WO2013053365A1

WO2013053365A1 - Methods of and nodes for locating an emergency voice session anchoring node in a serving communication network for transferring an emergency voice session

Info

Publication number: WO2013053365A1
Application number: PCT/EP2011/005042
Authority: WO
Inventors: Ralf Keller; Afshin Abtin
Original assignee: Telefonaktiebolaget L M Ericsson (Publ)
Priority date: 2011-10-10
Filing date: 2011-10-10
Publication date: 2013-04-18
Also published as: AR088281A1

Abstract

A method of locating an emergency voice session anchoring node (212) of a plurality of emergency voice session anchoring nodes in an Internet Protocol Multimedia Subsystem of a serving communication network (200) for transferring an emergency voice session of a terminal from a packet switched source radio access network (260) of the serving communication network (200) to a target radio access network (278) of the serving communication network (200) associated with a circuit switched domain of the serving communication network (200) is described. The emergency voice session is anchored in the emergency voice session anchoring node (212) and the transferring of the emergency voice session employs voice session continuity. The emergency voice session anchoring node (212) is identified by a node identifier. The method is executed by the emergency voice session anchoring node (212). The method comprises (240) sending the node identifier of the emergency voice session anchoring node (212) towards a control node (206) of the circuit switched domain of the serving communication network (200) for the locating of the emergency voice session anchoring node (212). Thereby, a support of a reliable session transfer of the emergency voice session from the packet switched source radio access network (260) to the target radio access network (278) associated with the circuit switched domain of the serving communication network (200) may be enabled.

Description

Methods of and nodes for locating an emergency voice session anchoring node in a serving communication network for transferring an emergency voice session

Technical field

The invention relates to telecommunications, and in particular to methods of and nodes for locating an emergency voice session anchoring node of a plurality of emergency voice session anchoring nodes in a serving communication network for transferring an emergency voice session from a packet switched source radio access network of the serving communication network to a target radio access network of the serving communication network associated with a circuit switched domain of the serving communication network, and a computer program.

Background An emergency voice session may be originated from a terminal to a destination via a packet switched radio access network and an emergency voice session anchoring node of an Internet Protocol Multimedia Subsystem (IMS) based serving

communication network.

The serving communication network may comprise at least two or more emergency voice session anchoring nodes, for example to meet capacity needs regarding an increased number of emergency voice sessions established in the serving

communication network or to provide geographical redundancy in the serving communication network. Further, it may be favorable to place the selected

emergency voice session anchoring node close to the involved destination, in order to reduce latency times of the originated emergency voice session.

In a case in which the terminal may lose coverage to the packet switched radio access network a session transfer of the emergency voice session from the packet switched radio access network towards another radio access network associated with a circuit switched domain of the IMS based serving communication network may allow for maintaining the established emergency voice session to the destination.

Since the serving communication network may comprise multiple emergency voice session anchoring nodes, a control node associated with the packet switched source radio access network and/or a control node associated with the target radio access network may have to be aware of the selected emergency voice session anchoring node linked in the session path of the established emergency voice session.

Summary It is an object of the present invention to provide methods and nodes for supporting a reliable session transfer of an emergency voice session from a packet switched source radio access network to a target radio access network associated with a circuit switched domain of a serving communication network which comprises multiple emergency voice session anchoring nodes. Further, it is an object of the present invention to provide a computer program using such a method.

The objects defined above are solved by methods of, a control node and an intermediate node for locating an emergency voice session anchoring node of a plurality of emergency voice session anchoring nodes in an Internet Protocol Multimedia Subsystem of a serving communication network, an emergency voice session anchoring node of a plurality of emergency voice session anchoring nodes in an Internet Protocol Multimedia Subsystem in a serving communication network for being located, and a computer program according to the independent claims.

According to an exemplary aspect of the invention, a method of locating an emergency voice session anchoring node of a plurality of emergency voice session anchoring nodes in an Internet Protocol Multimedia Subsystem of a serving communication network for transferring an emergency voice session of a terminal from a packet switched source radio access network of the serving communication network to a target radio access network of the serving communication network associated with a circuit switched domain of the serving communication network is provided. The emergency voice session is anchored in the emergency voice session anchoring node and the transferring of the emergency voice session employs voice session continuity. The emergency voice session anchoring node is identified by a node identifier. The method is executed by the emergency voice session anchoring node. The method comprises sending the node identifier of the emergency voice session anchoring node towards a control node of the circuit switched domain of the serving communication network for the locating of the emergency voice session anchoring node.

According to another exemplary aspect of the invention, a method of locating an emergency voice session anchoring node of a plurality of emergency voice session anchoring nodes in an Internet Protocol Multimedia Subsystem of a serving communication network for transferring an emergency voice session of a terminal from a packet switched source radio access network of the serving communication network to a target radio access network of the serving communication network associated with a circuit switched domain of the serving communication network is provided. The emergency voice session is anchored in the emergency voice session anchoring node and the transferring of the emergency voice session employs voice session continuity. The emergency voice session anchoring node is identified by a node identifier. The method is executed by a control node in the serving

communication network. The method comprises receiving the node identifier of the emergency voice session anchoring node, and sending the node identifier of the emergency voice session anchoring node for the locating of the emergency voice session anchoring node.

According to another exemplary aspect of the invention, a method of locating an emergency voice session anchoring node of a plurality of emergency voice session anchoring nodes in an Internet Protocol Multimedia Subsystem of a serving communication network for transferring an emergency voice session of a terminal from a packet switched source radio access network of the serving communication network to a target radio access network of the serving communication network associated with a circuit switched domain of the serving communication network is provided. The emergency voice session is anchored in the emergency voice session anchoring node and the transferring of the emergency voice session employs voice session continuity. The emergency voice session anchoring node is identified by a node identifier. The method is executed by an intermediate node of the serving communication network. The method comprises receiving the node identifier of the emergency voice session anchoring node, and sending the node identifier of the emergency voice session anchoring node towards the control node of the circuit switched domain of the serving communication network for the locating of the emergency voice session anchoring node.

According to another exemplary aspect of the invention, an emergency voice session anchoring node of a plurality of emergency voice session anchoring nodes in an Internet Protocol Multimedia Subsystem of a serving communication network for being located for transferring an emergency voice session of a terminal from a packet switched source radio access network of the serving communication network to a target radio access network of the serving communication network associated with a circuit switched domain of the serving communication network is provided. The emergency voice session is anchored in the emergency voice session anchoring node and the transferring of the emergency voice session employs voice session continuity. The emergency voice session anchoring node is identified by a node identifier. The emergency voice session anchoring node comprises a sending unit configured to send the node identifier of the emergency voice session anchoring node towards a control node of the circuit switched domain of the serving

communication network for the locating of the emergency voice session anchoring node.

According to another exemplary aspect of the invention, a control node for locating an emergency voice session anchoring node of a plurality of emergency voice session anchoring nodes in an Internet Protocol Multimedia Subsystem of a serving communication network for transferring an emergency voice session of a terminal from a packet switched source radio access network of the serving communication network to a target radio access network of the serving communication network associated with a circuit switched domain of the serving communication network is provided. The emergency voice session is anchored in the emergency voice session anchoring node and the transferring of the emergency voice session employs voice session continuity. The emergency voice session anchoring node is identified by a node identifier. The control node comprises a reception unit configured to receive the node identifier of the emergency voice session anchoring node, and a sending unit configured to send the node identifier of the emergency voice session anchoring node for the locating of the emergency voice session anchoring node.

According to another exemplary aspect of the invention, an intermediate node for locating an emergency voice session anchoring node of a plurality of emergency voice session anchoring nodes in an Internet Protocol Multimedia Subsystem of a serving communication network for transferring an emergency voice session of a terminal from a packet switched source radio access network of the serving communication network to a target radio access network of the serving

communication network associated with a circuit switched domain of the serving communication network is provided. The emergency voice session is anchored in the emergency voice session anchoring node and the transferring of the emergency voice session employs voice session continuity. The emergency voice session anchoring node is identified by a node identifier. The intermediate node is part of the serving communication network. The intermediate node comprises a reception unit configured to receive the node identifier of the emergency voice session anchoring node, and a sending unit configured to send the node identifier of the emergency voice session anchoring node towards the control node of the circuit switched domain of the serving communication network for the locating of the emergency voice session anchoring node. According to another exemplary aspect of the invention, a computer program is provided. The computer program, when being executed by a processor, is configured to carry out or control a method of locating an emergency voice session anchoring node of a plurality of emergency voice session anchoring nodes in an Internet Protocol Multimedia Subsystem of a serving communication network for transferring an emergency voice session of a terminal from a packet switched source radio access network of the serving communication network to a target radio access network of the serving communication network associated with a circuit switched domain of the serving communication network as described above.

Further embodiments of the methods, the nodes, and the computer program are described in the dependent claims. Brief description of the drawings

Embodiments of the invention will be described in more detail hereinafter with reference to examples, but to which the scope of the invention is not limited.

Figure 1 is a flowchart illustrating a method of locating an emergency voice session anchoring node of a plurality of emergency voice session anchoring nodes in an Internet Protocol Multimedia Subsystem of a serving communication network according to a first exemplary embodiment of the invention.

Figure 2 is an architecture diagram illustrating a serving communication network associated with a method of locating an emergency voice session anchoring node of a plurality of emergency voice session anchoring nodes in an Internet Protocol Multimedia Subsystem of a serving communication network according to a second exemplary embodiment of the invention.

Figure 3 is another architecture diagram illustrating a portion of the serving communication network of Figure 2. Figures 4 and 5 are flowcharts illustrating message flows of the method associated with the serving communication network of Figures 2 and 3.

Figure 6 is another architecture diagram illustrating data paths of the emergency voice session of a terminal established in the serving communication network of Figures 2 and 3 prior and subsequent to a session transfer of the emergency voice session.

Figure 7 is a block diagram illustrating an emergency voice session anchoring node of a plurality of emergency voice session anchoring nodes in an Internet Protocol Multimedia Subsystem of a serving communication network for being located according to an exemplary embodiment of the invention. Figure 8 is a block diagram illustrating a control node for locating an emergency voice session anchoring node of a plurality of emergency voice session anchoring nodes in an Internet Protocol Multimedia Subsystem of a serving communication network according to an exemplary embodiment of the invention. Figure 9 is a block diagram illustrating an intermediate node for locating an emergency voice session anchoring node of a plurality of emergency voice session anchoring nodes in an Internet Protocol Multimedia Subsystem of a serving communication network according to an exemplary embodiment of the invention.

Detailed description

The illustration in the drawing is schematic. In different drawings, similar or identical elements are provided with the same reference signs or with reference signs which are different from one another within the first digit. In the following, the methods, the emergency voice session anchoring node, the control node, the intermediate node, and the computer program according to the exemplary aspects of the invention will be explained in more detail.

Within the context of the present application, the term "emergency voice session anchoring node" may particularly denote a node or entity of a serving communication network which may represent a session anchor of an emergency voice session or emergency call. In particular, signaling data of the emergency voice session may be routed prior and subsequent to a session transfer of the emergency voice session via the emergency voice session anchoring node.

The term "voice session continuity" may particularly denote a functionality allowing for a session continuity of an Internet Protocol Multimedia Subsystem based emergency voice session originated in a packet switched source radio access network to be transferred to a target radio access network associated with a circuit switched domain of the serving communication network. In particular, the term "voice session continuity" may refer to Single Radio Voice Call Continuity (SRVCC) which may be particularly defined in accordance with the 3rd Generation Partnership Project (3GPP) Technical Specification (TS) 23.112 v11.1.0.

The term "node identifier" may particularly denote an identification indication uniquely assigned to a node in a serving communication network such that the node may be unambiguously addressable by one or more other nodes using this node identifier. In particular, the other nodes may be aware of the identity of the emergency voice session anchoring node in that the other nodes may deduce the identity of the node from the node identifier thereof. Alternatively, the other nodes may not be capable of associating the node identifier to a particular instance of the serving communication network, and may be therefore not aware of the identity of the node. In particular, the node identifier may comprise or may be configured as a (telephone) number.

The Internet Protocol Multimedia Subsystem (IMS) may be particularly defined in accordance with 3GPP TS 23.228 v11.1.0.

According to the exemplary aspects of the invention, an emergency voice session anchoring node may make its location available to a serving communication network, and in particular to one or more control nodes which may participate in a possible, later-on requested session transfer of an emergency voice session anchored in the emergency voice session anchoring node.

Thus, the serving communication network may comprise or support a communication architecture comprising multiple emergency voice session anchoring nodes, in order to enable a fast and reliable session transfer of the emergency voice session from a packet switched source radio access network to a target radio access network associated with a circuit switched domain of the serving communication network. The execution of the emergency voice session in the serving communication network may be improved, since latency times during a session transfer of the emergency voice session may be reduced. Further, the serving communication network may be enabled to serve an increased number of emergency voice sessions originated in the serving communication network. The serving communication network may also allow for geographical redundancy of the emergency voice session anchoring nodes, in order to compensate for a failure of one or more emergency voice session anchoring nodes.

Next, further exemplary embodiments of the method executed by the emergency voice session anchoring node will be explained. However, these embodiments also apply to the other methods, the emergency voice session anchoring node, the control node, the intermediate node, and the computer program. In particular, the step of sending the node identifier may comprise sending the node identifier via a control node associated with the packet switched radio access network. Thus, the control node requesting the session transfer for the handover of the terminal may be informed of the location of the emergency voice session anchoring node. Upon reception of the node identifier, the control node of the packet switched domain may send the node identifier in a handover request to the control node of the circuit switched domain of the serving communication network. Thus, conventional procedures of a handover of the terminal, for example protocols and/or interfaces usable by the involved nodes, may be employed for the execution of the method, thereby facilitating the implementation of the method in already existing communication networks.

In particular, the node identifier may be stored in the emergency voice session anchoring node, and may be retrieved upon request, particularly when receiving an establishment request for establishing an emergency voice session towards a destination. The node identifier may be a preconfigured identifier, and may be unique for each emergency voice session anchoring node in the serving communication network.

The node identifier may be configured as an identification number for a session transfer of the emergency voice session using voice session continuity, particularly as an Emergency Session Transfer Number for Single Radio Voice Call Continuity (E-STN-SR). In particular, the E-STN-SR may be particularly defined in accordance with TS 23.003 v9.0.0. Thus, the node identifier may comprise a simple form, for example a number assigned to the emergency voice session transfer capability of each emergency voice session anchoring node in the serving communication network. The number may be configured in the form of a telephone number. In particular, in the case of the E-STN-SR, a generic number usable by the control node of the CS domain when addressing the type of node in the serving communication network may be redefined to be a specific, distinct number assigned to a dedicated instance of a plurality of nodes of a particular node type. Here, the E-STN-SR may correspond to an E.164 compatible number. Thus, the interfaces and/or protocols supported by the control node and/or the intermediate node may be usable or deployable when executing the method, thus facilitating the architecture of the communication network. A terminal identifier of the terminal may be sent together with the node identifier in one message, wherein the terminal may be (uniquely) identified by the terminal identifier. In particular, the terminal identifier may comprise an identification number of the terminal in the serving communication network, particularly an International Mobile Station Equipment Identity (IMEI) of the terminal particularly defined in accordance with 3GGP TS 23.033 v10.2.0. The terminal identifier may be received in an establishment request of the terminal for requesting the establishment of the emergency voice session towards its destination in the serving communication network. The terminal identifier may be included in a header of the message or may be included in a message body. To this end, either the message header or the message body may be extended with a space for the respective information. The terminal identifier may be usable for identifying an established emergency voice session during the session transfer of the emergency voice session, and may thus act as a session identifier. In particular, the sending of the node identifier may comprise sending the node identifier in a response message of an emergency voice session establishment request message. The response message may be a Session Initiation Protocol (SIP) compatible message, particularly a 200 OK message, such that the node identifier may be embedded in a conventional signal flow in the serving communication network. The serving communication network may support SIP.

The sending of the node identifier may comprise sending the node identifier to (or towards) an intermediate node of the serving communication network. In particular, the node identifier may be sent via one or more intermediate nodes to the control node of the circuit switched domain. The intermediate node may represent a node related to a control of charging and/or of policy constraints in the serving

communication network. Alternatively or additionally, the intermediate node may be related to signaling and/or media data transfer in the serving communication network particularly in accordance with SIP, and may be interposed in a session path of the established emergency voice session between the terminal and the destination. The emergency voice session of the terminal may be established for an authenticated subscriber. For example, such an authenticated emergency voice session may relate to the terminal comprising a subscriber identity module

application which may be validly activated via unique subscriber data, and having successfully executed authentication procedures with the serving communication network. Such a subscriber identity module application may be a Subscriber Identity Module (SIM) application or a User Subscriber Identity Module (USIM) application activatable by a Personal Identity Number (PIN).

The emergency voice session of the terminal may be established for a non- authenticated subscriber, and the terminal may comprise a subscriber identity module application but may not have successfully executed authentication

procedures or may be free of a subscriber identity module application, which may particularly be defined as explained above. Thus, the session transfer may be accomplished for non-authenticated or unauthenticated emergency calls

corresponding to a usage of a terminal comprising a SIM card or Universal Integrated Circuit Card (UICC) without entering a valid PIN as well as for terminals without SIM cards or without UICCs.

Altogether, the method may be applicable to one or more standard use cases of emergency voice sessions such that implementation of the method in standardized network configurations may be possible.

The sending of the node identifier may comprise sending the node identifier during an establishment of the emergency voice session. Thus, the control node of the circuit switched domain may be prepared from an initiation of the emergency voice session on for an execution of a session transfer of the emergency voice session. Latency times resulting from requesting a location of the emergency voice session anchoring node in the serving communication network during the handover may be omitted.

The method may further comprise associating, in response to receiving a session transfer request (particularly from a control node of the circuit switched domain or an intermediate node), a terminal identifier with the emergency voice session for the transferring of the emergency voice session. The step of receiving the session transfer request may comprise receiving the session transfer request in a SIP Invite message for requesting the emergency voice session anchoring node to be part of the session path. The received terminal identifier may be identical to the above described terminal identifier sent by the terminal to the emergency voice session anchoring node during the session establishment. Thus, the emergency voice session anchoring node may find the anchored emergency voice session to complete the session transfer.

Next, further exemplary embodiments of the method executed by the control node will be explained. However, these embodiments also apply to the other methods, the emergency voice session anchoring node, the control node, the intermediate node, and the computer program.

The control node may be associated with the packet switched source radio access network, and the sending of the node identifier may comprise sending, to a control node of the circuit switched domain of the serving communication network, the node identifier in an handover request for handing over the terminal from the packet switched source radio access network to the target radio access network associated with the circuit switched domain of the serving communication network. Thus, making the node identifier available to the control node of the circuit switched domain of the serving communication network may be facilitated in that the step of the sending of the node identifier may be implemented in an already existing handover procedure, for example a handover request sent between the control node of the source radio access network and the control node of the target radio access network.

The control node may be part of the circuit switched domain of the serving

communication network, and the sending of the node identifier may comprise sending the node identifier in a session transfer request for transferring the emergency voice session from the packet switched source radio access network to the target radio access network associated with the circuit switched domain of the serving communication network. In particular, the session transfer request may be sent from the control node to an intermediate node of the serving communication network interfacing the control node and the emergency voice session anchoring node. Thus, the intermediate node may be informed of the location, for example the address and/or identify, of the emergency voice session anchoring node in the serving communication network for completing the session transfer of the emergency voice session. Here, the intermediate node may be explicitly informed about the address and/or identity of the emergency voice session anchoring node to instantly contact the emergency voice session anchoring node. Alternatively, the intermediate node may send a respective location information request comprising the received node identifier to a subscriber server, in order to obtain the location information of the emergency voice session anchoring node. By that, the intermediate node may be capable of sending a session transfer request to the emergency voice session anchoring node. Next, further exemplary embodiments of the method executed by the intermediate node will be explained. However, these embodiments also apply to the other methods, the emergency voice session anchoring node, the control node, the intermediate node, and the computer program.

In particular, the method may further comprise, upon reception of the node identifier, recording the node identifier and/or the terminal identifier. Thus, legal intercept of the emergency voice session may be also supported in the case of session voice continuity for emergency voice sessions. The terminal identifier may be usable for identification of the subscriber or user of the terminal for the legal intercept.

Next, further exemplary embodiments of the emergency voice session anchoring node, the control node, and the intermediate node will be explained. However, these embodiments also apply to the methods, the respective other nodes, and the computer program.

In particular, the emergency voice session anchoring node may be configured to execute one or more steps of one or more methods described above. The emergency voice session anchoring node may be configured as an Emergency Access Transfer Function (EATF).

In particular, the control node may be configured to execute one or more steps of one or more methods described above. The control node may be configured as a Mobility Management Entity (MME), a Serving General Packet Radio Service (GPRS) Support node (SGSN) or a Mobile Switching Centre Server (MSC-S). The MME may be part of a packet switched domain of a core network, for example an Evolved Packet Core (EPC), and may be associated with a Long Term Evolution (LTE) radio access network. The SGSN may be part of a packet switched domain of a core network, and may be associated with a High Speed Packet Access (HSPA) radio access network. The MSC-S may be part of a circuit switched domain of a core network, and may be associated with a Global System for Mobile Communications (GSM) Enhanced Data Rates for GSM Evolution (EDGE) Radio Access Network (GERAN) as well as an Universal Mobile

Telecommunications System (UMTS) Terrestrial Radio Access Network (UTRAN) or Wideband Code Division Multiple Access (WCDMA) radio access network.

In particular, the intermediate node may be configured to execute one or more steps of one or more methods described above. The intermediate node may be configured as a Serving Gateway (SGW), a Packet Data Network Gateway (PGW), a Policy Charging and Rules Function (PCRF), a Proxy-Call Session Control Function (P-CSCF) or an Emergency-Call Session Control Function (E-CSCF). The SGW, the PGW, the P-CSCF, and the E-CSCF may comprise functionalities regarding data transmission in the serving communication network, and the PCRF may execute functions related to the control of charging and policy constraints in the serving communication network.

In particular, the emergency voice session anchoring node may be co-located with at least one another node, particularly an intermediate node, of the serving

communication network. The node identifier of the emergency voice session anchoring node may thus point also to the location of the co-located node. For example, the EATF may be co-located with the E-CSCF and/or the P-CSCF, and the E-STN-SR may indicate the location of the EATF and the co-located node or nodes.

Referring to Figure 1 , a method of locating an emergency voice session anchoring node of a plurality of emergency voice session anchoring nodes in an IMS of a serving communication network according to a first exemplary embodiment of the invention will be explained. The serving communication network 100 comprises a terminal 102 which may originate an emergency voice session towards a destination 104, for example a Public Safety Answering Point (PSAP). The serving communication network 100 corresponds to the visited communication network of the terminal 102, for example a Visited Public Land Mobile Network (VPLMN). The emergency voice session represents an authenticated emergency voice session, i.e. a subscriber has entered a valid PIN into the terminal 102 for activating a SIM card arranged in the terminal 102, and has successfully executed authentication procedures with the serving communication network 100 using the terminal 102. Alternatively, the emergency voice session may be a non-authenticated emergency voice session.

The serving communication network 100 comprises a core network being split in a packet switched (PS) domain, a circuit switched (CS) domain, and an IMS domain. A control node 106, a MME, of the PS domain is associated with a PS source radio access network, for example a LTE source radio access network. Alternatively, the control node 106 may be a SGSN associated with a HSPA source radio access network.

The CS domain of the core network comprises a control node 108, for example a MSC-S, and is associated with a target radio access network of the serving communication network 100, for example a GERAN target radio access network. An intermediate node 110 forms part of the IMS domain, and is configured as an E- CSCF. The intermediate node 110 interfaces other entities of the IMS domain and a plurality of emergency voice session anchoring nodes 112 of the serving

communication network 100 each of which being configured as a EATF. For illustration purposes, only one emergency voice session anchoring node 112 is shown in Figure 1. Another intermediate node 114, for example an l-CSCF, of the IMS domain interfaces the control node 108 and the emergency voice session anchoring node 112. A subscriber server 116, a Home subscriber Server (HSS), also forms part of the serving communication network 100.

In order to establish the emergency voice session from the terminal 102 towards the destination 104, the terminal 102 sends in a step 120 an emergency voice session establishment request. Upon reception, the intermediate node 110 propagates in a step 122 the emergency voice session establishment request to the emergency voice session anchoring node 112, which, in a step 124, anchors the emergency voice session, and associates in a step 126 the anchored emergency voice session with a terminal identifier. To this end, the terminal identifier, an IMEI, is included in the latter mentioned messages and in all messages mentioned in the following. The inclusion of the terminal identifier in a message may enable a recipient of the respective message to be informed about the address of the entity originating the emergency voice session. In a step 128, the emergency voice session anchoring node 112 sends a response message to the intermediate node 110, which, in a next step 130, sends a respective establishment request for the emergency voice session to the destination 104. Thereupon, a response message is sent in a step 132 between the destination 104 and the intermediate node 110, which then sends in a step 134 an acknowledgment or response message to the emergency voice session anchoring node 112.

In order to prepare a later on requested session transfer of the emergency voice session, the emergency voice session anchoring node 112 makes its location available to the control nodes 106 and 108 by sending in a step 140 an

acknowledgment message to the intermediate node 110 for accepting the emergency voice session. A unique node identifier of the emergency voice session anchoring node is added to this acknowledgement message. The intermediate node 110 then sends in a step 142 the node identifier in an acknowledgment message to the control node 106 of the PS domain. Upon reception of the message, the emergency voice session is successfully established between the terminal 102 and the destination 104, as indicated by the reference numeral 144.

Upon reception of a handover request from the source radio access network for the terminal 02 to be handed over from the source radio access network to the target radio access network, the control node 106 sends in a step 146 a handover request including the node identifier to the control node 108 of the CS domain. By this, the control node 108 is informed about the identification of the emergency voice session anchoring node 112 such that the control node 108 is enabled to locate the emergency voice session anchoring node 112 during the requested session transfer procedure. The control node 108 sends in a step 148 a transfer request message comprising the node identifier to the intermediate node 114. Next, the intermediate node 114 sends in a step 149a a request for location information of the emergency voice session anchoring node 1 12 to the subscriber server 1 16, and receives in a step 149b the requested location information. To this end, the request comprises the node identifier such that the subscriber server 1 16 is capable of converting the node identifier to respective location information usable for the routing of a message to the emergency voice session anchoring node 1 12. In a step 150, a transfer request message is sent from the intermediate node 1 14 to the emergency voice session anchoring node 1 12 which, upon reception of this request message, associates the received terminal identifier with the emergency voice session anchored in the emergency voice session anchoring node 1 12 in a step 152, in order to identify the emergency voice session to be transferred from the source radio access network to the target radio access network. In a step 154, the emergency voice session anchoring node 1 12 sends an update request to the intermediate node 1 14 which forwards the update request in a step 156 to the destination 104. Subsequent to an acknowledgment procedure between the destination 104 and the terminal 102 in which the latter mentioned nodes are also involved, the session transfer is

completed, and the media and signaling data of the emergency voice session are routed between the terminal 102, which is attached to the control node 108 via the target radio access network, and the destination 104. As the emergency voice session anchoring node 1 12 represents the session anchor, the signaling data of the transferred emergency voice session are still routed via the emergency voice session anchoring node 1 12.

Referring to Figures 2 to 6, a method of locating an emergency voice session anchoring node of a plurality of emergency voice session anchoring nodes in an IMS of a serving communication network according to a second exemplary embodiment of the invention will be explained. Figures 2, 3, and 6 show the serving

communication network 200 which is associated with the method and corresponds to the home communication network of a terminal 202, for example a Home Public Land Mobile Network HPLMN. A message flow sequence of the method is indicated in Figure 3 by numbers 1 to 21 . Figures 4, 5 show a signaling message flow of the method.

The serving communication network 200 comprises a PS domain of a core network, for example an Evolved Packet Core (EPC), interfacing a PS based LTE radio access network 260 and a session destination, a PSAP 204. A node 262 of the source radio access network 260 is configured as eNodeB. The PS domain

comprises a control node 206, a MME. Further, the serving communication network 200 comprises an IMS domain comprising intermediate nodes configured as a SGW and a PGW, both being denoted by the common reference numeral 264 in Figures 2 and 6 and by the reference numerals 264a, b in Figures 3 to 5. A media node 266 of the PS domain is an ATGW, and another intermediate node 268 of the serving communication network 200 is a PCRF. A further intermediate node of the IMS domain is a P-CSCF which is co-located with a signaling controlling node, an ATCF. The latter nodes are denoted by the common reference numeral 270. Further intermediate nodes of the IMS domain are configured as l-CSCF and S-SCFC, and are denoted by the common reference numeral 214 in Figures 2, 3 and 6 and by the reference numerals 214a, b in Figures 4, 5. Further, for the sake of completeness, the serving communication network comprises a Service Centralization and

Continuity Application Server (SCC AS) and a Multimedia Telephony Service

(MMTel) node, both being denoted by the common reference numeral 272. An intermediate node 210, for example an E-CSCF, and a plurality of emergency voice session anchoring nodes 212, for example ETAFs, are also part of the IMS domain of the serving communication network 200. For ease of illustration, only one EATF 212 is shown throughout the Figures 2, 4 to 6. A location retrieval node 274, a Location Retrieval Function (LRF), and a subscriber server 216, for example a HSS, are also part of the serving communication network 200. The CS domain of the core network comprises a control node, a MSC-S, and a media node, a Media Gateway (MGW), with both being denoted by the common reference numeral 208 and by the

abbreviation MSS in Figures 2, 3, and 6 and the MSC-S being solemnly denoted by the reference numeral 208 in Figures 4 and 5. An intermediate node 276, a Media Gateway Control Function (MGCF), is also part of the IMS domain. The MSS 208 interfaces a target radio access network 278, for example a GSM or a WCDMA radio access network, and the PS domain. A node 280 of the target radio access network 278 is configured as a Base Transceiver Station in case of the GSM target radio access network 278 or as a NodeB in case of the WCDMA target radio access network 278. Another node 282 of the target radio access network 278 interfaces the node 280 and the MSS 208, and is configured as a Base Station Controller in case of the GSM target radio access network 278 or as a Radio Network Controller in case of the WCDMA target radio access network 278. The terminal 202 originates a non-authenticated emergency voice session, as the terminal 202 is free of a SIM card or UICC. Alternatively, the terminal 202 may originate an authenticated emergency voice session subsequent to a registration of the subscriber of the terminal 202 in the HSS 216. In both cases, the P-CSCF 270, the E-CSCF 210, and the EATF 212 are linked in the session path of the emergency voice session during the session setup.

In a first step 220a of the method, the terminal 202 sends an emergency call setup request in the form of a SIP Invite message to the SGW 264a, which message comprises the IMEI of the terminal 202 in the contact header of the message and an Uniform Resource Name (urn) of the requested emergency service called "sos". The SGW 264a propagates this message in a step 220b to the PGW 264b. Upon reception, the PGW 264b sends a respective SIP Invite message in a step 220c to the P-CSCF 270 which in turn sends in a step 220d another SIP Invite message to the E-CSCF 210. In a step 222, a SIP Invite message is sent from the E-CSCF 210 to the EATF 212 which additionally comprises the Uniform Resource Identifier (URI) of the EAFT 212 and of the E-CSCF 210 in the message header for routing purposes. In a step 224, the EATF 212 anchors the requested emergency voice session subsequent to the reception of the Invite message, and in a step 226 associates the received IMEI with the anchored emergency voice session to be established. Thereupon, in a step 228, the EATF 212 continues the session setup by generating a request message, namely a SIP Invite message, which is sent to the E- CSCF 210. The SIP Invite message comprises the emergency service urn. In a step 484, a location and routing information retrieval procedure is executed between the E-CSCF 210 and the LRF 274, in order to obtain address information of the PSAP 204. In a step 230, a SIP Invite message requesting the establishment of the emergency voice session and including the IMEI is sent to the PSAP 204. Next, in a step 232, a response message, for example a 200 OK message, is sent from the PSAP 204 to the E-CSCF 210. A further 200 OK response message is then propagated in a step 234 to the EATF 210. Next, in a step 240, the EATF 212 sends a 200 OK message to the E-CSCF 212 comprising its node identifier, namely a uniquely assigned telephone number, an E- STN-SR, and the IMEI. To this end, the EATF 212 retrieves its E-STN-SR stored in the EATF 212. In steps 242a-e, the E-STN-SR and IMEI is sent in an AAR message, a RAR message, an Update Bearer Request message and an Update Bearer Request message between the E-CSCF 210 and the P-CSCF 270 via a Mw interface, between the P-CSCF 270 and the PCRF 268 via a Rx interface, between the PCRF 268 and PGW 264b via a Gx interface, between the PGW 264b and the SGW 264a via an S5 interface, and between the SGW 264a and the MME 206 via a S1 1 interface, respectively. Thus, the MME 206 is informed about the location of the EATF 212 in the serving communication network 200 in terms of being informed of the identity of the latter for routing purposes. The AAR message, the RAR message, and the Update Bearer Request messages may be extended with the E-STN-SR and the IMEI to include this information in the message header.

Further, the P-CSCF 270 may be capable of recording the IMEI, in order to support a legal intercept for a SRVCC based emergency voice session handover. The E-STN- SR may be also recorded by the P-CSCF 270 in addition to the IMEI.

As indicated by the reference numeral 244, an emergency voice session is now on- going between the terminal 202 and the PSAP 204. The media data of the

emergency voice session are routed via the PS source radio access network 260, the

EPC of the serving communication network 200, the SGW 264a and the PGW 264b.

In one embodiment, the media data may be PS routed between the EATF 212 and the PSAP 204 via the IMS. In another embodiment, the media data may be PS and CS routed via the MGW 268 and an Interconnect Boarder Control Function (IBCF), i.e. there may be a breakout of the emergency voice session towards the CS network domain towards the PSAP 204.

In a step 586, the eNodeB 262 initiates SRVCC for the emergency voice session in accordance with 3GPP TS 23.216 v1 1.1.0, and the MME 206 determines that an emergency voice session SRVCC PS to CS handover of the terminal 202 has to be executed to maintain a sufficient strong signaling quality of the on-going emergency voice session. In a subsequent step 246, the MME 206 sends a SRVCC PS to CS handover request to the MSC-S 208 via a Sv interface. The handover request comprises the E-STN-SR of the EATF 212 and the IMEI of the terminal 202, both of which having been received in the step 242d. To this end, the handover request is extended with the E-STN-SR. Thereupon, in a step 248, the MSC 208 sends a SIP Invite Request-URI message towards the IMS domain, and in particular to the I- CSCF 214 via an I2/Mw interface. The Invite message includes the E-STN-SR and the IMEI in the message header. The IMEI is supported on this interface, for example, in accordance with 3GPP TS 23.237 v1 1 .1.0, TS 24.237 vl O.3.0, and TS 23.216 v1 1.1 .0. Optionally, in particular, if available in the case of authenticated emergency voice sessions, a Correlation- Mobile Station (MS) International Internet Service Digital Network (C-MSISDN) of the terminal subscriber is also included in the sent Invite Request message for identifying the originating entity of the emergency voice session. In the case of a non-authenticated emergency voice session, the C- MSISDN is not included in the SIP Invite message. In a location information request procedure executed in subsequent steps 249a, b between the l-CSCF 214 and the HSS 216, location information of the EATF 212 is obtained by sending a Location Information Request LIR for requesting respective location information of the EATF 212 in the step 249a and a Location Information Answer LIA comprising the requested location information in a step 249b. The LIR comprises the E-STN-SR, and the HSS 216 converts the E-STN-SR to an Internet Protocol (IP) based location information usable for the routing. This procedure enables Public Service Identifier (PSI) direct routing to the EATF 212.

In a step 250, the l-CSCF 214 sends a SIP Invite message to the EATF 212 including the E-STN-SR, the IMEI, and a Session Description Protocol (SDP) tag indicating the routing via the MGW. Optionally, if available, the C-MSISDN of the terminal subscriber is also included in the Invite message, as described above. In a step 252, the EATF 2 2 associates the received IMEI with the emergency voice session anchored in the EATF 212 based on the contact header of the received Invite message to find the involved emergency voice session particularly in accordance with 3GPP TS 23.237 v.1 1 .1.0, and sends thereupon in a step 254 an update request, namely a SIP Re-Invite message, to the E-CSCF 210. Next, the E-CSCF 210 sends in a step 256 another update request, which is also embodied as a SIP Re-Invite message, to the PSAP 204. Optionally, the Re-Invite message is also sent via the MGCF 276 and the IBCF to enable CS data exchange towards the PSAP 204. Subsequent acknowledgement procedures between the MSC-S 208 and the PSAP 204 are negotiated between the involved entities using 200 OK and 200 ACK messages. Next, in a step 590, the source access leg is released by the EATF 212 such that the session transfer of the emergency voice session from the source radio access network 260 to the target radio access network 278 is completed. Thus, the media data subsequent to the session transfer are CS based routed between the terminal 202 and the MSS 208, and PS routed between the MSS 208 and the PSAP 204. To this end, as detailed in connection with the on-going session depicted by the reference numeral 244, the media data may be routed directly between the EATF 212 and the PSAP 204 or via the MGCF 276 and the IBCF.

Referring to Figure 6, session paths of signaling data and media data of the established emergency voice session prior and subsequent to the SRVCC PS to CS handover of the terminal 202 from the source radio access network 260 to the target radio access network 278 are shown. Initially, a session path 691 of the signaling data runs from the terminal 202 to the PSAP 204 via the node 262 of the source radio access network 260, the SGW/PWG 264, the P-CSCF 270, the E-CSCF 210 and the ETAF 212. A session path 692 of the media data runs between the terminal 202 and the PSAP 204 via the eNodeB 262, the SGW/PGW 264, and the ATGW 266. Subsequent to the handover of the terminal 202, the session path 696 of the signaling data runs between the terminal 202 and the PSAP 204 via the nodes 280, 282 of the target radio access network 278, the MSC-S of the MSS 208, the I- CSCF/S-CSCF 210, the ETAF 212, and the E-CSCF 210. The session path 696 of the media data includes the nodes 280, 282, and the MGW of the MSS 208. Referring to Figure 7, an emergency voice session anchoring node 712 of a plurality of emergency voice session anchoring nodes in an IMS of a serving communication network for being located for transferring an emergency voice session of a terminal from a PS source radio access network of the serving communication network to a target radio access network of the serving communication network associated with a CS domain of the serving communication network is illustrated. The emergency voice session is anchored in the emergency voice session anchoring node and the transferring of the emergency voice session employs voice session continuity. The emergency voice session anchoring node 712 is identified by a node identifier. The emergency voice session anchoring node 712 comprises a sending unit TUI configured to send the node identifier of the emergency voice session anchoring node 712 towards a control node of the CS domain of the serving communication network for the locating of the emergency voice session anchoring node 712. The emergency voice session anchoring node 712 further comprises a reception unit RU1 configured to receive data related to a method of locating an emergency voice session anchoring node of a plurality of emergency voice session anchoring nodes in an IMS of a serving communication network as described above, and a processing unit PU1 configured to process data or information related to the method. The emergency voice session anchoring node 712 further comprises a storage unit SU1 configured to store data related to the method. The emergency voice session anchoring node 712 is configured as an ETAF.

The emergency voice session anchoring node 712 also comprises an associating unit configured to associate, in response to receive a session transfer request, a terminal identifier with the emergency voice session for the transferring of the emergency voice session. The associating unit is part of the processing unit PU1.

Referring to Figure 8, a control node 806 for locating an emergency voice session anchoring node of a plurality of emergency voice session anchoring nodes in an IMS of a serving communication network for transferring an emergency voice session of a terminal from a PS source radio access network of the serving communication network to a target radio access network of the serving communication network associated with a CS domain of the serving communication network is illustrated. The emergency voice session is anchored in the emergency voice session anchoring node and the transferring of the emergency voice session employs voice session continuity. The emergency voice session anchoring node is identified by a node identifier. The control node 806 comprises a reception unit RU2 configured to receive the node identifier of the emergency voice session anchoring node, and a sending unit TU2 configured to send the node identifier of the emergency voice session anchoring node for the locating of the emergency voice session anchoring node. Further, the control node 806 comprises a processing unit PU2 configured to process data related to a method of locating an emergency voice session anchoring node of a plurality of emergency voice session anchoring nodes in an IMS of a serving communication network as described above, and a storage unit SU2 configured to store data related to the method. The control node 806 is configured as a MME of a PS domain of a core network of the serving communication network. In another embodiment, the control node 806 is configured as a MSC-S of the CS domain of the core network of the serving communication network.

Referring to Figure 9, an intermediate node 910 for locating an emergency voice session anchoring node of a plurality of emergency voice session anchoring nodes in an IMS of a serving communication network for transferring an emergency voice session of a terminal from a PS source radio access network of the serving communication network to a target radio access network of the serving

communication network associated with a CS domain of the serving communication network is illustrated. The emergency voice session is anchored in the emergency voice session anchoring node and the transferring of the emergency voice session employs voice session continuity. The emergency voice session anchoring node is identified by a node identifier. The intermediate node 910 is part of the serving communication network. The intermediate node 910 comprises a reception unit RU3 configured to receive the node identifier of the emergency voice session anchoring node, and a sending unit TU3 configured to send the node identifier of the

emergency voice session anchoring node towards the control node of the CS domain of the serving communication network for the locating of the emergency voice session anchoring node. Further, the intermediate node 910 comprises a processing unit PU3 configured to process data related to a method of locating an emergency voice session anchoring node of a plurality of emergency voice session anchoring nodes in an IMS of a serving communication network as described above, and a storage unit SU3 configured to store data related to the method.

The intermediate node 910 is an E-CSCF. In further embodiments, the intermediate node 910 is configured as a SGW, a PGW, a PCRF or a P-CSCF. It is noted that an association between the described functionalities of the nodes in Figures 7 to 9 and the functionality based units of these nodes may be different. For example, the associating unit of the emergency voice session anchoring node 712 may also form part of the sending unit TU1 of the emergency voice session anchoring node 712. While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive; the invention is not limited to the disclosed embodiments, and in particular not to those embodiments in accordance with the cited 3GPP Technical Specifications. Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the claims, the word "comprising" does not exclude other elements or steps, and the use of indefinite articles "a" or "an" does not exclude a plurality. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. Any reference signs in the claims should not be construed as limiting the scope.

Claims

A method of locating an emergency voice session anchoring node of a plurality of emergency voice session anchoring nodes (112, 212, 712) in an Internet Protocol Multimedia Subsystem of a serving communication network (100, 200) for transferring an emergency voice session of a terminal (102, 202) from a packet switched source radio access network (260) of the serving communication network to a target radio access network (278) of the serving communication network associated with a circuit switched domain of the serving communication network, wherein the emergency voice session is anchored in the emergency voice session anchoring node and wherein the transferring of the emergency voice session employs voice session continuity, wherein the emergency voice session anchoring node is identified by a node identifier, the method being executed by the emergency voice session anchoring node, the method comprising:

• sending (140, 240) the node identifier of the emergency voice session anchoring node towards a control node (106, 108, 206, 208, 806) of the circuit switched domain of the serving communication network for the locating of the emergency voice session anchoring node.

2. The method according to claim 1 , wherein the node identifier is configured as an identification number for a session transfer of the emergency voice session using voice session continuity, particularly an Emergency Session Transfer Number for Single Radio Voice Call Continuity.

3. The method according to claim 1 or 2, wherein a terminal identifier of the terminal (102, 202) is sent together with the node identifier in one message, wherein the terminal is identified by the terminal identifier.

4. The method according to anyone of claims 1 to 3, wherein the sending (140, 240) of the node identifier comprises sending the node identifier to an intermediate node (110, 210, 264-270, 910) of the serving communication network.

5. The method according to anyone of claims 1 to 4, wherein the emergency voice session of the terminal (102, 202) is established for an authenticated subscriber.

6. The method according to anyone of claims 1 to 5, wherein the emergency voice session of the terminal (102, 202) is established for a non- authenticated subscriber, and wherein the terminal comprises a subscriber identity module application or is free of a subscriber identity module application.

7. The method according to anyone of claims 1 to 6, wherein the sending (140, 240) of the node identifier comprises sending the node identifier during an establishment of the emergency voice session.

8. The method according to anyone of claims 1 to 7, the method further

comprises associating (152, 252), in response to receiving (150, 250) a session transfer request, a terminal identifier with the emergency voice session for the transferring of the emergency voice session.

9. A method of locating an emergency voice session anchoring node of a plurality of emergency voice session anchoring nodes (112, 212, 712) in an Internet Protocol Multimedia Subsystem of a serving communication network (100, 200) for transferring an emergency voice session of a terminal (102, 202) from a packet switched source radio access network (260) of the serving communication network to a target radio access network (278) of the serving communication network associated with a circuit switched domain of the serving communication network, wherein the emergency voice session is anchored in the emergency voice session anchoring node and wherein the transferring of the emergency voice session employs voice session continuity, wherein the emergency voice session anchoring node is identified by a node identifier, the method being executed by a control node (106, 108, 206, 208, 806) in the serving communication network, the method comprising: • receiving (142, 146, 242, 246) the node identifier of the emergency voice session anchoring node (112, 212, 712), and

• sending (146, 148, 246, 248) the node identifier of the emergency voice session anchoring node (112, 212, 712) for the locating of the emergency voice session anchoring node (1 2, 212, 712).

10. The method according to claim 9, wherein the control node (106, 206) is associated with the packet switched source radio access network (260), wherein the sending (146, 148, 246, 248) of the node identifier comprises sending (146, 246), to a control node (108, 208) of the circuit switched domain of the serving communication network (100, 200), the node identifier in an handover request for handing over the terminal (102, 202) from the packet switched source radio access network (260) to the target radio access network (278) associated with the circuit switched domain of the serving communication network.

11. The method according to claim 9, wherein the control node (108, 208) is part of the circuit switched domain of the serving communication network (100, 200), wherein the sending (146, 148, 246, 248) of the node identifier comprises sending (148, 248) the node identifier in a session transfer request for transferring the emergency voice session from the packet switched source radio access network (260) to the target radio access network (278) associated with the circuit switched domain of the serving communication network.

12. A method of locating an emergency voice session anchoring node of a plurality of emergency voice session anchoring nodes (112, 212, 712) in an Internet Protocol Multimedia Subsystem of a serving communication network (100, 200) for transferring an emergency voice session of a terminal (102, 202) from a packet switched source radio access network (260) of the serving communication network to a target radio access network (278) of the serving communication network associated with a circuit switched domain of the serving communication network, wherein the emergency voice session is anchored in the emergency voice session anchoring node and wherein the transferring of the emergency voice session employs voice session continuity, wherein the emergency voice session anchoring node is identified by a node identifier, the method being executed by an intermediate node (110, 210, 264-270, 910) of the serving communication network (100, 200), the method comprising:

• receiving (140, 240, 242a-e) the node identifier of the emergency voice session anchoring node, and

• sending (142, 242a-e) the node identifier of the emergency voice

session anchoring node towards the control node (108, 208) of the circuit switched domain of the serving communication network for the locating of the emergency voice session anchoring node.

13. An emergency voice session anchoring node of a plurality of emergency voice session anchoring nodes (112, 212, 712) in an Internet Protocol Multimedia Subsystem of a serving communication network (100, 200) for being located for transferring an emergency voice session of a terminal (102, 202) from a packet switched source radio access network (260) of the serving communication network to a target radio access network (278) of the serving communication network associated with a circuit switched domain of the serving communication network, wherein the emergency voice session is anchored in the emergency voice session anchoring node and wherein the transferring of the emergency voice session employs voice session continuity, wherein the emergency voice session anchoring node is identified by a node identifier, the emergency voice session anchoring node comprises a sending unit (TU1) configured to send the node identifier of the emergency voice session anchoring node towards a control node (108, 208) of the circuit switched domain of the serving communication network for the locating of the emergency voice session anchoring node.

14. The emergency voice session anchoring node according to claim 13,

wherein the emergency voice session anchoring node is configured as an Emergency Access Transfer Function.

15. A control node (106, 108, 206, 208, 806) for locating an emergency voice session anchoring node of a plurality of emergency voice session anchoring nodes (112, 212, 712) in an Internet Protocol Multimedia

Subsystem of a serving communication network (100, 200) for transferring an emergency voice session of a terminal (102, 202) from a packet switched source radio access network (260) of the serving communication network to a target radio access network (278) of the serving

communication network associated with a circuit switched domain of the serving communication network, wherein the emergency voice session is anchored in the emergency voice session anchoring node and wherein the transferring of the emergency voice session employs voice session continuity, wherein the emergency voice session anchoring node is identified by a node identifier, the control node comprising:

• a reception unit (RU2) configured to receive the node identifier of the emergency voice session anchoring node, and

• a sending unit (TU2) configured to send the node identifier of the

emergency voice session anchoring node for the locating of the emergency voice session anchoring node.

16. The control node according to claim 15, wherein the control node is

configured as an Mobility Management Entity (106, 206, 806), a Serving General Packet Radio Service Support Node or a Mobile Switching Center Server (108, 208, 908).

17. An intermediate node (110, 210, 264-270, 910) for locating an emergency voice session anchoring node of a plurality of emergency voice session anchoring nodes (112, 212, 712) in an Internet Protocol Multimedia

communication network associated with a circuit switched domain of the serving communication network, wherein the emergency voice session is anchored in the emergency voice session anchoring node and wherein the transferring of the emergency voice session employs voice session continuity, wherein the emergency voice session anchoring node is identified by a node identifier, the intermediate node being part of the serving communication network, the intermediate node comprising:

• a reception unit (RU3) configured to receive the node identifier of the emergency voice session anchoring node, and

• a sending unit (TU3) configured to send the node identifier of the

emergency voice session anchoring node towards the control node of the circuit switched domain of the serving communication network for the locating of the emergency voice session anchoring node.

The intermediate node according to claim 17, wherein the intermediate node is configured as a Serving Gateway (264, 264a), a Packet Data Network Gateway (264, 264b), a Policy Charging and Rules Function (268), a Proxy Call Session Control Function (270) or an Emergency Call Session Control Function (110, 210, 910).

A computer program, which, when being executed by a processor, is configured to carry out or control a method of locating an emergency voice session anchoring node of a plurality of emergency voice session anchoring nodes (112, 212, 712) in an Internet Protocol Multimedia Subsystem of a serving communication network (100, 200) for transferring an emergency voice session of a terminal (102, 202) from a packet switched source radio access network (260) of the serving communication network to a target radio access network (278) of the serving

communication network associated with a circuit switched domain of the serving communication network according to anyone of claims 1 to 12.