WO2021013353A1

WO2021013353A1 - Fallback during call set-up

Info

Publication number: WO2021013353A1
Application number: PCT/EP2019/070055
Authority: WO
Inventors: Ralf Keller; Afshin Abtin; Jörgen AXELL
Original assignee: Telefonaktiebolaget Lm Ericsson (Publ)
Priority date: 2019-07-25
Filing date: 2019-07-25
Publication date: 2021-01-28

Abstract

There is provided mechanisms for fallback during call set-up. A method is performed by a terminal device. The method comprises performing a call set-up procedure for a call with a first network. The first network is a 5GS network. The method comprises, whilst performing the call set-up procedure, performing a fallback procedure to a second network. The method comprises continuing the call set-up procedure with the second network despite having obtained an indication of no VoPS support in the second network.

Description

FALLBACK DURING CALL SET-UP

TECHNICAL FIELD

Embodiments presented herein relate to a method, a terminal device, a computer program, and a computer program product for fallback during call set-up. BACKGROUND

In general terms, a fifth generation (5G) system (5GS) is a telecommunication system using the 5G New Radio (NR) air interface, or the Evolved Universal Terrestrial Radio Access (E-UTRA) air interface connected to a 5G core network (5GC). Support for voice services has recently been defined for 5GS. Voice services in 5GS can be supported either by means of Voice over NR (VoNR) when voice services are supported in the terminal device, the radio network (i.e., the NR air interface) and the 5GC, or by means of Evolved Packet System (EPS) fallback when voice service are not supported in the radio network or if VoNR has not been deployed. VoNR and EPS fallback are defined in the documents 3GPP TS 23.501 entitled“System architecture for the 5G System (5GS)”, version 16.1.0, and 3GPP TS 23.502 entitled“Procedures for the 5G System (5GS)”, version 16.1.1, respectively.

EPS fallback implies that for a terminal device camping on a cell of a 5GS (NR/5GC) having an Internet Protocol Multimedia Subsystem (IMS) Protocol Data Unit (PDU) session established and IMS registration performed already, during an IMS call setup procedure (e.g. when a flow with QoS with 5QI value equal to 1 is established, where 5QI is short for 5G QoS Indicator), the network triggers a mobility procedure to move the terminal device from the 5GS to the EPS where the actual call will be established. During the EPS fallback procedure, the voice media bearer, a bearer with a QoS Class Identifier (QCI) with value 1 (i.e., QCI=i), will be established on the EPS. For a voice centric terminal device to camp on a 5GS (5GC/NR), a Voice over PS

(VoPS; where PS is short for Packet Switched) support indication provided from the network to the terminal device during registration with the 5GC is needed. This is specified in clause 5.16.3.5 of aforementioned 3GPP TS 23.501.

Additionally, for Voice over Long Term Evolution (VoLTE) and VoNR, the call of a terminal device with a voice session that moves to an area, tracking area, or network without VoPS support, should not be terminated because a bearer with QCI=i (or a QoS flow with 5QI=I) has already been established in the network where the call set up procedure originated. This is further explained in clause 7.2d in document 3GPP TS 23.221 entitled“Architectural requirements”, version 16.2.0, clause 5.5.3.24 in document 3GPP TS 24.301 entitled“Non-Access-Stratum (NAS) protocol for Evolved Packet System (EPS); Stage 3”, version 16.1.1, in relation to“Persistent EPS bearer context”, and clause 4.3.2 in document 3GPP TS 24.501 entitled“Non-Access- Stratum (NAS) protocol for 5G System (5GS); Stage 3”, version 16.1.0, in relation to “Persistent PDU session”. Persistent EPS bearer context implies that the terminal device has an established EPS bearer with QCI=i (i.e. a voice call is about to be established or is already ongoing).

The behavior of a terminal device when it has“Persistent PDU session/Persistent EPS bearer context” is specified in aforementioned documents 3GPP TS 24.301 and 3GPP TS 24.501. However, when the terminal device is subject for an EPS fallback to a second network, the terminal device having an IMS PDU session on the 5GS

(5GC/NR) is moved to EPS, the terminal device does not have any“Persistent PDU session”, i.e. the terminal device does not yet have a QoS flow with 5QI=i. This is because the bearer for the voice media is established on LTE/EPC (where EPC is short for Evolved Packet Core, i.e., the Core Network (CN) in the LTE / System

Architecture Evolution (SAE) system), being the second network, as part of the EPS fallback procedures.

However, there might be fallback scenarios for which the behavior of the terminal device is not well defined. A consequence of this is that the call set-up procedure might be terminated.

Hence, there is still a need for mechanisms avoiding calls to be dropped during fallback.

SUMMARY

An object of embodiments herein is to provide mechanisms for efficient fallback during a call-set up procedure that does not suffer from the issues noted above or at least where the above noted issues are mitigated or reduced. According to a first aspect there is presented a terminal device for fallback during call set-up. The terminal device comprises processing circuitry. The processing circuitry is configured to cause the terminal device to perform a call set-up procedure for a call with a first network. The first network is a 5GS network. The processing circuitry is configured to cause the terminal device to, whilst the call set-up procedure is performed, perform a fallback procedure to a second network continue the call set- up procedure with the second network despite having obtained an indication of no VoPS support in the second network.

According to a second aspect there is presented a terminal device for fallback during call set-up. The terminal device comprises a call set-up module configured to perform a call set-up procedure for a call with a first network. The first network is a 5GS network. The terminal device comprises a fallback module configured to, whist the call set-up procedure is performed, perform a fallback procedure to a second network. The terminal device comprises a continue module configured to continue the call set-up procedure with the second network despite having obtained an indication of no VoPS support in the second network. According to a third aspect there is presented a method for fallback during call set up. The method is performed by a terminal device. The method comprises

performing a call set-up procedure for a call with a first network. The first network is a 5GS network. The method comprises, whilst performing the call set-up procedure, performing a fallback procedure to a second network. The method comprises continuing the call set-up procedure with the second network despite having obtained an indication of no VoPS support in the second network.

According to a fourth aspect there is presented a computer program for fallback during call set-up, the computer program comprising computer program code which, when run on a terminal device, causes the terminal device to perform a method according to the third aspect.

According to a fifth aspect there is presented a computer program product

comprising a computer program according to the fourth aspect and a computer readable storage medium on which the computer program is stored. The computer readable storage medium could be a non-transitory computer readable storage medium. Advantageously this provides efficient fallback during a call-set up procedure for the terminal device.

Advantageously this does not result in that the call is terminated during the call set up procedure. Advantageously this ensures that the fallback succeeds even if IMS Voice over PS is not supported for the terminal device in the second network.

Advantageously this enables the behaviour of the terminal device to be uniform during call setup and after call completion, regardless of the type of radio access technology used by the first network towards which the call set-up procedure is initiated.

Other objectives, features and advantages of the enclosed embodiments will be apparent from the following detailed disclosure, from the attached dependent claims as well as from the drawings.

Generally, all terms used in the claims are to be interpreted according to their ordinary meaning in the technical field, unless explicitly defined otherwise herein. All references to "a/an/the element, apparatus, component, means, module, step, etc." are to be interpreted openly as referring to at least one instance of the element, apparatus, component, means, module, step, etc., unless explicitly stated otherwise. The steps of any method disclosed herein do not have to be performed in the exact order disclosed, unless explicitly stated.

BRIEF DESCRIPTION OF THE DRAWINGS

The inventive concept is now described, by way of example, with reference to the accompanying drawings, in which:

Figs. 1 and 2 are schematic diagrams illustrating a communications network according to embodiments;

Fig. 3 is a flowchart of methods according to embodiments;

Fig. 4 is a signalling diagram according to embodiments; Fig. 5 is a schematic diagram showing functional units of a terminal device according to an embodiment;

Fig. 6 is a schematic diagram showing functional modules of a terminal device according to an embodiment; and Fig. 7 shows one example of a computer program product comprising computer readable storage medium according to an embodiment.

DETAILED DESCRIPTION

The inventive concept will now be described more fully hereinafter with reference to the accompanying drawings, in which certain embodiments of the inventive concept are shown. This inventive concept may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided by way of example so that this disclosure will be thorough and complete, and will fully convey the scope of the inventive concept to those skilled in the art. Like numbers refer to like elements throughout the

description. Any step or feature illustrated by dashed lines should be regarded as optional.

Fig. 1 is a schematic diagram illustrating a communication network 100a where embodiments presented herein can be applied. The communications network 100a comprises a networks 110a, 110b, 110c, 120. Each of the networks 110a, 110b, 110c is operatively connected to the PDN 120 over a respective link 140a, 140b, 140c which might be a wireless link, a wired link, or any combination thereof. In general terms, each of the networks 110a, 110b, 110c is a cellular network. Network 120 is a Packet Data Network (PDN) 120. Network 110a will hereinafter be denoted a first network, and networks 110b, 110c will hereinafter collectively be denoted a second network, or second networks. Network 110a will hereinafter be denoted a first network, and networks 110b, 110c will hereinafter collectively be denoted a second network, or second networks.

In some examples the first network 110a is a 5GS. In some examples the second network 110b is an EPS network 110b. In some examples the second network 110c is a radio access network connected to (and including) a 5GC network 110c. Each of the first network noa, the second network nob, and the third network lioc provides network access to terminal devices, as represented by terminal device (TD) 200 in Fig. l. Non-limiting examples of terminal devices are portable wireless devices, mobile stations, mobile phones, handsets, wireless local loop phones, user equipment (UE), smartphones, laptop computers, tablet computers, and the like.

Fig. 2 is a schematic diagram illustrating a communication network loob where embodiments presented herein can be applied. The communications network loob discloses details of network noa and network nob of Fig. l and also shows the terminal device 200 and the PDN 120. As the skilled person understands, some of the entities in Fig. 2 are part of network noa, some entities are part of network nob, and some entities in Fig. 2 might be shared between network noa and network nob.

The Evolved Universal Terrestrial Radio Access Network (E-UTRAN) comprises radio access network nodes in terms of eNodeBs, providing radio access between the terminal device and the EPC in terms of User Plane (Packet Data Convergence Protocol (PDCP), Radio Link Control (RLC), Medium Access Control (MAC) and PHYsical (PHY) layers) and Control Plane (Radio Resource Control (RRC)) protocol terminations towards the terminal device.

The Next Generation Radio Access Network (NG-RAN) comprises radio access network nodes in terms of gNBs or ng-eNBs, providing radio access between the terminal device 200 and the 5GC in terms of User Plane and Control Plane protocol terminations towards the terminal device.

The Mobility Management Entity (MME) is a control node which processes the signalling between the terminal device and the EPC and provides Visitor Location Register (VLR) functionality for the EPS.

The SGW routes and forwards user data packets, while also acting as the mobility anchor for the user plane during inter-eNodeB handovers and as the anchor for mobility between LTE and other 3GPP technologies.

The Access and Mobility Management function (AMF) supports termination of NAS signalling, NAS ciphering and integrity protection, registration management, connection management, mobility management, access authentication and

authorization, security context management The Session Management function (SMF) supports session management (session establishment, modification, release), terminal device Internet protocol (IP= address allocation and management, DHCP functions, termination of Non-Access Stratum (NAS) signalling related to session management, downlink data notification, traffic steering configuration for the UPF for proper traffic routing.

The User Plane Function (UPF) supports packet routing and forwarding, packet inspection, QoS handling, acts as external PDU session point of interconnect to a PDN, and is an anchor point for intra- and inter- RAT mobility.

The Policy Control Function (PCF) supports unified policy framework, providing policy rules to Control Plane (CP) functions, access subscription information for policy decisions in a User data Repository (UDR).

The Home Subscriber Server (HSS) holds subscription-related information to support the network entities handling calls/sessions.

The Unified Data Management (UDM) supports generation of Authentication and Key Agreement (AKA) credentials, user identification handling, access authorization, and subscription management.

The Internet Protocol Multimedia Subsystem (IMS) provides an architectural framework for delivering IP multimedia services via UTRAN and E- UTRAN.

The PDN 120 is the network through which the terminal device obtains a packet data connection to the Internet.

These entities are assumed to be configured to perform operations as known in the art unless modified as described hereinafter in accordance with the herein disclosed embodiments. A detailed description of these entities is therefore omitted.

Some of the interfaces facilitating communication between the entities in Fig. 2 are the following. Interface N26 is between the MME and the AMF. Interface S5-U is between the SGW and the SMF. Interface S5-C is between the SGW and the UPF. Interface N4 is between the SMF and the UPF. Interface N7 is between the SMF and the PCF. Interface N6 is between the UPF and the PDN. Interface Rx is between the PCF and the IMS. Interface NNI is between the IMS of one network and the IMS of another network. As noted there might be fallback scenarios for which the behavior of the terminal device is not well defined. A consequence of this is that the call set-up procedure might be terminated.

As an example, behavior of the terminal device 200 is moving to a target EPS with EPS fallback where there is not any indication from the EPS that VoPS is not supported is not well defined.

The embodiments disclosed herein relate to mechanisms for fallback during call set- up. In order to obtain such mechanisms there is provided a terminal device 200, a method performed by the terminal device 200, a computer program product comprising code, for example in the form of a computer program, that when run on a terminal device 200, causes the terminal device 200 to perform the method.

Fig. 3 is a flowchart illustrating embodiments of methods for fallback during call set- up. The methods are performed by the terminal device 200. The methods are advantageously provided as computer programs 720.

S102: The terminal device 200 performs a call set-up procedure for a call with a first network 110a. The first network is a 5GS network.

S104: The terminal device 200 performs fallback procedure to a second network 110b, 110c. S104 is performed whilst the call set-up procedure is performed.

S106: The terminal device 200 continues the call set-up procedure with the second network 110b, 110c despite having obtained an indication of no VoPS support in the second network 110b, 110c.

Embodiments relating to further details of fallback during call set-up as performed by the terminal device 200 will now be disclosed.

In some embodiments, during a registration procedure with the first network 110a performed before or in conjunction with the call set-up procedure, the terminal device 200 has obtained an indication that VoPS is supported in the first network noa. There may be different ways for the terminal device 200 to obtain the indication of no VoPS support in the second network 110b, 110c. In some aspects the terminal device 200 receives a VoPS indicator which indicates“not supported”. That is, according to an embodiment, the indication of no VoPS support in the second network 110b, 110c is obtained by receiving, from the second network 110b, 110c, an VoPS indicator indicating that VoPS is not supported. The VoPS indicator might be an IMS voice over PS session indicator.

There could be different parts of the call set-up procedure during which the indication of no VoPS support in the second network 110b, 110c is obtained.

According to a first embodiment, the VoPS indicator is received upon the terminal device 200 initiating a Tracking Area Update (TAU) procedure. This will be described in further detail with reference to step S2o6a below. According to a second

embodiment, the VoPS indicator is received upon the terminal device 200 initiates an attach procedure with a Packet Data Network (PDN) connectivity request with request type“handover”. This will be described in further detail with reference to step S2o6b below.

In other aspects the terminal device 200 does not receive an explicit indication of no VoPS support in the second network 110b, 110c. The indication of no VoPS support in the second network 110b, 110c is then obtained by means of absence of such an explicit indication. That is, according to an embodiment, the indication of no VoPS support in the second network 110b, 110c is obtained by the terminal device 200 not receiving, from the second network 110b, 110c, any VoPS indicator indicating that VoPS is supported.

There could be different types of fallback procedures, e.g. depending on the type of second network 110b, 110c. As disclosed above with reference to Fig. 1, according to a first example the second network 110b, 110c is an Evolved Packet System (EPS) network 110b. The fallback procedure could then be an EPS fallback procedure to the EPS network 110b. As disclosed above with reference to Fig. 1, according to a second example the second network 110b, 110c is a radio access network connected to (and including) a 5GC network 110c. The fallback procedure could then be a RAT fallback procedure to this other 5G network 110c (possibly using the same 5GC as the first network 110a). In some aspects the terminal device 200, as part of continuing the call set-up procedure with the second network 110b, 110c, considers a PDU session established with the first network as persistent in the second network 110b, 110c. Particularly, according to an embodiment, a PDU session with the first network 110a is established as part of the call set-up procedure, and the PDU session is by the terminal device 200 considered as persistent when continuing the call set-up procedure with the second network 110b, 110c.

In some aspects the thus persistent PDU session is turned into a persistent PDN connection. That is, according to an embodiment, the thus persistent PDU session is turned into a persistent PDN connection when continuing the call set-up procedure with the second network 110b, 110c.

Examples of scenarios where the PDU session is by the terminal device 200 considered to be persistent will be disclosed next.

In some scenarios the terminal device 200 considers the PDU session to the IMS DNN as persistent if a voice call is being established, even if the terminal device 200 notices that a handover is initiated and even if a Quality of Service (QoS) flow with 5G Quality Indicator (5QI) of value 1, i.e., sQI=i, is not established. In particular, according to an embodiment, the PDU session is by the terminal device 200 considered as persistent even if obtaining an indication that a handover procedure is initiated for the terminal device 200 and even if a QoS flow with sQI= 1 is not established.

In some scenarios the terminal device 200 considers the PDU session to the IMS DNN as persistent if a voice call is being established, even if the terminal device 200 notices that a redirect is initiated even if the QoS flow with sQI=i is not established. In particular, according to an embodiment, the PDU session is by the terminal device 200 considered as persistent even if obtaining an indication that a redirect procedure is initiated for the terminal device 200 and even if a QoS flow with sQI= 1 is not established.

In some scenarios the terminal device 200 has a persistent PDU session and has moved to EPS during the fallback procedure. Then the terminal device 200 considers the EPS bearer context as persistent until the EPS bearer with QCI=i is released or until call establishment failed (e.g. because the setup of the EPS bearer with QCI=i failed). That is, according to an embodiment where the fallback procedure is an EPS fallback procedure to the EPS network, the EPS bearer context is considered as persistent until an EPS bearer with QCI=i EPS bearer is released. In some scenarios the terminal device 200 has a persistent PDU session and has moved to a radio access network connected to (and including) a 5GC network 110c during the fallback procedure. Then the terminal device 200 continues to consider the PDU session as persistent until a QoS flow with sQI=i QoS is released or until call establishment failed (e.g. because the setup of the QoS flow with sQI=i failed). That is, according to an embodiment where the fallback procedure is a RAT fallback procedure to another 5G network 110c, the PDU session is considered as persistent until the QoS flow with sQI=i is released.

In some aspects the terminal device 200 considers the lack of support for VoPS for the next call by re-selecting to another network than the first network 110a. That is, the call set-up procedure might be regarded as a first call set-up procedure, and in some embodiments the terminal device 200 is configured to, upon having terminated the call for which the first call set-up procedure was performed, perform (optional) step S108:

S108: The terminal device 200 performs a second call set-up procedure for another call with another network 110b, 110c than the first network 110a.

This another network 110b, 110c could be the network to which the fallback was performed.

Reference is now made to the signalling diagram of Fig. 4 illustrating one particular embodiment for for fallback during call set-up based on at least some of the above disclosed embodiments, aspects, and examples. In addition to showing entities illustrated in Fig. 2, also a PGW (short for Packet Data Network) Gateway is shown.

In general terms, in the EPC the PGW Gateway provides connectivity from the terminal device 200 to the PDN 120 by being the point of exit and entry of traffic for the terminal device 200. The terminal device 200 might have simultaneous connectivity with more than one PGW for accessing multiple PDNs. 5201. Terminal device 200 camps on an NG-RAN in a first network 110a being an 5GS. A mobile originating (MO) or mobile terminating (MT) IMS voice session establishment has been initiated for terminal device 200.

5202. A network initiated PDU session modification to setup a QoS flow for the voice reaches the NG-RAN.

5203. The NG-RAN is configured to support EPS fallback for IMS voice services and decides to trigger fallback to EPS, taking into account capabilities of the terminal device 200, an indication from the AMF that redirection for EPS fallback for voice service is possible (e.g. received as part of initial context setup as defined in document 3GPP TS 38.413“NG-RAN; NG Application Protocol (NGAP)”, version 15.3.0), network configurations (e.g. N26 interface availability configuration) and radio conditions. If the NG-RAN decides not to trigger fallback to EPS, then the procedure is terminated and thus the following steps are not executed.

In terms of radio conditions, the NG-RAN may initiate measurement report solicitation from the terminal device 200, including E-UTRAN (as candidate for a second network to which fallback is to be made) as target.

If the AMF has indicated that redirection for EPS fallback for voice service is not possible, then an AN release via inter-system redirection to EPS is not performed in below step S205. S204. The NG-RAN responds by indicating rejection of the PDU Session modification to set up the QoS flow for IMS voice received in step S202 by sending a PDU Session Response message towards the PGW-C and the SMF (or H-SMF and P-GW-C via V- SMF, in case of a roaming scenario) via the AMF with an indication that mobility due to fallback for IMS voice is ongoing. The PGW-C+SMF maintains the PCC rule(s) associated with the QoS flow.

S205. The NG-RAN initiates either a handover (as in clause 4.11.1.2.1 of document 3GPP TS 23.502 entitled“Procedures for the 5G System (5GS)”, version 16.1.1), or AN release via inter-system redirection to EPS (as clause 4.2.6 and clause 4.11.1.3.2 of aforementioned document 3GPP TS 23.502), taking into account capabilities of the terminal device 200. The PGW-C+SMF reports change of the RAT type if subscribed by the PCF as specified in clause 4.11.1.2.1, or clause 4.11.1.3.2.6 of aforementioned document 3GPP TS 23.502.

When a connection has been established between the terminal device 200 and the EPS, either step S2o6a or step S2o6b is executed. S2o6a. The terminal device 200 initiates a TAU procedure.

S2o6b. The terminal device 200 initiates an attach procedure with a PDN

connectivity request with request type“handover”. S2o6b is performed when the terminal device 200 supports request type“handover” for PDN connectivity request during the attach procedure and has received the indication that interworking without N26 is supported.

According to the herein disclosed embodiments, the terminal device 200 will in either step S2o6a or step S2o6b (depending on which of these steps is performed) obtain from the MME (or similar entity) in the second network 110b, 110c that VoPS is not supported. For example, in case of an explicit indication, the terminal device 200 might in S2o6a receive a VoPS support indicator in the TAU accept message.

As the terminal device 200 is moved from the 5GS to the EPS according to the EPS fallback procedure, the terminal device 200 continues its EPS fallback procedure and the call set-up procedure is continued despite the lack of VoPS support indicator in the second network 110b, 110c. This aligns with the behavior for VoLTE and VoNR regarding Persistent PDU session and EPS bearer context. The terminal device 200 might take into account the lack of support for VoPS for the next voice call by re selecting to another network 110b, 110c than the first network 110a, as in S108 when performing a call set-up procedure for the next voice call.

S207. An IMS voice media bearer with QCI=i is established for the terminal device 200 in the second network 110b, 110c. At that point in time the terminal device 200 has a persistent EPS bearer context. After completion of the mobility procedure to the EPS, or as part of the 5GS to EPS handover procedure, the SMF/PGW re-initiates the set-up of the dedicated bearer for IMS voice services, mapping the 5G QoS

parameters to EPC QoS parameters. The PGW-C+SMF reports Successful Resource Allocation and Access Network Information to the PCF, if subscribed to by the PCF. S208. The IMS voice session establishment is continued.

In summary, by means of the herein disclosed embodiments the terminal device 200 is enhanced to support persistent EPS bearer context also in case a bearer with sQI=i is not yet established but the terminal device 200 has to perform a fallback procedure to second network 110b, 110c where IMS VoPS is not supported for the terminal device 200.

Fig. 5 schematically illustrates, in terms of a number of functional units, the components of a terminal device 200 according to an embodiment. Processing circuitry 210 is provided using any combination of one or more of a suitable central processing unit (CPU), multiprocessor, microcontroller, digital signal processor

(DSP), etc., capable of executing software instructions stored in a computer program product 710 (as in Fig. 7), e.g. in the form of a storage medium 230. The processing circuitry 210 may further be provided as at least one application specific integrated circuit (ASIC), or field programmable gate array (FPGA). Particularly, the processing circuitry 210 is configured to cause the terminal device 200 to perform a set of operations, or steps, as disclosed above. For example, the storage medium 230 may store the set of operations, and the processing circuitry 210 may be configured to retrieve the set of operations from the storage medium 230 to cause the terminal device 200 to perform the set of operations. The set of operations may be provided as a set of executable instructions.

Thus the processing circuitry 210 is thereby arranged to execute methods as herein disclosed. The storage medium 230 may also comprise persistent storage, which, for example, can be any single one or combination of magnetic memory, optical memory, solid state memory or even remotely mounted memory. The terminal device 200 may further comprise a communications interface 220 at least configured for

communications with the networks 110a, 110b, 110c. As such the communications interface 220 may comprise one or more transmitters and receivers, comprising analogue and digital components. The processing circuitry 210 controls the general operation of the terminal device 200 e.g. by sending data and control signals to the communications interface 220 and the storage medium 230, by receiving data and reports from the communications interface 220, and by retrieving data and

instructions from the storage medium 230. Other components, as well as the related functionality, of the terminal device 200 are omitted in order not to obscure the concepts presented herein.

Fig. 6 schematically illustrates, in terms of a number of functional modules, the components of a terminal device 200 according to an embodiment. The terminal device 200 of Fig. 6 comprises a number of functional modules; a call set-up module 210a configured to perform step S102, a fallback module 210b configured to perform step S104, and a continue module 210c configured to perform step S106. The terminal device 200 of Fig. 6 may further comprise a number of optional functional modules, such as a call set-up module 2iod configured to perform step S108. In general terms, each functional module 2ioa-2iod may in one embodiment be implemented only in hardware and in another embodiment with the help of software, i.e., the latter embodiment having computer program instructions stored on the storage medium 230 which when run on the processing circuitry makes the terminal device 200 perform the corresponding steps mentioned above in conjunction with Fig 6. It should also be mentioned that even though the modules correspond to parts of a computer program, they do not need to be separate modules therein, but the way in which they are implemented in software is dependent on the programming language used. Preferably, one or more or all functional modules 2ioa-2iod may be

implemented by the processing circuitry 210, possibly in cooperation with the communications interface 220 and/or the storage medium 230. The processing circuitry 210 may thus be configured to from the storage medium 230 fetch

instructions as provided by a functional module 2ioa-2iod and to execute these instructions, thereby performing any steps as disclosed herein.

Fig. 7 shows one example of a computer program product 710 comprising computer readable storage medium 730. On this computer readable storage medium 730, a computer program 720 can be stored, which computer program 720 can cause the processing circuitry 210 and thereto operatively coupled entities and devices, such as the communications interface 220 and the storage medium 230, to execute methods according to embodiments described herein. The computer program 720 and/or computer program product 710 may thus provide means for performing any steps as herein disclosed. In the example of Fig. 7, the computer program product 710 is illustrated as an optical disc, such as a CD (compact disc) or a DVD (digital versatile disc) or a Blu-Ray disc. The computer program product 710 could also be embodied as a memory, such as a random access memory (RAM), a read-only memory (ROM), an erasable programmable read-only memory (EPROM), or an electrically erasable

programmable read-only memory (EEPROM) and more particularly as a non-volatile storage medium of a device in an external memory such as a USB (Universal Serial Bus) memory or a Flash memory, such as a compact Flash memory. Thus, while the computer program 720 is here schematically shown as a track on the depicted optical disk, the computer program 720 can be stored in any way which is suitable for the computer program product 710.

The inventive concept has mainly been described above with reference to a few embodiments. However, as is readily appreciated by a person skilled in the art, other embodiments than the ones disclosed above are equally possible within the scope of the inventive concept, as defined by the appended patent claims.

Claims

1. A terminal device (200) for fallback during call set-up, the terminal device (200) comprising processing circuitry (210), the processing circuitry (210) being configured to cause the terminal device (200) to: perform a call set-up procedure for a call with a first network (110a), the first network being a 5GS network; and whilst the call set-up procedure is performed: perform a fallback procedure to a second network (110b, 110c); and

continue the call set-up procedure with the second network (110b, 110c) despite having obtained an indication of no VoPS support in the second network (110b, 110c).

2. The terminal device (200) according to claim 1, wherein the indication of no VoPS support in the second network (110b, 110c) is obtained by receiving, from the second network (110b, 110c), an VoPS indicator indicating that VoPS is not

supported.

3. The terminal device (200) according to claim 2, wherein the VoPS indicator is received upon the terminal device (200) initiating a Tracking Area Update, TAU, procedure.

4. The terminal device (200) according to claim 2, wherein the VoPS indicator is received upon the terminal device (200) initiates an attach procedure with a Packet Data Network, PDN, connectivity request with request type“handover”.

5. The terminal device (200) according to claim 1, wherein the indication of no VoPS support in the second network (110b, 110c) is obtained by not receiving, from the second network (110b, 110c), any VoPS indicator indicating that VoPS is supported.

6. The terminal device (200) according to claim 1, wherein the second network (110b, 110c) is an Evolved Packet System, EPS, network (110b).

7. The terminal device (200) according to claim 6, wherein the fallback procedure is an EPS fallback procedure to the EPS network (110b).

8. The terminal device (200) according to claim 1, wherein the second network (110b, 110c) is a radio access network connected to a 5GC network (110c).

9. The terminal device (200) according to claim 8, wherein the fallback procedure is a Radio Access Technology, RAT, fallback procedure to said radio access network connected to a 5GC network (110c).

10. The terminal device (200) according to claim 1, wherein a Protocol Data Unit, PDU, session with the 5G network (110a) is established as part of the call set-up procedure, and wherein the PDU session is by the terminal device (200) considered as persistent when continuing the call set-up procedure with the second network (110b, 110c).

11. The terminal device (200) according to claim 10, wherein the thus persistent PDU session is turned into a persistent Packet Data Network, PDN, connection when continuing the call set-up procedure with the second network (110b, 110c).

12. The terminal device (200) according to claim 10, wherein the PDU session is by the terminal device (200) considered as persistent even if obtaining an indication that a handover procedure is initiated for the terminal device (200) and even if a Quality of Service, QoS, flow with a 5G Quality Indicator, 5QI, with value 1 is not established.

13. The terminal device (200) according to claim 10, wherein the PDU session is by the terminal device (200) considered as persistent even if obtaining an indication that a redirect procedure is initiated for the terminal device (200) and even if a Quality of Service, QoS, flow with a 5G Quality Indicator, 5QI, with value 1 is not established.

14. The terminal device (200) according to a combination of claim 7 and claim 10, wherein EPS bearer context is considered as persistent until an EPS bearer with QCI=i EPS bearer is released.

15. The terminal device (200) according to a combination of claim 9 and claim 10, wherein the PDU session is considered as persistent until the QoS flow with 5QI=i is released.

16. The terminal device (200) according to claim 1, wherein, during a registration procedure with the first network (110a) performed before or in conjunction with the call set-up procedure, the terminal device (200) has obtained an indication that VoPS is supported in the first network (110a).

17. The terminal device (200) according to claim 1, wherein the call set-up procedure is a first call set-up procedure, the terminal device (200) further being configured to, upon having terminated the call for which the first call set-up procedure was performed:

perform a second call set-up procedure for another call with another network

(110b, 110c) than the first network (110a).

18. A terminal device (200) for fallback during call set-up, the terminal device (200) comprising: a call set-up module (210a) configured to perform a call set-up procedure for a call with a first network (110a), the first network being a 5GS network;

a fallback module (210b) configured to, whilst the call set-up procedure is performed, perform a fallback procedure to a second network (110b, 110c); and

a continue module (210c) configured to continue the call set-up procedure with the second network (110b, 110c) despite having obtained an indication of no VoPS support in the second network (110b, 110c).

19. A method for fallback during call set-up, the method being performed by a terminal device (200), the method comprising:

performing (S102) a call set-up procedure for a call with a first network (110a), the first network being a 5GS network; and whilst performing the call set-up procedure:

performing (S104) a fallback procedure to a second network (110b, 110c); and continuing (S106) the call set-up procedure with the second network (110b, 110c) despite having obtained an indication of no VoPS support in the second network (110b, 110c).

20. The method according to claim 19, wherein the call set-up procedure is a first call set-up procedure, the method further comprising, upon having terminated the call for which the first call set-up procedure was performed:

Performing (S108) a second call set-up procedure for another call with another network (110b, 110c) than the first network (110a).

21. A computer program (720) for fallback during call set-up, the computer program comprising computer code which, when run on processing circuitry (210) of a terminal device (200), causes the terminal device (200) to: perform (S102) a call set-up procedure for a call with a first network (110a), the first network being a 5GS network; and whilst performing the call set-up procedure: perform (S104) a fallback procedure to a second network (110b, 110c); and continue (S106) the call set-up procedure with the second network (110b, 110c) despite having obtained an indication of no VoPS support in the second network (110b, 110c).

22. A computer program product (710) comprising a computer program (720) according to claim 21, and a computer readable storage medium (730) on which the computer program is stored.