But with 11ac, beam-forming 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 "Beam-forming: 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 affect speed and throughput, he says.
At the distances most users are from access points, "you can't use 256 QAM and so you'll 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.GHz band. One of the most significant but underappreciated 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 say 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 nonoverlapping channels in the 2.4 GHz band.
The 5GHz swath has more channels available; in the U.S., between 20 and 25 channels that are 20MHz wide are avaiable (vendors and bloggers all cited different numbers). 11n can bond two of these channels to create 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) to avoid channels claimed by systems such as weather radars. With second-generation 11ac silicon, to ship in 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 notes, 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.