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.
256-QAM
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.
MIMO
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
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
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
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.
Conclusion
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
