Unknown error code 1651

hi i have an iphone 4 with the latest OS in it and the latest itunes downloaded in my laptopn windows as well.

recently i am having some prob which i never faced before. once i connect the iphone it syncs automatically that means it takes the first back up and then suppose to sync it but what happens now is....it backs up and stops halfway ..........u disconnect the phone it says there showing backing up halfway,,.......but then i tried checking on updates and tried clicking on the check for updates from itunes and i was given this message...."itunes could not check for an updat to the carrier settings for your iphone . an unknown error has occured (1651)...make sure your network settings are correct and your network connection is active or try again later.

so that means i cannot sync fully and no updates i am confused what is wrong with it?

pls suggest or help

thank you