WO2016159841A1

WO2016159841A1 - Service continuity

Info

Publication number: WO2016159841A1
Application number: PCT/SE2015/050388
Authority: WO
Inventors: Pontus Wallentin; Qianxi Lu; Fredrik Gunnarsson
Original assignee: Telefonaktiebolaget Lm Ericsson (Publ)
Priority date: 2015-03-31
Filing date: 2015-03-31
Publication date: 2016-10-06

Abstract

According to an aspect, there is provided a method of operating a first radio access network, RAN, node (13) to maintain service continuity for a data session from a first terminal device (11) to a second terminal device (12) during a switch from routing the data session via the first RAN node (13) to routing the data session directly from the first terminal device (11) to the second terminal device (12). The method comprises establishing a first forwarding bearer to the first terminal device (11); and forwarding, to the first terminal device (11) via the first forwarding bearer, data packets of the data session from the first terminal device (11) that have not been received by the second terminal device (12) following initiation of the switch.

Description

SERVICE CONTINUITY

Technical Field

The techniques described herein relate to maintaining service continuity for a terminal device during a switch between an infrastructure routing path and a direct routing path.

Background

Device-to-device communication is a well-known and widely used component of many existing wireless technologies, including ad hoc and cellular networks. Examples include Bluetooth and several variants of the IEEE 802.1 1 standards suite such as Wi- Fi Direct. These systems operate in the unlicensed spectrum.

Recently, device-to-device (D2D) communications (also known as ProSe - Proximity Service - Direct Communication) have been proposed as an underlay to cellular networks to take advantage of the proximity of communicating terminal devices and at the same time to allow terminal devices to operate in a controlled interference environment. It has been suggested that such device-to-device communication share the same spectrum as the cellular system, for example by reserving some of the cellular uplink resources for device-to-device purposes. Allocating dedicated spectrum for device-to-device purposes is a less likely alternative as spectrum is a scarce resource and (dynamic) sharing between the device-to-device services and cellular services is more flexible and provides higher spectrum efficiency.

The transmission mode when sending data during D2D communication may be either: · Unicast - a specific terminal device (also known as a user equipment, UE) is the receiver

Multicast (may also be denoted groupcast) - a group of UEs are receivers Broadcast - all UEs are receivers With connectionless D2D communication, data can be sent from one terminal device to another terminal device without prior arrangement, thereby reducing the overhead and increasing the communication capacity which is crucial in emergency situations. The source terminal device transmits data to one (unicast) or more (multicast/groupcast/broadcast) other terminal devices, without first ensuring that the recipients are available and ready to receive the data. Connectionless communication may be used for one-to-one or one-to-many communication, but it is particularly effective for multicast and broadcast transmissions and thus well-suited for broadcast and group communication. The connectionless communication may be realized, e.g., via PHY (physical) unicast/multicast/groupcast/broadcast transmissions; with PHY broadcast transmissions, the transmissions may still be turned into unicast/groupcast/multicast at higher layers. For example, in the medium access control, MAC, layer, multicast or even unicast addresses may be used. Alternatively, if using broadcast on both PHY and MAC, multicast or unicast Internet Protocol, IP, addresses may be used at the IP layer. When a UE has coverage from the network, any D2D communication is controlled by the network nodes (such as the eNB). Since the radio resources in a cell (especially the uplink resources) are shared between traditional cellular communication and D2D communication, the eNB should divide and assign the radio resources for D2D communication.

In 3GPP (3^rd Generation Partnership Project) Release 12, a Sidelink UE Information message has been introduced as part of the radio resource control (RRC) protocol (3GPP TS 36.331 v12.5.0). This message is used whenever the UE needs to inform the eNB about need for E-UTRAN transmission and/or reception radio resources for ProSe communication or ProSe Discovery. For communication, the message contains a list of ProSe destinations, and an index associated to each of these. In case of multicast communication, a ProSe destination is a ProSe Layer 2 Group identity. The index may be used as a 4-bit short reference to a given destination, e.g. as used in a MAC Buffer Status Report when transmitting data to the destination.

Moreover, a given unicast traffic session between two UEs may use either a direct communication path or an infrastructure communication path. When using the direct communication path (also known as "sidelink"), the data is transmitted directly between the UEs using D2D communication channels. On the other hand, when using the infrastructure communication path, the data is instead transmitted via one or more network nodes that use non-D2D legacy physical (uplink and downlink) channels, and the packets are transmitted over an EPS (evolved packet system) bearer, which is effectively a tunnel between the UE and the packet data network (PDN) gateway (GW) network node. The latter case is only available when both UEs are in coverage of the network. A service continuity switch, in the context of ProSe communication, is the procedure for moving a user traffic session from the direct communication path to the infrastructure communication path, or vice versa. Service continuity switching for ProSe will likely be included in 3GPP Release 13.

There are two scenarios for service continuity switching. In the first scenario, which is known as a "two UE" scenario (an example of which is shown in Figure 1), two UEs can switch to using a ProSe Direct Communication path when in coverage by the network and the UEs are within proximity of each other. As shown in Figure 1 (a), a first UE (UE1) 11 is in coverage of a first eNB (eNB1) 13 and is communicating via an infrastructure path with a second UE (UE2) 12 that is in coverage of a second eNB (eNB2) 14). This path is shown by arrow 16. However, as shown in Figure 1 (b), when the UEs are close enough to each other, the UEs 12, 16 can switch to using a direct communication path, as indicated by dashed arrow 18. For this scenario, UE2 is referred to as the "peer UE" of UE1 , and vice versa.

In the second scenario, which is known as a "one UE" scenario (an example of which is shown in Figure 2), a user traffic session is maintained even when a UE goes between being in coverage of the network and out of coverage of the network. In this scenario, a first UE (UE1) 21 is communicating with an eNB (eNB1) 22 that generally has a coverage area indicated by dotted line 28 can communicate 'normally' with eNB 22 while in coverage (indicated by the solid arrow). Communications to/from UE1 therefore pass via eNB1 22, a packet data network (PDN) gateway (PGW) 23 (PGW1 ) and a PDN 24. If UE1 21 loses coverage from eNB1 22, a second UE (UE2) 25 can act as a relay between UE1 and the PDN 24. In this example, UE2 25 is served by a different eNB to UE1 (eNB2 26) and a different PGW (PGW2 27). In this scenario UE2 25 is referred to as the "relay UE" of UE1 21 , and UE1 21 the "remote UE" of UE2 25. The relay path is shown by the dashed lines. The objective of service continuity is to keep the session between the two UEs (in the first scenario) or the session between the first UE (UE1) and the network (in the second scenario) considering the UE mobility (either both UE1 and UE2 in the first scenario or only UE1 in the second scenario). In particular, it is an objective to provide lossless switching in the above scenarios (i.e. to perform switching while ensuring that all data packets reach the intended destination). Summary

In the two UE scenario, there may be some packets that have been sent by a transmitting UE to the network that have not yet arrived at the receiving UE at the point that the infrastructure path to the receiving UE fails (e.g. due to the receiving UE moving out of coverage of the network). Therefore during the switching from the infrastructure path to the direct path the packets needs to be forwarded to the direct path in order to achieve lossless switching.

In the one UE scenario, there may be some downlink packets in the RAN that have been sent by a PGW into the corresponding GPRS (General Packet Radio Service) tunnel protocol (GTP) tunnel for a remote UE in the infrastructure path or for a relay UE in the relay path, and that have not yet arrived at the UE (remote UE or relay UE), which may be lost if the path switching only happens above the RAN (e.g. in the core network). Therefore during switching between the infrastructure path and the direct path, the packets in the source path would need to be forwarded to target path in order to achieve lossless switching.

The aspects set out below provide ways to address the problem of achieving lossless switching in the two UE and one UE scenarios.

Therefore, according to a first aspect, there is provided a method of operating a first radio access network, RAN, node to maintain service continuity for a data session from a first terminal device to a second terminal device during a switch from routing the data session via the first RAN node to routing the data session directly from the first terminal device to the second terminal device. The method comprises establishing a first forwarding bearer to the first terminal device; and forwarding, to the first terminal device via the first forwarding bearer, data packets of the data session from the first terminal device that have not been received by the second terminal device following initiation of the switch.

According to a second aspect, there is provided a method of operating a first terminal device to maintain service continuity for a data session from the first terminal device to a second terminal device during a switch from routing the data session via a first radio access network, RAN, node to routing the data session directly from the first terminal device to the second terminal device. The method comprises establishing a forwarding bearer with the first RAN node; receiving, from the first RAN node via the forwarding bearer, data packets of the data session previously sent from the first terminal device to the second terminal device that have not been received by the second terminal device following initiation of the switch; and forwarding the received data packets directly to the second terminal device.

According to a third aspect, there is provided a method of operating a first radio access network, RAN, node to maintain service continuity for a data session to a first terminal device during a switch from a first routing path to a second routing path, the first and second routing paths comprising a routing path in which the data session is routed via a RAN node that is serving the first terminal device, and a routing path in which the data session is routed to the first terminal device via a relay terminal device. The first RAN node is in the first routing path and the method comprises establishing a first forwarding bearer to a gateway node in the first routing path; and forwarding, to the gateway node via the first forwarding bearer, data packets of the data session that have not been received by the first terminal device following initiation of the switch.

According to a fourth aspect, there is provided a method of operating a first gateway node to maintain service continuity for a data session to a first terminal device during a switch from a first routing path to a second routing path, the first and second routing paths comprising a routing path in which the data session is routed via a radio access network, RAN, node that is serving the first terminal device, and a routing path in which the data session is routed to the first terminal device via a relay terminal device. The first gateway node is in the first routing path and the method comprises establishing a first forwarding bearer with a RAN node in the first routing path; receiving, from the RAN node via the first forwarding bearer, data packets of the data session that have not been received by the first terminal device following initiation of the switch; and forwarding the received data packets to a node in the second routing path.

According to further aspects, there are provided terminal devices, radio access network nodes, gateway nodes, and computer program products corresponding to the methods recited above.

Brief Description of the Drawings

Exemplary embodiments of the techniques introduced in this document are described below with reference to the following figures, in which: Figure 1 illustrates communications in a two UE scenario;

Figure 2 illustrates communications in a one UE scenario; Figure 3 is a non-limiting example block diagram of a Long Term Evolution (LTE) cellular communications network;

Figure 4 is a block diagram of a radio access network (RAN) node according to an embodiment;

Figure 5 is a block diagram of a terminal device according to an embodiment;

Figure 6 is a block diagram of a gateway node according to an embodiment; Figure 7 is a signalling diagram illustrating the signalling during a switch from routing data via an infrastructure path to routing data via a direct path in the two UE scenario;

Figure 8 is a flow chart illustrating a method of operating a RAN node according to an embodiment;

Figure 9 is a flow chart illustrating a method of operating a terminal device according to an embodiment;

Figure 10 is a signalling diagram illustrating the signalling during a switch from routing data via an infrastructure path to routing data via a direct (relay) path in the one UE scenario;

Figure 11 is a signalling diagram illustrating the signalling during a switch from routing data via a direct (relay) path to routing data via an infrastructure path in the two UE scenario;

Figure 12 is a flow chart illustrating a method of operating a RAN node according to an embodiment; and Figure 13 is a flow chart illustrating a method of operating a gateway node according to an embodiment. Detailed Description

The following sets forth specific details, such as particular embodiments for purposes of explanation and not limitation. But it will be appreciated by one skilled in the art that other embodiments may be employed apart from these specific details. In some instances, detailed descriptions of well-known methods, nodes, interfaces, circuits, and devices are omitted so as not obscure the description with unnecessary detail. Those skilled in the art will appreciate that the functions described may be implemented in one or more nodes using hardware circuitry (e.g., analog and/or discrete logic gates interconnected to perform a specialized function, ASICs, PLAs, etc.) and/or using software programs and data in conjunction with one or more digital microprocessors or general purpose computers. Nodes that communicate using the air interface also have suitable radio communications circuitry. Moreover, the technology can additionally be considered to be embodied entirely within any form of computer-readable memory, such as solid-state memory, magnetic disk, or optical disk containing an appropriate set of computer instructions that would cause a processor to carry out the techniques described herein.

Hardware implementation may include or encompass, without limitation, digital signal processor (DSP) hardware, a reduced instruction set processor, hardware (e.g., digital or analog) circuitry including but not limited to application specific integrated circuit(s) (ASIC) and/or field programmable gate array(s) (FPGA(s)), and (where appropriate) state machines capable of performing such functions. In terms of computer implementation, a computer is generally understood to comprise one or more processors, one or more processing modules or one or more controllers, and the terms computer, processor, processing module and controller may be employed interchangeably. When provided by a computer, processor, or controller, the functions may be provided by a single dedicated computer or processor or controller, by a single shared computer or processor or controller, or by a plurality of individual computers or processors or controllers, some of which may be shared or distributed. Moreover, the term "processor" or "controller" also refers to other hardware capable of performing such functions and/or executing software, such as the example hardware recited above. Although the description is given for user equipment (UE), it should be understood by the skilled in the art that "UE" is a non-limiting term comprising any mobile or wireless device or node equipped with a radio interface allowing for at least one of: transmitting signals in uplink (UL), receiving and/or measuring signals in downlink (DL), and transmitting and/or receiving signals in a D2D/sidelink mode. A UE herein may comprise a UE (in its general sense) capable of operating or at least performing measurements in one or more frequencies, carrier frequencies, component carriers or frequency bands. It may be a "UE" operating in single- or multi-radio access technology (RAT) or multi-standard mode. As well as "UE", the terms "mobile device" and "terminal device" may be used interchangeably in the following description, and it will be appreciated that such a device does not necessarily have to be 'mobile' in the sense that it is carried by a user. Instead, the term "mobile device" encompasses any device that is capable of communicating with communication networks that operate according to one or more mobile communication standards, such as the Global System for Mobile communications, GSM, Universal Mobile Telecommunications System (UMTS), Long- Term Evolution, LTE, etc.

A cell is associated with a base station, where a base station comprises in a general sense any network node transmitting radio signals in the downlink (DL) and/or receiving radio signals in the uplink (UL). Some example base stations, or terms used for describing base stations, are eNodeB, eNB, NodeB, macro/micro/pico/femto radio base station, home eNodeB (also known as femto base station), relay, repeater, sensor, transmitting-only radio nodes or receiving-only radio nodes. A base station may operate or at least perform measurements in one or more frequencies, carrier frequencies or frequency bands and may be capable of carrier aggregation. It may also be a single-radio access technology (RAT), multi-RAT, or multi-standard node, e.g., using the same or different base band modules for different RATs.

It should be noted that use of the term "network node" as used herein can refer to a base station, such as an eNodeB, a network node in the RAN responsible for resource management, such as a radio network controller (RNC), or, in some cases, a core network node, such as a mobility management entity (MME), a ProSe function (ProSe- F) node or a ProSe Application Server.

Unless otherwise indicated herein, the signalling described is either via direct links or logical links (e.g. via higher layer protocols and/or via one or more network nodes). Figure 3 shows an example diagram of an evolved UMTS Terrestrial Radio Access Network (E-UTRAN) architecture as part of a Long Term Evolution (LTE)-based communications system 32. Nodes in the core network 34 include one or more Mobility Management Entities (MMEs) 35, a key control node for the LTE access network, one or more Serving Gateways (SGWs) 36 which route and forward user data packets while acting as a mobility anchor, a ProSe Function node 37, a ProSe Application Server 38 and a home subscriber server (HSS) 39. The ProSe Function node 37 is used for network related actions required for ProSe, such as provisioning the UEs with necessary parameters to use ProSe, and network support for ProSe direct discovery and EPC-level discovery. The Application server 38 provides network functionality required by the application in the UEs based on ProSe communication and/or discovery, for example a Mission-Critical Push-To-Talk (MCPTT) application server. The application server 38 is connected with the ProSe-Function node 37, which in turn is connected with the HSS 39 that is a database that contains user-related and subscriber-related information. The HSS 39 is connected to the MME 35. The MME(s) 35 and SGW(s) 36 communicate with base stations 40 referred to in LTE as eNBs, over an interface, for example an S1 interface. The eNBs 40 can include the same or different categories of eNBs, e.g. macro eNBs, and/or micro/pico/femto eNBs. The eNBs 40 communicate with each other over an interface, for example an X2 interface. The S1 interface and X2 interface are defined in the LTE standard. A UE 42 is shown, and a UE 42 can receive downlink data from and send uplink data to one of the base stations 40 with that base station 40 being referred to as the serving base station of the UE 42.

In various embodiments, UE 42 is configured or adapted to be able to communicate with another UE in D2D mode (i.e. communicate directly with the other UE without routing the communications via one or more eNBs 40). UE 42 may also be configured or adapted to be able to communicate with another UE in local routing mode (i.e. communicate with the other UE via one or more eNBs 40). In this case, the eNBs are configured or adapted to enable local routing between UEs 40. Figure 4 shows a radio access network, RAN, node that can be adapted or configured to operate according to the example embodiments described. In some embodiments the network node can be a base station 40 (for example a NodeB or an eNodeB as shown in Figures 1 and 2) or other node in the radio access network (RAN) of the network 32.

The RAN node 40 comprises a processor or processing module 60 that controls the operation of the RAN node. The processing module 60 can be connected to a transceiver module 62 (which comprises a receiver and a transmitter) with associated antenna(s) 64 which are used to transmit signals to, and receive signals from, UEs 42 in the network 32. The RAN node also comprises a memory or memory module 66 that is connected to the processing module 60 and that contains instructions or computer code executable by the processor 60 and other information or data required for the operation of the network node. The RAN node also includes components and/or circuitry 68 for allowing the RAN node to exchange information with other network nodes. For example the circuitry 68 can allow the RAN node to communicate with other RAN nodes via an X2 interface and/or communicate with network nodes in the core network via an S1 or other type of interface. It will be appreciated that RAN nodes for use in other types of network (e.g. UMTS Terrestrial Radio Access Network (UTRAN) or Wideband Code Division Multiple Access (WCDMA) RAN) will include similar components to those shown in Figure 4 and appropriate interface circuitry 68 for enabling communications with the other network nodes in those types of networks (e.g. other base stations, mobility management nodes and/or nodes in the core network).

Figure 5 shows a terminal device 42 or user equipment (UE) that can be adapted for use in one or more of the non-limiting example embodiments described. The terminal device 42 comprises a processing module 70 that controls the operation of the terminal device 42. The processing unit 70 is connected to receiver circuitry, transmitter circuitry or transceiver circuitry 72 (which comprises a receiver and a transmitter) with associated antenna(s) 74 which are used to receive signals from or both transmit signals to and receive signals from a radio access network, such as RAN node 40 in the LTE network 32. Circuitry 72 is also configured to enable the terminal device 42 to communicate with other terminal devices 42 using D2D communications. The terminal device 42 also comprises a memory module 76 that is connected to the processing module 70 and that stores computer program code and other information and data required for the operation of the terminal device 42.

Figure 6 shows a gateway node 23; 27 that can be used in the example embodiments described. The gateway node 23; 27 can be a node that controls the operations of one or more RAN nodes 40. The gateway node 23; 27 can be a PGW 23; 27 or a SGW 36. The gateway node 23; 27 comprises a processing module 82 that controls the operation of the gateway node 23; 27. The processing module 82 is connected to inter-node interface circuitry 84 for allowing the gateway node 23; 27 to exchange information with RAN nodes 40 with which it is associated. The gateway node 23; 27 also comprises a memory module 86 that is connected to the processing unit 82 and that stores computer program code and other information and data required for the operation of the gateway node 23; 27. It will be appreciated that only the components of the terminal device 42, RAN node 40 and gateway node 23; 27 required to explain the embodiments presented herein are illustrated in Figures 4, 5 and 6.

Two UE scenario

As noted above and shown in Figures 1 (a) and (b), in this scenario two UEs that are communicating with each other via an infrastructure path (e.g. via one or more eNBs and also possibly a core network) can switch to using a ProSe Direct Communication path when the UEs are within proximity of each other. At the point that the infrastructure path fails (e.g. where UE2 12 moves out of coverage of eNB2) - which can be the trigger to switch to the direct routing path 18, there may be some packets sent by UE1 11 that have not yet arrived at UE2 12.

To provide lossless switching, the packets need to be forwarded to the direct path during the switching from the infrastructure path to the direct path. Figure 7 illustrates the signalling used to achieve lossless signalling according to an embodiment.

Figure 7 shows the signalling between UE1 1 1 , the serving eNB 13 of UE1 (eNB1), UE2 12 (the UE2 that loses coverage with the network), and eNB2 14 (the serving eNB of UE2 before UE2 loses coverage). As indicated by arrow 701 , the two UEs have a communication session that uses conventional cellular mode communication for the packet transmission (which is also known as the infrastructure routing path). At step 702 a decision is made to switch to use D2D communication for the packets in this communication session. The trigger for the decision can be one or more measurement reports from one or both UEs, and the entity taking the decision can be a network node on either the UE1 side or UE2 side (e.g. an eNB, an MME, a PGW, or even a ProSe-F node). The switching decision, in the general case, is performed per direction. In this example, the switching decision is made for the packets being sent from UE1 to UE2. The packets being sent in the opposite direction can therefore be subject to a separate switching decision (although this does not have to be the case).

To avoid packets sent by UE1 that have not yet reached UE2 when the switch is triggered from being lost, forwarding bearers are established between eNB2, eNB1 and UE1 to enable the packets to be returned to UE1 so that they can be retransmitted by UE1 to UE2 when the direct communication path is established. eNB2 therefore sends a signal 703 to eNB1 to establish a forwarding bearer to eNB1 (although in some embodiments eNB1 can send signal 703 to eNB2 instead), and eNB1 sends a signal 704 to UE1 to establish a forwarding bearer to UE1.

Signal 703 can be sent via the X2 interface between the eNBs (although optionally it can be sent through SGW 36 via the S1 interface), and can make use of existing X2 HO preparation signalling.

The forwarding bearer to UE1 can reuse the existing data radio bearer (DRB) for UE1 , which means it possibly co-exists with other Internet Protocol, IP, flows in the same bearers, so that UE1 can only differentiate the packet at IP layer or higher, e.g., based on the source/destination IP address. Alternatively, the forwarding bearer can be a dedicated DRB for UE1 which means it can have dedicated QoS (quality of service) control with respect to the direct path, so that UE1 can differentiate the packets at a lower layer, based on the LCID (logical channel ID). So that UE1 is aware that data packets received via the forwarding bearer are to be sent to UE2, eNB1 provides an indication to UE1 that data packets received via the bearer are to be sent to UE2. In some embodiments, eNB1 can indicate to UE1 that the forwarding bearer is linked to a bearer used for the retransmission of packets to UE2, and vice versa, and this link can be signalled when the forwarding bearer and/or the other bearer is established.

Once the forwarding bearers have been established, eNB2 forwards any data packets from UE1 that have not been received by UE2 (which can include packets stored or buffered in UE2 that were not sent to UE2 before the switch was triggered, and/or packets that were sent to UE2 but not acknowledged by UE2) to eNB1 (signal 705). eNB1 then forwards the data packets to UE1 (signal 706) via the forwarding bearer to UE1.

Once the direct communication path is established between UE1 and UE2, UE1 forwards the data packets received from eNB1 directly to UE2 (indicated by signal 707).

Once those data packets have been forwarded, UE1 can start to transmit new data packets to UE2 using the direct communication path (signal 708).

In some embodiments, there may be signalling to indicate that the switch has or is about to occur, and/or that data forwarding has started. This signalling may indicate to UE1 to stop the transmission along the cellular path, and be ready to send the forwarded packets (received from eNB1) to UE2 via the direct path. This signalling may indicate to UE2 (assuming that eNB2 can still send signals to UE2) that it should stop the reception of data packets along the cellular path, and be ready to receive data packets (included any forwarded data packets) from UE1 via a direct path.

In some embodiments, eNB1 may indicate the end of the forwarded packets (i.e. the last of the forwarded packets) to UE1 so that UE1 knows when to start the transmission of new data packets to UE2 via the direct communication path 708. In this case, an 'End marker' or other signal indicating a last data packet can be sent from eNB1 to UE1 at the point that uplink transmission of data packets from UE1 to UE2 is prevented. After an 'End marker' is rerouted from eNB2 to eNB1 , UE1 can be notified of the end marker in the form of Uu data/signalling, e.g., RRC or MAC CE signalling, empty packet data convergence protocol (PDCP)/radio link control (RLC)/MAC packets, etc.

Although in the embodiment of Figure 7 the forwarding bearers are established after the switching decision (step 702) is taken, it will be appreciated that in some embodiments the forwarding bearers may already be established when the switching decision is taken. For example, the forwarding bearers may be established when the data session with UE1 is set up. In addition or alternatively, forwarding bearers can be established when a switch may be imminent, for example based on a measurement report from a UE.

The flow chart in Figure 8 illustrates a method of operating a RAN node according to an embodiment. The RAN node corresponds to eNB1 13 in Figure 7. The RAN node 13 is operated to maintain service continuity for a data session from a first terminal device 1 1 to a second terminal device 12 during a switch from routing the data session via the RAN node 13 to routing the data session directly from the first terminal device 1 1 to the second terminal device 12.

In a first step, step 801 , a forwarding bearer is established to the first terminal device.

Once the forwarding bearer is established, data packets of the data session from the first terminal device 1 1 that have not been received by the second terminal device 12 following initiation of the switch are forwarded to the first terminal device 11 via the first forwarding bearer (step 803).

The flow chart in Figure 9 illustrates a method of operating a terminal device according to an embodiment. The terminal device corresponds to UE1 in Figure 7. The terminal device is operated to maintain service continuity for a data session from the terminal device 1 1 to a second terminal device 12 during a switch from routing the data session via a first RAN node 13 to routing the data session directly from the terminal device 1 1 to the second terminal device 12.

In a first step, step 901 , a forwarding bearer is established with the first RAN node 13. Then, the terminal device 11 receives, from the first RAN node 13 via the forwarding bearer, data packets of the data session previously sent from the terminal device 1 1 to the second terminal device 12 that have not been received by the second terminal device 12 following initiation of the switch (step 903).

The received data packets are then forwarded directly to the second terminal device 12 (step 905).

One UE scenario - switching from an infrastructure path to a direct (relay) path

As noted above and shown in Figure 2, in this scenario data being sent to and/or from a first UE (UE1) 21 passes via eNB1 22, PGW1 23 and PDN 24. This is referred to as the infrastructure path. If UE1 21 loses coverage from eNB1 22, a second UE (UE2) 25 can act as a relay between UE1 and the PDN 24, and in Figure 2 UE2 25 is served by eNB2 26 and PGW2 27. In this scenario UE2 25 is referred to as the "relay UE" of UE1 21 , and UE1 21 the "remote UE" of UE2 25. Thus, when UE1 loses coverage from the network, or a switch to a direct path is otherwise triggered, there may be some downlink packets in the RAN (e.g. at eNB1) that have not yet arrived at UE1 (or that UE1 has not yet acknowledged receipt of). To avoid these packets being lost during the switch from the infrastructure path to the direct path, the packets in the infrastructure path need to be forwarded to the direct path.

Figure 10 illustrates the signalling used to achieve lossless signalling according to an embodiment. Figure 10 shows the signalling between UE1 21 (the remote UE that loses coverage from the network), the serving eNB of UE1 (eNB1 22), the serving PGW of UE1 (PGW1 23), the PDN 24, UE2 25 (the relay UE), eNB2 26 (the serving eNB of UE2) and the serving PGW of UE2 (PGW2 27). As indicated by the group of signals labelled 1001 , UE1 has a PDN connection towards the network in which data packets for UE1 are transmitted over evolved packet system (EPS) bearers using an infrastructure communication path (i.e. from PDN 24 to PGW1 , from PGW1 to eNB1 and from eNB1 to UE1). At step 1002 a decision is made to switch UE1 to using UE2 as a relay UE for data packets in UE1's communication session. The trigger for the decision can be one or more measurement reports from one or both UEs, and the entity taking the decision can be a network node on either the UE1 side or UE2 side (e.g. an eNB, an MME, a PGW, or even a ProSe-F node). To avoid packets sent to UE1 from the PDN 24 that have not yet reached UE1 when the switch is triggered from being lost, forwarding bearers are established between eNB1 , PGW1 and PGW2 to enable the packets to be rerouted to PGW2 and then to UE1 via the relay UE (UE2) once the direct communication path is established. PGW1 therefore sends a signal 1003 to PGW2 to establish a forwarding bearer to PGW2 (although in some embodiments PGW2 can send signal 1003 to PGW1 instead), and PGW1 sends a signal 1004 to eNB1 to establish a forwarding bearer from eNB1 to PGW1 (although in some embodiments eNB1 can send signal 1004 to PGW1 instead).

In some embodiments signal 1003 can reuse the registration request/response signalling in a mobile IP procedure to establish the forwarding bearer (e.g. IP tunnel) between the two PGWs. Once the forwarding bearers have been established, eNB1 forwards any data packets from PDN 24 that have not been received by UE1 (which can include packets stored or buffered in eNB1 that were not sent to UE1 before the switch was triggered, and/or packets that were sent to UE1 but not acknowledged by UE1) back up to PGW1 via the forwarding bearer (signal 1005).

PGW1 then forwards the data packets to PGW2 (signal 1006) via the forwarding bearer between the PGWs, and PGW2 reroutes the packets back to the GTP tunnel of the relay UE. Thus, PGW2 sends the data packets to the serving eNB of UE2 (eNB2 26), signal 1007. In turn eNB2 transmits the data packets to UE2 (signal 1008), and UE2 transmits the data packets to UE1 (signal 1009) when the direct communication path is established between UE1 and UE2.

Once the direct communication path is established between UE1 and UE2 and the forwarding of data packets via the forwarding bearers is completed, new data packets from PDN 24 are sent to UE1 via PGW2, eNB2 and UE2 (indicated by the group of signals labelled 1010). In some embodiments, there may be signalling to indicate that the switch has or is about to occur and/or that data forwarding has started. This signalling may be from PGW1 to eNB1 , or from an MME 35 to eNB1. On receiving this or another indication eNB1 stops the downlink transmission of data packets to UE1 , and data packets received from PGW1 are sent back to PGW1 on the forwarding bearer, so that PGW1 can forward them to the relay UE side.

In some embodiments, when the switch is triggered, PGW1 may indicate the end of the downlink data packets (i.e. the last of the data packets for UE1 sent to eNB1) to eNB1. When eNB1 forwards the last of the data packets to PGW1 , eNB1 includes an 'end marker' or other signal indicating a last data packet to indicate this to PGW1. PGW1 can send the end marker to PGW2 after the last data packet is forwarded from PGW1 to PGW2 so PGW2 knows when to start the transmission of new data packets to UE1 via UE2. In some embodiments the 'end marker' might go through multiple PGWs to reach UE1 , so PGWs require the capability to forward the end marker information as a special IP packet.

It will be appreciated that in the event that both UE1 and UE2 are being served by the same PGW (e.g. PGW1 ), no inter-PGW forwarding bearer is required and signal 1006 is not required. In this case, eNB1 establishes the forwarding bearer with PGW1 , forwards any data packets to PGW1 , and then PGW1 sends them to eNB2.

Although in the embodiment of Figure 10 the forwarding bearers are established after the switching decision (step 1002) is taken, it will be appreciated that in some embodiments the forwarding bearers may already be established when the switching decision is taken. For example, the forwarding bearers may be established when the data session with UE1 is set up. In addition or alternatively, forwarding bearers can be established when a switch may be imminent, for example based on a measurement report from a UE.

One UE scenario - switching from a direct (relay) path to an infrastructure path

As noted above and shown in Figure 2, in this scenario data being sent to and/or from a first UE (UE1) 21 passes via a relay UE (UE2 25), eNB2 26 (the serving eNB of UE2), PGW2 27 (the serving PGW of UE2) and PDN 24. This is referred to as the direct or relay path. The direct path may have been established due to UE1 losing coverage from the network (although other reasons are possible). If UE1 21 subsequently regains coverage from the network (e.g. it is within the coverage of eNB1 22), it can switch back to using an infrastructure path via eNB1. Thus, when UE1 regains coverage from the network, or a switch to an infrastructure path is otherwise triggered, there may be some downlink packets in the RAN (e.g. at eNB2) or at UE2 that have not yet arrived at UE1 (or that UE1 has not yet acknowledged receipt of). To avoid these packets being lost during the switch from the direct path to the infrastructure path, the packets in the direct path need to be forwarded to the infrastructure path.

Figure 1 1 illustrates the signalling used to achieve lossless signalling according to an embodiment. Figure 1 1 shows the signalling between UE1 21 (the remote UE that is receiving data packets via a relay UE), the serving eNB of UE1 (eNB1 22), the serving PGW of UE1 (PGW1 23), the PDN 24, UE2 25 (the relay UE), eNB2 26 (the serving eNB of UE2) and the serving PGW of UE2 (PGW2 27). As indicated by the group of signals labelled 1101 , UE1 is receiving data packets from PDN 24 via UE2 25 (i.e. from PDN 24 to PGW2, from PGW2 to eNB2, from eNB2 to UE2 and from UE2 to UE1).

At step 1102 a decision is made to switch UE1 to using the infrastructure path (i.e. to stop using UE2 as a relay UE for data packets in UE1 's communication session). The trigger for the decision can be one or more measurement reports from one or both UEs, and the entity taking the decision can be a network node on either the UE1 side or UE2 side (e.g. an eNB, an MME, a PGW, or even a ProSe-F node). To avoid packets sent to UE1 from the PDN 24 that have not yet reached UE1 when the switch is triggered from being lost, forwarding bearers are established between eNB2, PGW2 and PGW1 to enable the packets to be rerouted to PGW1 and then to UE1 once the infrastructure communication path is established. PGW2 therefore sends a signal 1 103 to PGW1 to establish a forwarding bearer to PGW1 (although in some embodiments PGW1 can send signal 1 103 to PGW2 instead), and PGW2 sends a signal 1104 to eNB2 to establish a forwarding bearer from eNB2 to PGW2 (although in some embodiments eNB2 can send signal 1104 to PGW2 instead). The forwarding bearer from eNB2 to PGW2 requires bearer differentiation (between the relay UE (UE2) and the remote UE (UE1), or between multiple remote UEs) capability at eNB2, so that different GTP tunnels would be used by different UEs (relay UE and remote UEs), and to map to a single DRB between eNB2 and UE1. Thus in some embodiments signal 1 103 can reuse the registration request/response signalling in a mobile IP procedure to establish the forwarding bearer (e.g. IP tunnel) between the two PGWs.

Once the forwarding bearers have been established, eNB2 forwards any data packets from PDN 24 that have not been received by UE1 (which can include packets stored or buffered in eNB2 that were not sent to UE2 for relaying to UE1 before the switch was triggered, and/or packets that were sent to UE1 but not acknowledged by UE1) back up to PGW2 via the forwarding bearer (signal 1 105).

PGW2 then forwards the data packets to PGW1 (signal 1 106) via the forwarding bearer between the PGWs, and PGW1 reroutes the packets back to UE1 via the infrastructure path. Thus, PGW1 sends the data packets to the serving eNB of UE1 (eNB1 22), signal 1 107. eNB1 then transmits the data packets to UE1 (signal 1108) when the infrastructure communication path is established between UE1 and eNB1. Once the infrastructure communication path is established between UE1 and eNB1 and the forwarding of data packets via the forwarding bearers is completed, new data packets from PDN 24 are sent to UE1 via PGW1 and eNB1 (indicated by the group of signals labelled 1109). In some embodiments, there may be signalling to indicate that the switch has or is about to occur and/or that data forwarding has started. This signalling may be from PGW2 to eNB2, or from an MME 35 to eNB2. On receiving this or another indication eNB2 stops the downlink transmission of data packets to UE1 via UE2, and data packets received from PGW2 are sent back to PGW2 via the forwarding bearer, so that PGW2 can forward them to the remote UE side. In some embodiments, when the switch is triggered, PGW2 may indicate the end of the downlink data packets (i.e. the last of the data packets for UE1 sent to eNB2) to eNB2. When eNB2 forwards the last of the data packets to PGW2, eNB2 includes an 'end marker' or other signal indicating a last data packet to indicate this to PGW2. PGW2 can send the end marker to PGW1 after the last data packet is forwarded from PGW2 to PGW1 so PGW1 knows when to start the transmission of new data packets to UE1 via eNB1. In some embodiments the 'end marker' might go through multiple PGWs to reach UE1 , so PGWs require the capability to forward the end marker information as a special IP packet.

It will be appreciated that in the event that both UE1 and UE2 are being served by the same PGW (e.g. PGW1 ), no inter-PGW forwarding bearer is required and signal 1 106 is not required. In this case, eNB2 establishes the forwarding bearer with PGW1 , forwards any data packets to PGW1 , and then PGW1 sends them to eNB1.

Although in the embodiment of Figure 1 1 the forwarding bearers are established after the switching decision (step 1 102) is taken, it will be appreciated that in some embodiments the forwarding bearers may already be established when the switching decision is taken. For example, the forwarding bearers may be established when the data session with UE1 is set up. In addition or alternatively, forwarding bearers can be established when a switch may be imminent, for example based on a measurement report from a UE.

The flow chart in Figure 12 illustrates a method of operating a RAN node according to an embodiment. The RAN node corresponds to eNB1 22 in Figure 10 and eNB2 26 in Figure 1 1.

The RAN node is operated to maintain service continuity for a data session to a first terminal device 21 during a switch from a first routing path to a second routing path (the RAN node is in the first routing path). The first and second routing paths comprises a routing path in which the data session is routed via a RAN node 22 that is serving the first terminal device 21 , and a routing path in which the data session is routed to the first terminal device 21 via a relay terminal device 25. In a first step, step 1201 , a forwarding bearer is established to a gateway node 23, 27 in the first routing path.

Then, in step 1203, data packets of the data session that have not been received by the first terminal device 21 following initiation of the switch are forwarded to the gateway node 23, 27 via the first forwarding bearer.

The flow chart in Figure 13 illustrates a method of operating a gateway node according to an embodiment. The gateway node corresponds to PGW1 23 in Figure 10 and PGW2 27 in Figure 11.

As with the RAN node above, the gateway node is operated to maintain service continuity for a data session to a first terminal device 21 during a switch from the first routing path to the second routing path (with the gateway node being in the first routing path).

In a first step, step 1301 , a forwarding bearer is established with a RAN node 22, 26 in the first routing path. The gateway node then receives, from the RAN node via the first forwarding bearer, data packets of the data session that have not been received by the first terminal device 21 following initiation of the switch (step 1303).

The received data packets are then forwarded to a node in the second routing path, such as a RAN node or another gateway node (step 1305).

Thus, the techniques described above allow for service continuity during switching between infrastructure and direct routing paths to reduce packet loss and session interruptions due to packet re-transmission.

Modifications and other variants of the described embodiment(s) will come to mind to one skilled in the art having the benefit of the teachings presented in the foregoing descriptions and the associated drawings. Therefore, it is to be understood that the embodiment(s) is/are not to be limited to the specific examples disclosed and that modifications and other variants are intended to be included within the scope of this disclosure. Although specific terms may be employed herein, they are used in a generic and descriptive sense only and not for purposes of limitation.

Various exemplary embodiments are set out in the following statements:

1. A method of operating a first radio access network, RAN, node to maintain service continuity for a data session from a first terminal device to a second terminal device during a switch from routing the data session via the first RAN node to routing the data session directly from the first terminal device to the second terminal device, the method comprising

establishing a first forwarding bearer to the first terminal device; and

forwarding, to the first terminal device via the first forwarding bearer, data packets of the data session from the first terminal device that have not been received by the second terminal device following initiation of the switch.

2. A method as defined in statement 1 , wherein the first RAN node is a serving node for the first terminal device and the second terminal device.

3. A method as defined in statement 1 , wherein the first RAN node is a serving node for the first terminal device, and the second terminal device is served by a second

RAN node.

4. A method as defined in statement 3, further comprising:

establishing a second forwarding bearer with the second RAN node;

wherein the data packets of the data session from the first terminal device that have not been received by the second terminal device following initiation of the switch are received from the second RAN node via the second forwarding bearer and sent to the first terminal device via the first forwarding bearer. 5. A method as defined in any of statements 1-4, wherein the first forwarding bearer is an existing data radio bearer, DRB, to the first terminal device that is used for the data session.

6. A method as defined in any of statements 1-4, wherein the first forwarding bearer is a data radio bearer, DRB, to the first terminal device that is dedicated to the forwarding of data packets that have not been received by the second terminal device following initiation of the switch.

7. A method as defined in any of statements 1-6, further comprising:

sending a first signal to the first terminal device after initiation of the switch to indicate that transmission of data packets from the first terminal device to the second terminal device should be stopped until all forwarded data packets are received by the first terminal device and resent by the first terminal device directly to the second terminal device.

8. A method as defined in statement 7, further comprising:

sending a second signal to the first terminal device to indicate a last data packet of the forwarded data packets so that the first terminal device can resume transmitting data packets of the data session directly to the second terminal device after the last data packet has been resent by the first terminal device directly to the second terminal device.

9. A first radio access network, RAN, node for use in maintaining service continuity for a data session from a first terminal device to a second terminal device during a switch from routing the data session via the first RAN node to routing the data session directly from the first terminal device to the second terminal device, the first RAN node being adapted to:

establish a first forwarding bearer to the first terminal device; and

forward, to the first terminal device via the first forwarding bearer, data packets of the data session from the first terminal device that have not been received by the second terminal device following initiation of the switch.

10. A first RAN node as defined in statement 9, wherein the first RAN node is a serving node for the first terminal device and the second terminal device.

1 1. A first RAN node as defined in statement 9, wherein the first RAN node is a serving node for the first terminal device, and the second terminal device is served by a second RAN node. 12. A first RAN node as defined in statement 11 , further adapted to:

establish a second forwarding bearer with the second RAN node; and receive, from the second RAN node via the second forwarding bearer, the data packets of the data session from the first terminal device that have not been received by the second terminal device following initiation of the switch. 13. A first RAN node as defined in any of statements 9-12, wherein the first forwarding bearer is an existing data radio bearer, DRB, to the first terminal device that is used for the data session.

14. A first RAN node as defined in any of statements 9-12, wherein the first forwarding bearer is a data radio bearer, DRB, to the first terminal device that is dedicated to the forwarding of data packets that have not been received by the second terminal device following initiation of the switch.

15. A first RAN node as defined in any of statements 9-14, further adapted to:

send a first signal to the first terminal device after initiation of the switch to indicate that transmission of data packets from the first terminal device to the second terminal device should be stopped until all forwarded data packets are received by the first terminal device and resent by the first terminal device directly to the second terminal device.

16. A first RAN node as defined in statement 15, further adapted to:

send a second signal to the first terminal device to indicate a last data packet of the forwarded data packets so that the first terminal device can resume transmitting data packets of the data session directly to the second terminal device after the last data packet has been resent by the first terminal device directly to the second terminal device.

17. A method of operating a first terminal device to maintain service continuity for a data session from the first terminal device to a second terminal device during a switch from routing the data session via a first radio access network, RAN, node to routing the data session directly from the first terminal device to the second terminal device, the method comprising:

establishing a forwarding bearer with the first RAN node;

receiving, from the first RAN node via the forwarding bearer, data packets of the data session previously sent from the first terminal device to the second terminal device that have not been received by the second terminal device following initiation of the switch; and

forwarding the received data packets directly to the second terminal device. 18. A method as defined in statement 17, wherein the first forwarding bearer is an existing data radio bearer, DRB, that is used for the data session.

19. A method as defined in statement 17, wherein the first forwarding bearer is a data radio bearer, DRB, to the first terminal device that is dedicated to the forwarding of data packets that have not been received by the second terminal device following initiation of the switch.

20. A method as defined in any of statements 17-19, further comprising:

receiving a first signal from the first RAN node after initiation of the switch to indicate that transmission of new data packets from the first terminal device to the second terminal device should be stopped until all forwarded data packets are received by the first terminal device via the forwarding bearer and resent directly to the second terminal device; and

stopping the transmission of new data packets from the first terminal device.

21. A method as defined in statement 20, wherein new data packets are data packets in a transmission buffer of the first terminal device that have not yet been sent to the second terminal device. 22. A method as defined in statement 20 or 21 , further comprising:

receiving a second signal from the first RAN node to indicate a last data packet of the forwarded data packets; and

following receipt of the second signal and forwarding the indicated last data packet directly to the second terminal device, transmitting new data packets of the data session directly to the second terminal device.

23. A first terminal device for use in maintaining service continuity for a data session from the first terminal device to a second terminal device during a switch from routing the data session via a first radio access network, RAN, node to routing the data session directly from the first terminal device to the second terminal device, the first terminal device being adapted to: establish a forwarding bearer with the first RAN node;

receive, from the first RAN node via the forwarding bearer, data packets of the data session previously sent from the first terminal device to the second terminal device that have not been received by the second terminal device following initiation of the switch; and

forward the received data packets directly to the second terminal device.

24. A first terminal device as defined in statement 23, wherein the first forwarding bearer is an existing data radio bearer, DRB that is used for the data session.

25. A first terminal device as defined in statement 23, wherein the first forwarding bearer is a data radio bearer, DRB, to the first terminal device that is dedicated to the forwarding of data packets that have not been received by the second terminal device following initiation of the switch.

26. A first terminal device as defined in any of statements 23-25, further adapted to: receive a first signal from the first RAN node after initiation of the switch to indicate that transmission of new data packets from the first terminal device to the second terminal device should be stopped until all forwarded data packets are received by the first terminal device via the forwarding bearer and resent directly to the second terminal device; and

stop the transmission of new data packets from the first terminal device.

27. A first terminal device as defined in statement 26, wherein new data packets are data packets in a transmission buffer of the first terminal device that have not yet been sent to the second terminal device.

28. A first terminal device as defined in statement 27 or 28, further adapted to:

receive a second signal from the first RAN node to indicate a last data packet of the forwarded data packets; and

transmitting new data packets of the data session directly to the second terminal device following receipt of the second signal and forwarding the indicated last data packet directly to the second terminal device. 29. A computer program product having computer readable code embodied therein, the computer readable code being configured such that, on execution by a suitable computer or processor, the computer or processor performs the method as defined in any of statements 1-8 or 17-22.

30. A first radio access network, RAN, node for use in maintaining service continuity for a data session from a first terminal device to a second terminal device during a switch from routing the data session via the first RAN node to routing the data session directly from the first terminal device to the second terminal device the first RAN node comprising a processor and a memory, said memory containing instructions executable by said processor whereby said first RAN node is operative to:

establish a first forwarding bearer to the first terminal device; and

forward, to the first terminal device via the first forwarding bearer, data packets of the data session from the first terminal device that have not been received by the second terminal device following initiation of the switch. 31. A first RAN node as defined in statement 30, wherein the first RAN node is a serving node for the first terminal device and the second terminal device.

32. A first RAN node as defined in statement 30, wherein the first RAN node is a serving node for the first terminal device, and the second terminal device is served by a second RAN node.

33. A first RAN node as defined in statement 32, further operative to:

establish a second forwarding bearer with the second RAN node; and

receive, from the second RAN node via the second forwarding bearer, the data packets of the data session from the first terminal device that have not been received by the second terminal device following initiation of the switch.

34. A first RAN node as defined in any of statements 30-33, wherein the first forwarding bearer is an existing data radio bearer, DRB, to the first terminal device that is used for the data session.

35. A first RAN node as defined in any of statements 30-33, wherein the first forwarding bearer is a data radio bearer, DRB, to the first terminal device that is dedicated to the forwarding of data packets that have not been received by the second terminal device following initiation of the switch. 36. A first RAN node as defined in any of statements 30-35, further operative to: send a first signal to the first terminal device after initiation of the switch to indicate that transmission of data packets from the first terminal device to the second terminal device should be stopped until all forwarded data packets are received by the first terminal device and resent by the first terminal device directly to the second terminal device.

37. A first RAN node as defined in statement 36, further operative to:

send a second signal to the first terminal device to indicate a last data packet of the forwarded data packets so that the first terminal device can resume transmitting data packets of the data session directly to the second terminal device after the last data packet has been resent by the first terminal device directly to the second terminal device. 38. A first terminal device for use in maintaining service continuity for a data session from the first terminal device to a second terminal device during a switch from routing the data session via a first radio access network, RAN, node to routing the data session directly from the first terminal device to the second terminal device, the first terminal device comprising a processor and a memory, said memory containing instructions executable by said processor whereby said first terminal device is operative to:

establish a forwarding bearer with the first RAN node;

forward the received data packets directly to the second terminal device.

39. A first terminal device as defined in statement 38, wherein the first forwarding bearer is an existing data radio bearer, DRB that is used for the data session.

40. A first terminal device as defined in statement 38, wherein the first forwarding bearer is a data radio bearer, DRB, to the first terminal device that is dedicated to the forwarding of data packets that have not been received by the second terminal device following initiation of the switch. 41. A first terminal device as defined in any of statements 38-40, further operative to: receive a first signal from the first RAN node after initiation of the switch to indicate that transmission of new data packets from the first terminal device to the second terminal device should be stopped until all forwarded data packets are received by the first terminal device via the forwarding bearer and resent directly to the second terminal device; and

stop the transmission of new data packets from the first terminal device.

42. A first terminal device as defined in statement 41 , wherein new data packets are data packets in a transmission buffer of the first terminal device that have not yet been sent to the second terminal device.

43. A first terminal device as defined in statement 41 or 42, further operative to:

transmitting new data packets of the data session directly to the second terminal device following receipt of the second signal and forwarding the indicated last data packet directly to the second terminal device. 44. A method of operating a first radio access network, RAN, node to maintain service continuity for a data session to a first terminal device during a switch from a first routing path to a second routing path, the first and second routing paths comprising a routing path in which the data session is routed via a RAN node that is serving the first terminal device, and a routing path in which the data session is routed to the first terminal device via a relay terminal device, wherein the first RAN node is in the first routing path, the method comprising:

establishing a first forwarding bearer to a gateway node in the first routing path; and

forwarding, to the gateway node via the first forwarding bearer, data packets of the data session that have not been received by the first terminal device following initiation of the switch.

45. A method as defined in statement 44, wherein the first routing path is the routing path in which the data session is routed via a RAN node that is serving the first terminal device and the second routing path is the routing path in which the data session is routed to the first terminal device via a relay terminal device; and wherein the first RAN node is the RAN node serving the first terminal device.

46. A method as defined in statement 44, wherein the first routing path is the routing path in which the data session is routed to the first terminal device via a relay terminal device and the second routing path is the routing path in which the data session is routed via a RAN node that is serving the first terminal device; and wherein the first RAN node is a RAN node serving the relay terminal device. 47. A method as defined in any of statements 44-46, further comprising:

receiving a first signal from the gateway node, the first signal indicating a last data packet that is to be sent to the first terminal device; and

sending a second signal to the gateway node to indicate that the last data packet has been forwarded via the first forwarding bearer.

48. A first radio access network, RAN, node for use in maintaining service continuity for a data session to a first terminal device during a switch from a first routing path to a second routing path, the first and second routing paths comprising a routing path in which the data session is routed via a RAN node that is serving the first terminal device, and a routing path in which the data session is routed to the first terminal device via a relay terminal device, wherein the first RAN node is in the first routing path, the first RAN node being adapted to:

establish a first forwarding bearer to a gateway node in the first routing path; and forward, to the gateway node via the first forwarding bearer, data packets of the data session that have not been received by the first terminal device following initiation of the switch.

49. A first RAN node as defined in statement 48, wherein the first routing path is the routing path in which the data session is routed via a RAN node that is serving the first terminal device and the second routing path is the routing path in which the data session is routed to the first terminal device via a relay terminal device; and wherein the first RAN node is the RAN node serving the first terminal device.

50. A first RAN node as defined in statement 48, wherein the first routing path is the routing path in which the data session is routed to the first terminal device via a relay terminal device and the second routing path is the routing path in which the data session is routed via a RAN node that is serving the first terminal device; and wherein the first RAN node is a RAN node serving the relay terminal device.

51. A first RAN node as defined in any of statements 48-50, the RAN node being further adapted to:

receive a first signal from the gateway node, the first signal indicating a last data packet that is to be sent to the first terminal device; and

send a second signal to the gateway node to indicate that the last data packet has been forwarded via the first forwarding bearer.

52. A method of operating a first gateway node to maintain service continuity for a data session to a first terminal device during a switch from a first routing path to a second routing path, the first and second routing paths comprising a routing path in which the data session is routed via a radio access network, RAN, node that is serving the first terminal device, and a routing path in which the data session is routed to the first terminal device via a relay terminal device, wherein the first gateway node is in the first routing path, the method comprising:

establishing a first forwarding bearer with a RAN node in the first routing path; receiving, from the RAN node via the first forwarding bearer, data packets of the data session that have not been received by the first terminal device following initiation of the switch; and

forwarding the received data packets to a node in the second routing path.

53. A method as defined in statement 52, wherein the first routing path is the routing path in which the data session is routed via a RAN node that is serving the first terminal device and the second routing path is the routing path in which the data session is routed to the first terminal device via a relay terminal device; and wherein the first forwarding bearer is established with the RAN node that is serving the first terminal. 54. A method as defined in statement 53, wherein the step of forwarding the received data packets comprises forwarding the received data packets to a second gateway node that is serving the relay terminal device.

55. A method as defined in statement 53, wherein the first gateway node is serving the first terminal device and the relay terminal device and wherein the step of forwarding the received data packets comprises forwarding the received data packets to a RAN node that is serving the relay terminal device.

56. A method as defined in statement 52, wherein the first routing path is the routing path in which the data session is routed to the first terminal device via a relay terminal device and the second routing path is the routing path in which the data session is routed via a RAN node that is serving the first terminal device; and wherein the first forwarding bearer is established with a RAN node that is serving the relay terminal device.

57. A method as defined in statement 56, wherein the step of forwarding the received data packets comprises forwarding the received data packets to a second gateway node that is serving the first terminal device. 58. A method as defined in statement 56, wherein the first gateway node is serving the first terminal device and the relay terminal device and wherein the step of forwarding the received data packets comprises forwarding the received data packets to the RAN node that is serving the first terminal device. 59. A method as defined in any of statements 52-58, further comprising:

receiving a first signal from the RAN node in the first routing path that indicates the last data packet has been forwarded via the first forwarding bearer.

60. A method as defined in statement 59, further comprising:

sending a second signal to the RAN node in the first routing path after transmission of data packets to the first terminal device is prevented, the first signal indicating a last data packet sent to the RAN node that was to be sent to the first terminal device. 61. A first gateway node for use in maintaining service continuity for a data session to a first terminal device during a switch from a first routing path to a second routing path, the first and second routing paths comprising a routing path in which the data session is routed via a radio access network, RAN, node that is serving the first terminal device, and a routing path in which the data session is routed to the first terminal device via a relay terminal device, wherein the first gateway node is in the first routing path, the first gateway node being adapted to: establish a first forwarding bearer with a RAN node in the first routing path;

receive, from the RAN node via the first forwarding bearer, data packets of the data session that have not been received by the first terminal device following initiation of the switch; and

forward the received data packets to a node in the second routing path.

62. A first gateway node as defined in statement 61 , wherein the first routing path is the routing path in which the data session is routed via a RAN node that is serving the first terminal device and the second routing path is the routing path in which the data session is routed to the first terminal device via a relay terminal device; and wherein the first forwarding bearer is established with the RAN node that is serving the first terminal.

63. A first gateway node as defined in statement 62, wherein the first gateway node is adapted to forward the received data packets to a second gateway node that is serving the relay terminal device.

64. A first gateway node as defined in statement 62, wherein the first gateway node is serving the first terminal device and the relay terminal device and wherein the first gateway node is adapted to forward the received data packets to a RAN node that is serving the relay terminal device.

65. A first gateway node as defined in statement 61 , wherein the first routing path is the routing path in which the data session is routed to the first terminal device via a relay terminal device and the second routing path is the routing path in which the data session is routed via a RAN node that is serving the first terminal device; and wherein the first forwarding bearer is established with a RAN node that is serving the relay terminal device. 66. A first gateway node as defined in statement 65, wherein the first gateway node is adapted to forward the received data packets to a second gateway node that is serving the first terminal device.

67. A first gateway node as defined in statement 65, wherein the first gateway node is serving the first terminal device and the relay terminal device and wherein the first gateway node is adapted to forward the received data packets to the RAN node that is serving the first terminal device.

68. A first gateway node as defined in any of statements 61-67, wherein the gateway node is further adapted to:

receive a first signal from the RAN node in the first routing path that indicates the last data packet has been forwarded via the first forwarding bearer.

69. A first gateway node as defined in statement 68, wherein the gateway node is further adapted to:

send a second signal to the RAN node in the first routing path after transmission of data packets to the first terminal device is prevented, the first signal indicating a last data packet sent to the RAN node that was to be sent to the first terminal device. 70. A computer program product comprising a computer readable medium having computer readable code embodied therein, the computer readable code being configured such that, on execution by a suitable computer or processor, the computer or processor is caused to perform the method of any of statements 44-47 and 52-60. 71 . A first radio access network, RAN, node for use in maintaining service continuity for a data session to a first terminal device during a switch from a first routing path to a second routing path, the first and second routing paths comprising a routing path in which the data session is routed via a RAN node that is serving the first terminal device, and a routing path in which the data session is routed to the first terminal device via a relay terminal device, wherein the first RAN node is in the first routing path, the first RAN node comprising a processor and a memory, said memory containing instructions executable by said processor whereby said first RAN node is operative to:

72. A first RAN node as defined in statement 71 wherein the first routing path is the routing path in which the data session is routed via a RAN node that is serving the first terminal device and the second routing path is the routing path in which the data session is routed to the first terminal device via a relay terminal device; and wherein the first RAN node is the RAN node serving the first terminal device.

73. A first RAN node as defined in statement 71 , wherein the first routing path is the routing path in which the data session is routed to the first terminal device via a relay terminal device and the second routing path is the routing path in which the data session is routed via a RAN node that is serving the first terminal device; and wherein the first RAN node is a RAN node serving the relay terminal device. 74. A first RAN node as defined in any of statements 71-73, the RAN node being further operative to:

send a second signal to the gateway node indicate that the last data packet has been forwarded via the first forwarding bearer.

75. A first gateway node for use in maintaining service continuity for a data session to a first terminal device during a switch from a first routing path to a second routing path, the first and second routing paths comprising a routing path in which the data session is routed via a radio access network, RAN, node that is serving the first terminal device, and a routing path in which the data session is routed to the first terminal device via a relay terminal device, wherein the first gateway node is in the first routing path, the first gateway node comprising a processor and a memory, said memory containing instructions executable by said processor whereby said first gateway node is operative to:

establish a first forwarding bearer with a RAN node in the first routing path;

forward the received data packets to a node in the second routing path.

76. A first gateway node as defined in statement 75, wherein the first routing path is the routing path in which the data session is routed via a RAN node that is serving the first terminal device and the second routing path is the routing path in which the data session is routed to the first terminal device via a relay terminal device; and wherein the first forwarding bearer is established with the RAN node that is serving the first terminal.

77. A first gateway node as defined in statement 76, wherein the first gateway node is operative to forward the received data packets to a second gateway node that is serving the relay terminal device.

78. A first gateway node as defined in statement 76, wherein the first gateway node is serving the first terminal device and the relay terminal device and wherein the first gateway node is operative to forward the received data packets to a RAN node that is serving the relay terminal device.

79. A first gateway node as defined in statement 75, wherein the first routing path is the routing path in which the data session is routed to the first terminal device via a relay terminal device and the second routing path is the routing path in which the data session is routed via a RAN node that is serving the first terminal device; and wherein the first forwarding bearer is established with a RAN node that is serving the relay terminal device. 80. A first gateway node as defined in statement 79, wherein the first gateway node is operative to forward the received data packets to a second gateway node that is serving the first terminal device.

81. A first gateway node as defined in statement 79, wherein the first gateway node is serving the first terminal device and the relay terminal device and wherein the first gateway node is operative to forward the received data packets to the RAN node that is serving the first terminal device.

82. A first gateway node as defined in any of statements 75-81 , wherein the gateway node is further operative to:

83. A first gateway node as defined in statement 82, wherein the gateway node is further operative to: send a second signal to the RAN node in the first routing path after transmission of data packets to the first terminal device is prevented, the first signal indicating a last data packet sent to the RAN node that was to be sent to the first terminal device. 84. A first radio access network, RAN, node for use in maintaining service continuity for a data session from a first terminal device to a second terminal device during a switch from routing the data session via the first RAN node to routing the data session directly from the first terminal device to the second terminal device, the first RAN node comprising:

a first module configured to establish a first forwarding bearer to the first terminal device; and

a second module configured to forward, to the first terminal device via the first forwarding bearer, data packets of the data session from the first terminal device that have not been received by the second terminal device following initiation of the switch.

85. A first terminal device for use in maintaining service continuity for a data session from the first terminal device to a second terminal device during a switch from routing the data session via a first radio access network, RAN, node to routing the data session directly from the first terminal device to the second terminal device, the first terminal device comprising:

a first module configured to establish a forwarding bearer with the first RAN node; a second module configured to receive, from the first RAN node via the forwarding bearer, data packets of the data session previously sent from the first terminal device to the second terminal device that have not been received by the second terminal device following initiation of the switch; and

a third module configured to forward the received data packets directly to the second terminal device.

86. A first radio access network, RAN, node for use in maintaining service continuity for a data session to a first terminal device during a switch from a first routing path to a second routing path, the first and second routing paths comprising a routing path in which the data session is routed via a RAN node that is serving the first terminal device, and a routing path in which the data session is routed to the first terminal device via a relay terminal device, wherein the first RAN node is in the first routing path, the first RAN node comprising: a first module configured to establish a first forwarding bearer to a gateway node in the first routing path; and

a second module configured to forward, to the gateway node via the first forwarding bearer, data packets of the data session that have not been received by the first terminal device following initiation of the switch.

87. A first gateway node for use in maintaining service continuity for a data session to a first terminal device during a switch from a first routing path to a second routing path, the first and second routing paths comprising a routing path in which the data session is routed via a radio access network, RAN, node that is serving the first terminal device, and a routing path in which the data session is routed to the first terminal device via a relay terminal device, wherein the first gateway node is in the first routing path, the first gateway node comprising:

a first module configured to establish a first forwarding bearer with a RAN node in the first routing path;

a second module configured to receive, from the RAN node via the first forwarding bearer, data packets of the data session that have not been received by the first terminal device following initiation of the switch; and

a third module configured to forward the received data packets to a node in the second routing path.

Claims

1. A method of operating a first radio access network, RAN, node (13) to maintain service continuity for a data session from a first terminal device (11) to a second terminal device (12) during a switch from routing the data session via the first RAN node (13) to routing the data session directly from the first terminal device (11) to the second terminal device (12), the method comprising:

establishing (801) a first forwarding bearer to the first terminal device (11); and forwarding (803), to the first terminal device (11) via the first forwarding bearer, data packets of the data session from the first terminal device (1 1) that have not been received by the second terminal device (12) following initiation of the switch.

2. A method as claimed in claim 1 , wherein the first RAN node (13) is a serving node for the first terminal device (1 1) and the second terminal device (12).

3. A method as claimed in claim 1 , wherein the first RAN node (13) is a serving node for the first terminal device (11), and the second terminal device (12) is served by a second RAN node (14).

4. A method as claimed in claim 3, further comprising:

establishing a second forwarding bearer with the second RAN node (14);

wherein the data packets of the data session from the first terminal device (11) that have not been received by the second terminal device (12) following initiation of the switch are received from the second RAN node (14) via the second forwarding bearer and sent to the first terminal device (1 1) via the first forwarding bearer.

5. A method as claimed in any of claims 1-4, wherein the first forwarding bearer is an existing data radio bearer, DRB, to the first terminal device (11) that is used for the data session.

6. A method as claimed in any of claims 1-4, wherein the first forwarding bearer is a data radio bearer, DRB, to the first terminal device (11) that is dedicated to the forwarding of data packets that have not been received by the second terminal device (12) following initiation of the switch.

7. A method as claimed in any of claims 1-6, further comprising: sending a first signal to the first terminal device (1 1) after initiation of the switch to indicate that transmission of data packets from the first terminal device (11) to the second terminal device (12) should be stopped until all forwarded data packets are received by the first terminal device (1 1) and resent by the first terminal device (11) directly to the second terminal device (12).

8. A method as claimed in claim 7, further comprising:

sending a second signal to the first terminal device (1 1) to indicate a last data packet of the forwarded data packets so that the first terminal device (11) can resume transmitting data packets of the data session directly to the second terminal device (12) after the last data packet has been resent by the first terminal device (11) directly to the second terminal device (12).

9. A first radio access network, RAN, node (13) for use in maintaining service continuity for a data session from a first terminal device (1 1) to a second terminal device (12) during a switch from routing the data session via the first RAN node (13) to routing the data session directly from the first terminal device (11) to the second terminal device (12), the first RAN node being adapted to:

establish a first forwarding bearer to the first terminal device (1 1); and

forward, to the first terminal device (11) via the first forwarding bearer, data packets of the data session from the first terminal device (11) that have not been received by the second terminal device (12) following initiation of the switch.

10. A first RAN node (13) as claimed in claim 9, wherein the first RAN node (13) is a serving node for the first terminal device (1 1) and the second terminal device (12).

1 1. A first RAN node (13) as claimed in claim 9, wherein the first RAN node (13) is a serving node for the first terminal device (1 1), and the second terminal device (12) is served by a second RAN node (14).

12. A first RAN node (13) as claimed in claim 11 , further adapted to:

establish a second forwarding bearer with the second RAN node (14); and receive, from the second RAN node (14) via the second forwarding bearer, the data packets of the data session from the first terminal device (1 1) that have not been received by the second terminal device (12) following initiation of the switch.

13. A first RAN node (13) as claimed in any of claims 9-12, wherein the first forwarding bearer is an existing data radio bearer, DRB, to the first terminal device (11) that is used for the data session.

14. A first RAN node (13) as claimed in any of claims 9-12, wherein the first forwarding bearer is a data radio bearer, DRB, to the first terminal device (1 1) that is dedicated to the forwarding of data packets that have not been received by the second terminal device (12) following initiation of the switch.

15. A first RAN node (13) as claimed in any of claims 9-14, further adapted to:

send a first signal to the first terminal device (11) after initiation of the switch to indicate that transmission of data packets from the first terminal device (11) to the second terminal device (12) should be stopped until all forwarded data packets are received by the first terminal device (1 1) and resent by the first terminal device (11) directly to the second terminal device (12).

16. A first RAN node (13) as claimed in claim 15, further adapted to:

send a second signal to the first terminal device (1 1) to indicate a last data packet of the forwarded data packets so that the first terminal device (11) can resume transmitting data packets of the data session directly to the second terminal device (12) after the last data packet has been resent by the first terminal device (11) directly to the second terminal device (12).

17. A method of operating a first terminal device (11) to maintain service continuity for a data session from the first terminal device (11) to a second terminal device (12) during a switch from routing the data session via a first radio access network, RAN, node (13) to routing the data session directly from the first terminal device (11) to the second terminal device (12), the method comprising:

establishing (901) a forwarding bearer with the first RAN node (13);

receiving (903), from the first RAN node (13) via the forwarding bearer, data packets of the data session previously sent from the first terminal device (11) to the second terminal device (12) that have not been received by the second terminal device (12) following initiation of the switch; and

forwarding (905) the received data packets directly to the second terminal device (12).

18. A method as claimed in claim 17, wherein the first forwarding bearer is an existing data radio bearer, DRB, that is used for the data session.

19. A method as claimed in claim 17, wherein the first forwarding bearer is a data radio bearer, DRB, to the first terminal device (1 1) that is dedicated to the forwarding of data packets that have not been received by the second terminal device (12) following initiation of the switch.

20. A method as claimed in any of claims 17-19, further comprising:

receiving a first signal from the first RAN node (13) after initiation of the switch to indicate that transmission of new data packets from the first terminal device (1 1) to the second terminal device (12) should be stopped until all forwarded data packets are received by the first terminal device (11) via the forwarding bearer and resent directly to the second terminal device (12); and

stopping the transmission of new data packets from the first terminal device (11).

21. A method as claimed in claim 20, wherein new data packets are data packets in a transmission buffer of the first terminal device (11) that have not yet been sent to the second terminal device (12).

22. A method as claimed in claim 20 or 21 , further comprising:

receiving a second signal from the first RAN node (13) to indicate a last data packet of the forwarded data packets; and

following receipt of the second signal and forwarding the indicated last data packet directly to the second terminal device (12), transmitting new data packets of the data session directly to the second terminal device (12).

23. A first terminal device (11) for use in maintaining service continuity for a data session from the first terminal device (11) to a second terminal device (12) during a switch from routing the data session via a first radio access network, RAN, node (13) to routing the data session directly from the first terminal device (11) to the second terminal device (12), the first terminal device (11) being adapted to:

establish a forwarding bearer with the first RAN node (13);

receive, from the first RAN node (13) via the forwarding bearer, data packets of the data session previously sent from the first terminal device (11) to the second terminal device (12) that have not been received by the second terminal device (12) following initiation of the switch; and

forward the received data packets directly to the second terminal device (12).

24. A first terminal device (11) as claimed in claim 23, wherein the first forwarding bearer is an existing data radio bearer, DRB that is used for the data session.

25. A first terminal device (11) as claimed in claim 23, wherein the first forwarding bearer is a data radio bearer, DRB, to the first terminal device (11) that is dedicated to the forwarding of data packets that have not been received by the second terminal device (12) following initiation of the switch.

26. A first terminal device (11) as claimed in any of claims 23-25, further adapted to: receive a first signal from the first RAN node (13) after initiation of the switch to indicate that transmission of new data packets from the first terminal device (1 1) to the second terminal device (12) should be stopped until all forwarded data packets are received by the first terminal device (11) via the forwarding bearer and resent directly to the second terminal device (12); and

stop the transmission of new data packets from the first terminal device (11).

27. A first terminal device (1 1) as claimed in claim 26, wherein new data packets are data packets in a transmission buffer of the first terminal device (11) that have not yet been sent to the second terminal device (12).

28. A first terminal device (1 1) as claimed in claim 27 or 28, further adapted to:

receive a second signal from the first RAN node (13) to indicate a last data packet of the forwarded data packets; and

transmitting new data packets of the data session directly to the second terminal device (12) following receipt of the second signal and forwarding the indicated last data packet directly to the second terminal device (12).

29. A computer program product having computer readable code embodied therein, the computer readable code being configured such that, on execution by a suitable computer or processor, the computer or processor performs the method as claimed in any of claims 1-8 or 17-22.

establish a first forwarding bearer to the first terminal device; and

31. A first RAN node as claimed in claim 30, wherein the first RAN node is a serving node for the first terminal device and the second terminal device.

32. A first RAN node as claimed in claim 30, wherein the first RAN node is a serving node for the first terminal device, and the second terminal device is served by a second RAN node.

33. A first RAN node as claimed in claim 32, further operative to:

establish a second forwarding bearer with the second RAN node; and

34. A first RAN node as claimed in any of claims 30-33, wherein the first forwarding bearer is an existing data radio bearer, DRB, to the first terminal device that is used for the data session.

35. A first RAN node as claimed in any of claims 30-33, wherein the first forwarding bearer is a data radio bearer, DRB, to the first terminal device that is dedicated to the forwarding of data packets that have not been received by the second terminal device following initiation of the switch.

36. A first RAN node as claimed in any of claims 30-35, further operative to:

37. A first RAN node as claimed in claim 36, further operative to:

38. A first terminal device for use in maintaining service continuity for a data session from the first terminal device to a second terminal device during a switch from routing the data session via a first radio access network, RAN, node to routing the data session directly from the first terminal device to the second terminal device, the first terminal device comprising a processor and a memory, said memory containing instructions executable by said processor whereby said first terminal device is operative to:

establish a forwarding bearer with the first RAN node;

forward the received data packets directly to the second terminal device.

39. A first terminal device as claimed in claim 38, wherein the first forwarding bearer is an existing data radio bearer, DRB that is used for the data session.

40. A first terminal device as claimed in claim 38, wherein the first forwarding bearer is a data radio bearer, DRB, to the first terminal device that is dedicated to the forwarding of data packets that have not been received by the second terminal device following initiation of the switch.

41. A first terminal device as claimed in any of claims 38-40, further operative to: receive a first signal from the first RAN node after initiation of the switch to indicate that transmission of new data packets from the first terminal device to the second terminal device should be stopped until all forwarded data packets are received by the first terminal device via the forwarding bearer and resent directly to the second terminal device; and

stop the transmission of new data packets from the first terminal device.

42. A first terminal device as claimed in claim 41 , wherein new data packets are data packets in a transmission buffer of the first terminal device that have not yet been sent to the second terminal device.

43. A first terminal device as claimed in claim 41 or 42, further operative to:

transmitting new data packets of the data session directly to the second terminal device following receipt of the second signal and forwarding the indicated last data packet directly to the second terminal device.

44. A method of operating a first radio access network, RAN, node (22; 26) to maintain service continuity for a data session to a first terminal device (21) during a switch from a first routing path to a second routing path, the first and second routing paths comprising a routing path in which the data session is routed via a RAN node (22) that is serving the first terminal device (21), and a routing path in which the data session is routed to the first terminal device (21) via a relay terminal device (25), wherein the first RAN node (22; 26) is in the first routing path, the method comprising: establishing (1201) a first forwarding bearer to a gateway node (23; 27) in the first routing path; and

forwarding (1203), to the gateway node (23; 27) via the first forwarding bearer, data packets of the data session that have not been received by the first terminal device (21) following initiation of the switch.

45. A method as claimed in claim 44, wherein the first routing path is the routing path in which the data session is routed via a RAN node (22) that is serving the first terminal device (21) and the second routing path is the routing path in which the data session is routed to the first terminal device (21) via a relay terminal device (25); and wherein the first RAN node (22; 26) is the RAN node serving the first terminal device (21).

46. A method as claimed in claim 44, wherein the first routing path is the routing path in which the data session is routed to the first terminal device (21) via a relay terminal device (25) and the second routing path is the routing path in which the data session is routed via a RAN node (22) that is serving the first terminal device (21); and wherein the first RAN node (22; 26) is a RAN node serving the relay terminal device (25).

47. A method as claimed in any of claims 44-46, further comprising:

receiving a first signal from the gateway node (23; 27), the first signal indicating a last data packet that is to be sent to the first terminal device (21); and

sending a second signal to the gateway node (23; 27) to indicate that the last data packet has been forwarded via the first forwarding bearer.

48. A first radio access network, RAN, node (22; 26) for use in maintaining service continuity for a data session to a first terminal device (21) during a switch from a first routing path to a second routing path, the first and second routing paths comprising a routing path in which the data session is routed via a RAN node (22) that is serving the first terminal device (21), and a routing path in which the data session is routed to the first terminal device (21) via a relay terminal device (25), wherein the first RAN node (22; 26) is in the first routing path, the first RAN node being adapted to:

establish a first forwarding bearer to a gateway node (23; 27) in the first routing path; and

forward, to the gateway node (23; 27) via the first forwarding bearer, data packets of the data session that have not been received by the first terminal device (21) following initiation of the switch.

49. A first RAN node as claimed in claim 48, wherein the first routing path is the routing path in which the data session is routed via a RAN node (22) that is serving the first terminal device (21) and the second routing path is the routing path in which the data session is routed to the first terminal device (21) via a relay terminal device (25); and wherein the first RAN node (22; 26) is the RAN node serving the first terminal device (21).

50. A first RAN node as claimed in claim 48, wherein the first routing path is the routing path in which the data session is routed to the first terminal device (21) via a relay terminal device (25) and the second routing path is the routing path in which the data session is routed via a RAN node (22) that is serving the first terminal device (21); and wherein the first RAN node (22; 26) is a RAN node serving the relay terminal device (25).

51. A first RAN node as claimed in any of claims 48-50, the RAN node being further adapted to:

receive a first signal from the gateway node (23; 27), the first signal indicating a last data packet that is to be sent to the first terminal device (21); and

send a second signal to the gateway node (23; 27) to indicate that the last data packet has been forwarded via the first forwarding bearer.

52. A method of operating a first gateway node (23; 27) to maintain service continuity for a data session to a first terminal device (21) during a switch from a first routing path to a second routing path, the first and second routing paths comprising a routing path in which the data session is routed via a radio access network, RAN, node (22) that is serving the first terminal device (21), and a routing path in which the data session is routed to the first terminal device (21) via a relay terminal device (25), wherein the first gateway node (23; 27) is in the first routing path, the method comprising:

establishing (1301) a first forwarding bearer with a RAN node (22; 26) in the first routing path;

receiving (1303), from the RAN node (22; 26) via the first forwarding bearer, data packets of the data session that have not been received by the first terminal device (21) following initiation of the switch; and

forwarding (1305) the received data packets to a node (22; 26; 23; 27) in the second routing path.

53. A method as claimed in claim 52, wherein the first routing path is the routing path in which the data session is routed via a RAN node (22) that is serving the first terminal device (21) and the second routing path is the routing path in which the data session is routed to the first terminal device (21) via a relay terminal device (25); and wherein the first forwarding bearer is established with the RAN node (22) that is serving the first terminal (21).

54. A method as claimed in claim 53, wherein the step of forwarding the received data packets comprises forwarding the received data packets to a second gateway node (27) that is serving the relay terminal device (25).

55. A method as claimed in claim 53, wherein the first gateway node (23) is serving the first terminal device (21) and the relay terminal device (25) and wherein the step of forwarding the received data packets comprises forwarding the received data packets to a RAN node (26) that is serving the relay terminal device (25).

56. A method as claimed in claim 52, wherein the first routing path is the routing path in which the data session is routed to the first terminal device (21) via a relay terminal device (25) and the second routing path is the routing path in which the data session is routed via a RAN node (22) that is serving the first terminal device (21); and wherein the first forwarding bearer is established with a RAN node (26) that is serving the relay terminal device (25).

57. A method as claimed in claim 56, wherein the step of forwarding the received data packets comprises forwarding the received data packets to a second gateway node (23) that is serving the first terminal device (21).

58. A method as claimed in claim 56, wherein the first gateway node (23) is serving the first terminal device (21) and the relay terminal device (25) and wherein the step of forwarding the received data packets comprises forwarding the received data packets to the RAN node (22) that is serving the first terminal device (21).

59. A method as claimed in any of claims 52-58, further comprising:

receiving a first signal from the RAN node (22; 26) in the first routing path that indicates the last data packet has been forwarded via the first forwarding bearer.

60. A method as claimed in claim 59, further comprising:

sending a second signal to the RAN node (22; 26) in the first routing path after transmission of data packets to the first terminal device (21) is prevented, the first signal indicating a last data packet sent to the RAN node (22; 26) that was to be sent to the first terminal device (21).

61. A first gateway node (23; 27) for use in maintaining service continuity for a data session to a first terminal device (21) during a switch from a first routing path to a second routing path, the first and second routing paths comprising a routing path in which the data session is routed via a radio access network, RAN, node (22) that is serving the first terminal device (21), and a routing path in which the data session is routed to the first terminal device (21) via a relay terminal device (25), wherein the first gateway node (23; 27) is in the first routing path, the first gateway node being adapted to:

establish a first forwarding bearer with a RAN node (22; 26) in the first routing path;

receive, from the RAN node (22; 26) via the first forwarding bearer, data packets of the data session that have not been received by the first terminal device (21) following initiation of the switch; and

forward the received data packets to a node (22; 26; 23; 27) in the second routing path.

62. A first gateway node as claimed in claim 61 , wherein the first routing path is the routing path in which the data session is routed via a RAN node (22) that is serving the first terminal device (21) and the second routing path is the routing path in which the data session is routed to the first terminal device (21) via a relay terminal device (25); and wherein the first forwarding bearer is established with the RAN node (22) that is serving the first terminal (21).

63. A first gateway node as claimed in claim 62, wherein the first gateway node is adapted to forward the received data packets to a second gateway node (27) that is serving the relay terminal device (25).

64. A first gateway node as claimed in claim 62, wherein the first gateway node (23) is serving the first terminal device (21) and the relay terminal device (25) and wherein the first gateway node is adapted to forward the received data packets to a RAN node (26) that is serving the relay terminal device (25).

65. A first gateway node as claimed in claim 61 , wherein the first routing path is the routing path in which the data session is routed to the first terminal device (21) via a relay terminal device (25) and the second routing path is the routing path in which the data session is routed via a RAN node (22) that is serving the first terminal device (21); and wherein the first forwarding bearer is established with a RAN node (26) that is serving the relay terminal device (25).

66. A first gateway node as claimed in claim 65, wherein the first gateway node is adapted to forward the received data packets to a second gateway node (23) that is serving the first terminal device (21).

67. A first gateway node as claimed in claim 65, wherein the first gateway node (23) is serving the first terminal device (21) and the relay terminal device (25) and wherein the first gateway node is adapted to forward the received data packets to the RAN node (22) that is serving the first terminal device (21).

68. A first gateway node as claimed in any of claims 61-67, wherein the gateway node is further adapted to:

receive a first signal from the RAN node (22; 26) in the first routing path that indicates the last data packet has been forwarded via the first forwarding bearer.

69. A first gateway node as claimed in claim 68, wherein the gateway node is further adapted to:

send a second signal to the RAN node (22; 26) in the first routing path after transmission of data packets to the first terminal device (21) is prevented, the first signal indicating a last data packet sent to the RAN node (22; 26) that was to be sent to the first terminal device (21).

70. A computer program product comprising a computer readable medium having computer readable code embodied therein, the computer readable code being configured such that, on execution by a suitable computer or processor, the computer or processor is caused to perform the method of any of claims 44-47 and 52-60.

71 . A first radio access network, RAN, node for use in maintaining service continuity for a data session to a first terminal device during a switch from a first routing path to a second routing path, the first and second routing paths comprising a routing path in which the data session is routed via a RAN node that is serving the first terminal device, and a routing path in which the data session is routed to the first terminal device via a relay terminal device, wherein the first RAN node is in the first routing path, the first RAN node comprising a processor and a memory, said memory containing instructions executable by said processor whereby said first RAN node is operative to:

72. A first RAN node as claimed in claim 71 wherein the first routing path is the routing path in which the data session is routed via a RAN node that is serving the first terminal device and the second routing path is the routing path in which the data session is routed to the first terminal device via a relay terminal device; and wherein the first RAN node is the RAN node serving the first terminal device.

73. A first RAN node as claimed in claim 71 , wherein the first routing path is the routing path in which the data session is routed to the first terminal device via a relay terminal device and the second routing path is the routing path in which the data session is routed via a RAN node that is serving the first terminal device; and wherein the first RAN node is a RAN node serving the relay terminal device.

74. A first RAN node as claimed in any of claims 71-73, the RAN node being further operative to:

establish a first forwarding bearer with a RAN node in the first routing path;

receive, from the RAN node via the first forwarding bearer, data packets of the data session that have not been received by the first terminal device following initiation of the switch; and forward the received data packets to a node in the second routing path.

76. A first gateway node as claimed in claim 75, wherein the first routing path is the routing path in which the data session is routed via a RAN node that is serving the first terminal device and the second routing path is the routing path in which the data session is routed to the first terminal device via a relay terminal device; and wherein the first forwarding bearer is established with the RAN node that is serving the first terminal.

77. A first gateway node as claimed in claim 76, wherein the first gateway node is operative to forward the received data packets to a second gateway node that is serving the relay terminal device.

78. A first gateway node as claimed in claim 76, wherein the first gateway node is serving the first terminal device and the relay terminal device and wherein the first gateway node is operative to forward the received data packets to a RAN node that is serving the relay terminal device.

79. A first gateway node as claimed in claim 75, wherein the first routing path is the routing path in which the data session is routed to the first terminal device via a relay terminal device and the second routing path is the routing path in which the data session is routed via a RAN node that is serving the first terminal device; and wherein the first forwarding bearer is established with a RAN node that is serving the relay terminal device.

80. A first gateway node as claimed in claim 79, wherein the first gateway node is operative to forward the received data packets to a second gateway node that is serving the first terminal device.

81. A first gateway node as claimed in claim 79, wherein the first gateway node is serving the first terminal device and the relay terminal device and wherein the first gateway node is operative to forward the received data packets to the RAN node that is serving the first terminal device.

82. A first gateway node as claimed in any of claims 75-81 , wherein the gateway node is further operative to: receive a first signal from the RAN node in the first routing path that indicates the last data packet has been forwarded via the first forwarding bearer.

83. A first gateway node as claimed in claim 82, wherein the gateway node is further operative to:

send a second signal to the RAN node in the first routing path after transmission of data packets to the first terminal device is prevented, the first signal indicating a last data packet sent to the RAN node that was to be sent to the first terminal device.