Tuesday, June 30, 2015

802.11ac – Evolution or revolution

There has been much talk of 11ac W2 recently. After getting into one such discussion and giving my own view on it, someone said to me, I must hate 11ac, which is definitely not the case. I definitely like it, but what I seriously dislike is the faceless user exploitation of some of the marketing practices that are branding 11ac as something it definitely is not and probably will not be, and making it seem that it’s a solution for all our woes. It was the same with 11n before it came out and then after it did, we immediately started looking for the next thing.
Using words as switch-like, gigabit wireless and what have you are wrong and don’t represent the technology correctly at all. I thought I’d give it my own view on what 11ac means to me and what I expect and not expect from it.
For a much deeper understanding into the below list of functions I seriously suggest you get the book "802.11ac A survival guide" from Matthew S. Gast.

Prior to 11ac the highest modulation was 64-QAM which is 4 times lower. This increased the throughput by about 20% which is always welcome, but the problem here is achieving it and consistently. Those who have done extensive tests on it say that the distance from the AP where this modulation can be achieved is only a few meters. This makes it very unpractical for almost all uses. The only one I can think of is a high density deployment where APs are placed under users, like under seats, tables or floors where the distance to users is very short and even then the only users that will probably be able to use it are the once in the immediate vicinity that don’t have the signal blocked by theirs or others body mass.

Wider channels
11n brought us 40MHz wide channels, which is a 2 fold increase to before and 11ac gives us 160MHz wide channels which is a 4 fold increase to 11n capability.
The question here is why anyone would ever go beyond the 40MHz mark for regular enterprise use or even home use. I can see 80 and 160MHz channels maybe being used in P2P links but other than that not really. Even having a Gigabit link is a rare case.
In the sense of channel widths 11ac is not revolutionary neither evolutionary, it will probably prove to be self-destructive.

11n brought us multiple-in-multiple-out radio architecture. In fact it defined that a radio chain can be designed of up to 4 radios, hence the 4x4 nomenclature. This is an increase of times 4 to 11a/b/g. 11ac evolved from that to allow for up to 8 such chains which is a factor of 2 compared to 11n.
The gains of having more than 4x4 is highly questionable due to power requirements, design, return of investment and ugliness of such APs. But the more chains an AP has the more options it has with regards to beamforming, but I have reservations about that also.
So having more radios will not bring anything new to WiFi so it’s hard to call it even an evolution. IT’s just something the standard allows I guess.

Spatial streams
Prior to 11n WiFi was a one stream to one client affair, but 11n brought with it the ability to send 4 distinct streams to one station at a time, which is an increase of 4 times. 11ac continues with this trend and evolves an option of 8 simultaneous streams, which is an increase of 2 times of 11n.
But one needs to understand much more than just numbers here and realize that most client devices are at most 2x2. So whatever your AP is it will basically at best only match what the client is capable of, which means that majority of chains are wasted most of the time if APs don’t employ a different technique of sending data through redundant chains like STBC, and getting stable 3x3:3 connection to even capable clients is difficult and costs power and most APs and/or client devices will rather disable a chain or two or at most employ MRC to enable a better reception.
Another thing to also realize is that phones and phablets will only ever be 1x1 devices due to size and also power restrictions. Tablets will be at most 2x2 devices for the same reason. To integrate more radios and therefore antennas a device has to be the right size for it to even work and integrating an 8x8 chain the size of the device has to be enormous and even then it wouldn’t matter much.
Having more that 2x2 chains is marginally useful so in that respect 11ac is not a revolution at all, it’s hardly an evolution.

Beamforming was introduced with 11n as a big revolutionary idea that would increase the signal strength at the client device and/or would lower the amount of RF propagation lowering CCI an AP causes. But as the standard didn’t specify which BF method to use no one used any.
The only thing 11ac changed in that respect is that TxBF (as it’s called) now has a standard way of defining how to get information on a channel to employ proper weights to each radio in the chain. The catch here is that client devices must support it, which again is still rare, but given that there is only one way defined in 11ac as opposed to about 9 that were defined in 11n maybe we could see something there in the future.
As a side note I have my doubts about beamforming actually contributing in any big way in the real world either by lowering CCI or providing higher RSSI, but I don’t have much data to go on here. It’s more of a hunch and I could be wrong.

Throughput and efficiency
With 11n speeds increased from a “mere” 54Mbps to up to 600Mbps of throughput, a factor of about 10. 11ac promises speeds of up to 7Gbps which is s factor of 12. So 11ac hit and passed the Gigabit mark, a revolutionary step indeed… or is it? The fact is that these speeds can only be achieved through the use of multiple radio chains, spatial streams, wider channels and higher coding rates, all of which are very hard or should I say impossible to achieve due to many restrictions like power and size requirements, price and just pure laws of physics. So don’t expect 11ac capable devices to reach the Gigabit mark anytime soon if at all. But what one should be always striving at is to optimize their network to get devices on and off the medium as fast as possible with as little retries as possible and in turn get the highest average speed possible.

MU-MIMO is a very revolutionary idea. Up until now all standards defined PHY operation as one station occupying the channel at one time which with 11ac they would like to change through the use of MIMO and beamforming to get a better channel reuse.
The trick with this one is for all the receiving stations to be able to differentiate between different streams because every receiving station will receive every others data too. The analogy here is one of identical twins (or triplets) and being able to know which one is which. If one can’t tell the difference from them, how will they know from whom they need to take the data to get the information that’s meant for them?

Band operation
11n operates in both bands, whereas 11ac operates only in the 5GHz band. Although 80 and 160MHz channels can’t even be used in the 2,4G band (they don’t even fit there), 20 and 40MHz can and IEEE could have made the amendment stay in the “dead-band”, but took the opportunity of a new standard and decided against it.
This is a very big thing and for me it’s revolutionary. Even if you don’t agree it’s at least a very big evolutionary step.

Chipsets, as mostly everything anywhere else, evolve. But the rate of evolution is always dependent on outside factors. In that respect 11ac at least sped up this rate and with it every AP and client device that supports it is better off by it. If every node on the network can get on and off the channel faster more can use it and therefore speeding up the network for all.
Nothing revolutionary there but the speed of evolution was probably helped by 11ac introduction and that’s a very good thing for sure.

At the end of the day everyone is looking for more speed. At the start of 11n days it was touted as the cure for all our woes standard, due to the much higher speeds all the bells and whistles brought with it. And I would agree the throughput increase and the efficiencies put in the amendment solved some issues, but those speeds can only be achieved IF proper design principles are employed which, let’s be honest, are still few and far between. The real revolution won’t come with technology, but with realizing that knowledge is the essential ingredient that enables higher throughput, reliability and in the end happy users… or you can talk to your local sales representative to give you the right low-down.

Definite improvement:
  • Chipsets will evolve faster which means better with regards to RF characteristics that will enable faster throughput
  • Mandatory use of 5GHz only

Marginal improvement:

  • Beamforming is standardized, but requires sounding which requires bandwidth and I have reservations about beamforming effectiveness in general
  • Throughput will be higher but only if networks are designed properly
  • 256-QAM

Most likely useless features:

  • 80/160MHZ channels – self-destructive; maybe useful only in P2P links
  • Radio chains beyond 4x4 are unlikely due to power requirements, investment return and just shier ugliness of such APs
  •  MU-MIMO – possible to achieve in the long run, but what's the volume of scenarios where employed