802.11ac (VHT) - Just the Facts

802.11ac (VHT) - Just the Facts

By CWNP On 07/05/2012 - 20 Comments

When a new development comes along, related to any technology, the proper foundation must exist if we are to understand it well. Today, I want to present the basic facts related to the new 802.11ac draft amendment of the 802.11-2012 standard. The purpose is to answer the five most common questions asked about 802.11ac:

  • Why create a new physical layer (PHY)?
  • In what frequency band will the PHY operate?
  • What new capabilities or technologies are introduced?
  • Who will benefit from it?
  • When will we see it?

Why Create a New PHY?

The answer to this first question will be somewhat generic as the reasons for creating new PHYs have remained fairly constant over the 15 year history of the 802.11 standard. The primary motivators for the creation of a new PHY or the modification of an existing one are as follows:
  • Improve throughput
  • Utilize additional frequency bands
  • Improve stability/reliability
The very first 802.11 PHYs (FHSS and DSSS) operated at a whopping maximum of 2 Mbps data rate and even lower throughput rates. OFDM was introduced through the 802.11a amendment and provided a maximum of 54 Mbps data rate, but it also added the use of a new frequency block – the 5 GHz U-NII bands. The HR/DSSS PHY introduced through the 802.11b amendment provided 11 Mbps. So, 802.11 was ratified in 1997 and before the decade was out (in 1999), data rates had increased by a factor of 27.
The next PHY added was the ERP PHY introduced through the 802.11g amendment. It simply brought OFDM operations into the 2.4 GHz ISM band. The ERP PHY was ratified in 2003. In the first six years of the standard’s existence, three new PHYs were added (OFDM, HR/DSSS and ERP). It would be another six years before the next PHY was introduced.
In 2009, the 802.11n amendment was ratified giving us the new potential for 600 Mbps (with 4 streams operating at the highest data rate of 150 Mbps per stream). Now, we are expecting the ratification of 802.11ac within the next 12-24 months (the IEEE currently says the fourth quarter of 2013). It will provide a new potential aggregate capacity of nearly 7 Gbps (assuming MU-MIMO is implemented).
DISCLAIMER: Starting with 802.11n, due to the complexity of the needed hardware design and cost factors, vendors have not implemented the full potential of the standard. We are likely to see this hardware and software limitation with 802.11ac as well.
All of this shows one important fact: new PHYs are often motivated by the desire for faster data rates. This is still true for 802.11ac.
While 802.11ac does not introduce the use of new frequency bands (it will operate in the 5 GHz U-NII bands), it does take advantage of the less congested license-free band. Though there is nothing that would prevent the standard concepts from being implemented in the 2.4 GHz ISM band, it is simply not practical from a real-world perspective. One 802.11ac AP or wireless LAN router could easily consume all of the available frequency space within an area.
An additional motivator for new PHYs is the improved stability or reliability desired. For example, the HT PHY (802.11n) introduced improved stability for links at greater distances through the use of MIMO technology. 802.11ac (the VHT PHY) will continue this and also adds enhanced reliability, stability and range features such as Multi-User MIMO. The new capabilities will be described in more detail later in this post. As you can see, these three factors are important to the evolution the 802.11 standard and will continue to be in the future.

In What Frequency Band Will the PHY Operate?

The 802.11ac-D1.2 document (which is draft version 1.2 from October 2011) states that, “This clause is concerned with the below 65 GHz frequency bands excluding the 2.4 GHz frequency band…” The message is clear: 802.11ac will not support the 2.4 GHz ISM band. This is why you will see many people talking about the fact that 802.11ac will be implemented in the 5 GHz U-NII bands, but not in the 2.4 GHz ISM bands.
In actual implementations, many vendors (both consumer and enterprise) will implement dual-band APs and wireless LAN routers. The devices will implement 802.11n (HT) in the 2.4 GHz band and, therefore, compatibility with older wireless LAN clients. They will implement 802.11ac in the 5 GHz band with backwards compatibility all the way back to 802.11a (OFDM) devices. This model will be used for several years during the transition period. Eventually, we can remove our wireless LANs from the 2.4 GHz bands (sometime in the next couple of decades, maybe, possibly) and use the 5 GHz bands exclusively; but we have to wait for the client devices to catch up.

What New capabilities or Technologies Are Introduced?

The VHT PHY introduces faster data rates and MU-MIMO. On of the ways that the VHT PHY provides higher data rates is through the use of wider channels. The widest channel available in the HT PHY is 40 MHz. The VHT PHY will support 80 MHz and even 160 MHz channels. Logically, with the use of channels up to four times wider, we should see a potential increase in data rates of four times as well. So why is there potential for even more than four times the data rate?
Another data rate enhancement feature is the introduction of 256 QAM (quadrature amplitude modulation). QAM techniques use a constellation chart to identify bytes of data to be encoded with values smaller (in transmission size) than the data itself. HT PHYs support up to 64 QAM so the constellation includes 64 identified bits encodes. The result is that each bit encode was a 6 bit chunk and there were 64 of them defined. With 256 QAM, each bit encode is an 8 bit chunk and there are 256 of them defined. Stated differently, with 8 bits there are 256 possible combinations, with 6 bits, there are 64 possible combinations. Therefore, 256 QAM can represent more data than 64 QAM, but it also requires a very good link and can only be used in most environments over short distances (a few meters).
VHT also supports more spatial streams. HT supported 4 spatial streams and VHT supports twice that at 8. However, given that few HT devices support more than 3x3:3, we are not likely to see 8x8:8 anytime soon. In fact, most client devices are very unlikely to support more than 3-4 antennas in the near future due to space and power consumption and the conflict with consumer desire to have every smaller and lighter devices and more battery life. However, a tablet with just 2 spatial streams connected to a 4x4 VHT AP would be much better than what most of us have today.
NOTE: The term 3x3x3 is a traditional reference to 3 transmitters (Tx), 3 receivers (Rx) with 3 spatial data streams. The more recent notation, and the one used commonly in documentation, is 3x3:3 and means the same thing.
Finally, Multi-User MIMO (MU MIMO) is introduced. This concept allows the AP to transmit to multiple client stations simultaneously. This is accomplished using information received from the clients to aim or form the signal specifically for them when sent from the AP. I will describe MU-MIMO in more specific detail in a whitepaper later this year. For now, just know that it allows for more aggregate throughput in a coverage area by optimally using antennas and the frequency space.

Who Will Benefit From It?

At the moment, many organization could benefit from faster data rates. Rather than answering this question from the benefit of “who,” let’s talk about the benefit of “what.” What technologies can benefit from this? Certainly video delivery is a key factor. Organizations are using videoconferencing and other video delivery technologies (elearning, virtual presentations, etc.) that require streaming video. They also require reliable delivery of the video – particularly for live streams. Video consumes a significant amount of bandwidth and when multiplied by all the users accessing the video from mobile devices as well as desktops and laptops, high throughput Wi-Fi is important.
Eventually, the MU-MIMO technology will benefit wireless LAN engineers in that the bigger pipe provided by 802.11ac can be better used within coverage areas. Of course, we will need the hardware and software that implements MU-MIMO, but that will come.
The key technologies to explore are high bandwidth technologies and technologies that are real-time and low latency in nature. For example, VoIP and streaming desktops and applications. Citrix, VMware and other organizations are pushing for more and more desktop virtualization. While each individual desktop is not a significant load on the typical network , dozens or hundreds of users becomes very significant. And with BYOD, many users will want to stream these desktops to their iPADs and Android tablets (eventually Windows 8 tablets?) and many of them will want one virtual desktop running on their “desktop computer” and another on their mobile device. While the desktop computer may be wired (with a huge emphasis on maybe), the mobile device most certainly will not. BYOD and desktop and application virtualization are certainly important areas that can benefit from 802.11ac.

When Will We See It?

We are already seeing consumer devices come to market that support 802.11ac. NETGEAR was first out of the gate with an 802.11ac wireless LAN router and Buffalo released a wireless router and a “client” at the same time. The client is what we have traditionally called a workgroup bridge, but it is a client to the 802.11ac wireless LAN router just the same. Eventually, we will see USB 3.0 client adapters as well as integrated wireless LAN clients. NETGEAR currently predicts that they will have an 802.11ac USB adapter in August of this year.
The current round of devices are, of course, based on the draft specification. According to the IEEE 802.11 timeline, final ratification is expected in December of 2013. While it is possible that it could be ratified early, it is more likely that it will either be ratified on time or a little late. Either way, the first flood of devices based on the final standard will not provide the full benefits. We are not seeing MU-MIMO in the first draft devices and may see many devices based on the ratified standard that do not implement it as well. The chips have to support it and the software hardware has to be configured and implemented to provide the value it brings.
As with 802.11n, devices should be upgradeable through firmware to support any changes made to the amendment before it is ratified. Even a NETGEAR wireless LAN router purchased today should be upgradeable to support the final ratified amendment when it is released. Of course, whether the vendors choose to support this or not will depend on the value it brings to them and their clients.
The purpose of this post was not to argue for or against VHT in the enterprise, but I can certainly see its value there. I would love to hear from all of you. What do you think about VHT? How quickly do you expect organizations to adopt it once vendors make it available? Will we use dual-band devices in the enterprise and, if so, for how long?
In the very near future, I will provide videos of frame captures against an 802.11ac device so that you can see the information transmitted from the draft-based implementation.
For now, remember, Frames Are Food. Tom Tagged with: 802.11ac, VHT, spatial streams, tom carpenter, mu-mimo

20 Responses to 802.11ac (VHT) - Just the Facts

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09/26/2012 at 08:54am

Tom, thanks for a very clear article. The one piece of information I haven't found so far anywhere is whether 11n devices will allow a VHT network operate at the full data rates, or will the whole network be drawn to the lowest common denominator.That is, can an 11ac AP communicate with other ac clients at ac speeds, and with 11n clients at 11n speeds, simultaneously?

Primoz Marinsek Says:
09/05/2012 at 07:15am

For my 0.02$ I think that it's going to be quite a while before 11ac will be main stream. This I think for 2 reasons

1. I'm very involved with the WLAN for SMBs, hotels and other customers and I see that the biggest issue in all of them is the cable network or the DS I should probably say. I haven't come across a company that had more that 1G wired network backbone. Even if we put 11g APs there, we quickly come to that 1G barrier. The same applies to the WAN connection. If you have  an FTTH/FTTO you need at least 50/50 for a hotel of a size of about 200 rooms as the demand is just that hihg at certain times of the day.

2. The other is that I haven't even seen a 11n STA with a 4x4:4 MIMO. I think there are power and size issues there for clients and APs alike.

I think the technology will come into it's own and there will be use cases early on, but the general usage will most likely take some times. I predict 7 years from now :)

08/10/2012 at 04:51am

It's really useful info. thanks a lot.
I have a question about MU-MIMO.
MU-MIMO would be able to work on 80MHz.
 I saw the MU-MIMO wiki.That said MU-MIMO works on 160MHz CB.
I'm really wondering MU-MIMO could work on 80MHz CB(channel bandwidth)

08/10/2012 at 00:30am
What are your views on the 802.11ad spec running on the 60ghz band?  I have noticed things like signal loss through cable just between the 2.4 to 5.8 bands is noticeable.  Like all technology, seems like once everyone is on the ac there will be the ad and one more wifi frequency band.

Tom Carpenter Says:
07/19/2012 at 06:20am

Single stream clients (11n, 11a) will function as 11n or 11a clients. However, eventual 11ac clients could use MU-MIMO.

Also, on another note, I just noticed that the draft 2.0 specification now states below 6 GHz instead of 60 GHz as draft 1.3 did. Looks like they caught it. So clause has been repaired.

07/17/2012 at 14:22pm
Loved the article Tom, thanks for the informative read.

What are your thoughts on a lot of byod type devices benefiting from the new standard given that most only use a single spatial stream?

Tom Carpenter Says:
07/17/2012 at 13:19pm

Yes, as with 11n at first release, we are seeing the flood of consumer-grade devices. However, they are great for getting a first look at frames and operations for those of us who eat this stuff for breakfast, lunch and dinner :)

Glenn Cate Says:
07/17/2012 at 10:51am

Tom, great article. Thanks for a very understandable description of 802.11ac.

Cisco has their Linksys EA6500 router with 802.11ac (http://homestore.cisco.com/viewproduct.htm?productId=148919965) and just today (7/17/12) I see that DLink is offering their DIR-865L model (http://www.dlink.com/us/en/home-solutions/press-centre/dlink-ships-its-first-802-11ac-router)  Both appear to be home-based and not enterprise routers.

Tom Carpenter Says:
07/16/2012 at 10:41am

loganmoreno, it will likely be more like 18 months before the final 802.11ac amendment is ratified. The hardware in the current draft devices will be very similar (if not identical) to the final hardware based on the ratified amendment. It is more likely that you will be able to upgrade through firmware to the completely compatible with the final amendment; however, a device purchased in draft may not ever be made Wi-Fi Alliance certified. It just depends on the vendor's desire to do so. It's not uncommon for the vendor to come out with a "new" device that is really just the old device with new firmware and minor changes (like different RAM amounts and such). I hope this helps.

Frames are food,


07/14/2012 at 19:46pm
Great info, just what I was looking for.  Thank you!
 am thinking of putting Wifi in our house, and I figure if I am going to do that then I should set it up the fastest I could get it.
Lots of wireless routers out there, so I started doing some research and found that 802.11n is currently the latest, but 802.11ac will be much faster.
Here is where I am confused, the 802.11ac certification won’t be issued till early 2013, yet I am seeing 802.11ac products like the Buffalo AirStation and NETGEAR R6300 on the market claiming they are 802.11ac – even though they can’t be ‘Wi-Fi CERTIFIED’ since the 11ac standard isn’t finalized yet.
Some of the reviews of these first 802.11ac product mention that they are based on a first draft of the 802.11ac spec, and might have some performance issues with products that are based on the final 802.11ac spec
From what I read, the ‘Wi-Fi CERTIFIED’ certification makes sure that the devices will be fully compatible and function correctly with other 11ac products. The just releaced 802.11ac routers from several manufactures don’t have the Wi-Fi Certification.
I feel it is best to wait on 802.11ac, since I have waited this long, another 6 month shouldn’t kill me.
Am I crazy for wanting to wait till early next year to buy an 802.11ac router?

Tom Carpenter Says:
07/11/2012 at 10:04am

A quote of the below 65 GHz statement in draft 1.2

Thanks for all the feedback. Dan, the 65 GHz reference comes from the draft standard. Interestingly, the title of the draft says, "below 6 GHz," but I wanted to include that quote in the post to see what feedback I received. Everywhere else it says 6 GHz, so I'm thinking it's either a typo or they really wanted to be inclusive for future developments.

I will be posting about the 256 QAM distances in the next few weeks.

Thanks again,


07/10/2012 at 16:22pm
Another excellent article.
I did notice a typo."Therefore, 256 QAM can represent more date than 64-QAM" should say "data."
Also "64 QAM" and "64-QAM" are not consistent.  Personally I prefer the notation with the dash but you only used that once.
And I was confused by "“This clause is concerned with the below 65 GHz frequency ..."  Doesn't that make it sound like VHT will operate in the 65 GHz frequency?
Enough nit picking :-)
Like Albert I am looking forward to the MU MIMO whitepaper.  Could you (or someone) write a little about what we should expect 256-QAM to behave like?  I would like to know what "a few meters" really means and what the fallback rates will be like.   Is 128-QUAM supported?  Of course radio quality will play a huge role in the real world.
Thanks again for a great article and I look forward to learning more about MU MIMO.

07/09/2012 at 00:42am

Hi Tom,

  really a nice stuff. and very informational too. this give us the good start and a long vision of when 2.4GHz will be obslete from enterprise. As you said if the enterprises will accept the .11ac or not. I heard that cisco also come up with their own AP, I do not know if that is enterprise or consumer one.

 Any way thanks  a lot and will keep you following for next capture...

07/06/2012 at 16:50pm
Thank you for the short, but informational article.

I do not foresee widespread implementation of 802.11ac in
organizations, mostly because of the very wide channels. Office
buildings with many tenants, many times on the same floor, do not
provide needed "clean" air. Let us remember also of the short range of
VHT (more APs needed!) coupled with probably high costs of the devices.

Could you write more about MU-MIMO? I believe that is the most beneficial part of 802.11ac.

07/05/2012 at 20:51pm
Good stuff Tom. Keep up the superb work!

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