Windows 8.1 phone has a useful feature of restricting data usage when using mobile network.
What Data Sense basically does is help in setting a data usage limit both in terms of amount of data and schedule. Also, this is a great way to keep track of how much cellular data is being used.
To enable it, first go to Settings.  Then scroll down to “data sense”.
Tap on Set Limit. Also, if needed, turn on the slider for settings “Restrict background data usage” and “Restrict background data usage when roaming”.
Next will be a configuration screen where data amount as well as limit type can be selected. Also, the date when this will be resetted can be selected.
Tap the tick mark icon for these changes to take place.
This is a useful way to limit those large data usage charges!