I just installed Ubuntu on my mid 2012 MacBook Air. Everything worked out of the box, but the wifi is showing some weird behavior.

When I first login it's really fast, loading google.com is near instant, and browsing in general feels at least as smooth as it did on Mac OS. However, after a couple minutes the connection slows down dramatically, sometimes it takes over 5s to load google.com, a simple reboot fixes the problem for another couple minutes.

Specs:

• Wifi: 02:00.0 Network controller: Broadcom Corporation BCM43224 802.11a/b/g/n (rev 01)
• Driver: open-source brcmsmac driver
• Kernel: Linux wega 3.8.0-21-generic #32-Ubuntu SMP Tue May 14 22:16:46 UTC 2013 x86_64 x86_64 x86_64 GNU/Linux
• Distro: Ubuntu 13.04 (uptodate)

I tried a number of things, none of which actually helped

• Use proprietary sta driver from broadcom
• Installed firmware into /lib/firmware/brcms (which, as far as I can tell from logs, does not get loaded at all)
• Switch router to only use 2.4 OR 5 GHz
• Set router to only use a OR g OR n
• Set router to use AES encryption only
• Turned off power management on the adapter
• Set regulatory region to the correct value (NL) on both router and laptop
• Disable ipv6

Nothing seems to help, the slowdown always occurs. I did notice that the latency (ping google.com) stays roughly the same (around 9ms).

Below is some more information that might be of use.

$ lspci -nnk | grep -iA2 net
02:00.0 Network controller [0280]: Broadcom Corporation BCM43224 802.11a/b/g/n [14e4:4353] (rev 01)
    Subsystem: Apple Inc. Device [106b:00e9]
    Kernel driver in use: bcma-pci-bridge

$ rfkill list
0: hci0: Bluetooth
    Soft blocked: no
    Hard blocked: no
1: phy0: Wireless LAN
    Soft blocked: no
    Hard blocked: no

$ lsmod
Module                  Size  Used by
dm_crypt               22820  1 
arc4                   12615  2 
brcmsmac              550698  0 
coretemp               13355  0 
kvm_intel             132891  0 
parport_pc             28152  0 
kvm                   443165  1 kvm_intel
ppdev                  17073  0 
cordic                 12574  1 brcmsmac
brcmutil               14755  1 brcmsmac
mac80211              606457  1 brcmsmac
cfg80211              510937  2 brcmsmac,mac80211
bnep                   18036  2 
rfcomm                 42641  12 
joydev                 17377  0 
applesmc               19353  0 
input_polldev          13896  1 applesmc
snd_hda_codec_hdmi     36913  1 
microcode              22881  0 
snd_hda_codec_cirrus    23829  1 
nls_iso8859_1          12713  1 
uvcvideo               80847  0 
btusb                  22474  0 
snd_hda_intel          39619  3 
videobuf2_vmalloc      13056  1 uvcvideo
snd_hda_codec         136453  3 snd_hda_codec_hdmi,snd_hda_intel,snd_hda_codec_cirrus
bcm5974                17347  0 
bluetooth             228619  22 bnep,btusb,rfcomm
snd_hwdep              13602  1 snd_hda_codec
lpc_ich                17061  0 
videobuf2_memops       13202  1 videobuf2_vmalloc
videobuf2_core         40513  1 uvcvideo
videodev              129260  2 uvcvideo,videobuf2_core
bcma                   41051  1 brcmsmac
snd_pcm                97451  3 snd_hda_codec_hdmi,snd_hda_codec,snd_hda_intel
snd_page_alloc         18710  2 snd_pcm,snd_hda_intel
snd_seq_midi           13324  0 
snd_seq_midi_event     14899  1 snd_seq_midi
snd_rawmidi            30180  1 snd_seq_midi
snd_seq                61554  2 snd_seq_midi_event,snd_seq_midi
snd_seq_device         14497  3 snd_seq,snd_rawmidi,snd_seq_midi
snd_timer              29425  2 snd_pcm,snd_seq
snd                    68876  16 snd_hwdep,snd_timer,snd_hda_codec_hdmi,snd_pcm,snd_seq,snd_rawmidi,snd_hda_codec,snd_hda_intel,snd_seq_device,snd_hda_codec_cirrus
mei                    41158  0 
soundcore              12680  1 snd
apple_bl               13673  0 
mac_hid                13205  0 
lp                     17759  0 
parport                46345  3 lp,ppdev,parport_pc
usb_storage            57204  0 
hid_apple              13237  0 
hid_generic            12540  0 
ghash_clmulni_intel    13259  0 
aesni_intel            55399  399 
aes_x86_64             17255  1 aesni_intel
xts                    12885  1 aesni_intel
lrw                    13257  1 aesni_intel
gf128mul               14951  2 lrw,xts
ablk_helper            13597  1 aesni_intel
cryptd                 20373  4 ghash_clmulni_intel,aesni_intel,ablk_helper
i915                  600351  3 
ahci                   25731  3 
libahci                31364  1 ahci
video                  19390  1 i915
i2c_algo_bit           13413  1 i915
drm_kms_helper         49394  1 i915
usbhid                 47074  0 
drm                   286313  4 i915,drm_kms_helper
hid                   101002  3 hid_generic,usbhid,hid_apple

$ dmesg | egrep 'b43|bcma|brcm|[F]irm'
[    0.055025] [Firmware Bug]: ioapic 2 has no mapping iommu, interrupt remapping will be disabled
[    0.152336] [Firmware Bug]: ACPI: BIOS _OSI(Linux) query ignored
[    2.187681] pci_root PNP0A08:00: [Firmware Info]: MMCONFIG for domain 0000 [bus 00-99] only partially covers this bridge
[   12.553600] bcma-pci-bridge 0000:02:00.0: enabling device (0000 -> 0002)
[   12.553657] bcma: bus0: Found chip with id 0xA8D8, rev 0x01 and package 0x08
[   12.553688] bcma: bus0: Core 0 found: ChipCommon (manuf 0x4BF, id 0x800, rev 0x22, class 0x0)
[   12.553715] bcma: bus0: Core 1 found: IEEE 802.11 (manuf 0x4BF, id 0x812, rev 0x17, class 0x0)
[   12.553764] bcma: bus0: Core 2 found: PCIe (manuf 0x4BF, id 0x820, rev 0x0F, class 0x0)
[   12.605777] bcma: bus0: Bus registered
[   12.852925] brcmsmac bcma0:0: mfg 4bf core 812 rev 23 class 0 irq 17
[   13.085176] brcmsmac bcma0:0: brcms_ops_bss_info_changed: qos enabled: false (implement)
[   13.085186] brcmsmac bcma0:0: brcms_ops_config: change power-save mode: false (implement)
[   20.862617] brcmsmac bcma0:0: brcmsmac: brcms_ops_bss_info_changed: associated
[   20.862622] brcmsmac bcma0:0: brcms_ops_bss_info_changed: arp filtering: enabled true, count 0 (implement)
[   20.862625] brcmsmac bcma0:0: brcms_ops_bss_info_changed: qos enabled: true (implement)
[   20.897957] brcmsmac bcma0:0: brcms_ops_bss_info_changed: arp filtering: enabled true, count 1 (implement)


$ iwconfig
lo        no wireless extensions.

wlan0     IEEE 802.11abgn  ESSID:"wlan"  
          Mode:Managed  Frequency:5.22 GHz  Access Point: E0:46:9A:4E:63:9A   
          Bit Rate=65 Mb/s   Tx-Power=17 dBm   
          Retry  long limit:7   RTS thr:off   Fragment thr:off
          Power Management:off
          Link Quality=63/70  Signal level=-47 dBm  
          Rx invalid nwid:0  Rx invalid crypt:0  Rx invalid frag:0
          Tx excessive retries:13  Invalid misc:56   Missed beacon:0