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Content for  TS 24.501  Word version:  17.0.0

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4  GeneralWord‑p. 37

4.1  Overview

The non-access stratum (NAS) described in the present document forms the highest stratum of the control plane between UE and AMF (reference point "N1" see TS 23.501) for both 3GPP and non-3GPP access.
Main functions of the protocols that are part of the NAS are:
  • support of mobility of the user equipment (UE) including also common procedures such as authentication, identification, generic UE configuration update and security control mode procedures;
  • support of session management procedures to establish and maintain data connectivity between the UE and the data network; and
  • NAS transport procedure to provide a transport of SMS, LPP, LCS, UE policy container, SOR transparent container and UE parameters update information payload.
Principles for the handing of 5GS security contexts and for the activation of ciphering and integrity protection, when a NAS signalling connection is established, are provided in subclause 4.4.
For the support of the above functions, the following procedures are supplied within this specification:
  • elementary procedures for 5GS mobility management in clause 5; and
  • elementary procedures for 5GS session management in clause 6.
Signalling procedures for the control of NAS security are described as part of the 5GMM common procedures in subclause 5.4.
Complete NAS transactions consist of specific sequences of elementary procedures. Examples of such specific sequences can be found in TS 23.502.
The NAS for 5GS follows the protocol architecture model for layer 3 as described in TS 24.007.
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4.2  Coordination between the protocols for 5GS mobility management and 5GS session management

A 5GS session management (5GSM) message is piggybacked in specific 5GS mobility management (5GMM) transport messages. To this purpose, the 5GSM messages can be transmitted in an information element in the 5GMM transport messages. In this case, the UE, the AMF and the SMF execute the 5GMM procedure and the 5GSM procedure in parallel. The success of the 5GMM procedure is not dependent on the success of the piggybacked 5GSM procedure.
The UE can only initiate the 5GSM procedure when there is a 5GMM context established at the UE.
During 5GMM procedures, the UE and the AMF shall suspend the transmission of 5GSM messages, except when:
  1. the 5GMM procedure is piggybacking 5GSM messages; or
  2. the UE is in 5GMM-CONNECTED mode and a service request procedure for re-establishing user-plane resources of PDU session(s) is initiated without including PDU session status IE or Allowed PDU session status IE. In this case, the UE and the AMF need not suspend the transmission of 5GSM messages related to other PDU session(s) than the one(s) for which the user- plane resources re-establishment is requested.
A 5GMM message piggybacking a 5GSM message for a PDU session shall be delivered via the access associated with the PDU session, if any, with the following exceptions:
  1. the AMF shall send, via 3GPP access, a DL NAS TRANSPORT message piggybacking a downlink 5GSM message of a network-requested 5GSM procedure for a PDU session associated with non-3GPP access if the conditions specified in subclause 5.5.1.3.4 or subclause 5.6.1.4 are met;
  2. the UE shall send an UL NAS TRANSPORT message piggybacking a response message to the 5GSM message described in a) via either:
    1. 3GPP access; or
    2. non-3GPP access if the UE is in 5GMM-CONNECTED mode over non-3GPP access; and
  3. the UE shall send, via the target access, an UL NAS TRANSPORT message piggybacking a 5GSM message associated with a request type set to "existing PDU session" or "existing emergency PDU session" for handover of an existing PDU session between 3GPP access and non-3GPP access.
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4.3  UE domain selectionWord‑p. 38

4.3.1  UE's usage setting

The UE's usage setting defined in TS 24.301 applies to voice capable UEs in 5GS and indicates whether the UE has preference for voice services over data services or vice-versa, where:
  1. voice services include IMS voice; and
  2. data services include any kind of user data transfer without a voice media component.
The UE's usage setting can be set to:
  1. "voice centric"; or
  2. "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.
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4.3.2  Domain selection for UE originating sessions / calls

The behaviour of the UE for domain selection is determined by:
  1. the UE usage setting;
  2. the availability of IMS voice; and
  3. whether the UE operates in single-registration mode or dual-registration mode (see TS 23.501).
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:
  1. the UE does not support IMS voice;
  2. the UE supports IMS voice, but the network indicates in the REGISTRATION ACCEPT message that IMS voice over PS sessions are not supported; or
  3. 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.
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:
  1. 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
  2. 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:
  1. 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 TS 24.301; and
  2. 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:
  1. 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
  2. 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 shall disable the N1 mode capability for non-3GPP access (see subclause 4.9.3).
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4.3.3  Change of UE's usage settingWord‑p. 40
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:
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)


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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:
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)
"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
"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)


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