WO2021136651A1

WO2021136651A1 - Method and apparatus for terminal device availability management

Info

Publication number: WO2021136651A1
Application number: PCT/EP2020/086122
Authority: WO
Inventors: Yunjie Lu; Juying GAN; Qian Chen
Original assignee: Telefonaktiebolaget Lm Ericsson (Publ)
Priority date: 2019-12-31
Filing date: 2020-12-15
Publication date: 2021-07-08

Abstract

Embodiments of the present disclosure provide methods and apparatuses for terminal device availability management. A method performed by a first access and mobility management function entity comprises receiving a first message from a first network function entity. The first message requests a first notification toward a second network function entity when a terminal device becomes available after downlink data notification failure for the terminal device and the first message includes at least one of a traffic descriptor identifying a source of the downlink data and network information associated to a session related to the terminal device. The method further comprises sending a second message to at least one session management function entity associated to the session related to the terminal device. The second message requests a second notification toward the first access and mobility management function entity when the downlink data notification failure for the terminal device is detected and the second message includes the traffic descriptor when the first message includes the traffic descriptor.

Description

METHOD AND APPARATUS FOR TERMINAL DEVICE AVAILABILITY

MANAGEMENT

TECHNICAL FIELD

[0001] The non-limiting and exemplary embodiments of the present disclosure generally relate to the technical field of communications, and specifically to methods and apparatuses for terminal device availability management.

BACKGROUND

[0002] This section introduces aspects that may facilitate a better understanding of the disclosure. Accordingly, the statements of this section are to be read in this light and are not to be understood as admissions about what is in the prior art or what is not in the prior art.

[0003] The terminal device availability management has been introduced in some communication systems. For example, availability after DDN (Downlink Data Notification) failure event is specified in 3rd Generation Partnership Project (3GPP) TS 23.682 V16.4.0, the disclosure of which is incorporated by reference herein in their entirety. In this feature, the AS (application server) subscribes once and then gets notification only when there has been some data delivery failure followed by the UE (user equipment) becoming reachable. This feature involves an entry in the subscription for a UE for "network application triggering when the UE is available after a DDN failure". This is a different monitoring event from the "UE is reachable" monitoring event. This information is provided to the serving node (MME(Mobile Management Entity)/SGSN (Serving GPRS(General Packet Radio Service) Support Node)) at registration. The serving node notes this and sets a Notify-on-available-after-DDN-failure flag after a DDN failure. If the flag is set when the UE next contacts the network, the serving node notifies the SCEF (Service Capability Exposure Function) that the UE is reachable, and will clear the flag. A use case for this feature is the application that wants to communicate with a UE that sleeps for a long time. If downlink packets from the application are not delivered, the application recognizes that the UE is not available by lack of response within a reasonable time from the UE, and then await notification from the network (i.e. from the MME/S4-SGSN via the SCEF) of UE reachability.

SUMMARY

[0004] This summary is provided to introduce a selection of concepts in a simplified form that are further described below in the detailed description. This summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used to limit the scope of the claimed subject matter. [0005] Some solutions for availability after DDN failure event have been proposed in some communication systems such as EPS (Evolved Packet System) and 5GS (the fifth generation (5G) system). However the existing solutions have some problems, such as heavy and expensive traffic for subscription update when access and mobility management function entity such as AMF (Access and Mobility Management Function) changes, potential doubled signaling over some interfaces such as N16/N16a when a session management function entity such as V-SMF(Visited SMF (session management function))/l-SMF (Intermediate SMF) is changed/inserted, etc.

[0006] To overcome or mitigate at least one of above mentioned problems or other problems, the embodiments of the present disclosure propose an improved solution of terminal device availability management.

[0007] In an embodiment, the proposed solution can simplify the system level complexity of the exposure handling for availability after DDN failure event by introducing an access and mobility management entity centric process for availability after DDN failure event.

[0008] In an embodiment, the access and mobility management entity such as AMF may manage the configuration and lifecycle of availability after DDN failure event, as a single exposure point.

[0009] In an embodiment, for the session (such as PDU (protocol data unit) Session) matches network information such as data network name (DNN) and single network slice selection assistance information (S-NASSI) provided by a data management entity such as UDM (unified data management) for availability after DDN failure event, the access and mobility management entity such as AMF may directly subscribe to the session management function entity associated to the session related to the terminal device and with the network information with the AMF’s notification endpoint to receive the event for ongoing PDU Sessions. [0010] In an embodiment, the access and mobility management entity may directly subscribe to the SMF associated to the session related to the terminal device and, optionally, with the network information when a new PDU Session establishment.

[0011] In an embodiment, the access and mobility management entity such as AMF may directly subscribe to a new I-SMFA/-SMF when the new l-SMF/V-SMF is installed in the PDU Session.

[0012] In an embodiment, the proposed solution can reuse existing service operations to save traffic and simplify the AMF/SMF logic.

[0013] In an embodiment, the access and mobility management entity such as AMF may subscribe to the DDN Failure Status on SMF as a part of create SM (session management) Context service operation, during PDU Session establishment or any SMF change scenarios.

[0014] In an embodiment, the access and mobility management entity such as AMF may unsubscribe to DDN Failure Status when invoking Release SM Context service operation. DDN failure Status reporting may be implicitly ended when SM Context Resource is released due to any reason. [0015] In an embodiment, the access and mobility management entity such as AMF may subscribe/unsubscribe to DDN Failure Status for ongoing PDU Sessions by using Update SM Context service operation, when the event configuration is provisioned/removed.

[0016] In an embodiment, SMF may inform the access and mobility management entity such as AMF for DDN Failure Status via SM Context Status Notification.

[0017] In an embodiment, AMF/SMF may use the normal SMF providing event exposure service operation to achieve less signaling optimization.

[0018] In a first aspect of the disclosure, there is provided a method performed by a first access and mobility management function entity. The method comprises receiving a first message from a first network function entity. The first message requests a first notification toward a second network function entity when a terminal device becomes available after downlink data notification failure for the terminal device and the first message includes at least one of a traffic descriptor identifying a source of the downlink data and network information associated to a session related to the terminal device. The method further comprises sending a second message to at least one session management function entity associated to the session related to the terminal device. The second message requests a second notification toward the first access and mobility management function entity when the downlink data notification failure for the terminal device is detected and the second message includes the traffic descriptor when the first message includes the traffic descriptor. [0019] In an embodiment, sending the second message to the at least one session management function entity associated to the session related to the terminal device may comprise: when the first message includes the network information, sending the second message to the at least one session management function entity associated to the session related to the terminal device and with the network information. [0020] In an embodiment, the network information may include a data network name and single network slice selection assistance information.

[0021] In an embodiment, the first network function entity may be a unified data management entity and the second network function entity may be a network exposure function entity.

[0022] In an embodiment, the first message may be a subscribe request or a subscribe update request. [0023] In an embodiment, the first message may be an Namf_EventExposure_Subscribe request.

[0024] In an embodiment, the first network function entity may be a second access and mobility management function entity and the second network function entity may be a network exposure function entity.

[0025] In an embodiment, the second message may be a subscribe request or a subscribe update request. [0026] In an embodiment, the second message may be an Nsmf_EventExposure_Subscribe request. [0027] In an embodiment, the subscribe request may be included in an smf_PDUSession_CreateSMContext Request or Nsmf_PDUSession_UpdateSMContext Request. The subscribe update request may be included in an Nsmf_PDUSession_UpdateSMContext Request.

[0028] In an embodiment, the method may further comprise receiving the second notification from a session management function entity; setting a flag of the first notification; detecting that the terminal device is reachable; and sending the first notification to the second network function entity.

[0029] In an embodiment, the second notification may be received in an Nsmf_EventExposure_Notify message or an Nsmf_PDUSession_SMContextStatusNotify message.

[0030] In an embodiment, the method may further comprise sending a third message for unsubscribing to the second notification to at least one of the at least one session management function entity.

[0031] In an embodiment, the third message may be sent when the subscription to the first notification expires; when the subscription to the first notification is removed; when the session related to the terminal device is released; when the at least one of the at least one session management function entity is changed or removed; when the access and mobility management function entity serving the terminal device is changed from the first access and mobility management function entity to a third access and mobility management function entity; or when a new session management function entity associated to the session related to the terminal device is inserted.

[0032] In an embodiment, when a new session management function entity associated to the session related to the terminal device is inserted, the method may further comprise sending a fourth message to the new session management function entity, wherein the fourth message requests the second notification and the fourth message includes the traffic descriptor when the first message includes the traffic descriptor.

[0033] In an embodiment, when the access and mobility management function entity serving the terminal device is changed from the first access and mobility management function entity to a third access and mobility management function entity, the method may further comprise sending a fifth message to the third access and mobility management function entity, wherein the fifth message requests the first notification and includes at least one of the traffic descriptor identifying the source of the downlink data and the network information associated to a session related to the terminal device.

[0034] In a second aspect of the disclosure, there is provided a method performed by a first network function entity. The method comprises determining to send a first message to a first access and mobility management function entity, wherein the first message requests a first notification toward a second network function entity when a terminal device becomes available after downlink data notification failure for the terminal device and the first message includes at least one of a traffic descriptor identifying a source of the downlink data and network information associated to a session related to the terminal device. The method further comprises sending the first message to the first access and mobility management function entity. [0035] In an embodiment, the first network function entity is a unified data management entity, the second network function entity is a network exposure function entity, and determining to send the first message to the first access and mobility management function entity may be in response to receiving a message for subscribing to the first notification from the network exposure function entity.

[0036] In an embodiment, the first network function entity is a second access and mobility management function entity, the second network function entity is a network exposure function entity, and determining to send the first message to the first access and mobility management function entity may be in response to the access and mobility management function entity serving the terminal device is changed from the second access and mobility management function entity to the first access and mobility management function entity. [0037] In a third aspect of the disclosure, there is provided a method performed by a session management function entity. The method comprises receiving a message from an access and mobility management function entity. The message requests a notification when a downlink data notification failure for a terminal device is detected. The method further comprises sending the notification to the access and mobility management function entity when the downlink data notification failure for the terminal device is detected. [0038] In an embodiment, the message includes a traffic descriptor identifying a source of the downlink data, and the sending step may comprise sending the notification to the access and mobility management function entity when the downlink data notification failure matching the traffic descriptor is detected.

[0039] In an embodiment, the message may be a subscribe request or a subscribe update request.

[0040] In an embodiment, the message may be an Nsmf_EventExposure_Subscribe request.

[0041] In an embodiment, the subscribe request may be included in an Nsmf_PDUSession_CreateSMContext Request or Nsmf_PDUSession_UpdateSMContext Request. The subscribe update request may be included in an Nsmf_PDUSession_UpdateSMContext Request.

[0042] In an embodiment, the notification may be sent in an Nsmf_EventExposure_Notify message or an Nsmf_PDUSession_SMContextStatusNotify message.

[0043] In an embodiment, the method may further comprise: receiving a message for unsubscribing to the notification from the session management function entity; and performing an unsubscribing operation.

[0044] In an embodiment, the method may further comprise: when session management context associated to a session related to the terminal device is released, performing an unsubscribing operation to the notification.

[0045] In a fourth aspect of the disclosure, there is provided an apparatus implemented in a first access and mobility management function entity. The apparatus comprises a processor; and a memory coupled to the processor, said memory containing instructions executable by said processor, whereby said apparatus is operative to receive a first message from a first network function entity. The first message requests a first notification toward a second network function entity when a terminal device becomes available after downlink data notification failure for the terminal device and the first message includes at least one of a traffic descriptor identifying a source of the downlink data and network information associated to a session related to the terminal device. Said apparatus is further operative to send a second message to at least one session management function entity associated to the session related to the terminal device. The second message requests a second notification toward the first access and mobility management function entity when the downlink data notification failure for the terminal device is detected and the second message includes the traffic descriptor when the first message includes the traffic descriptor.

[0046] In a fifth aspect of the disclosure, there is provided an apparatus implemented in a first network function entity. The apparatus comprises a processor; and a memory coupled to the processor, said memory containing instructions executable by said processor, whereby said apparatus is operative to determine to send a first message to a first access and mobility management function entity. The first message requests a first notification toward a second network function entity when a terminal device becomes available after downlink data notification failure for the terminal device and the first message includes at least one of a traffic descriptor identifying a source of the downlink data and network information associated to a session related to the terminal device. Said apparatus is further operative to send the first message to the first access and mobility management function entity.

[0047] In a sixth aspect of the disclosure, there is provided an apparatus implemented in a session management function entity. The apparatus comprises a processor; and a memory coupled to the processor, said memory containing instructions executable by said processor, whereby said apparatus is operative to receive a message from an access and mobility management function entity. The message requests a notification when a downlink data notification failure for a terminal device is detected. Said apparatus is further operative to send the notification to the access and mobility management function entity when the downlink data notification failure for the terminal device is detected.

[0048] In a seventh aspect of the disclosure, there is provided a first access and mobility management function entity. The first access and mobility management function entity may be AMF of 5GS. The first access and mobility management function entity comprises a receiving module configured to receive a first message from a first network function entity. The first message requests a first notification toward a second network function entity when a terminal device becomes available after downlink data notification failure for the terminal device and the first message includes at least one of a traffic descriptor identifying a source of the downlink data and network information associated to a session related to the terminal device. The first access and mobility management function entity further comprises a sending module configured to send a second message to at least one session management function entity associated to the session related to the terminal device. The second message requests a second notification toward the first access and mobility management function entity when the downlink data notification failure for the terminal device is detected and the second message includes the traffic descriptor when the first message includes the traffic descriptor. [0049] In an eighth aspect of the disclosure, there is provided a first network function entity according to an embodiment of the present disclosure. The first network function entity may be UDM or AMF of 5GS. The first network function entity comprises a determining module configured to determine to send a first message to a first access and mobility management function entity. The first message requests a first notification toward a second network function entity when a terminal device becomes available after downlink data notification failure for the terminal device and the first message includes at least one of a traffic descriptor identifying a source of the downlink data and network information associated to a session related to the terminal device. The first network function entity further comprises a sending module configured to send the first message to the first access and mobility management function entity.

[0050] In a ninth aspect of the disclosure, there is provided a session management function entity according to an embodiment of the present disclosure. The session management function entity may be SMF of 5GS. The session management function entity comprises a receiving module configured to receive a message from an access and mobility management function entity, wherein the message requests a notification when a downlink data notification failure for a terminal device is detected. The session management function entity further comprises a sending module configured to send the notification to the access and mobility management function entity when the downlink data notification failure for the terminal device is detected.

[0051] In a tenth aspect of the disclosure, there is provided a computer program product comprising instructions which, when executed on at least one processor, cause the at least one processor to carry out the method according to the first aspect of the disclosure.

[0052] In an eleventh aspect of the disclosure, there is provided a computer program product comprising instructions which, when executed on at least one processor, cause the at least one processor to carry out the method according to the second aspect of the disclosure.

[0053] In a twelfth aspect of the disclosure, there is provided a computer program product comprising instructions which, when executed on at least one processor, cause the at least one processor to carry out the method according to the third aspect of the disclosure.

[0054] In a thirteenth aspect of the disclosure, there is provided a computer-readable storage medium storing instructions which, when executed on at least one processor, cause the at least one processor to carry out the method according to the first aspect of the disclosure.

[0055] In fourteenth aspect of the disclosure, there is provided a computer-readable storage medium storing instructions which, when executed on at least one processor, cause the at least one processor to carry out the method according to the second aspect of the disclosure. [0056] In fifteenth aspect of the disclosure, there is provided a computer-readable storage medium storing instructions which, when executed on at least one processor, cause the at least one processor to carry out the method according to the third aspect of the disclosure.

[0057] Embodiments herein afford many advantages, of which a non-exhaustive list of examples follows. Some embodiments herein may simplify the realization on system level, for example reducing the complexity from UDM and SMF. Some embodiments herein may support implicit subscription using existing SM Context service operation, saving network traffic especially inter-PLMN traffic for Home Routed PDU sessions. Some embodiments herein may reusing SM Context Update service operation and AMF notification URI (Uniform Resource Identifier) for SMF Status Notification, which is already updated by AMF in existing procedures when AMF changes. The embodiments herein are not limited to the features and advantages mentioned above. A person skilled in the art will recognize additional features and advantages upon reading the following detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

[0058] The above and other aspects, features, and benefits of various embodiments of the present disclosure will become more fully apparent, by way of example, from the following detailed description with reference to the accompanying drawings, in which like reference numerals or letters are used to designate like or equivalent elements. The drawings are illustrated for facilitating better understanding of the embodiments of the disclosure and not necessarily drawn to scale, in which:

FIG.1 schematically shows a high level architecture of the 5G network;

FIG. 2 shows a flowchart of a procedure of the configuration of availability after DDN failure event in EPS;

FIG. 3 shows a flowchart of a procedure of the detection and reporting of availability after DDN failure event in EPS;

FIG. 4 shows a flowchart of a procedure of the configuration of availability after DDN failure event in 5GS;

FIG.5 shows a flowchart of a procedure of the detection and reporting of availability after DDN failure event in 5GS;

FIG.6 shows a flowchart of a procedure of the configuration of availability after DDN failure event in 5GS when AMF changes;

FIG.7 shows a flowchart of a procedure of the configuration of availability after DDN failure event in 5GS when V-SMF/I-SMF is changed/inserted;

FIG. 8 shows a flowchart of a method according to an embodiment of the present disclosure;

FIG. 9 shows a flowchart of a method according to another embodiment of the present disclosure;

FIG. 10 shows a flowchart of a method according to an embodiment of the present disclosure; FIG. 11 shows a flowchart of a method according to another embodiment of the present disclosure;

FIG. 12 shows a flowchart of a method according to another embodiment of the present disclosure;

FIG. 13 shows a flowchart of a method according to another embodiment of the present disclosure;

FIG. 14 shows a flowchart of a method according to another embodiment of the present disclosure;

FIG. 15 shows a flowchart of a method according to another embodiment of the present disclosure;

FIG. 16 shows a flowchart of a method according to another embodiment of the present disclosure;

FIG. 17 shows a flowchart of a method according to another embodiment of the present disclosure;

FIG. 18 is a block diagram showing an apparatus suitable for use in practicing some embodiments of the disclosure;

FIG. 19 illustrates a simplified block diagram of a first access and mobility management function entity according to an embodiment of the present disclosure;

FIG. 20 illustrates a simplified block diagram of a first network function entity according to an embodiment of the present disclosure; and

FIG. 21 illustrates a simplified block diagram of a session management function entity according to an embodiment of the present disclosure.

DETAILED DESCRIPTION

[0059] The embodiments of the present disclosure are described in detail with reference to the accompanying drawings. It should be understood that these embodiments are discussed only for the purpose of enabling those skilled persons in the art to better understand and thus implement the present disclosure, rather than suggesting any limitations on the scope of the present disclosure. Reference throughout this specification to features, advantages, or similar language does not imply that all of the features and advantages that may be realized with the present disclosure should be or are in any single embodiment of the disclosure. Rather, language referring to the features and advantages is understood to mean that a specific feature, advantage, or characteristic described in connection with an embodiment is included in at least one embodiment of the present disclosure. Furthermore, the described features, advantages, and characteristics of the disclosure may be combined in any suitable manner in one or more embodiments. One skilled in the relevant art will recognize that the disclosure may be practiced without one or more of the specific features or advantages of a particular embodiment. In other instances, additional features and advantages may be recognized in certain embodiments that may not be present in all embodiments of the disclosure.

[0060] As used herein, the term “network” refers to a network following any suitable wireless communication standards such as new radio (NR), long term evolution (LTE), LTE-Advanced, wideband code division multiple access (WCDMA), high-speed packet access (HSPA), Code Division Multiple Access (CDMA), Time Division Multiple Address (TDMA), Frequency Division Multiple Access (FDMA), Orthogonal Frequency-Division Multiple Access (OFDMA), Single carrier frequency division multiple access (SC-FDMA) and other wireless networks. A CDMA network may implement a radio technology such as Universal Terrestrial Radio Access (UTRA), etc. UTRA includes WCDMA and other variants of CDMA. A TDMA network may implement a radio technology such as Global System for Mobile Communications (GSM). An OFDMA network may implement a radio technology such as Evolved UTRA (E-UTRA), Ultra Mobile Broadband (UMB), IEEE 802.11 (Wi-Fi), IEEE 802.16 (WiMAX), IEEE 802.20, Flash-OFDMA, Ad-hoc network, wireless sensor network, etc. In the following description, the terms “network” and “system” can be used interchangeably. Furthermore, the communications between two devices in the network may be performed according to any suitable communication protocols, including, but not limited to, the communication protocols as defined by a standard organization such as 3GPP. For example, the communication protocols as may comprise the first generation (1G), 2G, 3G, 4G, 4.5G, 5G communication protocols, and/or any other protocols either currently known or to be developed in the future.

[0061] The term “network entity” used herein refers to a network device or network node or function in a communication network. For example, in a wireless communication network such as a 3GPP-type cellular network, the network entity such as a core network device may offer numerous services to customers who are interconnected by an access network device. Each access network device is connectable to the core network device over a wired or wireless connection.

[0062] The term “network function (NF)” refers to any suitable function which can be implemented in a network entity (physical or virtual) of a communication network. For example, the 5G system (5GS) may comprise a plurality of NFs such as AMF (Access and mobility Function), SMF (Session Management Function), AUSF (Authentication Service Function), UDM (Unified Data Management), PCF (Policy Control Function), AF (Application Function), NEF (Network Exposure Function), UPF (User plane Function) and NRF (Network Repository Function), RAN (radio access network), SCP (service communication proxy), etc. In other embodiments, the network function may comprise different types of NFs for example depending on a specific type of network.

[0063] The term “terminal device” refers to any end device that can access a communication network and receive services therefrom. By way of example and not limitation, the terminal device refers to a mobile terminal, user equipment (UE), or other suitable devices. The UE may be, for example, a Subscriber Station (SS), a Portable Subscriber Station, a Mobile Station (MS), or an Access Terminal (AT). The terminal device may include, but not limited to, a portable computer, an image capture terminal device such as a digital camera, a gaming terminal device, a music storage and a playback appliance, a mobile phone, a cellular phone, a smart phone, a voice over IP (VoIP) phone, a wireless local loop phone, a tablet, a wearable device, a personal digital assistant (PDA), a portable computer, a desktop computer, a wearable terminal device, a vehicle-mounted wireless terminal device, a wireless endpoint, a mobile station, a laptop-embedded equipment (LEE), a laptop-mounted equipment (LME), a USB dongle, a smart device, a wireless customer-premises equipment (CPE) and the like. In the following description, the terms “terminal device”, “terminal”, “user equipment” and “UE” may be used interchangeably. As one example, a terminal device may represent a UE configured for communication in accordance with one or more communication standards promulgated by the 3GPP, such as 3GPP’ LTE standard or NR standard. As used herein, a “user equipment” or “UE” may not necessarily have a “user” in the sense of a human user who owns and/or operates the relevant device. In some embodiments, a terminal device may be configured to transmit and/or receive information without direct human interaction. For instance, a terminal device may be designed to transmit information to a network on a predetermined schedule, when triggered by an internal or external event, or in response to requests from the communication network. Instead, a UE may represent a device that is intended for sale to, or operation by, a human user but that may not initially be associated with a specific human user.

[0064] As yet another example, in an Internet of Things (IOT) scenario, a terminal device may represent a machine or other device that performs monitoring and/or measurements, and transmits the results of such monitoring and/or measurements to another terminal device and/or network equipment. The terminal device may in this case be a machine-to-machine (M2M) device, which may in a 3GPP context be referred to as a machine-type communication (MTC) device. As one particular example, the terminal device may be a UE implementing the 3GPP narrow band internet of things (NB-loT) standard. Particular examples of such machines or devices are sensors, metering devices such as power meters, industrial machinery, or home or personal appliances, for example refrigerators, televisions, personal wearables such as watches etc. In other scenarios, a terminal device may represent a vehicle or other equipment that is capable of monitoring and/or reporting on its operational status or other functions associated with its operation.

[0065] As used herein, a downlink, DL, transmission refers to a transmission from a network device to a terminal device, and an uplink, UL, transmission refers to a transmission in an opposite direction.

[0066] References in the specification to “one embodiment,” “an embodiment,” “an example embodiment,” and the like indicate that the embodiment described may include a particular feature, structure, or characteristic, but it is not necessary that every embodiment includes the particular feature, structure, or characteristic. Moreover, such phrases are not necessarily referring to the same embodiment. Further, when a particular feature, structure, or characteristic is described in connection with an embodiment, it is submitted that it is within the knowledge of one skilled in the art to affect such feature, structure, or characteristic in connection with other embodiments whether or not explicitly described.

[0067] It shall be understood that although the terms “first” and “second” etc. may be used herein to describe various elements, these elements should not be limited by these terms. These terms are only used to distinguish one element from another. For example, a first element could be termed a second element, and similarly, a second element could be termed a first element, without departing from the scope of example embodiments. As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed terms.

[0068] The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of example embodiments. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises”, “comprising”, “has”, “having”, “includes” and/or “including”, when used herein, specify the presence of stated features, elements, and/or components etc., but do not preclude the presence or addition of one or more other features, elements, components and / or combinations thereof. [0069] It is noted that these terms as used in this document are used only for ease of description and differentiation among nodes, devices or networks etc. With the development of the technology, other terms with the similar/same meanings may also be used.

[0070] In the following description and claims, unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skills in the art to which this disclosure belongs.

[0071] Although the subject matter described herein may be implemented in any appropriate type of system using any suitable components, the embodiments disclosed herein are described in relation to a communication system complied with the exemplary system architecture illustrated in FIG.1. For simplicity, the system architecture of FIG.1 only depicts some exemplary elements. In practice, a communication system may further include any additional elements suitable to support communication between terminal devices or between a wireless device and another communication device, such as a landline telephone, a service provider, or any other network node or terminal device. The communication system may provide communication and various types of services to one or more terminal devices to facilitate the terminal devices’ access to and/or use of the services provided by, or via, the communication system.

[0072] FIGURE 1 schematically shows a high level architecture of the 5G network. The system architecture of FIG.1 may comprise some exemplary elements such as AUSF, AMF, DN (data network), NEF, NRF, NSSF, PCF, SMF, UDM, UPF, AF, UE, (R)AN, SCP, UE, Intermediate NEF (l-NEF), visited Security Edge Protection Proxy (vSEPP), home Security Edge Protection Proxy (hSEPP), etc.

[0073] In accordance with an exemplary embodiment, the UE can establish a signaling connection with the AMF over the reference point N1, as illustrated in FIG.1. This signaling connection may enable NAS (Non-access stratum) signaling exchange between the UE and the core network, comprising a signaling connection between the UE and the (R)AN and the N2 connection for this UE between the (R)AN and the AMF. The (R)AN can communicate with the UPF over the reference point N3. The UE can establish a protocol data unit (PDU) session to the DN (data network, e.g. an operator network or Internet) through the UPF over the reference point N6.

[0074] As further illustrated in FIG.1 , the exemplary system architecture also contains the service-based interfaces such as Nnrf, Nnef, Nausf, Nudm, Npcf, Namf and Nsmf exhibited by NFs such as the NRF, the NEF, the AUSF, the UDM, the PCF, the AMF and the SMF. In addition, FIG.1 also shows some reference points such as N1, N2, N3, N4, N6, N9 and N32, which can support the interactions between NF services in the NFs. For example, these reference points may be realized through corresponding NF service-based interfaces and by specifying some NF service consumers and providers as well as their interactions in order to perform a particular system procedure.

[0075] Various NFs shown in Fig.1 may be responsible for functions such as session management, mobility management, authentication, security, etc. The AUSF, AMF, DN, NEF, NRF, NSSF, PCF, SMF, UDM, UPF, AF, UE, (R)AN, SCP, SEPP, etc. may include the functionality for example as defined in 3GPP TS23.501 V16.2.0 and 3GPP TS 33.501 V16.0.0, the disclosures of which are incorporated by reference herein in their entirety.

[0076] Some solutions for availability after DDN failure event have been proposed in some communication systems such as EPS and 5GS.

[0077] FIGURE 2 shows a flowchart of a procedure of the configuration of availability after DDN failure event in EPS. In EPS, availability after DDN failure event is configured and detected in MME.

[0078] At step 202, the SCS (Services Capability Server)/AS executes step 1 of clause 5.6.1.9 of 3GPP TS 23.682 V16.4.0. For example, SCS (Services Capability Server)/AS sends a trigger request (“notify on availability after DDN failure”, UEID (UE identifier)) to SCEF.

[0079] At step 204, the SCEF sends a Monitoring Request message to the HSS (Flome Subscriber Server) to request notification when the UE becomes available after a DDN failure. The Monitoring Request message includes the parameters specified in clause 5.6.1.2 of 3GPP TS 23.682 V16.4.0. For example, SCEF sends monitoring requestfnotify on availability after DDN failure”, UEID) to HSS.

[0080] At step 206, the HSS provides the monitoring event request to the MME/SGSN according to steps 5-7 in Figure 5.6.1.1-1 of 3GPP TS 23.682 V16.4.0. For example, the HSS provides the monitoring event request to the MME/S4-SGSN per steps 5-7 in Figure 5.6.1.1-1 of 3GPP TS 23.682 V16.4.0.

[0081] At step 208, the HSS internally notes the request, and sends Monitoring Response message to the SCEF.

[0082] At step 210, the SCEF executes step 9 of clause 5.6.1.9 of 3GPP TS 23.682 V16.4.0. For example, SCEF sends a trigger response to SCS/AS.

[0083] The event configured via SCEF to HSS and downloaded to MME as shown in FIG.2 is similar to clause 5.7.1.3 of 3GPP TS 23.682 V16.4.0. [0084] FIGURE 3 shows a flowchart of a procedure of the detection and reporting of availability after DDN failure event in EPS. EUTRAN denotes evolved universal terrestrial radio access network.

[0085] At step 302, the application (SCS/AS) sends downlink data to PGW (packet data network gateway) which forwards the downlink data to the SGW (serving gateway).

[0086] At step 304, the SGW sends a Downlink Data Notification (DDN) message to the MME requesting that UE be paged.

[0087] At step 306, the UE is in Power Saving Mode or Extended idle mode DRX (Discontinuous Reception), or the MME initiates paging for the UE but receives no response.

[0088] At step 308, the MME sends a DDN failure indication to the SGW.

[0089] At step 310, the MME notes the subscription option for notification of availability after DDN failure for the UE, and sets a Notify-on-available-after-DDN-failure flag. Not every DDN failure triggers this event. This event may be triggered only when the UE is in PSM or Extended idle mode DRX.

[0090] At step 312, at some later time, the UE contacts the network (e.g. to perform a TAU(Tracking Area Update), or a service request), or the UE becomes or is about to become reachable for paging (e.g. an eDRX Paging Transmission Window is reached).

[0091] At step 314, the MME notes that UE is available and that the Notify-on-available-after-DDN-failure flag for the UE is set.

[0092] At step 316, the MME sends a Monitoring Indication to the SCEF that the UE is available, according to clause 5.6.3.1 of 3GPP TS 23.682 V16.4.0. The MME also resets the Notify-on-available-after-DDN-failure flag for the UE.

[0093] At step 318, the SCEF notifies the SCS/AS that the UE is available.

[0094] At step 320, the application may decide to resend data it has for the UE.

[0095] At step 322, UE transitions to idle mode according to 3GPP TS 23.401 V16.3.0, e.g. CM(Connection Management)-IDLE with PSM or eDRX.

[0096] At step 324, if Idle Status Indication was requested during Monitoring Event configuration, and the MME/SGSN supports Idle Status Indication, then MME executes step 2a of clause 5.6.3.8 of 3GPP TS 23.682 V16.4.0, and includes the time at which the UE transitioned into idle mode, its granted active time (if PSM is enabled), the periodic TAU/RAU (Routing Area Update) timer granted to the UE by the MME, the eDRX cycle length (if extended idle mode DRX is enabled), and the Suggested number of downlink packets if a value was provided to the S-GW in the message.

[0097] The event detection and reporting as shown in FIG.3 is similar to clause 5.7.1.3 of 3GPP TS 23.682 V16.4.0.

[0098] In 3GPP SA2#136, S2-1912606 was agreed and proposes the following realization: UDM subscribes separately to AMF and SMF, and SMF notifies to AMF for DNN failure. [0099] FIGURE 4 shows a flowchart of a procedure of the configuration of availability after DDN failure event in 5GS. In 5GS, since the AM (Access Management)/MM (Mobility Management) and SM function are distributed to AMF and SMF, the event detection may work in a collaborative way, i.e., SMF detects the DDN Failure Status and informs AMF, AMF sets the Notify-o n-avai I abl e-after- D D N-fai I u re flag after the DDN failure and detects subsequent UE activities to trigger the complete event reporting.

[00100] At step 402, the AF sends Nnef_EventExposure_Subscribe Request to the NEF requesting notifications for "Availability after DDN Failure" for a UE or group of UEs and providing a traffic descriptor identifying the source of the downlink IP (Internet protocol) or Ethernet traffic. If the reporting event subscription is authorized by the NEF, the NEF records the association of the event trigger and the requester identity.

[00101] At step 404, the NEF sends the Nudm_EventExposure_Subscribe Request to UDM. Identifier of the UE or group of UEs, the traffic descriptor, monitoring event received from AF at step 402, and notification endpoint of the NEF are included in the message. If the reporting event subscription is authorized by the UDM, the UDM records the association of the event trigger and the requester identity.

[00102] At step 406, UDM invokes Namf_EventExposure_Subscribe to AMF and AMF provides a notification endpoint (to receive the Nsmf_EventExposure_Notify message) to UDM.

[00103] At step 408, UDM invokes Nsmf_EventExposure_Subscribe to SMF and provides the AMF notification endpoint received in step 406.

[00104] At step 410, for PDU Session with I-SMFA/-SMF, the SMF (e.g., H-SMF) further subscribes to I-SMFA/-SMF and provides the AMF notification endpoint received in step 408.

[00105] FIGURE 5 shows a flowchart of a procedure of the detection and reporting of availability after DDN failure event in 5GS.

[00106] At step 502, if DDN failure is detected, SMF/I-SMF/V-SMF sends Nsmf_EventExposure_Notify message to the AMF notification endpoint received in the subscribe operation. For example, the SMF is informed that the UE is unreachable via a Namf_Communication_N1 N2MessageTransfer service operation. The SMF then decides to discard downlink packets received from the UPF. By comparing those discarded downlink packets received from the UPF with the traffic descriptor(s) received in the event subscription(s), the SMF determines whether any traffic from an AF having an event subscription occurred and if so, the SMF sends the event report, by means of Nsmf_EventExposure_Notify message (including NEF reference ID), to the AMF indicated as notification endpoint. If the UE is not reachable after the AMF received the notification from the SMF, the AMF shall set a Notify-on-available-after-DDN-failure flag corresponding to the NEF Reference ID(identifier).

[00107] At step 504, the AMF detects the UE is reachable and sends the event report(s) based on the Notify-on-available-after-DDN-failure flag, by means of Namf_EventExposure_Notify message(s), only to the NEF(s) indicated as notification endpoint(s) identified via the corresponding subscription. In this way, only the AF(s) for which DL traffic transmission failed are notified.

[00108] At step 506, the NEF sends Nnef_EventExposure_Notify message with the "Availability after DDN Failure" event to AF.

[00109] The event configuration, detection and reporting as shown in FIGs.4-5 are similar to S2-1912606 of 3GPP SA2#136.

[00110] The above solutions as shown in FIGs.4-5 have the some issues. An issue is the heavy and expensive traffic for subscription update when AMF is changed. Another issue is potential doubled signaling over N16/N16a when V-SMF/I-SMF change/insertion. For example, when AMF is changed, AMF notification endpoint for SMF event (to receive the DDN Failure Status) will be relocated to the new AMF. The new AMF needs to update the new notification endpoints in subscriptions on all SMFs.

[00111] FIGURE 6 shows a flowchart of a procedure of the configuration of availability after DDN failure event in 5GS when AMF is changed. At step 602, the new AMF may inform UDM for new notification URI for SMF Event. At step 604, UDM may update subscription to SMF. At step 606, for PDU Session with I-SMFA/-SMF, the SMF (e.g., H-SMF) may further update subscription to I-SMFA/-SMF. The procedure of FIG.6 may generate quite heavy network signaling traffics, especially for FIR (Flome Routed (roaming)) PDU Sessions. Both step 602 and step 604 are expensive inter-PLMN (Public Land Mobile Network) traffic involving SEPPs via N32.

[00112] FIGURE 7 shows a flowchart of a procedure of the configuration of availability after DDN failure event in 5GS when V-SMF/I-SMF is changed/inserted. When a new l-SMF/V-SMF is inserted/used for the PDU Session, the SMF/FI-SMF will subscribe to the DDN Failure event to the l-SMF/V-SMF on behalf of the AMF at step 702. If the l-SMF/V-SMF change/insertion is part of the inter-AMF mobility procedure, AMF will subsequently update the notification endpoint at step 704, UDM will update the notification endpoint at step 706 and the SMF/FI-SMF subscribes to the event with new AMF notification endpoint again over N16/N16a towards the I-SMFA/-SMF at step 708. Therefore there may be the potential doubled signaling issue at step 702 and step 708. In addition, the procedure of FIG.7 may also generate extra network signaling traffics. [00113] To overcome or mitigate at least one of above mentioned problems or other problems, the embodiments of the present disclosure propose an improved solution of terminal device availability management which will be described in FIGs.8-21.

[00114] FIGURE 8 shows a flowchart of a method according to an embodiment of the present disclosure, which may be performed by an apparatus implemented in/as or communicatively coupled to a first access and mobility management function entity. As such, the apparatus may provide means or modules for accomplishing various parts of the method 800 as well as means or modules for accomplishing other processes in conjunction with other components. The first access and mobility management function entity may be any suitable entity or node which can implement the access and mobility management function. For example, the first access and mobility management function entity may be AMF.

[00115] At block 802, the first access and mobility management function entity receives a first message from a first network function entity. The first message may request a first notification toward a second network function entity when a terminal device becomes available after downlink data notification failure for the terminal device and the first message may include at least one of a traffic descriptor identifying a source of the downlink data and network information associated to a session related to the terminal device.

[00116] The first network function entity may be any suitable network function entity which can send various messages to the first access and mobility management function entity. For example, the first network function entity may be a unified data management entity such as UDM or another access and mobility management function entity. The second network function entity may be any suitable network function entity which requires the first notification. For example, the second network function entity may be a network exposure function entity such as NEF.

[00117] The first message may be a subscribe request or a subscribe update request or other message which can implement the role of the first message. For example, in 5GS, the first message may be Namf_EventExposure_Subscribe request as described in 3GPP TS 23.502 V16.1.1 , the disclosure of which is incorporated by reference herein in their entirety.

[00118] The traffic descriptor may contain any suitable information for identifying the source of the downlink data. For example, depending on the type of the session related to the terminal device, the traffic descriptor may contain the following components:

For IPv4 (Internet protocol version 4), or IPv6 (Internet protocol version 6), or IPv4v6 type: Application descriptors and/or IP descriptors.

For Ethernet type: Application descriptors and/or Non-IP descriptors.

[00119] The network information may include any suitable network information associated to the session related to the terminal device. For example, the network information may include a data network name (e.g., DNN in 5GS) and single network slice selection assistance information (e.g., S-NSSAI in 5GS). For example, for each PDU Session, UE Context in AMF may include S-NSSAI(s) associated to the PDU Session and associated DNN for the PDU Session.

[00120] In an embodiment, the first network function entity may be a unified data management entity (e.g., UDM in 5GS) and the second network function entity may be a network exposure function entity (e.g., NEF in 5GS). For example, in 5GS, the AF sends Nnef_EventExposure_Subscribe Request to the NEF requesting notifications for "Availability after DDN Failure" for a UE or group of UEs and providing a traffic descriptor identifying the source of the downlink IP or Ethernet traffic. If the reporting event subscription is authorized by the NEF, the NEF records the association of the event trigger and the requester identity. The NEF sends the Nudm_EventExposure_Subscribe Request to UDM. Identifier of the UE or group of UEs, the traffic descriptor, monitoring event received from AF, and notification endpoint of the NEF are included in the Nudm_EventExposure_Subscribe Request. If the reporting event subscription is authorized by the UDM, the UDM records the association of the event trigger and the requester identity. The UDM sends an Namf_EventExposure_Subscribe message (i.e., the first message) to the AMF(s) which serve the UE or group of UEs to subscribe to "Availability after DDN Failure". A separate subscription may be used for each UE. The NEF notification endpoint and at least one of the traffic descriptor identifying the source of the downlink data and the network information are included in the Namf_EventExposure_Subscribe message. If the reporting event subscription is not authorized by the UDM, the UDM may indicate failure to NEF.

[00121] In an embodiment, the first network function entity may be a second access and mobility management function entity (e.g., AMF in 5GS) and the second network function entity may be a network exposure function entity (e.g., NEF in 5GS). The access and mobility management function entity serving the terminal device may be changed due to various reasons. For example, when the access and mobility management function entity serving the terminal device is changed from the second access and mobility management function entity to the first access and mobility management function entity, the first access and mobility management function entity may receive the first message from the second access and mobility management function entity. The second access and mobility management function entity may receive the first message from the unified data management entity or another access and mobility management function entity in case of access and mobility management function entity change. For example, in 5GS, the second (old) AMF may transfer the subscription to the first (new) AMF during the AMF change. The event subscription related to the first notification may be moved as part of UE context transferring between AMFs, during UE Context Transfer (idle mobility) or Create UE Context (handover).

[00122] At block 804, the first access and mobility management function entity sends a second message to at least one session management function entity associated to the session related to the terminal device. The second message may request a second notification toward the first access and mobility management function entity when the downlink data notification failure for the terminal device is detected and the second message may include the traffic descriptor when the first message includes the traffic descriptor. The session management function entity may be any suitable network function which implement session management function. For example, The session management function entity may be SMF in 5GS.

[00123] The first access and mobility management function entity may store UE context. For each session, the UE context may include the network information associated to the session related to the terminal device and session management function entity information such as associated session management function entity identifier and session management function entity address for the each session. The first access and mobility management function entity may determine the at least one session management function entity associated to the session related to the terminal device based on the UE context and the first message, and send the second message to the at least one session management function entity.

[00124] In an embodiment, when the first message includes the network information, the first access and mobility management function entity may send the second message to the at least one session management function entity associated to the session related to the terminal device and with the network information. [00125] In an embodiment, the second message may be a subscribe request or a subscribe update request or other message which implement the role of the second message.

[00126] In an embodiment, the second message may be an Nsmf_EventExposure_Subscribe request as described in 3GPP TS 23.502 V16.1.1 .

[00127] In an embodiment, the second message may be a subscribe request which may be included in an Nsmf_PDUSession_CreateSMContext Request or Nsmf_PDUSession_UpdateSMContext Request as described in 3GPP TS 23.502 V16.1.1.

[00128] In an embodiment, the second message may be a subscribe update request which may be included in an Nsmf_PDUSession_UpdateSMContext Request as described in 3GPP TS 23.502 V16.1.1. [00129] In an embodiment, the first access and mobility management function entity may send the second message to at least one session management function entity associated to the session related to the terminal device when the first access and mobility management function entity receives the first message and there is a matching session related to the terminal device.

[00130] In an embodiment, the first access and mobility management function entity may send the second message to at least one session management function entity associated to the session related to the terminal device when the first message is active and a matching session related to the terminal device is established.

[00131] FIGURE 9 shows a flowchart of a method according to another embodiment of the present disclosure, which may be performed by an apparatus implemented in/as or communicatively coupled to a first access and mobility management function entity. As such, the apparatus may provide means or modules for accomplishing various parts of the method 900 as well as means or modules for accomplishing other processes in conjunction with other components. The first access and mobility management function entity may be any suitable entity or node which can implement the access and mobility management function. For example, the first access and mobility management function entity may be AMF. For some parts which have been described in the above embodiments, the description thereof is omitted here for brevity.

[00132] At block 902, the first access and mobility management function entity receives the second notification from a session management function entity. The session management function entity may be the session management function entity to which the first access and mobility management function entity has sent the second message. The second notification may be received in any suitable message. [00133] In an embodiment, the second notification is received in an Nsmf_EventExposure_Notify message as described in 3GPP TS 23.502 V16.1.1. For example, in 5GS, the SMF may be informed that the UE is unreachable via a Namf_Communication_N1 N2MessageTransfer service operation as described in 3GPP TS 23.502 V16.1.1. The SMF then decides to discard downlink packets received from the UPF. SMF may compare those discarded downlink packets received from the UPF with the traffic descriptor(s) (if any) received in the second message. The SMF determines whether any traffic having an event subscription occurred and if so, the SMF sends the event report by means of Nsmf_EventExposure_Notify message to the AMF indicated as notification endpoint.

[00134] In an embodiment, the second notification is received in an Nsmf_PDUSession_SMContextStatusNotify message as described in 3GPP TS 23.502 V16.1.1.

[00135] At block 904, the first access and mobility management function entity sets a flag of the first notification. For example, in 5GS, if the UE is not reachable after the AMF received the second notification from the SMF, the AMF shall set a Notify-on-available-after-DDN-failure flag corresponding to the NEF Reference ID.

[00136] At block 906, the first access and mobility management function entity detects that the terminal device is reachable. For example, at some later time, the UE contacts the network (e.g. to perform a TAU, or a service request), or the UE becomes or is about to become reachable for paging (e.g. an eDRX Paging Transmission Window is reached), the first access and mobility management function entity detects that the terminal device is reachable.

[00137] At block 908, the first access and mobility management function entity sends the first notification to the second network function entity. For example, in 5GS, AMF may send the event report(s) (i.e., the first notification) based on the Notify-on-available-after-DDN-failure flag, by means of Namf_EventExposure_Notify message(s), to the NEF(s) indicated as notification endpoint(s). In an embodiment, only the AF(s) for which downlink traffic transmission failed are notified.

[00138] FIGURE 10 shows a flowchart of a method according to another embodiment of the present disclosure, which may be performed by an apparatus implemented in/as or communicatively coupled to a first access and mobility management function entity. As such, the apparatus may provide means or modules for accomplishing various parts of the method 1000 as well as means or modules for accomplishing other processes in conjunction with other components. The first access and mobility management function entity may be any suitable entity or node which can implement the access and mobility management function. For example, the first access and mobility management function entity may be AMF. For some parts which have been described in the above embodiments, the description thereof is omitted here for brevity.

[00139] At block 1002, the first access and mobility management function entity sends a third message for unsubscribing to the second notification to at least one of the at least one session management function entity. The first access and mobility management function entity may send the third message due to various reasons, for example, when the subscription to the first notification expires; when the subscription to the first notification is removed; when the session related to the terminal device is released; when the at least one of the at least one session management function entity is changed or removed; when the access and mobility management function entity serving the terminal device is changed from the first access and mobility management function entity to a third access and mobility management function entity; or when a new session management function entity associated to the session related to the terminal device is inserted. When at least one above condition is fulfilled, the first access and mobility management function entity may send the third message to at least one affected session management function entity. Block 1002 can be performed at any suitable time after block 804 of FIG.8.

[00140] FIGURE 11 shows a flowchart of a method according to another embodiment of the present disclosure, which may be performed by an apparatus implemented in/as or communicatively coupled to a first access and mobility management function entity. As such, the apparatus may provide means or modules for accomplishing various parts of the method 1100 as well as means or modules for accomplishing other processes in conjunction with other components. The first access and mobility management function entity may be any suitable entity or node which can implement the access and mobility management function. For example, the first access and mobility management function entity may be AMF. For some parts which have been described in the above embodiments, the description thereof is omitted here for brevity.

[00141] At block 1102, when a new session management function entity associated to the session related to the terminal device is inserted, the first access and mobility management function entity may send a fourth message to the new session management function entity. The fourth message requests the second notification and the fourth message includes the traffic descriptor when the first message includes the traffic descriptor. The fourth message may be similar to the second message as described above. Block 1102 can be performed at any suitable time after block 802 of FIG.8.

[00142] FIGURE 12 shows a flowchart of a method according to another embodiment of the present disclosure, which may be performed by an apparatus implemented in/as or communicatively coupled to a first access and mobility management function entity. As such, the apparatus may provide means or modules for accomplishing various parts of the method 1200 as well as means or modules for accomplishing other processes in conjunction with other components. The first access and mobility management function entity may be any suitable entity or node which can implement the access and mobility management function. For example, the first access and mobility management function entity may be AMF. For some parts which have been described in the above embodiments, the description thereof is omitted here for brevity.

[00143] At block 1202, when the access and mobility management function entity serving the terminal device is changed from the first access and mobility management function entity to a third access and mobility management function entity, the first access and mobility management function entity may send a fifth message to the third access and mobility management function entity. The fifth message requests the first notification and includes at least one of the traffic descriptor identifying the source of the downlink data and the network information associated to a session related to the terminal device. The fifth message may be any suitable message which can be transferred between two access and mobility management function entities. For example, the fifth message may be Namf_Communication_N1 N2MessageTransfer message as described in 3GPP TS 23.502 V16.1.1. The event configuration information related to the first message may be stored and managed in UE context in the first access and mobility management function entity, and the UE context may be transferred between access and mobility management function entities when the access and mobility management function entity is changed. Block 1202 can be performed at any suitable time after block 802 of FIG.8.

[00144] FIGURE 13 shows a flowchart of a method according to another embodiment of the present disclosure, which may be performed by an apparatus implemented in/as or communicatively coupled to a first network function entity. As such, the apparatus may provide means or modules for accomplishing various parts of the method 1300 as well as means or modules for accomplishing other processes in conjunction with other components. The first network function entity may be any suitable entity or node which can implement the data management function or access and mobility management function. For example, the first network function entity may be UDM or AMF. For some parts which have been described in the above embodiments, the description thereof is omitted here for brevity.

[00145] At block 1302, the first network function entity determines to send a first message to a first access and mobility management function entity. The first message requests a first notification toward a second network function entity when a terminal device becomes available after downlink data notification failure for the terminal device and the first message includes at least one of a traffic descriptor identifying a source of the downlink data and network information associated to a session related to the terminal device.

[00146] In an embodiment, the first network function entity is a unified data management entity, the second network function entity is a network exposure function entity, and the first network function entity may determine to send the first message to the first access and mobility management function entity in response to receiving a message for subscribing to the first notification from the network exposure function entity. In this embodiment, the first message may be similar to the first messages as described in the above embodiments.

[00147] In an embodiment, the first network function entity is a second access and mobility management function entity, the second network function entity is a network exposure function entity, and the first network function entity may determine to send the first message to the first access and mobility management function entity in response to the access and mobility management function entity serving the terminal device is changed from the second access and mobility management function entity to the first access and mobility management function entity. In this embodiment, the first message may be similar to the fifth messages as described in the above embodiments.

[00148] At block 1304, the first network function entity sends the first message to the first access and mobility management function entity.

[00149] FIGURE 14 shows a flowchart of a method according to another embodiment of the present disclosure, which may be performed by an apparatus implemented in/as or communicatively coupled to a session management function entity. As such, the apparatus may provide means or modules for accomplishing various parts of the method 1400 as well as means or modules for accomplishing other processes in conjunction with other components. The session management function entity may be any suitable entity or node which can implement the session management function. For example, the session management function entity may be SMF. For some parts which have been described in the above embodiments, the description thereof is omitted here for brevity.

[00150] At block 1402, the session management function entity receives a message from an access and mobility management function entity. The message requests a notification when a downlink data notification failure for a terminal device is detected. For example, the access and mobility management function entity may send the message to the session management function entity as described in block 804 of FIG.8, and then the session management function entity may receive this message. The message is similar to the second message or the fourth message as described above.

[00151] At block 1404, the session management function entity sends the notification to the access and mobility management function entity when the downlink data notification failure for the terminal device is detected. The session management function entity may detect the downlink data notification failure for the terminal device in various ways. For example, the session management function entity may detect this failure during network triggered service request. In 5GS, the SMF may be informed that the UE is unreachable via a Namf_Communication_N1 N2MessageTransfer service operation.

[00152] In an embodiment, the message includes a traffic descriptor identifying a source of the downlink data, and the session management function entity may send the notification to the access and mobility management function entity when the downlink data notification failure matching the traffic descriptor is detected. For example, in 5GS, the session management function entity may compare those discarded downlink packets received from the UPF with the traffic descriptor and determine whether any discarded downlink packet matching the traffic descriptor is occurred and if so, the SMF sends the notification to AMF. [00153] In an embodiment, the message is a subscribe request or a subscribe update request.

[00154] In an embodiment, the message is an Nsmf_EventExposure_Subscribe request. [00155] In an embodiment, the subscribe request is included in an Nsmf_PDUSession_CreateSMContext Request or Nsmf_PDUSession_UpdateSMContext Request.

[00156] In an embodiment, the subscribe update request is included in an Nsmf_PDUSession_UpdateSMContext Request.

[00157] In an embodiment, the notification is sent in an Nsmf_EventExposure_Notify message or an Nsmf_PDUSession_SMContextStatusNotify message.

[00158] At block 1406, optionally, the session management function entity receives a message for unsubscribing to the notification from the session management function entity.

[00159] At block 1408, optionally, the session management function entity performs an unsubscribing operation.

[00160] At block 1410, optionally, when session management context associated to a session related to the terminal device is released, the session management function entity may perform an unsubscribing operation to the notification. For example, AMF may unsubscribe to DDN failure Status when invoking Release SM Context service operation. DDN Failure Status reporting may be implicitly ended when SM Context Resource is released by any reason.

[00161] According to various embodiments, when the network information associated to a session related to the terminal device is provided, the session related to the terminal device refers to the session related to the terminal device and with the network information.

[00162] FIGURE 15 shows a flowchart of a method according to another embodiment of the present disclosure. This embodiment can be implemented in 5GS. In this embodiment, AMF manages the configuration/provisioning and lifecycle of availability after DDN Failure event. As shown in FIG.15, availability after DDN Failure event is configured via NEF -> UDM -> AMF, with information available (traffic descriptor (to identify AF or the source of the downlink data), DNN and S-NASSI if UDM can derive the information). Step 1502 and step 1504 are similar to the step 402 and step 404 of FIG.4 respectively. Step 1506 is similar to the step 804 of FIG.8. If DDN and slice information is received, AMF monitors the DDN Failure status for PDU Session(s) identified by the received information. Otherwise, the AMF monitors DDN Failure status for all PDU Sessions of the UE. The event configuration may be stored and managed in UE context in AMF. UE context may be transferred between AMFs when AMF is changed. No communication between UDM and SMF is needed for this event configuration.

[00163] FIGURE 16 shows a flowchart of a method according to another embodiment of the present disclosure. This embodiment can be implemented in 5GS. In this embodiment, DDN Failure Status Monitoring is implemented by using SMF Event Exposure Service.

[00164] At step 1602, AMF sends an Nsmf_EventExposure_Subscribe message to SMF/I-SMFA/-SMF. For example, a UE can be registered in 5GS without any PDU Session. If there is ongoing PDU Session, AMF checks if the PDU Session is established towards the DNN+S-NSSAI indicated by the UDM. When establishing a new PDU Session, AMF will check if the new PDU Session will be established towards the DNN+S-NSSAI indicated by the UDM. For example, when there are two AFs, i.e., AF1, AF2, if traffic descriptor for AF1 is provided (by AF->NEF->UDM->AMF->SMF/UPF), only when DL packets from AF1 are dropped, UPF/SMF reports DDN Failure (if the event is subscribed by AMF). If traffic descriptor is not provided, any DL packets dropped will trigger UPF/SMF reporting the DDN Failure (if the event is subscribed by AMF). When event configuration is active and a matching PDU Session is established, AMF subscribe to DDN Failure status for matching PDU Session(s) if existing, by invoking Nsmf_EventExposure_Subscribe service operation.

[00165] At step 1604, AMF sends an Nsmf_EventExposure_Unsubscribe message to SMF/I-SMF/V-SMF. For example, when event configuration expires or removed or a PDU Session is released, AMF unsubscribes to all corresponding DDN Failure status subscription(s) by invoking Nsmf_EventExposure_Unsubscribe service operation.

[00166] When I-SMFA/-SMF change/insertion/removal for a monitored PDU Session, (new) AMF subscribe to the DDN Failure status to new I-SMFA/-SMF by invoking Nsmf_EventExposure_Subscribe service operation on the new l-SMF/V-SMF, and (old) AMF unsubscribe to removed l-SMF/V-SMF. When AMF change without I-SMFA/-SMF change/insertion/removal, the old AMF may transfer the subscription(s) on SMF for DDN Failure status to the new AMF and the new AMF may update the subscription(s) with the new notification URI. Alternatively, the new AMF may subscribe to the SMF(s) independently and the old AMF unsubscribe to the SMF(s).

[00167] At step 1606, when DDN Failure is detected, the SMF/I-SMFA/-SMF sends Nsmf_EventExposure_Notify to AMF, with indication that DDN Failure occurs.

[00168] At step 1608, AMF sets the Notify-on-available-after-DDN-failure flag.

[00169] At step 1610, AMF reports the Availability after DDN failure event when UE activity is detected by AMF subsequently.

[00170] FIGURE 17 shows a flowchart of a method according to another embodiment of the present disclosure. This embodiment can be implemented in 5GS. In this embodiment, DDN Failure Status Monitoring is implemented by using SMF PDU Session Service.

[00171] At step 1702, AMF sends SM Context Create/Update message (DDN Failure Status, traffic descriptor) to SMF/I-SMFA/-SMF. For example, during PDU session establishment, or when l-SMF/V-SMF change/insertion/removal scenarios, AMF subscribes to reporting of DDN Failure status to SMF in Create SM Context service operation based on the DNN and S-NSSAI received from UDM. SMF stores the subscription locally (not transferred to another SMF). When AMF change without SMF change, AMF updates the new SM context status notification URI (Existing). When event configuration is changed, AMF may subscribe/update/unsubscribe to DDN Failure status reporting by using Update SM context.

[00172] At step 1704, when an SM Context resource is released (due to any reason), the associated DDN Failure Status reporting is ended.

[00173] At step 1706, when DDN Failure is detected, the SMF sends SM Context Status Change Notification to AMF, with indication that DDN Failure occurs.

[00174] At step 1708, AMF sets the Notify-on-available-after-DDN-failure flag.

[00175] At step 1710, AMF reports the Availability after DDN failure event when UE activity is detected by AMF subsequently.

[00176] FIGURE 18 is a block diagram showing an apparatus suitable for use in practicing some embodiments of the disclosure. For example, any one of the first access and mobility management function entity, the first network function entity and the session management function entity described above may be implemented through the apparatus 800.

[00177] The apparatus 1800 comprises at least one processor 1821 , such as a DP, and at least one MEM 1822 coupled to the processor 1821. The apparatus 1820 may further comprise a transmitter TX and receiver RX 1823 coupled to the processor 1821. The MEM 1822 stores a PROG 1824. The PROG 1824 may include instructions that, when executed on the associated processor 1821, enable the apparatus 1820 to operate in accordance with the embodiments of the present disclosure. A combination of the at least one processor 1821 and the at least one MEM 1822 may form processing means 1825 adapted to implement various embodiments of the present disclosure.

[00178] Various embodiments of the present disclosure may be implemented by computer program executable by one or more of the processor 1821, software, firmware, hardware or in a combination thereof. [00179] The MEM 1822 may be of any type suitable to the local technical environment and may be implemented using any suitable data storage technology, such as semiconductor based memory devices, magnetic memory devices and systems, optical memory devices and systems, fixed memories and removable memories, as non-limiting examples.

[00180] The processor 1821 may be of any type suitable to the local technical environment, and may include one or more of general purpose computers, special purpose computers, microprocessors, digital signal processors DSPs and processors based on multicore processor architecture, as non-limiting examples.

[00181] In an embodiment where the apparatus is implemented as or at the first access and mobility management function entity, the memory 1822 contains instructions executable by the processor 1821, whereby the first access and mobility management function entity operates according to any of the methods 800, 900, 1000, 1100 and 1200 as described in reference to FIGs. 8-12. [00182] In an embodiment where the apparatus is implemented as or at the first network function entity, the memory 1822 contains instructions executable by the processor 1821, whereby the first network function entity operates according to the method 1300 as described in reference to FIG.13.

[00183] In an embodiment where the apparatus is implemented as or at the session management function entity, the memory 1822 contains instructions executable by the processor 1821, whereby the session management function entity operates according to the method 1400 as described in reference to FIG.14. [00184] FIGURE 19 illustrates a simplified block diagram of a first access and mobility management function entity according to an embodiment of the present disclosure. The first access and mobility management function entity may be AMF of 5GS. The first access and mobility management function entity 1900 comprises a receiving module 1902 configured to receive a first message from a first network function entity. The first message requests a first notification toward a second network function entity when a terminal device becomes available after downlink data notification failure for the terminal device and the first message includes at least one of a traffic descriptor identifying a source of the downlink data and network information associated to a session related to the terminal device. The first access and mobility management function entity 1900 further comprises a sending module 1904 configured to send a second message to at least one session management function entity associated to the session related to the terminal device. The second message requests a second notification toward the first access and mobility management function entity when the downlink data notification failure for the terminal device is detected and the second message includes the traffic descriptor when the first message includes the traffic descriptor.

[00185] FIGURE 20 illustrates a simplified block diagram of a first network function entity according to an embodiment of the present disclosure. The first network function entity may be UDM or AMF of 5GS. The first network function entity 2000 comprises a determining module 2002 configured to determine to send a first message to a first access and mobility management function entity. The first message requests a first notification toward a second network function entity when a terminal device becomes available after downlink data notification failure for the terminal device and the first message includes at least one of a traffic descriptor identifying a source of the downlink data and network information associated to a session related to the terminal device. The first network function entity 2000 further comprises a sending module 2004 configured to send the first message to the first access and mobility management function entity.

[00186] FIGURE 21 illustrates a simplified block diagram of a session management function entity according to an embodiment of the present disclosure. The session management function entity may be SMF of 5GS. The session management function entity 2100 comprises a receiving module 2102 configured to receive a message from an access and mobility management function entity, wherein the message requests a notification when a downlink data notification failure for a terminal device is detected. The session management function entity 2100 further comprises a sending module 2104 configured to send the notification to the access and mobility management function entity when the downlink data notification failure for the terminal device is detected.

[00187] According to an aspect of the disclosure it is provided a computer program product being tangibly stored on a computer readable storage medium and including instructions which, when executed on at least one processor, cause the at least one processor to carry out the method related to the first access and mobility management function entity as described above.

[00188] According to an aspect of the disclosure it is provided a computer program product being tangibly stored on a computer readable storage medium and including instructions which, when executed on at least one processor, cause the at least one processor to carry out the method related to the first network function entity as described above.

[00189] According to an aspect of the disclosure it is provided a computer program product being tangibly stored on a computer readable storage medium and including instructions which, when executed on at least one processor, cause the at least one processor to carry out the method related to the session management function entity as described above.

[00190] According to an aspect of the disclosure it is provided a computer-readable storage medium storing instructions which when executed by at least one processor, cause the at least one processor to perform the method related to the first access and mobility management function entity as described above. [00191] According to an aspect of the disclosure it is provided a computer-readable storage medium storing instructions which when executed by at least one processor, cause the at least one processor to perform the method related to the first network function entity as described above.

[00192] According to an aspect of the disclosure it is provided a computer-readable storage medium storing instructions which when executed by at least one processor, cause the at least one processor to perform the method related to the session management function entity as described above.

[00193] Embodiments herein afford many advantages, of which a non-exhaustive list of examples follows. Some embodiments herein may simplify the realization on system level, for example reducing the complexity from UDM and SMF. Some embodiments herein may support implicit subscription using existing SM Context service operation, saving network traffic especially inter-PLMN traffic for Home Routed PDU sessions. Some embodiments herein may reusing SM Context Update service operation and AMF notification URI for SMF Status Notification, which is already updated by AMF in existing procedures when AMF changes. The embodiments herein are not limited to the features and advantages mentioned above. A person skilled in the art will recognize additional features and advantages upon reading the following detailed description.

[00194] In addition, the present disclosure may also provide a carrier containing the computer program as mentioned above, wherein the carrier is one of an electronic signal, optical signal, radio signal, or computer readable storage medium. The computer readable storage medium can be, for example, an optical compact disk or an electronic memory device like a RAM (random access memory), a ROM (read only memory), Flash memory, magnetic tape, CD-ROM, DVD, Blue-ray disc and the like.

[00195] The techniques described herein may be implemented by various means so that an apparatus implementing one or more functions of a corresponding apparatus described with an embodiment comprises not only prior art means, but also means for implementing the one or more functions of the corresponding apparatus described with the embodiment and it may comprise separate means for each separate function, or means that may be configured to perform two or more functions. For example, these techniques may be implemented in hardware (one or more apparatuses), firmware (one or more apparatuses), software (one or more modules), or combinations thereof. For a firmware or software, implementation may be made through modules (e.g., procedures, functions, and so on) that perform the functions described herein.

[00196] Exemplary embodiments herein have been described above with reference to block diagrams and flowchart illustrations of methods and apparatuses. It will be understood that each block of the block diagrams and flowchart illustrations, and combinations of blocks in the block diagrams and flowchart illustrations, respectively, can be implemented by various means including computer program instructions. These computer program instructions may be loaded onto a general purpose computer, special purpose computer, or other programmable data processing apparatus to produce a machine, such that the instructions which execute on the computer or other programmable data processing apparatus create means for implementing the functions specified in the flowchart block or blocks.

[00197] Further, while operations are depicted in a particular order, this should not be understood as requiring that such operations be performed in the particular order shown or in sequential order, or that all illustrated operations be performed, to achieve desirable results. In certain circumstances, multitasking and parallel processing may be advantageous. Likewise, while several specific implementation details are contained in the above discussions, these should not be construed as limitations on the scope of the subject matter described herein, but rather as descriptions of features that may be specific to particular embodiments. Certain features that are described in the context of separate embodiments may also be implemented in combination in a single embodiment. Conversely, various features that are described in the context of a single embodiment may also be implemented in multiple embodiments separately or in any suitable sub-combination.

[00198] While this specification contains many specific implementation details, these should not be construed as limitations on the scope of any implementation or of what may be claimed, but rather as descriptions of features that may be specific to particular embodiments of particular implementations. Certain features that are described in this specification in the context of separate embodiments can also be implemented in combination in a single embodiment. Conversely, various features that are described in the context of a single embodiment can also be implemented in multiple embodiments separately or in any suitable sub-combination. Moreover, although features may be described above as acting in certain combinations and even initially claimed as such, one or more features from a claimed combination can in some cases be excised from the combination, and the claimed combination may be directed to a sub-combination or variation of a sub-combination.

[00199] It will be obvious to a person skilled in the art that, as the technology advances, the inventive concept can be implemented in various ways. The above described embodiments are given for describing rather than limiting the disclosure, and it is to be understood that modifications and variations may be resorted to without departing from the spirit and scope of the disclosure as those skilled in the art readily understand. Such modifications and variations are considered to be within the scope of the disclosure and the appended claims. The protection scope of the disclosure is defined by the accompanying claims.

[00200]

SOME EMBODIMENTS DESCRIBED ABOVE MAY BE SUMMARIZED IN THE FOLLOWING MANER:

1. A method (800) performed by a first access and mobility management function entity, comprising: receiving (802) a first message from a first network function entity, wherein the first message requests a first notification toward a second network function entity when a terminal device becomes available after downlink data notification failure for the terminal device and the first message includes at least one of a traffic descriptor identifying a source of the downlink data and network information associated to a session related to the terminal device; and sending (804) a second message to at least one session management function entity associated to the session related to the terminal device, wherein the second message requests a second notification toward the first access and mobility management function entity when the downlink data notification failure for the terminal device is detected and the second message includes the traffic descriptor when the first message includes the traffic descriptor.

2. The method according to embodiment 1, wherein sending the second message to the at least one session management function entity associated to the session related to the terminal device comprises: when the first message includes the network information, sending the second message to the at least one session management function entity associated to the session related to the terminal device and with the network information. 3. The method according to embodiment 1 or 2, wherein the network information includes a data network name and single network slice selection assistance information.

4. The method according to any of embodiment 1-3, wherein the first network function entity is a unified data management entity and the second network function entity is a network exposure function entity.

5. The method according to any of embodiment 1-4, wherein the first message is a subscribe request or a subscribe update request.

6. The method according any of embodiment 1-4, wherein the first message is an Namf_EventExposure_Subscribe request.

7. The method according to any of embodiment 1-3, wherein the first network function entity is a second access and mobility management function entity and the second network function entity is a network exposure function entity.

8. The method according to any of embodiment 1-7, wherein the second message is a subscribe request or a subscribe update request.

9. The method according to any of embodiment 1-7, wherein the second message is an Nsmf_EventExposure_S u bscri be req uest.

10. The method according to embodiment 8, wherein the subscribe request is included in an Nsmf_PDUSession_CreateSMContext Request or Nsmf_PDUSession_UpdateSMContext Request, wherein the subscribe update request is included in an Nsmf_PDUSession_UpdateSMContext Request.

11. The method according to any of embodiment 1-10, further comprising: receiving (902) the second notification from a session management function entity; setting (904) a flag of the first notification; detecting (906) that the terminal device is reachable; and sending (908) the first notification to the second network function entity.

12. The method according to embodiment 11, wherein the second notification is received in an Nsmf_EventExposure_Notify message or an Nsmf_PDUSession_SMContextStatusNotify message.

13. The method according to any of embodiment 1-12, further comprising: sending (1002) a third message for unsubscribing to the second notification to at least one of the at least one session management function entity.

14. The method according to embodimentl 3, wherein the third message is sent: when the subscription to the first notification expires; when the subscription to the first notification is removed; when the session related to the terminal device is released; when the at least one of the at least one session management function entity is changed or removed; when the access and mobility management function entity serving the terminal device is changed from the first access and mobility management function entity to a third access and mobility management function entity; or when a new session management function entity associated to the session related to the terminal device is inserted.

15. The method according to any of embodiment 1-14, wherein when a new session management function entity associated to the session related to the terminal device is inserted, the method further comprises: sending (1102) a fourth message to the new session management function entity, wherein the fourth message requests the second notification and the fourth message includes the traffic descriptor when the first message includes the traffic descriptor.

16. The method according to any of embodiment 1-15, wherein when the access and mobility management function entity serving the terminal device is changed from the first access and mobility management function entity to a third access and mobility management function entity, the method further comprises: sending (1202) a fifth message to the third access and mobility management function entity, wherein the fifth message requests the first notification and includes at least one of the traffic descriptor identifying the source of the downlink data and the network information associated to a session related to the terminal device.

17. A method (1300) performed by a first network function entity, comprising: determining (1302) to send a first message to a first access and mobility management function entity, wherein the first message requests a first notification toward a second network function entity when a terminal device becomes available after downlink data notification failure for the terminal device and the first message includes at least one of a traffic descriptor identifying a source of the downlink data and network information associated to a session related to the terminal device; and sending (1304) the first message to the first access and mobility management function entity.

18. The method according to embodiment 17, wherein the first network function entity is a unified data management entity, the second network function entity is a network exposure function entity, and determining to send the first message to the first access and mobility management function entity is in response to receiving a message for subscribing to the first notification from the network exposure function entity.

19. The method according to embodiment 17, wherein the first network function entity is a second access and mobility management function entity, the second network function entity is a network exposure function entity, and determining to send the first message to the first access and mobility management function entity is in response to the access and mobility management function entity serving the terminal device is changed from the second access and mobility management function entity to the first access and mobility management function entity.

20. The method according to any of embodiment 17-19, wherein the network information includes a data network name and single network slice selection assistance information.

21. The method according to any of embodiment 17-20, wherein the first message is a subscribe request or a subscribe update request.

22. The method according to any of embodiment 17-20, wherein the first message is an Namf_EventExposure_Subscribe request.

23. A method (1400) performed by a session management function entity, comprising: receiving (1402) a message from an access and mobility management function entity, wherein the message requests a notification when a downlink data notification failure for a terminal device is detected; and sending (1404) the notification to the access and mobility management function entity when the downlink data notification failure for the terminal device is detected.

24. The method according to embodiment 23, wherein the message includes a traffic descriptor identifying a source of the downlink data, and the sending step comprises: sending the notification to the access and mobility management function entity when the downlink data notification failure matching the traffic descriptor is detected.

25. The method according to any of embodiment 23-24, wherein the message is a subscribe request or a subscribe update request.

26. The method according to any of embodiment 23-24, wherein the message is an Nsmf_EventExposure_S u bscri be req uest.

27. The method according to embodiment25, wherein the subscribe request is included in an Nsmf_PDUSession_CreateSMContext Request or Nsmf_PDUSession_UpdateSMContext Request, wherein the subscribe update request is included in an Nsmf_PDUSession_UpdateSMContext Request.

28. The method according to any of embodiment 23-27, wherein the notification is sent in an Nsmf_EventExposure_Notify message or an Nsmf_PDUSession_SMContextStatusNotify message.

29. The method according to any of embodiment 23-28, further comprising: receiving (1406) a message for unsubscribing to the notification from the session management function entity; and performing (1408) an unsubscribing operation.

30. The method according to any of embodiment 23-28, further comprising: when session management context associated to a session related to the terminal device is released, performing (1410) an unsubscribing operation to the notification.

31. An apparatus (1800) implemented in a first access and mobility management function entity, comprising: a processor (1821); and a memory (1822) coupled to the processor (1821), said memory (1822) containing instructions executable by said processor (1821), whereby said apparatus (1800) is operative to: receive a first message from a first network function entity, wherein the first message requests a first notification toward a second network function entity when a terminal device becomes available after downlink data notification failure for the terminal device and the first message includes at least one of a traffic descriptor identifying a source of the downlink data and network information associated to a session related to the terminal device; and send a second message to at least one session management function entity associated to the session related to the terminal device, wherein the second message requests a second notification toward the first access and mobility management function entity when the downlink data notification failure for the terminal device is detected and the second message includes the traffic descriptor when the first message includes the traffic descriptor.

32. The apparatus according to embodiment 31, wherein the apparatus is further operative to perform the method of any one of embodiment 2 to 16.

33. An apparatus (1800) implemented in a first network function entity, comprising: a processor (1821); and a memory (1822) coupled to the processor (1821), said memory (1822) containing instructions executable by said processor (1821), whereby said apparatus (1800) is operative to: determine to send a first message to a first access and mobility management function entity, wherein the first message requests a first notification toward a second network function entity when a terminal device becomes available after downlink data notification failure for the terminal device and the first message includes at least one of a traffic descriptor identifying a source of the downlink data and network information associated to a session related to the terminal device; and send the first message to the first access and mobility management function entity.

34. The apparatus according to embodiment 33, wherein the apparatus is further operative to perform the method of any one of embodiment 18 to 22.

35. An apparatus (1800) implemented in a session management function entity, comprising: a processor (1821); and a memory (1822) coupled to the processor (1821), said memory (1822) containing instructions executable by said processor (1821), whereby said apparatus (1800) is operative to: receive a message from an access and mobility management function entity, wherein the message requests a notification when a downlink data notification failure for a terminal device is detected; and send the notification to the access and mobility management function entity when the downlink data notification failure for the terminal device is detected.

36. The apparatus according to embodiment 35, wherein the apparatus is further operative to perform the method of any one of embodiment 24 to 30.

37. A computer-readable storage medium storing instructions which when executed by at least one processor, cause the at least one processor to perform the method according to any one of embodiment 1 to 30.

38. A computer program product comprising instructions which when executed by at least one processor, cause the at least one processor to perform the method according to any of embodiment 1 to 30.

Claims

CLAIMS WHAT IS CLAIMED IS:

2. The method according to claim 1, wherein sending the second message to the at least one session management function entity associated to the session related to the terminal device comprises: when the first message includes the network information, sending the second message to the at least one session management function entity associated to the session related to the terminal device and with the network information.

3. The method according to claim 1 or 2, wherein the network information includes a data network name and single network slice selection assistance information.

4. The method according to any of claims 1-3, wherein the first network function entity is a unified data management entity and the second network function entity is a network exposure function entity.

5. The method according to any of claims 1 -4, wherein the first message is a subscribe request or a subscribe update request.

6. The method according any of claims 1-4, wherein the first message is an Namf_EventExposure_Subscribe request.

7. The method according to any of claims 1-3, wherein the first network function entity is a second access and mobility management function entity and the second network function entity is a network exposure function entity.

8. The method according to any of claims 1-7, wherein the second message is a subscribe request or a subscribe update request.

9. The method according to any of claims 1-7, wherein the second message is an Nsmf_EventExposure_S u bscri be req uest.

10. The method according to claim 8, wherein the subscribe request is included in an Nsmf_PDUSession_CreateSMContext Request or Nsmf_PDUSession_UpdateSMContext Request, wherein the subscribe update request is included in an Nsmf_PDUSession_UpdateSMContext Request.

11. The method according to any of claims 1-10, further comprising: receiving (902) the second notification from a session management function entity; setting (904) a flag of the first notification; detecting (906) that the terminal device is reachable; and sending (908) the first notification to the second network function entity.

12. The method according to claim 11, wherein the second notification is received in an Nsmf_EventExposure_Notify message or an Nsmf_PDUSession_SMContextStatusNotify message.

13. The method according to any of claims 1-12, further comprising: sending (1002) a third message for unsubscribing to the second notification to at least one of the at least one session management function entity.

14. The method according to claim 13, wherein the third message is sent: when the subscription to the first notification expires; when the subscription to the first notification is removed; when the session related to the terminal device is released; when the at least one of the at least one session management function entity is changed or removed; when the access and mobility management function entity serving the terminal device is changed from the first access and mobility management function entity to a third access and mobility management function entity; or when a new session management function entity associated to the session related to the terminal device is inserted.

15. The method according to any of claims 1-14, wherein when a new session management function entity associated to the session related to the terminal device is inserted, the method further comprises: sending (1102) a fourth message to the new session management function entity, wherein the fourth message requests the second notification and the fourth message includes the traffic descriptor when the first message includes the traffic descriptor.

16. The method according to any of claims 1-15, wherein when the access and mobility management function entity serving the terminal device is changed from the first access and mobility management function entity to a third access and mobility management function entity, the method further comprises: sending (1202) a fifth message to the third access and mobility management function entity, wherein the fifth message requests the first notification and includes at least one of the traffic descriptor identifying the source of the downlink data and the network information associated to a session related to the terminal device.

18. The method according to claim 17, wherein the first network function entity is a unified data management entity, the second network function entity is a network exposure function entity, and determining to send the first message to the first access and mobility management function entity is in response to receiving a message for subscribing to the first notification from the network exposure function entity.

19. The method according to claim 17, wherein the first network function entity is a second access and mobility management function entity, the second network function entity is a network exposure function entity, and determining to send the first message to the first access and mobility management function entity is in response to the access and mobility management function entity serving the terminal device is changed from the second access and mobility management function entity to the first access and mobility management function entity.

20. The method according to any of claims 17-19, wherein the network information includes a data network name and single network slice selection assistance information.

21. The method according to any of claims 17-20, wherein the first message is a subscribe request or a subscribe update request.

22. The method according to any of claims 17-20, wherein the first message is an Namf_EventExposure_Subscribe request.

24. The method according to claim 23, wherein the message includes a traffic descriptor identifying a source of the downlink data, and the sending step comprises: sending the notification to the access and mobility management function entity when the downlink data notification failure matching the traffic descriptor is detected.

25. The method according to any of claims 23-24, wherein the message is a subscribe request or a subscribe update request.

26. The method according to any of claims 23-24, wherein the message is an Nsmf_EventExposure_S u bscri be req uest.

27. The method according to claim 25, wherein the subscribe request is included in an Nsmf_PDUSession_CreateSMContext Request or Nsmf_PDUSession_UpdateSMContext Request, wherein the subscribe update request is included in an Nsmf_PDUSession_UpdateSMContext Request.

28. The method according to any of claims 23-27, wherein the notification is sent in an Nsmf_EventExposure_Notify message or an Nsmf_PDUSession_SMContextStatusNotify message.

29. The method according to any of claims 23-28, further comprising: receiving (1406) a message for unsubscribing to the notification from the session management function entity; and performing (1408) an unsubscribing operation.

30. The method according to any of claims 23-28, further comprising: when session management context associated to a session related to the terminal device is released, performing (1410) an unsubscribing operation to the notification.

32. The apparatus according to claim 31, wherein the apparatus is further operative to perform the method of any one of claims 2 to 16.

34. The apparatus according to claim 33, wherein the apparatus is further operative to perform the method of any one of claims 18 to 22.

36. The apparatus according to claim 35, wherein the apparatus is further operative to perform the method of any one of claims 24 to 30.

37. A computer-readable storage medium storing instructions which when executed by at least one processor, cause the at least one processor to perform the method according to any one of claims 1 to 30.

38. A computer program product comprising instructions which when executed by at least one processor, cause the at least one processor to perform the method according to any of claims 1 to 30.