Deploying next-generation PoE-powered networks

With the ratification of the IEEE802.3at standard this month, Power over Ethernet will be ready to deliver 30W of power per port to a broad range of gear -- everything from 802.11n equipment to IP video and thin clients. As such, you need to establish an enterprise-grade POE plan that optimizes flexibility, scalability, energy efficiency, reliability, safety, maintenance and management.

One of the biggest predictors of PoE flexibility and scalability is whether it is delivered via the network switch or through a piece of equipment called a midspan. PoE switches and midspans both deliver the same amount of power over the same distances, in support of the same IEEE specifications. However, today's PoE-enabled switches deliver basic industry-standard PoE capabilities to some or all network ports, with very few options over how that power is managed or budgeted.

In contrast, a midspan is a small, stand-alone piece of standards-compliant PoE equipment that sits between the existing switch and powered devices, and injects power into the data line using significantly more energy-efficient intelligent power management and allocation techniques than are available with PoE switches. Midspans also offer a variety of cost-saving power-infrastructure monitoring and maintenance capabilities that PoE switches do not include.

Typically, midspans offer the most flexible, scalable and energy-efficient solution, especially for first-time PoE deployment. Unless the existing data network infrastructure is inadequate (from a feature, capacity or performance perspective), or you need to simultaneously upgrade both the data and power infrastructure with a new PoE-capable switch, midspans are the best upgrade choice. They require no changes to the existing switch or CAT5 (and above) cabling, and are generally compatible with any Ethernet switch.

Midspans can be used to deploy PoE when PoE-capable switches aren't yet available, and/or if available PoE switches don't support all necessary data features. For instance, many enterprises have upgraded to high-power capability with midspans, even when switches were not yet available.

Finally, midspans decouple the power and data infrastructures for optimal flexibility and scalability. They enable PoE ports to be incrementally added, as needed. This contrasts with the installation of a new switch, for which the prescribed approach is to deploy PoE on as many ports as possible to support future growth.

The latest midspans also incorporate a number of enterprise-grade features that enhance energy efficiency, reliability and safety, flexibility and scalability, and management and maintenance.* Energy efficiency: A poorly designed PoE infrastructure can be a large energy drain. PoE is already the greatest heat generator in most switching closets. Cooling challenges are considered the biggest potential roadblock for widespread high-power PoE deployment.

Midspans solve this problem because they diminish heat concentration. By delivering only the power necessary, they are a green alternative to PoE switches. Midspans can be used alone or combined with PoE switches to power both low- and high-power devices for the most energy-efficient solution.

Slideshow: What does a real green data center look like?

The latest enterprise-grade midspans owe much of their efficiency to their distributed power architecture. A 48-port switch with 800W of full IEEE802.3af power per port might use only 20 ports at once and waste 80W of quiescent power. In high-power IEEE802.3at applications, it's rare for any single port to require full power. Therefore, today's enterprise-grade PoE midspans augment smaller, more economical internal default power supplies with external power supplies for incremental additional power, or for redundancy.

This distributed architecture improves system efficiency and reduces cooling costs since smaller supplies require smaller and/or lower-speed fans. Using this approach, midspans also can back each other up, with one or multiple additional power supplies serving the highest-priority system ports. This improves reliability and safety -- the second major decision category for enterprise-grade PoE.

* Reliability and Safety: Next-generation distributed PoE power architectures also improve reliability through prioritized, per-port backup. And there are reliability and safety considerations. For instance, be wary of solutions that promise greater than 60W/port. Anything above 60W/port can pose standards-compliance problems, and anything over 100W/port poses significant safety risks.

It should be noted, however, that 60W/port can often be an attractive option, and standards compliance for this option is possible by delivering power over all four pairs of Ethernet CAT5 cable, which also improves efficiency compared with two-pair products.

Four-pair powering enables 60W of power to be delivered with a low 600mA current rather than the 1.2A level of two-pair midspans. This means for the same 60W at the source, 51W can be delivered over CAT5 cable via four-pair solution, compared with 42W for two-pair solutions. Additionally, this same four-pair configuration can be used to power two-pair devices with 30W of power, while dissipating up to half the power and consuming almost 15% less energy than conventional two-pair solutions. This translates into savings of approximately $25/year per powered device, assuming energy costs of $0.10 per kilowatt hour (KWH).

* Flexibility and scalability: Flexibility is ensured through the inclusion of a gigabit interface so midspans can support high-power gigabit video phones, WiMAX transmitters or 802.11n access points. These and other devices, such as Pan-Tilt-Zoom (PTZ) cameras and thin clients, also require two-event classification support per IEEE 802.3at specifications.

Additionally, the inclusion of an interlocking feature enables enterprise-grade midspans to scale the power infrastructure in one-port midspan increments as new powered devices are added. Among 1U multi-port options, there also are configurations that deliver 36W/port and a total of 864W in up-to-24-port versions, or up to 48 ports of IEEE802.3af power. There also is the virtual 48-port 2U IEEE802.3at option to consider.

Other enterprise-grade midspan options that enhance flexibility include the ability to use DC inputs with external power supplies for incremental power capacity or redundancy, and flexible powering from AC, DC or another midspan. Interconnected midspans can also back each other up.

* Management and maintenance: Today's enterprise-grade PoE infrastructure must also have remote power-management capabilities that support both IPv4 and IPv4/6 addressing. This allows simple and efficient monitoring and control of powered devices, which increases in importance with network size and complexity. A key requirement is the availability of a remote power-off/-power-on feature so selected ports may be shut down during the day, which can reduce power consumption by 70%. Each device's power consumption can be measured and its average power consumption can be actively reduced.Remote power management also enables unit scheduling, UPS power monitoring and Web-based monitoring. Malfunctioning remote devices can be reset, eliminating an expensive service call. Enterprise-grade midspans also enable centralized control of multi-site or multi-building installations, with support for immediate alert (e.g. E911) and response if IP phone status changes. When the midspan is integrated with a UPS system, the remote power-off/power-on capability also enables low-priority ports to be disconnected during power failures.

Remote power management must be performed in a secure fashion. SNMPv3 management is recommended to prevent malefic agents from interfering with network operations.

Preparing for the future

As you lay the foundation for your power infrastructure, it is important to consider all of these factors. Enterprise-grade midspans deliver these capabilities while offering a superior alternative to wholesale switch upgrades. They also make it possible to plan ahead -- by using midspans that power on spare pairs as required by the PoE standard, a network will be ready for four-pair powering with future PoE switches. These and other future-proofing strategies will maximize operational effectiveness over the life of the installation.

Ronen is product marketing manager, PowerDsine Midspans, with Microsemi's  Analog Mixed Signal Group. Contact him at

Learn more about this topic

More power yet more green

IEEE task forces move forward on Ethernet standards

PoE powers Ethernet switch revenues
Join the Network World communities on Facebook and LinkedIn to comment on topics that are top of mind.

Copyright © 2009 IDG Communications, Inc.