MacBook Air real-world 802.11ac speeds throttled

Following the release of the new 2013 MacBook Air, AnandTech, in looking at its Wi-Fi performance, found that even though the new systems include advanced high-speed Wi-Fi controllers, speeds are severely limited in software.

(Credit: Apple)

The new MacBook Air was announced by Apple at the recent Worldwide Developers Conference (WWDC), 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.

However, there appears to be some limitation in the software currently shipping on these systems that severely limits real-world data transfer speed.

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 showed 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 that the fix should be easy, and it may be only a matter of a quick update for these systems to realise their full potential.


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