03.353GPPImmediate Service Termination (IST)Stage 2TS
This clause describes the implementation of IST using CAMEL. CAMEL can be used to terminate all the mobile originated (MO), mobile terminated (MT) and forwarded (CF) calls of a subscriber, provided there is a control relationship between the CAMEL server (the gsmSCF) in the HPLMN and the MSC (visited MSC or GMSC) (the gsmSSF) controlling the call or forwarding leg.
5.1 Subscriber settings
The subscriber is marked as a CAMEL subscriber by setting Originating CAMEL Subscription Information (O‑CSI) and Terminating CAMEL Subscription Information (T-CSI) in the subscriber data stored in the HLR of the HPLMN. The O‑CSI is sent to the VPLMN when the subscriber first registers in the VPLMN; the T-CSI is sent to the GMSC in the response to a request for routeing information. If the subscriber is being monitored using FIGS, it will already be marked as a CAMEL subscriber.
If the HPLMN wishes to mark a subscriber as a CAMEL subscriber when the subscriber is already registered in the VPLMN, it modifies the subscriber data in the VPLMN using the command Insert Subscriber Data.
5.2 DP Settings
A call cannot be terminated using CAMEL unless there is a control relationship between the gsmSCF and the gsmSSF controlling the call. To ensure that the IST command can be used at any point in the duration of a call, right up to the end of the call, there must be a control relationship until the end of the call.
A "control relationship" exists where there is at least one armed DP in the gsmSSF. This can be achieved if the DP O/T_Disconnect (which will trigger the sending of an Event_Report_BCSM to the gsmSCF) is set for the call.
If the subscriber is being monitored using FIGS level 2 or 3, the subscriber will already be marked as a CAMEL subscriber and DP O/T_Disconnect will already have been set.
If the subscriber is not being monitored using FIGS levels 2 or 3 then O/T_Disconnect is set with command Request_Report_BCSM_Event sent to the gsmSSF by the gsmSCF after the gsmSCF has received notification of a call attempt via the Initial_DP message received from the gsmSSF.
Initial_DP notifies the gsmSCF of call attempts. If the gsmSCF wishes to be notified of the success or failure of a call attempt, and so not be keeping a register of "calls" that may not have commenced, DP O/T_Answer can be set to inform the gsmSCF when a call is answered by the calling party.
If the non-reception of an Event_Report_BCSM indicating call answer is not sufficient indication to the HPLMN of the failure of a call attempt, DPs O_Route_Select_Failure, O/T_Busy, O/T_No_Answer, O/T_Abandon, O/T_Not_Reachable can be set to inform the gsmSCF explicitly if the call attempt fails. If the various failure DPs are not armed, the gsmSCF can still deduce that the call attempt has failed because the gsmSSF will terminate the relationship when the call fails by sending message Abort to the gsmSCF.
All these DPs can be set via the Request_Report_BCSM_Event sent to the gsmSSF by the gsmSCF after receiving the InitialDP message. If all these DPs are set then the subscriber is in fact being monitored with the equivalent of FIGS level 2 monitoring.
5.3 Call termination
The HPLMN will be informed of the call attempts (MO, MT and CF) of the subscriber with message InitialDP. This message will give the address of the gsmSSF (=MSC) controlling the call. The call can be terminated by the gsmSSF at any time by sending message ReleaseCall to the controlling gsmSSF. The gsmSSF will then terminate the call.