Real World 802.11ac Performance Under OS X

A good friend of mine recently bought an older house and had been contemplating running a bunch of Cat6 through the crawlspace in order to get good, high-speed connectivity through his home. Pretty stoked about what I found with 802.11ac performance on the MacBook Air, I thought I came across a much easier solution to his problem. I shared my iPerf data with him, but he responded with a totally valid request: was I seeing those transfer rates in real world file copies?

I have an iMac running Mountain Lion connected over Gigabit Ethernet to my network. I mounted an AFP share on the MacBook Air connected over 802.11ac and copied a movie over.

21.2MB/s or 169.6Mbps is the fastest I saw.

Hmm. I connected the iMac to the same ASUS RT-AC66U router as the MacBook Air. Still 21.2MB/s.

I disabled all other wireless in my office. Still, no difference. I switched ethernet cables, I tried different Macs, I tried copying from a PC, I even tried copying smaller files - none of these changes did anything. At most, I only saw 21.2MB/s over 802.11ac.

I double checked my iPerf data. 533Mbps. Something weird was going on.

I plugged in Apple’s Thunderbolt Gigabit Ethernet adaptor and saw 906Mbps, clearly the source and the MacBook Air were both capable of high speed transfers.

What I tried next gave me some insight into what was going on. I setup web and FTP servers on the MacBook Air and transferred files that way. I didn’t get 533Mbps, but I broke 300Mbps. For some reason, copying over AFP or SMB shares was limited to much lower performance. This was a protocol issue.

Digging Deeper, Finding the Culprit

A major component of TCP networking, and what guarantees reliable data transmission, is the fact that all transfers are acknowledged and retransmitted if necessary. How frequently transfers are acknowledged has big implications on performance. Acknowledge (ACK) too frequently and you’ll get terrible throughput as the sender has to stop all work and wait for however long an ACK takes to travel across the network. Acknowledge too rarely on the other hand and you run the risk of doing a lot of wasted work in sub optimal network conditions. The TCP window size is a variable that’s used to define this balance.

TCP window size defines the max amount of data that can be in flight before an acknowledgement has to be sent/received. Modern TCP implementations support dynamic scaling of the TCP window in order to optimize for higher bandwidth interfaces.

If you know the round trip latency of a network, TCP window size as well as the maximum bandwidth that can be delivered over the connection you can actually calculate maximum usable bandwidth on the network.

The ratio of the network’s bandwidth-delay product to the TCP window size gives us that max bandwidth number.

The 2-stream 802.11ac in the new MacBook Air supports link rates of up to 867Mbps. My iPerf data showed ~533Mbps of usable bandwidth in the best conditions. Round trip latency over 50 ping requests between the MBA client and an iMac wired over Gigabit Ethernet host averaged 2.8ms. The bandwidth-delay product is 533Mbps x 2.8ms or 186,550 bytes. Now let’s look at the maximum usable bandwidth as a function of TCP window size:

Impact of TCP Window Size on 802.11ac Transfer Rates, 533Mbps Link, 2.8ms Latency
Window Size Bandwidth-Delay Product TCP Window/BDP Percentage Link Bandwidth Max Realized Bandwidth
32KB 186550B 32768/186550B 17.6% 533Mbps 93.6Mbps
64KB 186550B 65536/186550B 31.1% 533Mbps 187.2Mbps
128KB 186550B 131072/186550B 70.3% 533Mbps 374.5Mbps
256KB 186550B 262144/186550B 140.5% 533Mbps 533Mbps

The only way to get the full 533Mbps is by using a TCP window size that’s at least 256KB.

I re-ran my iPerf test and sniffed the packets that went by to confirm the TCP window size during the test. The results came back as expected. OS X properly scaled up the TCP window to 256KB, which enabled me to get the 533Mbps result:

I then monitored packets going by while copying files over an AFP share and found my culprit:

OS X didn’t scale the TCP window size beyond 64KB, which limits performance to a bit above what I could get over 5GHz 802.11n on the MacBook Air. Interestingly enough you can get better performance over HTTP or FTP, but in none of the cases would OS X scale TCP window size to 256KB - thus artificially limiting 802.11ac.

I spent a good amount of time trying to work around this issue, even manually setting TCP window size in OS X, but came up empty handed. I’m not overly familiar with the networking stack in OS X so it’s very possible that I missed something, but I’m confident in saying that there’s an issue here. At a risk of oversimplifying, it looks like the TCP window scaling algorithm features a hard limit in OS X’s WiFi networking stack optimized for 802.11n and unaware of ac’s higher bandwidth capabilities. I should also add that the current developer preview of OS X Mavericks doesn’t fix the issue, nor does using an Apple 802.11ac router.

The bad news is that in its shipping configuration, the new MacBook Air is capable of some amazing transfer rates over 802.11ac but you won’t see them when copying files between Macs or PCs. The good news is the issue seems entirely confined to software. I’ve already passed along my findings to Apple. If I had to guess, I would expect that we’ll see a software update addressing this.

802.11ac: 533Mbps Over WiFi Display
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  • Blindsay - Tuesday, July 16, 2013 - link

    So i would say the only thing that dissapoints me about the screen are the viewing angles. I find the resolution of 1440x900 quite fine for a screen of this size but i am spoiled by the viewing angles of my IPS screens. Other than that i am quite happy with my MBA so far (i7 8GB, 256GB) Reply
  • Risas - Thursday, July 18, 2013 - link

    Macs vs Macs... what a usefulness comparisons. Is the Mac from 2013 better than the one from 2011? I hope so!

    When will you compare the ultrabooks in stores now against the Macs? Some comparison like "what you get for that price"... Maybe some people could find some good ideas for spending their money... Not just Macs against Macs as there was no other option in the world... inbreeding?
    Reply
  • gentux - Friday, July 26, 2013 - link

    Good review. One thing I noticed was that the speakers are much better than on the 2011 MacBook Air. I have no idea if this was already improved in 2012 but the 2013 really rocks and actually has a nice bass. I think they haven't been stereo in the past. Reply
  • lobisme11 - Tuesday, August 06, 2013 - link

    This computer is simply amazing if I could change on thing it would be to add retina display. However the screen resolution is still great! The battery dosen't last the 12 hours it is supposed to when editing movies/videos which is what I use it for. For typing/surfing the web the battery is great! Some times while editing a movie the computer freezes normally for about 30 seconds however it rarely ever has to be restarted when it freezes. Overall Simply Amazing! A definite Buy! Great phone support to. Buy a warranty Overall: 8.5/10 Reply
  • antonio22m - Saturday, August 10, 2013 - link

    Macbook Air is undoubtedly a very good notebook
    Price and lack of optical drive can affect a large number of users whose decision during the judgment can be negative so that they can decide to choose another manufacturer.
    Air is perfect and the best "second computer" that you can wish for.
    His task was not to be the main and only computer we can possess.
    If you want excellent laptop computer that will be able to carry it with you wherever you go, the Air is an excellent choice for perfectly reasonable size and more pronounced weight that barely exceeds one kilo.
    Take a look at this comparison at http://www.squidoo.com/apple-macbook-air-133 and You will see comparison to the another Apple laptops.Anyone considering purchasing this laptop needs to see the information in this chart.
    Reply
  • strafejumper - Tuesday, August 20, 2013 - link

    I went to the apple store to get this haswell macbook air but the tn panel - while good for a tn panel - looked bad next to all the iphones, mini ipads, and ipads all around it
    To me an ultrabook should not have a tn panel - i couldn't buy it
    The Macbook pro 13" was too heavy (and i cannot wait for the impending refresh)
    It's really tough though because the windows ultrabooks (only 2 out so far - Sony Pro and Asus S7) have half the battery life of the macbook air.
    Half! My guess is this is mostly due to Mac OS and Mavericks isn't even out yet!
    If the Macbook air had an IPS I think it would be a slam dunk -
    As it is I think i'm going to be stuck with a crappy 6hr Sony Vaio
    Reply
  • thinkpanda - Friday, September 13, 2013 - link

    Apple delivered an update of OS X (10.8.5) which claim to fix the file transfer speed with 802.11ac network. Would Anand update the test result with that path? Reply
  • Stofken - Monday, November 25, 2013 - link

    Resolution is even lower on the rMBP 13, 1280x800px. Come on, really? You can call this Pro without blushing? Yes, those 800 pixels are cristal sharp, but still only 800, on a premium priced notebook! Reply
  • appliance5000 - Friday, December 20, 2013 - link

    It's a good looking screen.
    It's pro in the sense that it can do heavy graphics editing without a hiccup (I have a fully tricked out 11") and seems to be magnitudes more powerful than my first gen mac pro tower. It pretty effing pro.

    It's a powerful computer.
    Reply

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