Q

What power supply problems do 802.11n wireless networks pose?

The power supply requirements demanded of 802.11n wireless networks won't be affordable for enterprises until the IEEE standard and supporting network hardware are approved and have caught up. Here are some options for fixing this problem while you wait for next-generation Ethernet switches and other equipment to appear.

We're planning to deploy 802.11n later this year. I've heard there will be power supply problems. Can you explain what the issue is and how we can solve it?

Many enterprises supply power to access points (APs) using 802.3af Power over Ethernet (PoE)-capable switches. Unfortunately, standard 802.3af delivers 12.95 watts per port, which is not enough to drive the extra transmit/receive chains used by 802.11n APs with Multiple Input Multiple Output (MIMO) antennas. For example, today's best selling dual-radio 802.11n AP, configured to use 3x3 MIMO requires 18 watts.

Eventually, next-generation Ethernet switches will support the emerging 802.3at PoE standard, being specified to deliver 30 watts per port. But that IEEE specification -- not to mention supporting hardware and enterprise deployment -- will take time. For now, what are your options?

Option 1: The easiest solution may be to configure new 802.11n APs to operate within your existing 802.3af power budget. For example, configure both radios in a dual-radio AP to use 2x2 instead of 3x3 MIMO. Or use one radio in legacy 802.11g mode, while using the other in 3x3 MIMO mode to support new 802.11n clients. AP options and power consumption vary, so workable combinations differ from product to product.

Option 2: Alternatively, depending on the products that you buy and already own, you might find a way to deliver more power to your new 802.11n APs. Companies with Cisco Ethernet switches can use Cisco's proprietary enhanced PoE. Others can add PoE injectors to supply additional power mid-span, or double up on 802.3af PoE switch ports and cables to deliver 2 x 12.95 watts to each new 802.11n AP. Some installations might even power selected APs from in-wall/ceiling outlets.

Option 3: Finally, as 802.11n chipsets mature, AP power consumption is falling. Some vendors now ship MIMO APs that can run full-bore with only 802.3af power. New hardware will undoubtedly become more efficient, but newer products will also add features that consume even more power (4x4 MIMO, advanced signal processing). The bottom line: Don't assume that you will or will not have sufficient power to drive 802.11n -- talk to your vendor and ask about the specific configurations that you plan to use.

For more information:
You may also be interested in this tip on Power over Ethernet and 802.3af.

This was first published in October 2008

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