Tariffs usually cap you to so many GB of data.

Does anyone know if this is the mobile data you receive, or send, or both ?

Does anyone know how to measure this volume, in real-time ?

 

My Data Manager is a very useful app to monitor mobile data usage on Android devices. You can use this App to see real time data transfer.

 

Data caps are on all data, both receive and send....
Some phones allow you to set warning limits on the "phone measured" data usage, which may vary with what your phone company measures (by a bit).