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I have a new Macbook Air with 802.11ac, and a new Airport Extreme with 802.11ac. I'm trying to transfer a large file across my LAN (I have a network drive attached to network via wired ethernet).

If I transfer the file over (wired) ethernet, I'm seeing transfer speeds of 60-70MB/sec, which seems reasonable. But if I transfer the same file via wi-fi, I'm only seeing rates of 15-17MB/sec. If have tried to remove as many variables as possible by connecting my computer to the same cable that is connected to the Airport Extreme, and in that case I'm seeing the fast (60-70MB/sec) speeds, but the same transfer over wi-fi -- even if my laptop is right next to the Airport Extreme -- is much slower. I have confirmed that the laptop is connecting to the Airport Extreme via 802.11ac.

My assumption is that 802.11ac operating properly should have produced transfer speeds just as fast as what I was seeing over wired ethernet. What am I missing? Is it possible there's a software bug, or perhaps a configuration problem?

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I would run /System/Library/CoreServices/Wireless Diagnostics to see if the tool can pick up on any problems with channel or interference. The drop off in speed seems too large to account for a switch from wired ethernet MTU to the overhead for WiFi, but it could be something that will get tuned with an update.

The good news is you get free support within 90 days of purchase so you could call AppleCare or visit a genius bar to review the Wireless Diagnostic scans and have them escalate the case to engineering at no cost to you if this slowdown persists. I'm sure they would be very interested in a test case where the hardware and software isn't working as expected.

In the mean time, you might have more details on SNR and Transmit Rate that might point to a reason why your transfers are slower over wireless than over wired.

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You can even see how this tool can be used in action in this answer - where I turned on a microwave to show that there was some interference from that device.

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AnandTech has done some troubleshooting on this issue and has found some problems with the TCP/IP stack in Mac OS X. Hopefully Apple can release a software patch soon to fix the issue.

http://anandtech.com/show/7085/the-2013-macbook-air-review-13inch/10

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  • Wow - great find. I wonder if you removed the TCP window limitation if there would be a second bottleneck or that was the sole reason for the slow file transfer.
    – bmike ♦
    Commented Feb 9, 2014 at 21:07

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