Five keys to successful PoE deployment

Feb. 1, 2005
If you were to put together a list of the hottest technologies in today's networking industry, WiFi (wireless networking) and VoIP (Voice over Internet Protocol) would be near the top.

If you were to put together a list of the hottest technologies in today's networking industry, WiFi (wireless networking) and VoIP (Voice over Internet Protocol) would be near the top. Recently, several major corporations announced plans for a global rollout of IP phones. By the same token, WiFi hotspots have cropped up everywhere.

Before deploying a midspan powering device like this one from ADC, make sure it complies with the IEEE 802.3af Power over Ethernet specifications and the TlA Category cabling specifications that apply to your cabling system.
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In addition to being current industry buzzwords, WiFi and VoIP also share another common attribute. They both require Ethernet and power to function. In June 2003, the Institute of Electrical and Electronics Engineers ( released IEEE 802.3af, also known as the Power over Ethernet (PoE) standard, which was established to provide a universal way to provide power and Ethernet-based data to a remote powered device over standard Ethernet cabling, such as Category 5e or Category 6.

Before 802.3af, vendors and consumers of IP phones were faced with two options: Provide local power through an AC (alternating current) electrical outlet, or use non-standards-based proprietary solutions to provide inline powering. Both choices were equally unappealing.

The release of 802.3af has enabled technologies such as VoIP and WiFi, and has paved the way for literally hundreds of new product ideas that require the combination of both data and power. Examples include radio-frequency identification (RFID) scanners, IP-security cameras, alarm systems, and smart-building controls.

Planning for integration

Given the future impact PoE-based devices will have on your business, it is important to begin planning how to integrate PoE into your current and future network. The IEEE standard recognizes two methods for deploying PoE in your network. The first requires that Ethernet switches inject power and data at the same time. This approach may work well for new installations, in which a new switch must be purchased anyway; however, it likely will require a much larger power supply and will be more expensive than a standard Ethernet switch. These devices are referred to in the 802.3af standard as "endspan" devices, since the power is injected at the end of the structured cabling where it terminates on the Ethernet switch.

The second method involves using a dedicated power-injecting device in between the Ethernet switch and the remote powered device. These power-injecting devices are known in the standard as "midspan" devices, and work well for both new and existing installations with non-PoE-enabled Ethernet switches.

How to ensure PoE success

When deployed correctly, PoE is safe, reliable, and cost-effective. But because it is such a new networking technology, it is important to consider five keys that will ensure successful PoE deployments:

1. Provide sufficient power to the remote powered device. This may sound simple, but in practice, it is difficult. The IEEE 802.3af standard identifies four possible power classifications. At maximum, the powered remote device can draw up to 12.95 watts of power. Factoring loss through the length of the cable, this means that, at maximum, the power-sourcing equipment must have the ability to provide 15.4 watts of power to each port. In a 24-port Ethernet switch or midspan device, this means that approximately 370 watts of power must be available to supply the necessary power to each port. For the Ethernet switches, additional power above and beyond that required for PoE must be available for its switching functions. With this in mind, pay close attention to the size of the power supply used in each device. Midspan devices should have in excess of 370 watts available and Ethernet switches should have significantly more, depending on how much power their switching functions require. A rule of thumb is to check the power-supply size used on a non-PoE-enabled switch and add 15.4 watts per port to determine if the power supply is large enough.

2. Connect the power source to uninterruptible and redundant power. The remote devices fed by PoE typically are mission-critical devices. For instance, an IP phone that loses power is a lost voice circuit. Think about that for a moment. Regardless of whether it is a traditional circuit-switched analog phone or an IP phone, it is a lifeline circuit. It has to work. You must consider how you are going to design your network to ensure that consistent and reliable data transfer and power are maintained to this critical device. Connect the critical power-sourcing devices to an uninterruptible power supply, and use devices with dual redundant power supplies to ensure that your critical devices never lose power.

3. Deploy only IEEE 802.3af-compliant devices. Before release of 802.3af, many companies chose to develop products to various proprietary standards in order to provide PoE cabling. Many of these devices are still being sold, even though the standard is now released. Be sure to verify that both the sourcing device and the powered remote device are compatible with each other. If both devices list IEEE 802.3af in their specifications, you are covered. But if any of the devices you are using do not explicitly state compliance to IEEE 802.3af, be cautious. Carefully read the technical documentation and contact the technical-support number to determine compatibility. Failure to do so will leave you frustrated and will cost you time and money.

4. Pay attention to cabling-performance specifications. What Category rating does your cabling infrastructure have? Probably Category 5e or 6. Now if you deploy Power over Ethernet with a midspan device, how does that affect your performance? Pay close attention to the manufacturer's specifications. Look for Category 5e and 6 compliance. Midspans should be treated just like another patch panel in your channel; if they are of an unkown Category or one lower than the rest of your cabling, you may be significantly disregarding the performance of your infrastructure. Also, remember that per TIA standards, only four connectors can exist between the switch or hub and the network interface card (NIC). A midspan device should be counted and treated as one of these connection points. People commonly ask how far they can run Power over Ethernet. The answer is 100 meters from the switch or hub to the NIC, regardless of where the power is injected. The limitation is not the power; it's the Ethernet cabling standards that limit the total length of cabling to 100 meters.

5. Use the most cost-effective PoE method for your network. The business motivation behind deploying IP-based technologies like WiFi and VoIP is to decrease networking costs. A significant benefit of PoE is that it runs on your existing infrastructure. If you have recently purchased or upgraded your Ethernet switches, but they are not compliant to the IEEE 802.3af standard, before you run out and buy all new Ethernet switches (which you probably won't do), and before you abandon or delay your project (which you don't want to do), consider using midspan devices to inject power. These devices are economical to purchase and install relative to Ethernet switches, and they let you insert standards-compliant PoE using your existing infrastructure, including the existing Ethernet switch.

Know the potential and limitations

To deploy Power over Ethernet successfully, it is important to realize both the potential and the limitations of the devices you are putting into your network. Properly deployed Power over Ethernet technology in your network will lead to a safe, reliable, and economical solution for delivering consistent and dependable power to such common networking devices as VoIP phones and WiFi access points.

Invest the time today to learn about this new technology and you will thank yourself tomorrow as you successfully deploy your next-generation networking equipment.

JOHN SCHMIDT is a product manager with ADC (

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