[Technology Report]
Evolving PoE Promises Rich Landscape Of Power Innovation
Developments surrounding Power over Ethernet include a number of unexpected apps, many spec-compliant products from chips to midspan hubs, and efforts to boost the maximum current that's deliverable via Ethernet cable.
Developments surrounding Power over Ethernet include a number of unexpected apps, many spec-compliant products from chips to midspan hubs, and efforts to boost the maximum current that's deliverable via Ethernet cable.
It has been barely a year since the Power-over-Ethernet (PoE) IEEE 802.3af standard, DTE Power via MDI, was formally approved. However, the birth of the standard effort for PoE, a backward-compatible replacement for previous proprietary power-over-LAN (local-area network) schemes, occurred back in 1999, with 3Com, Intel, PowerDsine, Nortel, Mitel, and National Semiconductor as its proponents. Originally conceived as a means of powering Voice-over-Internet-Protocol (VoIP) phones, the number of PoE applications expanded dramatically once the standard was approved (Fig. 1). These range from the predictable (wireless access points and RFID tag readers) to the revolutionary (PoE and the garage band). (For a guide to terminology, see "IEEE 802.3AF Voltage, Power, And Equipment Designations," p. 68.)
PoE uses standard Ethernet cable, which has four twisted pairs. However, only two of those pairs are used for 10BaseT or 100BaseT. Because Ethernet data pairs are transformer-coupled at each end of the cable, either the spare pairs or the data pairs can be used to power powered-device (PD) equipment.
The PD must be able to accept power from either distribution configuration, though it may use only one at a time. At the power-source end of the cable, the power-source equipment (PSE) may apply power to either the spare pairs or the data pairs in a cable, but not to both simultaneously. Also, the PSE may not apply power to non-PoE Ethernet devices if they're connected to the cable.
The reasoning behind whether the power is carried over the spare pair or the data pair takes some explaining. According to Linear Technology design manager David Dwelley, some existing Ethernet installations don't have continuity through the spare pairs. So, the IEEE 802.3af committee specified that midspan PSE equipment could only power the spare pairs, while endpoint equipment could power either pair. (In practice, all endpoints power the data pair.)
That may seem counterproductive at this point in PoE deployment, because midspan PSE can't be used to power legacy infrastructure if it has no spare-pair continuity. However, says Dwelley, the committee thought that in the long term, most PoE would be endpoint powered, 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, 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. In either case, the sense of the dc voltage applied to the pairs doesn't matter, because the standard requires a diode bridge ahead of the dc-dc converter in the PD.
BENEFITS OF A SINGLE CABLE Ultimately, using a single cable for data and power provides simplicity, safety, and economic advantages (Fig. 2). There's no need to install an ac outlet next to a LAN socket, and the low-voltage LAN cable can be installed by individuals with fewer skills than electricians with regular electrical wiring experience. In addition, PoE's 48 V is designated as a Safety Extra-Low Voltage (SELV), providing an additional safety factor.
Forty-eight volts is also the standard backplane bus in telephone central offices and data centers, so a standard uninterruptible power system can keep PoE appliances running in the event of a power failure. Furthermore, there's a large selection of dc-dc power converters designed for 48-V inputs. On top of that, power can be controlled remotely from the Ethernet switch (e.g., from a system controller via I2C) while Simple Network Management Protocol (SNMP) commands from the switch or from further upstream offer additional PD-control options.
A disadvantage of PoE may be its 13-W load-power limitation. However, this wasn't anticipated to be a liability in 1999 when the standard was seen primarily as a means of powering VoIP enterprise telephone sets.
The first PoE killer app to show up after enterprise phones was surveillance cameras. It's far easier to put a camera up on a pole in the company parking lot if the only wire needed to run to it is a CAT5 Ethernet cable. On the other hand, notes PowerDsine president Rich Bauer, cameras also challenge PoE's power limit when users want motors for pan, tilt, and zoom functions. "A basic camera may use five to seven watts," he says. "Add some motors and you quickly come up against that power limitation."
Less susceptible to the 13-W blues are the two wireless PoE killer apps—Wi-Fi access points and RFID tag readers. PoE lets a company put access points in lobbies, lunchrooms, conference rooms, and even in outdoor dining areas more or less ad hoc. If a particular location doesn't provide adequate coverage, it's relatively simple to shift to another without a great deal of fuss.
A UHF (15- to 20-foot range) RFID tag reader with a 1-W RF output draws only around 120 mA at 48 V. Shorter-range low-frequency readers draw even less, turning tag readers into another hot candidate for PoE implementation. High unit prices will probably see readers implemented first in the enterprise for supply-chain management, but they will eventually be competitive with alternative loss-control methods in retail stores. Long term, their combined advantages in loss-control and inventory management should make their use widespread.
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