Sunday, June 16, 2024

Power over Ethernet — Ready to Power On?

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Consider the following scenarios:

  • You’re planning to install the latest Voice over IP (VoIP) phone system to minimize cabling build-out costs when your company moves into new offices next month. Just imagine if you didn’t need to be concerned about losing your telecommunications system every time there’s a power outage.
  • The company staff has been clamoring for a wireless access point in the picnic area behind the building so they can work on their laptops through lunch, but the cost of running electrical power to the outside is prohibitive.
  • Management has been asking for security cameras and business access systems throughout the facility, but you would rather avoid yet another electrician’s bill.

Have you ever imagined what you could do if the network cabling you already have in place (or that you’re going to have to install anyway) could also support electrical power? Think of the possibilities for simplifying your infrastructure support.

You no longer need to imagine such a scenario, because with the forthcoming new IEEE standard 802.3af, also known as Power over Ethernet (PoE), your dreams of power over network wiring are now a gigantic step closer to reality. Power over Ethernet promises to enable these applications and many more by providing up to 12.95 watts of power (at 48 volts) over the same Category 5 cable that already delivers your standard 10/100/1000Mb Ethernet service.

You’re probably thinking this all sounds so great that there must be a catch somewhere, when in fact this technology is real and is rapidly becoming more widely available. With the new standard’s official approval expected in the next few months, let’s explore the technological frontier and find out what PoE is all about and how it can make your life easier.

Page 2: What is Power over Ethernet?

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.

Page 3: Getting Started

Getting Started

At this point, you might be saying to yourself, “All this sounds fantastic! How can I start using this technology?” Introducing Power over Ethernet to your network is trivial. All of the equipment goes into your wiring closet, where the edge switches connect to networked devices. If you are expanding or upgrading your network equipment, you can purchase Ethernet switches or modules that integrate PoE into their 10/100 or 10/100/1000 ports. These ports are said to have “inline power.”

Alternatively, you can add power to existing ports using a mid-span insertion device, sometimes alternatively called a mid-span insertion panel or a “power hub.” When using a power hub, a patch cable connects the switch port to an input port on the power hub. The matching output port on the power hub is connected to the Powered Device. A Power over Ethernet adaptor is similar to a power hub. It adds PoE capability to a single existing Ethernet port or networked appliance.

When do you use switch ports with inline power, and when do you use a mid-span insertion device? The correct choice depends on how many powered ports you will be deploying and how much flexibility you need or want to locate or move the powered devices. You will also need to work within the space, power, and cooling constraints in your wiring closet. Of course, the biggest factor, as always, will be what your budget can support.

If you will only need to administer a handful of PoE ports in an existing or new wiring closet, a power hub may be the simplest and most cost-effective approach. You pay for PoE only where you need it, and you maintain your investment in your current switches. You have the flexibility to connect one mid-span device to ports on multiple switches. However, using mid-span insertion devices results in having three ports for every one that you would need if you used a switch that integrates PoE on its ports. This means more rack space and higher power requirements. You should take care, for example, by tagging or using color-coded cables when making the connections from switch port to the mid-span insertion device to ensure that the correct connections are made between switch ports and networked devices, and to speed and simplify the process the next time the configuration is changed.

On the other hand, if you plan to support many PoE ports — or if space, power, or cooling in the wiring closet is at a premium — purchasing new switch ports with inline PoE may be preferable. Note that due to the limitations of the available power and cooling, not all existing chassis-based switches can support modules with inline power. Similarly, some systems may limit the number of powered ports for the same reasons. Be sure to check the product specifications before purchasing any additional devices.

Your UPS configuration may need to be updated to support Power over Ethernet. You might discover the need to expand your existing UPS capacity or to add UPS capability to wiring closets where it is not currently present. You will want to coordinate UPS configuration with PoE configuration to ensure that power is available to the switches and power hubs where it is most needed. Likewise, you must take care to match configurations of switches and mid-span devices if you are managing power on a port-by-port basis during a power outage.

Vendors are releasing management tools to complement their hardware devices and enterprise solutions. For small PoE deployments, device-level management may be adequate. However, for large installations of Power over Ethernet, such as IP telephony, network managers may prefer a management solution that integrates PoE with the management of the application itself. This approach reduces the total configuration required and eliminates configuration errors due to inconsistency between the application and the network ports. Enterprise solution vendors have a natural advantage over device vendors in this area because they are able to fully integrate the hardware and software.

Page 4: What Is the Catch?

What Is the Catch?

Power over Ethernet equipment has been available for a number of years, so what has changed? The most important change is that a stable, official specification will soon exist that the vendors can build to. Many current products claim to already comply with the forthcoming standard; however, as with any emerging technology, these claims should be taken cautiously, especially for products that have been released significantly in advance of the final version of the standard.

The newly established Power over Ethernet consortium conducted its first round of interoperability testing on a matrix of Powered Devices and Power Sourcing Equipment in April 2003, at the University of New Hampshire InterOperability Laboratory. 3Com, Extreme Networks, Nortel Networks, PowerDsine, and Texas Instruments are members of the consortium. Additional companies that participated in the event included Avaya and Foundry Networks. The results were only made available to the participants.

When considering the purchase of Power over Ethernet equipment, be sure to ask your vendor about interoperability with other vendors’ equipment. Even if all your data switches come from a single vendor, the Powered Devices that you and your users want to deploy may come from other manufacturers.

As with Gigabit Ethernet over copper, deploying Power over Ethernet depends on the proper use of Category 5 cable. Some older networks may still have remnants of Category 3 cable or connections in them. Another gotcha to watch for is, on occasion, some installations have economized on cable by “splitting” Cat-5 cable in two, connecting two end-devices with a single cable. Power over Ethernet will not work in such deployments. This is an opportunity to bring your cable infrastructure up to standard.


While the IEEE standard for Power over Ethernet has yet to be completely formalized, a final draft is available for review and the last step in the ratification process is expected in June of this year. The final ratification of the standard will quickly lead to the introduction of many new types and varieties of network appliances. Numerous PoE products have already been announced by all major Enterprise network equipment vendors, making now an ideal time to evaluate the advantages that PoE can bring to your organization. With the capital cost of a pilot deployment running as little as the cost of a few adaptors or a power hub and some powered devices, what are you waiting for?

Resources – The Power over Ethernet Consortium website – Most recent IETF documentation – IEEE Power over Ethernet web pages

Beth Cohen is president of Luth Computer Specialists, Inc., a consulting practice specializing in IT infrastructure for smaller companies. She has been in the trenches supporting company IT infrastructure for over 20 years in a number of different fields including architecture, construction, engineering, software, telecommunications, and research. She is currently consulting, teaching college IT courses, and writing a book about IT for the small enterprise.

Debbie Deutsch is a data networking industry veteran with 25 years experience as a technologist, product manager, and consultant, and has participated in the development of national and international data communications standards. Her expertise spans wired and wireless technologies for Enterprise, Carrier, and DoD markets. She is currently a freelance writer and consultant.


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Beth Cohen

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