Q: mountain lion wifi problem

Look to the Right, More Like this. You are not alone.

I changed my MTU to 1300 on the WiFi network and it solved my problem.  Please try it and let us know if that solved it for you.

Go to System Preferences -> Network -> Advanced -> Hardware

check to Manually, MTU: 1300

Check the signal level and noise level using this cool tool you already have:

http://www.macworld.com/article/1162117/monitor_wi_fi_with_lions_hidden_tool.htm l

I had the same problem.  I upgraded from Snow Leopard to Mountain Lion and immediately had Wifi problems on my iMac.  It would connect but then drop out - sometimes within a couple of minutes, other times lasting maybe half an hour or so.  I kept having to use Network Preferences > Diagnostics to reconnect.  Other devices (MBP running Snow Leopard and iPhones etc have had no problems).

POSSIBLE SOLUTION: I have an old router (BT Home Hub v1.0) and was running WEP security.  I have switched it over to WPA2 protocol and so far no problems.  Wifi has been connected now for a few hours without dropping.

I had previously tried the MTU 1300 change (above) but that didn't fix it for me.

I have same problem too.. I change the rounter wifi security to WEP (I was using WPA2), then the problem is fixed... but hey, Apple, this is serious bug... Not everywhere is using WEP... if I am in airport, i can't change their rounter setting... Please fix it

One other possible work around that is coming forward is to adjust the MTU size to smaller than 1500 default size. 1453 is proffered as a number that seems to work.

System preferences > network > WiFi/Airport > Advanced > Ethernet >  Manually > MTU: [Custom:] 1453 

Thanks, 1300 seems to be working for me (I'm 10 minutes into the change).  I had previously tried 1453 but that didn't work for me.

I previously posted that changing MTU to 1300 fixed the problem for me, but I changed BACK to MTU=1500 and my system is still "fixed": wi-fi performance is at same level as it was pre-10.8 upgrade. (My original problem was that wi-fi speed drastically decreased after 10.8 upgrade.  Handheld devices on same wi-fi network worked fine.)

I got suspicious when I read someone's post that said that they switched to WEP from WPA2 and that "fixed" their problem.  I have been using WPA2 all along. 

I now suspect that something in the process of MAKING a change causes some sort of reset and that is the actual fix.  But not all changes that I tried worked. 

Somewhere along this process I stumbled on a "fix" for my system:

1.  A few days ago cycled wi-fi off/on (that worked for someone else in another thread). No joy.  Didn't work.

2.  A few days ago changed MTU from 1500 to 1453. No joy.

3.  Turned wi-fi off.  Used ethernet hardline to wireless router for a few days.  Everything fine - bypassed problem.

4.  Today, created new wi-fi network.  Didn't end up using it.  Using original wi-fi network, changed MTU from 1453 to 1300.  Speedtest nominal.  "Fixed" problem.

5.  Less than 1/2 hour later changed MTU from 1300 back to 1500 (which was working fine pre-10.8 upgrade).  Speedtest nominal.  System still "fixed".

Again, I suspect some sort of reset occurred causing internal wi-fi hardware, driver, and osx 10.8 to play nice with each other.  Someone smarter than me will have to figure this out and explain it.

Your experience supports the theory that there are old preference files that are being mis-handled by the new software. Another "blow them all away and make new ones" suggestion that accomplishes this is to use:

System Preferences > Network > Airport/WiFi > ( Assist Me ) > Assistant ...

... and set it up again from scratch, including a NEW "Location"

Hey, I have the same problem here since I upgraded to Lion - which is no fun at all. Because sometimes you can't just change the router or have access to the settings. Try especially at work to explain the network admin that he has to change the router because your mac doesn't work (while everything else is working fine).

Well...

Adding a new Location didn't work for me. After that I had the same problems. I am connected to the router but after 2-10 minutes no internet signal is coming in anymore. I could have access to the router though. 

I will try the MTU settings. But does it have a big effect on the speed?

Another friend has the same problem since he updated to Lion. He even can't use his Wifi at home anymore and can only connect to the router.

I hope Apple is going to have a solution...

Interesting.  I am running Mountain Lion 10.8, on an rMBP, and was unable to connect to an older wifi network *at all*.  I changed it to MTU 1300, and voila, connected.  After reading your post, I changed it back to MTU 1500 (Automatically, after cycling through and making sure I was back to 1500 Manually first, saving and Applying between each change.)

Still works!

I happened to run wifi diagnostics *before* I did this, so if anyone at Apple reads this and wants me to forward those, I'm happy to (let me know if you want me to run them again, now that it's working)...

Anyway, might be useful to others until Apple figures out how to fix this issue.