Came across an interesting problem last week. Paid Apple a few pounds for the Wireless N firmware upgrade for my MacBook Pro. Then a few days after I noticed that my iPhone was unable to sync with my GTD app Things on the laptop over wireless.
Then I tried using the Apple Remote app, for controlling iTunes. Again nothing.
Yet both the laptop and iPhone were clearly connected to the wireless network, and both had communications with the Internet, and could communicate directly with each other (I tested browsing to a website I have running on the laptop using the iPhone and all was well.)
After much head scratching I decide to turn the router (O2 Wireless Box III) back to B/G wireless mode only. BAM! Everything back to normal. Switched back to B/G/N mode again and sure enough comms were lost.
It seems that the bridge between the N clients and G clients blocks multicast traffic. This was a known issue on the older O2 routers between the wireless and wired networks, but I never thought the seperate channels of wireless would be so widely separated!
So stick to B/G mode for now, I am going to contact O2 and also Thomson who make the modem/router for O2 and see if there is a workaround. I liked having 130Mbps link to the network!!
#1 by Matt Smart on June 21, 2009 - 10:54 am
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Hi! Just thought I’d say, thanks so much for this. I had no idea that the b/g/n thing on the wireless box III would affect multicast, and couldn’t work out why Firefly wasn’t being picked up in iTunes. Changed this setting and it worked immediately.
You’ve saved me a lot of hassle, and I’ll be sure to link back to you (when my blog actually appears). Thank you!
Matt
#2 by Allan Clark on October 13, 2009 - 8:17 pm
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Hey, to “AOL!” Matt, me too, me too. Thanks for posting the fix, I just did the same with my Thompson TG582 AKA “O2 Wireless III” with HP Photosmart C6380 use of MDNS to find things. MDNS requires broadcast, but both laptop and printer being opposite G/N channels, were not finding each other. Dropped N, (http://Administrator@o2wireless.lan/, “Home Network”, select the WLAN, “Configure”, “Interface Type”, Save) works like a charm.
Yeah, I added some searchable keywords in this comment, maybe it’ll help another O2 victim find it faster
Funny, all I can do to help is try to get people to your good work faster 
Thanks!
#3 by Allan Clark on October 13, 2009 - 8:22 pm
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Also found http://forum.o2.co.uk/viewtopic.php?p=90489 which might describe a way to deactivate any kind of filtering.