We previously reported about an issue with Apple’s latest MacBook Air having Wi-Fi issues. One of the major upgrades found in the new iteration of the hardware was the inclusion of 802.11ac Wi-Fi but a recent discovery points toward an issue in OS X 10.8 Mountain Lion found to be 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 discovering speeds hitting a cap of 21.2MBps or 169.9MBps over 802.11ac, much lower than the boasted 533Mbps throughput seen with networking testing tool iPerf, Shimp narrowed down the problem to Apple Filing Protocol (AFP) and Microsoft’s Server Message Block (SMB). After investigating the issue further, it was found that OS X doesn’t scale TCP windows to the appropriate size, therefore limiting the performance of the newly implemented Wi-Fi protocol. According to Shimpi:
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.
Source: AnandTech, Ars Technica