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February 2007
Volume 10 / Number 2

Power to the People

By Tony Rybczynski, Inside Networking

The drivers behind the development of PoE are twofold: Firstly, to power phones (eliminating the need for a separate AC plug), and secondly, to provide continued telephone operation in case of power failures (a regulatory requirement in some environments). Given this important role, let’s deep dive into the technology and identify some important deployment strategies.

PoE — Standard or Forget It
We have all experienced the challenges of finding the right AC power plug and contending with 110 vs. 220 Volt standards when traveling internationally, and the value of having a single standard across North America. PoE standards have, for the first time, created a single globally deployed physical and electrical standard for the delivery of power for electronic devices, such as IP phones, wireless LAN Access Points, and video surveillance cameras.

The PoE standard is called IEEE802.3af. The standard was completed in 2003, so enterprises can deploy the technology, since it is well known and there are many interoperable products from which to choose. This standard provides 48V over two out of four available pairs over a Category 3 or 5 cable, with a maximum load of 15.4 Watts. This level is adequate for most, though not necessarily all, IP phones, WLAN APs, and IP cameras, though it is generally inadequate to drive laptops and PCs. A common practice is to use the Link Layer Discovery Protocol (IEEE 802.3ab) at start-up to determine one of four power classes to which a particular device belongs. While 10/100 Megabit Ethernet (i.e., 10 and 100BASE-T respectively) only requires two pairs for data transmission, the standard allows powered pairs to be used for data transfer, as is required by Gigabit Ethernet or GigE (i.e., 1000BASE-T). The powering source is the Ethernet Switch in the wiring closet, which dynamically detects whether the device at the end of the line requires powering or not (thus protecting devices that do not support the standard) and the supported power levels.

Avoiding Deployment Shocks
While electrical shocks are not a concern with PoE, there are a number of strategies that need to be followed, if post-deployment shocks are to be avoided. By planning in advance, your network can meet today’s requirements and be prepared for the future. Firstly, decide on a strategy for the deployment of PoE, with a multi-year view of the deployment of IP phones, access points, surveillance cameras, and any other PoE devices. The absence of such a plan could result in not having enough PoE ports to cover demand. That said, many enterprises take the opportunity when moving to GigE in the wiring closet to preequip all ports with GigE PoE, arguing that a less than a $20 per port price premium is easily offset by reductions in the ongoing OPEX cost of network engineering.

Secondly, purchase only PoE switches going forward as an insurance policy, since you can’t add PoE with a software change. IP phones typically draw 4 to 8 Watts. Some Ethernet switches cannot deliver PoE on all ports or full power concurrently on all ports; others are specified to deliver PoE on a subset of ports. With high density IP Telephony deployment, and increasing penetration of WLAN APs and IP cameras, this could drive the deployment of additional switches in the wiring closet to meet power demands, while leaving ports unused on existing switches. In addition, power levels delivered to the wiring closet will be higher with PoE, in some cases driving the need for DC power.

Thirdly, roll out GigE Switches supporting the GigE PoE standard, given the fact that most desktop PCs and laptops are being shipped with Gigabit Ethernet capabilities. Additionally, in order to allow one Ethernet jack per desk, IP phones should support integrated GigE QoS-enabled switching. Today, many IP phones will force the PC to slow down to 100 Mbps speeds, negating the value of GigE operation.

Fourthly, remember that PoE alone does not provide telephony continuity in the case of power failures, so you need to plan for UPS in the wiring closet. What is needed is a backup strategy driven by a business case, to either provide battery backup for some period of time or optionally to provide some form of longer term power generation (e.g., a diesel generator). For example, a hospital will opt for the latter, while a school may have a policy for one-hour backup, after which children are sent home. In the event that there is insufficient backup power, a priority scheme could be used, along with 802.3ab, to optimally allocate power to devices.

Finally, your PoE business continuity strategy should take into account wiring closet space and environmental requirements. For example, you don’t want to blow your budget by discovering too late the need for wiring closet air conditioning.

PoE — Power to the People and Beyond
The number of devices connected to the network is exploding, not only because of multiple devices per employee, but also because of everything is being networked opening up new asset and environmental monitoring and tracking, security and surveillance, and location-based applications. While driven by IP Telephony, the global PoE standard is becoming a key technology enabler for many of these applications.

Plan for PoE and you’ll have a powerful technology that not only brings power to the people for IP Telephony, but is also a critical infrastructure for in-building mobility, environmentally- aware business applications, and enhanced security.

Tony Rybczynski is Director of Strategic Enterprise Technologies at Nortel. (quote - news - alert) He has over 20 years experience in the application of packet network technology. For more information, please visit



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