The 2013 MacBook Air Review (13-inch)
by Anand Lal Shimpi on June 24, 2013 12:01 AM EST802.11ac: 533Mbps Over WiFi
Haswell isn’t all that’s new with the 2013 MacBook Air, Apple also integrated support for 802.11ac. I wrote a primer on 802.11ac last year, but I’ll provide a quick recap here. 802.11ac is a 5GHz-only WiFi standard, with support for wider channels (80/160MHz vs. 40MHz in 802.11n) and better spatial efficiency within those channels (256QAM vs. 64QAM in 802.11n). Today, that means a doubling of channel bandwidth and a 4x increase in data encoded on a carrier, which are responsible for the significant increase in bandwidth. Usable bandwidth should also see improvements on 802.11ac as high-end access points are all expected to ship with beam forming enabled.
The first 802.11ac implementations we saw were on the smartphone side with HTC’s One and Samsung’s Galaxy S 4. Both of these devices were single antenna/single spatial stream implementations with 80MHz channels and 256QAM, resulting in a max PHY rate of 433Mbps. In his review of the HTC One, Brian documented peak performance using iPerf and a TCP transfer. In a smartphone, such high bandwidth from WiFi is really useful for improving battery life (race to sleep). In a notebook, you get the same potential improvement in battery life but there’s one more: a wireless alternative to Gigabit Ethernet.
In a 3-stream configuration given what’s available today, we’re talking about a 1.33Gbps PHY rate. Assuming better link efficiency on a notebook compared to what we’ve seen in smartphones thus far, we could be talking about a real alternative to Gigabit Ethernet - at least close to an AP. While wired GbE is always going to give you a more consistent experience, the vast majority of homes aren’t pre-wired with Gigabit Ethernet. In living situations where you can’t just run a bunch of Cat6 everywhere, but still want high speed networking, 802.11ac could be a real alternative.
The 2013 MacBook Air adds support for 802.11ac via Broadcom’s BCM4360. The controller is capable of supporting up to 3 spatial streams, but in its implementation in the MacBook Air we see a maximum of 2 used. I fully expect the 2013 rMBPs to use a third antenna to leverage all 3 streams. BCM4360 supports 80MHz channels, 256QAM and short guard intervals. The result is a max PHY rate of 867Mbps.
ASUS sent me its RT-AC66U based on the same BCM4360 silicon (coincidentally the same controller that’s in the new Airport Extreme), which I promptly used for testing the new MacBook Air. The ASUS router and MacBook Air combination worked perfectly. In the same room as the AP, I had no issues seeing the maximum 867Mbps PHY rate (above).
Within about 5 - 8 feet of the AP, I saw an average of 533Mbps using iPerf. That’s real data sent over TCP:
A 3-stream solution could definitely rival wired GbE, at least for short distances.
I then went about characterizing 802.11ac performance vs. distance to get an idea for how performance fell off as I moved away from the AP. My desk and test area is in the corner of my office, which is where I put the ASUS 802.11ac router. Performance around my desk was always up around 533Mbps.
Move around 18 feet away but remain in the same room and measured performance dropped to 450Mbps. One set of walls and another 10 - 15 feet dropped performance to between 250Mbps - 340Mbps. Another set of walls without moving much further and I was looking at 200Mbps. When I went one more set of rooms away, or dropped down to a lower level, I saw pretty consistent falloff in performance - dropping down to 145Mbps. Note that my setup is pretty much the worst case scenario for longer distances. The AP isn’t centrally located at all. If I were setting up an 802.11ac network for max coverage, I’d probably see 300 - 400Mbps in most immediately adjacent rooms.
So 802.11ac on the new MacBook Air is pretty awesome, there's just one issue...
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seapeople - Tuesday, June 25, 2013 - link
Brightness is pretty much the number one power consumer in a laptop like this (which is actually mentioned in the review). If you expect to run anything at 100% brightness and get anywhere near ideal battery life then you are bound to be disappointed.name99 - Monday, June 24, 2013 - link
"802.11ac ... better spatial efficiency within those channels (256QAM vs. 64QAM in 802.11n). Today, that means a doubling of channel bandwidth and a 4x increase in data encoded on a carrier"This is a deeply flawed statement in two ways.
(a) The modulation form describes (essentially) how many bits can be packed into a single up/down segment of a sinusoid wave form, ie how many bits/Hz. It is constrained by the amount of noise in the channel (ie the signal to noise ratio) which smeers different amplitudes together so that you can't tell them apart.
It can be improved somewhat over 802.11n performance by using a better error correcting code (which essentially distributes the random noise level over a number of bits, so that a single large amount of noise rather than destroying that bit information gets spread into a smaller amount of noise over multiple bits).
802.11ac uses LDPC, a better error correcting code, which allows it to use more aggressive modulation.
Point is, in all this the improved modulation has nothing to do with spatial encoding and spatial efficiency.
(b) The QAM64 and QAM256 refer to the number of possible states encoded per bit, not in any way to the number of bits encoded. So QAM64 encodes 6 bits per Hz, QAM256 encodes 8 bits per Hz. the improvement is 8/6=1.33 which is nice, but is not "a 4x increase in data encoded on a carrier".
We are close to the end of the line with fancy modulation. From now on out, pretty much all the heavy lifting comes from
(1) wider spectrum (see the 80 and 160MHz of 802.11ac) and
(2) smaller, more densely distributed base stations.
We could move from 3 up to 4 spatial streams (perhaps using polarization to help out) but that's tough to push further without much larger antennas (and a rapidly growing computational budget).
There is one BIG space for a one-time 2x improvement, namely tossing the 802.11 distributed MAC, which wastes half the time waiting randomly for one party or another to talk, and switching to a centrally controlled MAC (like the telcos) along with a very narrow RACH (random access channel) for lightweight tasks like paging and joining.
My guess/hope is that the successor to 802.11ac will consist primarily of the two issues I've described above (and so will look a lot more like new SW than new DSP algorithms), namely a central arbiter for a network along with the idea that, from the start, the network will consist of multiple small low-power cells working together, about one per room, rather than a single base station trying to reach out to 100 yards or more.
bittwiddler - Monday, June 24, 2013 - link
• The keyboard key size and spacing is the same on the 11 and 13" MBAs.• The 11" MBA is exempt from being removed from luggage during TSA screenings, unlike the 13.
• The 11" screen is lower height than most and doesn't get caught by the clip for the airplane seat tray table.
• When it comes to business travel computing, I'm not interested in a race to the bottom.
Sabresiberian - Monday, June 24, 2013 - link
One thing I would NOT like is for Apple to make a move to a 16:9 screen. I'd certainly rather have 1440x900 on a 13" screen than anything denser that was 16:9. I mean, I'm one of the guys that has been harping on pixel density and refresh rates since before we had modern smart phones (the move to LCDs set us back a decade or more in that regard), but on a screen smaller than 27", 16:9 is just bad. In my not-so-humble opinion.4:3 is better for something smaller than 17", but I can live with 16:10. :)
Kevin G - Monday, June 24, 2013 - link
Re-reading trough the review I have a question about the display: does it use panel self refresh? I recall Intel hyping up this technology several years ago and the Haswell slides in this review indicate support for it. The question is, does Apple take advantage of it?Kevin G - Monday, June 24, 2013 - link
I think that I can answer my own question. I couldn't find the data sheet for the review panel LSN133BT01A02 but references on the web point towards an early 2012 release for it. Thus it looks like it appeared on the market before panel self refresh was slated for wide spread introduction alongside Haswell.hobagman - Monday, June 24, 2013 - link
Hi Anand & all -- could I ask a more CPU related question I've been wondering about a lot -- how come the die shots always look so colorful and diverse, when isn't the top layer all just interconnects? Or are the die shots actually taken before they do the interconnects, consisting in the top 10-15 layers? Would really appreciate an explanation of this ...hobagman - Monday, June 24, 2013 - link
I mean, what are we actually seeing when we look at the die shot? Are those all different transistor regions, and if so, we must be looking at the bottom layers. Or is it that the interconnects in the different regions look different ... or ... ?SkylerSaleh - Tuesday, June 25, 2013 - link
When making the ASIC, thin layers of glass are grown on the silicon, etched, and filled with metal to build the interconnects. This leaves small sharp geometric shapes in the glass, which reacts with the light similarly to how a prism would, causing the wafer to appear colorful.cbrownx88 - Monday, June 24, 2013 - link
Please please please revisit with the i7 config - been wanting to make a purchase but have been waiting for this review (and now waiting on the update lol).