Apple's OS X Mountain Lion reportedly hindering MacBook Air 802.11ac speeds

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One of the major upgrades found in Apple's latest MacBook Air is the inclusion of 802.11ac Wi-Fi, but a discovery on Monday appears to show an issue in OS X 10.8 Mountain Lion is hampering the laptop from reaching peak data transfer speeds.

In a review of the mid-2013 13-inch MacBook Air, Anand Lai Shimpi of AnandTech found real world 802.11ac file transfer speed to be artificially slowed by an apparent software issue in OS X Mountain Lion.

After finding speeds hitting a cap of 21.2MBps or 169.6Mbps over 802.11ac, much lower than the 533Mbps throughput seen with network testing tool iPerf, Shimpi narrowed down the problem to Apple Filing Protocol (AFP) and Microsoft's Server Message Block (SMB). Further investigation showed OS X does not scale TCP windows to the appropriate size, thus limiting the performance of the newly implemented 11ac protocol.

"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," Shimpi wrote. "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."

A separate report from Ars Technica claims an 802.11ac-enabled MacBook Air running Windows 8 in Boot Camp will reach higher file transfer speeds than Apple's own operating system. While transferring files in Windows is relegated to Microsoft's SMB protocol, the publication saw speeds 9 percent faster than OS X over Ethernet, 30 percent over 802.11n and 218 percent faster on 802.11ac.

The issue also presents itself in the Developer Preview build of OS X Mavericks, suggesting the TCP limitation was not purposely instituted.