The VLR is the location and management data base for the mobile subscribers roaming in the area controlled by the associated MSC(s). Whenever the MSC needs data related to a given mobile station currently located in its area, it interrogates the VLR.
When a mobile station initiates a location updating procedure with an MSC, the MSC informs its VLR which stores the relevant information. This procedure occurs whenever an MS roams to another location area.
Also, when a subscriber activates a specific supplementary service or modifies some data attached to a service, the MSC informs (via the VLR) the HLR which stores these modifications and updates the VLR if required.
This interface is internal to the MSC server /VLR; signalling on it is not standardised.
By definition, dealing with CDR files only implies that Bx is solely related to offline charging.
The CGF in each network domain, service or subsystem is relevant for the network side of
the Bx reference point and is connected with the Billing Domain that forms the receiving end of the Bx reference point.
Further details of the Billing Domain, i.e. beyond terminating Bx, are outside the scope of 3GPP standardization.
Note that the OCS can also generate CDRs and transfer them to the BD across the Bo reference point.
Furthermore the GSM SCF, defined in CAMEL, can also generate CDRs and transfer them to the BD across the Bs