The latest edition of the MacBook Air includes 802.11ac Wi-Fi capabilities. However, Anand Lai Shimpi of AnandTech found real world 802.11ac file transfer speed to be 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.”
Another report from Ars Technica says an 802.11ac equipped MacBook Air running Windows 8 via Boot Camp will reach higher file transfer speeds than the same computer running Apple’s flagship OS. 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 appears to be present in the Developer Preview build of OS X Mavericks.