Power over Ethernet/Stackable Switches FAQ

Updated: April 30, 2009

What is Power over Ethernet?

Power over Ethernet is known by the acronym PoE and sometimes termed PoL (Power over LAN) or Inline Power. Power over Ethernet allows devices to transfer electrical power via Ethernet cable. A network switch or computer can supply power to components such as IP telephones , wireless LAN access points, network video or still cameras, and other appliances. With PoE, these external devices don't need a separate power cord or outlet.

How much power can this technique supply?

PoE transfers 48 volts of DC current across two of the four pairs on a CAT 5 cable, with a maximum current of 400mA and a maximum load power of 15.4 W. However, most devices lose at least 10 to 20 percent of the available energy during the transfer. Using a "phantom power" technique, the cables can supply both data and power. The powered devices must operate with either polarity or pair 45+78 or 12+36 with the use of a bridge rectifier. This process is defined by the IEEE standard 802.3af, ratified in 2005.

What role does PoE play in the stackable-switch market?

Stackable switches are less than 2 inches tall, but they supply gigabit speeds and make it possible for networks to handle voice, video and other multimedia applications at quality speeds. They are installed closer to client computers than the network backbone, which allows users to benefit from the better speeds. In most networks, the switch has one Ethernet link to each PC, offers a redundant power supply and can help support features such as IP telephony, IP messaging and IP conferencing.

Do all switches offer PoE?

Each large vendor in this market offers its own family of products, some which support PoE and some which don't. When you're evaluating these switches, look for the "PWR" or "P" in the model name — indicating support for PoE. Usually the switches offer a variety of 48 ports, optional PoE support, 10/100 Ethernet interfaces, 10/100/1000 Mbps interfaces, or even up to 10,000 Mbps interfaces. On average, the number of switches in a stack is eight but this can vary among vendors. With eight switches, administrators have access to about 400 different ports, including PoE as well as other applications.

What are the benefits of PoE and stackable switches?

Stackable switches help improve network efficiency and quality by offering the following capabilities:

  • Quality of Service improvements
  • Accommodation of 10-gigabit Ethernet uplinks
  • A migration path to IPv6
  • Proprietary security that integrates with vendor's infrastructure
  • Scalability because they are small and stackable

Where do I learn more about these switches and PoE?

If you're looking to invest in stackable switches, review IT Management's Buying a Power over Ethernet Checklist to evaluate what kind of product will best fit your network architecture. Then, take a read through IT Management's Power over Ethernet Buyer's Guide to learn more about the market as well as the available products. You might want to check out the models available from the large vendors in this market as well, including 3Com Corp ., Alcatel-Lucent , Cisco Systems Inc ., Enterasys Networks Inc ., Hewlett-Packard 's ProCurve and Nortel Networks .

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