, and higher bandwidth. Ethernet has become the de facto standard for highly distributed systems, including data acquisition applications and programmable automation controllers (PACs). One reason for Ethernet’s rapid adoption was the fact that the infrastructure already existed. Ethernet and even Wi-Fi connectivity is near ubiquitous in corporate networks, and it was only a matter of time before engineering departments began using it for instrumentation and control.

Today, many IT groups are already upgrading to PoE-based systems, though you may not have noticed. The proliferation of wireless networking and Voice over IP (VoIP) telephony have been the primary drivers behind PoE adoption for the last several years. For example, take a look at the nearest wireless access point (WAP) mounted in the ceiling of your office building. If there are fewer than two cables coming out of it, the power is coming over Ethernet.

Figure 1. A PoE-enabled device can transmit or receive power over the four twisted-pair copper wires of a CAT 3 (minimum) Ethernet cable in two different configurations to carry power, data, or both.

In fact, according to a research study by Venture Development Corporation, 95 percent of the enterprise WAPs shipped in 2007 were PoE-enabled. This trend is likely to continue, as it is estimated that 25 percent of all new Ethernet ports will be PoE-enabled by 20101.

  2007 2008 2009 2010 2011 2012 CAGR
PoE-Enabled Switch Ports 47.0 57.8 71.1 87.4 107.4 132.1 23.0%
Total Switch Ports 237.9 268.2 302.2 340.6 383.9 432.7 12.7%

Table 1. VDC estimates that by 2012 up to 30 percent of all enterprise Ethernet ports shipped worldwide will have PoE capabilities.1

PoE for Measurement and Control Applications

Though not originally designed for measurement and automation, PoE technology offers several potential benefits for scientists and engineers working with networked equipment such as data acquisition devices or programmable automation controllers. This is especially true considering the higher power available with the upcoming IEEE 802.3at PoE+ standard. When designing a networked measurement application, consider the following advantages and disadvantages.

Advantages of Power over Ethernet

Simplicity: PoE uses standard CAT 3 or CAT 5 Ethernet cables and is no different than installing a typical network – the only specialized equipment needed is a PoE-compliant switch. Powering devices over the network eliminates the need for separate data and power cables, significantly simplifying the installation and maintenance of networked devices. In addition, a centralized power supply for the power sourcing equipment (PSE) eliminates the need for each powered device (PD) to have its own AC-to-DC converter or uninterruptable power supply (UPS).

Flexibility: With the option to install virtually anywhere, PoE technology allows for a scalable and flexible networking infrastructure. Using PoE, you can place PDs where they are needed, which may or may not be near existing power sources. Because devices no longer need to be located near an AC outlet, individual devices and even entire measurement systems can be moved and adapted to suit the changing needs of an application.

Cost Savings: PoE cuts expenses on two fronts: network installation and energy management. Taking into account materials, labor, and installation time, wiring costs can vary between as much as tens and hundreds of dollars per foot. You can potentially cut this cost in half by requiring only one wire (or cable) to be routed to each device. However, you can realize even larger savings through effective energy management. Most enterprise class PoE switches support the simple network management protocol (SNMP), which gives network administrators the ability to remotely monitor and manage power consumption of PSEs. For example, PSEs and PDs may be put into low-power or sleep modes during off-peak hours during the night or weekends. Rather than have a separate power supply for each device, centralized dynamic power control in a PoE network is analogous to a mini smart grid – you can design for power efficiency.

Reliability: PoE enables centralized power back-up solutions using UPS, which ensures the reliability and availability of powered devices. When each PSE is combined with a UPS, a network is fully optimized for continuous operation, virtually eliminating power interruptions and downtime. During a power failure, a UPS continues to provide power to the PSE, which in turn continues to power the PDs. Furthermore, a combination of SNMP and control logic can troubleshoot, restart, shutdown, or put measurement equipment into a safe state if the need arises.

Disadvantages of Power over Ethernet

Capital Investment: Taking advantage of the benefits of PoE can require you to make significant capital expenditures. Ethernet switches that use PoE are typically 10 to 20 percent more expensive than traditional switches. A few additional costs include more powerful UPS systems and power supplies, and an increase in cooling and electrical capacity in data rooms to address the additional heat generated by PSEs. Thus, PoE often makes more sense for greenfield applications. However, the use of injectors and splitters allows you to add PoE equipment to an existing network or vice versa.

Standardization: Most measurement and control applications will most fully realize the potential of power over Ethernet when the higher power IEEE 802.3at standard is ratified by the IEEE. Many vendors have started releasing hardware in anticipation of ratification, but many of these are proprietary solutions that may not be interoperable with compliant devices. With ratification scheduled for the second half of 2009, it may be prudent to start with now and add 802.3at later.

How to Use PoE with NI Devices

Several National Instruments devices provide Ethernet connectivity for distributed or remote measurement and control applications. Some devices, such as NI CompactRIO programmable automation controllers (PACs), may have to wait for IEEE 802.3at (PoE+) ratification, but several others are within the 12.95 W power budget offered by the existing IEEE 802.3af standard. For example, NI ENET-9xxx Ethernet data acquisition (DAQ) devices and NI Smart Cameras can be made compatible with PoE networks by using a simple adapter.

  Input Voltage (V) Maximum Power (W) 802.3af Classification
NI ENET-9xxx 9 to 30 4.5 Class 2
NI Smart Camera 24 10.8 Class 3

Table 2. Power specifications for NI Ethernet DAQ and NI Smart Camera devices.

To take advantage of PoE with these devices, you must use a PoE splitter, which separates the power lines from the data lines in the CAT 3 or CAT 5 cable. To set up your system, connect the energized Ethernet cable to the PoE input port on the splitter. Then connect the regular Ethernet output port and power out lines to your device.

Figure 2. Use a PoE splitter to provide data and power separately to an NI Ethernet DAQ device.

This approach is analogous to a long USB cable or USB extender, particularly for NI C Series DAQ devices. For example, you could use an NI USB-9234 to reach up to 5 m from your PC or laptop. However, with PoE you can extend that reach to up to 100 m with an NI ENET-9234.

Using PoE in Your Next Application

The simplicity, reliability, and cost savings make a compelling argument for combining power and data into one cable – even more so with the recent emphasis on energy conservation. With the growing adoption of power over Ethernet and the migration trend of PC-based technology into measurement and automation applications, it is only a matter of time before most engineers and scientists are presented with the option of using PoE in their projects. And with the ratification of IEEE 802.3at (PoE+) the most common barrier to adoption will be removed. When designing your next system, take a holistic approach and consider power over Ethernet – it could save you both time and money.

-Charlie Stiernberg

Charlie Stiernberg is an NI data acquisition product manager. He received his bachelor’s degree in electrical engineering, with a focus on embedded systems and VLSI design, from The University of Texas at Austin.

Read a tutorial on connecting NI measurement and automation hardware to PoE-enabled networks

1 Brian Greenberg, Venture Development Corporation. Power Over Ethernet: Global Market Opportunity Analysis, Third Edition. March 2008.