MOBILE TECHNOLOGIES AND TRENDS
Going the distance with MIMO
Lisa Phifer
03.16.2005
Rating: -3.81- (out of 5)
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[TABLE]If you've used a wireless LAN of any kind, you've experienced this phenomenon: throughput declines with distance between transmitter and receiver. 802.11g may have a top link speed of 54 Mbps, but maximum throughput (about half link speed) can only be achieved close to the AP. A station pushing 20 Mbps over a distance of 10 meters may well deliver just one Mbps at 40 meters.
The IEEE 802.11n task group is now developing a next-generation WLAN standard known as MIMO -- Multiple-Input, Multiple-Output -- that promises a top speed of at least 100 Mbps. Some people salivate at the thought of having all that bandwidth to deliver rich content like multimedia data, video, and voice over wireless. But others wonder: Do I really need that much speed? Perhaps 20 Mbps would be sufficient if it were available reliably over greater distances. Fortunately, MIMO will address that wish too. In fact, some pre-standard MIMO products already do so today.
Many faces of MIMO
The principal behind MIMO is what the name suggests: receiving more than one data stream (multiple inputs), transmitted over several paths (multiple outputs) within a single radio channel. The result is similar to opening new lanes on a heavily-traveled road. If you're crawling along a one-lane stretch at 25 mph, a second lane might double your speed. By the end of an hour, you'll have traveled twice as far on the two-lane highway. This analogy isn't perfect, but it helps to see why MIMO can increase both WLAN throughput and distance.
When 802.11n is completed -- perhaps by the end of 2006 -- there will be one agreed-upon standard MIMO approach. As for 802.11a, b, and g, the Wi-Fi Alliance will then offer 802.11n interoperability testing and certification, so that we'll be able to buy an 802.11n AP from Linksys and an 802.11n adapter from D-Link and be reasonably confident that they'll work together. But today, this is not the case.
There are several competing definitions of MIMO
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vying to become the basis of the 802.11n standard. Proposals vary in number of transmitters, receivers, antennas, and the way in which data streams are sent over multiple paths. As a result, today's "pre-N" products that utilize MIMO technologies are not (necessarily) interoperable with other-vendor "pre-N" products. They are also unlikely to interoperate with products based on the final 802.11n standard, whatever that ends up being. But if you're willing to purchase your (new) WLAN gear from one vendor, you don't have to wait until 2006 to go MIMO.
Early to market
The first MIMO chipset to hit the market was manufactured by Airgo Networks. Airgo's approach splits data into two streams, transmitted concurrently at 54 Mbps through two radios and two antennas. These paths are received through two antennas and recombined, using a technique called spacial multiplexing. Airgo's chipset is being currently used by several WLAN equipment vendors, including Belkin, Linksys, and SOHOware.
Atheros competes in the MIMO market with its AR5005VL chipset. This chipset's VLocity technology uses dual radios and up to four antennas to send data simultaneously over two paths. These MIMO techniques can be used by the transmitter, receiver, or both, over either the 2.4 or 5 GHz band. This Atheros "Super G with MIMO" chipset is now used by D-Link.
Video54's MIMO chipset uses an approach called BeamFlex. BeamFlex employs just one radio at the sender and receiver, but transmits the same data simultaneously through seven directional smart antennas "that adapt on the fly to the network environment." This approach is analogous to saving the same file on floppy, USB, and CD-R -- if one copy has errors, another probably won't. Over wireless, fewer lost or errored frames translates into higher speed and better throughput. Video54's chipset is now used by Netgear.
Finding MIMO
Most analysts expect MIMO chipsets to be widely used in enterprise WLAN products once the 802.11n standard is completed -- or when fundamental agreement is reach regarding which flavor(s) of MIMO will be adopted by 802.11n. In the meantime, the first products to use MIMO have been those marketed largely to SOHOs. If you're shopping for MIMO today, here are some products you may want to consider.
These are not the only MIMO products today, and new products will continue to emerge, so look beyond this list for new WLAN gear that incorporates MIMO. Our list cites test results to provide insight into early experiences with these new products. However, bear in mind that each reviewer's test environment and methodology differs. You generally cannot compare numbers from one review with numbers from another. For direct comparison, consult a head-to-head test like PC World's April 2005 review of D-Link, Linksys, and Netgear MIMO offerings.
Ultimately, what really matters is how well a product fares in your own network environment. If you're creating a residential or small business WLAN and struggling with distance limitations, you may want to give MIMO a try. Don't expect miracles, but do expect noticeable improvement -- especially if you're trying to cover a small building with a single 802.11b/g AP.
About the author: Lisa Phifer is vice president of Core Competence Inc., a consulting firm specializing in network security and management technology. Phifer has been involved in the design, implementation, and evaluation of data communications, internetworking, security, and network management products for nearly 20 years. She teaches about wireless LANs and virtual private networking at industry conferences and has written extensively about network infrastructure and security technologies for numerous publications. She is also a site expert to SearchMobileComputing.com and SearchNetworking.com.
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