FM can be registered / erased / interrogated by an initiating subscriber A with respect to a remote party B if both parties are provisioned with FM. To enable forced erasure by an FM service supervisor, the FM service shall be provisioned to the FM service supervisor. Additionally, she needs the subscription entitlement to perform the forced erasure. If remote party B is a GSM subscriber and if the forwarding of calls for remote party B to initiating subscriber A is achieved by invoking the Call Forwarding Unconditional Supplementary Service, provision of CFU for remote party B is required.

The initiating subscriber registers the FM feature with respect to a particular remote party. If an initiating subscriber A successfully registers FM with respect to a remote party B then FM becomes registered, active and operative for remote party B. As a result of the registration subsequent calls directed to remote party B are forwarded to initiating subscriber A.

If an initiating subscriber A or the FM service supervisor successfully erases FM with respect to a remote party B then FM becomes not registered and not active for remote party B. For forced erasure by the FM service supervisor the previously registered subscriber shall be informed of the successful forced erasure via a network initiated USSD Notify message with appropriate contents. This message is sent by the FFN. If remote party B is a GSM subscriber and successfully erases FM then FM becomes not registered and not active for remote party B.

If an initiating subscriber A or the FM service supervisor successfully interrogates FM with respect to a remote party B then this procedure interrogates the FM data of subscriber B. If remote party B is a GSM subscriber and successfully interrogates FM then this procedure interrogates her own FM data.

Figure 4.1 shows the Information Flow for the control of FM (registration, erasure, forced erasure and interrogation) by the initiating subscriber. For any control operation on FM, the initiating subscriber (MSa) enters a Follow Me Request (FM-Request). This is a USSD string containing the requested FM operation and the remote number. The Follow Me Request is routed via the MSC/VLR to the HLR of the initiating subscriber (HLRa). The HLRa performs a series of checks as described in the SDLs (clause 4.3). If these checks fail, the MSa receives a response (FM-Response) indicating the error. If the checks pass, the HLRa forwards the operation request (HLR-FM-Request) to the FFN of the remote party (FFNb). FFNb carries out the appropriate control operation and checks as described in the SDLs (clause 4.3) for the remote party. The result of this operation (success or error) is reported back in a USSD Response to the initiating subscriber. For successful forced erasure by a service supervisor, the FFN shall send a Network Initiated USSD notify message with the corresponding USSD string to the HLR of the previously registered subscriber who had registered the Follow Me data. The HLR shall forward the USSD notify to VLR which will relay the USSD Notify towards the MS. Upon receipt of the USSD Notify, the MS shall respond by sending a FACILITY message with empty return result component. An error response with corresponding reason can be returned from any entity, when error happens at the entity TS 23.090.

NOTE 1: NOTE 2:

Control of FM by the remote party is possible if the remote party is a GSM subscriber. The information flow for control of FM by the remote party (erasure and interrogation of her own FM data) is the same as the information flow for control of FM by the initiating subscriber. If a remote party tries to register FM to herself the registration is rejected and an error is reported.

The FM USSD application in HLRa is the process FM_initiating_subscriber_handling_in_HLR (Figure 4.2). It receives the FM-Request from the initiating subscriber. This FM-Request is an USSD-string containing:

• the operation code (register, erase, interrogate);
• the remote number;
• an additional operator specific information field.

The HLR checks:

• the provisioning of FM to the initiating subscriber;
• whether the FFN can be deduced from the remote number;
• whether any operator specific restrictions to engage in FM activity with the remote party apply;
• if the initiating subscriber requires forced erasure, the HLR checks Whether the initiating subscriber is entitled to do it, i.e. Whether the initiating subscriber is a FM service supervisor.

The basic MSISDN of the initiating subscriber is sent together with the original USSD string to the FFN of the remote party. The HLR forwards the response from the FFN to the initiating subscriber. For successful forced erasure by a service supervisor, the HLR of the previously registered subscriber (HLRp) shall relay the USSD Notify to the VLR when the USSD Notify from the FFN is received. The VLR will then forward the USSD Notify towards the MS of the previously registered service subscriber. On receipt of an USSD response from the MS of the previously registered subscriber, the HLRp shall relay it to the FFN.

If the FFN is an HLR, the FFN is responsible for handling the interactions between FM and CFU. Two kinds of request may be received in an FFN which deals with forwarding services:

• CFU requests sent by the VLR for CFU operations (only if the FFN is a HLR);
• FM-HLR-Requests which are USSD strings sent by HLRa for FM operations.

When the control process in the FFN receives a CFU request, it shall either pass the CFU operation request directly to a CFU process or reject it depending on the registration and/or activation states of both FM and CFU services (see Table A.1 for permission checks). On receipt of an HLR-FM request, the control process in the FFN performs a series of FM specific checks and checks the states of both FM and CFU. If the checks are successful, a CFU operation request is sent to a CFU process. On receipt of an HLR-FM-Request from HLRa, the FFN performs a series of checks. e.g.:

• if the remote party is a GSM subscriber:
  • provisioning of FM to the remote party;
  • provisioning of CFU to the remote party;
  • illegal interaction with CFU registered or active to remote party.
• if the remote number is registered in the FFN;
• if any operator specific restrictions to engage in FM activity with the initiating subscriber apply;
• specific checks for forced erasure.

Depending on the requested operation, one of the following procedures is performed:

• registration with implicit Activation (procedure Handle_Remote_Party_Registration, Figure 4.6);
• erasure with implicit Deactivation (procedure Handle_Remote_Party_Erasure, Figure 4.7);
• interrogation (procedure Handle_Remote_Party_Interrogation, Figure 4.8).

For successful forced erasure by a service supervisor, the FFN shall generate an USSD-Notify message and send it to the HLRp, which will relay the USSD Notify towards the MS of the previously registered subscriber (MSp). On receipt of an error response that the USSD Notify message could not be transferred to the MS, the FFN shall check the error code of the response. Depending on the error types and the specific implementation the process shall decide to resend the USSD message after a predefined time. For the resend procedure the process shall start a timer. On timer expiry it shall send the message again. The FFN shall repeat the messages up to 5 times. The length of the timer is defined by operator and has the value in the range of 1 - 10 minutes. Figure 4.3 shows the message flow between the process Forwarding_Service_Control and the processes handling CFU operation requests, defined in TS 23.082.