Power over Ethernet — Ready to Power On?

Power over Ethernet — Ready to Power On?

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Written By Eric Sandler

By Beth Cohen and Debbie Deutsch

May 19, 2003

What is Power over Ethernet?

A version of electrical power over network connections has been utilized in the telecommunications industry for many years. It’s what allows your telephone service to continue when you experience power outages. So what is so exciting and unique about Power over Ethernet?

Power over Ethernet extends the reliability that the telecommunications industry has enjoyed for years and enables lifeline service for IP telephones. It has the ability to connect and power wireless access points and web-based security cameras. Even more exciting, PoE opens the door to a new generation of networked appliances. Because there is no need for the PoE appliance (called a “Powered Device” or PD in the standard) to be anywhere near a wall socket, the PoE vendors foresee a plethora of innovative applications, from building access systems and retail point-of-information systems to “smart” signs and vending and gaming machines.

There are two system components in PoE — the Power Sourcing Equipment (PSE) initiates the connection to the second component, the Powered Device (PD). The current is transmitted over two of the four twisted pairs of wires in a Category-5 cable. The standard defines two choices for which pairs of wires are used to transmit the power. In one method, the power goes over the spare pairs that are not used by 10BASE-T and 100BASE-T. In the other method, the data pairs are used (without negatively affecting data transfer performance). The Power Sourcing Equipment (PSE) can take either approach. All Powered Devices must support both.

When used in conjunction with a UPS system and integrated with network management tools, mission-critical network-based facilities can maintain high availability, even though the electrical power may be down. You may be able to unplug devices with sufficiently low power requirements and rely on the network to provide power with UPS reliability.

Any network manager understands the pain of those midnight visits to the data center or wiring closet to reset some piece of equipment. Power over Ethernet eliminates the need to push a reset or power switch on remote, possibly difficult-to-reach PoE-powered devices. They can be turned on or off or reset by a network manager sitting at his or her desk. This has the potential to save your company the huge overhead costs of on-site service calls, the maintenance of dispatch centers, and late night administration trips.

The Internet Engineering Task Force (IETF) has been working in parallel with the IEEE to extend its Simple Network Management Protocol (SNMP) to apply to PoE ports as well. It has developed an Internet Draft that extends the Ethernet-like Interfaces MIB (RFC 2665) with a set of objects for managing Power Source Equipment and Powered Devices. IEEE 802.3af defines the hardware registers that would be used by a management interface. The IETF draft defines management data objects based on the information read from and written to these registers.

The real beauty of the standard is that Power over Ethernet is completely compatible with existing Ethernet switches and networked devices. Because the Power Sourcing Equipment (PSE) tests whether a networked device is PoE-capable, power is never transmitted unless a Powered Device is at other end of the cable. It also continues to monitor the channel. If the Powered Device does not draw a minimum current, perhaps because it has been unplugged or physically turned off, the PSE shuts down the power to that port. Optionally, the standard permits Powered Devices to signal to the PSEs exactly how much power they need.

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