The recent news that the IEEE had approved 802.11n within the 802.11 Working Group, and ratification was likely a few months away, led the Wi-Fi Alliance to explain its roadmap for adding more steps to the certification process. When the Wi-Fi group certifies a device, it runs it through tests that are supposed to ensure that the equipment responds in a standard manner. (The group also does plugfests in which equipment makers bring lots of gear together outside of lab conditions.)
When the word hit, the alliance identified four optional areas of certification that it would add. I knew about some of these areas, but I spoke with the group today to clarify what this meant for both equipment makers and end users. The Wi-Fi Alliance said it would offer tests for coexistence in 2.4 GHz, space-time block coding, transmit MPDU, and three spatial streams. Scratching your head? After 8 years of covering Wi-Fi, I admit I was in that position over a couple of those.
Coexistence has to do with the use of double-wide channels--40 MHz instead of the roughly 20 MHz regular channels--in both 2.4 and 5 GHz bands.
The 5 GHz band isn't a problem, because 20 MHz channels don't overlap; Wi-Fi selectable channels in 5 GHz are staggered by intervals of 4 band channels (5 MHz each), such as 36, 40, 44, and 48. In 2.4 GHz, channels are staggered only by a single 5 MHz band channel, meaning that the use of 40 MHz will nearly always conflict with other existing networks.Ennis said that 2.4 GHz coexistence terms weren't fully settled until recently, even though manufacturers have built in some methods of using 40 MHz in 2.4 GHz. The Wi-Fi Alliance discouarged the use; Apple, for one, doesn't allow its gear to use wide channels in 2.4 GHz.
In the new testing regime, "not everybody is required to support 40 MHz operation--but if they do support 40 MHz operation, they must go through the testing that we've defined," Ennis said.
The mechanisms that require an access point backing off to 20 MHz channels are so broad and severe that it's unlikely you could use a wide channel in any environment in which other Wi-Fi networks operate. Still, Ennis says, it may be of use in enteprise situations, or with future gear that's all 802.11n with these modes enabled that can be more respectful of each other automatically.
Space-time block coding. This term makes my head hurt every time I read it. I go off to the Web and read up on the principle, and it's above my paygrade. All wireless communication has to allot slots in some fashion--through contention or scheduling--for bits to go through. That's the basis of all wireless standards.
What STBC does is extend that beyond time into the domain of space. An access point can, through some complicated encoding, send different information simultaneously using multiple spatial streams so that receivers (stations in Wi-Fi parlance) that have single-spatial stream receivers can separately but at the same time decode their unique package.
The utility of this complicated feature is that we're likely to start seeing lots of single-stream N devices.
Chipmakers are most likely now delivering quantities of these lower-powered, cheaper 802.11n chips that can't offer two streams--and thus double the bandwidth--as laptop and desktop 802.11n modules can. With STBC, an access point can utilize the full available 802.11n bandwidth by splitting it spatially between two devices instead of halving bandwidth by speaking to a single-stream device solely.
Ennis noted that STBC also improves the signal-to-noise ratio, which makes faster rates and farther distances possible. "I think this is going to be a popular optional feature," he said.
Aggregation MPDUs (MAC Protocol Data Units). While sounding obscure, this is yet another way by which 802.11n can eke out improved speeds. For long sequences of data, aggregation MPDUs lets a Wi-Fi system create a long frame, reducing all the overhead required to send a packet. (Every packet has origin and destination information, a preamble, and other data that adds overhead.)For video, for instance, Ennis says that this kind of aggregation can improve throughput, although probably not by double-digit percentages. "It's not as dramatic an improvement as say using more spatial streams, or using 40 MHz channels," he said.
Currently, the Wi-Fi Alliance tests aggregation only if a manufacturer's access point sends these aggregated frames; it checks that a station can properly receive such frames, which can be interpreted under earlier 802.11n drafts. The new optional certification tests for aggregated frames sent by both stations and access points. (If included, it must be tested.)
Three spatial streams. This last one is quite simple. The Wi-Fi Alliance can now test for devices that send three streams of data across space up from two streams of data. Ultimately, we should see devices that can handle four, with a maximum raw symbol rate of 600 Mbps with wide channels in 5 GHz.
Those are the technical bits. The a/b/g/draft n labeling can only go so far and there should be more news on that front soon.
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