If you've grown accustomed to viewing the familiar LAN connector as a data port, think again. That RJ-45 jack makes a great power outlet. At least that's the message being promoted these days by Power-over-Ethernet (PoE) proponents. They want equipment designers of every ilk to know that standard CAT 5 unshielded twisted-pair cabling makes a great medium for transmitting dc power, along with the usual data, to network peripherals. Although this concept grew out of a need to eliminate the power adapter required by voice-over-IP (VoIP) phones, PoE technology has broad implications well beyond the phones. As a consequence, manufacturers of power semiconductors are currently gearing up development and production of numerous PoE control chips.
These control chips will help designers of networking hardware and network peripherals to realize the many benefits of PoE. One advantage is reliability. With power emanating from a central point on the LAN, the 48-V power source can be backed up by a UPS, ensuring a higher degree of reliability for peripherals than those operating off the ac grid. That feature is critical to VoIP phones, because users expect these devices to have similar reliability to traditional POTS phones, which have always operated off a battery-backed power source.
Another PoE advantage is convenience. Peripherals only need one chord for plugging into the LAN, so the requirement for an ac-dc adapter is eliminated. Even more compelling may be the ability to place peripherals in spots that currently have no ac outlets. Peripherals such as WLAN access points and security cameras can be located in ceilings and other remote sites without running ac power cabling, which require the services of an electrician (Fig. 1).
However, another PoE benefit may be even more compelling. With the emergence of 802.3af as a global standard, the LAN outlet becomes a universal power jack. As Igal Rotem, CEO of PowerDsine, observes, "The [PoE-enabled] RJ-45 is the only standard power connector in the world."
That gives a PoE power supply a capability not offered by power supplies with universal, 85- to 265-V ac input ranges. Although the latter can accommodate all commonly encountered line voltages, they still require different plugs to mate with all of the various power jacks. In contrast, the PoE-powered device will operate anywhere using the same RJ-45 connector. Of course, the caveat is the limit on available power. The new IEEE 802.3af PoE standard only allows a powered device to draw up to 12.95 W.
Nevertheless, that level is sufficient for a variety of products, including battery chargers for portable devices such as cell phones, PDAs, and notebooks. Moreover, PC vendors are said to be developing low-power notebooks that can be powered directly off the PoE port.
Other possible power devices (PDs) include factory automation equipment, RF ID scanners, security systems, time card systems, credit card terminals, and keyless entry systems. One of the more exotic applications is Gibson Guitar's use of the LAN to transmit digitized audio from the guitar back to a PC. In this application, getting rid of the ac power chord with PoE improved the SNR of the audio, in turn lowering bit error rates. This benefit could be extended to other areas of data acquisition.
Efforts to develop PoE equipment and components were given a boost last June when the IEEE 802.3af-2003 PoE standard was formally ratified, establishing uniform requirements for managing and distributing 48-V dc power over the LAN. That standard defines the protocols for the power-sourcing equipment (PSE), the Ethernet hubs and switches, to transmit power over CAT 5 and other cabling to the PDs, such as the VoIP phones and other network peripherals.
The PSE encompasses two categories of equipment: midspan and endspan devices. A midspan device is a standalone unit that adds the PoE capability to existing networking equipment. The unit is inserted into the LAN between the Ethernet switch and the peripherals. However, a midspan device requires additional cabling, takes up extra space, and adds cost to the system. These problems are lessened by incorporating PoE functionality directly into the Ethernet switches. Such pieces of equipment are called endspan devices.
Today, semiconductor vendors are doing their part to craft power-management ICs that implement the functions needed to build IEEE802.3af-compliant PSE and PDs. Although the most recently developed PoE chips aren't the first chips aimed at this application, they generally offer higher levels of functional integration than their predecessors. By increasing the level of integration, these components reduce the cost and development effort associated with adding PoE capabilities to network equipment and peripherals.
Naturally, PoE chip development is an ongoing, industry-wide effort with promises from vendors that the chips to come will offer even more integration, smaller size, and lower cost. In the meantime, semiconductor vendors with PoE offerings are tackling the pressing system design issues, such as the need for compatibility with legacy systems.
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