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Next-generation 802.11ax wi-fi: Dense, fast, delayed

The 802.11ax standard addresses wi-fi challenges such as client density and latency, but it's still early days, with final approval not expected until late 2019.

The 21-year-old IEEE 802.11 wireless network family of standards, more widely known as 'wi-fi', has a new member: 802.11ax. In its two decades, wi-fi has gone from a curiosity to a major social force, having brought easy, fast, cheap internet access to home, office and public spaces.

Each new version of the standard has primarily promoted itself on faster speeds, and 802.11ax does promise up to 30 percent faster top speeds over its forebear 802.11ac. But that's not its major selling point. Instead, 802.11ax uses a suite of new and extended technologies to solve some of wi-fi's more enduring problems, including client density and latency. With four times as much data deliverable simultaneously to multiple clients and latency cut by 75 percent, the user experience of 802.11ax should be much improved. It also retains full backwards compatibility with older standards -- an essential feature, but one that comes at a cost.

The new wi-fi standard has had a rocky road. The first two drafts of the standard were rejected, while the third was passed by the IEEE committee running the standard on July 1 2018. All this means that final approval won't happen until late 2019.

SEE: IT pro's guide to the evolution and impact of 5G technology (free PDF)

If the promises come true, the wait will be worth it. In particular, 802.11ax may solve one of technology's greater ironies -- that any large conference dedicated to wireless communication has unusable wi-fi. The same problem occurs at stadium events, busy concourses, and large events. An 802.11ax access point (AP) will not have much greater throughput than the 1Gbps or so that an 802.11ac device could manage, but it can split that total efficiently between many more simultaneous connections.

Key technologies

Radio

At the base of the 802.11ax innovation tree is the way it handles radio frequencies. It has to use the same spectrum allocations on 2.4GHz and 5GHz as before, with sets of 20MHz-wide channels that can be grouped together in blocks up to 160MHz wide. But within those 20MHz channels, 802.11ax subdivides the frequency space into 256 subchannels -- four times more than the 64 subchannels previously used. This improves the resolution with which a link can cope with interference, frequency-dependent fading and so on.

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The greater density of narrower subcarriers gives 802.11ax more flexibility to control each channel, equalising characteristics with more precision and allocating groups of subchannels to many clients simultaneously.

Image: Qorvo

A bigger change is in the way 802.11ax uses the subchannels. Before, all subchannels were used in parallel to talk to an individual device, which monopolised the channel until it was handed over to another device. 802.11ax allocates subchannels into resource units (RUs) that can be used to talk simultaneously with multiple 802.11ax clients -- up to nine on one 20MHz channel, or 74 on a 160MHz channel group. This means much lower latency and fairer distribution of bandwidth between 802.11ax clients. The AP can send clients trigger frames that query what sort of service is required, while the clients reply with buffer status reports that the AP uses to allocate RUs.

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By combining different sents of subchannels, 802.11ax makes much better use of the spectrum with a high density of clients. However, one non-802.11ax client in the mix can severely downgrade this capability.

Image: Aruba Networks

The old 64-subcarrier system was called Orthogonal Frequency Division Multiplex (OFDM) -- 'orthogonal' meaning the frequencies were related to each other so as to minimise interference between subcarriers. The new 802.11ax version is called Multi-User Orthogonal Frequency Division Multiple Access (MU-OFDMA). This may prove useful if you go to the wrong sort of pub quiz.

Colours

Another major cause of slow working in dense wi-fi environments is mutual interference between access points that share the same channel, or whose channel groupings overlap. Wi-fi copes with this co-channel interference by Carrier Sense with Multiple Access Collision Avoidance (CSMA/CA): a radio wanting to transmit first listens on its frequency, and if it hears another transmission in process it waits a while before trying again.

Even if two access points are too far apart from them to detect each other's transmissions directly, a client of either in between them can effectively trigger collision avoidance when one access point hears it talking to the other. However, it's possible that the client is close enough to its access point that the other AP could reuse the frequency without causing interference. This is called 'spatial reuse'.

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With BSS Color, different links on the same frequency can decide to ignore each other and continue if they decide the risk of interference is low, instead of backing off and potentially not using available spectrum.

Image: Aruba Networks

To this end, 802.11ax has added a concept called BSS Color. BSS stands for Basic Service Set, which doesn't matter: it's the colour that's important. This is a number between 0 and 7, and APs that are close together on the same channel should be configured to use different colours. When an AP or a client that wants to transmit picks up a signal on its channel, it can check the colour code and if it's different and the signal strength is low enough to indicate a low chance of interference, go ahead with its transmission anyway. If the colour is the same, or if there's no colour value because the conversation is taking place between pre-802.11ax radios, then the old rules apply.

MIMO

Multiple Input/Multiple Output (MIMO) technology has been around for a while, and refers to using multiple antennas to create simultaneous links on the same frequency but separated by a combination of the time each signal is transmitted and the space it occupies. Although this was part of the preceding 802.11ac standard, it hasn't met with much success. The process of establishing MIMO links involves sending sounding frames to clients, which respond with the conditions they observe, and then building up a matrix of how to group and configure the clients to use MIMO.

In 802.11ac this was slow and the results tepid; 802.11ax incorporates a number of changes to address multiple clients simultaneously and to use other kinds of frames to help build MIMO groups. But other problems with MIMO -- clients frequently don't have multiple antennas, for example, and MIMO just doesn't work well in dense situations -- remain. Although 802.11ax's MIMO will work better in some circumstances, don't expect much of it. Some aspects, such as adding MIMO enhancements to the uplink from clients as opposed to just the link down from the AP, have been put off until the next version of the standard, 802.11ax Wave 2.

Minor arcana

A variety of smaller tweaks in 802.11ax address specific situations. One is 1024-QAM, for Quadrature Amplitude Modulation -- the 1024 is the number of combinations of size and relative phase each individual chunk of radio energy can have. The previous highest number was 256-QAM; higher numbers mean higher data rates for a given radio bandwidth, but are more susceptible to interference and thus are only useful in high signal strength, low-noise situations. 1024-QAM will give a boost of around 20 percent over 256-QAM in ideal situations, thus giving its most significant benefit to marketing efforts.

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Each dot represents a possible state of the radio signal. The more dots, the more information can be conveyed, but the more interference can mask the signal state. The receiver has to work out the probable message, and greater dot density needs more processing power to decode.

Image: Aerohive

TWT -- Target Wake Time -- is a negotiated agreement between an AP and a client for when the AP will next query the client for traffic. This allows clients to go into low-power modes between awakenings; it also lets the AP create efficient patterns of use to maximise the number of clients it can handle over time.

802.11ax: when and why

Some of 802.11ax's new features have very specific use cases. TWT is a clear nod to IoT, where power management and reliability are more important than high bandwidth. IoT on previous generations of 802.11 is very inefficient, tying up lots of resources to deliver occasional small bursts of data. By contrast, 802.11ax will be able to devote a small slice of spectrum to multiple IoT devices, freeing up the rest of the channel for reuse.

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You can buy an enterprise-grade 802.11ax access point such as Aerohive's AP650X now, but no 802.11ax clients are available yet.

Image: Aerohive

But most of the benefits of 802.11ax won't be realized for many years. Although you can buy '802.11ax-compliant' access points from multiple vendors right now, there are no clients -- and until something like 30 to 40 percent of clients are 802.11ax compliant, access points will have to fall back to older standards nearly all of the time, in order to service the older clients.

Naturally, 802.11ax access points will have the latest CPUs and highest RAM, so there'll be a performance boost under some circumstances even with older clients. But the APs will also need more power -- a problem if you're using Power-over-Ethernet with its 15-watt limit -- and faster wired backhaul via multiple 1Gbps or 5Gbps Ethernet connections.

There will also be stability issues with all the new code, and a lack of analytics, diagnostic and site-planning equipment, which will affect access points and, when they arrive, the clients. If you have to ask 'should I buy 802.11ax now?', you already know the answer. Unless your job or passion involves evaluating cutting-edge networking technologies, you should wait.

The first deployments will come in large stadiums, where the audiences can be expected to have the latest and greatest smartphones -- Apple fans, in other words. They led the charge in 1999, with Apple's adoption of wi-fi for its iBooks being the first mass-market consumer product to use the standard, and they'll do it again.

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