Page 82 - From GMS to LTE

68 From GSM to LTE-Advanced Pro and 5G

triggered a DP, the SCP is able to influence the way the call should proceed. The SCP
can take the call down, change the number that was called or return information to the
MSC, which is put into the billing record of the call for later analysis on the billing
system.
For the prepaid service, for example, the CAMEL protocol can be used between the
MSC and the SCP as follows.
If a subscriber wants to establish a call, the MSC detects during the setup of the call
that the ‘authorize origination’ DP is activated in the subscriber’s HLR entry. Therefore,
the MSC sends a message to the SCP and waits for a reply. As the message contains the
IMSI of the subscriber as well as the CAMEL service number, the SCP recognizes that
the request is for a prepaid subscriber. By using the destination number, the current
time and other information, the SCP calculates the price per minute for the connection.
If the subscriber’s balance is sufficient, then the SCP allows the call to proceed and
informs the MSC about the duration for which the authorization is valid. The MSC then
continues and connects the call. At the end of the call, the MSC sends another message
to the SCP to inform it of the total duration of the call. The SCP then modifies the
subscriber’s balance. If the time that the SCP initially granted for the call expires, the
MSC has to contact the SCP again. The SCP then has the possibility of sending an
additional authorization to the MSC which is again limited to a determined duration.
Other options for the SCP to react are to send a reply in which the MSC is asked to
terminate the call or to return a message in which the MSC is asked to play a tone as an
indication to the user that the balance on the account is almost depleted.

Initial state Error condition
Originator terminates call Outgoing call

authorized
by the SCP

Call establishment Busy
No reply
Error

Call is accepted

Call is Error
terminated Active call

Figure 1.54 Simplified state model for an originator (O‐BCSM) according to 3GPP TS 23.078 [37].