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802.11n WLANs Can Work on Standard PoE

Tue, 10/21/2008 - 1:56pm
Luc Roy

Wireless LANs based on the IEEE 802.11 or Wi-Fi standards have been one of the great success stories in networking. Since their commercial debut in 1999, WLANs have been freeing users from wired network connections and  allowing them to work more productively in and out of the office.

Luc Roy
Roy: Power over the Ethernet may not be sufficient for most 802.11n access points.

One of the most exciting developments in the wireless LAN arena has been the introduction of the new higher capacity 802.11n radio link. Where the earlier 802.11a or g WLAN radio interfaces supported a maximum transmission capacity of 54 Mbps, an 802.11n interface can boost the transmission rate to almost 150 Mbps in the same 20 MHz channel, or to 300 Mbps in a double-wide (i.e. 40 MHz) channel. Not only does the 802.11n interface boost the transmission rate, it also can increase the transmission range and improve the overall reliability.

As enterprise users have begun to explore their options for upgrading existing WLAN infrastructures to 802.11n, many have been met with an unpleasant surprise: most 802.11n-capable access points will not operate on existing 802.3af Power over Ethernet (PoE) solutions. That can leave users choosing among options that are costly, inefficient or proprietary.

POWER POINTS
The power requirement for 802.11n is directly related to the technology that provides its higher transmission rate. The major capacity impact of 802.11n comes from the use of a Multiple Input-Multiple Output (MIMO) antenna system. The basic idea of MIMO is that the transmitter divides the bit stream into a number of independent transmit chains; the Draft 2.0 standard defines up to four transmit chains. Each transmit chain uses a separate radio transmitter, so the power source may now have to drive up to four transmit radios. Current MIMO implementations support two or three transmit chains.

In a MIMO system, all of the transmitters send in the same frequency band. Normally, if we have multiple transmitters sending on the same channel, their signals will interfere. A MIMO system addresses this by spacing the transmitting antennas some distance apart, so the signals are originating from different points in space. The receiver is then able to distinguish each of the signals by their unique arrangement of multipath images (i.e. the original signal and the delayed echoes of that signal produced by its bouncing off obstacles in the environment).

The MIMO receive process involves distinguishing and correlating the received signals, which is computationally intense and requires a more power-hungry processor. Further, data frames may be arriving at six times the rate increasing the MAC frame processing requirements. As a result, if the access point is not designed to minimize power consumption while delivering the required performance, an 802.11n access point can surpass the power limits of 802.3af.

OPTIONS
To reduce the cost of deploying wireless LANs, most organizations use Power over Ethernet (PoE) to deliver electrical power to their WLAN access points over the LAN connection. In that way, an access point can be deployed without an electrical outlet; unfortunately, the 12.95 W supplied by PoE is not sufficient to power most 802.11n access points.

WLAN vendors have developed a number of different strategies to address this difficulty. The first option was to develop proprietary powering mechanisms for 802.11n APs. However, those non-standard solutions may require costly infrastructure modifications when the standard becomes final. Other vendors have proposed using two PoE-enabled Ethernet connections to the access point. However, that requires two switch ports and two cable runs to support each access point and significantly increases the cost of the wired network.

The most detrimental approach is to reduce the data rate if there is not sufficient power to support the access point. In this arrangement, the access point will shut off one of the radios reducing the number of transmit chains from three to two if the power feed is insufficient. While that does take care of the power problem, it also reduces the transmission rate by one-third.

Wireless LANs have been one of the great technology successes in networking, and that success has been driven by progressive advances in technology like 802.11n. When used in an overall architecture that provides robust and manageable WLAN service, 802.11n can deliver a solution that rivals wired networks in both capacity and reliability. The most cost-effective implementation is one that delivers full 802.11n capability over standard PoE.

Roy is vice president of Enterprise Mobility for Siemens Communications.

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