802.11ac (VHT) - Just the FactsBy Tom Carpenter On 07/05/2012 - 23 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). We expect 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 nothing 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 MIMO technology. 802.11ac (the VHT PHY) will continue this and add 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 of 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 backward 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. One 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 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. Now, 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 organizations 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 become 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.
Editor's Note: This post was originally published in July 2012 and has been updated for accuracy and comprehensivenessTagged with: 802.11ac, VHT, spatial streams, tom carpenter, mu-mimo