Electronic Design

Preparing for Power over Ethernet Plus

The draft standard for IEEE 802.3at Power over Ethernet Plus (PoE Plus) remains on track for an August release. The original 802.3af PoE standard offered a fairly straightforward way to supply loads with 13 W or so of usable power delivered at 48 V dc. But IEEE 802.3at PoE Plus, which ups usable power to something over 50 W, introduces some wrinkles that designers and even IT managers must understand.

One catch is that designers can still supply power in a limited fashion in some existing Ethernet installations via a mid-span bridge. But in that case, designers can’t implement power negotiations between a powered device (PD) and power source equipment (PSE). This implies dedicated PoE Plus ports and relatively high duty-cycle power supplies in midspans.

Something else to watch out for are PDs that dynamically negotiate power requirements with the PSE via their Ethernet connection. This requires more code in the PD micro-controller and a greater understanding of dynamic power requirements on the part of the engineer writing that code.

A potential pitfall for end users is that PDs can meet the standard by operating in a fall-back mode if there’s not enough power for full functionality. (For example, a video phone could fall back to operating voice-only, with- out a video display.) Alternatively, a PD application could meet the standard simply by signaling “insufficient power.” IT managers who bought a lot of “compliant” video phones could find themselves embarrassed by a system that didn’t work as expected if a “compliant” switch didn’t possess a sufficiently robust power supply.

To get comfortable with PoE Plus, it helps to understand its genesis and subsequent evolution. In the beginning, Cisco had a proprietary approach for powering Voice over Internet Protocol (VoIP) business phones that involved powering some pairs in the router with 48 V.

The rest of the industry saw that this was good and wished for an open standard, which became IEEE 802.3af. To be conservative, the IEEE subcommittee limited power to 15 W at the PSE, which was enough for the non-video VoIP phones that then dominated the market. They also expanded Cisco’s idea by allowing the “spare pairs” in an Ethernet cable to be powered by a midspan, making it possible to retrofit PoE to legacy Ethernet plant.

When PoE hit the streets, many potential vendors saw its advantages and jumped on the bandwagon. VoIP phones would no longer need power plugs, making them more like old-fashioned public-branch-exchange (PBX) phones. Wireless hotspots could be located anywhere someone could pull a CAT5 cable. Supermarket shelves would twinkle with up-to-date price tags that would always match the prices in the cash register. And, PoE musical instruments, mixers, and recording equipment would displace the MIDI bus and revolutionize the music business.

Obviously, some of these goals were more realistic than others. In the three years since basic PoE was released, three killer applications have taken hold: VoIP phones, Wi-Fi hotspots, and security cameras. Within those applications, though, there immediately appeared a need for power beyond 13 W.

For example, there’s an anticipated demand for video conferencing using VoIP phones, and backlighting a video screen takes power. Simple short-range Wi-Fi is happy with 13 W, but WiMAX takes more power. And while fixed security cameras don’t require much power, once motors are added for panning, tilting, and zooming, power does become an issue.

But the makers of Ethernet switches, concerned about over-specifying power supplies, pointed out that video phones and pan/zoom/tilt cameras don’t need full power all of the time. Most of the time, the phone is just sitting there. Even when there’s a call, video isn’t always necessary. Unless it’s a formal conference, most people would prefer to remain invisible to the other party. Similarly, those high-end security cameras only move when a guard touches a joystick. In other words, the requirement for higher power changes continuously.

The dynamic-power issue transformed the questions facing the IEEE 802.3at task force from simply “How much current can a bundle of CAT5 cables and their associated RJ45 connectors safely handle?” to “How can we create a protocol that allows PDs to dynamically negotiate for power with a PSE?”

Before we get into that, let’s take a look at basic PoE as described in the IEEE 802.3af, DTE Power via MDI, which was formally approved on June 12, 2003 (Fig. 1). PoE uses either the data pairs or the spare pairs in an Ethernet cable to carry 48 V dc from the PSE in an endpoint switch or midspan hub to the PD appliance at the other end of the cable. Data pairs are powered via center tap, while spare pairs are simply paralleled. The sense of the dc voltage doesn’t matter, thanks to a diode bridge ahead of the PD controller chip.

To take advantage of Power over Ethernet, PSEs must be able to detect the presence of a PD on any port. And, PD appliances must be able to assert their PoE compatibility and may assert their maximum power requirements. Under the 802.3af standard, PSEs may not apply power to the Ethernet cable unless there’s a PoE-enabled PD on the other end. PoE PDs are identified by the presence of a 25-kΩ resistor across their input.

The PSE measures resistance by applying two voltages (separated by 1 V and a 20-ms interval) and using the resulting currents to determine the resistance value. This part of the handshake is called the “discovery” phase. Next, there’s an optional “classification” phase. In the 15- W maximum world of basic PoE, classification allows the PSE to decide whether it has enough capacity to supply the PD. If it doesn’t, it can refuse to power up the Ethernet pair.

During the classification phase, the PSE briefly asserts a 15.5- to 20-V pulse on the pair, and the PD can opt to signal the PSE by placing a load on the line. Doing nothing, not putting a load online, automatically identifies the PD as Class 0, and the PSE would expect it to limit current to 400 mA. Class 1 PDs must self-limit to 120 mA, Class 2 to 210 mA, and Class 3 to 310 mA. A Class 4 was reserved for future use. Timing relationships were 500 ms (max) for detection, 10 to 75 ms for classification, and 400 ms for power turn-on.

Originally, PoE was intended for standard Ethernet cable, which has four twisted pairs. But only two of those pairs carry data. Under basic PoE, powering is an either/or situation—only one pair may be used at a time. This enables the seam- less use of new endpoint routers with built-in PoE or power via midspan bridges in legacy systems. Midspans would only power the spare pairs, while endpoint equipment could power either pair. (In practice, all endpoints power the data pair.)

That arrangement eliminates the possibility of midspan PoE for legacy plant in which the spare pairs are left unconnected. However, the expectation was that most PoE would be endpoint-powered in the long term. So, endpoints should be the type of equipment that could handle any kind of infrastructure, including legacy sites with unconnected spare pairs.

When using the spare pairs in basic PoE, pins 4 and 5 are paralleled for one side of the dc supply and pins 7 and 8 are paralleled for the other side. When using the data pairs, the PSE applies dc power to the center tap of each isolation transformer so that pins 3 and 6 supply one side of the dc and pins 1 and 2 supply the other. At the PD, data-pair power is recovered via center taps on each of its transformers.

With that background, it’s possible to understand PoE Plus. Part of the IEEE 802.3at Task Force’s job was to decide whether the additional power would be delivered by simply increasing the maximum current rating or by paralleling the spare pairs with the data pairs. The more challenging part involved expanding the classification scheme for PDs to dynamically negotiate with the PSE for more or less power.

Resolving the first issue was relatively simple, once the task force agreed that CAT5 cable and RJ45 connectors could handle more current and that it would be acceptable to use both sets of twisted pairs. One could have it both ways: more current and four active pairs. That decision had an impact on midspan makers, though. If there’s no continuity through the spare pairs, they can only supply half as much power as an endpoint switch.

That leads to a situation in which PoE Plus-compliant midspans can be configured with a dedicated mix of basic PoE, PoE Plus, and non-PoE ports—without the versatility inherent in a full PoE Plus endpoint. This isn’t a big disappointment to midspan makers, however. There was never going to be a way to deliver as much power through two pairs as was delivered through four. All they ever wanted was to deliver more than 13 W on dedicated ports at prices below those for new endpoint switches or routers.

So, what’s the real maximum power that can be supplied by PoE Plus? “The IEEE 802.3at current has been established at 360 mA per conductor, 720-mA delivered current by the TIA TR-42 working group. This current is good for up to a 45 C environment and must be de-rated to 0 mA at 60 C,” says Clay Stanford of Linear Technology.

“There are some concerns about the 45 C and the derating. Because of this, it would be my opinion that the 720-mA current limit isn’t set in stone. It might be reduced to something like 500 mA so that a higher ambient temperature could be allowed,” he notes.

With respect to voltage, Stanford says, “The IEEE 802.3at committee has tentatively established the PSE output voltage as 50 to 57 V. The committee has established the total round-trip 100-meter cable resistance to be 12.5 max.” With this voltage and resistance, the power is:

Ppse min = 0.72 A 50 V = 36 W 
Ppd min = 0.72 A 41 V = 29.5 W 
Cable loss max = I2R = 0.72 0.72 12.5 = 6.5 W max 

The challenge for the 802.3at Task Force was to determine how to extend the classification system. It came up with a two-level classification method. All PSE chips will now send the first classification pulses, as in basic PoE, but PoE Plus PSEs will also send a second pulse (Fig. 2). Standard PoE PDs will respond in the usual fashion to the first pulse. A PoE-Plus PD will respond by asserting that it is Class 4.

The second pulse from the PSE tells the PD that a high-power PSE is present. The PD response of Class 4 for each of the pulses tells the PSE that a high-power PD is connected.

The PoE-Plus PSE can power up the PD using conventional 802.3.af methods. Once the PD is powered up and data communication is established, the PSE can communicate with the PD using the Link Layer Data Protocol (LLDP) to determine if the PD installed is a high-power device desiring power above the 13-W 802.3.af limit.

The original method of communicating power information is referred to as Layer 1 classification, and the new data path method is called Layer 2 classification. The PSE has the option of using either method, and the PD is required to support both Layer 1 and Layer 2 methods.

The big question is whether PoE Plus will be limited to video phones, high-speed wireless hotspots, and pan/zoom/tilt security cameras. The market’s response to PoE Plus remains to be seen, but some interesting ideas are afloat.

One sleeper PoE/PoE Plus application enables laptops to top off their batteries while connected to the corporate local-area network (LAN). That way, there’s no need for a separate power connection, nor do you have to crawl under your workbench to plug in and remove your power adapter.

Many experts are skeptical of the idea, noting that people these days tend to roam their company’s offices with their laptops untethered, maintaining their LAN connections via Wi-Fi. Faster wired connections and a few serious security problems with wireless could modify that behavior, though. Along those same lines, at May’s Interop conference, Phihong USA demonstrated a PoE- powered workstation based on a midspan and splitter (Fig. 3).

Kiosks provide a further possibility for midspan-based applications, since PoE Plus provides sufficient electrical power to run a respectable display. In Asia, kiosks are nearly as ubiquitous as giant LED advertising displays, offering all kinds of help to travelers and shoppers. For kiosks, PoE Plus provides opportunities for easy siting and relocation.

In the past few months, several companies have announced products that anticipate the release of the IEEE 802.3at draft standard.

Microsemi Corp. (formerly PowerDsine) unveiled its PD64012GH 12-port PSE PoE Manager and the PD64004AH four-port PSE PoE Manager with integrated power FETs. These allow OEMs to build switches that can drive 36 W for every two pairs, as well as interface to devices consuming up to 30 W. Customers requiring up to 60 W can use the same Microsemi ICs in a four-pair configuration. Operating in pre-standard IEEE 802.3at mode, they can be managed by Microsemi’s PD63000G PoE MCU.

Linear Technology’s LTC4264 PD chip incorporates a precision dual-current limiting circuit with an on-board power MOSFET and sense resistor to provide a complete inrush control circuit without additional external components (Fig. 4). The dual-level current limit enables legacy PSE with limited current-sourcing capability to power up the device. It also allows the device to draw full power from custom PSEs that exceed the limit of the existing standard. The chip can handle 750 mA. Pricing begins at $1.80 each for 1000-piece quantities.

Broadcom Corp. announced two four-port PSE controllers, the BCM59101 and BCM59103, the latter being “pre-IEEE802.3at-compliant.” Both chips are pin-compatible, facilitating product migration for OEMs. They integrate power MOSFETS, dc-dc controllers, ac disconnect, and LED control circuitry. Pricing is $10 each in 10,000-unit lots.

On the equipment side, Phihong USA introduced a 60-W midspan, the POE60U-560G, and a 52-W PoE Gigabit high-power splitter, the POE60D, for high-power wireless access points that require up to 52 W. The splitter, a gigabit-compatible device designed to be mounted right next to a powered device, performs connect identification as well as separates the power and data. Both are designed around the specifications of the proposed IEEE 802.3at standard as of March 2007.

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