4.3 UE domain selection

24.5013GPPNon-Access-Stratum (NAS) protocol for 5G System (5GS)Release 18Stage 3TS

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

4.3.1 UE’s usage setting

The UE’s usage setting defined in 3GPP TS 24.301 [15] applies to voice capable UEs in 5GS and indicates whether the UE has preference for voice services over data services or vice-versa, where:

a) voice services include IMS voice; and

b) data services include any kind of user data transfer without a voice media component.

The UE’s usage setting can be set to:

a) "voice centric"; or

b) "data centric".

If the UE is capable of S1 mode, there is a single UE’s usage setting at the UE which applies to both 5GS and EPS.

4.3.2 Domain selection for UE originating sessions / calls

The behaviour of the UE for domain selection is determined by:

a) the UE usage setting;

b) the availability of IMS voice; and

c) whether the UE operates in single-registration mode or dual-registration mode (see 3GPP TS 23.501 [8]).

In the present document the condition "the UE supports IMS voice over 3GPP access" evaluates to "true" if at least one of the following is fulfilled:

1) the UE supports IMS voice over NR connected to 5GCN;

2) the UE supports IMS voice over E-UTRA connected to 5GCN; or

3) the UE supports IMS voice in EPS.

In the present document the condition "the UE does not support IMS voice over 3GPP access" evaluates to "true" if the condition "the UE supports IMS voice over 3GPP access" evaluates to "false".

In the present document the condition "the UE supports IMS voice over non-3GPP access" evaluates to "true" if the UE supports IMS voice over non-3GPP access connected to 5GCN.

In the present document the condition "the UE does not support IMS voice over non-3GPP access" evaluates to "true" if the condition "the UE supports IMS voice over non-3GPP access" evaluates to "false".

In the present document, "IMS voice not available" is determined per access type independently, i.e. 3GPP access or non-3GPP access.

In the present document, "IMS voice not available" refers to one of the following conditions:

a) the UE does not support IMS voice;

b) the UE supports IMS voice, but the network indicates in the REGISTRATION ACCEPT message that IMS voice over PS sessions are not supported; or

c) the UE supports IMS voice, the network indicates in the REGISTRATION ACCEPT message that IMS voice over PS sessions are supported, but the upper layers:

1) provide no indication that the UE is available for voice call in the IMS within a manufacturer determined period of time; or

2) indicate that the UE is not available for voice calls in the IMS.

NOTE 1: If conditions a and b evaluate to false, the upper layers need time to attempt IMS registration. In the event an indication from the upper layers that the UE is available for voice calls in the IMS takes longer than the manufacturer determined period of time (e.g. due to delay when attempting IMS registration or due to delay in obtaining a QoS flow for SIP signalling), the NAS layer assumes the UE is not available for voice calls in the IMS.

Other conditions may exist but these are implementation specific.

In the present document, "IMS voice available" applies when "IMS voice not available" does not apply.

When IMS voice is not available over 3GPP access, if the UE’s usage setting is "voice centric", the UE operates in single-registration mode, and the UE:

a) does not have a persistent PDU session, and:

1) if the UE is only registered over 3GPP access, or if the UE is registered over both 3GPP access and non-3GPP access and IMS voice is not available over non-3GPP access, the UE shall disable the N1 mode capability for 3GPP access and proceed as specified in subclause 4.9.2 with modifications described below;. or

2) if the UE is registered over both 3GPP access and non-3GPP access and IMS voice is available over non-3GPP access, the UE may disable the N1 mode capability for 3GPP access and proceed as specified in subclause 4.9.2 with modifications described below; or

b) has a persistent PDU session, then the UE waits until the radio bearer associated with the persistent PDU session has been released. When the radio bearer associated with the persistent PDU session has been released, then:

1) if the UE is only registered over 3GPP access, or if the UE is registered over both 3GPP access and non-3GPP access and IMS voice is not available over non-3GPP access,the UE shall disable the N1 mode capability for 3GPP access and proceed as specified in subclause 4.9.2 with modifications described below; or

2) If the UE is registered over both 3GPP access and non-3GPP access and IMS voice is available over non-3GPP access, the UE may disable the N1 mode capability for 3GPP access and proceed as specified in subclause 4.9.2 with modifications described below.

The following modifications are applied to the procedure in subclause 4.9.2 for disabling the N1 mode capability for 3GPP access, if the UE’s usage setting is "voice centric" and the UE operates in single-registration mode:

a) in item a) of subclause 4.9.2, the UE shall attempt to select an E-UTRA cell connected to EPC. If such a cell is found, the UE shall then perform voice domain selection procedures as defined in 3GPP TS 24.301 [15]; and

b) in item b) of subclause 4.9.2, if an E-UTRA cell connected to EPC cannot be found, the UE shall attempt to select another supported radio access technology which supports voice services.

When IMS voice is not available over non-3GPP access, if the UE’s usage setting is "voice centric" and the UE operates in single-registration mode, then:

a) if the UE is only registered over non-3GPP access, the UE shall disable the N1 mode capability for non-3GPP access (see subclause 4.9.3); or

b) if the UE is registered over both 3GPP access and non-3GPP access and IMS voice is not available also over 3GPP access, the UE may disable the N1 mode capability for non-3GPP access (see subclause 4.9.3).

NOTE 2: The UE can register over 3GPP access in another mode, e.g., S1 mode, for voice service, and in this case the UE can keep the N1 mode capability for non-3GPP access enabled.

4.3.3 Change of UE’s usage setting

If the UE operates in single-registration mode, whenever the UE’s usage setting changes, the UE shall execute procedures according to table 4.3.3.1:

Table 4.3.3.1: Change of UE’s usage setting for a UE in single-registration mode

UE’s usage setting change	Procedure to execute
From "data centric" to "voice centric" and "IMS voice not available" over 3GPP access only	Disable the N1 mode capability for 3GPP access (see subclause 4.9.2), if the UE is only registered over 3GPP access (NOTE)
From "data centric" to "voice centric", and "IMS voice not available" over both 3GPP access and non-3GPP access	Disable the N1 mode capability for 3GPP access (see subclause 4.9.2) and non-3GPP access (see subclause 4.9.3), if the UE is registered over both 3GPP access and non-3GPP access Disable the N1 mode capability for 3GPP access (see subclause 4.9.2), if the UE is only registered over 3GPP access. Disable the N1 mode capability for non-3GPP access (see subclause 4.9.3), if the UE is only registered over non-3GPP access. (NOTE)
From "voice centric" to "data centric" and the N1 mode capability for 3GPP access is disabled at the UE due to "IMS voice not available"	Re-enable the N1 mode capability for 3GPP access (see subclause 4.9.2)
From "data centric" to "voice centric" and "IMS voice not available" over non-3GPP access only	Disable the N1 mode capability for non-3GPP access (see subclause 4.9.3), if the UE is only registered over non-3GPP access
From "voice centric" to "data centric", and the N1 mode capability for non-3GPP access is disabled at the UE due to "IMS voice not available"	Re-enable the N1 mode capability for non-3GPP access (see subclause 4.9.3)
NOTE: If the UE is registered over 3GPP access and has a persistent PDU session, then the UE waits until the radio bearer associated with the persistent PDU session has been released.

4.3.4 Change or determination of IMS voice availability

If the UE operates in single-registration mode, whenever the IMS voice availability is determined or changes, the UE shall execute procedures according to table 4.3.4.1:

Table 4.3.4.1: Change of IMS voice availability for a UE in single-registration mode

Change of IMS voice available condition	Procedure to execute
"IMS voice not available" over 3GPP access only and the UE’s usage setting is "voice centric"	Disable the N1 mode capability for 3GPP access, if the UE is only registered over 3GPP access (see subclause 4.9.2). (NOTE 2)
"IMS voice not available" over non-3GPP access only and the UE’s usage setting is "voice centric"	Disable the N1 mode capability for non-3GPP access (see subclause 4.9.3), if the UE is only registered over non-3GPP access. (NOTE 2)
"IMS voice not available" over both 3GPP access and non-3GPP access, and the UE’s usage setting is "voice centric"	Disable the N1 mode capability for 3GPP access (see subclause 4.9.2) and non-3GPP access (see subclause 4.9.3), if the UE is registered over both 3GPP access and non-3GPP access. Disable the N1 mode capability for 3GPP access (see subclause 4.9.2), if the UE is only registered over 3GPP access Disable the N1 mode capability for non-3GPP access (see subclause 4.9.3), if the UE is only registered over non-3GPP access. (NOTE 1, NOTE 2)
NOTE 1: If the UE is registered over 3GPP access and has a persistent PDU session, then the UE waits until the radio bearer associated with the persistent PDU session has been released. NOTE 2: If the UE determines "IMS voice not available" upon receipt of a 5GS session management reject message including a back-off timer value, and the re-attempt indicator indicates that the UE is not allowed to re-attempt the procedure in S1 mode then, upon inter-system change from N1 mode to S1 mode, the UE proceeds as specified in 3GPP TS 24.301 [15], subclause 4.3.2.4, Change or determination of IMS registration status.