Even in the event that the Wi-Fi connectivity issues on the mid-2013 MacBook Air notebooks are hardware-based, the underlying operating system software is apparently throttling potential data speeds on the new networking protocol.
Per AnandTech and CNET, a series of tests by AnandTech confirms that much of the Wi-Fi speed throttling is software-based. The new MacBook Air was announced by Apple at the recent Worldwide Developers Conference, and in addition to extended battery life, the new systems include support for 802.11ac Wi-Fi networking, which supports up to three times faster data rates in comparison with 802.11n.
In testing, AnandTech noticed that the link speeds of the systems are very high, at an average of about 533Mbps, but when transferring files over standard networking protocols, the speed drops to about 169Mbps at its maximum. This is over three times less than the expected speeds.
In investigating the issue, AnandTech discovered that an apparent bug in OS X limits the TCP window size (the maximum data that can be sent at a time) to a maximum of 64KB, which is far less than the 256KB needed to meet the speed capabilities of the 802.11ac connection.
As a result of this finding, AnandTech shows that in its current state, while the 2013 MacBook Air will still give fast file transfers, these will be limited to about 21MBps, instead of the more than 50MBps expected. Luckily, this limitation being in software means the fix should be easy, and it may be only a matter of a quick update for these systems to realize their full potential.
Stay tuned for additional details as they become available.