User Plane and Control Plane
The Mobile Core is partitioned into a Control Plane and User Plane, an architectural feature known as Control and User Plane Separation (CUPS) from 3GPP.
Think of the control plane (CP) as everything related to authentication. User Plane (UP) takes care of exchanging the actual core data (like the voice messages, sms data, etc.) and connecting to the internet. See [[4G Mobile Core]] to see diagram. 
Traditionally, UP and CP have different .
They still end up in the same place. However, we can layer the network as hardware gets cheaper.
Relationship is no longer 1 to 1.