Wireless Systems Design

Looking For Ethernet Power? Find The Outlet, Nevermore

Power-over-Ethernet technology gives the world a standard way to easily connect all Internet-based devices while possibly providing the world’s first global power standard.

Designers and customers alike are just beginning to appreciate the far-reaching effects of Power-over-Ethernet (PoE) technology. The products that meet the IEEE 802.3af PoE standard may revolutionize the installation of any device that's based on the Internet Protocol (IP). Imagine if wireless access points (APs), Voice-over-IP (VoIP) phones, cameras, and other devices were instantly ready for use. The user would merely have to plug the devices into a traditional Category-5, Ethernet-cable wall socket (FIG. 1).

Many wireless designers may only view PoE technology as a boon to the proliferation of wireless access points. But IP phones were the first drivers in the development of Power-over-Ethernet systems. One of the early developers of IP-based phones was Cisco Systems (www.cisco.com). Its goal was to bring voice communication into the same network as data structures, according to Steve Shalita, Cisco Systems' Senior Manager.

Customers found those early IP-based phones difficult to install. They required a connection to an AC power source as well as to the network. One way to simplify these installations was to include power through the existing Ethernet cable. Shalita notes that these same problems were faced years later when wireless access points first became popular.

Many vendors shared these challenges, so an IEEE standard was developed to universalize the creation of Power-over-Ethernet devices. This standard, which was published in June of 2003, is known as 802.3af. Not surprisingly, Cisco was one of its main participants.

What is involved in the design of PoE products? Do the benefits of PoE technology outweigh any potential risks? First of all, many obvious as well as subtle benefits can be derived from standardized PoE technology. Perhaps the most important benefit—at least to information-technology (IT) departments—is cost savings. PoE-enabled devices don't require any accompanying AC power source, such as a wall socket. This aspect reduces the need for certified electricians to install conduit, wiring, and outlets throughout a facility.

Instead, engineers with low-voltage licenses can perform installations, observes Nigel Ballard, Wireless Director for Matrix Networks (www.mtrx.com). The use of less expensive labor translates into cost effectiveness and faster deployment times. Ballard explains that the installations are much neater in both the ceiling space—a typical location of access points—and the LAN-switch cupboard.

Keith Hopwood, Vice President of Marketing for Phihong USA (www.phihong.com), says that although cost savings may vary, an electrician who installs power to an access point may charge around $100 per unit. Others place the cost of access-point power installations at several hundreds of dollars per unit. Either way, the return-on-investment (ROI) for PoE-enabled devices can be significant.

Power-over-Ethernet technology also offers flexibility. Instead of locating APs near existing AC power outlets, wireless-LAN designers can place them where they maximize coverage or increase bandwidth. A less obvious benefit is PoE's capability to provide power through data cables to dangerous locations. For example, IP-based security cables sometimes need to be routed to "wet locations," such as a building's exterior. Running a separate power supply to such locations can be a safety hazard. IEEE 802.3af-certified PoE cables can reduce those hazards.

PoE products also boast power reliability. For applications that use AC wall power, PoE offers a source of backup power when outages occur. This characteristic could prove extremely valuable to IP phones. They would still work even if the main supply line goes down.

PoE has even produced benefits outside of the usual venue of wireless-LAN office deployments. One application area that's gaining interest is in tower tops for wireless-Internet service providers (WISPs). According to Mike Tadros of Antenna Systems & Supplies Company (www.antennasystems.com), it's often much more efficient to run 200 ft. of CAT-5 Ethernet cable instead of running a 1/2 in. or 5/8 in. of hardline coaxial cable. Tadros cautions that by "efficient," he's referring to the energy (measured in dBm) that's being broadcast from the antenna(s). As he explains, "There are plenty of online and commercial microwave-link calculators available that will help an end user determine the theoretical distance achievable utilizing the two previously mentioned scenarios."

Almost all LAN-switch vendors endorse the 802.3af standard. In fact, Cisco—one of the largest manufacturers of LAN switches—recently announced support for the IEEE 802.3af PoE standard across almost all of its switching products.

The 802.3af PoE specification details all of the requirements for designing PoE equipment. Two types of devices are specified in the standard: Power-Sourcing Equipment (PSE) and a Powered Device (PD). The PSE provides 48-V DC power, with a current limit of 350 mA, to the PD—be it a VoIP phone or wireless access point. The PSE is limited to a continuous maximum power output of 15.4 W.

As one might expect, different devices require different power levels. For example, a VoIP phone typically consumes 4 to 6 W of power. Yet a dual-radio wireless access point typically requires closer to 14 W. Several power-classification levels can be specified, as will be discussed shortly.

Power passes from the PSE to a PD over standard Ethernet CAT-5 cable. How is this done? First, remember that Ethernet signals travel along two twisted pairs—one pair for data transfer in each direction. There are four twisted pairs in each CAT-5 cable. So one option for delivering power is to use one spare pair for the positive DC supply and the other spare pair for the negative return. The other way to supply power is by "floating" it over a pair of wires that's already being used to pass data. The remaining pair of wires is then left available for another data connection port (FIG. 2).

One of the interesting decisions made in the 802.3af standard was the selection of which device would support which power-delivery option. Cisco's Steve Shalita explains that the PSE or LAN switch in the PoE connection only needs to implement one of the two power-delivery techniques. A given PSE can either send power over an unused twisted pair of wires or float power over the twisted pair that's being used for data. The Powered-Device unit, on the other hand, must be designed to support both methods of power delivery.

What handshakes occur between the PSE and the PD? A PD product, such as a wireless access point, is first connected to the PoE-enabled cable. Then, the PSE sends a test voltage to determine if the PD has a valid IEEE 802.3af signature. This detection signature results from a small current-limited voltage that's applied to the CAT-5 copper wire. The voltage will respond to the presence of a 25-KΩ resistor in the PD.

Having the PSE perform such probing can be a difficult task, cautions Todd Nelson, Product Marketing Manager for Linear Technology Corp. (www.linear.com). He explains that variations in the cable length and the presence of diode bridges at the PD interface can potentially lead to confusing results. The diode bridge is an IEEE requirement, notes Nelson. "It is there for polarity protection. It is there in case someone wires the connector backwards, and also because the IEEE does not strictly mandate whether the PSE sources power as −48 V or +48 V. So all PD circuits should include a diode bridge."

If the PD responds to the PSE's detection signal with a valid signature, another test is performed to determine the PD's power-consumption classification, explains Rex Caballero, Field Applications Engineer at Supertex (www.supertex.com). "During the classification test, the PD must sink a known current according to the IEEE 802.3af classification table," observes Mr. Caballero (see table). If the PD doesn't provide a proper current sink, the PSE will assume a default type of Class 0 for the PD. Only after the classification test does the PSE provide 48 V to the PD.

Power isolation is another of the many requirements that are covered under the IEEE standard, notes Thomas Sng, Product Manager for Agilent's Isolation Products Division (www.agilent.com). He says that Section 33.4.1 of the specification states that a PD must be able to withstand either:

a) 1500 V rms steady-state at 50 to 60 Hz for 60 sec., or:

b) An impulse test consisting of a 1500-V, 10/700 microsecond waveform—applied 10 times—with a 60-sec. interval between pulses.

In either case, there is to be no insulation breakdown. One way of achieving this electrical strength test is through the use of optocouplers.

CHALLENGES LIE AHEAD
Are there any downsides to PoE technology? One constant concern is electromagnetic interference (EMI) between the Ethernet and power cables as well as within the PSE and PD units. Although few PoE equipment vendors or installers have noticed many EMI-related problems, such issues are possible. Linear's Todd Nelson agrees that EMI hasn't been a strong concern for most customers. But he points out that Gigabit Ethernet over copper wires is challenging because the environment in which the Ethernet cable is routed is subject to considerable noise. This noise will affect all of the detection, classification, and disconnect sensing circuits.

Many of the targeted PoE devices, such as IP phones, wireless APs, and web cameras, are related to the telecommunications infrastructure. The distortions of the output signals caused by EMI generation are a critical concern for such highly integrated devices. Thomas Sng of Agilent suggests the use of optocouplers as a proven method for isolation from magnetic fields. He observes that unlike magnetic isolators, transformers, or signal coils, there is no EMI in optocoupling.

Clean DC power is another way to reduce EMI challenges. National Semiconductor (www.national.com) has recently introduced a new high-voltage, single-ended converter with a MOSFET driver that is directly targeted at PoE Powered-Device applications.

Perhaps one of the least considered issues with power deployment is the power itself. There must be enough cumulative power available to the LAN switch to support all of the connected PoE devices. If these devices don't support the optional power-classification feature listed in the 802.3af standard, the switch must assume that the full 15.4 W is required. This cumulative power can quickly add up to a large amount—much more than the amount that's provided by a standard 110-V AC wall power switch. For a large PoE installation, new 110 or 220 AC power lines may be required.

PoE designers also must be cautious when selecting PSE vendors. Some manufacturers won't include the necessary power field-effect transistors (FETs) and current-sensing resistors in their multi-channel PSE products. Thankfully, many vendors do include these essential components. Supertex, for example, has designed both the FETs and current-sensing circuits that are internal to their devices. In addition, PowerDsine (www.powerdsine.com) has worked with Motorola (www.motorola.com) to develop a design that integrates the per-channel FET directly into an ASIC.

To provide the necessary power, PoE devices require either a power-enabled LAN switch or DC injector. Most major manufacturers now offer a wide range of PoE LAN switches and related devices. As with any relatively new technology, these switches are expensive. To avoid the high cost and simultaneously deal with legacy infrastructures, many IT departments have chosen to inject DC power into their existing non-PoE cable line (FIG. 3). This approach will provide unmanaged power to the PD units. But it also will act as a potential point of failure.

Even with these challenges, the benefits of PoE technology far outweigh its potential drawbacks. In fact, many experts predict that PoE technology will become a global power standard. The RJ-45 Ethernet connectors are already uniform worldwide. With PoE, Ethernet moves from a basic data-connectivity mechanism to powering all Ethernet-based devices. It's very likely that Ethernet could become the world's only universal power standard— the same plug (RJ-45), same voltage, and same range anywhere in the world.

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