If you are having connection issues and/or MW3 says you have a moderate router connection, try following this link to set up port-fowarding for Xbox Live and MW3.
Portforward.com for Xbox LiveThis link will help you open all ports required for Xbox & MW3.
Before you start you will need to no a little bit about your network (router ip, internal and external just in case), and your router make and model which should be on a sticker underneath, and finally your router password..
Once you have finished, MW3 should show your router as being open.
These ports are also used for all the other Call of Duty series and all Xbox Live services.
Also check out this site for information on other games that may use different ports and also for other platforms, such as PS3 or PC.
And finally, although you Xbox may be wireless, a wireless connection will never beat wired connection.