The site previously tested the MacBook Air's 802.11ac performance using iPerf and found that the connection offered ~533Mbps of usable bandwidth in good conditions. However, after being asked by a friend what the real world transfer speeds were, Anand tested a file transfer between two Macs and was only able to obtain speeds of 21.2MB/s or 169.6Mbps.
"I disabled all other wireless in my office. Still, no difference. I switched ethernet cables, I tried different Macs, I tried copying from a PC, I even tried copying smaller files - none of these changes did anything. At most, I only saw 21.2MB/s over 802.11ac. I double checked my iPerf data. 533Mbps. Something weird was going on."
After some thorough testing, it appears the TCP window size being used by OS X for the AFP file transfer is at fault. The larger the window the more data can be sent before an acknowledgement needs to be received from the recipient. Modern TCP implementation support dynamic scaling of the window. This allows faster connections to send ACK packets less often in order to prevent the transfer from being slowed down.
The only way to get the full 533Mbps is by using a TCP window size that’s at least 256KB. I re-ran my iPerf test and sniffed the packets that went by to confirm the TCP window size during the test. The results came back as expected. OS X properly scaled up the TCP window to 256KB, which enabled me to get the 533Mbps result.
I then monitored packets going by while copying files over an AFP share and found my culprit: OS X didn’t scale the TCP window size beyond 64KB, which limits performance to a bit above what I could get over 5GHz 802.11n on the MacBook Air. Interestingly enough you can get better performance over HTTP or FTP, but in none of the cases would OS X scale TCP window size to 256KB - thus artificially limiting 802.11ac.
Unfortunately, a fix for this issue isn't available as of yet. Anand could not find one and the issue is still present in OS X Mavericks. Apple has been informed and hopefully will take steps to address the problem in a future update.
Many more details at the link below...
Read More [via Vic]