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11ac will be faster, but how much faster really?

Six months from now, enterprise IT groups will be facing a big change for their Wi-Fi networks: the shift to 802.11ac, which promises wireless data rates that start at 433Mbps.

But what's on paper and what happens in the real world are two different things.

RESEARCH: Resources for 802.11ac 'Gigabit' Wi-Fi

The 11ac overview

The IEEE 802.11ac standard: builds on some of the technologies introduced in 802.11n; makes mandatory some 11n options; leverages several ways to dramatically boost Wi-Fi throughput; and works solely in the under-used 5GHz band.

Yet at least one of the most important features, dubbed multi-user MIMO, which will let an access point 11ac radio talk to as many as four clients simultaneously, won't be available until 2014, with the second-generation 11ac chipsets. Today's access point can only talk to one connected client at a time.

With the current draft of the new standard considered stable, some consumer-grade routers and adapters already are available, or have been announced. Cisco, in an unusual move, announced in May that it will introduce in the first quarter of 2013 an 802.11ac radio module that will snap into its popular enterprise access point, the Aironet 3600. Other WLAN vendors and device makers are expected to introduce enterprise-class products starting in that same period.

The 11ac radios are backward compatible with 802.11n and 11a radios in the 5GHz bands; and new two-radio access points will likely have the single-band 11ac radio and a dual-band 11n radio to handle legacy clients.

But there are a lot of variables, and together, they add up to a wide range of performance possibilities for end users and the network.

Why is 11ac so much faster?

Vendors say a single data stream 11ac radio, which is what most 11ac-equipped mobile devices will have, will yield maximum data rate of 433Mbps using an 80MHz-wide channel, compared to 150Mbps for single-stream 11n with a 40MHz channel.

The new Wi-Fi does several things to be faster.

First, the first-generation 11ac chips will support 20MHz, 40MHz and 80MHz channel widths, compared to 20MHz and 40MHz for 11n. The wider channel is like a fatter pipe: You can push more through it in the same amount of time.

Second, there's a new modulation scheme, called 256 QAM, which essentially lets 11ac pack more information into the radio signal. "Inside a given finite 'space,' the whole process [of 256 QAM] lets you get more data transmitted, with an improved possibility that you send a '1' and receive a '1' on other end," says Dino Bekis, senior director of wireless connectivity at Broadcom.

Third, beamforming will be standard feature in 11ac instead of the rarely implemented option in 11n. Today, some vendors such as Ruckus use specially designed antennas with multiple components that can be used in different combinations to create an optimal signal for each associated client.

But with 11ac, beamforming will be done at the silicon level, as a function of the baseband chip and a protocol that learns about the path between the two radios and automatically recalibrates to optimize the signal for the radio at the other end. [See "Beamforming: 802.11ac promises great Wi-Fi enhancements, but you can get a jump today"]

The optimal conditions for use of the new modulation and the wider channels are likely to be in home Wi-Fi deployments, according to Ruckus CTO William Kish. But in "challenging" WLAN deployments with longer distances and lots of client devices -- in corporate networks -- 11ac will make adjustments that will impact speed and throughput, he says.

At the distances most users are from access points, "you can't use 256 QAM and so fall back to existing 11n modulations," Kish says. "Similarly in dense environments or, again, at real-world distances, you fall back to 20MHz channelization."

Learning to love the 5GHz band

Today, 11n networks can operate in either the 2.4GHz or 5GHz bands, although the latter is much less used. Partly, that's because many clients, such as most current Wi-Fi-equipped smartphones, only run over the 2.4 band. One of the most significant but under-appreciated changes in the new iPhone 5 is that it now supports 5GHz Wi-Fi. [See "FAQ: iPhone 5 and 5GHz Wi-Fi"]

Kish and others think that the most important immediate impact of 11ac is that it runs only in this band.

The higher frequency has, for now, far fewer devices using it -- none of the Bluetooth peripherals, microwave ovens, embedded Wi-Fi devices or baby monitors, for example, that crowd the three non-overlapping channels in 2.4.

The 5GHz swath has more channels available, in the U.S. from 20 to 25 (vendors and bloggers all cited different numbers) channels that are 20MHz wide. 11n can bond two of these channels together creating a 40MHz pipe, with an attendant big boost in throughput. That reduces the number of channels to half as many. The first-generation 11ac chipsets will be able to also use an 80MHz channel, again reducing the channel number in half.

The channel picture for 11ac is complicated further by the fact that to use many of the 5GHz channels, the Wi-Fi radio has to be certified to support Dynamic Frequency Selection (DFS) in order to avoid those channels that are being claimed by systems such as weather radars. With second-generation 11ac silicon, in 2014-15, the radios eventually will be able to support 160MHz channels, leaving the band with essentially two channels should that configuration be used.

But as Kish noted, in high-density 11n environments today, vendors routinely recommend the use of 20MHz channels. One example can be found in the "Cisco High Density Wireless LAN Design Guide." 

More data streams possible

One of the big breakthroughs in 802.11n was in creating multiple pairs of transmit-receive antennas that supported multiple data streams, up to four in all. The result was a dramatic increase in data rate and throughput.

By contrast, 11ac can support as many as eight spatial streams. Yet no one expects to see that many, at least not soon and possibly ever. "Handheld clients in 11ac will probably continue to be single stream, because of size and power constraints," says Greg Ennis, technical director for the Wi-Fi Alliance, the industry group that's creating an 11ac interoperability testing and certification program.

Broadcom's Bekis says that most of the chip vendor's customers see three streams as offering the best cost-benefit tradeoff. But he emphasizes those will be three 11ac streams, for a total capacity of 1Gbps compared to 300Mbps-400Mbps for a three-stream 11n implementation.

One impact of 11ac that's not been widely noted is that 11ac radios can save battery power. Apart from efficiencies derived from advances in silicon processes and power management, 11ac's power boost is due to one simple fact: Because it can move the same amount of data so much faster, the radio shifts back into low-power mode much faster as a result. Broadcom, for example, claims its 11ac combo chip is six times more power efficient "than equivalent 802.11 solutions."

That doesn't mean your battery will last six times longer, but depending on your data behavior, it can extend battery life and it gives mobile device designers a bigger "power budget" to work with.

The IEEE draft specification has already gone through several rounds of balloting, says Greg Ennis of the Wi-Fi Alliance. Final IEEE approval isn't expected until early 2014. But products based on the draft are already shipping: Both Netgear and Buffalo Technology released routers in May. 

The WFA plans to launch its 11ac certification program in early 2013, and products are expected to be announced if not shipping within weeks after that.

John Cox covers wireless networking and mobile computing for Network World. Twitter: http://twitter.com/johnwcoxnww Email: [email protected]

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