Content for  TS 24.501  Word version:  18.5.0

The network may activate congestion control for transport of user data via the control plane, as specified in TS 23.501. If the UE has indicated support for the control plane CIoT 5GS optimizations and the network decides to activate the congestion control for transport of user data via the control plane, the network may include a value for the control plane data back-off timer T3448 in REGISTRATION ACCEPT, SERVICE ACCEPT or SERVICE REJECT message, and shall store an control plane data back-off time on a per UE basis. The UE starts the timer T3448 with the value informed in the message. To avoid that large numbers of UEs simultaneously initiate deferred requests, the network should select the value for the timer T3448 for the informed UEs so that timeouts are not synchronised. The network sends REGISTRATION ACCEPT message or SERVICE ACCEPT message without T3448 value IE to stop the timer T3448 running in the UE as specified in subclause 5.5.1.3.4 and subclause 5.6.1.4. Based on the stored control plane data back-off time for the UE, the network may reject the transfer of user data via the control plane initiated by the UE. While the timer T3448 is running, the UE in 5GMM-IDLE mode does not initiate the transport of user data via the control plane procedure, except if the UE is allowed to use exception data reporting (see the ExceptionDataReportingAllowed leaf of the NAS configuration MO in TS 24.368 or the USIM file EFNASCONFIG in TS 31.102) and the user data is related to an exceptional event. The UE is allowed:

1. to respond to paging with CONTROL PLANE SERVICE REQUEST message without uplink data; or
2. to send a CONTROL PLANE SERVICE REQUEST message for emergency services or for emergency services fallback;

even if the timer T3448 is running. Upon entering the state 5GMM-DEREGISTERED or a new PLMN which is not equivalent to the PLMN where the UE started the timer T3448, or upon being switched off while the timer T3448 is running, the UE stops the timer T3448.

CIoT 5GS optimizations provide improved support of small data and SMS transfer. A UE supporting CIoT 5GS optimizations can indicate the 5GS CIoT network behaviour the UE can support and prefers to use during the registration procedure (see TS 23.502). The UE may indicate the support for control plane CIoT 5GS optimization, user plane CIoT 5GS optimization, N3 data transfer and header compression (see subclause 9.11.3.1). Furthermore, the UE may, separately from the indication of support, indicate preference for control plane CIoT 5GS optimization or user plane CIoT 5GS optimization (see subclause 9.11.3.9A). The indication of preference is also considered as the request to use. A UE supporting CIoT EPS optimizations can also indicate the EPS CIoT network behaviour the UE can support during the registration procedure. Furthermore, the UE may, separately from the indication of support, indicate preference for control plane CIoT EPS optimization or user plane CIoT EPS optimization. The UE can be in NB-N1 mode or WB-N1 mode when requesting the use of CIoT 5GS optimizations during the registration procedure. A UE in NB-N1 mode always indicates support for control plane CIoT 5GS optimization. In NB-N1 mode, the UE, when requesting the use of CIoT 5GS optimizations, does not:

• request an initial registration for emergency services;
• request a PDU session establishment for emergency PDU session; or
• indicate UE's usage setting during the registration procedure.

The network does not indicate to the UE support of emergency services when the UE is in NB-N1 mode (see subclause 5.5.1.2.4 and 5.5.1.3.4). The control plane CIoT 5GS optimization enables support of efficient transport of user data (IP, Ethernet and Unstructured) or SMS messages over control plane via the AMF without triggering user-plane resources establishment. The support of control plane CIoT 5GS optimization is mandatory for the network in NB-N1 mode and optional in WB-N1 mode. Optional header compression of IP data and Ethernet data can be applied to PDU sessions with IP PDU session type and Ethernet PDU session type that are configured to support header compression. For IP header compression, Robust Header Compression (ROHC) protocol specified in RFC 5795 is used. For Ethernet header compression, Ethernet Header Compression (EHC) protocol specified in TS 38.323 is used. For a UE that supports Location Services (LCS) notification mechanisms in N1 mode, the control plane CIoT 5GS optimization also enables the transport of location services messages from 5GMM-IDLE mode using the CONTROL PLANE SERVICE REQUEST message when location services are requested (see subclause 6.7.1 in TS 23.273). The user plane CIoT 5GS optimization enables support for change from 5GMM-IDLE mode over 3GPP access to 5GMM-CONNECTED mode over 3GPP access without the need for using the service request procedure (see subclause 5.3.1.5). If the UE supports user plane CIoT 5GS optimization, it shall also support N3 data transfer. If the UE indicates support of one or more CIoT 5GS optimizations and the network supports one or more CIoT 5GS optimizations and decides to accept the registration request, the network indicates the supported CIoT 5GS optimizations to the UE per registration area when accepting the UE request. Network indication of support is interpreted by the UE as the acceptance to use the respective feature. After completion of the registration procedure, the UE and the network can then use the accepted CIoT 5GS optimizations for the transfer of user data (IP, Ethernet, Unstructured and SMS). A UE in NB-N1 mode or WB-N1 mode can request the use of SMS over NAS by setting the SMS requested bit of the 5GS update type IE in the REGISTRATION REQUEST message as specified in subclauses 5.5.1.2.2 and 5.5.1.3.2. The AMF indicates whether it allows the use of SMS over NAS for a UE in NB-N1 mode or WB-N1 mode by setting the SMS allowed bit of the 5GS registration result IE in the REGISTRATION ACCEPT message as specified in subclauses 5.5.1.2.4 and 5.5.1.3.4. If the UE and the network support both the control plane CIoT 5GS optimization and N3 data transfer, then when receiving the UE's request for a PDU session establishment, the AMF decides whether the PDU session should be NEF PDU session or N6 PDU session as specified in TS 23.501 and then:

1. if NEF PDU session is to be established for unstructured data type, the AMF includes control plane only indication for the requested PDU session to the SMF;
2. if N6 PDU session is to be established and the DNN or S-NSSAI of the newly requested N6 PDU session supports interworking with EPS as specified in TS 23.502:
   1. if there are existing N6 PDU sessions supporting interworking with EPS for this UE that were established with the control plane only indication, the AMF includes the control plane only indication for the newly requested N6 PDU session to the SMF; or
   2. if there are existing N6 PDU sessions supporting interworking with EPS for this UE that were established without the control plane only indication, the AMF does not include the control plane only indication for the newly requested N6 PDU session to the SMF;
   3. if there is no existing N6 PDU session supporting interworking with EPS for this UE, the AMF determines whether to include the control plane only indication for the newly requested N6 PDU session to the SMF based on local policies, the UE's preferred CIoT network behaviour and the supported CIoT network behaviour; and
3. if N6 PDU session is to be established and the DNN or S-NSSAI of the N6 PDU session does not support interworking with EPS as specified in TS 23.502, the AMF determines whether to include the control plane only indication for the newly requested N6 PDU session to the SMF based on local policies, the UE's preferred CIoT network behaviour and the supported CIoT network behaviour.

In NB-N1 mode, if the UE or the network does not support N3 data transfer, then when receiving the UE's request for a PDU session establishment, the AMF decides whether the PDU session should be NEF PDU session or N6 PDU session as specified in TS 23.501 and then includes the control plane only indication for the requested PDU session to the SMF. If the network supports user plane CIoT 5GS optimization, it shall also support N3 data transfer. Broadcast system information may provide information about support of CIoT 5GS optimizations (see TS 36.331). At reception of new broadcast system information, the lower layers deliver it to the 5GMM layer in the UE. The information provided by lower layers is per PLMN and used by the UE to determine whether certain CIoT 5GS optimizations are supported in the cell. The UE shall not attempt to use CIoT 5GS optimizations which are indicated as not supported. In NB-N1 mode, at any given time, there cannot be user-plane resources established for a number of PDU sessions that exceeds the UE' s maximum number of supported user-plane resources. The UE in NB-N1 mode shall not:

1. request the establishment of user-plane resources for a number of PDU sessions that exceeds the UE' s maximum number of supported user-plane resources; or
2. initiate the establishment of a new PDU session, or request the transfer of a PDU session from the non-3GPP access to the 3GPP access, if:
   1. the UE has indicated preference for user plane CIoT 5GS optimization;
   2. the network accepted the use of user plane CIoT 5GS optimization; and
   3. the UE currently has user-plane resources established fora number of PDU sessions that is equal to the UE' s maximum number of supported user-plane resources.

The AMF enforces a limit on the number of PDU sessions with active user-plane resources for a UE in NB-N1 mode based on the UE's maximum number of supported user-plane resources as follows:

1. there can be a maximum of one PDU session with active user-plane resources when the Multiple user-plane resources support bit is set to "Multiple user-plane resources not supported", or
2. there can be a maximum of two PDU sessions with active user-plane resources when the Multiple user-plane resources support bit is set to "Multiple user-plane resources supported".

A PDU session for a UE in NB-N1 mode shall only have one QoS rule and that is the default QoS rule. Reflective QoS is not supported in NB-N1 mode. Reflective QoS is not applicable for a PDU session with control plane only indication. In NB-N1 mode, when the UE requests the lower layer to establish a RRC connection and the UE requests the use of user plane CIoT 5GS optimization, the UE shall pass an indication of the requested CIoT 5GS optimizations to the lower layers. If the UE requests the use of N3 data transfer without user plane CIoT 5GS optimization, then the UE shall also pass an indication of user plane CIoT 5GS optimization to lower layers. In WB-N1 mode, when the UE requests the lower layer to establish a RRC connection and the UE requests the use of control plane CIoT 5GS optimization or user plane CIoT 5GS optimization, the UE shall pass an indication of the requested CIoT 5GS optimizations to the lower layers.

The UE can request the use of both MICO mode with active time and eDRX during a registration procedure but it is up to the network to decide to enable none, one of them or both (see TS 23.501). If the network accepts the use of both MICO mode with active time (see subclause 5.3.6) and eDRX (see subclause 5.3.16), the extended DRX parameters provided to the UE should allow for multiple paging occasions before the active timer expires.

The AMF shall send a REGISTRATION REJECT, SERVICE REJECT or DEREGISTRATION REQUEST message over a wireline access network with the 5GMM cause #77 "wireline access area not allowed", if conditions specified in TS 23.316 for AMF's enforcement of forbidden area are fulfilled. If a REGISTRATION REJECT, SERVICE REJECT or DEREGISTRATION REQUEST message is received over a wireline access network with the 5GMM cause #77 "wireline access area not allowed":

1. the 5G-RG shall not access 5GCN over the wireline access network until the 5G-RG is switched off, the UICC containing the USIM is removed, or the 5G-RG starts using another wireline access network;
2. the W-AGF acting on behalf of the FN-CRG shall not access 5GCN until the W-AGF acting on behalf of the FN-CRG determines that the FN-CRG is switched off; and
3. the W-AGF acting on behalf of the N5GC device shall not access 5GCN until the W-AGF acting on behalf of the FN-CRG determines that the FN-CRG serving the N5GC device is switched off.

A UE supporting reception of WUS assistance information indicates its capability for reception of WUS assistance information during registration procedure (see TS 23.501). The UE supporting WUS assistance information may include its UE paging probability information in the Requested WUS assistance information IE in the REGISTRATION REQUEST message (see TS 23.501). The UE shall not include its UE paging probability information during:

1. a registration procedure for initial registration for emergency services (see subclause 5.5.1.2);
2. a registration procedure for initial registration for initiating an emergency PDU session (see subclause 5.5.1.2);
3. a registration procedure for mobility and periodic registration update (see subclause 5.5.1.3) for initiating an emergency PDU session if the UE is in the state 5GMM-REGISTERED.ATTEMPTING-REGISTRATION-UPDATE; or
4. a registration procedure for mobility and periodic registration update (see subclause 5.5.1.3) when the UE has an emergency PDU session established.

The UE and the network may negotiate the UE paging probability information during a registration procedure when the UE does not have an emergency PDU session. The UE paging probability information is an assistance information used to determine the WUS group for paging UE (see TS 23.501, TS 36.300). If a UE supporting WUS assistance information did not receive the Negotiated WUS assistance information IE during the last registration procedure due to an active emergency PDU session over 3GPP access, the UE shall initiate a registration procedure for mobility and periodic registration update to request WUS assistance information after the emergency PDU session is released over 3GPP access. If the UE does not have an emergency PDU session and the network accepts the use of the WUS assistance information for the UE, the network determines the negotiated UE paging probability information for the UE based on the requested UE paging probability information, if any, local configuration or previous statistical information for the UE, and then indicates the negotiated UE paging probability information in the Negotiated WUS assistance information IE to the UE in the REGISTRATION ACCEPT message. The network shall store the negotiated UE paging probability information in the 5GMM context of the UE for paging. The UE shall use WUS assistance information only if the UE received the Negotiated WUS assistance information IE during the last registration procedure. If the UE did not receive the Negotiated WUS assistance information IE during the last registration procedure, the UE shall delete any existing WUS assistance information received from the network. If the network did not accept the request to use WUS assistance information, the network shall delete the stored negotiated UE paging probability information for the UE, if available. When an emergency PDU session is successfully established after the UE received the Negotiated WUS assistance information IE during the last registration procedure, the UE and the AMF shall not use WUS assistance information until:

• the successful completion of the PDU session release procedure of the emergency PDU session;
• the UE receives WUS assistance information during a registration procedure with PDU session status IE or upon successful completion of a service request procedure, if the UE or the network locally releases the emergency PDU session; or
• the successful completion of the handover of the emergency PDU session to non-3GPP access.

A UE may indicate its capability to support NR paging subgrouping during registration procedure when the UE:

• initiates a registration procedure with 5GS registration type IE not set to "emergency registration"; and
• does not have an active emergency PDU session.

If a UE supporting NR paging subgrouping did not indicate its capability to support NR paging subgrouping during the last registration procedure due to an active emergency PDU session over 3GPP access, the UE shall initiate a registration procedure for mobility and periodic registration update procedure to indicate its capability to support NR paging subgrouping after the emergency PDU session is released over 3GPP access. If the UE indicates support of NR paging subgrouping the UE may include its paging probability information in the Requested PEIPS assistance information IE in the REGISTRATION REQUEST message. If the UE indicates support of NR paging subgrouping and the network supports and accepts the use of the PEIPS assistance information for the UE, the network provides to the UE the Negotiated PEIPS assistance information, including the Paging subgroup ID, in the REGISTRATION ACCEPT message or the CONFIGURATION UPDATE COMMAND message. The Paging subgroup ID is used to determine the NR paging subgroup for paging the UE. The UE NAS layer shall indicate the Paging subgroup ID to the access stratum layer. The network shall store the Paging subgroup ID in the 5GMM context of the UE. The UE shall use PEIPS assistance information only if the UE received the Negotiated PEIPS assistance information IE during the last registration procedure. If the UE did not receive the Negotiated PEIPS assistance information IE during the last registration procedure, the UE shall delete any existing PEIPS assistance information received from the network. If the network did not accept the request to use PEIPS assistance information during the registration procedure, the network shall delete the stored PEIPS assistance information for the UE, if available. If the UE supports the use of the PEIPS assistance information and the network supports and accepts the use of the PEIPS assistance information, the network may provide the PEIPS assistance information to the UE by including the Updated PEIPS assistance information IE in the CONFIGURATION UPDATE COMMAND message. When an emergency PDU session is successfully established over 3GPP access after the UE received the Negotiated PEIPS assistance information IE during the last registration procedure, the UE and the AMF shall not use PEIPS assistance information until:

• the successful completion of the PDU session release procedure of the emergency PDU;
• the UE receives PEIPS assistance information during a registration procedure with PDU session status IE or upon successful completion of a service request procedure, if the UE or the network locally releases the emergency PDU session;
• the successful completion of handover of emergency PDU session to non-3GPP access; or
• the successful transfer of the emergency PDU session in 5GS to the EPS or ePDG connected to EPC.

If the UE and network support unavailability period and an event is triggered in the UE making the UE unavailable for a certain period of time, the UE may store its 5GMM and 5GSM context in USIM or non-volatile memory in the ME to be able to reuse it after the unavailability period. To activate the unavailability period, the UE provides unavailability information, including a type of unavailability, an unavailability period duration if known, and the start of the unavailability period if known, during the registration procedure or during the de-registration procedure (see TS 23.501 and TS 23.502). The support for the unavailability period is negotiated in the registration procedure. If the UE is registered to a PLMN via a satellite NG-RAN cell, the AMF may provide the unavailability period duration of the UE during the registration procedure. The AMF may consider the unavailability period duration provided by the UE if available and determine the unavailability period duration of the UE as described in subclause 5.5.1.3.4. If the UE provided unavailability information in the last registration procedure or de-registration procedure, the AMF considers the UE unreachable until the UE re-register for a normal service without providing unavailability information. If the UE did not include a start of the unavailability period, the AMF shall consider the start of the unavailability period to be the time at which AMF received the REGISTRATION REQUEST message or the DEREGISTRATION REQUEST message from the UE. During the registration procedure, the AMF may determine the values of the negotiated extended DRX parameters, the timer T3324, and the periodic registration update timer (T3512) to be provided to the UE based on the unavailability period duration and the start of the unavailability period based on their availability. The AMF should set the value of the mobile reachable timer and implicit de-registration timer based on the unavailability period duration and the start of the unavailability period. The AMF releases the N1 signalling connection after the completion of the registration procedure in which the UE provided unavailability information without providing the start of the unavailability period. If for discontinuous coverage the UE has stored a discontinuous coverage maximum time offset as described in subclause 5.4.4.3, 5.5.1.2.4, and 5.5.1.3.4, the UE shall set the discontinuous coverage maximum time offset value to a random value up to and including the stored discontinuous coverage maximum time offset for this PLMN and satellite NG-RAN RAT Type, determine a time point equal to the time when the UE is about to lose satellite coverage minus the discontinuous coverage maximum time offset value, and send the REGISTRATION REQUEST message for mobility registration update to the AMF indicating out of coverage at the determined time point. If for discontinuous coverage the UE has stored a maximum time offset as described in subclause 5.4.4.3, 5.5.1.2.4, and 5.5.1.3.4, upon returning in coverage of a TA in the current registration after being out of coverage due to discontinuous coverage, and in the last REGISTRATION ACCEPT message the end of unavailability period report was set to "UE needs to report end of unavailability period", the UE shall start the discontinuous coverage maximum time offset timer with a random value up to and including the stored maximum time offset value, if available, for this PLMN and NR satellite access, and starts the discontinuous coverage maximum time offset timer. The UE shall enter 5GMM-REGISTERED.ATTEMPTING-REGISTRATION-UPDATE state and shall not initiate any NAS signalling on that NR satellite access and PLMN while the maximum time offset timer is running. The UE shall stop the maximum time offset timer and initiate NAS signalling if the UE receives paging message, has pending emergency services, is establishing an emergency PDU session or is performing emergency services fallback procedure or when the UE enters a TAI outside the registration area. At expiry of the discontinuous coverage maximum time offset timer the UE shall perform a registration procedure for mobility registration update. If the end of unavailability period report indicates "UE does not need to report end of unavailability period" then upon returning in coverage of a TAI in the current registration area the UE shall choose an appropriate substate of 5GMM-REGISTERED and does not need to perform a registration procedure for mobility registration update due to end of the unavailability period. If for discontinuous coverage the AMF sets the End of unavailability report bit in Unavailability configuration IE value to "UE does not need to report end of unavailability", the UE is not required to trigger the registration procedure for mobility registration update when the unavailability period duration has ended. If the AMF does not provide the Unavailability configuration IE or the AMF sets the End of unavailability report bit in Unavailability configuration IE value to "UE needs to report end of unavailability", the UE should trigger registration procedure for mobility registration update when the unavailability period duration has ended. When the unavailability period is activated, all NAS timers are stopped and associated procedures aborted except for timers T3512, T3346, T3447, T3448, T3396, T3584, T3585, any back-off timers, T3245, T3247, the timer T controlling the periodic search for HPLMN or EHPLMN or higher prioritized PLMNs, and the timer TSENSE controlling the periodic search for PLMNs satisfying the operator controlled signal level threshold (see TS 23.122). When the UE activates the unavailability period using registration procedure without providing the start of the unavailability period, then after successful completion of the procedure the UE shall enter the state 5GMM-REGISTERED.NO-CELL-AVAILABLE and may deactivate AS layer. Otherwise, if the UE provided the start of unavailability period in the registration procedure, the UE shall enter the state 5GMM-REGISTERED.NO-CELL-AVAILABLE and may deactivate AS layer only after the UE activates the unavailability period. When the UE activates the unavailability period using the de-registration procedure, then after successful completion of the procedure the UE shall enter the state 5GMM-DEREGISTERED.NO-CELL-AVAILABLE and deactivate the AS layer. When the UE comes out of the unavailability period the UE shall activate the AS layer if deactivated and perform registration procedure as described in subclause 5.5.1.2.2 and 5.5.1.3.2.