4.3.3 Terminating side

23.1723GPPRelease 17Stage 2Technical realization of Circuit Switched (CS) multimedia service UDI/RDI fallback and service modificationTS

Tools: ARFCN - Frequency Conversion for 5G NR/LTE/UMTS/GSM

4.3.3.1 HLR Interrogation

The GMSC sends the Send Routing Information request with both the Network Signal Information and Network Signal Information 2 parameters. The Network Signal Information shall include the ISDN BC IE for the preferred service, and the Network Signal Information 2 includes the ISDN BC IE for the less preferred service.

The functional behaviour of the HLR is described in 3GPP TS 23.018 [10]. The procedures specific to SCUDIF calls are specified in the subclause 4.3.3.1.3 for the HLR and 4.3.3.1.4 for the GMSC. The information elements specific to SCUDIF between the GMSC and the HLR are specified in subclauses 4.3.3.1.1 and 4.3.3.1.2.

4.3.3.1.1 Send Routing Info

The following element specific to SCUDIF calls is defined for Send Routing Info:

Information element name	Required	Description
ISDN BC 2	C	ISDN bearer capability 2. Shall be present for a SCUDIF call to indicate the less preferred service.
ISDN LLC 2	C	ISDN lower layer compatibility 2. Shall be present for a SCUDIF call if necessary for the less preferred service, otherwise shall be absent.
ISDN HLC 2	C	ISDN higher layer compatibility 2. Shall be present for a SCUDIF call if necessary for the less preferred service, otherwise shall be absent.

4.3.3.1.2 Send Routing Info Ack

The following elements specific to SCUDIF calls are defined for Send Routing Info Ack:

Information element name	Required	Description
Roaming number 2	C	E.164 number required to route the call to VMSCB (see 3GPP TS 23.003 [11]) for the less preferred service of a SCUDIF call. Shall be present for a SCUDIF call if the preferred service is not forwarded (i.e. either Roaming Number is present or the preferred service is not allowed) and the less preferred service is allowed and not forwarded, otherwise shall be absent.
Forwarded-to number 2	C	E.164 number of the C subscriber for the less preferred service of a SCUDIF call. Shall be present if the HLR has determined that the less preferred service of a SCUDIF call is to be forwarded, otherwise shall be absent.
Long forwarded-to number 2	C	Number of the C subscriber (see 3GPP TS 23.082 [15]) for the less preferred service of a SCUDIF call. Shall be present if the HLR has determined that the less preferred service of a SCUDIF call is to be forwarded, and GMSC and HLR support Long Forwarded-to Numbers, otherwise shall be absent.
Forwarded-to subaddress 2	C	Subaddress of the C subscriber (see 3GPP TS 23.003 [11]) for the less preferred service of a SCUDIF call. Shall be present if the HLR has determined that the less preferred service of a SCUDIF call is to be forwarded and a forwarded-to subaddress is stored in the HLR in association with the forwarded-to number 2, otherwise shall be absent.
Notification to calling party 2	C	Indication of whether the calling party is to be notified that the call has been forwarded if the less preferred service of a SCUDIF call is kept. Shall be present if the HLR has determined that the less preferred service of a SCUDIF call is to be forwarded, otherwise shall be absent.
Forwarding reason 2	C	Indication of why the call has been forwarded (unconditionally or on mobile subscriber not reachable) for the less preferred service of a SCUDIF call. Shall be present if the HLR has determined that the less preferred service of a SCUDIF call is to be forwarded, otherwise shall be absent.
SS-List 2	C	List of SS-codes for the less preferred service of a SCUDIF call. Shall be present if the HLR has determined that the less preferred service of a SCUDIF call is allowed, otherwise shall be absent.
Basic Service Code 2	C	Indicates the type of the basic service for the less preferred service, i.e. teleservice or bearer service.
Allowed Services	C	Shall be present for SCUDIF calls. Indicates which services are available for that call.
Unavailability Cause	C	Indicates the reason for which one of the services of a SCUDIF call is not available. Shall be present for SCUDIF calls if one service is not available.

4.3.3.1.3 Handling of mobile terminating calls in the HLR

The procedures specific to SCUDIF calls in the HLR are specified within this subclause:

– Procedure SCUDIF_Subscription_Check_HLR

– Procedure SCUDIF_CAMEL_CSI_Check_HLR

– Procedure SCUDIF_Set_Second_Service_when_Forwarded

– Procedure SCUDIF_Set_Correct_PLMN_BC

– Procedure SCUDIF_Check_Second_Service_before_Negative_Response

– Procedure SCUDIF_Check_Second_Service_after_PRN

– Procedure SCUDIF_Check_Second_Service_when_Forwarded

Figure 4.16A: Procedure SCUDIF_Subscription_Check_HLR (sheet 1)

Figure 4.16B: Procedure SCUDIF_Subscription_Check_HLR (sheet 2)

Figure 4.16C: Procedure SCUDIF_CAMEL_CSI_Check_HLR

Figure 4.16D: Procedure SCUDIF_Set_Second_Service_when_Forwarded

Figure 4.16E: Procedure SCUDIF_Set_Correct_PLMN_BC

Figure 4.16F: Procedure SCUDIF_Check_Second_Service_before_Negative_Response

Figure 4.16G: Procedure SCUDIF_Check_Second_Service_after_PRN

Figure 4.16H: Procedure SCUDIF_Check_Second_Service_when_Forwarded

4.3.3.1.4 Handling of mobile terminating calls in the GMSC

The procedures specific to SCUDIF calls in the GMSC are specified within this subclause:

– Procedure SCUDIF_Negative_SRI_Response_Handling

– Procedure SCUDIF_Check_Service_Availability

– Procedure SCUDIF_Check_Service_Compatibility

Figure 4.16I: Procedure SCUDIF_Negative_SRI_Response_Handling

Figure 4.16J: Procedure SCUDIF_Check_Service_Availability

Figure 4.16K: Procedure SCUDIF_Check_Service_Compatibility

4.3.3.1a GMSC Handling at Terminating Side

Depending on operator policy, the GMSC may remove the 3G-324.M2 codec from the list of supported codec types if the call is received from selected other PLMNs.

NOTE: This enables the operator to block a network initiated upgrade if inter-operator accounting agreements do not allow this functionality, e.g. because billing schemes are not compatible for the network-initiated upgrade.

4.3.3.2 Terminating MSC Handling

The terminating MSC receives the list of supported codec types, including the 3G-324.M codec and possibly the 3G-324.M2 codec. It shall then send a SETUP message towards the terminating UE including a Repeat Indicator with the value "service change and fallback" and two BC-IEs, according to the following rule:

– if the 3G-324.M codec is the first (preferred) codec in the list of supported codecs, then the first BC-IE in the SETUP message is the multimedia BC-IE, and the second BC-IE is the speech BC-IE (see figure 4.17);

– if the 3G-324.M codec is in the list of supported codec types, but not in the first position, then the first BC-IE in the SETUP message is the speech BC-IE, and the second BC-IE is the multimedia BC-IE (see figure 4.18).

The terminating UE answers according to its capabilities in the CALL CONFIRMED message. The terminating MSC shall determine the Selected Codec and construct the list of available codecs according to the following rules:

– If no Repeat Indicator is included, and only a speech BC-IE is received, the MSC shall choose a speech codec as the Selected Codec according to the normal mechanism, and no 3G-324.M codec shall be inserted in the list of available codecs (see figure 4.19).

– If no Repeat Indicator is included, and only a multimedia BC-IE is received, the MSC shall choose the 3G‑324.M codec as the Selected Codec, and only the 3G-324.M codec shall be inserted in the list of available codecs (see figure 4.20).

– If the Repeat Indicator is included, and the speech BC_IE is the first BC-IE and the multimedia BC-IE is the second BC-IE, the MSC shall choose a speech codec as the Selected Codec according to the normal mechanism, and both the 3G-324.M codec and speech codecs shall be inserted in the list of available codecs (see figure 4.21) If the terminating MSC supports incoming requests for a network-initiated service change from speech to multimedia and was offered the 3G-324.M2 codec and the terminal had indicated in the CALL CONFIRMED message with the"Enhanced Network-initiated ICM" (ENICM) Capability for the support of Network-initiated service upgrade to multimedia, the 3G-324.M codec shall be the last but one codec in the list and the 3G-324.M2 codec shall be the last codec in the list.

– If the Repeat Indicator is included, and the multimedia BC-IE is the first BC-IE and the speech BC-IE is the second BC-IE, the Selected Codec shall be the 3G-324.M codec, and both the 3G-324.M codec and speech codecs shall be inserted in the list of available codecs (see figure 4.22). If the terminating MSC supports incoming requests for a network-initiated service change from speech to multimedia and was offered the 3G-324.M2 codec and the terminal had indicated in the CALL CONFIRMED message with the"Enhanced Network-initiated ICM" (ENICM) Capability for the support of Network-initiated service upgrade to multimedia, the terminating MSC it shall include the 3G-324.M2 codec as second codec in the list of available codecs.

The Selected Codec and the list of available codecs shall be sent back to the originating MSC according to the normal codec negotiation procedure.

NOTE: The 3G-324.M2 codec may be included the list in addition to 3G-324.M. This is not depicted in the figure.

Figure 4.17: 3G-324M codec first

NOTE: The 3G-324.M2 codec may be included the list in addition to 3G-324.M. This is not depicted in the figure.

Figure 4.18: Speech codec first

NOTE: The actual speech codec is selected according to OoBTC procedures.

Figure 4.19: Speech only

NOTE: The 3G-324.M2 codec may be included in the list of available codecs after the 3G-324.M codec. This is not depicted in the figure.

Figure 4.20: Multimedia only

NOTE 1: The actual speech codec is selected according to OoBTC procedures.

NOTE 2: If the terminating MSC supports incoming requests for a network-initiated service change from speech to multimedia and was offered the 3G-324.M2 codec and the terminal had indicated in the CALL CONFIRMED message with the"Enhanced Network-initiated ICM" (ENICM) Capability for the support of Network-initiated service upgrade to multimedia, the terminating MSC shall include the 3G-324.M2 codec as last codec in the list of available codecs after 3G-324.M. This is not depicted in the figure.

Figure 4.21: Speech preferred

NOTE 1: The actual list of speech codecs is built according to OoBTC procedures.

NOTE 2: If the terminating MSC supports incoming requests for a network-initiated service change from speech to multimedia and was offered the 3G-324.M2 codec and the terminal had indicated in the CALL CONFIRMED message with the"Enhanced Network-initiated ICM" (ENICM) Capability for the support of Network-initiated service upgrade to multimedia, the terminating MSC shall include the 3G-324.M2 codec as second codec in the list of available codecs after 3G-324.M. This is not depicted in the figure.

Figure 4.22: Multimedia preferred