Tuesday, October 27, 2009

Snow Leopard, Time Capsule and Network Issues

For a few weeks now I have been seeing weird network behavior, but only at home and only on my Macs. Both Macs connect to a WRT150N wireless router running DD-WRT VPN firmware. As described in an earlier post (its early and I am too lazy to link) I have set up a fake TimeCapsule on an Ubuntu box sitting on my home network for net-based backups. Everything was working fine until a few weeks ago...

Suddenly when attempting to move large amounts of data across the network, such as when backing up, my network connection would drop. My connection to the network remained, but throughput went to zero on the local network. Connectivity was completely lost. Many times I couldn't even reconnect to the wireless network to reestablish connectivity. However, if I used a hard-wired connection I didn't see the same problem. Ultimately this corrupted my TimeCapsule data twice in 2 weeks.

After some frantic Googling I found somewhere which indicated that the problem may have to do with auto channel switching in 802.11N networks. So last night I configured my wireless network to choose a specific channel (8, FWIW) instead of allowing it to automatically choose the best channel. My network problems appear to have now been resolved, no more loss of connectivity over night or this morning. I created a local backup on a USB disk overnight and I am now recreating my online backup to the Ubuntu TimeCapsule as I write.

This seems to be an unacknowledged bug in most recent versions of MacOS Snow Leopard. Apple, I hope you're listening.

7 comments:

Cameron said...

Alot of devices share spectrum with 802.11 devices. Could be a new device in your home has been interfering just enough to make the base constantly seek new channels. Among devices overlapping or adjacent to that frequency are - cordless phones, bluetooth, wireless USB, certain wireless video transmitters, and (apparently) microwave ovens. I wonder if you introduced some new non-802.11 electronic device in your home 2 weeks ago that triggered this?

dhs said...

Cam, no, I haven't. This seems to be a common issue right now with Snow Leopard installations.

dhs said...

Of course I spoke too soon and I am still seeing problems. Time to fire up wireshark and see what's happening. Now I am not sure its the channel issue at all...

Jake said...

My Coworker is having the same issue.
He is going to try this fix later tonight when he gets home. I will let you know if it works.

http://www.andrewgrant.org/2009/09/22/fix-for-broken-time-machine-backups-under-snow-leopard.html

dhs said...

Thanks Jake. That won't help me because I am too cheap to buy a TimeCapsule and use an Ubuntu box as described here: http://www.fullfrontalnerdity.com/2008_12_01_archive.html

I spent a few minutes watching network traffic this morning and now I think there may be a problem with AFP over TCP in 10.6.1, but more research is needed. Non-AFP over TCP traffic seems to be just fine...

Jake said...

I read a post that the Mac OS X Server 10.6 was having issues with AFP.

Here is the site.
http://www.macwindows.com/snowleopard-filesharing.html

It might shed some light on things.

dhs said...

A bit more digging tonight reveals that the problem is for any bulk data transfer over the local network using wireless (802.11n). If I switch to use 802.11g, disabling 802.11n, things work just fine. So it appears that there is some issue in the 10.6.1 network stack for 802.11n. Digging into configuration of the router now to see if I can figure out any configuration that works.