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
POST A COMMENT

229 Comments

View All Comments

  • darwinosx - Monday, July 08, 2013 - link

    Hah! Lots of developers use Airs without issue. Reply
  • cscordo - Thursday, August 08, 2013 - link

    What a ridiculous comment from someone who clearly doesn't develop software.

    I'm still on a 13" MBA from two gens ago and run multiple IDE's and SQL Server 2010, and it doesn't skip a beat.

    What type of "heavy" software development do you perform that it can't handle? I'd be very interested to know.
    Reply
  • josef195 - Tuesday, August 20, 2013 - link

    So, you're saying the Macbook Air isn't for pro users? I must say, if only they had some Macbook for professional users; they could even call it a Macbook Pro. Reply
  • Calista - Thursday, June 27, 2013 - link

    I agree with your ideas of adding the option of 16 GB of RAM and 4G support. Virtualization is growing all the time, the memory usage can grow very quickly if starting to run a few VM in parallel. And not having the option of 4G in this day and age is just embarrassing. For a machine *built* to be used on the go, with every component (battery life, weight, size) adopted to this task not adding 3G/4G just doesn't make sense. Reply
  • Dave DeCo - Saturday, June 29, 2013 - link

    I have never given much thought to the 4G thing. But now that it's brought up I too wonder why the MBA doesn't offer it. My incredibly portable iPad has it. iPhone too of course. Why not the wafer thin MacBook Air? Oh Apple. Always dangling that carrot on the longest stick possible while telling us we don't need carrots. If that last line makes sense to anyone please tell me. Because I don't know what the Helsinki I just said. Reply
  • australianm8 - Saturday, July 06, 2013 - link

    A few considerations for everyone fussing about 4g support.
    1. Apple likes to keep a simple product line
    2. Look at a teardown, not much room in there for much more to add. (already pushing it's thermal limits)
    3. There are a plethora of cell carriers that offer 4g USB sticks
    Reply
  • darwinosx - Monday, July 08, 2013 - link

    That was a childish and ignorant display of petulance. Obviously a particularly immature teenager. Reply
  • jaycee1970 - Saturday, August 24, 2013 - link

    "Dump your apple stock now, it will bounce back"

    Wow, that's fantastic financial advice, genius. I'll definitely sell something that will go up in value. The rest of your post was just as intelligent. Thank you.
    Reply
  • othernet - Monday, November 04, 2013 - link

    Uhm... there are hybrid notebook/tablets out there. Did you buy one? I don't know of anyone that did.

    Apple sells Retina 4, 10, 13 and 15 inch products. Soon, an 8 inch product too. You must have missed the keynotes.

    Wireless charging? Wait, do you still need to plug the wireless charger somewhere to make it work? When it doesn't have to sign me up!
    Reply
  • darwinosx - Monday, July 08, 2013 - link

    If you read the review you are commenting on you would know its not a terrible display. Reply

Log in

Don't have an account? Sign up now