WO2016159850A1

WO2016159850A1 - Proximity service

Info

Publication number: WO2016159850A1
Application number: PCT/SE2015/050592
Authority: WO
Inventors: Pontus Wallentin; Qianxi Lu
Original assignee: Telefonaktiebolaget Lm Ericsson (Publ)
Priority date: 2015-03-31
Filing date: 2015-05-22
Publication date: 2016-10-06

Abstract

A network node and a method of operating the same An aspect provides a method of operating a first network node in a communication network. The method comprising receiving (101) information about a proximity service, ProSe, communication session between a first terminal device and a second terminal device, the information comprising a respective ProSe user equipment, UE, identity, ID, for each of the first terminal device and the second terminal device;determining (103) a serving node identity of a network node in the communication network that is serving the first terminal device and a serving node identity of a network node in the communication network that is serving the second terminal device; and determining (105) a network-level identity for the first terminal device and a network-level identity for the second terminal device, wherein the network-level identity identifies a terminal device to the network node that is serving the terminal device.

Description

PROXIMITY SERVICE

Technical Field

The present disclosure relates to network nodes in a communication network, and in particular relates to techniques for identifying terminal devices involved in Proximity Service, ProSe, communication sessions.

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 "one UE" scenario and shown in Figure 1 , 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) 2 communicating with an eNB 4 that generally has a coverage area indicated by dashed line 6 can communicate 'normally' with eNB 4 while in coverage (indicated by solid arrow 7), and via a second UE (UE2) 8 (indicated by dashed arrows 9), which acts as a relay between UE1 and eNB 4, when UE1 is outside the coverage of eNB 4. In this scenario UE2 is referred to as the "relay UE" of UE1 , and UE1 the "remote UE" of UE2.

In the second scenario, which is known as a "two UE" scenario and shown in Figure 2, 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. In this scenario, a first UE (UE1) 12 is in coverage of a first eNB (eNB1) 14 and is communicating via an infrastructure path with a second UE (UE2) 16 that is in coverage of a second eNB (eNB2) 18 (as indicated by solid arrows 19) and the UEs 12, 16 can switch to using a direct communication path when they are close enough to each other (as indicated by dashed arrow 20). For this scenario, UE2 is referred to as the "peer UE" of UE1 , and vice versa.

The objective of service continuity is to keep the session between the first UE (UE1) and the network (in the first scenario) or the session between the two UEs (in the second scenario) considering the UE mobility (either only UE1 in the first scenario, or both UE1 and UE2 in the second scenario).

Summary

In either scenario above, it is envisaged that for service continuity to be effective network nodes involved in the communications and/or control and/or management of the UEs will need to be aware of the identities of the UEs involved in the communication session and whether the UEs have coverage from the network. There are also other situations involving direct communications or communications via an infrastructure path that require a network node to be aware of the identities of the UEs involved. The existing Sidelink UE Information message includes (when applied also for unicast communication) a list of remote ProSe UE identities. These identities cannot directly be used by network nodes as identities of UEs, since the eNB and EPC does not know how these relate to network-level UE identities, such as IMSI (International Mobile Subscriber Identity).

Thus, there is a need for a network node to be made aware of the identities of UEs involved in a ProSe communication session with each other.

Therefore, according to a first aspect, there is provided a method of operating a first network node in a communication network. The method comprises receiving information about a proximity service, ProSe, communication session between a first terminal device and a second terminal device, the information comprising a respective ProSe user equipment, UE, identity, ID, for each of the first terminal device and the second terminal device; determining a serving node identity of a network node in the communication network that is serving the first terminal device and a serving node identity of a network node in the communication network that is serving the second terminal device; and determining a network-level identity for the first terminal device and a network-level identity for the second terminal device, wherein the network-level identity identifies a terminal device to the network node that is serving the terminal device.

According to a second aspect, there is provided 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 described above.

According to a third aspect, there is provided a first network node for use in a communication network. The first network node is adapted to receive information about a proximity service, ProSe, communication session between a first terminal device and a second terminal device, the information comprising a respective ProSe user equipment, UE, identity, ID, for each of the first terminal device and the second terminal device; determine a serving node identity of a network node in the communication network that is serving the first terminal device and a serving node identity of a network node in the communication network that is serving the second terminal device; and determine a network-level identity for the first terminal device and a network-level identity for the second terminal device, wherein the network-level identity identifies a terminal device to the network node that is serving the terminal device. According to a fourth aspect, there is provided a first network node for use in a communication network. The first network node comprises a processor and a memory, the memory containing instructions executable by said processor whereby said first network node is operative to receive information about a proximity service, ProSe, communication session between a first terminal device and a second terminal device, the information comprising a respective ProSe user equipment, UE, identity, ID, for each of the first terminal device and the second terminal device; determine a serving node identity of a network node in the communication network that is serving the first terminal device and a serving node identity of a network node in the communication network that is serving the second terminal device; and determine a network-level identity for the first terminal device and a network-level identity for the second terminal device, wherein the network-level identity identifies a terminal device to the network node that is serving the terminal device.

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 between a UE and an eNB that can use ProSe communications;

Figure 2 illustrates communications between two UEs that can use ProSe communications;

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 network node according to an embodiment;

Figure 5 is a flow chart illustrating a method of operating a network node according to an embodiment;

Figures 6-8 are signalling diagrams illustrating the signalling between nodes according to exemplary embodiments;

Figure 9 is a flow chart illustrating an exemplary method of operating a network node; and

Figures 10-13 are signalling diagrams illustrating the signalling between nodes according to further exemplary embodiments. 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 network 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) or other node in the radio access network (RAN) of the network 32. In other embodiments the network node can be a node in the core network of the network 32, for example a Mobility Management Entity (MME) 35, a Serving Gateway (SGW) 36, a ProSe Function node 37 or a ProSe Application Server 38. The network node 35; 36; 37; 38; 40 comprises a processor or processing module 60 that controls the operation of the network node. Where the network node is a node in the RAN, 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 network 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 network node also includes components and/or circuitry 68 for allowing the network node to exchange information with other network nodes. For example where the network node is a RAN node the circuitry 68 can allow the network 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. Where the network node is a node in the core network, circuitry 68 can allow the network node to communicate with other nodes in the core network and/or with nodes in the RAN of network 32 (for example via X2). It will be appreciated that network 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).

It will be appreciated that only the components of network node required to explain the embodiments presented herein are illustrated in Figure 4.

As noted above, there is a need for a network node to be made aware of the identity of UEs involved in a ProSe communication session with each other. In this document, this awareness by the network node is referred to as 'binding' or 'associating' the two UEs (the relay UE and remote UE in the first scenario described above and the peer UEs in the second scenario described above) involved in the same ProSe Communication Session.

In the case of performing service continuity switching for a particular session between a first UE (UE1) and a second UE (UE2):

· The serving MME of UE1 needs to know the EPC-level identity of UE2, in order to determine the status of UE2, e.g. to determine whether a service continuity switch is possible.

• To perform a switch from a direct path to an infrastructure (EPC) path, the serving MME of UE1 needs to know whether UE2 is within coverage and thus is able to use the EPC path.

• To perform a switch from an infrastructure (EPC) path to a direct path, the serving MME of UE1 needs to know whether UE2 would be able to receive the packets transmitted from UE1 over the direct path.

The serving eNB of UE1 needs to know the RAN-level identity (e.g. Cell Radio Network Temporary Identifier, C-RNTI) and serving eNB of UE2, in order to setup a measurement performed by UE2, e.g. on a signal transmitted from UE1 that is used to trigger a service continuity switch.

Thus, the techniques described herein provide a way for a network node to 'bind' two UEs involved in a ProSe communication session, e.g. UE1 that has a ProSe communication session with UE2. This 'binding' corresponds to the network node being aware of the following information:

1. The ProSe UE IDs of UE1 and UE2, which are used for identifying the UEs over the direct path, e.g. in ProSe direct data packets.

2. The identities of the respective serving network nodes of UE1 and UE2 (e.g. eNB identity or MME identity), e.g. the network nodes the provide access to the RAN network 32.

3. The network-level identities of UE1 and UE2, which are used for identifying the UEs in the serving network nodes, and which enable the serving network nodes to find the contexts of the UEs (e.g. IMSI, C-RNTI).

As used herein, a successful 'binding' of UE1 and UE2 implies that at least one of the serving network nodes of UE1 and UE2, respectively, have the above UE binding information as part of their contexts for UE1 and UE2.

The flow chart in Figure 5 illustrates a method of operating a network node (for example a RAN node, such as an eNB 40, or a core network node, such as an MME 35, a SGW 36, a ProSe-F node 37 or a ProSe Application Server 38) according to an embodiment. The network node is also referred to a 'first network node' below.

In a first step, step 101 , the network node receives information about a ProSe communication session between a first terminal device 42 (UE1) and a second terminal device 42 (UE2). This information comprises a ProSe UE ID for each of the first terminal device and the second terminal device.

In a second step, step 103, the network node determines a serving node identity of a network node 40 (e.g. an eNB) in the communication network 32 that is serving the first terminal device 42 and a serving node identity of a network node 40 (e.g. an eNB) in the communication network 32 that is serving the second terminal device 42. In some embodiments the serving node identity is an eNB identity and/or an MME identity.

In a third step, step 105 (which may take place before, after, or at the same time as step 103), the network node determines a network-level identity for the first terminal device 42 and a network-level identity for the second terminal device 42. The network- level identity identifies a terminal device to the network node (e.g. eNB 40) that is serving the terminal device 42. In some embodiments the network-level identity is an IMSI and/or C-RNTI.

On completion of these steps, the network node is aware of the ProSe UE IDs for each of the first terminal device and the second terminal device, the serving node identities of the network node(s) that is/are serving the first terminal device and the second terminal device, and the network-level identities for the first terminal device and the second terminal device. In some embodiments the method further comprises storing the ProSe UE IDs for the first terminal device and the second terminal device, the serving node identities for the first terminal device and the second terminal device and the network-level identities for the first terminal device and the second terminal device. This information can be stored in the memory module 66 of the network node. More particularly, the network node can store this information in, or associate this information with, a UE context for the first terminal device and/or a UE context for the second terminal device.

In some embodiments the method further comprises the network node sending the ProSe UE IDs for the first terminal device and the second terminal device, the serving node identities for the first terminal device and the second terminal device and the network-level identities for the first terminal device and the second terminal device to a second network node. The network node can send this information using the inter- node interface circuitry 68. As with the first network node, the second network node can be a RAN node, such as an eNB 40, or a core network node, such as an MME 35, a SGW 36, a ProSe-F node 37 or a ProSe Application Server 38. The second network node can be the same or a different type of node to the first network node.

In some embodiments one or both of step 103 and 105 can comprise translating a ProSe UE ID for a terminal device received in step 101 into a serving node identity and/or a network-level identity for the terminal device. This translating can comprise looking up the ProSe UE ID for a terminal device in a ProSe UE ID/network level identity table stored in the network node.

In some embodiments one or both of step 103 and 105 can comprise receiving the serving node identities and/or the network-level identities from another node (e.g. a terminal device, RAN node or core network node) in the communication network. Where the first network node is the serving network node for the first terminal device, one or both of step 103 and 105 can comprise receiving the information about the ProSe communication session from the first terminal device. In some embodiments the network node (as the serving network node of the first terminal device) can determine the serving node identity and network-level identity for the second terminal device by determining if the first network node is a serving network node of the second terminal device. If the first network node is the serving network node of the second terminal device, then the first network node will already be aware of the network-level identity for the second terminal device and can obtain the network- level identity from a memory 66 of the first network node.

In some embodiments the first network node can send a request for information on the second terminal device to a second network node (for example another RAN node, such as an eNB 40). The request can comprise the ProSe UE IDs for the first terminal device and the second terminal device.

The first network node may receive the serving node identity and the network-level identity for the second terminal device from another network node (whether in response to an explicit request from the first network node or otherwise). In some embodiments, the first network node may receive the serving node identity and network-level identity for the second terminal device from the second terminal device (for example in the case where the first network node is the serving network node for the second terminal device.

In some embodiments the network node can use the information obtained about the first terminal device and second terminal device to provide service continuity switching for the ProSe communication session. More specifically, possible uses for the information include identifying the communication session as a candidate for service continuity switching; associating a direct bearer used by the communication session with an EPS bearer to be used for the service continuity switch, and vice versa; establishing contact with a serving network node (e.g. the serving MME) of the second terminal device (e.g. UE2), for example to check the status of UE2; and/or triggering the setup of measurement reporting in the terminal devices, in order to prepare for a service continuity switch. Further or other uses for the stored 'binding' information for two UEs involved in a ProSe communication session include:

a. use by an MME 35 in setting up an associated EPS bearer to the UE with a configuration that is based on the stored information;

b. use by an eNB 40 in setting up measurements in UE1 and/or UE2;

c. use in deciding on the switching between an infrastructure path and a direct path based on, for example, the coverage status (by either eNB 40 or MME 35 or both); d. performing a user data switch of the transmitted packets from UE1 to UE2 from using a direct path to instead using an infrastructure path (i.e. the associated EPS bearer). When switching to an infrastructure path, the IMSI of UE2 and the IDs of the serving network nodes are used to find the UE context in order to configure the EPS bearer and the ProSe bearer accordingly (by either the eNB 40 or MME 35 or both); e. performing a user data switch of the transmitted packets from UE1 to UE2 from using an infrastructure path to using a direct path. The IMSI of UE2 and the IDs of the serving network nodes are used to find the UE context in order to configure the EPS bearer and the ProSe bearer accordingly (by either eNB 40 or MME 35 or both); and/or f. triggering a switch of the user data transmitted from UE1 to UE2;

g. collecting statistics about ProSe communication sessions;

h. collecting information used for charging of the ProSe communication session.

In a first specific embodiment of the techniques described herein, a UE 42, UE1 transmits its own ProSe UE ID and also the ProSe UE ID of UE2 (e.g. a relay UE/remote UE for the first service continuity scenario described above or a peer UE in the second service continuity scenario described above) in a ProSe Communication Session Information message to its serving network node (e.g. a serving eNB 40, MME 35 or ProSe-F node 37).

The serving network node receiving this message stores the information in the UE1 context. The serving network node then checks whether it can find a UE context, served by itself, which has information of the same ProSe Communication Session (e.g. the same ProSe UE IDs). If it finds that UE context, it knows that the context is UE2 (in terms of the ProSe communication), and the binding is successful. If it doesn't find that UE context, the binding can be completed by receiving a ProSe Communication Session Information message from UE2. Alternatively, the binding can be completed when the UE context of UE2 is relocated to the same serving network node.

Another alternative is for the serving network node to find the UE2 context by inquiring with the neighbour network nodes, and including the ProSe UE IDs of the two UEs in the inquiry message.

In a second specific embodiment, a first network node which has already obtained the UE binding information provides the UE binding information to a second network node. For example, the first network node may be an application server 38 which is aware of ProSe communication sessions and the second network node is the MME 35 serving UE1 and/or UE2. As another example, the first network node may be a first MME 35 and the second network node may be a second MME 35 (this can occur in the case of UE1 or UE2 changing its serving MME 35 from the first MME 35 to the second MME 35).

As noted above, when a given network node, e.g. a serving MME 35 of UE1 , has received the UE binding information using either of these specific embodiments, it may use this information to prepare for and/or execute service continuity switching for that communication session, and/or to check the status of the other UE with a serving MME 35 of that other UE. Similarly, when a serving eNB has received the ProSe Communication Session information, it can use the information to, for example, trigger measurements performed by UE1 and/or UE2. Figures 6-9 illustrate various exemplary embodiments of the techniques described herein in which ProSe Communication Session Information is transferred to a network node from a UE.

Figure 6 illustrates the signalling according to an exemplary embodiment. UE1 and UE2 both have coverage from the network 32 (indicated by step 601) and a ProSe unicast communication session is established between UE1 and UE2 (indicated by step 602). UE1 transmits a ProSe Communication Session Information message 603 to a network node serving UE1 (ProSe-F node 37 in this embodiment) which includes UE1 's ProSe UE Identity, UE2's ProSe UE Identity and relay information (in the case where UE2 is used as a relay UE for UE-network relay). The ProSe Communication Session Information message 603 may also include other information about the ProSe Bearer, such as quality of service (QoS) information, a TFT (Traffic Flow Template), layer 2 information (radio link control, RLC, configuration, logical channel identity etc.).

The transmitting of message 603 can be triggered when UE1 is in coverage and sets up the ProSe Direct Bearer to UE2.

The network node (ProSe-F node 37 in this case, although it could alternatively be an eNB 40, an MME 35), stores the ProSe Communication Session information in the received message as part of UE1 's context (step 604).

In some cases the network node is able to translate UE2's ProSe UE ID in message 603 into a network-level identity of UE2 (such as the I MSI or some other identity of UE2 used within the EPS network, such as a Temporary Mobile Subscriber Identity (TMSI), Bearer ID, ... ), and stores that too. As part of that translation, the network node may use a ProSe UE ID > network-level ID translation table stored within the network node. The network node may keep the information of the ID/address of other network nodes serving the UE2 in the EPS network (e.g., eNB, MME), and update that when there is a mobility of UE2.

The network node may keep the information of the coverage status (or generally the cellular link quality) of UE2, and update that when there is a mobility of UE2.

Similarly to UE1 , UE2 also transmits a ProSe Communication Session Information message 605 to the network node, and a serving network node of UE2 (ProSe-F node 37 in this embodiment) stores the ProSe Communication Session information in the received message, as part of UE2's context (shown by step 606).

As with message 603 from UE1 , in some cases the network node also translates UE1 's ProSe UE ID to, for example, an IMSI, and stores this identity. The network node may also maintain IDs/addresses of serving network nodes, and store these identities. The network node may also store information about UE coverage status. The binding of UE1 and UE2 in the network node is completed if the network node has UE contexts for both UE1 and UE2 (step 607).

Figure 7 illustrates the signalling in a variation to the embodiment in Figure 6. In this variation, the sending of the ProSe Communication Session Information messages is triggered by the relevant UE entering coverage of the network 32 (i.e. starting to be served. Thus, UE1 and UE2 are initially out of coverage of the network 32 (indicated by step 701), and a ProSe communication session is set up between them (step 702). When UE1 enters network coverage (i.e. it starts being served by a network node (e.g. eNB 40, MME 35 and/or ProSe-F node 37)), as indicated by step 703, UE1 sends the ProSe Communication Session Information message 704 to the network node. Message 704 is similar to message 603 in Figure 6. When UE2 enters network coverage (indicated by step 706), UE2 sends ProSe Communication Session Information message 707 to the network node. Message 707 is similar to message

605 in Figure 6. The network node stores the ProSe Communication Session Information messages 704 and 707, as indicated by steps 705 and 708 respectively, and completes the binding by finding UE contexts for both UE1 and UE2 (step 709). The operations in steps 705, 708 and 709 correspond generally to the operations in steps 604, 606 and 607 respectively.

Figure 8 illustrates the signalling in another variation to the embodiment in Figure 6. In this variation, UE1 is initially within the coverage of the network 32 (indicated by step 801) and UE2 is initially outside the coverage of the network 32 (indicated by step 802). A ProSe communication session is set up between UE1 and UE2 (step 803).

UE1 then sends the ProSe Communication Session Information message 804 to the network node. Message 804 is similar to message 603 in Figure 6. The ProSe Communication Session Information is stored by the network node (step 805, which is similar to step 604 in Figure 6).

When UE2 enters coverage of the network (indicated by step 806), UE2 sends the ProSe Communication Session Information message 807 to the network node. Message 807 is similar to message 605 in Figure 6. The ProSe Communication Session Information is stored in the network node (step 808, which is similar to step

606 in Figure 6). The binding of UE1 and UE2 is complete when the network node can find UE contexts for both UE1 and UE2 based on the ProSe communication session information.

The flow chart in Figure 9 illustrates an exemplary method of operating a network node to complete a binding of two UEs engaged in a ProSe communication session. Thus, in step 901 the network node (a serving node of UE1) receives a ProSe Communication Session Information message from UE1 (e.g. message 603 in Figure 6). The network node stores the received ProSe UE IDs for UE1 and UE2 as part of UE1's context information (step 902).

After receiving a ProSe Communication Session Information message, the network node checks if there is another UE context, having the information of the same ProSe session (step 903). For example, if the message received from UE1 contains the ProSe UE IDs of UE1 and UE2, the network node checks whether another UE context has the same ProSe UE IDs as the received ProSe Communication Session Information (step 904). If another UE context is found, the binding is successful (step 905). If not, the method passes to step 906 in which the network node awaits a ProSe Communication Session message from UE2 or from a neighbouring serving network node.

For example, if a relocation is performed of a UE context where the serving network node is the target for the relocation, the serving network node may receive ProSe Communication Session Information as part of the UE context being relocated to the network node (step 906).

After receiving a ProSe Communication Session Information message from UE2 in step 906, the network node stores the information in a UE context for UE2 (step 907). The network node then checks if it has a UE context containing a ProSe Communication Session Information with matching ProSe UE IDs (step 908). It is necessary to do this since UE2 may be engaged in another ProSe communication session with a UE other than UE1. If another UE context is found (step 909), the binding is successful (step 905).

Although not illustrated in Figure 9, in addition or alternatively to awaiting a ProSe Communication Session Information message from UE2, the serving network node of UE1 may transmit a request to neighbouring serving network nodes (e.g. a ProSe Communication Session Inquiry message) for this information, with the request including the ProSe UE IDs of UE1 and UE2, and the network-level identity of UE1. The neighbouring network nodes then check whether they have a UE context with ProSe Communication Session Information matching the ProSe UE IDs. If so, the binding is successful and the particular neighbouring network node transmits a response to the request (e.g. a ProSe Session Inquiry Response message) back to the serving network node of UE1 , with the response including a network-level identity of UE2. This action also completes the binding of UE1 in the serving network node of UE1.

Figures 10-13 illustrate various exemplary embodiments of the techniques described herein in which ProSe Communication Session information is transferred between serving network nodes. A typical case is to transfer the information to a serving MME 35 of UE1 from another serving network node (such as a ProSe-F node 37, an eNB 40 or another MME 35).

In the exemplary embodiment of Figure 10, after a ProSe Communication Session between UE1 and UE2 is set up (indicated by step 1001), a first serving network node (e.g. ProSe Application Server 38) of UE1 and/or UE2 detects the setup of the session, and transmits a ProSe Communication Session Information message 1002 to a second serving network node (e.g. ProSe-F node 37) of UE1.

The second serving network node (e.g. ProSe-F node 37) forwards the information (message 1003) to a third serving network node (e.g. the serving MME 35) of UE1. In some embodiments message 1003 is sent via HSS 39. The serving MME 35 of UE1 stores the ProSe Communication Session Information for UE1 (step 1004) and in turn creates a new ProSe Communication Session message 1005 and transmits it to the serving eNB 40 of UE1. The serving eNB 40 of UE1 stores the ProSe Communication Session Information for UE1 (step 1006).

Thus, in the embodiment of Figure 10 the ProSe Communication Session Information for UE1 effectively propagates from the ProSe Application Server 38 down to the other serving network nodes of UE1 (MME 35 and eNB 40). It will be appreciated that in alternative embodiments, the information can propagate from a different serving node of UE1 to the other serving network nodes. In addition to the signalling shown in Figure 10, the ProSe Application Server 38 can transmit a ProSe Communication Session information message to the serving ProSe-F node 37 of UE2, and so on. In this way, all serving nodes of UE1 and UE2 will become aware of a ProSe Communication Session between UE1 and UE2, with that information including the ProSe UE identities and network-level identities of these UEs.

In the exemplary embodiment of Figure 1 1 , a UE1 that is in network coverage (indicated by step 1101) and that sets up a ProSe Communication Session with UE2 (indicated by step 1102) will send a Sidelink UE Information message 1103 to the serving eNB 40 of UE1. The eNB 40 stores the ProSe Communication Session Information for UE1 (step 1104) and transmits a ProSe Communication Session information message 1105 to the serving MME 35 of UE1 , with message 1105 including the ProSe UE identity of UE2. The serving MME 35 stores the information (step 1 106).

The exemplary embodiment of Figure 12 illustrates signalling between serving MMEs 35 when there is a change in serving MME 35. Thus, when a decision (step 1201) is made to change the serving network node of UE1 from a first serving network node (MME1) to a second serving network node (MME2), a ProSe Communication Session information message 1202 is sent from MME1 to MME2. MME2 stores the ProSe Communication Session information (step 1203).

In an alternative embodiment, the ProSe Communication Session Information is transferred within existing MME relocation messages.

The exemplary embodiment of Figure 13 illustrates signalling between serving eNBs 40 when there is a change in serving eNB 40. Thus, when a decision (step 1301) is made to change the serving network node of UE1 from a first serving network node (eNB1) to a second serving network node (eNB2), a ProSe Communication Session information message 1302 is sent from eNB1 to eNB2. eNB2 stores the ProSe Communication Session information (step 1303).

In an alternative embodiment, the ProSe Communication Session Information is transferred within existing eNB relocation messages. Thus the techniques described above and illustrated in the Figures provide ways for a network node to be made aware of the identity of UEs involved in a ProSe communication session with each other. 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 network node in a communication network, the method comprising:

receiving information about a proximity service, ProSe, communication session between a first terminal device and a second terminal device, the information comprising a respective ProSe user equipment, UE, identity, ID, for each of the first terminal device and the second terminal device;

determining a serving node identity of a network node in the communication network that is serving the first terminal device and a serving node identity of a network node in the communication network that is serving the second terminal device; and determining a network-level identity for the first terminal device and a network- level identity for the second terminal device, wherein the network-level identity identifies a terminal device to the network node that is serving the terminal device.

2. A method as defined in statement 1 , the method further comprising:

storing the ProSe UE IDs for the first terminal device and the second terminal device, the serving node identities for the first terminal device and the second terminal device and the network-level identities for the first terminal device and the second terminal device.

3. A method as defined in statement 1 or 2, the method further comprising:

sending the ProSe UE IDs for the first terminal device and the second terminal device, the serving node identities for the first terminal device and the second terminal device and the network-level identities for the first terminal device and the second terminal device to a second network node.

4. A method as defined in any of statements 1-3, wherein one or both of the steps of determining comprise translating a ProSe UE ID for a terminal device into a serving node identity and/or a network-level identity for the terminal device.

5. A method as defined in any of statements 1-4, wherein one or both of the steps of determining comprise receiving the serving node identities and/or the network-level identities from another node in the communication network.

6. A method as defined in any of statements 1-5, wherein the first network node is the serving network node for the first terminal device. 7. A method as defined in statement 6, wherein the step of receiving comprises receiving the information about the ProSe communication session from the first terminal device.

8. A method as defined in statement 6 or 7, wherein the step of determining a serving node identity and a network-level identity for the second terminal device comprises determining if the first network node is a serving network node of the second terminal device; and if the first network node is the serving network node of the second terminal device, obtaining the network-level identity for the second terminal device from a memory of the first network node.

9. A method as defined in any of statements 6-8, wherein the steps of determining a serving node identity and a network-level identity for the second terminal device comprise sending a request for information on the second terminal device to a second network node, the request comprising the ProSe UE IDs for the first terminal device and the second terminal device.

10. A method as defined in any of statements 6-9, wherein the step of determining a serving node identity and a network-level identity for the second terminal device comprises receiving the serving node identity and the network-level identity for the second terminal device from another network node. 1 1. A method as defined in any of statements 6-9, wherein the step of determining a serving node identity and a network-level identity for the second terminal device comprises receiving the serving node identity and the network-level identity for the second terminal device from the second terminal device.

12. A method as defined in any of statements 1-5, wherein the step of determining a serving node identity and a network-level identity for the first terminal device comprises receiving the serving node identity and the network-level identity for the first terminal device from another network node.

13. A method as defined in any of statements 1-5 or 12, wherein the step of determining a serving node identity and a network-level identity for the second terminal device comprises receiving the serving node identity and the network-level identity for the second terminal device from another network node.

14. A method as defined in any of statements 1-13, further comprising:

using the ProSe UE IDs for the first terminal device and the second terminal device, the serving node identities for the first terminal device and the second terminal device and the network-level identities for the first terminal device and the second terminal device to provide service continuity switching for the ProSe communication session.

15. 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 1-14.

16. A first network node for use in a communication network, the first network node being adapted to:

receive information about a proximity service, ProSe, communication session between a first terminal device and a second terminal device, the information comprising a respective ProSe user equipment, UE, identity, ID, for each of the first terminal device and the second terminal device;

determine a serving node identity of a network node in the communication network that is serving the first terminal device and a serving node identity of a network node in the communication network that is serving the second terminal device; and determine a network-level identity for the first terminal device and a network-level identity for the second terminal device, wherein the network-level identity identifies a terminal device to the network node that is serving the terminal device. 17. A first network node as defined in statement 16, the first network node being further adapted to:

store the ProSe UE IDs for the first terminal device and the second terminal device, the serving node identities for the first terminal device and the second terminal device and the network-level identities for the first terminal device and the second terminal device.

18. A first network node as defined in statement 16 or 17, the first network node being further adapted to:

send the ProSe UE IDs for the first terminal device and the second terminal device, the serving node identities for the first terminal device and the second terminal device and the network-level identities for the first terminal device and the second terminal device to a second network node.

19. A first network node as defined in any of statements 16-18, wherein being adapted to determine the serving node identity of a network node that is serving the first terminal device and the serving node identity of the network node that is serving the second terminal device and/or determine the network-level identity for the first terminal device and the network-level identity for the second terminal device comprises being adapted to translate a ProSe UE ID for a terminal device into a serving node identity and/or a network-level identity for the terminal device.

20. A first network node as defined in any of statements 16-19, wherein being adapted to determine the serving node identity of a network node that is serving the first terminal device and the serving node identity of the network node that is serving the second terminal device and/or determine the network-level identity for the first terminal device and the network-level identity for the second terminal device comprises being adapted to receive the serving node identities and/or the network-level identities from another node in the communication network. 21. A first network node as defined in any of statements 16-20, wherein the first network node is the serving network node for the first terminal device. 22. A first network node as defined in statement 21 , wherein the first network node is adapted to receive the information about the ProSe communication session from the first terminal device.

23. A first network node as defined in statement 21 or 22, wherein being adapted to determine the serving node identity of the network node that is serving the second terminal device and determine the network-level identity for the second terminal device comprises being adapted to determine if the first network node is a serving network node of the second terminal device; and to obtain the network-level identity for the second terminal device from a memory of the first network node if the first network node is the serving network node of the second terminal device.

24. A first network node as defined in any of statements 21-23, wherein being adapted to determine the serving node identity of the network node that is serving the second terminal device and determine the network-level identity for the second terminal device comprises being adapted to send a request for information on the second terminal device to a second network node, the request comprising the ProSe UE IDs for the first terminal device and the second terminal device.

25. A first network node as defined in any of statements 21-24, wherein being adapted to determine the serving node identity of the network node that is serving the second terminal device and determine the network-level identity for the second terminal device comprises being adapted to receive the serving node identity and the network- level identity for the second terminal device from another network node.

26. A first network node as defined in any of statements 21-24, wherein being adapted to determine the serving node identity of the network node that is serving the second terminal device and determine the network-level identity for the second terminal device comprises being adapted to receive the serving node identity and the network- level identity for the second terminal device from the second terminal device.

27. A first network node as defined in any of statements 16-20, wherein being adapted to determine the serving node identity of the network node that is serving the first terminal device and determine the network-level identity for the first terminal device comprises being adapted to receive the serving node identity and the network-level identity for the first terminal device from another network node.

28. A first network node as defined in any of statements 16-20 or 27, wherein being adapted to determine the serving node identity of the network node that is serving the second terminal device and determine the network-level identity for the second terminal device comprises being adapted to receive the serving node identity and the network- level identity for the second terminal device from another network node. 29. A first network node as defined in any of statements 16-28, the first network node being further adapted to:

use the ProSe UE IDs for the first terminal device and the second terminal device, the serving node identities for the first terminal device and the second terminal device and the network-level identities for the first terminal device and the second terminal device to provide service continuity switching for the ProSe communication session.

30. A first network node for use in a communication network, the first network node comprising a processor and a memory, said memory containing instructions executable by said processor whereby said first network node is operative to:

determine a serving node identity of a network node in the communication network that is serving the first terminal device and a serving node identity of a network node in the communication network that is serving the second terminal device; and determine a network-level identity for the first terminal device and a network-level identity for the second terminal device, wherein the network-level identity identifies a terminal device to the network node that is serving the terminal device.

31. A first network node as defined in statement 30, wherein the first network node is further operative to:

32. A first network node as defined in statement 30 or 31 , wherein the first network node is further operative to:

33. A first network node as defined in any of statements 30-32, wherein being operative to determine the serving node identity of a network node that is serving the first terminal device and the serving node identity of the network node that is serving the second terminal device and/or determine the network-level identity for the first terminal device and the network-level identity for the second terminal device comprises being operative to translate a ProSe UE ID for a terminal device into a serving node identity and/or a network-level identity for the terminal device.

34. A first network node as defined in any of statements 30-33, wherein being operative to determine the serving node identity of a network node that is serving the first terminal device and the serving node identity of the network node that is serving the second terminal device and/or determine the network-level identity for the first terminal device and the network-level identity for the second terminal device comprises being operative to receive the serving node identities and/or the network-level identities from another node in the communication network.

35. A first network node as defined in any of statements 30-34, wherein the first network node is the serving network node for the first terminal device. 36. A first network node as defined in statement 35, wherein the first network node is operative to receive the information about the ProSe communication session from the first terminal device.

37. A first network node as defined in statement 35 or 36, wherein being operative to determine the serving node identity of the network node that is serving the second terminal device and determine the network-level identity for the second terminal device comprises being operative to determine if the first network node is a serving network node of the second terminal device; and to obtain the network-level identity for the second terminal device from a memory of the first network node if the first network node is the serving network node of the second terminal device.

38. A first network node as defined in any of statements 35-37, wherein being operative to determine the serving node identity of the network node that is serving the second terminal device and determine the network-level identity for the second terminal device comprises being operative to send a request for information on the second terminal device to a second network node, the request comprising the ProSe UE IDs for the first terminal device and the second terminal device.

39. A first network node as defined in any of statements 35-37, wherein being operative to determine the serving node identity of the network node that is serving the second terminal device and determine the network-level identity for the second terminal device comprises being operative to receive the serving node identity and the network- level identity for the second terminal device from another network node.

40. A first network node as defined in any of statements 35-38, wherein being operative to determine the serving node identity of the network node that is serving the second terminal device and determine the network-level identity for the second terminal device comprises being operative to receive the serving node identity and the network- level identity for the second terminal device from the second terminal device. 41. A first network node as defined in any of statements 30-34, wherein being operative to determine the serving node identity of the network node that is serving the first terminal device and determine the network-level identity for the first terminal device comprises being operative to receive the serving node identity and the network-level identity for the first terminal device from another network node.

42. A first network node as defined in any of statements 30-34 or 41 , wherein being operative to determine the serving node identity of the network node that is serving the second terminal device and determine the network-level identity for the second terminal device comprises being operative to receive the serving node identity and the network- level identity for the second terminal device from another network node. 43. A first network node as defined in any of statements 30-42, wherein the first network node is further operative to:

44. A first network node for use in a communication network, the first network node comprising:

a first module configured to receive information about a proximity service, ProSe, communication session between a first terminal device and a second terminal device, the information comprising a respective ProSe user equipment, UE, identity, ID, for each of the first terminal device and the second terminal device;

a second module configured to determine a serving node identity of a network node in the communication network that is serving the first terminal device and a serving node identity of a network node in the communication network that is serving the second terminal device; and

a third module configured to determine a network-level identity for the first terminal device and a network-level identity for the second terminal device, wherein the network-level identity identifies a terminal device to the network node that is serving the terminal device.

45. A first network node as defined in statement 44, the first network node further comprising:

a fourth module configured to store the ProSe UE IDs for the first terminal device and the second terminal device, the serving node identities for the first terminal device and the second terminal device and the network-level identities for the first terminal device and the second terminal device.

46. A first network node as defined in statement 44 or 45, the first network node further comprising:

a fifth module configured to send the ProSe UE IDs for the first terminal device and the second terminal device, the serving node identities for the first terminal device and the second terminal device and the network-level identities for the first terminal device and the second terminal device to a second network node. 47. A first network node as defined in any of statements 44-46, wherein the second module being configured to determine the serving node identity of a network node that is serving the first terminal device and the serving node identity of the network node that is serving the second terminal device and/or the third module being configured to determine the network-level identity for the first terminal device and the network-level identity for the second terminal device comprises the second module being configured to translate a ProSe UE ID for a terminal device into a serving node identity and/or the third module being configured to translate a ProSe UE ID for a terminal device into a network-level identity for the terminal device.

48. A first network node as defined in any of statements 44-47, wherein the second module being configured to determine the serving node identity of a network node that is serving the first terminal device and the serving node identity of the network node that is serving the second terminal device and/or the third module being configured to determine the network-level identity for the first terminal device and the network-level identity for the second terminal device comprises the second module being configured to receive the serving node identities from another node in the communication network and/or the third module being configured to receive the network-level identities from another node in the communication network.

49. A first network node as defined in any of statements 44-48, wherein the first network node is the serving network node for the first terminal device. 50. A first network node as defined in statement 49, wherein the first module is configured to receive the information about the ProSe communication session from the first terminal device.

51. A first network node as defined in statement 49 or 50, wherein the second module being configured to determine the serving node identity of the network node that is serving the second terminal device and the third module being configured to determine the network-level identity for the second terminal device comprises the second module and the third module being configured to determine if the first network node is a serving network node of the second terminal device and to obtain the network-level identity for the second terminal device from a memory of the first network node if the first network node is the serving network node of the second terminal device.

52. A first network node as defined in any of statements 49-51 , wherein the second module being configured to determine the serving node identity of the network node that is serving the second terminal device and the third module being configured to determine the network-level identity for the second terminal device comprises the second module and the third module being configured to send a request for information on the second terminal device to a second network node, the request comprising the ProSe UE IDs for the first terminal device and the second terminal device.

53. A first network node as defined in any of statements 49-52, wherein the second module being configured to determine the serving node identity of the network node that is serving the second terminal device and the third module being configured to determine the network-level identity for the second terminal device comprises the second module and the third module being configured to receive the serving node identity and the network-level identity for the second terminal device from another network node. 54. A first network node as defined in any of statements 49-52, wherein the second module being configured to determine the serving node identity of the network node that is serving the second terminal device and the third module being configured to determine the network-level identity for the second terminal device comprises the second module and the third module being configured to receive the serving node identity and the network-level identity for the second terminal device from the second terminal device.

55. A first network node as defined in any of statements 44-48, wherein the second module being configured to determine the serving node identity of the network node that is serving the first terminal device and the third module being configured to determine the network-level identity for the first terminal device comprises the second module and the third module being configured to receive the serving node identity and the network-level identity for the first terminal device from another network node. 56. A first network node as defined in any of statements 44-48 or 55, wherein the second module being configured to determine the serving node identity of the network node that is serving the second terminal device and the third module being configured to determine the network-level identity for the second terminal device comprises the second module and the third module being configured to receive the serving node identity and the network-level identity for the second terminal device from another network node.

57. A first network node as defined in any of statements 44-56, the first network node further comprising:

a sixth module configured to use the ProSe UE IDs for the first terminal device and the second terminal device, the serving node identities for the first terminal device and the second terminal device and the network-level identities for the first terminal device and the second terminal device to provide service continuity switching for the ProSe communication session. In some embodiments, the modules of statements 44-57 may be implemented as a computer program running on a processor (e.g. processor/processing module 60 in Fig. 4).

Claims

1. A method of operating a first network node in a communication network, the method comprising:

receiving (101) information about a proximity service, ProSe, communication session between a first terminal device and a second terminal device, the information comprising a respective ProSe user equipment, UE, identity, ID, for each of the first terminal device and the second terminal device;

determining (103) a serving node identity of a network node in the communication network that is serving the first terminal device and a serving node identity of a network node in the communication network that is serving the second terminal device; and

determining (105) a network-level identity for the first terminal device and a network-level identity for the second terminal device, wherein the network-level identity identifies a terminal device to the network node that is serving the terminal device.

2. A method as claimed in claim 1 , the method further comprising:

3. A method as claimed in claim 1 or 2, the method further comprising:

4. A method as claimed in any of claims 1-3, wherein one or both of the steps of determining comprise translating a ProSe UE ID for a terminal device into a serving node identity and/or a network-level identity for the terminal device.

5. A method as claimed in any of claims 1-4, wherein one or both of the steps of determining comprise receiving the serving node identities and/or the network-level identities from another node in the communication network.

6. A method as claimed in any of claims 1-5, wherein the first network node is the serving network node for the first terminal device.

7. A method as claimed in claim 6, wherein the step of receiving comprises receiving the information about the ProSe communication session from the first terminal device.

8. A method as claimed in claim 6 or 7, wherein the step of determining a serving node identity and a network-level identity for the second terminal device comprises determining if the first network node is a serving network node of the second terminal device; and if the first network node is the serving network node of the second terminal device, obtaining the network-level identity for the second terminal device from a memory of the first network node.

9. A method as claimed in any of claims 6-8, wherein the steps of determining a serving node identity and a network-level identity for the second terminal device comprise sending a request for information on the second terminal device to a second network node, the request comprising the ProSe UE IDs for the first terminal device and the second terminal device.

10. A method as claimed in any of claims 6-9, wherein the step of determining a serving node identity and a network-level identity for the second terminal device comprises receiving the serving node identity and the network-level identity for the second terminal device from another network node.

1 1. A method as claimed in any of claims 6-9, wherein the step of determining a serving node identity and a network-level identity for the second terminal device comprises receiving the serving node identity and the network-level identity for the second terminal device from the second terminal device.

12. A method as claimed in any of claims 1-5, wherein the step of determining a serving node identity and a network-level identity for the first terminal device comprises receiving the serving node identity and the network-level identity for the first terminal device from another network node.

13. A method as claimed in any of claims 1-5 or 12, wherein the step of determining a serving node identity and a network-level identity for the second terminal device comprises receiving the serving node identity and the network-level identity for the second terminal device from another network node.

14. A method as claimed in any of claims 1-13, further comprising:

15. 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 1-14.

16. A first network node (35; 36; 37; 38; 40) for use in a communication network (32), the first network node being adapted to:

receive information about a proximity service, ProSe, communication session between a first terminal device (42) and a second terminal device (42), the information comprising a respective ProSe user equipment, UE, identity, ID, for each of the first terminal device and the second terminal device;

determine a serving node identity of a network node (40) in the communication network that is serving the first terminal device and a serving node identity of a network node (40) in the communication network that is serving the second terminal device; and determine a network-level identity for the first terminal device and a network-level identity for the second terminal device, wherein the network-level identity identifies a terminal device to the network node that is serving the terminal device.

17. A first network node as claimed in claim 16, the first network node being further adapted to:

18. A first network node as claimed in claim 16 or 17, the first network node being further adapted to:

19. A first network node as claimed in any of claims 16-18, wherein being adapted to determine the serving node identity of a network node that is serving the first terminal device and the serving node identity of the network node that is serving the second terminal device and/or determine the network-level identity for the first terminal device and the network-level identity for the second terminal device comprises being adapted to translate a ProSe UE ID for a terminal device into a serving node identity and/or a network-level identity for the terminal device.

20. A first network node as claimed in any of claims 16-19, wherein being adapted to determine the serving node identity of a network node that is serving the first terminal device and the serving node identity of the network node that is serving the second terminal device and/or determine the network-level identity for the first terminal device and the network-level identity for the second terminal device comprises being adapted to receive the serving node identities and/or the network-level identities from another node in the communication network.

21. A first network node as claimed in any of claims 16-20, wherein the first network node is the serving network node for the first terminal device.

22. A first network node as claimed in claim 21 , wherein the first network node is adapted to receive the information about the ProSe communication session from the first terminal device.

23. A first network node as claimed in claim 21 or 22, wherein being adapted to determine the serving node identity of the network node that is serving the second terminal device and determine the network-level identity for the second terminal device comprises being adapted to determine if the first network node is a serving network node of the second terminal device; and to obtain the network-level identity for the second terminal device from a memory of the first network node if the first network node is the serving network node of the second terminal device.

24. A first network node as claimed in any of claims 21-23, wherein being adapted to determine the serving node identity of the network node that is serving the second terminal device and determine the network-level identity for the second terminal device comprises being adapted to send a request for information on the second terminal device to a second network node, the request comprising the ProSe UE IDs for the first terminal device and the second terminal device.

25. A first network node as claimed in any of claims 21-24, wherein being adapted to determine the serving node identity of the network node that is serving the second terminal device and determine the network-level identity for the second terminal device comprises being adapted to receive the serving node identity and the network-level identity for the second terminal device from another network node.

26. A first network node as claimed in any of claims 21-24, wherein being adapted to determine the serving node identity of the network node that is serving the second terminal device and determine the network-level identity for the second terminal device comprises being adapted to receive the serving node identity and the network-level identity for the second terminal device from the second terminal device.

27. A first network node as claimed in any of claims 16-20, wherein being adapted to determine the serving node identity of the network node that is serving the first terminal device and determine the network-level identity for the first terminal device comprises being adapted to receive the serving node identity and the network-level identity for the first terminal device from another network node.

28. A first network node as claimed in any of claims 16-20 or 27, wherein being adapted to determine the serving node identity of the network node that is serving the second terminal device and determine the network-level identity for the second terminal device comprises being adapted to receive the serving node identity and the network- level identity for the second terminal device from another network node.

29. A first network node as claimed in any of claims 16-28, the first network node being further adapted to: use the ProSe UE IDs for the first terminal device and the second terminal device, the serving node identities for the first terminal device and the second terminal device and the network-level identities for the first terminal device and the second terminal device to provide service continuity switching for the ProSe communication session.

31. A first network node as claimed in claim 30, wherein the first network node is further operative to:

32. A first network node as claimed in claim 30 or 31 , wherein the first network node is further operative to:

33. A first network node as claimed in any of claims 30-32, wherein being operative to determine the serving node identity of a network node that is serving the first terminal device and the serving node identity of the network node that is serving the second terminal device and/or determine the network-level identity for the first terminal device and the network-level identity for the second terminal device comprises being operative to translate a ProSe UE ID for a terminal device into a serving node identity and/or a network-level identity for the terminal device.

34. A first network node as claimed in any of claims 30-33, wherein being operative to determine the serving node identity of a network node that is serving the first terminal device and the serving node identity of the network node that is serving the second terminal device and/or determine the network-level identity for the first terminal device and the network-level identity for the second terminal device comprises being operative to receive the serving node identities and/or the network-level identities from another node in the communication network.

35. A first network node as claimed in any of claims 30-34, wherein the first network node is the serving network node for the first terminal device.

36. A first network node as claimed in claim 35, wherein the first network node is operative to receive the information about the ProSe communication session from the first terminal device.

37. A first network node as claimed in claim 35 or 36, wherein being operative to determine the serving node identity of the network node that is serving the second terminal device and determine the network-level identity for the second terminal device comprises being operative to determine if the first network node is a serving network node of the second terminal device; and to obtain the network-level identity for the second terminal device from a memory of the first network node if the first network node is the serving network node of the second terminal device.

38. A first network node as claimed in any of claims 35-37, wherein being operative to determine the serving node identity of the network node that is serving the second terminal device and determine the network-level identity for the second terminal device comprises being operative to send a request for information on the second terminal device to a second network node, the request comprising the ProSe UE IDs for the first terminal device and the second terminal device.

39. A first network node as claimed in any of claims 35-37, wherein being operative to determine the serving node identity of the network node that is serving the second terminal device and determine the network-level identity for the second terminal device comprises being operative to receive the serving node identity and the network-level identity for the second terminal device from another network node.

40. A first network node as claimed in any of claims 35-38, wherein being operative to determine the serving node identity of the network node that is serving the second terminal device and determine the network-level identity for the second terminal device comprises being operative to receive the serving node identity and the network-level identity for the second terminal device from the second terminal device.

41. A first network node as claimed in any of claims 30-34, wherein being operative to determine the serving node identity of the network node that is serving the first terminal device and determine the network-level identity for the first terminal device comprises being operative to receive the serving node identity and the network-level identity for the first terminal device from another network node.

42. A first network node as claimed in any of claims 30-34 or 41 , wherein being operative to determine the serving node identity of the network node that is serving the second terminal device and determine the network-level identity for the second terminal device comprises being operative to receive the serving node identity and the network- level identity for the second terminal device from another network node.

43. A first network node as claimed in any of claims 30-42, wherein the first network node is further operative to: