3GPP 24.501 v20.0.0 — the document's own text

5.3.1.3 Release of the N1 NAS signalling connection

Taught in 4. Inside TS 24.501, the document at the centre (CT1 — the group that writes what your phone says, overview).

The signalling procedure for the release of the N1 NAS signalling connection is initiated by the network.

In N1 mode, upon indication from lower layers that the access stratum connection has been released, the UE shall enter 5GMM-IDLE mode and consider the N1 NAS signalling connection released.

If the UE in 3GPP access is configured for eCall only mode as specified in 3GPP TS 31.102 [22] then:

if the N1 NAS signalling connection that was released had been established for eCall over IMS was released, the UE shall stop and restart timer T3444 with its initial value; and
if the N1 NAS signalling connection that was released had been established for a call to an HPLMN designated non-emergency MSISDN or URI for test or terminal reconfiguration service was released, the UE shall stop and restart timer T3445 with its initial value.

The UE shall start the timer T3447, if not already running, when the N1 NAS signalling connection is released as specified in clause 5.3.17.

To allow the network to release the N1 NAS signalling connection, the UE:

shall start the timer T3540 if the UE receives any of the 5GMM cause values #7, #11, #12, #13, #15, #27, #31, #62, #72, #73, #74, #75, #76, #78, #80 and the UE does not consider the received 5GMM cause value as abnormal case as specified in clauses 5.5.1.2.7, 5.5.1.3.7 and 5.5.2.3.4;

a1) shall start the timer T3540 if the UE receives a SERVICE REJECT message, case i) in clause 5.6.1.7 is applicable and the procedure was started from 5GMM-IDLE mode;

shall start the timer T3540 for a UE in 3GPP access if:

the UE receives a REGISTRATION ACCEPT message which does not include a Pending NSSAI IE or UE radio capability ID deletion indication IE;
the UE has set the Follow-on request indicator to "No follow-on request pending" in the REGISTRATION REQUEST message;
the UE has not included the Uplink data status IE in the REGISTRATION REQUEST message, or the UE has included the Uplink data status IE in the REGISTRATION REQUEST message but the REGISTRATION ACCEPT message indicates that no user-plane resources of any PDU sessions are to be re-established;
the UE has not included the Allowed PDU session status IE or has included the Allowed PDU session status IE indicating there is no PDU session(s) for which the UE allowed the user-plane resource to be re-established over 3GPP access in the REGISTRATION REQUEST message, or the UE has included the Allowed PDU session status IE in the REGISTRATION REQUEST message but the REGISTRATION ACCEPT message does not indicate that any user-plane resources of any PDU sessions are to be re-established;
the registration procedure has been initiated in 5GMM-IDLE mode, or the UE has set Request type to "NAS signalling connection release" in the UE request type IE in the REGISTRATION REQUEST message and the N1 NAS signalling connection release bit is set to "N1 NAS signalling connection release supported" in the 5GS network feature support IE of the REGISTRATION ACCEPT message;
the user-plane resources for PDU sessions have not been set up, except for the case the UE has set Request type to "NAS signalling connection release" in the UE request type IE in the REGISTRATION REQUEST message and the N1 NAS signalling connection release bit is set to "N1 NAS signalling connection release supported" in the 5GS network feature support IE of the REGISTRATION ACCEPT message;
the UE need not request resources for V2X communication over PC5 reference point (see 3GPP TS 23.287 [6C]);
void;
the UE need not request resources for 5G ProSe direct discovery over PC5 or 5G ProSe direct communication over PC5 (see 3GPP TS 23.304 [6E]); and
the UE need not request resources for A2X communication over PC5 reference point (see 3GPP TS 23.256 [6AB]);

NOTE 1: The lower layers indicate when the user-plane resources for PDU sessions are successfully established or released.

b1) may start the timer T3540 at completion of the registration procedure if the UE has included unavailability information and has not included a start of the unavailability period in the REGISTRATION REQUEST message;

shall start the timer T3540 if the UE receives a REGISTRATION REJECT message indicating:

the 5GMM cause value #9 or #10;

shall start the timer T3540 if the UE receives a SERVICE REJECT message indicating:

the 5GMM cause value #9, #10; or

the 5GMM cause value #28 and with no emergency PDU session has been established;

shall start the timer T3540 if:

the UE receives a CONFIGURATION UPDATE COMMAND message containing the Configuration update indication IE with the Registration bit set to "registration requested" and with:

either new allowed NSSAI information or new configured NSSAI information or both included;
the network slicing subscription change indication; or

iii) no other information elements; and

the user-plane resources for PDU sessions have not been set up;

shall start the timer T3540 for a UE in 3GPP access if:

the UE receives a SERVICE ACCEPT message;
for the case that the UE sent the:

SERVICE REQUEST message, the UE did not set the Service type IE to "signalling" or "high priority access", the UE has not included the Uplink data status IE in the SERVICE REQUEST message, or the UE has included the Uplink data status IE in the SERVICE REQUEST message but the SERVICE ACCEPT message indicates that no user-plane resources of any PDU sessions are to be re-established; or
CONTROL PLANE SERVICE REQUEST message, the UE did not set the Control plane service type IE to "emergency services fallback", the UE has not included the Uplink data status IE in the CONTROL PLANE SERVICE REQUEST message, or the UE has included the Uplink data status IE in the CONTROL PLANE SERVICE REQUEST message but the SERVICE ACCEPT message indicates that no user-plane resources of any PDU sessions are to be re-established;

the UE has not included the Allowed PDU session status IE or has included the Allowed PDU session status IE indicating there is no PDU session(s) for which the UE allowed the user-plane resource to be re-established over 3GPP access in the SERVICE REQUEST message or the CONTROL PLANE SERVICE REQUEST message, or the UE has included the Allowed PDU session status IE in the SERVICE REQUEST message or the CONTROL PLANE SERVICE REQUEST message but the SERVICE ACCEPT message does not indicate that any user-plane resources of any PDU sessions are to be re-established;
the service request procedure has been initiated in 5GMM-IDLE mode;
the user-plane resources for PDU sessions have not been set up;
the UE need not request resources for V2X communication over PC5 reference point (see 3GPP TS 23.287 [6C]);
the UE need not request resources for 5G ProSe direct discovery over PC5 or 5G ProSe direct communication over PC5 (see 3GPP TS 23.304 [6E]); and
the UE need not request resources for A2X communication over PC5 reference point (see 3GPP TS 23.256 [6AB]);

NOTE 2: The lower layers indicate when the user-plane resources for PDU sessions are successfully established or released.

may start the timer T3540 if the UE receives any of the 5GMM cause values #3 or #6 or if it receives an AUTHENTICATION REJECT message;
shall start the timer T3540 for a UE in 3GPP access upon completion of the configuration update procedure or the registration procedure if the UE does not have an emergency PDU session and:

the UE received a CONFIGURATION UPDATE COMMAND message or a REGISTRATION ACCEPT message while camping on a CAG cell and none of the CAG-ID(s) supported by the current CAG cell is authorized based on "Allowed CAG list" of the entry for the current PLMN in the received "CAG information list";
the UE received a CONFIGURATION UPDATE COMMAND message or a REGISTRATION ACCEPT message while camping on a non-CAG cell and the entry for the current PLMN in the received "CAG information list" includes an "indication that the UE is only allowed to access 5GS via CAG cells"; or
the UE received a CONFIGURATION UPDATE COMMAND message while camping on a CAG cell and the entry for the current PLMN in not included in the received "CAG information list"; or

shall start the timer T3540 for a UE in 3GPP access if:

the UE receives a SERVICE ACCEPT message; and
the UE:

has set Request type to "NAS signalling connection release" in the UE request type IE in the SERVICE REQUEST message or CONTROL PLANE SERVICE REQUEST message; or
has set Request type to "Rejection of paging" in the UE request type IE in the SERVICE REQUEST message or the CONTROL PLANE SERVICE REQUEST message and the UE receives a CONFIGURATION UPDATE COMMAND message;

shall start the timer T3540 if:

the UE receives the 5GMM cause value #22 along with a T3346 value in the SERVICE REJECT message, the value indicates that the timer T3346 is neither zero nor deactivated and the service request procedure has been initiated in 5GMM-IDLE mode; or
the UE receives the 5GMM cause value #22 along with a T3346 value in a message different from the SERVICE REJECT message, and the value indicates that the timer T3346 is neither zero nor deactivated;

shall start the timer T3540 if the UE receives a DEREGISTRATION ACCEPT message;
shall start the timer T3540 after the completion of the de-registration procedure, if the UE receives a DEREGISTRATION REQUEST message and the De-registration type IE indicates "re-registration required";
shall start the timer T3540 if the UE receives:

a SERVICE REJECT or SERVICE ACCEPT message with timer T3448 value IE, the UE starts the timer T3448 and the service request procedure has been initiated in 5GMM-IDLE mode; or
REGISTRATION ACCEPT message with timer T3448 value IE and the UE starts the timer T3448; or

may start the timer T3540 if the UE is using 5GS services with control plane CIoT 5GS optimization and:

the UE sends a UL NAS TRANSPORT message and payload container IE is set to "CIoT user data container" and the Release assistance indication IE is included in the UL NAS TRANSPORT message and the DDX field of the Release assistance indication UE indicates "No further uplink and no further downlink data transmission subsequent to the uplink data transmission is expected"; or
the UE receives a SERVICE ACCEPT message and payload container IE was set to "CIoT user data container" in the CONTROL PLANE SERVICE REQUEST message and the Release assistance indication IE was included in the CONTROL PLANE SERVICE REQUEST message and the DDX field of the Release assistance indication UE indicates "No further uplink and no further downlink data transmission subsequent to the uplink data transmission is expected".

Upon expiry of T3540,

in cases a), a1), b), b1), f), g), h), i), j), k), m and n) the UE shall locally release the established N1 NAS signalling connection;
in cases c) and d) the UE shall locally release the established N1 NAS signalling connection and the UE shall initiate the registration procedure as described in clause 5.5.1.3 or 5.6.1.5;
in case e), the UE shall locally release the established N1 NAS signalling connection and perform a new registration procedure as specified in clause 5.5.1.3.2; or
in case l), the UE shall locally release the established N1 NAS signalling connection and initiate the registration procedure as specified in clause 5.5.1.2.2.

In case a) and m),

upon receiving a request from the upper layers to perform emergency services fallback only for a UE in 3GPP access or establishing an emergency PDU session, the UE shall stop timer T3540 and shall locally release the N1 NAS signalling connection, before proceeding as specified in clause 5.5.1.

In case a1),

upon receiving a request from the upper layers to perform:

an emergency services fallback or establishing an emergency PDU session; or
an MO MMTEL voice call, MO SMS or IMS registration related signalling if the request was not rejected by the SERVICE REJECT message,

the UE may stop timer T3540 and locally release the N1 NAS signalling connection, before proceeding as specified in clause 5.6.1.

In case b) and f),

upon an indication from the lower layers that the user-plane resources for PDU sessions are set up, the UE shall stop timer T3540 and may send uplink signalling via the existing N1 NAS signalling connection or user data via user plane. If the uplink signalling is associated with emergency services fallback only for a UE in 3GPP access or establishing an emergency PDU session, the UE shall stop timer T3540 and send the uplink signalling via the existing N1 NAS signalling connection;

In case b), f) and i),

upon receipt of a DEREGISTRATION REQUEST message, the UE shall stop timer T3540 and respond to the network-initiated de-registration request via the existing N1 NAS signalling connection as specified in clause 5.5.2.3;
upon receipt of a message of a network-initiated 5GMM common procedure except a CONFIGURATION UPDATE COMMAND message without requesting acknowledgement or a 5GMM status message, the UE shall stop timer T3540 and respond to the network-initiated 5GMM common procedure via the existing N1 NAS signalling connection as specified in clause 5.4;
if there is no user-plane resources established for PDU sessions, upon receiving a request from the upper layers to perform emergency services fallback only for a UE in 3GPP access or establishing an emergency PDU session, the UE shall stop timer T3540 and shall locally release the N1 NAS signalling connection, before proceeding as specified in clause 5.6.1;
if there is no user-plane resources established for PDU sessions, upon receiving a request from the upper layers to perform services other than emergency services fallback only for a UE in 3GPP access or establishing an emergency PDU session, the UE shall wait for the local release of the established N1 NAS signalling connection upon expiry of timer T3540 or wait for timer T3540 being stopped, before initiating NAS signalling;
upon receipt of a DL NAS TRANSPORT message, the UE shall stop timer T3540 and may send uplink signalling via the existing N1 NAS signalling connection;
upon reception of NOTIFICATION message as specified in clause 5.6.3.1 case a), the UE shall stop timer T3540 and send uplink signalling via the existing N1 NAS signalling connection; or
upon initiation of registration procedure for mobility and periodic registration update as specified in clause 5.5.1.2.7 for cases h), i), j), clause 5.5.1.3.7 for cases j), k) or clause 5.5.1.3.2 for case a), the UE shall stop timer T3540.

In case c) and d),

upon an indication from the lower layers that the access stratum connection has been released, the UE shall stop timer T3540 and perform a new registration procedure as specified in clause 5.5.1.3.5 or 5.6.1.5.
upon receiving a request from the upper layers to perform emergency services fallback only for a UE in 3GPP access or establishing an emergency PDU session, the UE shall stop timer T3540 and shall locally release the N1 NAS signalling connection, before proceeding as specified in clause 5.5.1.

In case e),

upon an indication from the lower layers that the access stratum connection has been released, the UE shall stop timer T3540 and perform a new registration procedure as specified in clause 5.5.1.3.2.
upon an indication from the lower layers that the user-plane resources for PDU sessions are set up, the UE shall stop timer T3540 and may send user data via user plane.

NOTE 3: In this case, the new registration procedure is performed when the UE moves to the 5GMM-IDLE mode.

upon receiving a request from the upper layers to perform emergency services fallback only for a UE in 3GPP access or establishing an emergency PDU session, the UE shall stop timer T3540 and shall locally release the N1 NAS signalling connection, before proceeding as specified in clause 5.5.1.

In case m),

upon an indication from the lower layers that the user plane resources are set up or upon receiving a request from upper layers to send NAS signalling not associated with CIoT user data, the UE shall stop timer T3540; or
the UE shall not send CIoT user data via the control plane until expiry of timer T3540 or times T3540 being stopped.

In case b), j) and m),

upon receiving a request from the upper layers to send user data via control plane related to an exceptional event and the UE is allowed to use exception data reporting (see the ExceptionDataReportingAllowed leaf of the NAS configuration MO in 3GPP TS 24.368 [17] or the USIM file EFNASCONFIG in 3GPP TS 31.102 [22]), the UE may stop timer T3540 and locally release the N1 NAS signalling connection, before proceeding as specified in clause 5.6.1.2.2.

If the UE had set the Follow-on request indicator to "Follow-on request pending" in the REGISTRATION REQUEST message due to pending uplink signalling but cannot send the pending signalling due to new service area restrictions received or due to network not supporting the feature as indicated in the REGISTRATION ACCEPT message (for example UE set the "Follow-on request pending" to send SMS over NAS but the AMF notified "SMS over NAS not allowed") and if there is no further pending data or signalling and user plane resources have not been set up, the UE may locally release the established N1 NAS signalling connection upon completion of the registration procedure.

NOTE 4: The UE is allowed to inform the lower layers that there is no 5GMM or 5GSM messages need to be sent over non-3GPP access if the UE receives a REGISTRATION REJECT message over non-3GPP access or a SERVICE REJECT message over non-3GPP access.

If the timer T3540 is not running when the UE enters state 5GMM-DEREGISTERED.PLMN-SEARCH or 5GMM-REGISTERED.PLMN-SEARCH, the UE may locally release the N1 NAS signalling connection.

If the timer T3540 is not running and the UE has included unavailability information and has not included the start of unavailability period:

in the DEREGISTRATION REQUEST message, then at completion of the de-registration procedure the UE shall locally release the N1 NAS signalling connection and enter 5GMM-DEREGISTERED.NO-CELL-AVAILABLE state; or
in the REGISTRATION REQUEST message, then at completion of the registration procedure the UE shall locally release the N1 NAS signalling connection and enter 5GMM-REGISTERED.NO-CELL-AVAILABLE state.