IMS network also support offline and online charging.
As per my understanding of IMS, IMS network nodes take care of charging.
There are following three types of charging:
- Event based charging
- Bearer level charging
- Session level charging.
Proxy-Call Session Control Function (P-CSCF) takes care of offline charging. Other nodes of IMS such as I-CSCF, S-CSCF may send offline charging data records to charging domain.
I am just guessing, the Bearer level charging records sent by PGW to charging domain since VoLTE traffic carried out by QCI - 1 bearer. Please correct me on my this thought if I am wrong.