I've got a 2820n running 3.3.4.1 and am trying to get QoS configured so my family's internet browsing and downloads don't get in the way of online gaming - i.e. they can browse / download but the gaming traffic is prioritised. It doesn't seem to be working. All but one of the devices in this network are WiFi connected (and the Xbox is too).

Here's what I've done so far:
1. Set the Xbox to have the same DHCP address.
2. Set up a class for Xbox live gaming, listing what I've googled as being the relevant TCP and UDP ports (TCP/UDP 3074, UDP 88, TCP/UDP 53, and TCP/80). The class is restricted to the IP of the Xbox.
3. Set up a class for the rest of the IP address range.
4. Enabled QoS, and set class 1 (Xbox) to get 65% of the bandwidth (overkill, yes, and I can probably tune it down), class 2 (DHCP) to 33%.
5. Unticked "Enable UDP Bandwidth Control" and ticked "Outbound TCP ACK Prioritize".

I still get issues with download traffic interfering with the Xbox traffic - games like CoD get stuttery movement.

What am I doing wrong ? Or am I misguided in what I think the unit can do ?

Check the QoS status for WAN1 and you should see whether it's being classified correctly - by far the easiest way to classify QoS for the Xbox would be to put it's IP into one of the class rules (#1 in your case) but without specifying the ports.

In my original setup I had the Xbox IP address set via IP-to-MAC binding, and that was included in the class.

Still no joy. My wife was downloading iOS 4.3 at the same time as I was trying to play, and it was an abysmal experience for me.

If QoS works then surely I'm doing something wrong in the config ? Or am I doing something wrong in assuming QoS works ?

Is that for ADSL or WAN2? With WAN2 you've got to set the speeds to around 80-90% of the full speed of the line to get QoS to work correctly. The ADSL will work that out automatically though for QoS.

When the phone caused your Xbox to slow down, did you check the QoS stats for anything showing as Other when it shouldnt? Alternatively, maybe it's a wireless problem if they're both wireless, make sure to use WPA2 security for the full 802.11n speed and try testing the connection out with the wireless set to 802.11G mode or 802.11n mode specifically (don't do this if the devices you're testing arent 11n though) and see if that improves things?
TBH I think it's wireless because an iphone really can't eat up much bandwidth but if both are on wireless and the router is having to switch from 802.11n to 802.11g, that'll screw things up.

QoS is set up on WAN1 - i.e. the ADSL, and it's correctly reported the line rate as compared to the previous ADSL router.

It wasn't the iPhone downloading iOS 4.3, it was the wireless iMac - and the Xbox is on the same wireless network.

I'll force the WLAN to 802.11N and see what effect that has - thanks.