However, this is implementation specific and the network
supports either implementation.
Online Charging Architecture The online charging function within the IMS depends on
the charging trigger function (CTF) resident in the various elements to provide charging
data to the OCS. Remember that on the front end of the network entity, SIP messages
164 Chapter 7
are being provided with regard to a specific service request. Using our previous example
of a voice call, a media gateway controller (MGC) would send a SIP INVITE to the local
P-CSCF for routing through the IMS. The P-CSCF would then take specific headers from
the SIP INVITE and use this data to create charging data. This data is then sent using
the DIAMETER protocol to the specific address in the SIP P-CHARGING-ADDRESS
headers for the OCS.
The OCS has an OCF used for collecting the billing data. This interface then receives
the DIAMETER message and uses this information to rate the service being provided.
Figure 7.1 Charging interfaces for online and offline charging
CS-NE
Svc-NE
AS
MRFC
MGCF
BGCF
P-CSCF
I-CSCF
GGSN
TPF
SGSN
S-CSCF
WLAN
Charging Rules Function
(CRF)
Application Function (AF)
Online charging system (OCS)
O
C
F
ABMF
RF
C
D
F
C
G
F
Ga
Billing domain Bx Bx
Rf
Wf
CAP
Ro
Ro
Ro
IMS
GWF
ISC
Ro
Gy
Ro
CAP
Wo
Rc
Re
Online Charging Off line Charging
C
T
F
Online and Offline Charging in the IMS 165
The OCF interfaces to the rating function (RF), where the session is rated and the account
balance management function (ABMF), where the subscriber account is verified
for adequate funds (see Figure 7.
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