802.11n Promises Faster Wireless

But a long road awaits the contentious technology There’s a new kid on the wireless block: 802.11n, a protocol that recently vaulted the first hurdle en route to becoming a full-fledged IEEE standard. It promises higher data rates, increased range and better reliability than its 802.11 siblings, but it will be at least a year before it hits the mass market. In January the IEEE approved 11n’s first draft proposal, but it wasn’t without controversy. A number of competing interests battled to get their respective approach to the technology adopted. The 11n standard is expected to provide raw throughput speeds of 500 to 600 Mbps, about 10 times faster than 802.11g, which tops out at about 50 Mbps. Approval is only 11n’s first step, though. Members of the IEEE working group responsible for the technology must fine-tune it. That process could take up to a year, so genuine 11n products won’t hit store shelves until 2007 at the earliest. The protocol combines Orthogonal Frequency Division Multiplexing (OFDM), which 11a and 11g already use, and MIMO architecture, which is key to the new technology’s success. MIMO stands for multiple input, multiple output. It employs the wireless-data version of a linguistic trick to send and receive signals. "Think of it as a conversation," says Dave Borison, marketing manager at MIMO chipset maker Airgo Networks Inc. "We’re speaking; this is a single stream of data. But if we were more complex, maybe I could speak as if I had two mouths. I could say every other word at the same time. If you could process that, we’d get double the data throughput." In MIMO, signals are divided between at least two transmitters, which send the signal bits along various paths – a technique known as spatial multiplexing. On the other end, at least two receivers collect the signals and their echoes – the result of multipath, whereby signals essentially use buildings, furniture and other obstacles as ethereal trampolines, bouncing their way to the receivers. Then the receivers, with the help of smart digital processing, put the signals together again. Current MIMO products such as Netgear Inc.’s RangeMax 240 Wireless Router provide throughput near 100 Mbps. But MIMO isn’t perfect. Judging from a test at Tom’s Networking, a hardware examination centre (www.tomsnetworking.com), the technology doesn’t play well with other protocols. Tom’s tested Netgear’s RangeMax 240, which sports Airgo’s MIMO chipset. Although the device offered high throughput – 100 Mbps – it interfered with a neighbouring 11g network, causing the older technology to achieve no more than about 5 Mbps when both nets shared the same channel. That’s common when two 802.11 devices try to work on the same channel, but Airgo’s chipset is supposed to detect neighbouring networks and change channels to avoid interference, a technique known as Adaptive Channel Expansion (ACE). Airgo’s reps say portions of the test are flawed. They insist that ACE works, but only if the neighbouring network is already up and running. If the neighbour comes on later, ACE doesn’t move the MIMO network to a non-interfering channel – at least, it doesn’t yet. Borison says Airgo plans to release a new version of the interference-avoiding technology that moves MIMO even if the neighbouring network pops onto the scene later. Tim Higgins, managing editor of Tom’s Networking, says his test is a warning: "b/g gear owners should worry about…any in-range draft-11n products," he writes in an email to Telemanagement. "All these products use a contiguous 40 MHz-wide band to achieve greater than 100 Mbps application-level throughput. And even though that 40 MHz band can be moved around, it will always overlap the centre channels (5, 6, 7). "The only way to avoid the problem in the 2.4 GHz band is to drop back to using a 20 MHz band, which also drops throughput, which manufacturers will be reluctant to do." Still, Airgo says its chipsets are neighbour-friendly and so is 11n. Compare the draft spec’s use of 40 MHz channels to the way some manufacturers use these ultra-wide wireless lanes, Borison says. " maintain the centre on channel 6 and widen their channels by 10 MHz on either side. As a result they interfere with both channel 1 and 11….The way 11n is defined, you can pick any centre channel, 1, 6 or 11. If you pick 1, you widen 20 MHz to the right – to 1 and 5. If you pick 6, you can go either left or right 20 MHz. If you pick 11, you go 20 MHz down, to 7." The standard is in its infancy. The IEEE must lock down MAC details and many other things. And there’s no guarantee the war for control is over. As Borison says, "This is a very complex specification" – in more ways than one.

802.11n Promises Faster Wireless

News | 03/01/2006 5:00 am EST

To keep reading, get a free trial.

Already a Subscriber?

Reuse & Permissions

Inside The Wire Report

Information

Hill Times Publishing

Get in Touch With Us