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I've got a mid-2009 Macbook Pro running Lion over 802.11n with an Airport Extreme and cable internet. I've been seeing drop-outs while using video chat (be it Facetime, Skype, Google Chat) and doing anything else (gaming, Hulu, Netflix, etc) on the network.

I ran ICSI's Netalyzr and found that the network buffering was 4000ms upstream and 3200ms downstream, which is abysmal. Running the same test on my Windows 7 machine (over 802.11b) results in 220ms upstream and 1800ms downstream.

Aggravatingly enough, when I switch my Macbook to 802.11b, Netalyzr fails to complete. I initially chalked this up to a bad test, however, video still drops out and a ping somedomain.com in the background will show progressively increasing RTT's and packetloss.

No such deficiencies exist on my Win7 desktop.

I was unable to find any option under network preferences, and searches only brought up complaints about buffer-bloat without specifics. How do I "improve" the buffering behavior under Lion?

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I don't have any specific recommendations when tweaking network configurations. However, I did come across one blog post that discussed various network settings that could be adjusted in /etc/sysctl.conf:

kern.ipc.maxsockbuf=4194304
kern.ipc.somaxconn=512
kern.ipc.maxsockets=2048
kern.ipc.nmbclusters=2048
net.inet.tcp.rfc1323=1
net.inet.tcp.win_scale_factor=3
net.inet.tcp.sockthreshold=16
net.inet.tcp.sendspace=262144
net.inet.tcp.recvspace=262144
net.inet.tcp.mssdflt=1440
net.inet.tcp.msl=15000
net.inet.tcp.always_keepalive=0
net.inet.tcp.delayed_ack=0
net.inet.tcp.slowstart_flightsize=4
net.inet.tcp.blackhole=2
net.inet.udp.blackhole=1
net.inet.icmp.icmplim=50

You'll definitely want to back up the original file before fiddling with these system settings. Various readers also contributed to a lengthy discussion in the comments for the blog post.

Apple Support also has a KB for OS X 10.7 detailing changes that can be made to the MTU for Ethernet, which might also be adjusted for performance issues. However, there were no specific instructions for Wi-Fi.

There's an associated Support KB for older versions of OS X (10.4 and 10.5) that details changes for both Wi-Fi and Ethernet - you may be able to apply use the Wi-Fi instructions in Lion.

If you take the first path (adjusting the sysctl.conf settings), some trial and error may be necessary.

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  • I started playing with the values, but apparently since 10.5 there is autotuning which goes on and ignores some of those values.
    – user7116
    Mar 29 '12 at 14:21
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Restarting the router and cable modem had no effect, however, telling the Airport Extreme to use a separate SSID for N-connections has made a huge difference.

At the moment I cannot tell if this is due to other variables (such as local loop usage, etc), but it appears to have "solved" the problem. The upstream buffering is now ~200ms and the downstream buffering is ~100ms.

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  • No problems after changing the SSID for Wireless-N connections.
    – user7116
    May 14 '12 at 20:49
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Have you tried changing your wireless channels? Yesterday, I noticed that my VNC connection to another host on my LAN was horrible and when I ping'ed the IP address of my AirPort extreme, it was around 60ms just to get there. I changed it from channel 11 to channel 3 (2.4GHz, wide channels enabled), and my ping times dropped down to 2-3ms. I assume this is due to decreased interference with neighboring APs.

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  • I'm on 3 and the neighboring AP's are all on 11. This happened using 802.11b or 802.11n.
    – user7116
    Mar 30 '12 at 22:02

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