diego's weblog

there and back again

linksys wrt350n 802.11n mixed mode causes mac os x crashes

A couple of weeks ago I wrote about my macbook crashing (or rather, freezing) constantly. Sometimes it would freeze right after booting, sometimes later, sometimes when you left it alone for a while, sometimes when waking up from sleep.

At that point I reinstalled OS X from scratch and hoped that was it. Alas, it wasn’t. Very soon afterwards the Mac started freezing again, with a vengeance. In fact, the next day after the fix (a Monday) my Mac Pro at the office also went out to lunch and the 10.4.8 update had to be reinstalled from scratch through SSH. I thought I was cursed!

Then the Mac Pro came to life and I continued with the Mac entertainment at home. The poor thing would lock up like there was no tomorrow. I started to be paranoid about saving files again, something I hadn’t done since switching over from Windows a few months ago, and I was also getting more than annoyed.

Then it hit me: the Mac was only freezing at home. At the office, it had never frozen. Not once. It was only at home that this was happening.

I had already ruled out software: the Mac Pro had exactly the same config, and the freezes happened also on a different Macbook with the same disk. Disk checks showed the disk was fine.

So it had to be environmental.

And what was the only difference between home and the office? That’s right the router. In February or so I got a Linksys WRT-350N to use 802.11n, which even if it’s not “official” sounded (from reading the IEEE proceedings of meetings — yeah, I do that sometimes, I’m that crazy) like it was not going to experience major changes and so was a relatively safe bet. I had been using 802.11n and all was well.

But that was the only difference with the office — I’ve been using the same encryption, WPA-PSK, so that wasn’t it. It had to be 802.11n.

But could a wireless protocol, however badly implemented or incompatible, irreparably hang the machine?

Apparently, yes.

To test the theory, last Sunday I changed the configuration of my router to 802.11g, and since then I’ve been running the Mac at home with no problems whatsoever, making it sleep, not sleep, disable the display after a while, and so on, with no problems at all.

Turn on Mixed mode in the router (802.11n+g) and the thing locks up again so fast it makes your head spin. Turn on pure 802.11n and OS X seems to whitstand it better, although I had one weird situation with it that I attribute to that as well.
OS X doesn’t really let you choose the wireless protocol you’re using, at least not in the base options that I’ve seen. So the Macbook must be going bananas when trying to decide what to do in Mixed Mode, as well as sometimes in pure 802.11n mode (perhaps, I have to confirm this fully).

Anyway, if you have a Macbook or Mac with 802.11n (and the software update for it installed) and you’re experiencing weird lockups, take a look at your wireless router and see if it’s on Mixed mode. That may be the source of the problems.


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