Infrastructure equipment, core network apparatus, communications device and methods

Abstract

A method of operating infrastructure equipment of a radio access network of a wireless communications network is provided. The method comprises transmitting (S2) an inactive state indication to the communications device. The inactive state indication indicates to the communications device to transition to an inactive state for communications via a first communications path of an MA-PDU session. The first communications path is used for communications between the communications device and a core network of the wireless communications network via the infrastructure equipment of the radio access network. The MA-PDU session comprises a second communications path for communications between the communications device and the core network via infrastructure equipment of another radio access network of the wireless communications network. The method comprises communicating (S3) a small data transmission, SDT, along the first communications path when the communications device is in the inactive state. The communication of the SDT along the first communications path comprises receiving the SDT from the core network and transmitting the SDT to the communications device, or receiving the SDT from the communications device and transmitting the SDT to the core network.

Description

[0001] INFRASTRUCTURE EQUIPMENT, CORE NETWORK APPARATUS, COMMUNICATIONS DEVICE AND METHODS

[0002] BACKGROUND

[0003] Field of Disclosure

[0004] The present disclosure relates to infrastructure equipment, a core network apparatus, a communications device, and methods of operating infrastructure equipment, a core network apparatus, and a communications device.

[0005] The present application claims Paris Convention priority from European patent application number 23184824.3, the contents of which are hereby incorporated by reference in their entirety.

[0006] Description of Related Art

[0007] The “background” description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description which may not otherwise qualify as prior art at the time of filing, are neither expressly or impliedly admitted as prior art against the present invention.

[0008] Wireless communications networks are now supporting communications to a wider range of communications devices and user equipment for a variety of applications and data traffic profiles and types. For example, communications are now supported with devices including reduced complexity devices, machine type communication (MTC) devices, high resolution video displays, virtual reality headsets and so on. Some of these different types of devices may be deployed in very large numbers, for example low complexity devices for supporting the “The Internet of Things”, and may typically be associated with the transmissions of relatively small amounts of data with relatively high latency tolerance. Other types of device, for example supporting high-definition video streaming, may be associated with transmissions of relatively large amounts of data with relatively low latency tolerance.

[0009] In order to provide coverage for an increasing range of devices, such as loT, 5G radio access technologies (RAT), also referred to as new radio (NR) systems, includes aspects which are devised to support connectivity over a wide range of environments.

[0010] 5G NR has continuously evolved and the current work plan includes 5G-NR-advanced in which some further enhancements are expected, especially to support new use-cases / scenarios with higher requirements. The desire to support these new use-cases and scenarios gives rise to new challenges for efficiently handling communications and reducing power usage in wireless communications systems that need to be addressed.

[0011] SUMMARY OF THE DISCEOSURE

[0012] The present disclosure can help address or mitigate at least some of the issues discussed above.

[0013] Embodiments can provide a method of operating infrastructure equipment of a radio access network of a wireless communications network. The method comprises transmitting an inactive state indication to the communications device. The inactive state indication indicates to the communications device to transition to an inactive state for communications via a first communications path of an MA-PDU session. The first communications path is used for communications between the communications device and a core network of the wireless communications network via the infrastructure equipment of the radio access network. The MA-PDU session comprises a second communications path for communications between the communications device and the core network via infrastructure equipment of another radio access network of the wireless communications network. The method comprises communicating a small data transmission, SDT, along the first communications path when the communications device is in the inactive state. The communication of the SDT along the first communications path comprises receiving the SDT from the core network and transmitting the SDT to the communications device, or receiving the SDT from the communications device and transmitting the SDT to the core network.

[0014] Embodiments can also provide a method of operating a communications device. The method comprises receiving an inactive state indication from infrastructure equipment of a radio access network, the inactive state indication indicating to the communications device to transition to an inactive state for communications via a first communications path of an MA-PDU session. The first communications path is used for communications between the communications device and a core network of the wireless communications network via the infrastructure equipment of the radio access network. The MA-PDU session comprises a second communications path for communications between the communications device and the core network via infrastructure equipment of another radio access network of the wireless communications network. The method comprises communicating a small data transmission, SDT, along the first communications path when the communications device is in the inactive state. The communication of the SDT along the first communications path comprises transmitting the SDT to the infrastructure equipment of the radio access network, or receiving the SDT from the infrastructure equipment of the radio access network.

[0015] Embodiments can also provide a method of operating a core network apparatus of a wireless communications network. The method comprises communicating a small data transmission, SDT, along a first communications path of a multi-access protocol data unit, MA-PDU, session for communications between a communications device and the core network apparatus via infrastructure equipment of a radio access network of the wireless communications network. The MA-PDU session comprises a second communications path for communications between the communications device and the core network apparatus via infrastructure equipment of another radio access network. The communication of the SDT along the first communications path comprises transmitting the SDT to the infrastructure equipment of the radio access network for forwarding to the communications device when the communications device is in an inactive state for communications with the infrastructure equipment of the radio access network, or receiving the SDT from the infrastructure equipment of the radio access network.

[0016] By communicating an SDT in an MA-PDU session, small amounts of data can be communicated without communications devices having to transition into a connected state and thereby incurring increased signalling overhead and power usage. Therefore, embodiments can provide improved communications efficiency and reduced power usage for communications devices.

[0017] Respective aspects and features of the present disclosure are defined in the appended claims and include a communications device (for example, a UE), and infrastructure equipment, a core network apparatus and methods for operating the same.

[0018] It is to be understood that both the foregoing general description and the following detailed description are exemplary, but are not restrictive, of the present technology. The described embodiments, together with further advantages, will be best understood by reference to the following detailed description taken in conjunction with the accompanying drawings.

[0019] BRIEF DESCRIPTION OF THE DRAWINGS

[0020] A more complete appreciation of the disclosure and many of the atendant advantages thereof will be readily obtained as the same becomes beter understood by reference to the following detailed description when considered in connection with the accompanying drawings wherein like reference numerals designate identical or corresponding parts throughout the several views, and wherein:

[0021] Figure 1 schematically represents some aspects of a 5G / new radio access technology (RAT) wireless communications system which may be configured to operate in accordance with embodiments of the present disclosure;

[0022] Figure 2 is a schematic block diagram of a communications device communicating data to and / or from an infrastructure equipment (gNB) forming part of the wireless communication system shown in Figure 1;

[0023] Figure 3 schematically illustrates a mapping between radio bearers and QoS flows for a PDU session;

[0024] Figure 4 schematically illustrates an example of an MA-PDU session over a 3 GPP access network and a non-3GPP access network;

[0025] Figure 5 schematically illustrates an example of simplified protocol stacks for an MA-PDU session over a 3GPP and non-3GPP access network;

[0026] Figure 6 schematically illustrates an example of an MA-PDU session over two 3GPP access networks;

[0027] Figure 7 schematically illustrates an example of switching between communications paths in an MA-PDU session according to conventional techniques;

[0028] Figures 8A and 8B schematically illustrate how uplink small data transmissions (SDT) may be performed by a user equipment (UE) using random access (RACH) schemes while the UE is in an inactive state;

[0029] Figure 9 schematically illustrates a flow diagram showing a method of operating infrastructure equipment of an access network in accordance with example embodiments;

[0030] Figure 10 schematically illustrates a flow diagram showing an example of dual steering in accordance with example embodiments.

[0031] Figure I l a schematic diagram illustrating various Radio Resource Control (RRC) states which may be occupied by a UE.

[0032] DETAILED DESCRIPTION OF THE EMBODIMENTS

[0033] New Radio Access Technology (5G)

[0034] An example configuration of a wireless communications network which uses some of the terminology proposed for and used in NR and 5G is shown in Figure 1. In Figure 1 a plurality of transmission and reception points (TRPs) 10 are connected to distributed control units (DUs) 41, 42 by a connection interface represented as a line 16. Each of the TRPs 10 is arranged to transmit and receive signals via a wireless access interface within a radio frequency bandwidth available to the wireless communications network. Thus, within a range for performing radio communications via the wireless access interface, each of the TRPs 10, forms a cell of the wireless communications network as represented by a circle 12. As such, wireless communications devices 14 which are within a radio communications range provided by the cells 12 can transmit and receive signals to and from the TRPs 10 via the wireless access interface. Each of the distributed units 41, 42 are connected to a central unit (CU) 40 (which may be referred to as a controlling node) via an interface 46. The central unit 40 is then connected to the core network 20 which may contain all other functions required to transmit data for communicating to and from the wireless communications devices and the core network 20 may be connected to other networks 30

[0035] As will be appreciated by those acquainted with the wireless communications network according to a 5G standard as shown in Figure 1, the CU 40, DU 42 and TRPs 10 collectively refer to functions which are conventionally performed by a network base station or, in accordance with 5G terminology, a gNodeB (gNB). In terms of broad top-level functionality, the term network infrastructure equipment / access node may be used to encompass these elements and more conventional base station type elements of wireless telecommunications systems. Depending on the application at hand, the responsibility for scheduling transmissions which are scheduled on the radio interface between the respective DUs and the communications devices may lie with the controlling node / central unit and / or the distributed units / TRPs.

[0036] A communications device 14 is represented in Figure 1 within the coverage area of the first communication cell 12. This communications device 14 may thus exchange signalling with the first CU 40 in the first communication cell 12 via one of the distributed units / TRPs 10 associated with the first communication cell 12. The communications devices 14 may be referred to as mobile terminals, terminals or user equipment (UE), which encompasses chip sets and have a functionality corresponding to the UE devices known for operation with wireless communications networks.

[0037] Figure 2 provides a more detailed diagram of components shown in Figure 1. Components shown in Figure 2 which are also shown in Figure 1 bear the same numerical designations and so description of these parts will not be repeated for brevity. In Figure 2, a TRP 120, which broadly corresponds to TRP 10 in Figure 1, and comprises, as a simplified representation, a transmitter 126, a receiver 124 and a controller or controlling processor 122 which may operate to control the transmitter 126 and the receiver 124 to transmit and receive radio signals to one or more UEs within a cell (not shown in Figure 2 for clarity) provided by the TRP 120. As shown in Figure 2, the TRP 120 is connected to a DU 140 via a physical interface 130 which may be a fibre optic cable, for example. The physical interface 130 therefore provides a communications link for data and signalling traffic from the TRP 120 via the DU 140 and a CU 160 to a core network 400. An interface 150 between the DU 140 and the CU 160 is known as the Fl interface which can be a physical or a logical interface. The Fl interface 150 between the DU 140 and the CU 160 may operate in accordance with specifications 3GPP TS 38.470 and 3GPP TS 38.473, and may be formed from a fibre optic or other wired or wireless high bandwidth connection. The connection between a TRP 120 and the core network 400 can be generally referred to as a backhaul, which comprises the physical interface 130 from the TRP 120 to the DU 140 and the Fl interface 150 from the DU 140 to the CU 160.

[0038] As shown in Figure 2, the TRP 120 may be configured to transmit downlink radio signals and receive uplink radio signals from a UE 200 via a direct wireless communications link 250 which may be a Uu interface in one example. The UE 200 is shown to include a transmitter 226, a receiver 224 and a controller 222 which is configured to control the transmitter 226 and the receiver 224 to transmit uplink signals to the TRP 120 and to receive downlink signals from the TRP 120 over the wireless communications link 250 formed between the UE 200 and the TRP 120.

[0039] The transmitters 126, 226 and the receivers 124, 224, as well as other transmitters, receivers and transceivers described in relation to examples and embodiments of the present disclosure may include radio frequency filters and amplifiers as well as signal processing components and devices in order to transmit and receive radio signals in accordance for example with the 5G / NR standard. The controllers 122, 222, as well as other controllers described in relation to examples and embodiments of the present disclosure may be, for example, a microprocessor, a CPU, or a dedicated chipset, etc., configured to carry out instructions which are stored on a computer readable medium, such as a non-volatile memory. The processing steps described herein may be carried out by, for example, a microprocessor in conjunction with a random access memory, operating according to instructions stored on a computer readable medium. The transmitters, the receivers and the controllers are schematically shown in Figure 2 as separate elements for ease of representation. However, it will be appreciated that the functionality of these elements can be provided in various different ways, for example using one or more suitably programmed programmable computer(s), or one or more suitably configured application-specific integrated circuit(s) / circuitry / chip(s) / chipset(s).

[0040] As mentioned above, the TRP 120, DU 140 and the CU 160 may collectively form the gNB 202 which is an example of infrastructure equipment of a radio access network of a wireless communications network. Therefore, references to the UE 200 communicating with the TRP 120 can alternatively be considered as references to the UE 200 communicating with the gNB 202. Furthermore, it will be appreciated that the UE 200 is an example of a communications device or wireless transceiver unit. As will be appreciated the infrastructure equipment / TRP / base station / gNB as well as the UE / communications device will in general comprise various other elements associated with its operating functionality.

[0041] PDU Session

[0042] In 5G systems, a protocol data unit (PDU) session provides end-to-end user plane connectivity between a UE and a particular data network (DN). Figure 3 schematically illustrates an example of a mapping between data radio bearers (DRBs) and Quality of Service (QoS) flows in a PDU session. As shown in Figure 3, a PDU session 608 is established between the UE 200 and a User Plane Function 606 (UPF) of the core network 400 via the gNB 202. The UPF 606 is responsible for packet routing and forwarding, packet inspection, QoS handling, and external PDU sessions for an interconnecting data network, in the 5G architecture as will be appreciated by one skilled in the art. The UE 200 communicates with the UPF 606 via the PDU session 608. Three QoS flows 610, 612, 614 belong to the PDU session 608, and are associated with DRBs 616 and 620. One PDU session may be mapped to one or more DRBs and one PDU session may have multiple QoS flows.

[0043] Each Internet Protocol (IP) packet received from the 5G core network may be assigned a particular QoS such as QoS flows 610, 612, 614. Each of the QoS flows may be characterised by a QoS flow ID and may be associated with a quality of service requirement (such as one or more of a Guaranteed Bit Rate (GBR), a maximum bit rate, a maximum latency, a permitted packet loss ratio and the like). Therefore the UE 200 is aware of the parameters associated with each of the QoS flows 610, 612, 614. The gNB 202 connected to the 5G core network 400 is also aware of the QoS flows.

[0044] The gNB 202 establishes logical connections with the UE 200 which may be DRBs 616 and 620. In accordance with a 5G system architecture as shown in Figure 3, one DRB (such as the DRB 616) may be used to transport packet data associated with two QoS flows (such as the QoS flows 610 and 612). In order to accommodate this flexibility, the gNB 202 may maintain a mapping table to store a mapping between each of the QoS flows and the respective DRB. Using the table, the gNB 202 is able to assign packets received from the 5G core network over the QoS flows 610, 612, 614 to the appropriate data radio bearers 616, 620 for transmission to the UE 200.

[0045] MA-PDU Session

[0046] 5G systems also support UEs establishing multiple Protocol Data Unit (PDU) sessions to the same data network or to different data networks over a single or multiple access networks. Such sessions are known as “Multi-Access PDU (MA-PDU) sessions”. Session establishment procedures for MA-PDUs are described in [1], Generally, in an MA-PDU session, data may be transmitted via one or both of the multiple access networks. Typically, an MA-PDUs comprises two communications paths , or “legs”, between a UE and a core network. Each of the two communications paths is over a different access network.

[0047] An example of an MA-PDU session established over multiple access networks is shown in Figure 4. As shown in Figure 4, an MA-PDU session 318 is established between a UE 302 (which may broadly correspond to UE 202) and a UPF (PDU Session Anchor (PSA ) ) 310 of a core network (such as core network 400) via a first communications path 322 and a second communications path 324. As will understood by a person skilled in the art, a “UPF (PSA)” is a user plane function which terminates the N6 interface of a PDU session in a 5G core network. As will be understood by a person skilled in the art, in order to support selective data routing to a data network (DN), or to support Session and Service Continuity (SSC) mode 3, a session management function (SMF) (not shown in Figure 4) may control a data path of a PDU session so that the PDU session may simultaneously correspond to multiple N6 interfaces. A UPF that terminates each of these interfaces is said to support PDU session anchor functionality and is referred to as a “UPF (PSA)”. Further details of the UPF (PSA) can be found in TS23.501, v. 18, which is hereby incorporated by reference in its entirety.

[0048] In Figure 4, the UPF (PSA) 310 is connected to a server host 312. The MA-PDU comprises two PDU sessions - one PDU session over the first communications path 322 and one PDU session over the second communications path 324. The first communications path 322 comprises a “3GPP access network” 316 and a UPF 314 (which may broadly correspond to UPF 606). The second communications path 324 comprises a “non-3GPP access network” 304, a non-3GPP Inter-Working Function (N3IWF) 306 and another UPF 308 (which may broadly correspond to UPF 606).

[0049] 3GPP access networks include any access network which operate in accordance with 3GPP protocols Examples of networks which can include access networks configured to operate in accordance with 3GPP protocols include terrestrial networks, TNs, non-terrestrial networks, NTNs, Public Land Mobile Networks, PLMN and stand-alone non-public networks (SNPNs). For example, such networks may include gNBs which provide access for communications devices to a core network.

[0050] Non-3PP access networks include any access network which does not operate in accordance with 3GPP protocols. Examples of networks which can include access networks which are not configured to operate in accordance with 3GPP protocols include wireless local area networks, WLANs. For example, a WLAN may comprise a residential gateway implementing WiFi protocols which provides access for communications devices to a core network. In some examples, as described with reference to Figure 5 below, a residential gateway may comprise both WiFi protocols and a DU protocol stack and can therefore form part of both a 3GPP and non-3GPP access network

[0051] An example of simplified protocol stacks present in an MA-PDU session over a 3GPP and non-3GPP access network is shown in Figure 5. As shown in Figure 5, the UE 302 may have a 5G access stratum (AS) layer 1204 and a WiFi protocol stack 1206 which are connected to a common non-access stratum (NAS) layer 1202. A residential gateway (RG) 1002 comprises a peer WiFi protocol stack 1208 corresponding to the WiFi protocol stack 1206 in the UE 302 and the RG 1002 comprises a distributed unit (DU) protocol stack 1210. A gNB 1010 (which may broadly correspond to gNB 202) has a DU protocol stack 1214 and a CU protocol stack 1212. The DU protocol stack 1210 in the RG 1002 and the DU protocol stack 1214 in the gNB 1010 . The DU in the RG 1002 and the DU in the gNB 1010 are connected to a single CU in the gNB 1010. Protocol stacks in a core network 1012 (which may broadly correspond to core network 400) are not shown for clarity. As indicated in Figure 5, QoS flows for providing a service to the UE 302 are set up in the NAS layer 1202 of the UE 302 when the user requests a service. The dashed lines indicate a communications path for transmission of protocol data units between the UE 302 and core network 1012 via a non-3GPP access network. The solid lines indicate a communications path for transmission protocol data units between the UE 302 and core network 1012 via a 3GPP access network. In this example, the non-3GPP includes the RG 1002 (specifically, a part of the RG 1002 implementing the WiFi protocols 1208) and the 3GPP network includes the RG 1002 (specifically, a part of the RG 1002 implementing the DU 1210 protocols) and the gNB 1010. In this example, the 3GPP access network may be comprised in an SNPN.

[0052] Generally, transmissions in MA-PDU sessions may be split, switched and steered between 3GPP and non-3GPP access networks according to Access Traffic Steering, Switching and Splitting (ATSSS) rules (see [2] and [3]).

[0053] As will be understood by a person skilled in the art, data may be transmitted with high QoS requirements (such as data transmitted via a Guaranteed Bit Rate (GBR) bearer) or low QoS requirements (such as data transmitted via a non-GBR bearer). Currently, GBR data can only be transmitted via one of the two communications paths of an MA-PDU session at any one time. There are currently no ATSSS rules which permit splitting of GBR data between the two communications paths in order to ensure the stringent QoS requirements of GBR data are met. By contrast, non-GBR data may be split, switched and steered across both of the communications paths in an MA-PDU session according to ATSSS rules.

[0054] Currently, one 3GPP access network and at least one non-3GPP access network are required to support ATSSS. However, the requirement that at least one non-3GPP access network is used provides restrictions on capacity, coverage, reliability and Quality of Experience (QoE). It has therefore been proposed to configure MA-PDU sessions over two 3GPP access networks.

[0055] An example of an MA-PDU session established over two 3GPP access networks is shown in Figure 6. As shown in Figure 6, an MA-PDU session 318 is established between the UE 302 and the UPF (PSA) 310 via a first communications path 328 and a second communications path 330. The UPF (PSA) 310 is connected to the server host 312. The first communications path 328 comprises the 3GPP access network 316 and the UPF 314. The second communications path 330 comprises another 3GPP access network 326 and another UPF 308.

[0056] In some examples, at least one of the 3GPP access networks 316, 326 may be comprised in a PLMN. In such examples, the other of the 3GPP access networks 316, 326 may be comprised in the same or different PEMN, or an SNPN. In some examples, at least one of the 3GPP access networks 316, 326 may be an comprised in a SNPN. In such examples, the other of the 3GPP access networks 316, 326 may also be comprised in the same or different SNPN, or an PLMN. The UE 302 may use the same subscription credentials for access networks comprised in the same or different PLMNs, the same or different SNPNs, or a PLMN and an SNPN.

[0057] With the introduction of MA-PDU sessions with two 3GPP access networks, scenarios may arise where the QoS requirements for both 3GPP access networks are very similar or the same. For example, 3GPP access networks comprised in NTNs and TNs may be very similar or the same. In some examples, the QoS requirements for two 3GPP access networks may be the same but each of the 3GPP access networks may support a different type of service. This may be because different 3GPP access networks are optimised to support different types of service. Therefore, since QoS requirements for 3GPP access networks may be very similar or the same, non-GBR traffic in such MA-PDU sessions with two 3GPP access networks may be required to be switched, rather than split, between the two communications paths. Therefore, there is expected to be an increase in the frequency of scenarios where data transmission is switched between communications paths.

[0058] An existing procedure for switching the transmission of data from one communications path to transmission of data via another communications path is described with reference to Figure 7.

[0059] As shown in Figure 7, an MA-PDU session is established between a UE 502 and a core network 510 via a first gNB 506 and via a second gNB 514. The UE 502 may broadly correspond to UE 202 and UE 302. The core network 510 may broadly correspond to core network 400. The first gNB 506 and via a second gNB 514 may broadly correspond to gNB 202. A first communications path comprises a wireless communications link 504 between the UE 502 and the first gNB 506, and a communications link 508 between the first gNB 506 and the core network 510. A second communications path comprises a wireless communications link 516 between the UE 502 and the second gNB 514, and a communications link 512 between the second gNB 514 and the core network 510. Phase 1 of Figure 7 is a schematic representation before a path switch. In Phase 1, the UE 502 is communicating with the core network 510 via the first communications path and is in connected state (for example, an RRC connected state) for communications via the first gNB 506. The UE 502 is in an RRC inactive state for communications via the second gNB 514 and is not communicating with the core network 510 via the second communications path. Although the UE 502 is shown in an RRC inactive state for communications via the second gNB 514 in Phase 1, the UE 502 may alternatively be in an RRC connected state for communications with the second gNB 514 but with no communications on the second communications path in Phase 1 (i.e. the second communications path is not currently being used for communications). Alternatively, the UE 502 may be in an RRC idle state for communications via the second gNB 514 in Phase 1.

[0060] The decision of whether to transmit downlink data via the first or second communications path is generally performed by a UPF (PSA) (not shown) in the core network 510. Typically, the decision of whether to transmit data via the first or second communications path is performed by the UPF(PSA) based on one or more ATSSS rules. The decision of whether to transmit uplink data via the first or second communications path is generally performed by the UE 502 based on the one or more ATSSS rules. However, if the ATSSS rules are on standby, the decision to switch uplink or downlink data are based on jitter measurements. For example, data may be transmitted over the first communications path and the UPF(PSA) may determine, based on jitter measurements, that the communications paths should be switched such that the second communications path is used for transmissions rather than the first communications path. This may be because radio channel conditions over the first communications path deteriorated due to the UE 502 moving from indoors to outdoors, for example. The decision to switch paths may alternatively be based on the methods discussed in pending international application PCT / EP2022 / 051040, published as WO 2022 / 167216, which is hereby incorporated by reference in its entirety.

[0061] As will be understood by a person skilled in the art, jitter measurements are related to variation in a delay of received packets. In some examples, jitter measurements may account for processing delays due to gNB and core network internal processing delays (such as hardware processing) and delays over a transport network. For wireless interfaces, the variation in transmission delay may be introduced by Hybrid Automatic Repeat Request (HARQ) retransmissions over the radio if the UE 502 is in poor radio conditions for example. For wired interfaces, the variation in transmission delay may be due to congestion in the network. The first and second gNB 506, 514 may be configured to detect the type of transmission received or transmitted by the respective gNB (such as whether the transmission is a GBR or non-GBR transmission). For example, the first and second gNB 506, 514 may receive Allocation Retention Parameters (ARP) along with other QoS parameters from an AMF (not shown) in the core network 510 during the establishment of the MA-PDU session. The first and second gNB 506, 514 may prepare UE 502 context after the MA-PDU session has been established. Therefore, based on the UE 502 context and ARP parameters, the first and second gNB 506, 514 are able to detect the type of transmission received or transmitted by the respective gNB 506, 514. Accordingly, for example, when the first communications path is being used for the transmission of data, the first gNB 506 is able to detect the type of transmission being transmitted or received by the first gNB 506.

[0062] Phase 2 is a schematic representation immediately after the UPF(PSA) and / or the UE 502 has decided to switch the path used for communications with the core network 510 from the first communications path to the second communications path. The decision to perform the path switch may be based on one or more ATSSS rules. Since first gNB 506 is unaware that the UPF(PSA) and / or the UE 502 has decided to perform a path switch, the UE 502 remains in the RRC connected state for communications via the first gNB 506, even though no communication is occurring via the first communications path in the MA-PDU session due to the path switch. According to existing scenarios, the first gNB 506 implements a timer comprising a pre-defined time period. The timer may be an inactivity timer. For example, the first gNB 506 may restart a timer every time a PDU is received by the first gNB 506 along the first communications path. If no PDU is detected before the timer expires, then the first gNB 506 transmits an RRC release message to the UE 502 indicating to the UE 502 to switch to an RRC inactive state for communications via the first gNB 506. Alternatively, the first gNB 506 may transmit an RRC Release message to the UE 502 indicating to the UE 502 to switch to an RRC idle state for communications via the first gNB 506. The presence of a “suspend configuration” indication in the RRC Release message may indicate to the UE 502 to switch to an RRC inactive state for communications via the first gNB 506 whereas the absence of a “suspend configuration” indication in the RRC Release message may indicate to the UE 502 to switch to an RRC idle state for communications via the first gNB 506. RRC states for a UE are described in more detail with respect to Figure 11 in the Annex. Phase 3 of Figure 7 is a schematic representation after the pre-defined timer has expired and the first gNB 506 has transitioned the UE 502 to the RRC inactive state for communications via the first gNB 506. In Phase 3, the UE 502 and the core network 510 communicate in the MA-PDU session via the second communications path. The UE 502 knows the one or more ATSSS rules used by the core network 510 to perform the path switch. Therefore, the UE 502 determines, based on the one or more ATSSS rules, to transmit uplink transmissions via the second communications paths.

[0063] Small Data Transmissions (SDTs)

[0064] One feature of the RRC inactive state is the ability of a UE to perform small data transmissions (SDT) for uplink transmission specified in Rel-17 as Mobile Originated SDT (MO-SDT) and for downlink reception specified in Rel-18 as Mobile Terminated SDT (MT-SDT).

[0065] Some specific examples of small data transmission and infrequent data traffic may include the following use cases [4] :

[0066] - Smartphone applications:

[0067] - Traffic from Instant Messaging services;

[0068] - Heart-beat / keep-alive traffic from IM / email clients and other applications; and

[0069] - Push notifications from various applications; - Non-smartphone applications:

[0070] - Traffic from wearable devices (e.g. periodic positioning information);

[0071] - Sensors (e.g., Industrial Wireless Sensor Networks transmitting temperature or pressure readings, periodically or in an event-triggered manner); and

[0072] - Smart meters and smart meter networks sending periodic meter readings.

[0073] In Rel-17, uplink small data transmissions (MO-SDT) have been supported for UEs in the RRC inactive state (i.e. without the UE moving to a fully connected state with the network) in order to reduce the signalling overheads as well as power consumption at the UE, and primarily being for infrequent data traffic. SDT on the uplink for UEs in the RRC inactive state has been agreed for both RACH based schemes (i.e. 2-step and 4-step RACH) - known as Random Access SDT (RA-SDT) - and configured grant (CG) based schemes (CG-SDT), each of which is discussed in greater detail below. This includes general procedures to enable user plane data transmissions for small data packets on the uplink in the inactive state (for example using either message A of the 2-step RACH procedure or message 3 of the 4- step RACH procedure), and enables flexible payload sizes larger than the Release 16 Common Control Channel (CCCH) message size that is possible currently for a UE in the RRC inactive state to transmit small data in message A or message 3 to support user plane data transmission in the uplink.

[0074] As described above, three schemes have been agreed by 3GPP for the initiation of SDT on the uplink, onginating from a mobile UE in the inactive state. These are:

[0075] - 4-step RACH based scheme;

[0076] - 2-step RACH based scheme; and

[0077] - CG based scheme.

[0078] 4-Step RACH Scheme

[0079] Figure 8A shows an example of the 4-step RACH based scheme, and shows how MO SDTs can be initiated by such a scheme. When a UE has an UL SDT ready for transmission, it may start a 4-step RACH procedure as shown in Figure 8A, which comprises the following steps:

[0080] - A UE starts message 1 transmission 50 of a Physical Random Access (PRACH) preamble from a set of preambles allocated for SDT in the current cell. When a gNB receives the preambles, it identifies this as an SDT initiation, and responds with message 2.

[0081] - The gNB transmits 51 message 2, which contains UL timing alignment command and UL PUSCH scheduling for message 3.

[0082] - The UE transmits 52 message 3, which contains Radio Resource Control (RRC) signaling For example, the UE may transmit an RRC Resume Request. If there is any remaining space in message 3, the UE may also transmit SDT data in message 3. As will be explained in more detail below, the transmission of message 3 may trigger an SDT failure timer at the UE defining a period for which the UE can communicate SDTs with the gNB.

[0083] - Similarly to the general 4-step RACH procedure, the gNB then provides 53 the contention resolution after the UE that transmitted the preamble in the first step 50 is identified and confirmed. In this step 53, DL and UL feedback or acknowledgments are transmitted.

[0084] - For UL feedback received by a gNB from a UE in response to transmitting a DL PDSCH to that UE, a HARQ-ACK is transmitted on a cell-specific PUCCH resource configured within the system information (though it should be noted that, that from the third step 52, the UE is already UL-synchronised) . - For DL feedback received by a UE in response to transmitting the UL message 3 in the third step 52, the reception of message 4 in the fourth step 53 at the UE is considered as a positive acknowledgment.

[0085] - After the fourth step 53, the UE is now already identified by the network and is also UL- synchromsed. Hence, subsequent UL and DL SDT with dynamic scheduling can take place 54 as required while the UE remains in the INACTIVE state. Once SDT is completed, and neither the gNB or UE have any further small data to transmit, the gNB can choose to keep the UE in RRC inactive state by sending RRCRelease with suspend indication 55.

[0086] 2-Step RACH Scheme

[0087] Figure 8B shows an example of the 2-step RACH based scheme, and shows how MO SDTs can be initiated by such a scheme. When a UE has an UL SDT ready for transmission, it may start a 2-step RACH procedure as shown in Figure 8B, which comprises the following steps:

[0088] - A UE starts message A transmission 56 of a PRACH preamble and associated PUSCH for SDT in the current cell. The PUSCH contains RRC signaling. For example, message (i.e. RRCResumeRequest. If there is any remaining space in the PUSCH, the UE may transmit SDT data. As will be explained in more detail below, the transmission of message A may trigger an SDT failure timer at the UE defining a period for which the UE can communicate SDTs with the gNB.

[0089] - When the gNB receives 56 message A, it responds 57 with message B which contains both an UL timing alignment command and the contention resolution where the UE which transmitted 56 message A in the first step is identified and confirmed. In this step 57, DL and UL feedback or acknowledgments are transmitted.

[0090] - For UL feedback received by a gNB from a UE in response to a DL PDSCH being transmitted by the gNB to that UE, a HARQ-ACK is transmitted by the UE on a cell-specific PUCCH resource configured within the system information.

[0091] - For DL feedback received by a UE from the gNB in response to message A being transmitted 56 by that UE, the reception 57 of message B at the UE is considered as a positive acknowledgment; and

[0092] - After the second step 57, the UE is now already identified by the network and is also UL- synchronised. Hence, subsequent UL and DL SDT with dynamic scheduling can take place 54 as required while the UE remains in the RRC inactive state. Once SDT is completed, and neither the gNB or UE have any further small data to transmit, the gNB can choose to keep the UE in RRC inactive state by sending RRCRelease with suspend indication 59.

[0093] Configured Grant (CG) Scheme

[0094] When a UE remains in the same coverage area or cell for a period of time, it is possible that the UE can use some pre-configured UL resources for transmitting data, provided that the UE is UL synchronised, while remaining in the RRC inactive state. Hence in Release 17 of the 3GPP standards, it has been agreed that a network can configure dedicated CG PUSCH resource(s) for SDT on a dedicated BWP or initial BWP, just before a UE moves to the RRC inactive state.

[0095] The first message of CG contains RRC signalling. For example, the first message of CG may include an RRCResumeRequest. If there is remaining space in the first message of CG, the UE may include SDT data in the first message of CG. As will be explained in more detail below, the first message of CG may trigger an SDT failure timer at the UE defining a period for which the UE can communicate SDTs with the gNB. Therefore, SDT can enable UEs to transmit relatively small amounts of data whilst in the RRC inactive state. However, since MA-PDU sessions were originally intended to be across a 3GPP access network and anon-3GPP access network, MA-PDU sessions can only be used for services which are supported by both the 3GPP access network and the non-3GPP access network. Therefore, MA-PDU could be only be used for a limited number of services. Since SDT is not supported by non-3GPP access networks, then UEs in the RRC inactive state in an MA-PDU session do not transmit or receive SDTs.

[0096] As will be understood by a person skilled in the art, SDT imposes limitations on a volume of data which can be transmitted in an SDT. Typically, only a data volume below a pre-defmed threshold can be transmitted in an SDT. Furthermore, SDT requires good coverage to reduce the probability of transmission failure. A base station, such as a gNB, may provide parameters on a required coverage level for SDT to a UE based on a Reference Signal Received Power (RSRP) such as,

[0097] - sdt-RSRP-Threshold: RSRP threshold for UE to determine whether to perform SDT procedure;

[0098] - cg-SDT-RSRP-ThresholdSSB: an RSRP threshold configured for SSB selection for CG-SDT.

[0099] If the coverage requirements are not met, the UE may switch from the RRC inactive state to the RRC connected state to send the SDT. A UE operating in the connected state requires additional power consumption and additional radio resources compared to a UE operating in the inactive state.

[0100] In addition, mobile terminated SDT (MT-SDT) is supported in Rel-18 [6], An MT-SDT is initiated by the gNB with an indication to the UE in a paging message when downlink (DL) data awaits transmission for radio bearers configured for SDT. Based on the indication, the UE initiates the MT-SDT only if the DL RSRP is above a configured threshold. When MT-SDT is initiated by the UE, a resume cause indicating MT-SDT is included in the RRCResumeRequest / RRCResumeRequestl.

[0101] A maximum duration that the SDT procedure can last is dictated by a SDT failure detection timer that is configured by the gNB. The gNB can enable MO-SDT, MT-SDT, or both in a cell.

[0102] As explained above, there is expected to be an increase in the number of scenanos where data transmissions are switched, rather than split, across communications paths in MA-PDU sessions. Since the UE is transitioned into an RRC inactive state for the communications path which is not being used for communications in the MA-PDU session, UEs are expected to spend a greater amount of time in the RRC inactive state. However, if a UE is in an RRC inactive state for communications with a gNB, then the UE can generally not perform communications with that gNB. In order to perform communication with that gNB, the UE has to transition to an RRC connected state which incurs signalling overhead. Therefore, even if the UE has a relatively small amount of data to transmit, usually must incur signalling overhead to enter the RRC connected state before being able to transmit the data to the gNB.

[0103] There is therefore a need for communications devices, infrastructure equipment and core network apparatus which can improve communications efficiency.

[0104] ATSSS Steering Modes and Rules

[0105] As will be understood by one skilled in the art, a “Steering Mode” identifies how a service data flow (SDF) should be steered between 3GPP and non-3GPP access networks. ATSSS rules may be provided to one or both of a UE and a UPF(PSA) of a core network. As will be understood by one skilled in the art, an SDF flow is a flow of packets representing a service being delivered to a subscriber. The following Steering Modes are currently supported [1]:

[0106] - Active-Standby. According to this steering mode, an SDF is steered on one access network (the “Active” access network) when this access network is available, and the SDF is switched to another access network (the “Standby” access network) when the active access network becomes unavailable. When the active access network becomes available again, the SDF is switched back to the active access network. If the Standby access network is not defined, then the SDF is only allowed on the active access network and cannot be transferred to another access network.

[0107] - Smallest Delay. According to this steering mode, SDF is steered on the access network that is determined to have the smallest Round-Trip Time (RTT). Measurements may be conducted to determine the RTT over a 3GPP access network and over a non-3GPP access network.

[0108] - Load-Balancing. According to his steering mode, an SDF is split across both access networks. With a 50-50 load-balancing, the SDF traffic is equally split across the two access networks. With an 80-20 load-balancing, about 80% of the SDF traffic is sent on one access network and 20% on the other access network.

[0109] - Priority-based. According to this steering mode, SDF is steered using only a high priority access network until a congestion arises on the corresponding interface and, when the high priority access network is congested, additionally using a low priority access network by splitting the SDF over the two access networks. In addition, when the high priority access network becomes unavailable, all traffic is switched to the low priority access network. How UE and UPF(PSA) determine when a congestion occurs on an interface is implementation dependent.

[0110] Examples of existing ATSSS rules based on steering modes are described below: a) "Traffic Descriptor: UDP, DestAddr 1.2.3.4", "Steering Mode: Active-Standby, Active=3GPP, Standby=non-3GPP"

[0111] - This ATSSS rule can be interpreted as follows: "steer UDP traffic with destination IP address 1.2.3.4 to the active access (3GPP access network), if available. If the active access network is not available, use the standby access (non-3GPP access network)". b) "Traffic Descriptor: TCP, DestPort 8080", "Steering Mode: Smallest Delay"

[0112] - This ATSSS rule can be interpreted as follows: "steer TCP traffic with destination port 8080 to the access network with the smallest delay". The UE may occasionally measure the RTT over both access network, in order to determine which access network has the smallest delay. c) "Traffic Descriptor: Application- 1", "Steering Mode: Load-Balancing, 3GPP=20%, non- 3GPP=80%", "Steering Function: MPTCP"

[0113] - This ATSSS rule can be interpreted as follows: "send 20% of the traffic of Application-1 to 3GPP access network and 80% to non-3GPP access network by using MPTCP". d) "Traffic Descriptor: TCP, DestDomain Dreamy5G.com", "Steering Mode: Smallest Delay"

[0114] - This ATSSS rule can be interpreted as follows: "steer TCP traffic with destination domain "google.com" to the access network with the smallest delay".

[0115] However, as explained above, support for ATSSS currently requires at least one 3GPP access network and at least one non-3GPP access network. In view of the proposal for MA-PDU sessions with two 3GPP networks, there is also need for new ATSSS rules.

[0116] In view of the above, a method of operating infrastructure equipment of a radio access network of a wireless communications network is illustrated in Figure 9. The method begins in step S 1.

[0117] In step S2, the method comprises transmitting an inactive state indication to the communications device. The inactive state indication indicates to the communications device to transition to an inactive state for communications via a first communications path of an MA-PDU session. The first communications path is used for communications between the communications device and a core network of the wireless communications network via the infrastructure equipment of the radio access network. The MA-PDU session comprises a second communications path for communications between the communications device and the core network via infrastructure equipment of another radio access network of the wireless communications network.

[0118] In some embodiments, the communicating the SDT along the first communications path comprises determining that an amount of data for in the SDT received from the core network is below a pre-defmed data amount threshold.

[0119] The communications device may be in a connected, inactive or idle state for communications via the second communications path. Therefore, after the communications device has transitioned to the inactive state for communications via the first path, the communications device may be in a connected, inactive or idle state for communications via the second communications path.

[0120] In step S3, the method comprises communicating a small data transmission, SDT, along the first communications path when the communications device is in the inactive state. The communication of the SDT along the first communications path comprises receiving the SDT from the core network and transmitting the SDT to the communications device, or receiving the SDT from the communications device and transmitting the SDT to the core network.

[0121] In some embodiments, the infrastructure equipment of the radio access network may receive an SDT from the communications device (for example, an MO-SDT) and determine, based on the SDT, to transition the communications device to the connected mode. The infrastructure equipment of the radio access network may then transmit a connected state indication to the communications device indicating to the communications device to transition to a connected state for communications with the infrastructure equipment of the radio access network.

[0122] In some embodiments, the infrastructure equipment of the radio access network may receive data from the core network. The infrastructure equipment may then transmit the data as an SDT to the communications device (for example, an MT-SDT) or the infrastructure equipment may instead transmit a connected state indication to the communications device indicating to the communications device to transition to the connected state.

[0123] In some embodiments, the inactive state is an RRC inactive state, and the connected state is an RRC connected state.

[0124] The method ends in step S4.

[0125] By communicating SDT in an MA-PDU session, small amounts of data can be communicated without communications devices having to transition into a connected state and thereby incurring signalling overhead and power usage. Therefore, embodiments can provide improved communications efficiency and reduced power usage for communications devices. As explained above, the infrastructure equipment of the radio access network transmits the inactive state indication to the communications device indicating to the communications device to transition to the inactive state for communications with the infrastructure equipment of the radio access network. In some embodiments, the infrastructure equipment of the other radio access network may also transmit an inactive state indication to the communications device indicating to the communications device to transition to the inactive state for communications with the infrastructure equipment of the other radio access network. Therefore, the communications device is in an inactive state with respect to both the infrastructure equipment of the radio access network and the infrastructure equipment of the other radio access network.

[0126] In accordance with example embodiments, one or more ATSSS rules may be configured for the communications device and / or the core network. The one or more ATSSS rules may comprise a set of one or more ATSSS rules for use when the communications device is in the connected state with respect to at least one of the infrastructure equipment of the radio access networks (“Connected State ATSSS Rules”) and a set of one or more other ATSSS rules for use when the communications device is in the inactive state with respect to both infrastructure equipment (“Inactive State ATSSS Rules”). The inactive state ATSSS rules may be different to the connected state ATSSS rules. In some embodiments, the connected state ATSSS rules may comprise two subsets of ATSSS rules - one subset comprising one or more ATSSS rules for use when the communications device is in the connected state with respect to one of the infrastructure equipment of the radio access networks and in the inactive state with respect to the other infrastructure equipment of the radio access networks, and another subset comprising one or more ATSSS rules for use when the communications device is in the connected state with respect to both infrastructure equipment of the radio access networks.

[0127] For example, the radio access network may be a 3GPP access network and the other radio access network may be another 3GPP access network. In some embodiments, the 3GPP access network is comprised in a terrestrial network, TN, and the other 3GPP access network is comprised in anon-terrestrial network NTN. In other embodiments, the 3GPP access network is comprised in a stand-alone non-public network, SNPN, and the other 3GPP access network is comprised a public land mobile network, PLMN.

[0128] Examples of connected state ATSSS rules are shown in a) and b) below. a) "Traffic Descriptor: UDP, DestAddr 1.2.3.4", "Steering Mode: Active-Standby, Active=TN, Standby=NTN".

[0129] This ATSSS rule can be interpreted as "steer UDP traffic with destination IP address 1.2.3.4 to the active access (TN), if available. If the active access is not available, use the standby access (NTN)". b) "Traffic Descriptor: UDP, DestAddr 1.2.3.4", "Steering Mode: Active-Standby, Active=SNPN, Standby=PLMN".

[0130] This ATSSS rule can be interpreted as "steer UDP traffic with destination IP address 1.2.3.4 to the active access network (SNPN), if available. If the active access network is not available, use the standby access network (PLMN)".

[0131] However, when the communications device is inactive state with respect to both infrastructure equipment, these ATSSS rules may be modified in accordance with example embodiments. For example, the active and standby networks in the ATSSS rules may be reversed as shown in c) and d) below: c) "Traffic Descriptor: UDP, DestAddr 1.2.3.4", "Steering Mode: Active-Standby, Active=NTN, Standby=TN" .

[0132] This ATSSS rule can be interpreted as "steer UDP traffic with destination IP address 1.2.3.4 to the active access (NTN), if available. If the active access is not available, use the standby access (TN)". d) "Traffic Descriptor: UDP, DestAddr 1.2.3.4", "Steering Mode: Active-Standby, Active=PLMN, Standby=SNPN" .

[0133] This ATSSS rule can be interpreted as "steer UDP traffic with destination IP address 1.2.3.4 to the active access network (PUMN), if available. If the active access network is not available, use the standby access network (SNPN)".

[0134] The ATSSS rules for use when the communications device is in the inactive state with respect to both infrastructure equipment (i.e. the inactive state ATSSS rules) allows for increased flexibility in communications resource utilisation. For example, it may be desirable to reserve communications resources provided by a TN for important, or high throughput services, rather than SDTs. Therefore, as shown in state c) above, the active access network is the NTN rather than the TN so that, by default, SDTs are transmitted via the NTN. For example, the SDTs transmitted to the NTN may comprise emergency communications.

[0135] In another example, it may be desirable to reserve SNPN communications resources for URLLC services rather than SDTs. Therefore, as shown in state d) above, the active access network is the PLMN rather than the SNPN so that, by default, SDTs are transmitted via the PLMN.

[0136] In accordance with example embodiments, the communications device is in the inactive state with respect to the infrastructure equipment of the radio access network but in the connected state with respect to the infrastructure equipment of the other radio access network. In this case, the connected state ATSSS rules may specify that downlink heavy traffic should be transmitted via a TN with respect to which the communications device is in a connected state and that uplink light traffic should be transmitted via an NTN with respect to which the communications device is in an inactive state.

[0137] As discussed above, the one or more ATSSS rules configured for the communications device and / or the core network may comprise a set of one or more connected state ATSSS rules and a set of one or more inactive state ATSSS rules. In some embodiments, the one or more ATSSS rules configured for the communications device and / or the core network may comprise a set of one or more ATSSS rules which indicate that an SDT must be communicated via an infrastructure equipment of a radio access network with respect to which the communications device is in an inactive state (“SDT-inactive state ATSSS rules”). For example, it may be specified that both the active and standby access networks in an ATSSS rule are in an inactive state. An example is shown in ATSSS rule e) below. e) "Traffic Descriptor: UDP, DestAddr 1.2.3.4", "Steering Mode: Active-Standby, Active=PLMNl SDT, Standby=PLMN2 SDT".

[0138] This ATSSS rule can be interpreted as "steer UDP traffic with destination IP address 1.2.3.4 to the active access network (PLMN 1 SDT) with respect to which the UE is in an inactive state, if available. If the active access network is not available (e.g. poor coverage), use the standby access network (PLMN2 SDT) with respect to which the UE is in an inactive state". Therefore, in response to poor coverage, instead of switching the UE to a connected state with respect to the active access network, the UE switches to communicating via the standby network with respect to which the UE is also in an inactive state. This can reduce the power consumption and reduce the reserved radio resources / signalling because it can avoid switching to connected mode in the active access network. When a coverage level of both PEMN1 and PEMN2 is poor, UE may switch to connected state for either PLMN1 or PEMN2.

[0139] In an example, an loT UE may use more than one operator subscriber identity module (SIM) (multioperator SIM). This means that the UE can communicate with an infrastructure equipment of one radio access network if the infrastructure equipment of the other radio access network is providing a poor coverage level for the UE. The SDT-mactive state ATSSS rules may enable SDT communication to remain with multiple PLMNs.

[0140] A non-access stratum (NAS) layer of the communications device may provide one or more of the set of connected state ATSSS rules, the set of inactive state ATSSS rules, and the set of SDT-inactive state ATSSS rules to an Access Stratum (AS) layer of the communications device. In some embodiments, the NAS layer provides all of the sets of ATSSS rules to the communications device. In some embodiments, the communications device determines whether it needs to use the set of connected state of ATSSS rules or the set of inactive state ATSSS rules based on whether the communications device is in a connected state or inactive state with respect to each of the infrastructure equipment. In this case, the AS layer of the communications device sends a request to the NAS rules for the set of rules which it needs, and the NAS layer responds with the requested rules.

[0141] There may be no separate NAS layer state in the core network for a communications device being in an inactive state. Therefore, from the perspective of the NAS layer of core network, the communications device may be treated as if it were still in a connected state, for example, a GTP tunnel between a UPF of the core network and infrastructure equipment of a radio access network (such as a gNB) may be maintained even when the communications device is transitioned into an inactive state.

[0142] In one embodiment, the ATSSS rules (connected, inactive state, or SDT-inactive ATSSS rules) indicate not only which communications path should be used but specifically whether the communications path should be used for uplink or downlink transmissions. For example, an ATSSS rule may indicate that one radio access network is used for downlink transmissions and the other radio access network is used for uplink transmissions. In some examples, the ATSSS rule may also depend on how heavy the traffic is. For example, an ATSSS rule may specify that a radio access network in an NTN is to be used for heavy downlink traffic but that an access network of a TN is to be used for light uplink traffic (for example, an acknowledgement in response to the downlink transmission). The communications device may transmit an SDT via the TN if an amount of data to be transmitted on the uplink is below a data amount threshold.

[0143] Figure 10 schematically illustrates a flow diagram showing an example of dual steering in accordance with example embodiments. Figure 10 illustrates a communications flow between UE 502, the first gNB 506, the second gNB 514 and the core network 510.

[0144] During establishment of an MA-PDU session, the UE 502 (in particular a UE NAS layer 502a and a UE AS layer 502b) and the core network 510 (for example, a UPF(PSA)) are configured with a set of ATSSS rules for determining whether to UE 502 will communicate with the core network 510 via the first gNB 506 (i.e. via the first communications path) or via the second gNB 514 (i.e. via the second communications path) when the UE 502 is in a connected state with respect to the at least one of the first gNB 506 and with respect to the second gNB 514 (“Connected State ATSSS Rules”). Furthermore, both the core network 510 and the UE 502 may be configured with a set of ATSSS rules for determining whether to UE 502 will communicate with the core network 510 via the first gNB 506 (i.e. via the first communications path) or via the second gNB 514 (i.e. via the second communications path) when the UE 502 is in an inactive state with respect to the first gNB 506 and the second gNB 514 (“Inactive State ATSSS Rules”). In some embodiments, although not shown in Figure 10, both the core network 510 and the UE 502 may be configured with a set of ATSSS rules for determining whether to UE 502 will communicate with the core network 510 via the first gNB 506 (i.e. via the first communications path) or via the second gNB 514 (i.e. via the second communications path) and those rules indicate that SDT must be transmitted via a gNB with respect to which the UE 502 is in an inactive state (SDT-inactive state ATSSS rules).

[0145] As shown in step S802 of Figure 10, the UE 502 and / or the core network 510 determine that the UE 502 will communicate with the core network 510 via the first communications path based on the connected state ATSSS rules. Then, in step S804, the UE 502 and the core network 510 communicate via the first communications path.

[0146] In step S806, the core network 510 determines, based on the connected state ATSSS rules, to perform a path switch from communicating with the UE 502 via the first communications path to communicating with the UE 502 via the second communications path.

[0147] In step S808, in response to the communications path changing, the first gNB 506 transmits an inactive state indication to the UE 502 indicating to the UE to transition to an RRC inactive state for communications with the first gNB 506.

[0148] In step S810, in response to receiving the inactive state indication, the UE 502 transitions to an RRC inactive state for communications with the first gNB 506.

[0149] In step S812, the UE 502 and the core network 510 communicate via the second communication path.

[0150] At a later time, the second gNB 514 determines that communications are not expected along the second communications path. Therefore, in step S814, the second gNB 514 transmits an inactive state indication to the UE 02 indicating the UE 502 to transition to an RRC inactive state for communications with the second gNB 514. Alternatively, the second gNB 514 may detect that a PDU session on the second communications path has been released and transmits the inactive state indication to the UE 502 in response to this detection. Accordingly, the UE 502 is in an RRC inactive state with respect to both the first gNB 506 and the second gNB 514.

[0151] In step S818, the core network 510 transmits an indication ofthe inactive state ATSSS rules to the first gNB 506 and, in step S820, the core network 510 transmits an indication of the inactive state ATSSS to the second gNB 514. In some embodiments, although not shown in Figure 10, the core network 510 may in addition to, or instead of, transmitting the inactive state ATSSS rules to the first gNB 506 and the second gNB 514, transmit an indication of the SDT-inactive state rules to the first gNB 506 and the second gNB 514. Consequently, subsequent description of techniques involving inactive state ATSSS rules are equally applicable to SDT-inactive state ATSSS rules.

[0152] In step S820, the first gNB 506 and the second gNB 514 determine, based on the inactive state ATSSS rules, whether communications between the UE 502 and the core network 510 should occur via the first gNB 506 or via the second gNB 514. In the example shown in Figure 10, the first gNB 506 determines, based on the inactive state ATSSS rules, that communications between the UE 502 and the core network 510 should occur via the first gNB 506. In some embodiments, not shown in Figure 10, the first gNB 506 may transmit a signal the core network 510 (for example, a UPF(PSA) of the core network 510) indicating to the core network 510 to perform a path switch from the second communications path to the first communications path.

[0153] In some embodiments, not shown in Figure 10, instead of transmitting the inactive state ATSSS rules to the first gNB 506 and the second gNB 514, the core network 510 may determine, based on the inactive state ATSSS rules, whether communications between the UE 502 and the core network 510 should occur via the first gNB 506 or via the second gNB 514. For example, in accordance with example embodiments, if the core network 510 determines that communications between the UE 502 and the core network 510 should occur via the first communications path, the core network 510 transmits a downlink SDT to the first gNB 506 for forwarding onto the UE 502.

[0154] In some embodiments, not shown in Figure 10, the UE 502 may determine, based on the inactive state ATSSS rules, whether communications between the UE 502 and the core network 510 should occur via the first gNB 506 or via the second gNB 514. For example, the second gNB 514 may transmit a RAN paging message to the UE 502. However, the UE 502 may determine, based on the inactive state ATSSS rules, that communications between the UE 502 and the core network 510 should occur via the first communications path. Therefore, the UE 502 may transmit an SDT to the first gNB 506 even though the second gNB 514 transmitted a RAN paging message to the UE 502. In such embodiments, the UE 502 may transmit a RAN paging reject indication to the second gNB 514. In some embodiments, the second gNB 514 may forward the RAN paging message to the first gNB 506 for transmission to the UE 502.

[0155] In step S822, the first gNB 506 transmits an indication to the UE 502 indicating that the UE 502 should transmit SDTs via the first communications path. In the example, shown in Figure 10, this indication is a RAN paging message. In response to receiving the RAN paging message, in step S824, the UE NAS layer 502a forwards data to the UE AS layer 502b for transmission to the first gNB 506 as an SDT. The SDT is transmitted from the UE 502 to the first gNB S826 in step S826. Furthermore, after the RAN paging message has been sent, the first gNB 506 may transmit an SDT to the UE 502.

[0156] As will be appreciated by a person skilled in the art, RAN paging messages may be sent over RAN notification area which can include more than one cell. Therefore, cells provided by infrastructure equipment of different radio access networks may have different RAN notification areas. For example, access networks for an SNPN and a PLMN may have different RAN paging notification areas. In some embodiments, the core network 510 may, based on the inactive state ATSSS rules, transmit an SDT to the first gNB 506 and, in response, the first gNB 506 transmits a RAN paging message to the UE 502. In some embodiments, where the core network 510 is not aware that the UE 502 is in the inactive state, but the core network 510 has transmitted the inactive state ATSSS rules to the first gNB 506 and the second gNB 514, the then the core network 510 may transmit an SDT to the second gNB 514 (for example, based on the connected state ATSSS rules because the core network 510 is unaware the UE 502 is in the inactive state). The second gNB 514 may transmit a RAN paging message to the first gNB 506 based on a UE context stored at the second gNB 514. For example, the UE context may comprise information indicating that the UE 502 has an on-going MA-PDU session which involves the first gNB 506.

[0157] The following numbered paragraphs provide further example aspects and features of the present technique:

[0158] Paragraph 1. A method of operating infrastructure equipment of a radio access network of a wireless communications network, the method comprising transmitting an inactive state indication to the communications device, the inactive state indication indicating to the communications device to transition to an inactive state for communications via a first communications path of a multi-access protocol data unit, MA-PDU, session, the first communications path being used for communications between the communications device and a core network of the wireless communications network via the infrastructure equipment of the radio access network, the MA-PDU session comprising a second communications path for communications between the communications device and the core network via infrastructure equipment of another radio access network of the wireless communications network, and communicating a small data transmission, SDT, along the first communications path when the communications device is in the inactive state, the communication of the SDT along the first communications path comprising receiving the SDT from the core network and transmitting the SDT to the communications device, or receiving the SDT from the communications device and transmitting the SDT to the core network.

[0159] Paragraph 2. A method according to paragraph 1, wherein the transmitting the SDT to the communications device comprises determining that an amount of data in the SDT received from the core network is below a predefined data amount threshold.

[0160] Paragraph 3. A method according to paragraph 1 or paragraph 2, comprising receiving an indication from the communications device that the communications device supports SDT when in the inactive state.

[0161] Paragraph 4. A method according to any of paragraphs 1 to 3, comprising transmitting an indication to the communications device indicating that the communications device may transmit SDTs via the infrastructure equipment of the radio access network.

[0162] Paragraph 5. A method according to any of paragraphs 1 to 4, wherein the communicating the SDT along the first communications path comprises transmitting an indication to the core network indicating that the communications device has been transitioned to the inactive state.

[0163] Paragraph 6. A method according to any of paragraphs 1 to 5, comprising receiving, from the core network, an indication of Access Traffic Steering, Switching and Splitting, ATSSS, rules, the ATSSS rules being for determining whether to communicate SDTs with the communications device via the first communications path or the second communications path when the communications device is in the inactive state for communications with the infrastructure equipment of the radio access network and in the inactive state for communications with the infrastructure equipment of the other radio access network, and transmitting, based on the ATSSS rules, an indication to the communications device indicating that the communications device should transmit SDTs via the first communications path. Paragraph 7. A method according to any of paragraphs 1 to 5, comprising receiving, from the core network, an indication of Access Traffic Steering, Switching and Splitting, ATSSS, rules, the ATSSS rules being for determining whether to communicate SDTs with the communications device via the first communications path or the second communications path, the one or more ATSSS rules indicating that SDTs must be communicated via a communications path for which the communications device is in an inactive state, and transmitting, based on the ATSSS rules, an indication to the communications device indicating that the communications device should transmit SDTs via the first communications path. Paragraph 8. A method of operating a communications device, the method comprising receiving an inactive state indication from infrastructure equipment of a radio access network, the inactive state indication indicating to the communications device to transition to an inactive state for communications via a first communications path of an MA-PDU session, the first communications path being used for communications between the communications device and a core network of the wireless communications network via the infrastructure equipment of the radio access network, the MA-PDU session comprising a second communications path for communications between the communications device and the core network via infrastructure equipment of another radio access network of the wireless communications network, and communicating a small data transmission, SDT, along the first communications path when the communications device is in the inactive state, the communication of the SDT along the first communications path comprising transmitting the SDT to the infrastructure equipment of the radio access network, or receiving the SDT from the infrastructure equipment of the radio access network.

[0164] Paragraph 9. A method according to paragraph 8, wherein the transmitting the SDT to the infrastructure equipment comprises determining that an amount of data in the SDT is below a pre-defined data amount threshold.

[0165] Paragraph 10. A method according to paragraph 8 or paragraph 9, comprising transmitting an indication to the infrastructure equipment of the radio access network that the communications device supports SDT when in the inactive state.

[0166] Paragraph 11. A method according to any of paragraphs 8 to 10, comprising receiving, from the infrastructure equipment of the radio access network, an indication indicating that the communications device may transmit SDTs via the infrastructure equipment of the radio access network.

[0167] Paragraph 12. A method according to any of paragraphs 8 to 11, wherein the transmitting the SDT to the infrastructure equipment of the radio access network comprises determining to transmit the SDT to the infrastructure equipment of the radio access network via the first communications path based on one or more Access Traffic Steering, Switching and Splitting, ATSSS, rules, the one or more ATSSS rules being for determining whether to communicate SDTs with the communications device via the first communications path or the second communications path when the communications device is in the inactive state for communications with the infrastructure equipment of the radio access network and in the inactive state for communications with the infrastructure equipment of the other radio access network.

[0168] Paragraph 13. A method according to any of paragraphs 8 to 12, wherein the transmitting the SDT to the infrastructure equipment of the radio access network comprises determining to transmit the SDT to the infrastructure equipment of the radio access network via the first communications path based on one or more Access Traffic Steering, Switching and Splitting, ATSSS, rules, the one or more ATSSS rules indicating that SDTs must be communicated via a communications path for which the communications device is in an inactive state Paragraph 14. A method of operating a core network apparatus of a wireless communications network, the method comprising communicating a small data transmission, SDT, along a first communications path of a multiaccess protocol data unit, MA-PDU, session for communications between a communications device and the core network apparatus via infrastructure equipment of a radio access network of the wireless communications network, the MA-PDU session comprising a second communications path for communications between the communications device and the core network apparatus via infrastructure equipment of another radio access network, the communication of the SDT along the first communications path comprising transmitting the SDT to the infrastructure equipment of the radio access network for forwarding to the communications device when the communications device is in an inactive state for communications with the infrastructure equipment of the radio access network, or receiving the SDT from the infrastructure equipment of the radio access network. Paragraph 15. A method according to paragraph 14, wherein the transmitting the SDT to the infrastructure equipment of the radio access network comprises determining that an amount of data in the SDT is below a pre-defined data amount threshold. Paragraph 16. A method according to paragraph 14 or paragraph 15, comprising receiving an indication from the infrastructure equipment of the radio access network indicating that the communications device supports SDT when in the inactive state.

[0169] Paragraph 17. A method according to any of paragraphs 14 to 16, comprising transmitting an indication to the infrastructure equipment of the radio access network, for forwarding to the communications device, indicating that communications device may transmit SDTs via the infrastructure equipment of the radio access network.

[0170] Paragraph 18. A method according to any of paragraphs 14 to 17, wherein the communicating the SDT along the first communications path comprises receiving an indication from the infrastructure equipment of the radio access network indicating that the communications device has been transitioned to the inactive state.

[0171] Paragraph 19. A method according to any of paragraphs 14 to 18, wherein the transmitting the SDT to the infrastructure equipment of the radio access network comprises determining to transmit the SDT to the infrastructure equipment of the radio access network based on one or more Access Traffic Steering, Switching and Splitting, ATSSS, rules, the ATSSS rules being for determining whether to communicate SDTs with the communications device via the first communications path or the second communications path when the communications device is in the inactive state for communications with the infrastructure equipment of the radio access network and in the inactive state for communications with the infrastructure equipment of the other radio access network. Paragraph 20. A method according to any of paragraphs 14 to 19, wherein the transmitting the SDT to the infrastructure equipment of the radio access network comprises determining to transmit the SDT to the infrastructure equipment of the radio access network based on one or more Access Traffic Steering, Switching and Splitting, ATSSS, rules, the one or more ATSSS rules indicating that SDTs must be communicated via a communications path for which the communications device is in an inactive state.

[0172] Paragraph 21. Infrastructure equipment for a radio access network of a wireless communications network, the infrastructure equipment comprising a transmitter configured to transmit signals, a receive configured to receive signals, and a controller configured in combination with the transmitter and the receiver to transmit an inactive state indication to the communications device, the inactive state indication indicating to the communications device to transition to an inactive state for communications via a first communications path of a multi-access protocol data unit, MA-PDU, session, the first communications path being used for communications between the communications device and a core network of the wireless communications network via the infrastructure equipment of the radio access network, the MA- PDU session comprising a second communications path for communications between the communications device and the core network via infrastructure equipment of another radio access network of the wireless communications network, and communicate a small data transmission, SDT, along the first communications path when the communications device is in the inactive state, the communication of the SDT along the first communications path comprising receiving the SDT from the core network and transmitting the SDT to the communications device, or receiving the SDT from the communications device and transmitting the SDT to the core network. Paragraph 22. Circuitry for infrastructure equipment for a radio access network of a wireless communications network, the circuitry comprising transmitter circuitry configured to transmit signals, receiver circuitry configured to receive signals, and controller circuitry configured in combination with the transmitter circuitry and the receiver circuitry to transmit an inactive state indication to the communications device, the inactive state indication indicating to the communications device to transition to an inactive state for communications via a first communications path of a multi-access protocol data unit, MA-PDU, session, the first communications path being used for communications between the communications device and a core network of the wireless communications network via the infrastructure equipment of the radio access network, the MA- PDU session comprising a second communications path for communications between the communications device and the core network via infrastructure equipment of another radio access network of the wireless communications network, and communicate a small data transmission, SDT, along the first communications path when the communications device is in the inactive state, the communication of the SDT along the first communications path comprising receiving the SDT from the core network and transmitting the SDT to the communications device, or receiving the SDT from the communications device and transmitting the SDT to the core network.

[0173] Paragraph 23. A communications device, the communications device comprising a transmitter configured to transmit signals, a receive configured to receive signals, and a controller configured in combination with the transmitter and the receiver to receive an inactive state indication from infrastructure equipment of a radio access network, the inactive state indication indicating to the communications device to transition to an inactive state for communications via a first communications path of an MA-PDU session, the first communications path being used for communications between the communications device and a core network of the wireless communications network via the infrastructure equipment of the radio access network, the MA-PDU session comprising a second communications path for communications between the communications device and the core network via infrastructure equipment of another radio access network of the wireless communications network, and communicate a small data transmission, SDT, along the first communications path when the communications device is in the inactive state, the communication of the SDT along the first communications path comprising transmitting the SDT to the infrastructure equipment of the radio access network, or receiving the SDT from the infrastructure equipment of the radio access network.

[0174] Paragraph 24. Circuitry for a communications device, the circuitry comprising transmitter circuitry configured to transmit signals, receiver circuitry configured to receive signals, and controller circuitry configured in combination with the transmitter circuitry and the receiver circuitry to receive an inactive state indication from infrastructure equipment of a radio access network, the inactive state indication indicating to the communications device to transition to an inactive state for communications via a first communications path of an MA-PDU session, the first communications path being used for communications between the communications device and a core network of the wireless communications network via the infrastructure equipment of the radio access network, the MA-PDU session comprising a second communications path for communications between the communications device and the core network via infrastructure equipment of another radio access network of the wireless communications network, and communicate a small data transmission, SDT, along the first communications path when the communications device is in the inactive state, the communication of the SDT along the first communications path comprising transmitting the SDT to the infrastructure equipment of the radio access network, or receiving the SDT from the infrastructure equipment of the radio access network.

[0175] Paragraph 25. A core network apparatus for a wireless communications network, the core network apparatus comprising a transmitter configured to transmit signals, a receive configured to receive signals, and a controller configured in combination with the transmitter and the receiver to communicate a small data transmission, SDT, along a first communications path of a multi-access protocol data unit, MA-PDU, session for communications between a communications device and the core network apparatus via infrastructure equipment of a radio access network of the wireless communications network, the MA-PDU session comprising a second communications path for communications between the communications device and the core network apparatus via infrastructure equipment of another radio access network, the communication of the SDT along the first communications path comprising transmitting the SDT to the infrastructure equipment of the radio access network for forwarding to the communications device when the communications device is in an inactive state for communications with the infrastructure equipment of the radio access network, or receiving the SDT from the infrastructure equipment of the radio access network.

[0176] Paragraph 26. Circuitry for a core network apparatus for a wireless communications network, the circuitry comprising transmitter circuitry configured to transmit signals, receiver circuitry configured to receive signals, and controller circuitry configured in combination with the transmitter circuitry and the receiver circuitry to communicate a small data transmission, SDT, along a first communications path of a multi-access protocol data unit, MA-PDU, session for communications between a communications device and the core network apparatus via infrastructure equipment of a radio access network of the wireless communications network, the MA-PDU session comprising a second communications path for communications between the communications device and the core network apparatus via infrastructure equipment of another radio access network, the communication of the SDT along the first communications path comprising transmitting the SDT to the infrastructure equipment of the radio access network for forwarding to the communications device when the communications device is in an inactive state for communications with the infrastructure equipment of the radio access network, or receiving the SDT from the infrastructure equipment of the radio access network.

[0177] Paragraph 27. A wireless communications network comprising infrastructure equipment according to paragraph 21, a core network apparatus according to paragraph 25 and a communications device according to paragraph 23.

[0178] Paragraph 28. A computer program comprising instructions which, when program is executed by a computer, cause the computer perform the method of any of paragraphs 1 to 20.

[0179] Paragraph 29. A non-transitory computer-readable storage medium storing a computer program according to paragraph 28.

[0180] Although the present disclosure has been described in connection with some embodiments, it is not intended to be limited to the specific form set forth herein. Additionally, although a feature may appear to be described in connection with particular embodiments, one skilled in the art would recognise that various features of the described embodiments may be combined in any manner suitable to implement the technique.

[0181] ANNEX

[0182] As shown in Figure 11, a UE may occupy one of three Radio Resource Control (RRC) states: the RRC connected state 706 (referred to herein as the “connected state”), the RRC inactive state 710 (referred to herein as the “inactive state”) and the RRC idle 702 state (referred to herein as the “idle state”).

[0183] In the idle state 702, the UE does not have dedicated resources and does not transmit or receive user data apart from performing actions necessary to manage its own mobility. The UE also performs monitoring for paging by monitoring broadcast signals transmitted on a Broadcast Control Channel (BCCH) by each gNB. The core network is aware of the UE’s location within a Tracking Area and does not have the UE AS (access stratum) context and cannot schedule physical resources for user data transmission because the UE does not have a unique identifier within a cell (C-RNTI).

[0184] In the connected state 702, the UE has established an RRC connection and has dedicated resources for transmitting or receiving data. The core network is aware of the UE’s location at the cell level and has the UE context thereby allowing scheduling of physical resources for user data transmissions because the UE has been assigned a temporary ID which is unique to that UE within the cell (C-RNTI) and hence the UE can be directly addressed by the core network.

[0185] In the inactive state 710, the UE can perform small data transmission (SDT) with the wireless communication network. The core network will not be aware that the UE is in the inactive state and will therefore not release any tunnels which have been established between the UE and the core network when the UE was in the connected state 706.

[0186] In the inactive state 710, the UE retains access stratum (AS) context. Storing of AS context in the UE is specified in [5] as:

[0187] “Store in the UE Inactive AS Context the current KgNB and KRRCint keys, the ROHC state, the stored QoS flow to DRB mapping rules, the C-RNTI used in the source PCell, the cellldentity and the physical cell identity of the source PCell, the spCellConfigCommon within ReconfigurationWithSync of the NR PSCell (if configured) and all other parameters configured except for: parameters within Reconfiguration WithSync of the PCell; parameters within ReconfigurationWithSync of the NR PSCell, if configured; parameters within MobilityControlInfoSCG of the E-UTRA PSCell, if configured; servingCellConfigCommonSIB. ”

[0188] Figure 11 illustrates how a UE may transition between the idle 702, inactive 710 and connected 706 states. If a UE in the idle state 702 establishes an RRC connection, the UE transitions from the idle 702 to the connected state 706 as shown by arrow 716. Conversely, a UE may transition from the connected state 706 to the idle state 702 by releasing its RRC connection (and hence releasing its C-RNTI) as shown by arrow 718.

[0189] A UE may transition from the connected state 706 to the inactive state if it receives an RRC suspend configuration message as shown by arrow 712. In this case, although the radio link between the UE and the base station in the connected state 706 is taken down for power conservation, the logical link to the Access Mobility and Management function (AMF) in the core network and the user data tunnel to the UPF remain in place. The core network is not aware that an inactive state exists for the UE and treats the UE as if it were in the connected state 706.

[0190] A UE transition from the inactive state 710 to the connected state 706 via a resume procedure as shown by arrow 714. A UE may initiate an RRC resume procedure to resume a previously suspended RRC connection. Specifically, the UE may resume SRB(s) or DRB(s) or perform an RNA update. Further details on the resume procedure may found in pending international application PCT / EP2022 / 051040, published as WO 2022 / 167216, which is hereby incorporated by reference in its entirety

[0191] It is advantageous for a UE to remain in an inactive state where there is no immediate traffic between the UE and the core network but there is likely to be so in the near future. As the UE in the inactive state retains AS context as explained above, the transition from the inactive state to the connected state quicker than the transition from the idle state to the connected state.

[0192] References:

[0193] [1] 3GPP TR 23.793 V16.0.0 (2018-12), "Study on access traffic steering, switch and splitting support in the 5G System (5GS) architecture”, 3rd Generation Partnership Project.

[0194] [2] 3GPP Technical Specification #23.501, VI 8.2.1, “System architecture for the 5G System (5GS);Stage 2”.

[0195] [3] 3GPP Technical Specification #24. 193, V0.1.0, “Technical Specification Group Core Network and Terminals; Access Traffic Steering, Switching and Splitting; Stage 3”.

[0196] [4] RP- 193252, ‘New Work Item on NR small data transmission in INACTIVE state,” ZTE Corporation, 3GPP TSG RAN Meeting #86.

[0197] [5] 3GPP TS 38.331 section 5.3.8.3, “Radio Resource Control (RRC) Specification”, 3rd Generation Partnership Project.

[0198] [6] 3GPP R2-2306963, Introduction of MT-SDT in Stage-2.

Claims

CLAIMS1. A method of operating infrastructure equipment of a radio access network of a wireless communications network, the method comprising transmitting an inactive state indication to the communications device, the inactive state indication indicating to the communications device to transition to an inactive state for communications via a first communications path of a multi-access protocol data unit, MA-PDU, session, the first communications path being used for communications between the communications device and a core network of the wireless communications network via the infrastructure equipment of the radio access network, the MA-PDU session comprising a second communications path for communications between the communications device and the core network via infrastructure equipment of another radio access network of the wireless communications network, and communicating a small data transmission, SDT, along the first communications path when the communications device is in the inactive state, the communication of the SDT along the first communications path comprising receiving the SDT from the core network and transmitting the SDT to the communications device, or receiving the SDT from the communications device and transmitting the SDT to the core network.

2. A method according to claim 1, wherein the transmitting the SDT to the communications device comprises determining that an amount of data in the SDT received from the core network is below a predefined data amount threshold.

3. A method according to claim 1, comprising receiving an indication from the communications device that the communications device supports SDT when in the inactive state.

4. A method according to claim 1, comprising transmitting an indication to the communications device indicating that the communications device may transmit SDTs via the infrastructure equipment of the radio access network.

5. A method according to claim 1, wherein the communicating the SDT along the first communications path comprises transmitting an indication to the core network indicating that the communications device has been transitioned to the inactive state.

6. A method according to claim 1, comprising receiving, from the core network, an indication of Access Traffic Steering, Switching and Splitting, ATSSS, rules, the ATSSS rules being for determining whether to communicate SDTs with the communications device via the first communications path or the second communications path when the communications device is in the inactive state for communications with the infrastructure equipment of the radio access network and in the inactive state for communications with the infrastructure equipment of the other radio access network, and transmitting, based on the ATSSS rules, an indication to the communications device indicating that the communications device should transmit SDTs via the first communications path.

7. A method according to claim 1, comprisingreceiving, from the core network, an indication of Access Traffic Steering, Switching and Splitting, ATSSS, rules, the ATSSS rules being for determining whether to communicate SDTs with the communications device via the first communications path or the second communications path, the one or more ATSSS rules indicating that SDTs must be communicated via a communications path for which the communications device is in an inactive state, and transmitting, based on the ATSSS rules, an indication to the communications device indicating that the communications device should transmit SDTs via the first communications path.

8. A method of operating a communications device, the method comprising receiving an inactive state indication from infrastructure equipment of a radio access network, the inactive state indication indicating to the communications device to transition to an inactive state for communications via a first communications path of an MA-PDU session, the first communications path being used for communications between the communications device and a core network of the wireless communications network via the infrastructure equipment of the radio access network, the MA-PDU session comprising a second communications path for communications between the communications device and the core network via infrastructure equipment of another radio access network of the wireless communications network, and communicating a small data transmission, SDT, along the first communications path when the communications device is in the inactive state, the communication of the SDT along the first communications path comprising transmitting the SDT to the infrastructure equipment of the radio access network, or receiving the SDT from the infrastructure equipment of the radio access network.

9. A method according to claim 8, wherein the transmitting the SDT to the infrastructure equipment comprises determining that an amount of data in the SDT is below a pre-defmed data amount threshold.

10. A method according to claim 8, comprising transmitting an indication to the infrastructure equipment of the radio access network that the communications device supports SDT when in the inactive state.

11. A method according to claim 8, comprising receiving, from the infrastructure equipment of the radio access network, an indication indicating that the communications device may transmit SDTs via the infrastructure equipment of the radio access network.

12. A method according to claim 8, wherein the transmitting the SDT to the infrastructure equipment of the radio access network comprises determining to transmit the SDT to the infrastructure equipment of the radio access network via the first communications path based on one or more Access Traffic Steering, Switching and Splitting, ATSSS, rules, the one or more ATSSS rules being for determining whether to communicate SDTs with the communications device via the first communications path or the second communications path when the communications device is in the inactive state for communications with the infrastructure equipment of the radio access network and in the inactive state for communications with the infrastructure equipment of the other radio access network.

13. A method according to claim 8, wherein the transmitting the SDT to the infrastructure equipment of the radio access network comprisesdetermining to transmit the SDT to the infrastructure equipment of the radio access network via the first communications path based on one or more Access Traffic Steering, Switching and Splitting, ATSSS, rules, the one or more ATSSS rules indicating that SDTs must be communicated via a communications path for which the communications device is in an inactive state14. A method of operating a core network apparatus of a wireless communications network, the method comprising communicating a small data transmission, SDT, along a first communications path of a multiaccess protocol data unit, MA-PDU, session for communications between a communications device and the core network apparatus via infrastructure equipment of a radio access network of the wireless communications network, the MA-PDU session comprising a second communications path for communications between the communications device and the core network apparatus via infrastructure equipment of another radio access network, the communication of the SDT along the first communications path comprising transmitting the SDT to the infrastructure equipment of the radio access network for forwarding to the communications device when the communications device is in an inactive state for communications with the infrastructure equipment of the radio access network, or receiving the SDT from the infrastructure equipment of the radio access network.

15. A method according to claim 14, wherein the transmitting the SDT to the infrastructure equipment of the radio access network comprises determining that an amount of data in the SDT is below a pre-defined data amount threshold.

16. A method according to claim 14, comprising receiving an indication from the infrastructure equipment of the radio access network indicating that the communications device supports SDT when in the inactive state.

17. A method according to claim 14, comprising transmitting an indication to the infrastructure equipment of the radio access network, for forwarding to the communications device, indicating that communications device may transmit SDTs via the infrastructure equipment of the radio access network.

18. A method according to claim 14, wherein the communicating the SDT along the first communications path comprises receiving an indication from the infrastructure equipment of the radio access network indicating that the communications device has been transitioned to the inactive state.

19. A method according to claim 14, wherein the transmitting the SDT to the infrastructure equipment of the radio access network comprises determining to transmit the SDT to the infrastructure equipment of the radio access network based on one or more Access Traffic Steering, Switching and Splitting, ATSSS, rules, the ATSSS rules being for determining whether to communicate SDTs with the communications device via the first communications path or the second communications path when the communications device is in the inactive state for communications with the infrastructure equipment of the radio access network and in the inactive state for communications with the infrastructure equipment of the other radio access network.

20. A method according to claim 14, wherein the transmitting the SDT to the infrastructure equipment of the radio access network comprisesdetermining to transmit the SDT to the infrastructure equipment of the radio access network based on one or more Access Traffic Steering, Switching and Splitting, ATSSS, rules, the one or more ATSSS rules indicating that SDTs must be communicated via a communications path for which the communications device is in an inactive state.

21. Infrastructure equipment for a radio access network of a wireless communications network, the infrastructure equipment comprising a transmitter configured to transmit signals, a receive configured to receive signals, and a controller configured in combination with the transmitter and the receiver to transmit an inactive state indication to the communications device, the inactive state indication indicating to the communications device to transition to an inactive state for communications via a first communications path of a multi-access protocol data unit, MA-PDU, session, the first communications path being used for communications between the communications device and a core network of the wireless communications network via the infrastructure equipment of the radio access network, the MA- PDU session comprising a second communications path for communications between the communications device and the core network via infrastructure equipment of another radio access network of the wireless communications network, and communicate a small data transmission, SDT, along the first communications path when the communications device is in the inactive state, the communication of the SDT along the first communications path comprising receiving the SDT from the core network and transmitting the SDT to the communications device, or receiving the SDT from the communications device and transmitting the SDT to the core network.

22. Circuitry for infrastructure equipment for a radio access network of a wireless communications network, the circuitry comprising transmitter circuitry configured to transmit signals, receiver circuitry configured to receive signals, and controller circuitry configured in combination with the transmitter circuitry and the receiver circuitry to transmit an inactive state indication to the communications device, the inactive state indication indicating to the communications device to transition to an inactive state for communications via a first communications path of a multi-access protocol data unit, MA-PDU, session, the first communications path being used for communications between the communications device and a core network of the wireless communications network via the infrastructure equipment of the radio access network, the MA-PDU session comprising a second communications path for communications between the communications device and the core network via infrastructure equipment of another radio access network of the wireless communications network, and communicate a small data transmission, SDT, along the first communications path when the communications device is in the inactive state, the communication of the SDT along the first communications path comprising receiving the SDT from the core network and transmitting the SDT to the communications device, or receiving the SDT from the communications device and transmitting the SDT to the core network.

23. A communications device, the communications device comprising a transmitter configured to transmit signals, a receive configured to receive signals, and a controller configured in combination with the transmitter and the receiver to receive an inactive state indication from infrastructure equipment of a radio access network, the inactive state indication indicating to the communications device to transition to an inactive state for communications via a first communications path of an MA-PDU session, the first communications path being used for communications between the communications device and a core network of the wireless communications network via the infrastructure equipment of the radio access network, the MA-PDU session comprising a second communications path for communications between the communications device and the core network via infrastructure equipment of another radio access network of the wireless communications network, and communicate a small data transmission, SDT, along the first communications path when the communications device is in the inactive state, the communication of the SDT along the first communications path comprising transmitting the SDT to the infrastructure equipment of the radio access network, or receiving the SDT from the infrastructure equipment of the radio access network.

24. Circuitry for a communications device, the circuitry comprising transmitter circuitry configured to transmit signals, receiver circuitry configured to receive signals, and controller circuitry configured in combination with the transmitter circuitry and the receiver circuitry to receive an inactive state indication from infrastructure equipment of a radio access network, the inactive state indication indicating to the communications device to transition to an inactive state for communications via a first communications path of an MA-PDU session, the first communications path being used for communications between the communications device and a core network of the wireless communications network via the infrastructure equipment of the radio access network, the MA-PDU session comprising a second communications path for communications between the communications device and the core network via infrastructure equipment of another radio access network of the wireless communications network, and communicate a small data transmission, SDT, along the first communications path when the communications device is in the inactive state, the communication of the SDT along the first communications path comprising transmitting the SDT to the infrastructure equipment of the radio access network, or receiving the SDT from the infrastructure equipment of the radio access network.

25. A core network apparatus for a wireless communications network, the core network apparatus comprising a transmitter configured to transmit signals, a receive configured to receive signals, and a controller configured in combination with the transmitter and the receiver to communicate a small data transmission, SDT, along a first communications path of a multi-access protocol data unit, MA-PDU, session for communications between a communications device and the core network apparatus via infrastructure equipment of a radio access network of the wireless communications network, the MA-PDU session comprising a second communications path for communications between the communications device and the core network apparatus via infrastructure equipment of another radio access network, the communication of the SDT along the first communications path comprisingtransmitting the SDT to the infrastructure equipment of the radio access network for forwarding to the communications device when the communications device is in an inactive state for communications with the infrastructure equipment of the radio access network, or receiving the SDT from the infrastructure equipment of the radio access network.

26. Circuitry for a core network apparatus for a wireless communications network, the circuitry comprising transmitter circuitry configured to transmit signals, receiver circuitry configured to receive signals, and controller circuitry configured in combination with the transmitter circuitry and the receiver circuitry to communicate a small data transmission, SDT, along a first communications path of a multi-access protocol data unit, MA-PDU, session for communications between a communications device and the core network apparatus via infrastructure equipment of a radio access network of the wireless communications network, the MA-PDU session comprising a second communications path for communications between the communications device and the core network apparatus via infrastructure equipment of another radio access network, the communication of the SDT along the first communications path comprising transmitting the SDT to the infrastructure equipment of the radio access network for forwarding to the communications device when the communications device is in an inactive state for communications with the infrastructure equipment of the radio access network, or receiving the SDT from the infrastructure equipment of the radio access network.

27. A wireless communications network comprising infrastructure equipment according to claim 21, a core network apparatus according to claim 25 and a communications device according to claim 23.

28. A computer program comprising instructions which, when program is executed by a computer, cause the computer perform the method of claim 1.

29. A non-transitory computer-readable storage medium storing a computer program according to claim 28.

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