Methods of handling a multi-access packet data unit (ma PDU) session with a non-3GPP access without 5g non-access stratum (NAS) communications in a 5g system (5GS)

Abstract

A method (2400) performed by a user equipment (102) in a wireless communication system (100) includes establishing (2450) a multi-access packet data unit, session with a network entity (130) of the wireless communication system, the MA PDU session using a 3GPP access and a non-3GPP access and having control signaling on the 3GPP access only. The method further includes communicating (2452) data via the non-3GPP access with a user plane function of the wireless communication system, for a retention time interval after the UE loses the 3GPP access.

Description

METHODS OF HANDLING A MULTI-ACCESS PACKET DATA UNIT (MA PDU) SESSION WITH A NON-3GPP ACCESS WITHOUT 5G NON-ACCESS STRATUM (NAS) COMMUNICATIONS IN A 5G SYSTEM (5GS)

[0001] This document generally describes methods and devices operating in wireless communication systems such as (but not limited to) fifth generation (5G) systems (5GS) described in 3rdGeneration Partnership Project (3GPP) technical specifications (TSs). More particularly, the application refers to methods of handling an MA PDU session with a 3GPP access and a non-3GPP access (e.g., WiFi access), with control signaling (i.e. , NAS communications) on the 3GPP access only.BACKGROUND

[0002] This background description is provided for the purpose of generally presenting the context of various embodiments later described, and the technical problems. Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure.

[0003] Nowadays, 5G wireless communication systems support MA PDU sessions between user equipment (UE) and a 5G core (5GC) via a 3GPP access and via a non-3GPP access simultaneously. Traditionally, both the 3GPP access and the non-3GPP access had individual control signaling (i.e., independent NAS signaling). However, recently, UEs use control signaling via the 3GPP access for managing both connections. This approach simplifies network operation over non-3GPP access by: (i) eliminating the control (NAS) signaling connection over non-3GPP access (i.e., both 3GPP access and non-3GPP access have a common NAS signaling connection over 3GPP access, and the non-3GPP access relies on the UE being authenticated via 3GPP access); (ii) establishing the MA PDU session over 3GPP access first and then adding non-3GPP access; and (iii) supporting multipath quick user datagram protocol internet connection (MPQUIC) connectivity to the user plane via the 3GPP access, thenon-3GPP access, or both, between the UE and user plane function (UPF) based on an existing MPQUIC interface between UE (operated as MPQUIC client) and PDU session anchor (PSA) UPF operated as MPQUIC proxy, when both the UE and the network support the MPQUIC functionality. Conventionally, due to both 3GPP access and non- 3GPP access have a common NAS signaling connection over 3GPP access, when 3GPP coverage is lost, the loss of 3GPP-only control signaling triggers discontinuing the 3GPP access and non-3GPP access, in spite of continued non-3GPP coverage.

[0004] In this context, an unresolved issue is whether and how the network and the UE can handle the MA PDU session via non-3GPP access if the UE loses 3GPP access. Another issue is whether the existing UE state machine for the 3GPP access is impacted when the non-3GPP access operates without control signaling (5G NAS) when the UE loses 3GPP access and the UE continues to use MA PDU session via non-3GPP access. A third open issue is how the access and mobility function (AMF) manages the two connection management (CM) states for both 3GPP access and non- 3GPP access, a CM state for 3GPP access and a CM state for non-3GPP access, when eliminating the control (NAS) signaling connection over non-3GPP access.SUMMARY

[0005] Methods and devices according to various embodiments enable, in a MA PDU session, maintaining the non-3GPP access after the 3GPP access is lost, which leaves the UE without control signaling for the non-3GPP access. A network entity (NE) of a 5GC, which intermediates the UE’s access to a data server via the 3GPP access and the non-3GPP access (which relies on control signaling via the 3GPP access), maintains the non-3GPP access for a retention time interval after the 3GPP access is lost. The NE, which executes one of the access and mobility function (AMF) or the session management function (SMF), has a retention timer for measuring the retention time interval based on a retention timer value. The retention timer value may be (1 ) provided to the NE by the UE during a user registration procedure, a service request procedure, or a session modification procedure, (2) stored locally or determined by the NE, or (3) retrieved by the NE from the other one of the AMF and SMF.BRIEF DESCRIPTION OF THE DRAWINGS

[0006] The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate one or more embodiments and, together with the description, explain these embodiments.

[0007] Fig. 1 is a block diagram of a wireless communication system including an NE and a UE that perform methods according to various embodiments.

[0008] Fig. 2 illustrates non-roaming 5G system architecture in reference point representation.

[0009] Fig. 3 illustrates conventional connection management (CM) states and transitions in a UE.

[0010] Fig. 4 illustrates conventional CM states and transitions in AMF.

[0011] Fig. 5 is a schematic representation of a wireless communication system in which a UE is connected to a core network via a 3GPP access and a non-3GPP access with each access having its own individual NAS signaling pathway.

[0012] Fig. 6 is a schematic representation of a baseline access traffic steering, switching and splitting (ATSSS) scenario.

[0013] Fig. 7 illustrates a user plane (UP) protocol stack for Nx interface.

[0014] Fig. 8 illustrates architecture for simplified ATSSS over non-3GPP access on direct multipath QUIC (MPQUIC) connection between UE and the user plane function (UPF).

[0015] Fig. 9 is a signal diagram illustrating a PDU session establishment.

[0016] Fig. 10 is a signal diagram illustrating adding a non-3GPP access (a QUIC path) without non-3GPP NAS signaling.

[0017] Fig. 11 is a signal diagram illustrating a policy and control function (PCF) or SMF initiated session release when the 3GPP access is interrupted.

[0018] Fig. 12 is a signal diagram illustrating UE-initiated PDU session release via the non-3GPP access when the UE is not registered via 3GPP access.

[0019] Fig. 13 illustrates UE’s CM states and transitions for non-3GPP access according to an embodiment.

[0020] Fig. 1 illustrates UE’s CM states and transitions for non-3GPP access according to another embodiment.

[0021] Fig. 15 illustrates UE’s CM states and transitions for non-3GPP access according to yet another embodiment.

[0022] Figs. 16A, 16B, and 16C illustrate UE’s CM states and transitions for non- 3GPP access based on QUIC path statuses according to various embodiments.

[0023] Fig. 17 is a signal diagram for a PCF / SMF-initiated PDU session release when 3GPP access is interrupted according to an embodiment.

[0024] Fig. 18 is a signal diagram illustrating procedures for maintaining the non- 3GPP access for a retention time interval after the 3GPP access is interrupted according to an embodiment.

[0025] Fig. 19 is a signal diagram illustrating AMF-initiated procedures for disconnecting a MA PDU session after maintaining the non-3GPP access for a retention time interval according to an embodiment.

[0026] Fig. 20 is a signal diagram illustrating a UE-initiated registration procedure according to an embodiment.

[0027] Fig. 21 is a signal diagram illustrating a UE-initiated deregistration procedure according to an embodiment.

[0028] Fig. 22 is a signal diagram illustrating a UE-initiated service request procedure according to an embodiment.

[0029] Fig. 23 is a signal diagram illustrating a PDU session modification procedure according to an embodiment.

[0030] Fig. 24 is a flowchart of a UE method according to an embodiment.

[0031] Fig. 25 is a flowchart of an NE method according to an embodiment.DETAILED DESCRIPTION

[0032] Methods and devices described in this section embody solutions that enable maintaining a non-3GPP access without control signaling (e.g., 5G NAS) over non- 3GPP access (a setup which is also known as “non-integrated non-3GPP access”) whenthe 3GPP access is lost, by resolving the open issues articulated in the background section.

[0033] Prior to discussing methods for maintaining the non-3GPP access according to various embodiments, Fig.1 schematically illustrates a wireless communication system 100 including an NE 130 and a UE 102, which are able and configured to perform these methods. The UE 102 can access the 5GC 110 via a first radio access network (RAN) node 104 or a second RAN node 106. The RAN 105 connects RAN nodes 104 and 106 to the 5GC 110. For the sake of simplicity and clarity, the following description refers mostly to 5G radio access technology (RAT), but this RAT is an illustration and should not be interpreted as a limitation; other RATs such as a sixth generation (6G) RAT may be employed. Thus, the RAN nodes 104 and 106 in Fig. 1 are NG-RAN nodes and the RAN 105 is a 5G RAN.

[0034] The first RAN node 104 serves (i.e. , intermediates communication with UEs located within) a first cell 107 and a second cell 108, and the second RAN node 106 serves a cell 109. These cells are New Radio (NR) cells and may be in the same Radio Access Network Notification Areas (RNA) or different RNAs. In general, the RAN 105 can include any number of RAN nodes, and each of the RAN node can serve one, two, three, or any other suitable number of cells. The UE 102 can support a 5G NR (or simply, "NR”) air interface to communicate with the RAN nodes 104 and / or 106. Each of the RAN nodes 104, 106 may connect to 5GC NEs (i.e., physical devices hosting 5G core network functions) via a 5GC-based interface (e.g., an S1 or an Ng interface). The RAN nodes 104 and 106 may also be interconnected via other specific interfaces (e.g., an X2 or an Xn interface).

[0035] The UE 102 is equipped with processing hardware 120 that includes one or more general-purpose processors and / or special-purpose processing units. The processing hardware 120 illustrated in Fig. 1 includes a processor 122 configured to process uplink (UL) data that the UE 102 transmits to the 5GC 110 via a RAN node, and / or downlink (DL) data the UE receives from or via a RAN node. The processing hardware 120 also includes a transmitter 124 configured to transmit UL data and a receiver 126 configured to receive DL data (or, alternatively, a transceiver performing both transmitting and receiving data). The UE may include (although not shown)transmission / reception hardware for other 3GPP RAT(s) besides 5G (e.g., LTE, 6G) and also for non-3GPP RAT(s) (e.g., WiFi, Bluetooth). The processing hardware 120 may also include a non-transitory computer-readable medium 128 (e.g., a memory) storing machine-readable instructions executable on the one or more general-purpose processors, and / or special-purpose processing units.

[0036] The first RAN node 104 is equipped with processing hardware 140 that may include one or more general-purpose processors and / or special-purpose processing units. The processing hardware 140 illustrated in Fig. 1 includes a processor 142 configured to process data that the first RAN node 104 transmits in DL direction (i.e., to a UE), or receives in the UL direction (i.e., from a UE). The processing hardware 140 also includes a transmitter 144 configured to transmit data in the DL direction and a receiver 146 configured to receive data in the UL direction (or, alternatively, a transceiver performing both transmitting and receiving data). The processing hardware 140 may also include a non-transitory computer-readable medium 148 (e.g., a memory) storing instructions that the one or more processors execute. The second RAN node 106 includes generally similar components.

[0037] As illustrated in Fig. 1 , the 5GC 110 includes an access and mobility management function (AMF) 112, a session management function (SMF) 114, a user plane function (UPF) 116, a policy control function (PCF) 118, a unified data management (UDM) 115, and a network slice selection function (NSSF) 117. The 5GC 110 may include other functions not illustrated in Fig. 1 . Each of the 5GC functions may be hosted by an NE 130 (i.e., processing hardware) that typically includes a processor 132, a transmitter 134, a receiver 136, and a memory 138 (which may store executable instructions for the processor to perform various methods described hereinafter). The same NE may execute one or more 5GC functions or instances of 5GC functions. For example, the 5GC 110 may have a plurality of UPF instances running on the same NE or on different NEs.

[0038] The AMF 112 is configured to manage authentication, registration, paging, and other related functions. The SMF 114 is configured to manage PDU sessions, and the UPF 116 is configured to transfer user-plane packets related to audio calls, video calls, Internet traffic, etc., between the UE and a data network. The UDM 115 is a cloud-based entity managing information for access authorization, user registration, data networkprofiles, generating credentials used during authentication, and sending credentials to other network functions based on user subscription. The NSSF 117 selects the network slicing instance, determines the allowed network slice selection assistance information, and sets an AMF to serve the UE. The PCF 118 is a network function that provides policy control and charging rules for 5G services and applications thereby facilitating network behavior control, network slicing, UE activities, and communication with other 5GC functions.

[0039] The current 3GPP technical specifications (TSs) describe a 5GS architecture reference model (e.g., 3GPP TS 23.501 ), a 5GS architecture reference model of policy and charging control framework for the 5GS (e.g., 3GPP TS 23.503), features of network functions are (e.g., 3GPP TS 23.501 ), and network function services and descriptions (e.g., 3GPP TS 23.501 ). The control and user plane protocol stacks are listed in 3GPP TS 23.501 , and procedures for the 5GS including 5GS mobility management and 5GS session management are described in 3GPP TS 23.502.

[0040] The interaction between network functions may be understood based on a service-based architecture or a reference point representation. Fig. 2 illustrates a nonroaming 5G system architecture in reference point representation. The 5GC functions in Fig. 2 (most of which have already been described relative to Fig. 1 ) enable other authorized network functions to access their services and data networks such as 219. Thus, the service-based architecture provides a modular framework for application functions, which can be in trusted domain or untrusted domain outside the 5GC, deployed using components from various sources and suppliers to provide services to users (e.g., UE 102)."

[0041] Fig. 3 illustrates CM state transitions in UE (e.g., UE 102) and Fig. 4 illustrates CM state transitions in AMF (e.g., AMF 112) according to 5GS CM state models described in 3GPP TS 23.501 . From UE’s perspective (illustrated in Fig. 3), the UE’s connection to a 5G network switches from the CM-idle state 310 to the CM-connected state 320 when access network (AN) signaling 313 (e.g., radio resource control (RRC) messages) for establishing the connection is exchanged between the UE (e.g., 102) and a RAN node (e.g., 104). The UE’s connection state then switches from the CM-connected state 320 to the CM-idle state 310 when AN signaling 317 for releasing the connection(i.e., disconnecting the UE) is exchanged between the UE and the RAN node managing the UE’s serving cell.

[0042] From AMF’s perspective (illustrated in Fig. 4), the UE via RAN connection to the 5GC switches from the CM-idle state 410 to the CM-connected state 420 when N2 context (i.e., control plane interface between the 5GC and a RAN node) is established 413. The connection state then switches from the CM-connected state 420 to the CM-idle state 410 when AN signaling for releasing the connection (i.e., disconnecting the UE) 417 is exchanged between the UE and the RAN node managing the UE’s serving cell. When a UE enters the CM-IDLE state, the user plane (UP) connection of the PDU sessions that were active on this access (either 3GPP access or non-3GPP access) are deactivated.

[0043] Further, 3GPP TS 23.501 describes support of a UE connected via both 3GPP and non-3GPP access. The AMF manages both a CM state for 3GPP access and a CM state for non-3GPP access for a UE. An N2 interface can serve the UE for either 3GPP access or for non-3GPP access. The UE connected over both 3GPP and non- 3GPP with independent NAS signaling has two N2 interfaces, one for each access. The UE may be in any combination of the CM states between 3GPP and non-3GPP access (e.g., a UE may be in CM-IDLE state for one access and CM-CONNECTED state for the other access, in CM-IDLE state for both accesses, or in CM-CONNECTED state for both accesses).

[0044] If the UE CM states in the AMF are CM-IDLE for 3GPP access and CM- CONNECTED for non-3GPP access, the AMF performs a network triggered service request (SR) procedure, when it has downlink data to be sent to this UE for 3GPP access, by sending either a paging request via 3GPP access or a NAS notification via non-3GPP access to this UE (as described in 3GPP TS 23.502).

[0045] Further, the connection management over a non-3GPP access is defined in 3GPP TS 23.501 . According to this description, a UE that successfully establishes a Non- 3GPP Access Connection to the 5GC over a non-3GPP access transitions to CM- CONNECTED state for the non-3GPP access. A UE does not establish multiple simultaneous non-3GPP access connections to the 5GC. The release of the non-3GPP access connection between the UE and the Non-3GPP InterWorking Function (N3IWF) and Trusted Non-3GPP Gateway Function (TNGF), trusted WLAN interconnectingfunction (TWIF), or wireline access gateway function (W-AGF) is interpreted (i) by the N3IWF, TNGF, TWIF, and W-AGF as a criterion to release the N2 connection, and (ii) by the UE as a criterion for the UE to transition to CM-IDLE.

[0046] A UE registered over non-3GPP access remains in registration management (RM) state RM-REGISTERED, unless the non-3GPP access connection release occurs as part of a deregistration procedure over non-3GPP access, in which case the UE enters the RM-DEREGISTERED state. When the UE in RM-REGISTERED state transitions to CM-IDLE, the UE non-3GPP deregistration timer starts running in the UE. The UE non- 3GPP deregistration timer stops when the UE moves to CM-CONNECTED state or to the RM-DEREGISTERED state. When moved to CM-IDLE state relative to one access, the UE can attempt to re-activate UP connections for the PDU sessions over another access, per UE policies and depending on the availability of these accesses.

[0047] In the case of non-3GPP access, when the AMF releases the N2 interface, the N3IWF, TNGF, TWIF, and W-AGF release all the resources associated with the UE, including the non-3GPP access connection with the UE and its corresponding N3 resources. A release of the N2 connection by the AMF sets the CM state for the UE in the AMF to CM-IDLE. It is assumed that a UE configured to receive services from a 5GC over non-3GPP access that is in the RM-DEREGISTERED or CM-IDLE state attempts to establish a non-3GPP access connection and transition to the CM-CONNECTED state whenever the UE successfully connects to a non-3GPP access unless prohibited by the network to make a non-3GPP access connection (e.g., due to network congestion). A UE cannot be paged on a non-3GPP access network.

[0048] When a UE, simultaneously registered over a 3GPP access and a non- 3GPP access, moves all the PDU sessions to one of the accesses, the UE may initiate a deregistration procedure in the access that has no PDU sessions (depending on the UE implementation). The release of PDU sessions over the non-3GPP access does not imply the release of N2 connection. When the UE has PDU sessions routed over the non-3GPP access and the UE state becomes CM-IDLE for the non-3GPP access, these PDU sessions are not released, which enables the UE to move the PDU sessions over to the 3GPP access based on UE policies. The core network maintains the PDU sessions but deactivates the N3 user plane connection for such PDU sessions.

[0049] Fig. 5, which is similar to a figure in 3GPP TS 23.501 , illustrates a UE 102 that supports N1 over 3GPP access 501 (which is illustrated in Fig. 1 via RAN 105 and the communication cells 107-109) and N1 over non-3GPP access 503 (e.g., an IP communication). The UE 102 and UPF 116 may embed one or more of multi-path transport control protocol (MPTCP) functionality, MPQUIC functionality, access traffic steering, switching and splitting lower layer (ATSSS-LL) functionality, and performance measurement functionality (PMF). This kind of functionalities require cooperation betweeen corresponding modules in the UE 102 and the UPF 116.

[0050] The 3GPP technical report (TR) 23.700-54 proposes the architecture for ATSSS-Lite illustrated in Fig. 6. In this architecture, the 3GPP access 601 is substantively the same as 501 in Fig. 5, but the non-3GPP access 503 (i.e. , non-3GPP access specific functions) has been replaced by an IP network -WiFi / IP access 605 (enabling UE’s Internet access). Note the steering functionality 645 included in the UE and the steering functionality 635 included the UPF 116 (i.e., the NE executing the UPF) operating in this architecture.

[0051] Fig. 7 is a schematic representation of user plane (UP) protocol stack for Nx interface enabling data exchange between the UE 102 and application server 719. Note that the non-integrated non-3GPP access (NIN3A) 705 provides direct IP connectivity between the UE and the UPF without any intermediate NF such as Non-3GPP InterWorking Function (N3IWF) or Trusted Non-3GPP Gateway Function (TNGF). The 3GPP TR 23.700-54 describes a simplified ATSSS over non-3GPP access based on direct MPQUIC connection between UE and UPF. Because QUIC is encrypted, the MPQUIC steering functionality between UE and UPF may be used without the underlying IPSec layer and without a gateway such as N3WIF or TNGF. IPSec is a set of protocols that integrate security in the internet protocol (IP) and provide data source authentication, data integrity, confidentiality, and protection against replay attacks. In this case there is no NAS signaling connection via non-3GPP access, and therefore the MA PDU session needs to be established and managed via 3GPP access. This approach is based on the following basic principles and assumptions: (i) N3IWF / TNGF is not used when accessing over non-3GPP access; (ii) the NAS procedures over 3GPP access are used to register in 5GC and establish an MA PDU session; (iii) the NAS procedures via 3GPP access arealso used to provision the UE with necessary information to connect towards the UPF with MPQUIC over non-3GPP access; (iv) the UE has no N1 (NAS) signaling connection with the 5GC over non-3GPP access; (v) in order to add non-3GPP access user plane resources for a MA PDU session, the UE first has to establish a PDU session with 5GC over 3GPP access; (vi) the UPF (PSA) has at least one transport address (i.e. , an IP address and a port number) that is reachable via the Internet; and / or (vii) this approach only supports MPQUIC Steering Functionality (i.e., ATSSS_LL and MPTCP are not supported).

[0052] Fig. 8 (which is also based on a figure in 3GPP TR 23.700-54) illustrates architecture for simplified ATSSS over non-3GPP based on direct MPQUIC connection between UE and UPF. The Nx reference point supports MPQUIC connectivity between UE and UPF and is based on an existing MPQUIC interface between UE and UPF. This approach does not use primary (e.g., authentication and key agreement (AKA), based) authentication via non-3GPP access and relies on the UE being authenticated via 3GPP access. In some embodiments, the MA PDU session establishment procedure is transparent to the AMF. In other embodiments, the AMF is enhanced to support simplified ATSSS procedures over non-3GPP access.

[0053] Fig. 9 (which is also based on a figure included in 3GPP TR 23.700-54) illustrates a PDU session establishment 950 over 3GPP access. The use of MPQUIC over non-3GPP access is based on the MPQUIC steering functionality implemented in 3GPP Release 18, with the clarifications and changes described in 3GPP TR 23.700-54. The UE sends 952 a PDU session establishment request to AMF 112 via RAN node 104. The AMF 112 then sends 954 a message to the SMF 114 to trigger the SMF to create a PDU session context. The SMF 114 then retrieves from the UDM 115 user subscription information and replies 955 with a response to the AMF 112. The SMF 114 then interacts 958 with the PCF 118 to establish the SM policy session, and then selects 960 the UPF instance (i.e., a PDU session anchor (PSA) UPF). Further, the SMF 114 sends 962 (via an N4 interface there-between) a session establishment request to the selected UPF 116 and receives 963 a response. The SMF 114 then informs 964 the AMF 112 that the PDU session has been established, which then prompts the AMF 112 to send 953 a PDU session establishment accept to the UE 102. The AMF 112 and the other 5GC functionsmay continue 966 interacting to complete PDU session establishment and an MPQUIC connection is then established 968 between the UE and the UPF.

[0054] Fig. 10 illustrates actions related to the addition of non-3GPP access userplane resources (i.e. , following the actions illustrated in Fig. 9). After the UE has established 1070 a QUIC connection over the 3GPP access (e.g., per establishment 950), the UE 102 connects the non-3GPP access 1005 and obtains 1072 the IP address on the non-3GPP access 1005. The procedure 1075 for adding the non-3GPP access includes the UE-UPF path validation 1074, and the UPF 116 sending 1076 a Session report to SMF 114. The SMF 114 and the UPF 116 may later modify 1077 this MA PDU session. After the session is established, the UE and the UPF exchange 1078 data via the QUIC path over the non-3GPP access. If the UE is not reachable via 3GPP access, the SMF may release the N4 Session and the UPF then releases the QUIC connections towards the UE.

[0055] Fig. 11 illustrates a conventional PCF-initiated or SMF-initiated PDU session release when the UE is not reachable via 3GPP access (actions occurring after the ones illustrated in Fig. 10). When the PCF 118 triggers or releases 1180 the PDU session or sua sponte upon discovering 1182 that the UE is not reachable via the 3GPP access, the SMF 114 sends 1184 a session release request to the UPF 116. After the UPF 116 and the UE 102 release the QUIC connection there-between, the UPF 116 replies 1185 with a session release response to the session release request. The SMF 114 then initiates (A) 1188 the SM policy association termination with the PCF 118, and (B) 1190 UE deregistration with the UDM 115.

[0056] The 3GPP TR 23.700-54 describes a UE-triggered PDU session release via non-3GPP access when a UE is not registered via 3GPP access. If the UE wants to release the MA PDU session and the 5GC is not reachable via 3GPP access, the UE releases the QUIC connection. If the SMF has requested the UPF to report access availability and unavailability, the UPF notifies the SMF that non-3GPP access is unavailable when the last QUIC connection for a N4 session is removed. Fig. 12 illustrates such a UE-triggered PDU session release via non-3GPP access when UE is not registered via 3GPP access. The UE 102 determines 1281 to trigger the PDU session release. The QUIC connection release 1286 is similar to 1186 except that it is UE-triggered. Similar to 1076 in Fig. 10, the UPF 116 then sends 1276 a session report to the SMF 114, which the SMF then acknowledges (no label).

[0057] Starting from the above-described context, the following embodiments are predicated on these assumptions: (A) the establishment / modification of an MA PDU session via non-3GPP access is not available, and (B) the UE does not have NAS capability over the non-3GPP access (i.e. , it is a non-integrated non-3GPP access).These assumptions consider an architecture that can support non-integrated WiFi (i.e., NIN3A). Conventionally, all non-3GPP access is supported with a network function, N3IWF, which can deliver NAS message to AMF. For such a NIN3A, the NAS signaling, and the UE’s state depends on the 3GPP access.

[0058] Focusing first on a UE state machine, when the UE deregisters 3GPP access explicitly via a deregistration request procedure or implicitly due to loss of 3GPP access, the UE enters 5GMM-deregistered state over 3GPP access (here MM stands for mobility management). Since there is no NAS signaling over the non-3GPP access, the UE does not maintain any 5GMM-registered or 5GMM-deregistered state over non-3GPP access or CM state over non-3GPP access at the AMF.

[0059] Fig. 13 illustrates a first option regarding the UE’s CM state for a UE supporting 3GPP access and non-3GPP access without NAS over non-3GPP access (i.e., NIN3A). In this case, the UE does not maintain a CM state for the non-3GPP access, states 1310 and 1320 being 3GPP access-related states. For a registered UE (e.g., 102), the UE switches 1313 from the CM-IDLE state 1310 to the CM-CONNECTED state 1320 if 3GPP access network (AN) signaling connection is established. The UE switches 1317 from the CM-CONNECTED state 1320 to the CM-IDLE state 1310 when the 3GPP AN signaling connection is released. Note again that in this first embodiment, the UE does not maintain non-3GPP access states.

[0060] Fig. 14 illustrates a second option according to which the UE and the AMF maintain a UE’s CM state over non-3GPP access that is aligned with (i.e., the same as) the UE’s CM state over 3GPP access. For a registered UE (e.g., 102), the UE switches 1413 from the (3GPP access and non-3GPP access) CM-IDLE state 1410 to the CM-CONNECTED state 1420 when a 3GPP AN signaling connection is established. The UE switches 1417 from the CM-CONNECTED state 1420 to the CM-IDLE state 1410when the 3GPP AN signaling connection is released. The CM-IDLE state 1410 and the CM-CONNECTED state 1420 refer to both the 3GPP and the non-3GPP accesses. Conventionally (e.g., for non-3GPP access with NAS over non-3GPP access) each connection has independent connection states that can be maintained independently.

[0061] Fig. 15 illustrates a third option according to which the UE and the SMF maintain a UE’s session management (SM) state over the non-3GPP access, which is aligned with (i.e. , the same as) the UE’s CM state over 3GPP access. Similar with the other options, for a registered UE (e g., 102), the UE switches 1513 from the CM-IDLE 3GPP access and SM-IDLE non-3GPP access 1510 to the CM-CONNECTED 3GPP access and SM-CONNECTED non-3GPP access state 1520 when a 3GPP AN signaling connection is established. The UE switches 1517 from the CM-CONNECTED 3GPP access and SM-CONNECTED non-3GPP access state 1520 to the CM-IDLE and SM- IDLE state 1510 when the 3GPP AN signaling connection is released. Here, the CM-IDLE state and the CM-CONNECTED state refer to the 3GPP access, where both the 3GPP and the non-3GPP access only have a common control signaling connection over 3GPP access. Further, for the registered UE, the UE enters the SM-CONNECTED state over non-3GPP access if a 3GPP AN signaling connection is established and then maintains the SM-CONNECTED state over non-3GPP access when it establishes an QUIC path for the MA PDU session. However, when the 3GPP AN signaling connection is released, the UE enters the SM-IDLE state over non-3GPP access.

[0062] In the states and transitions now described referring to Figs. 16A-C, a UE’s state over the non-3GPP access is maintained for a retention time period. When the UE deregisters 3GPP access explicitly via a deregistration request procedure or implicitly due to loss of 3GPP access, the UE enters a 5GMM-DEREGISTERED state over 3GPP access (here MM stands for “mobility management”). When there is no NAS signaling over non-3GPP access according to assumption (B), the UE does not maintain any 5GMM- REGISTERED or 5GMM-DEREGISTERED state over the non-3GPP access. However, when the UE successfully adds a new QUIC path over the non-3GPP access for the MA PDU session, the UE can maintain its state according to the following options. Fig. 16A is substantively the same as Fig. 13 except that in the scenario in Fig. 13, the states of theNIN3A access are the same as the ones for the 3GPP state while here the states of the NIN3A access are the ones in Figs. 16B and 16C, depending on the option.

[0063] Option A (Figure 16B) employs the AMF when the UE enters CM- CONNECTED state 1620 over the non-3GPP access. The AMF then maintains the CM- CONNECTED state 1620 over the non-3GPP access when the AMF gets 1618 a notification from the SMF about successfully adding user-plane resources for the MA PDU session for the QUIC path over the non-3GPP access. The AMF maintains CM- CONNECTED state over non-3GPP access when the AMF gets notification from the SMF about successfully adding user-plane resources for the MA PDU session for the QUIC path over non-3GPP access.

[0064] Option B (Figure 16C) employs the AMF when the UE enters SM- CONNECTED state 1621 over the non-3GPP access. The SMF maintains the SM- CONNECTED state 1621 over the non-3GPP access when the SMF successfully adds 1619 user-plane resources to the MA PDU session for the QUIC path over the non-3GPP access. The SMF maintains SM-CONNECTED state 1621 over the non-3GPP access when the SMF successfully adds user-plane resources to the MA PDU session for the QUIC path over non-3GPP access.

[0065] When the NE releases 1613 the 3GPP AN signaling connection, the UE releases QUIC path over non-3GPP access and the UE enters CM-Disconnected state 1610 over non-3GPP access per Option A or SM-Disconnected state 1611 over non- 3GPP access per Option B. As mentioned previously, the CM-IDLE state 1310 and CM- CONNECTED state 1320 shown in FIG. 16A coexist with either FIG. 16B per Option A or FIG. 16C per Option B.

[0066] According to embodiments in which the SMF operates with no retention times, the methods of handling a MA PDU session when the UE loses 3GPP access has the SMF performing the following: (i) based on operator policies, the SMF disconnects the MA PDU session when receiving a response message from the AMF which detects that the UE is unreachable (e.g. by sending a paging message but without getting a response from the UE); and (ii) if the UE is not reachable via 3GPP access and the SMF / PCF determines to release the MA PDU session, then the SMF releases the N4 Session andthe UPF releases the QIIIC connections towards the UE and other context for the N4 session.

[0067] Fig. 17 is a signal diagram for a PCF / SMF-initiated PDU session release when 3GPP access is interrupted according to an embodiment. In Fig. 17, the UE 102, which communicates via the RAN 104 with the AMF 112, in collaboration with the SMF 114, the UPF 116, the PCF 118, and the UDM 115, establishes 950 a PDU session over 3GPP access as illustrated in Fig. 9. Further, the UE 102 obtains 1772 (which is substantively similar to step 1072 in Fig. 10) an IP address after connecting to the non- 3GPP access 1705 (similar to 1005 in Fig. 10). Then, the UE 102 adds 1775 (which is substantively similar to 1075 in Fig. 10) a QUIC path over non-3GPP access for adding user-plane resources for the MA PDU session for the QUIC path over non-3GPP access. The UE 102 then has 1730 multiple QUIC connections with a QUIC path over 3GPP access and a QUIC path over non-3GPP access. Steps 950, 1772, 1775 and 1730 form a procedure 1750 for establishing an MA PDU session with multiple QUIC connections over 3GPP access and over non-3GPP access.

[0068] Next, the SMF 114 detects 1732 inactivity of the MA PDU session based on reports from the UPF 116, and the SMF / PCF 116 / 118 determines to release the MA PDU session. The SMF 114 sends 1733 an Namf_Communication_N1N2MessageTransfer Request (N1 : PDU session Resource Release Command) to the AMF 114, and then, the AMF 112 sends 1734 an N2 request message (N1 : PDU session Resource Release Command) to the RAN 104, whereby the N2 request message is PDU session Resource Release Command (e.g., as described in 3GPP TS 38.413). Further, if RAN 104 fails to deliver N1 message (PDU session Resource Release Command) to the UE 102 over 3GPP access due to loss 1731 of 3GPP access, the RAN 104 sends 1735, to the AMF 112, an N2 response message, including PDU session Resource Release Unsuccessful Transfer information element (IE) and a failure cause indicating that UE is unreachable. The AMF 112 then sends 1736 to the SMF 114 a Namf_N1N2TransferFailureNotification with N2 response message indicating that the UE is unreachable (i.e. , that the UE has lost 3GPP access and thus become unreachable).

[0069] The SMF 114 initiates a session release procedure 1745, by sending 1738 an N4 Session Release Request to the UPF 116. The UPF 116 the releases 1740 theQUIC connection(s) towards the UE and sends 1739 an N4 Session Release Response to the SMF 114. The SMF 114 terminates 1742 the SM policy association, if needed and deregisters 1744 the PDU session from the UDM 115.

[0070] If the UE regains 3GPP access connectivity via a Registration request procedure over 3GPP access, the UE 102 repeats the PDU session establishment procedure 1750.

[0071] In some embodiments now described and illustrated in Fig. 18, when the UE loses 3GPP access that provided sole NAS signaling connection, the SMF / PCF initiates MA PDU session retention over the non-3GPP access. The MA PDU session over the non-3GPP access is retained for a certain retention time period during which the UE continues to use the non-3GPP access for the MA PDU session. The retention time period needs to be agreed within the network system.

[0072] Upon receiving a notification from the AMF that the UE is unreachable, based on operator policies, the SMF / AMF can start a retention timer for non-3GPP access based on local configuration or retention timer information received from the AMF / SMF. Thus, before requesting to disconnect MA PDU session, the network function that maintains the UE’s state over a non-3GPP access manages a retention timer. For example, if the AMF maintains UE’s CM state between CM-connected and CM- disconnected, the AMF can determine starting / stopping the retention timer based on the local configuration or information received from the SMF / PCF. In another example, if the SMF maintains UE’s SM state between SM-connected and SM-disconnected, the SMF can determine starting / stopping the retention timer based on the local configuration or information received from the SMF / PCF. If the UE is not reachable via 3GPP access and the SMF / PCF determines to release the MA PDU session after the retention timer period expires, the SMF releases the N4 Session and the UPF releases the QUIC connections towards the UE and other context for the N4 session as illustrated in Fig. 18.

[0073] Procedure 1850, steps 1831 , and 1833-1835 in Fig. 18 are the same as procedure 1750, steps 1731 , and 1733-1735 in Fig. 17 and, therefore, their description is omitted. In step 1832, upon detecting the inactivity of the MA PDU session (e.g., based on reports from the UPF), the SMF 114 or the PCF 118 determines to enable retention over non-3GPP for the MA PDU session.

[0074] There are options regarding the origin of the retention timer value. In view of the N2 response message 1835 indicating UE unreachable (i.e., that the UE has lost 3GPP access and becomes unreachable), according to option A, the AMF provides the retention timer value to the SMF. That is, the AMF 112 sends 1846A an Namf_N1N2TransferFailureNotification that includes the retention timer value to the SMF 114, which then starts the retention timer. According to option B, the SMF 114 retrieves the retention value included in a local configuration. That is, the AMF 112 sends 1846B an Namf_N1N2TransferFailureNotification to the SMF 114, which then starts the retention timer based on local configuration. According to options C and D, the AMF starts the retention timer. In option C the SMF provides the retention timer value to the AMF. That is, the AMF 112 sends 1846C an Namf_N1N2TransferFailureNotification to the SMF 114, enables retention of the non-3GPP access of the MA PDU session for a certain retention time period and, therefore, the SMF 114 sends 1848, to the AMF 112, a notification message (e.g., Nsmf_PDUSeesion_SMContextStatusNotify) specifying a retention timer value. The AMF 112 then starts the retention timer. According to option D, the AMF 112 sends 1846D an Namf_N1N2TransferFailureNotification to the SMF 114 and starts the retention timer based on a value included in a local configuration at the AMF 112.

[0075] Regardless of which option is used, the SMF 114 then performs 1870 an N4 PDU session modification procedure to update the N4 rules to the UPF 116. Further, the UE 102 continues 1852 to use QUIC connection with QUIC path(s) over non-3GPP access. If the UE regains 3GPP access using a Registration Request procedure with indication for resuming MA PDU session over 3GPP access before the retention timer expires, the AMF / SMF stops 1854 the retention timer. The UE 102 then performs the following: (a) the UE sends, to the SMF, a PDU session modification Request message indicating the PDU session ID of the retained MA PDU session, triggering the SMF to send a N4 Session Modification Request to update N4 rules for the 3GPP access at the UPF; and (b) the UE may further perform UE-UPF path validation over 3GPP access to add 3GPP access user-plane resources at the UPF for adding QUIC path(s) for the MA PDU session. Further steps 1856, 1745 are skipped if the retention timer is stopped before it expires. When the retention timer expires, the SMF 114 determines to initiate a PDU session release procedure 1745 in the following cases: (i) if the retention timer at the SMFis expired and it maintains an SM-connected state over non-3GPP access; and (ii) if the SMF receives Namf_N1N2TransferFailureNotification from the AMF that the retention timer expired, the SMF maintains CM-connected state over non-3GPP access.

[0076] In the embodiments now described and illustrated in Fig. 19, after procedure 1950 (which is substantively the same as procedure 1750 and, therefore, its description is omitted), when the UE 102 loses 1931 the 3GPP access, the AMF 112 detects 1960 and then initiates MA PDU session retention over the non-3GPP access based on Option A, Option B, Option C, and Option D. The SMF / AMF 114 / 112 can start a retention timer for non-3GPP access based on operator policies, local configuration or retention timer information received from the AMF / SMF before requesting to disconnect MA PDU session. The network function (AMF or SMF) that maintains the UE’s state over non- 3GPP access manages the retention timer. For example, if the AMF maintains UE’s CM state between CM-connected and CM-disconnected, the AMF can determine starting / stopping the retention timer based on the local configuration or information received from the SMF / PCF. In another example, if the SMF maintains UE’s SM state between SM-connected and SM-disconnected, the SMF can determine starting / stopping the retention timer based on the local configuration or information received from the SMF / PCF. If the UE is not reachable via 3GPP access and the SMF / PCF determines to release the MA PDU session based on notification from the AMF, then: (a) the SMF releases the N4 Session, and (b) the UPF releases the QUIC connections towards the UE and other context for the N4 session.

[0077] Thus, after the AMF 112 detects that the UE 102 is not reachable before the implicit deregistration AMF timer expires and / or when not receiving the UE’s periodic registration request (i.e. , after the mobile reachability timer expires), different embodiments may then implement the following options. According to option A, during interaction 1962, the AMF 112 sends an Nsmf_PDUSession_UpdateContext request including a retention timer value for non-3GPP access to the SMF 114, triggering the SMF 114 to reply by sending an Nsmf_PDUSession_UpdateContext response and to start the retention timer. According to option B, the AMF 112 sends 1964 an Nsmf_PDUSession_UpdateContext request (including an indication of retention for non-3GPP access) to the SMF114, which then replies by sending an Nsmf_PDUSession_UpdateContext response and starts theretention timer based on a local configuration. According to option C, the AMF 112 sends 1966 an Nsmf_PDUSessionJJpdateContext request including an indication of retention over non-3GPP access, to the SMF 114, which then enables retention for the MA PDU session over the non-3GPP access and replies 1967 with an Nsmf_PDUSession_UpdateContext response specifying retention timer value. The AMF then starts the retention timer using the received value. According to option D, the AMF 112 determines to enable retention for the MA PDU session over non-3GPP access and sends 1968 an Nsmf_PDUSession_UpdateContext request including an indication of retention for non-3GPP access to the SMF 114, which then replies by sending an Nsmf_PDUSession_UpdateContext response. The AMF 112 then starts the retention timer based on a local configuration.

[0078] Regardless of the option, the SMF 112 then performs 1970 an N4 PDU session modification procedure to update the N4 rules to the UPF 116. Steps 1852, 1854, and 1856 and session release procedure 1745 follow similar to their counterparts in FIG. 18.

[0079] In the following described embodiments, the AMF / SMF handles an MA PDU session when the UE’s deregistration of the 3GPP access is imminent. First, in order to continue an MA PDU session over non-integrated non-3GPP access, the UE sends a NAS request message indicating the use of a retention timer for non-3GPP access. The UE sends this NAS request message over 3GPP access before or when it wants to deregister its 3GPP access. Based on the retention timer value (if available) from the network, the UE may continue to use an MA PDU session over non-3GPP access if losing 3GPP access (e.g., when moving out of the 3GPP coverage or explicitly deregistering 3GPP access). If the AMF receives a NAS message indicating use of a retention timer for non-3GPP access to continue the MA PDU session over non-3GPP access from the UE, the SMF / AMF can start a retention timer for non-3GPP access based on local configuration or retention timer value received from the AMF / SMF before requesting to disconnect the MA PDU session. The network function (AMF or SMF) that maintains the UE’s state over non-3GPP access manages the retention timer. For example, if the AMF maintains UE’s CM state between CM-connected and CM-disconnected, the AMF may determine starting / stopping the retention timer based on the local configuration or information received from theSMF / PCF. In another example, if the SMF maintains UE’s SM state between SM- connected and SM-disconnected, the SMF can determine starting / stopping the retention timer based on the local configuration or information received from the SMF / PCF.

[0080] Fig. 20 is a signal diagram illustrating a UE-initiated registration procedure 2051 according to an embodiment. Different from the conventional registration procedure, here the Registration Request message may indicate (i) a request type as mobile registration update, and a retention timer value (that input to a timer makes the timer to measure the retention duration period) or an indication of retention request, or (ii) a request type as non-integrated non-3GPP access update, and retention timer value or an indication of retention request. Procedure 2050 is the same as 1750 and, therefore, its description is omitted.

[0081] In one embodiment, the UE 102 sends 2072 a registration request message over 3GPP access to the AMF 112. The registration request message indicates a retention timer value for non-3GPP access (if available) or an indication of the retention request.The AMF 112 continues 2074 with steps of a conventional registration request procedure as described in 3GPP TS 23.502 performing the following: (i) updating the UECM Registration including suspend indication over 3GPP access to the UDM and / or (ii) updating the UE AM policy including suspend indication over 3GPP access to the PCF for the UE. The AMF 112 then sends 2076 an Nsmf_PDUSession_UpdateSMContext request specifying retention timer value for non-3GPP access if available or indicating a retention request, to the SMF 114. The SMF 114 and the PCF 118 may then perform 2078 an SM policy association update. Alternatively, the SMF 114 may perform 2080 an SM policy association update and determine to enable retention with retention timer for non-3GPP access based on at least one of the following: local configuration, a retention timer value received from the AMF 112 or received from the PCF 118.

[0082] The SMF 114 replies 2077 with an Nsmf_PDUSession_UpdateSMContext Response including a retention timer value for non-3GPP access if available, or an indication for the retention, to the AMF 112. The AMF 112 optionally determines 2082 retention timer value for non-3GPP access based on at least one of a local configuration, and / or a received retention timer value from the SMF. The AMF 112 sends 2073 a registration accept message, to the UE. This message may include a retention timer value(if available) based on operator’s policies with a retention timer configured locally at the AMF, obtained from the PCF for the UE, or obtained from the SMF. The UE 102 then starts a retention timer and continues to use the MA PDU session over non-3GPP access before the retention timer expires. Alternatively, if the UE does not receive the retention timer value in the registration accept message, it can continue to use the MA PDU session over non-3GPP access until the PDU session is released by the network. The following scenarios are based on Figure 20 and illustrate message flows of various alternatives.

[0083] According to a first scenario, the SMF maintains UE’s SM state over non- 3GPP access, hence it manages the retention timer over non-3GPP access. The UE also receives the configured retention timer information to synchronize with the SMF’s setting for retention timer. Then, the UE 102 sends 2072 Registration Request message including a retention timer value for non-3GPP access, to the AMF 112. The AMF 112 sends 2076 Nsmf_PDUSession_UpdateSMContext request message including the retention timer value, to the SMF. The SMF 114 determines 2080 a second retention timer value based on the UE indicated retention timer value, a local configuration, or received retention timer value obtained from the AMF or the PCF. The SMF 114 then sends 2077 the Nsmf_PDUSession_UpdateSMContext request message including the second retention timer value, to the AMF. The AMF 112 then sends 2073 the registration accept message including the second retention timer value to the UE.

[0084] According to a second scenario, the SMF 114 maintains UE’s SM state over non-3GPP access, hence the retention timer over non-3GPP access. The UE receives the configured retention timer value to synchronize with the SMF’s setting for retention timer. Then, the UE 102 sends 2072 a registration request message including an indication of retention for non-3GPP access, to the AMF 112. The AMF 112 then sends 2076 an Nsmf_PDUSession_UpdateSMContext request message including the indication of retention over non-3GPP access, to the SMF 114. The SMF 114 determines a first retention timer value based on: a local configuration or a retention timer value obtained from the PCF. The SMF 114 sends 2077 an Nsmf_PDUSession_UpdateSMContext response message including the first retention timer value, to the AMF 112. The AMF 112 then sends 2073 the registration accept message including the first retention timer value, to the UE 102.

[0085] According to a third scenario, the AMF maintains UE’s CM state over non- 3GPP access, hence it manages the retention timer over non-3GPP access. The UE also gets the configured retention timer information to synchronize with the AMF’s setting for retention timer. The UE 102 sends 2072 the registration request message including a retention timer value for non-3GPP access, to the AMF 112. The AMF 112 then sends 2076 an Nsmf_PDUSession_UpdateSMContext request message including the retention timer value, to the SMF 114. The SMF 114 determines to enable retention over non- 3GPP access and sends 2077 an Nsmf_PDUSession_UpdateSMContext request message including an indication of retention, to the AMF 112. The AMF determines 2082 a second retention timer value based on: the UE indicated retention timer value, local configuration, or a received retention indication from the SMF 114. The AMF 112 then sends 2073 the registration accept message including the second retention timer value, to the UE 102.

[0086] According to a fourth scenario, the AMF maintains UE’s CM state over non- 3GPP access, hence it manages the retention timer over non-3GPP access. The UE receives the configurated retention timer value to synchronize with the AMF’s setting for retention timer. The UE 102 sends 2072 the registration request message including the indication of retention for non-3GPP access, to the AMF 112. The AMF 112 sends 2076 an Nsmf_PDUSession_UpdateSMContext request message including an indication retention over non-3GPP access, to the SMF 114. The SMF 114 then determines 2080 a first retention timer value based on a local configuration, or a received retention timer value obtained from the PCF and sends 2077 an Nsmf_PDUSession_UpdateSMContext request message including the first retention timer value, to the AMF 112. The AMF 112 then determines 2082 a second retention timer value based on a UE indicated retention timer value, a local configuration, and / or a received retention timer value from the SMF 114 and sends 2073 the registration accept message including the second retention timer value, to the UE 102.

[0087] Regardless of the scenario, the SMF 114 performs 2070 an N4 PDU session modification procedure (similar to 1970) to update the N4 rules to the UPF 116. Steps 1852, 1854, and 1856 and session release procedure 1745 follow as previously described.

[0088] Fig. 21 illustrates the message flow for the UE initiated deregistration request 2153 when the UE explicitly requests to deregister from the 3GPP access with indicated retention time period over non-3GPP access. Procedure 2150 is the same as 1750 and, therefore, its description is omitted.

[0089] The UE 102 sends 2184 a deregistration request message over the 3GPP access, to the AMF 112. This deregistration request message may include a retention timer value for non-3GPP access (if available) or an indication of the retention request. The AMF 112 then sends 2186 an Nsmf_PDUSession_ReleaseSMContext request message including a retention timer value or an indication of retention or an Nsmf_PDUSession_UpdateSMContext request message including the retention timer value or the indication of retention, to the SMF 114. The SMF 114 may perform 2178 an SM policy association update (substantively the same as 2078) and determine 2180 to enable retention with retention timer for non-3GPP access based on at least one of a local configuration and a received retention timer value from the AMF or the PCF (2180 is substantively the same as 2080). The SMF 114 then sends 2187, to the AMF 112, (i) an Nsmf_PDUSession_ReleaseSMContext response including a retention timer value for non-3GPP access (if available) or an indication of the retention or (ii) an Nsmf_PDUSession_UpdateSMContext response specifying the retention timer value for non-3GPP access if available or including an indication for the retention. As in step 2082, the AMF 112 may determine 2182 a retention timer value for non-3GPP access based on at least one of a local configuration or a received retention timer value from the SMF 114. The AMF 112 sends 2185 a deregistration accept message, to the UE. This deregistration message may include a retention timer value (if available) based on operator’s policies with retention timer configured locally at the AMF, obtained from the PCF for the UE, or obtained from the SMF. Upon losing the 3GPP access, the UE starts the retention timer and continues to use the MA PDU session over non-3GPP access until the retention timer expires. Alternatively, if the UE does not receive the retention timer value in the deregistration accept message, it can continue to use the MA PDU session over non- 3GPP access until the PDU session is released by the network.

[0090] According to a first scenario, the SMF 114 maintains UE’s SM state over non-3GPP access, hence it manages the retention timer over non-3GPP access. The UEalso gets the configurated retention timer to synchronize with the SMF’s setting for retention timer. The UE 102 sends 2184 the deregistration request message including a retention timer value for non-3GPP access, to the AMF 112. The AMF 112 then sends2186 an Nsmf_PDUSession_ReleaseSMContext request message specifying the retention timer value, to the SMF 114. The SMF 114 determines 2180 a second retention timer value based on a UE indicated retention timer value, a local configuration or a retention timer value obtained from the AMF 112 or the PCF 118. The SMF 114 sends2187 an Nsmf_PDUSession_ReleaseSMContext response message including the second retention timer value, to the AMF 112. The AMF 112 then sends 2185 a deregistration accept message including the second retention timer value, to the UE 102.

[0091] According to a second scenario, the SMF maintains UE’s SM state over non-3GPP access, hence the retention timer over non-3GPP access. The UE receives the configured retention timer to synchronize with the SMF’s setting for retention timer. The UE sends 2184 a deregistration request message including an indication of retention for non-3GPP access, to the AMF 112. The AMF 112 then sends 2186 an Nsmf_PDUSession_UpdateSMContext request message including an indication retention over non-3GPP access, to the SMF 114. The SMF 114 determines 2180 a first retention timer value based on a local configuration or a retention timer value obtained from the PCF 118. The SMF 114 sends 2187 Nsmf_PDUSession_UpdateSMContext response message including the first retention timer value, to the AMF 112. The AMF 112 then sends 2185 a deregistration accept message including the first retention timer value, to the UE 102.

[0092] According to a third scenario, the AMF maintains UE’s CM state over non- 3GPP access, hence it manages the retention timer over non-3GPP access. The UE also gets the configured retention timer to synchronize with the AMF’s setting for retention timer. The UE 102 sends 2184 a deregistration request message including a retention timer value for non-3GPP access, to the AMF 112. The AMF 112 sends 2186 an Nsmf_PDUSession_ReleaseSMContext request message specifying the retention timer value, to the SMF 114. The SMF 114 then determines to enable retention over non-3GPP access and sends 2187 an Nsmf_PDUSession_ReleaseSMContext request message including an indication of retention, to the AMF 112. The AMF 112 then determines 2182a second retention timer value based on: UE indicated retention timer value, local configuration, or received retention indication from the SMF 114. The AMF sends a deregistration accept message including the second retention timer value, to the UE 102.

[0093] According to a fourth scenario, the AMF maintains UE’s CM state over non- 3GPP access, hence it manages the retention timer over non-3GPP access. The UE receives the configured retention timer to synchronize with the AMF’s setting for retention timer. The UE 102 sends 2184 a deregistration request message with an indication of retention for non-3GPP access, to the AMF 112. The AMF 112 then sends 2186 an Nsmf_PDUSession_ReleaseSMContext request message including an indication of retention over non-3GPP access, to the SMF 114. The SMF 114 determines 2180 a first retention timer value based on: local configuration or received retention timer value obtained from the PCF. The SMF 114 sends 2187 an Nsmf_PDUSession_ReleaseSMContext request message specifying the first retention timer value, to the AMF 112. The AMF 112 determines 2182 a second retention timer value based on a UE indicated retention timer value, a local configuration, and / or a retention timer value received from the SMF 114. The AMF 112 then sends 2185 a deregistration accept message including the second retention timer value, to the UE 102.

[0094] Regardless of the actual scenario, the SMF 114 then performs 2170 an N4 PDU session modification procedure (similar to 1970) to update the N4 rules to the UPF 116. Steps 1852, 1854, and 1856 and session release procedure 1745 follow as previously described.

[0095] Fig. 22 is a signal diagram illustrating a UE-initiated service request procedure 2255 according to an embodiment. Improvements to the conventional service request procedure are similar to the above-described improvements of the conventional deregistration procedure. A similar approach can be applied to the network-initiated service request procedure triggered by the SMF or the PCF. Procedure 2250 is the same as 1750 and, therefore, its description is omitted.

[0096] The UE 102 sends 2290 a Service request message over 3GPP access to the AMF 112. This service request message includes the MA PDU session ID, may include a retention timer value for non-3GPP access (if available) or an indication of retention. The AMF 112 then sends 2292, to the SMF, anNsmf_PDUSession_UpdateSMContext request including the MA PDU session ID, a retention timer value for non-3GPP access (if available), or an indication of retention. The SMF 114 may perform an SM policy association update 2278 (which is substantively similar to 2078) and may determine to enable retention with retention timer value for non- 3GPP access based on at least one of the following: local configuration, received retention timer value from the AMF or the PCF. In some scenarios the SMF 114 may determine 2280 the retention time value. The SMF 114 then sends 2293, to the AMF 112, an Nsmf_PDUSession_UpdateSMContext response including a retention timer value for non- 3GPP access (if available) or an indication of retention. The AMF 112 may determine retention timer value 2282 for non-3GPP access based on at least one of a local configuration, and / or a retention timer value received from the SMF 114. The AMF 112 then may send 2291 a service request accept message to the UE 112. Such a message may include a retention timer value (if available) based on operator’s policies with retention timer value configured locally at the AMF, obtained from the PCF for the UE, or obtained from the SMF. The UE 102 starts the retention timer and continues to use MA PDU session over non-3GPP access before the retention timer expires. Alternatively, if the UE does not receive the retention timer value in the service request accept message, it can continue to use the MA PDU session over non-3GPP access until the PDU session is released by the network.

[0097] According to a first scenario, the SMF maintains UE’s SM state over non- 3GPP access, hence it manages the retention timer over non-3GPP access. The UE also gets the configured retention timer to synchronize with the SMF’s setting for retention timer. The UE 102 sends 2290 a service request message including the MA PDU session ID and a retention timer value for non-3GPP access to the AMF 112. The AMF 112 then sends 2292 an Nsmf_PDUSession_UpdateSMContext request message (MA PDU session ID, retention timer value) to the SMF 114. The SMF 114 determines 2280 a second retention timer value based on: UE retention timer value, local configuration, or a retention timer value obtained from the AMF or the PCF. The SMF 114 then sends 2293 an Nsmf_PDUSession_UpdateSMContext response message including the second retention timer value to the AMF 112. The AMF 112 then sends 2291 a service request accept message including the second retention timer value to the UE 102.

[0098] According to a second scenario, the SMF maintains UE’s SM state over non-3GPP access, hence the retention timer over non-3GPP access. The UE receives the configured retention timer to synchronize with the SMF’s setting for retention timer. The UE 102 sends 2290 a service request message including the MA PDU session ID and an indication of retention for non-3GPP access to the AMF 112. The AMF 112 sends 2292 an Nsmf_PDUSession_UpdateSMContext request message including the MA PDU session ID and the indication of retention over non-3GPP access to the SMF 114. The SMF 114 determines 2280 a first retention timer value based on a local configuration, or a received retention timer value obtained from the PCF. The SMF 114 sends 2293 an Nsmf_PDUSession_UpdateSMContext response message including the first retention timer value to the AMF 112. The AMF 112 then sends a service request accept message including the first retention timer value to the UE 102.

[0099] According to a third scenario, the AMF maintains UE’s CM state over non- 3GPP access, hence it manages the retention timer over non-3GPP access. The UE also gets the configured retention timer to synchronize with the AMF’s setting for retention timer. The UE 102 sends 2290 a service request message including the MA PDU session ID and a retention timer value for non-3GPP access to the AMF 112. The AMF 112 then sends 2292 an Nsmf_PDUSession_UpdateSMContext request message including the MA PDU session ID and the retention timer value to the SMF 114. The SMF 114 determines to enable retention over non-3GPP access and sends 2293 an Nsmf_PDUSession_UpdateSMContext response message including an indication of retention to the AMF 112. The AMF 112 then determines 2282 a second retention timer value based on: UE indicated retention timer value, local configuration, and the retention indication received from the SMF 114. The AMF 112 sends 2291 to the UE 102 a service request accept message including the second retention timer value.

[0100] According to a fourth scenario, the AMF maintains UE’s CM state over non- 3GPP access, hence it manages the retention timer over non-3GPP access. The UE receives the configured retention timer to synchronize with the AMF’s setting for retention timer. The UE 102 sends 2290 a service request message including the MA PDU session ID and an indication of retention for non-3GPP access to the AMF 112. The AMF 112 sends 2292 an Nsmf_PDUSession_UpdateSMContext request message (including theMA PDU session ID, an indication for retention over non-3GPP access) to the SMF 114. The SMF 114 determines 2280 a first retention timer value based on a local configuration, or a retention timer value obtained from the PCF. The SMF 114 then sends 2293 an Nsmf_PDUSession_UpdateSMContext response message including the first retention timer value to the AMF 112. The AMF 112 determines 2282 a second retention timer value based on UE indicated retention timer value, a local configuration, and / or a received retention timer value from the SMF 114. The AMF 112 then sends 2291 a service request accept message including the second retention timer value to the UE 102.

[0101] Regardless of the scenario, the SMF 114 performs 2270 an N4 PDU session modification procedure to update the N4 rules to the UPF 116. Steps 1852, 1854, and 1856 and session release procedure 1745 follow as previously described.

[0102] Fig. 23 is a signal diagram illustrating a PDU session modification procedure 2357 according to an embodiment. Procedure 2350 is the same as 1750 and, therefore, its description is omitted.

[0103] The UE 102 sends 2394 a PDU session modification request message over 3GPP access to the AMF. The PDU session modification request message includes the MA PDU session ID, and a retention timer value for non-3GPP access (if available) or indication of retention. The AMF 112 then sends 2392, to the SMF 114, an Nsmf_PDUSession_UpdateSMContext request including the MA PDU session ID, plus a retention timer value for non-3GPP access (if available) or an indication of retention. The SMF 114 may perform 2378 an SM policy association update (which is substantively similar to 2078) and may determine 2380 (which is substantively similar to 2080) to enable retention with retention timer for non-3GPP access based on at least one of the following: local configuration, received retention timer value from the AMF, or received retention timer value from the PCF. The SMF 114 sends 2393, to the AMF, an Nsmf_PDUSession_UpdateSMContext response message including a retention timer value for non-3GPP access (if available) or an indication of retention. The AMF 112 may determine 2382 (which is similar to 2082) a retention timer value for non-3GPP access based on at least one of the following: local configuration, and / or received retention timer value from the SMF. The AMF 112 may send 2395 a PDU session modification command to the UE, which message may include a retention timer value (if available) based onoperator’s policies with the retention timer configured locally at the AMF, obtained from the PCF for the UE, or obtained from the SMF. The UE then starts the retention timer and continues to use the MA PDU session over non-3GPP access until the retention timer expires. Alternatively, if the UE does not receive the retention timer value from the AMF, it can continue to use the MA PDU session over non-3GPP access until the PDU session is released by the network.

[0104] According to a first scenario, the SMF maintains UE’s SM state over non- 3GPP access, hence it manages the retention timer over non-3GPP access. The UE also gets the configurated retention timer to synchronize with the SMF’s setting for retention timer. The UE 102 sends 2394 a PDU session modification request message including the MA PDU session ID and a retention timer value for non-3GPP access to the AMF 112. The AMF 112 then sends 2392 an Nsmf_PDUSession_UpdateSMContext request message including the MA PDU session ID and the retention timer value to the SMF 114. The SMF 114 determines 2380 a second retention timer value based on a UE indicated retention timer value, a local configuration, and / or a retention timer value obtained from the AMF or the PCF. The SMF 114 sends 2393 an Nsmf_PDUSession_UpdateSMContext response message including the second retention timer value to the AMF 112. The AMF 112 then sends 2395 a PDU session Modification Command message including the second retention timer value to the UE 102.

[0105] According to a second scenario, the SMF maintains UE’s SM state over non-3GPP access, hence the retention timer over non-3GPP access. The UE receives the configured retention timer to synchronize with the SMF’s setting for retention timer. The UE 102 sends a Service Request message including the MA PDU session ID and an indication of retention for non-3GPP access to the AMF 112. The AMF 112 then sends 2392 an Nsmf_PDUSession_UpdateSMContext request message including the MA PDU session ID and the indication of retention over non-3GPP access to the SMF 114. The SMF 114 determines 2380 a first retention timer value based on a local configuration, or a received retention timer value obtained from the PCF. The SMF 114 then sends an Nsmf_PDUSession_UpdateSMContext request message including the first retention timer value to the AMF 112. The AMF 112 sends 2395 a PDU session modification command message including the first retention timer value to the UE 102.

[0106] According to a third scenario, the AMF maintains UE’s CM state over non- 3GPP access, hence it manages the retention timer over non-3GPP access. The UE also gets the configured retention timer to synchronize with the AMF’s setting for retention timer. The UE 102 sends 2394 a PDU session modification request message including the MA PDU session ID and a retention timer value for non-3GPP access to the AMF 112. The AMF 112 then sends an Nsmf_PDUSession_UpdateSMContext request message including the MA PDU session ID and the retention timer value to the SMF 114. The SMF 114 determines to enable retention over non-3GPP access and sends 2393 an Nsmf_PDUSession_UpdateSMContext request message including an indication of retention to the AMF 112. The AMF 112 determines 2382 a second retention timer value based on a UE indicated retention timer value, a local configuration, and / or a received retention indication from the SMF 114. The AMF 112 then sends 2395 a PDU session modification command message including the second retention timer value to the UE 102.

[0107] According to a fourth scenario, the AMF maintains UE’s CM state over non- 3GPP access, hence it manages the retention timer over non-3GPP access. The UE receives the configured retention timer to synchronize with the AMF’s setting for retention timer. The UE 102 sends 2394 a PDU session modification request message including the MA PDU session ID and an indication of retention for non-3GPP access to the AMF 112. The AMF 112 then sends 2392 an Nsmf_PDUSession_UpdateSMContext request message including the MA PDU session ID and an indication retention over non-3GPP access to the SMF 114. The SMF 114 determines 2380 a first retention timer value based on a local configuration, or a received retention timer value obtained from the PCF. The SMF 114 sends 2393 an Nsmf_PDUSession_UpdateSMContext request message including the first retention timer value to the AMF 112. The AMF 112 determines 2382 a second retention timer value based on a UE indicated retention timer value, a local configuration, and / or a retention timer value received from the SMF. The AMF 112 sends 2395 a PDU session modification command message including the second retention timer value to the UE 102.

[0108] Regardless of the scenario, the SMF 114 performs 2370 (similar to 1970) an N4 PDU session Modification procedure to update the N4 rules to the UPF 116. Steps 1854, 1856 and session release procedure 1745 follow as previously described.

[0109] Fig. 24 is a flowchart of a UE method 2400 according to an embodiment. The method 2400 includes establishing 2450 an MA PDU session with an NE. This MA PDU session includes a 3GPP access and a non-3GPP access with control signaling on the 3GPP access only. This step corresponds to 1850, 1950, 2050, 2150, 2250, and 2350. The method 2400 further includes communicating 2452 via the non-3GPP access with an UPF of the wireless communication system, for a retention time interval after losing the 3GPP access. This step corresponds to 1852. The UE may receive a retention time value from the NE (e.g., AMF via RAN) and may then start a retention timer using the retention timer value to measure the retention time interval upon detecting that the 3GPP access is lost. The UE may transmit a UE-provided retention timer value to the NE during a registration procedure, a service request procedure, or a session modification procedure. The UE may transmit an indication of retention to the NE during the registration procedure or the service request procedure. The method may also include releasing the MA PDU session when the retention time ends. The wireless communication system may be a 5G system as specified in 3GPP technical specifications, the NE being a 5G core network device performing an SMF and the control signaling being a NAS signaling.

[0110] Fig. 25 is a flowchart of an NE method 2500 according to an embodiment. The method 2500 includes establishing 2550 an MA PDU session with a UE. This MA PDU session includes a 3GPP access and a non-3GPP access with control signaling on the 3GPP access only. This step corresponds to 1850, 1950, 2050, 2150, 2250, and 2350. The method 2500 further includes initiating 2552 an MA PDU session retention procedure that maintains the non-3GPP access for a retention time interval after the UE loses the 3GPP access. This step corresponds to step 1852. The method 2500 may also include starting a retention timer using a retention timer value to measure the retention time interval when detecting that the UE lost the 3GPP access. The retention timer value may be received from the UE during a registration procedure, a service request procedure, or a session modification procedure. The retention timer value may also be retrieved from a core network function or may be stored by the NE. The NE may determine the retention timer value. The method may further include stopping the retention timer when detecting that 3GPP access has been restored. The method mayalso include initiating an MA PDU session release procedure when the retention timer expires. The method 2500 may include receiving, from the UE, an indication of retention to enable initiating the MA PDU session retention procedure.

[0111] The embodiment descriptions in this section refer to the accompanying drawings. The same reference numbers in different drawings identify the same or similar elements. The detailed descriptions do not preclude other embodiments. The embodiments are not limited to the described configurations but may be extended to other arrangements.

[0112] Reference throughout this section to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with an embodiment is included in at least one embodiment. Thus, the appearances of the phrases “in one embodiment” or “in an embodiment” in various places throughout the specification are not necessarily all referring to the same embodiment. Further, the particular features, structures or characteristics may be combined in any suitable manner in one or more embodiments.

[0113] Numerical adjectives “first”, “second”, and “third” do not imply any order (are not ordinals) but are markers to distinguish separate instances of similar elements.References to the singular (e.g., “a” or “an”, “the”) should include the plural unless clearly indicated otherwise.

[0114] As used herein, a phrase referring to “at least one of’ or “one or more of’ a list of items refers to any combination of those items, including single members. For example, “at least one of: a, b, or c” is intended to cover the possibilities of: a only, b only, c only, a combination of a and b, a combination of a and c, a combination of b and c, and a combination of a and b and c.

[0115] Although the features and elements of the present embodiments are described in the embodiments in particular combinations, each feature or element can be used alone without the other features and elements of the embodiments or in various combinations with or without other features and elements disclosed herein. The methods or flowcharts may be implemented in a computer program, software or firmware tangibly embodied in a computer-readable storage medium for execution by a specifically programmed computer or processor.

Claims

WHAT IS CLAIMED IS:1 . A method (2400) performed by a user equipment, UE, (102) in a wireless communication system (100), the method comprising: establishing (2450, 1850) a multi-access packet data unit, MA PDU, session with a network entity, NE, (130) of the wireless communication system, the MA PDU session using a 3GPP access and a non-3GPP access and having control signaling on the 3GPP access only; and communicating (2452, 1852) data via the non-3GPP access with a user plane function, UPF, of the wireless communication system, for a retention time interval after losing the 3GPP access.

2. The method of claim 1 , further comprising: receiving a retention timer value from the NE; and starting a retention timer using the retention timer value to measure the retention time interval upon detecting that the 3GPP access is lost.

3. The method of claim 1 or 2, further comprising: transmitting a UE-provided retention timer value to the NE during a registration procedure, a service request procedure, or a session modification procedure.

4. The method of claim 3 further comprising: transmitting an indication of retention to the NE during the registration procedure or the service request procedure.

5. The method of any of claims 1 to 4, wherein the communicating of data via the non-3GPP access for the retention time interval after losing the 3GPP access, includes communicating of data via the non-3GPP access for the retention time interval after the UE indicates non-3GPP retention while deregistering the 3GPP access.

6. The method of any of claims 1 to 4, further comprising: releasing the MA PDU session when the retention time interval ends.

7. A method (2500) performed by a network entity, NE, (104, 130) in a wireless communication system (100), the method comprising: establishing (2550, 1850) a multi-access packet data unit, MA PDU, session with a user equipment, UE, (102), the MA PDU session including a 3GPP access and a non- 3GPP access with control signaling on the 3GPP access only; and initiating (2552, 1852) a MA PDU session retention procedure that maintains the non-3GPP access for a retention time interval after the UE loses the 3GPP access.

8. The method of claim 7, further comprising: when detecting that the UE lost the 3GPP access, starting a retention timer using a retention timer value to measure the retention time interval.

9. The method of claim 8, further comprising: receiving the retention timer value from the UE during a registration procedure, a service request procedure, or a session modification procedure.

10. The method of claim 8, further comprising: retrieving the retention timer value from a core network function.11 . The method of claim 8, wherein the NE stores the retention timer value.

12. The method of claim 8, further comprising: determining the retention timer value.

13. The method of any of claims 8 to 12, further comprising:stopping the retention timer when detecting that 3GPP access has been restored.

14. The method any of claims 8 to 13, further comprising: initiating an MA PDU session release procedure when the retention timer expires.

15. The method of any of claims 7 to 14, further comprising: receiving, from the UE, an indication of retention to enable the initiating of the MA PDU session retention procedure.

16. A wireless communication device (102, 104, 120, 130) comprising a processor (122, 132), a transceiver (122, 124, 132, 134), and computer readable recording medium (128, 138) storing executable codes that, when executed by the processor in collaboration with the transceiver, make the wireless communication device to perform any of the methods recited in claims 1 to 15.

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