What is using my cellular data?

Question

iPad 2, freshly reset, recently updated to iOS 6.1. The only installed app is Skype, which is set to go offline after 60 min. Location services are disabled, cellular data for Appstore ist also disabled.
I now had the iPad lying around for two days and according to my cell bill on both days around 3 am it used the internet, yesterday 15 kB, the day before 140 kB. This is a problem because in my cellular data plan any day where I use cellular data, I have to pay 2,50 €.
I know that I can disable cellular data completely, but I would rather have the iPad just don't use it when it's off (for more than an hour, when Skype shut down).
What can also cause this traffic?
Or are there any apps to block all applications from using cellular data except Safari and Skype?

Answer 1

If the phone is asleep and not connected to an external power source, it will use the cellular data connection for mail, push notifications, etc. This is by design. To keep the WiFi radio on with no external power source connected would drastically reduce battery life. Of course, it appears Safari and App Store are using WiFi, because they can only be used interactively... Mail will use WiFi as well IF the phone is not asleep and is connected to a WiFi network.
If you turn cellular data off, the phone will power up the WiFi radio to poll the mail server since there is no other option.
You can set mail to retrieve message manually instead of using push or polling. This will ensure that pulling down your mail can only be initiated by you, and if you make sure you have a live WiFi connection before doing so, it shouldn't be a problem.
Since you've seen a sharp increase in the last month, I'm assuming you're either getting much more mail than you had been previously, or you've been getting more 'large' messages or both.
I don't know what kind of data usage you're used to seeing or what you consider a sharp increase. Is it pushing you over your cap?