I took the opportunity during a recent briefing call with Qualcomm Atheros (QCA) about its new 802.11ac devices to dig into some of the technical details behind 802.11ac. I'm glad I did, because I learned a few things about the new higher-speed wireless standard that should be more widely known.
As you may know, 802.11ac is a 5 GHz only standard. The reason for this is that 11ac needs to use lots of 20 MHz wide channels to achieve its higher bandwidths. Like 802.11n, 11ac has a 40 MHz bandwidth mode that eats up two channels. But to reach its higher bandwidths, 80 MHz and even 160 MHz bandwidth modes are specified, which require four and eight channels, respectively.
Since the 2.4 GHz band has only three non-overlapping 20 MHz wide channels (1, 6, 11) and 83 MHz (ok, 83.5 MHz) in the entire band, it clearly can't support the two highest bandwidth modes in 802.11ac.
2.4 GHz band channel map
Source: FCC Tech Topic
The 5 GHz band, however, has 24 non-overlapping 20 MHz wide channels in North America (19 elsewhere). But due to possible interference with radar systems, only the bottom four and top five channels are commonly used by consumer Wi-Fi gear. The channels marked DFS required in the table below require a technology known as Dynamic Frequency Selection, which is usually implemented along with Transmit Power Control (TPC).
5 GHz band channel list
DFS enables a wireless device to detect the presence of a radar system on the channel they are using and, if the level of the radar is above a certain threshold, move to another channel.
The gist of all this is that 802.11ac, even in the first draft devices appearing this year, can reduce the effective number of available 5 GHz channels from nine to two! I've circled in yellow the channels typically available in consumer routers so that you can see the effect that 80 MHz channels have. (Note that some routers don't expose channel 165, although it doesn't matter in this case.)
802.11ac Channel map
Source: Rohde&Schwarz white paper
As with 802.11n, there is a relationship between number of transmit/receive pairs, number of spatial streams and the maximum data rates supported. Since product cost rises with the number of transmit/receive pairs, three-stream (perhaps four) designs are about the limit of what we see in consumer gear today.
Since the same cost/space constraints apply, 802.11ac consumer gear will probably max out at three streams. In fact, the first round of 802.11ac devices are at most 3X3 (three transmit, three receive).
The table below from a Broadcom presentation shows where the 1.3 Gbps maximum data rate that the CES product announcements trumpeted comes from. Given the previous discussion about available 5 GHz band channels, you can pretty much lop off the 160 MHz column for the foreseeable future.
Stream/Rate/Channel Width summary
We have been trained by consumer networking companies to lust after higher wireless speeds for our home networks in hopes of finally being able to achieve trouble-free wireless HD streaming and that's where they will continue to push with 11ac. Buffalo and TRENDnet have already announced their intent to ship three-stream dual-band 802.11acn routers by the end of this year.
But, true to its mobile roots, QCA sees a different push for 802.11ac. They see the primary beneficiary of 802.11ac as single antenna (1 stream) mobile products. By using 11ac's 80 MHz bandwidth mode, link rates of single-antenna mobile devices can be almost tripled (150 to 433 Mbps)!
This will enable faster peer-to-peer Wi-Fi Direct data transfer and higher quality screen-casting/throwing from phones and tablets to large flatscreens via Wi-Fi Display. But again, the higher speeds will come at the expense of using wider swaths of bandwidth in the currently relatively uncluttered 5 GHz band. There goes the neighborhood!
I was reminded in the Forums that I forgot to mention that 5 GHz signals don't travel as far as 2.4 GHz signals. So, depending on how thick your home/apartment walls are and how close your neighbors are, you may experience only slight interference from nearby 802.11ac networks. But having only two (maybe three) 80 MHz wide 802.11ac channels could make multiple AP installations in businesses more difficult.
QCA is also looking to 11ac to improve mobile device battery life, just as 11n can improve battery life over 11g. Since a given amount of data can be transmit in a shorter time, the radio is on less and so uses less power. Of course, if you end up using the higher bandwidth to transmit even more stuff, then your battery will still run down quickly!