WO2014139563A1

WO2014139563A1 - Universal access gateway

Info

Publication number: WO2014139563A1
Application number: PCT/EP2013/055155
Authority: WO
Inventors: Rogier August Caspar Joseph Noldus; Martien Huijsmans
Original assignee: Telefonaktiebolaget L M Ericsson (Publ)
Priority date: 2013-03-13
Filing date: 2013-03-13
Publication date: 2014-09-18

Abstract

The present invention relates to a method, performed by a universal access gateway (100), to carry out a hand over of a call of a mobile user entity (10) between a packet switched attachment to an access node (200) and a circuit switched attachment to the access node, a first control node (41 ) controlling the call during the packet switched attachment to the access node, a second control node (70) controlling the call during the circuit switched attachment to the access node (200). The method comprises the steps of: - receiving (S6) identification information allowing the other control node (70, 41 ) which will control the call after the hand over is completed, to be identified, wherein the user plane of the call is anchored in the universal access gateway (100), - receiving a request (S10) to set up a control connection to the universal access gateway for said call, - accepting the request to set up the control connection if the request is received from the other control node (70, 41 ) which is identified using the received identification information, - in response to accepting the request to set up the control connection, setting up a control connection between the universal access gateway (100) and the other control node (70, 41 ) and, - receiving instructions related to the user plane of the call from the other control node (70, 41 ) after the hand over is completed, wherein the user plane data of the call remains anchored in the universal access gateway (100) after the hand over.

Description

Universal Access Gateway

Technical Field

The invention relates to a method performed by a universal access gateway to carry out a hand over of a call of a mobile user entity between a packet switched

attachment to an access node and a circuit switched attachment to the access node. The invention relates furthermore to the universal access gateway configured to carry out the hand over and to a method, by a control node, to hand over the call and to the corresponding control node. The invention furthermore relates to a method to take over the call from another control node in a hand over process and to the corresponding control node taking over the call after the hand over.

Background

In Figure 1 the establishment of voice calls via LTE (Long-term Evolution) access, i.e. Voice over LTE (VoLTE) is shown. The voice call is established via the session border gateway SBG 40. The control plane for the VoLTE call breaks out from a packet data gateway, PDGw 30, and traverses a session gateway controller (SGC)

41 , the control plane entity in the SBG, P-CSCF 42 (Proxy Call Session Control Function), ATCF 43 (Access Transfer Control Function) and S-CSCF (Serving Call Session Control Function) and SCC-AS (Session Centralisation and Continuity Application Server) etc. not shown in Figure 1 . The user plane for the VoLTE also breaks out from the PDGw 30 and traverses a border gateway function BGF 41 , the user plane entity of the SBG 40. For the VoLTE call the PDGw 30, SBG 40, P-CSCF

42, ATCF 43 and the access transfer gateway ATGW are located in a visited network.

SGC 41 , P-CSCF 42 and ATCF 43 are separate functional entities, but may be combined in a single node, the SBG 40. The BGF 44 may be located in the SBG 40 or may be deployed as a stand-alone entity. Not all relevant entities and reference points are shown in Figure 1 . For example, the PCRF, the reference point between P-CSCF and PCRF and the reference point between PCRF and PDGw are not shown.

When the VoLTE call is subject to single radio voice call continuity (SR-VCC), the access leg of the call changes from PS (Packet Switched) access to CS (Circuit Switched) access. For the CS access leg, there will be a control plane established between the mobile user entity 10, the MSC (Mobile Switching Center) Server (MSC- S), towards ATCF 43 and SCC-AS. The user plane of the CS access leg is

established between UE, M-MGW and ATGW.

Figure 2 gives a schematic representation of the user plane in the circuit switched access leg. The figure is based on usage of the real-time transport protocol (RTP) over UDP/IP (User Datagram Protocol/Internet Protocol) for a media transport between the radio access network, RNS, shown by the radio network controller RNC 60 and symbolized by the UTRAN (Universal Terrestrial Radio Access Network) 50 in Figure 2 and to media gateway 80 in accordance with 3GPP specification TS 25.410. The MSC server 70 as in the embodiment shown built in MGCF (Media Gateway Control Function) with an Mg reference point towards IMS (IP Multimedia

Subsystem) network. Alternatively, the MSC is enhanced for IMS centralized services (ICS), for which the MSC 70 supports the I2 reference point towards the IMS network. The media gateway 80 that is controlled by the MSC server and MGCF 70 is actually composed of two media gateways. The MSC server 70a allocates a mobile media gateway (M-MGW) 80a for the user plane. The MGCF 70b co-located within the MSC server is allocated an IP multimedia media gateway (IM-MGW) 80b. The combination of MSC server 70c and MGCF 70b is embodied in the mobile soft switch MSS 70. The combination M-MGW and IM-IGW is generally referred to as MGW or M-MGW 80. It can be a blade server that contains pools of media handling functions of different kinds, including access BGF, network BGF also known as transition gateway, TrGW, MGW and MRFP (Media Resource Function Processor). The combination of M-MGW and IM-MGW is considered to represent a single logical MGW 80 controlled by MSS 70.

Figure 4 depicts this integrated media gateway 80 with further protocol details.

To the left of the M-MGw 80, the user plane transportation between M-MGW and radio access network (RAN or UTRAN) is depicted. The user plane utilizes, nowadays, commonly RTP, carried over UDP/IP, for encapsulating voice. The IP is, in its turn carried over ATM (Asynchronous Transfer Mode). Industry is generally moving away from using ATM for media transport in RAN, in favor of using Ethernet.

To the right of the M-MGw, the user plane transportation between M-MGw and GSM/3G core network or IMS network is depicted. RTP/UDP/IP is the commonly used protocol for encapsulating voice. ATM is, also in core network, gradually replaced by Ethernet.

M-MGW is equipped to transport media between (UT)RAN and core network. MGW may, hereto, have to transcribe between ATM based media transportation and Ethernet based media transportation.

Figure 5 shows an impact from SR-VCC. The figure especially shows which function entities are removed from the call and which function entities are added to the call. In the embodiment shown the hand over occurs from a packet switched attachment to a circuit switched attachment. Before SR-VCC the control plane is established between the mobile user entity UE 10 through LTE and EPC (jointly referred to as Enhanced Packet System, EPS 20), SGC 41 , P-CSCF 42, ATCF 43 and further. The user plane is located between the UE 10 through EPS 20, BGF 44 to ATGW 45 and further. After SR-VCC the control plane is located from the UE 10 through UTRAN 50, MSC- S/MGCF 70, ATCF 43 and further. The user plane is located between UE 10 through UTRAN 50, MGW 80 to ATGW 45 and further. ATCF 43 and ATGW 45 were introduced in 3GPP Release 10 as a means to anchor the media stream (user plane) in the visited network close the access network (EPS for a packet switched access and UTRAN + MSC-S for a circuit switched access). At access transfer, the ATGW 45 functions as user plane pivot. The media plane between ATGW 45 and the remote end depicted by the thick line in Figure 5 is not affected during access transfer. The ATGW 45 is controlled by ATCF 43. The user plane pivots in the ATGW 45, whereas the control plane pivots at the SCC-AS, in the IMS network. The control plane for the circuit switched access leg will, however, traverse the same ATCF node as the control plane for the packet switched access leg, enabling the ATCS 43 to keep control over the ATGW 45.

Considering the user plane view point, the change in collaboration during access is shown in Figure 6. At access transfer, the CS access leg is established by the MSC-S. The MSC-S allocates an MGW 80 for the user plane. The ATCF, through which the SIP session that is established by the MSC-S traverses, informs the MSC-S about the ATGW 45 through which the MSC-S shall establish the user plane. When access transfer occurs, it is attempted to use the same set of codecs in the CS access leg as is used in the PS access leg. In that manner, the necessity for media transcoding in the MGW is avoided.

The user plane for the PS access leg is released in the SR-VCC process. The BGF is de-allocated.

There are separate pools of MGWs 80 and BGF's in the mobile network. It is attempted to have a short media connection link between the respective media gateways in the call. This may be accomplished by selecting, for a particular call, BGF, ATGW 45 and M-MGW 80 in the same node (informally known as MRS, Media Resource Server in Figure 7) and, within the node, selecting adjacent blades or selecting one and the same blade. During the access transfer process, information can be exchanged between SGC, ATCF and MSC-C about the MRS 90 selected for this call and the blade(s) where the BGF and ATGW processes are running for this call. The MSC can then select an MGW on the same blade, in the same node, as the BGF and ATGW. This is depicted in Figure 7. The selection of media access gateway as depicted in Figure 7 is Optimized' since it ensures that ATGW 45 resides on the same blade as BGF 44 (for PS access) or MGW 80 (CS access). Hence, the user plane connection between the respective access gateway (BGF or MGW) and ATGW is in such case internal on a blade. During access transfer, the user plane in the PS access leg is replaced by a user plane in the CS access leg. This entails the de-allocation of one 'media anchor point', namely the BGF (in the PS access leg), and the seizing of another media anchor point, namely the MGW (in the CS access leg). Although the use of ATCF and ATGW reduces the impact on the user plane by the access transfer process, the procedure is not optimal. Re-routing the user plane, during an active call, from (i) being routed through one media anchor point to (ii) being routed through another media anchor point, will lead to noticeable interruption in speech transfer. Although it is possible in practice to have short and 'acceptable' voice interruption during SR- VCC (when using ATCF & ATGW), it's still desirable to improve this. Seamless voice calls is important for introduction of VoLTE.

The use of different gateways is disadvantageous and in a hand over process the speech may be interrupted noticeably for the user. Summary

Accordingly, a need exists to further improve a hand over between a circuit switched and a packet switched attachment to an access node, such as IMS. This need is met by the features of the independent claims. Further embodiments are described in the dependent claims. According to a first aspect of the invention, a Method performed by the gateway server is provided to carry out a hand over of a call of a mobile user entity between a packet switched attachment to an access node and a circuit switched attachment to the access node, wherein a first control node controls the call during the packet switched attachment to the access node and a second control node controls the call during the circuit switched attachment to the access node. The method comprises the step of receiving identification information allowing the other control node which will control the call after the hand over is completed, to be identified, the user plane of the call being anchored in the gateway server. Furthermore, a request to set up a control connection to the gateway server for said call is received. The request to set up the control connection is accepted if the request is received from the other control node, which is identified using the received identification information. In response to accepting the request to set up the control connection, a control connection between the gateway server and the other control node is set up. Furthermore, instructions related to the user plane of the call from the other control node are received after the hand over is completed, wherein the user plane of the call remains anchored in the gateway server after the hand over.

The gateway server anchors the user plane of the call during a circuit switched attachment, to the access node and during a packet switched attachment, to the access node. 'Anchoring' implies that the user plane of the call is established between the user terminal and the gateway server, acting as 'anchor point'. From gateway server, the user plane of the call is established through the core network, towards the remote party of the call. The use of gateway server as described above improves the allocation of the media anchor points for a call that may be subject to SR-VCC (single radio-voice call continuity). At the same time the method is applicable for reverse SR-VCC, in which case the call is transferred from a circuit switched access to a packet switched access. Furthermore, the method is also applicable for video SR-VCC or reverse video SR-VCC. In general the invention is not restricted to SR-VCC. In a period which includes the step of receiving the request to set up the control connection, and the step of receiving instructions related to the user plane from the other control node, the gateway server continues to exchange user plane data with the access network involved in the call before hand over and the gateway server detects a start of reception of user plane data from an access network to which the call is transferred during hand over. In this context it is possible that the gateway server continues to exchange data with the access network which handles the call before the hand over and listens to data from the other access network to detect a start of a media stream from the other access network through which the call is transmitted after hand over. As soon as user plane data are received from the other access network, this period, also called transition period, ends.

The gateway server can exchange the user plane data with the access network to which the call is transferred as soon as the reception of the user plane data is detected. The access transfer of the user plane is then accomplished without reallocating media gateway.

The gateway server acts as a border gateway function during a packet switched attachment to the access node, whereas the gateway server can act as a media gateway during a circuit switched attachment to the access node. The gateway server that is seized for a VoLTE call may switch from acting as a BGF to acting as a media gateway. There is hence no longer a need for switching the gateway during access transfer. An advantage of the procedure to switch between acting as a BGF and acting as a media gateway is a near seamless media switch by holding on to the existing media stream until the moment that the new media stream arrives.

In addition to the above-described method it is possible that the identification information allowing the other control node to be identified is received from the control node which controls the call before the hand over. The request to set up the control connection may then be received from the other control node. If the other control node is the same as the control node that was identified using the identification information received from the control node that handles the call before the hand over, the request to set up the control connection is accepted.

According to a further aspect of the invention, the invention relates to the gateway server configured to carry out the hand over between the packet switched attachment and the circuit switched attachment. The gateway server comprises an input/output module configured to receive the identification information allowing the other control node which will control the call after the hand over to be identified. A processing unit is configured to anchor the user plane of the call in the gateway server before the hand over. The input/output module is configured to receive the request to set up the control connection to the gateway server for said call and the processing unit is configured to accept the request to set up the control connection if the request is received from the other control node which is identified using the received

identification information. The processing unit is configured to set up the control connection between the gateway server and the other control node and is configured to apply the instructions related to the user plane of the call received from the other control node after the hand over and to keep the user plane of the call anchored in the gateway server after the hand over. During the hand over process, the gateway server is also configured to execute the instructions from the other control node.

The gateway server can comprise a first input/output buffer to buffer data packets for a data exchange with a packet data gateway during a packet switched attachment to the access node and a second input/output buffer to buffer data packets for a data exchange with a radio network control node during a circuit switched attachment to the access node. The processing unit can switch from using one of the buffers to using the other buffer during hand over.

The invention furthermore relates to a method, by a control node, to hand over the call of the mobile user entity between the packet switched attachment to the access node and the circuit switched attachment to the access node. The control node handing over the call establishes a user plane in the gateway server which handles the user plane data of the call before the hand over. Furthermore, a message is received to release the call connection between the mobile user entity and the control node, the message containing identification information allowing another control node which will control the call after hand over, to be identified. The received identification information is transmitted to the gateway server allowing the other control node to be identified by the gateway server. Furthermore, the control of the user plane of the call is released in the gateway server.

The control node controlling the call before hand over receives the identification information with which the other control node which will control the call after hand over can be identified. This identification information is then further transmitted to the gateway server so that the latter is informed which other control node may request to establish a control connection to control the user plane of the call after hand over.

In one example of the invention the message is received from an anchor control entity which controls the anchoring of the user plane of the call. The anchor control entity may have been informed by the other control node which initiates the hand over about the identification.

The control plane of the call connection between the mobile user entity and the control node can be released after a predefined delay.

Furthermore, it is possible that the control node handing over the call transmits a reference to the logical gateway server instance within the universal gateway to the anchor control entity which controls the anchoring of the user plane of the call. The reference to the logical gateway server instance can help the other control node to access the logical gateway server instance during and after the hand over.

The invention furthermore relates to the corresponding control node which controls the hand over of the call. The control node comprises a control unit configured to establish the user plane in the gateway server which handles the user plane data of the call before the hand over. Furthermore, an input/output module is provided. The input/output module is configured to receive a message to release the call, the message containing identification information allowing the other control node which will control the call after the hand over, to be identified. The input/output module is furthermore configured to transmit the identification information to the gateway server allowing the other control node to be identified by the gateway server. The control unit is further configured to release the control plane of the call connection between the mobile user entity and the control node.

According to a further aspect of the invention a method to take over from another control node a call of a mobile user entity is provided in a hand over process between a packet switched attachment to an access node and a circuit switched attachment to the access node. In one step a message is received containing an address of a gateway server which handles the call before the hand over. Furthermore, the control connection to the gateway server is established based on the received address. An acknowledgement message is transmitted to an anchor control entity which controls the anchoring of the user plane of the call, the message optionally containing a user plane address of the gateway server. Furthermore, the user plane of the call between the mobile user entity and the gateway server which handles the user plane of the call before the hand over for the call after the hand over, is updated. The control node taking over the control after the hand over can establish the control connection to the gateway server as the latter was informed before that the requesting control node will take over the control of the call.

In an example of the invention the control node taking over the call sends its own address as well as a reference to the call control process to the other control node. When it establishes the control relationship with the gateway server, it provides the same information to the gateway server.

The received message can contain a reference to the logical gateway server instance within the gateway server. Furthermore, the reference to the logical gateway server instance within the gateway server may be transmitted to the gateway server. The invention furthermore relates to the control node configured to take over the call from the other control node. The control node comprises an input/output module configured to receive a message containing a address of the gateway server which handles the user plane of the call before the hand over. A control unit is provided configured to establish the control connection with the gateway server based on the received address. The input/output module is furthermore configured to transmit an acknowledgement message to the anchor control entity which controls the anchoring of the user plane of the call, the message optionally containing a user plane address of the gateway server. The control node is configured to update the user plane between the mobile user entity and the gateway server which handled the call before the hand over for the call after the hand over.

It is an object to obviate at least some of the above disadvantages and to provide an improved node and method for a hand over between a packet switched and circuit switched network.

Brief description of the drawings

The invention will be described in further detail with reference to the accompanying drawings. In the drawings

Figure 1 is a schematic architecture illustrating an overview for a VoLTE call,

Figure 2 is a schematic view illustrating a VoLTE call after SR-VCC,

Figure 3 is a schematic view illustrating a combined mobile media gateway and IP multimedia media gateway,

Figure 4 is a diagram illustrating a user plane transportation in a media gateway,

Figure 5 is a block diagram illustrating an access transfer at SR-VCC, Figure 6 is a block diagram illustrating a user plane before and after SR-VCC,

Figure 7 is a block diagram illustrating the selection of a media access gateway, Figure 8 is a block diagram illustrating an SR-VCC including a gateway server according to the invention handling the user plane before and after hand over,

Figure 9 is a signaling diagram illustrating aspects of the invention during the handing over of the gateway control,

Figure 10 is a block diagram illustrating a schematic view of a control node according to the invention controlling the call before or after hand over,

Figure 1 1 is a block diagram illustrating a schematic view of a gateway server according to the invention anchoring the user plane before and after hand over,

Figure 12 is a block diagram illustrating the gateway server during a call with LTE access and UTRAN access, Figure 13 is a block diagram illustrating a schematic view of the switching of the access network media stream at the gateway server,

Figure 14 is a block diagram illustrating schematically the user plane transportation through the gateway server,

Figure 15 is a signaling diagram illustrating a message flow during SR-VCC,

Figure 16 is schematically a transfer of the gateway server control from SGC to MSS. Detailed Description The present invention improves the allocation of media anchor points in the border gateway function and media gateway for a call that may be subject to SR-VCC. The invention also allows for more effective deployment of user plane gateways in the network, as a single gateway server may be used for various forms of access, including, but not limited to, Circuit switched access (such as in GSM/3G networks) and Packet switched access (such as in 3G networks or LTE access networks). The gateway server or gateway node is called universal access gateway hereinafter. The invention is also applicable to reverse SR-VCC where a call is transferred from a circuit switched access to a packet switched access. Furthermore, the invention is usable for video SR-VCC or reverse video SR-VCC. The universal access gateway functions at the same time as a border gateway function entity and media gateway, but not for the same call. The universal access gateway that is seized for a VoLTE call may switch from acting as a BGF to acting as an M-MGW. Thus, there is no longer a need to switch gateway during access transfer. This provides a near seamless media switch by holding on to the existing media stream until the moment that the new media stream arrives.

During the packet switched attachment to an access node, e.g. the IP multimedia subsystem IMS, a VoLTE call is established with the user plane traversing the BGF and the BGF being controlled by SGC in the SBG as shown in Figure 1 . As shown in Figure 8, when access transfer occurs, the user plane remains anchored in the BGF which is now the universal access gateway of the present invention. The MSC server 70 receives from the ATCF 43 the address of the UAGw 100 that is used for packet switched access leg for the call. The MSC server 70 allocates this UAGw 100 to take the function of the media gateway for the circuit switched access leg for this call. In this process, the universal access gateway 100 remains allocated as media anchor point for the access leg. The MSC server 70 gains control over the universal access gateway 100 and the SGC 41 releases control of the universal access gateway 100. In Figure 8 it is shown that the ATCF 43, the ATGW 140 and the universal access gateway 100 remain in the constellation for the call when the SR-VCC takes place. As long as the VoLTE call is in a packet switched access, the UAGw 100 acts as BGF under control of the SGC 41 . When the call is transferred to a circuit switched access, the UAGw 100 takes the role of the media gateway under the control of the MSC server 70.

This is specified in further detail in Figure 9. The control over the universal access gateway 100 is handed over from the SGC to the MSS, in the case of SR-VCC, and from the MSS to SGC, in the case of rSR-VCC.

Figure 12 gives a more detailed view on the universal access gateway 100. The universal access gateway 100 contains, as explained in further detail below in connection with Figure 13, an input and output buffer for RTP media stream. Where possible, media transcoding in the universal access gateway is prevented just like the media transcoding in the media gateway and BGF is prevented where possible.

Assuming there is no need for media transcoding, the UAGw 100 forwards the RTP messages between two end points, without processing the contents of the RTP messages. When access transfer takes place, control of the UAGw 100 is transferred between SGC and MSS. The entity that has then gained control of the UAGw 100 updates the remote IP and port number for the RTP message transfer between UAGw 100 and access network. As is shown in Figure 12, the left hand side IP address and port number (one for RTP, another one for RTCP, if used) are updated on the access network side of the UAGw.

The update of the IP address and the port in the UAGw does not constitute the moment when the connection between the UAGw 100 and the packet data gateway terminates. There will be a transition period during which media from the RNC, the new source, may not be received yet. In the transition period the following behavior for the UAGw applies: Data received from the ATGW will be forwarded to the PDN gateway and data received from the PDN gateway will be forwarded to the ATGW. The UAGw continues listening to data from the RNC to detect the start of the media stream from RNC and thereby the end of the transition period. From that moment onwards the UAGw relays the media to the access network to which the call is transferred, i.e. to the RNC for an LTE to UTRAN access transfer or the PDN gateway (PDGw) for UTRAN to LTE access transfer. An access transfer for the user plane is now accomplished without re-allocating media gateway.

This is shown in further detail in Figure 13. The universal access gateway contains a processing unit 1 10 which is configured to carry out the instructions received from the SGC or MSC server. As shown by the arrow to the processing unit 1 10, instructions are received from the control plane (SGC or MSC server) about the pending change in the access network media stream. During the transition period, there are

effectively two RTP terminations at the universal access gateway for media transmission between the access network and the UAGw. First of all, the RTP termination towards the PDGw. This RTP termination is created during establishment of the VoLTE call. This RTP termination is released when the access transfer is complete. The second termination is the RTP termination towards the RNC. This RTP termination is created at access transfer and this RTP termination is retained for the remainder of the call which is now on UTRAN access. The processing unit switches over from sending and receiving media to or from the PD gateway to sending and receiving media to or from the RNC.

However, despite the temporary existence of the above-described two RTP terminations at the universal access gateway, the UAGw 100 will either relay media between the PDGw and ATGW or relay media between the RNC and ATGW. As furthermore shown in Figure 13, a first input/output buffer 130 is provided to buffer media stream to or from the PDGw and a second input/output buffer for the media stream to or from the RNC. A third input/output buffer 140 is provided for the media stream to and from the ATGW.

Figure 14 shows further details of the universal access gateway. On the left hand side the media transport between the UAGw and the subscriber terminal is shown, whereas on the right hand side the media transport to the IMS core network is shown. The universal access gateway has to take care of the adaptation between the lower layer transport protocols (ATM, Ethernet) used (a) between the UAGw 100 and subscriber terminal 10 and (b) between the UAGw 100 and the IMS core network. In this example ATM is shown for the lu-CS reference point. It may, however, also be Ethernet.

Although not explicitly depicted in Figure 14, the user plane over the SGi reference point terminates at the PDGw in the enhanced packet system, from where it is carried to the user terminal through the packet data connection. The user plane over the lu-CS reference point terminates at the RNC in the UTRAN from where it is carried to the terminal via the NodeB and radio access. In the following further aspects of the universal access gateway will be described. For an LTE access the SGC 41 needs to select a logical UAGw for the role of BGF. For UTRAN access the MSS 70 needs to select a logical UAGw for the role of media gateway. In an embodiment of the invention the SGC 41 and the MSS 70 are configured with the address or addresses of the UAGw node or nodes from which they may seize a logical UAGw instance. At access transfer, for SR-VCC the MSS 70 needs to receive from the SGC 41 the address of the UAGw node that is used for the user plane of this call as well as a reference to the logical UAGw instance within that node. For rSR-VCC the SGC 41 needs to receive from MSS 70 the address of the UAGw node that is used for the user plane of this call as well as a reference to the logical UAGw instance within said node. A logical UAGw instance is formed by a software process instance, with associated configuration data and dynamic

processing data, that fulfills the tasks as specified for the UAGw, for one specific call. For each call, a designated logical UAGw is instantiated. Hence, the logical UAGw is a dynamic functional component within the UAGw. The transfer of required

information is accomplished through SIP signaling between the SGC and MSS during access transfer. The SIP signaling between SGC 41 and MSS 70 may run via ATCF 43. Furthermore, at access transfer, for SR-VCC, the logical UAGw instance controlled by the SGC needs to be informed about the address of the MSS to which the control of this logical UAGw instance will be transferred. For rSR-VCC the logical UAGw instance controlled by MSS 70 needs to be informed about the address of the SGC 41 to which the control of the logical UAGw instance will be transferred. The UAGw is informed, by the SGC 41 , about the address of the MSS that will take over the control over this UAGw, the UAGw will then be prepared for accepting control from that MSS, i.e. it is a safety measure. This transfer of information, needed for informing UAGw about the address of the control node assuming control after hand over, is accomplished through message exchange between UAGw, MSS 70, ATCF 43 and SGC 41 during access transfer. The SIP signaling between SGC and MSS runs via ATCF.

Figure 15 shows a sequence diagram for a typical use case, namely SR-VCC, wherein an access transfer from LTE access to UTRAN access occurs. It should be understood that Figure 15 provides non-exhaustive functional listing of the messages that are transferred during SR-VCC. For a complete overview of other messages, reference is made to 3GPP 23.216, e.g. Figure 6.2.2.1 -1 . In the sequence diagram of Figure 15 we consider the SGC and P-CSCF to be combined, although they form separate logical entities.

In step S1 a session is established over EPS resulting in the setup of a control plane involving the SGC 41 , ATCF 43 and other entities, such as P-CSCF and SCC-AS, not being shown in Figure 15. Furthermore, a user plane is established comprising the UAGw 100 and other entities, such as the ATGW 140 not shown in Figure 15. In step S2 the MSS 70 has received from the mobility management entity, MME (not shown) in the EPS an instruction to transfer the access of the call towards UTRAN. In the next step S3 the MSS 70 sends a SIP Invite message towards ATCF 43 and SCC-AS for establishing the SIP session for the circuit switched access leg. The MSS does not allocate a media gateway yet at this stage. The Invite message contains a reference to the call handling process in the MSS. Especially the call handling process that will gain control over the logical UAGw instance.

In step S4 the ATCF 43 takes action to forward the Invite towards the SCC-AS (not shown in Figure 15) and to update the ATGW 45 (also not shown in Figure 15). In step S5 the ATCF releases the SIP session over the EPS access network, by sending a Bye request towards the SGC. This Bye request contains the address of the MSS 70 of the circuit switched access leg. This Bye request can contain the above-described reference to the call handling process in the MSS. This reference was received in the Invite message from MSS 70.

In step S8 the SGC 41 updates the UAGw 100. It conveys the following information to the UAGw 100:

- The address of the MSS that will take over the control of the UAGw logical instance, inside the UAGw;

- The reference of the call control process in the MSS that will take over the control of the UAGw logical instance.

The control of the UAGw will be transferred to a certain MSS. This means that the UAGw 100 is now prepared for receiving control from the MSS 70 and will accept this control. Furthermore, the SGC will provide the above-described MSS call handling process reference to the UAGw 100.

Additionally, the SGC 41 relinquishes control over the UAGw.

After the update/response between the SGC 41 and UAGw 100 the control relationship between SGC and UAGw is terminated and the UAGw 100 will wait for MSS to take ownership of the control and to provide further instructions. The MSS shall have taken ownership of the control prior to media stream starting to arrive from the RNC. In step S7 a 200 Ok message is sent to ATCF 43 in response to the Bye request, whereby the 200 Ok message contains the address of the UAGw 100 as well as a reference to the logical UAGw instance. The SGC has this information already available, it uses this information for controlling the logical UAGw. In step S8 the SGC 41 releases the SIP session towards the UE 10 over the EPS access network. It is possible that the release is performed after a preconfigured delay. With such a preconfigured delay it is possible to assure that the user plane over the EPS continues until the user plane over CS starts.

In step S9 the ATCF 43 sends a 200 Ok message to the MSS 70 in response to the Invite. The 200 Ok message contains the address of the UAGw 100 as well as a reference to the logical UAGw instance. The ATCF 43 had received that information, the UAGw node address and reference to the logical UAGw instance in step S7 as described above. In step S10 the MSS 70 establishes a control connection with UAGw 100 of which it had received the address from the ATCF 43. The MSS has also received a reference to the logical UAGw instance, so it can address the same logical UAGw instance in the UAGw 100. The UAGw will accept the MSS 70 to become controller of this logical UAGw instance, since the address of the MSC was previously provided to the UAGw by the SGC 41 when the SCG still had control over the UAGw instance. Additionally, the MSS 70 may provide in the signaling towards the UAGw 100 the reference of the call handling process, i.e. the call handling process that is now obtaining control over the UAGw 100. This enables the UAGw to verify that the control is gained by the process in the MSS that was authorized to gain control. Furthermore, instructions related to the user plane of the call are received from the control node taking over the call.

In step S1 1 an acknowledgement message is sent from the MSS 70 to ATCF 43 associated with the Invite transaction, this message containing the user plane address of the UAGw 100, more precisely the IP address and port number for the RTP and RTCP stream between this logical UAGw 100 and the ATGW. The logical UAGW has not changed in the SR-VCC process, so the ATCF 43 does not need to update the ATGW. For this reason it may not be needed for the MSS 70 to provide the information to the ATCF. In step S10 the MSS 70 also instructs the UAGw about updates of the user plane between the UE and the UAGw 100. For the circuit switched access leg this entails that the RNC is informed in S12 about the UAGw address and port number for the user plane of this call. The MSS 70 had received this information from UAGw when it gained control over the UAGw. In step S13 the update of the UAGw is complete and a user plane is established over UTRAN, between UE and UAGw. The logical UAGw can now process media transfer to or from the UE 10 via the circuit switched access network S10 in figure 15 includes:

1 ) the step of receiving from the control node a request to set up a control

connection;

2) the step of accepting this request;

3) the step of receiving instructions.

The above-described use case represents a typical case for SR-VCC. The above description especially relates to the most important aspects. The sequence diagram of Figure 15 can also be transformed into a use case for reverse SR-VCC in which an access transfer from UTRAN to LTE takes place. The method is the same, but then the role of the MSS 70 and the SGC 41 are exchanged and the control of the UAGw 100 is transferred from MSS 70 to SGC 41 . In the above-referenced description the UAGw 100 was used in the SR-VCC context. However, the UAGw may also be employed in a network for serving the following purposes: It may act as a BGF for a VoLTE call or it may act as a media gateway for a circuit switched call over UTRAN. Figure 16 provides information flows, wherein these information flows map amongst others to the SIP signaling. At A the SGC 41 has control over the UAGw 100. That control was gained during the establishment of the VoLTE call. Due to the SR-VCC, the MSS initiates a SIP signaling with SGC 41 via ATCF 43. The SIP signaling is meant for (a) establishing the MSS in the access control plane path and (b) obtaining control over the UAGw in the user plane. At B in Figure 16 the SGC 41 suspends control over the UAGw 100 and informs UE 10 that media transfer is temporarily suspended due to the SR-VCC process that was started.

At C, the SGC 41 signals back to MSS, via ATCF 43. This SIP signaling is for (a) confirming the release of the SGC 41 in the access control plane path, (b) confirming the release of the control over the UAGw 100 in the user plane and (c) providing to MSS details about the UAGw process, enabling MSS 70 to gain control over that UAGw process.

Furthermore, at D the MSS 70 has received indication that it can gain control over the user plane and has received the required information to actually claim that control from the UAGw 100. The MSS 70 establishes a relation with the UAGw 100 using information received from the SGC, to gain control over the UAGw process. The MSS also informs the UE 10 to resume media plane transportation, due to the completion of SR-VCC process.

For reverse SR-VCC the transfer of the UAGw 100 works in the opposite direction. In this case it is the SGC 41 that initiates the SIP signaling to gain control over the UAGw 100, the MSS 70 releases control over the UAGw 100 and hands over control to the SGC 41 .

When a voice call is transferred from LTE access to UTRAN access, it may not be guaranteed that the codec that has been agreed end-to-end is supported for the CS access leg. This may have the effect that the CS access leg is established by the MSC with a codec that's different from the codec that is used for the EPS access. In such situation, the ATCF 43 instructs the ATGW to apply transcoding on the user plane, for the user plane after access transfer.

A call that is established over CS access, will probably have a codec that may also be used over PS access. When access transfer to EPS access takes place, there would in any case no transcoding be needed. Since the UAGw is already acting as media anchor point, it is in this case not needed for the user plane to be anchored in the ATGW. This will give further improvement in the end-to-end call: fewer media gateways generally means smaller voice delay (due to fewer buffers). When the ATCF 43 determines during VoLTE call establishment that the codecs that are negotiated between the UE 10 (using EPS access) and the remote end are also supported by the MGW in the network, access transfer to UTRAN will not lead to the need for transcoding in ATGW. This removes the need for media anchoring in ATGW. Not anchoring the user plane in ATGW is known. 3GPP TS 23.237 describes various cases where the ATCF 43 may decide not to anchor the user plane in ATGW. When the user plane is not anchored in ATGW, access transfer between LTE access and UTRAN access will lead to the need for remote-end SDP (session description protocol) update. Not anchoring the user plane in ATGW may be decided even for a call that is subject to SR-VCC, under the condition that the remote-end SDP update is considered non-prohibitive. For example, the call takes a breakout to CS network and the user plane is anchored in the IM-MGW controlled by MGCF at the network boundary.

As can be deduced from the above examples described in connection with Figs. 15 and 16, the reference to the logical UAGw process is transferred from the UAGw 100 to the SGC 41 and from there to the ATCF 43. From the ATCF 43 it is then transferred to the MSS 70 from where it is transferred again to the UAGw 100. Thus, the UGAw gets the transmitted reference to the logical UAGw process back. When the UAGw 100 is addressed by the MSS 70 for the purpose of MSS gaining control over the user plane, the MSS can indicate to UAGw 100 exactly for which process it wants to obtain control.

In Figure 10 a schematic view of a control node is shown. This control node can be the call control node that controls the call before hand over or can be the control node which controls the call after hand over. In the example shown it is the SGC 41 . This control node comprises a control unit 410 which controls the functioning of the control node and which inter alia establishes the user plane in the universal access gateway, generates the instructions sent towards the other entities etc. Furthermore, a generalized input/output module 41 1 is schematically shown. This input/output module symbolizes that the control node communicates with all the different other entities involved. It should be understood that this input/output module contains many reference points to the different entities that are configured to support the different protocols. Examples of such reference points includes (not exhaustive): lu_CS interface, for control of RNC by MSC Server, Mc interface for control of MGw by MSC Server, Gm interface for control plane signaling between SGC and UE. It should be understood that a control node, such as the SGC 41 , may comprise additional modules. However, for the sake of clarity only the aspects are described which are relevant for the understanding of the present invention.

In Figure 1 1 an example of the universal access gateway is schematically shown. The universal access gateway comprises a processing unit 1 10 which is responsible for the functioning of the universal access gateway and which inter alia carries out the instructions received from a control node, such as control node 41 shown in Figure 10. The universal access gateway contains, as already explained in

connection with Figure 13, the different buffers 120-140 to buffer the media stream from and to the PDGw, the media stream from and to the RNC and the media stream from and to the ATGW. Furthermore, an input/output module 150 is schematically shown that is responsible for receiving messages and for transmitting messages. It should be understood that the input/output module covers the different interfaces or reference points with which the network access gateway communicates with other nodes of the network.

The modules shown in Figure 10 and in Figure 1 1 symbolize certain functional entities. The modules may be incorporated by hardware or software or by a combination of hardware and software. Furthermore, the different modules may be incorporated in a single physical device or may be incorporated in different devices, by hardware, software or a combination of hardware and software. Summarizing, for the end user the above-described invention has the advantage that an improved end user experience is provided at access transfer between LTE access and UTRAN access. The improved end user experience results from the fact that there is no need for transferring the user plane to another access gateway during the access transfer. Voice interruption during access transfer may therefore be shorter or may noticeably disappear. For the operator of the telecommunications network in which the hand over takes place the advantage can be seen in the fact that fewer resources may be needed. It is possible to use a single pool of UAGws. The UAGw may take the role of BGF or MGW and may switch role during SR-VCC. This entails more optimized resource (UAGw) allocation. Considering Erlang's rule of resource dimensioning, for certain grade of service the number of UAGws that are required can be less than the sum of BGFs and media gateways that would otherwise be required.

Claims

C LA I M S

1 . A method, performed by a gateway server (100), to carry out a hand over of a call of a mobile user entity (10) between a packet switched attachment to an access node (200) and a circuit switched attachment to the access node, a first control node (41 ) controlling the call during the packet switched attachment to the access node, a second control node (70) controlling the call during the circuit switched attachment to the access node (200), the method comprising the steps of:

- receiving (S6) identification information allowing the other control node (70, 41 ) which will control the call after the hand over is completed, to be identified, wherein the user plane of the call is anchored in the gateway server (100),

- receiving a request (S10) to set up a control connection to the gateway server for said call,

- accepting the request to set up the control connection if the request is received from the other control node (70, 41 ) which is identified using the received identification information,

- in response to accepting the request to set up the control connection, setting up a control connection between the gateway server (100) and the other control node (70, 41 ) and,

- receiving instructions related to the user plane of the call from the other control node (70, 41 ) after the hand over is completed, wherein the user plane data of the call remains anchored in the gateway server (100) after the hand over.

2. The method according to claim 1 , wherein the gateway server (100), in a period including the step of receiving the request to set up a control connection and the step of receiving instructions related to the user plane, continues to exchange user plane data with an access network involved in the call before hand over and detects start of reception of user plane data from an access network to which the call is transferred during hand over.

3. The method according to claim 2, wherein the gateway server (100) starts to exchange user plane data with the access network to which the call is transferred as soon as the reception of user plane data is detected.

4. The method according to any of the preceding claims, wherein the gateway server (100) acts as a border gateway function during a packet switched attachment to the access node, whereas the gateway server (100) acts as a media gateway during a circuit switched attachment to the access node.

5. A gateway server (100) configured to carry out a hand over of a call of a mobile user entity (10) between a packet switched attachment to an access node (200) and a circuit switched attachment to the access node (200), a first control node (41 ) controlling the call during a packet switched attachment to the access node, a second control node (70) controlling the call during the circuit switched attachment to the access node, the gateway server (100) comprising:

- an input/output module (150) configured to receive identification information allowing the other control node which will control the call after the hand over to be identified,

- a processing unit (1 10) configured to anchor the user plane of the call in the gateway server before the hand over,

wherein the input/output module (150) is further configured to receive a request to set up a control connection to the gateway server for said call, the processing unit (1 10) being further configured to accept the request to set up the control connection if the request is received from the other control node which is identified using the received identification information, the processing unit is further configured to set up a control connection between the gateway server (100) and the other control node, wherein the processing unit (1 10) is still further configured to apply the instructions related to the user plane of the call received from the other control node after the hand over, and to keep the user plane of the call anchored in the gateway server (100) after the hand over.

6. The gateway server (100) according to claim 5, further comprising a first input/output buffer (120) to buffer data packets for a data exchange with a packet data gateway during a packet switched attachment to the access node (200) and a second input/output buffer (130) to buffer data packets for a data exchange with a radio network controller during a circuit switched attachment to the access node, wherein the processing unit (1 10) is configured to switch from using one of the buffers to using the other buffer during hand over.

7. A method, by a control node (41 , 70), to hand over a call of a mobile user entity between a packet switched attachment to an access node and a circuit switched attachment to the access node, the method comprising the steps of: - establishing a user plane in a gateway server (100) which handles user plane data of the user plane of the call before the hand over,

- receiving a message to release the call connection between the mobile user entity (10) and the control node (41 , 70), the message containing identification information allowing another control node (70, 41 ) which will control the call after hand over to be identified,

- transmitting the identification information to the gateway server (100) allowing the other control node to be identified by the gateway server,

- releasing control of the user plane of the call in the gateway server (100).

8. The method according to claim 7, wherein the message is received from an anchor control entity (43) which controls the anchoring of the user plane of the call.

9. The method according to claim 7 or 8, wherein the control plane of the call connection between the mobile user entity and the control node is automatically released after a predefined delay.

10. The method according to any of claims 7 to 9, further comprising the step of transmitting a reference to the logical gateway server instance within the gateway server, to an anchor control entity (43) which controls the anchoring of the user plane of the call.

1 1 . A control node (41 , 70) configured to control a hand over of a call of a mobile user entity (10) between a packet switched attachment to an access node (200) and a circuit switched attachment to the access node, the control node comprising:

- a control unit (410) configured to establish a user plane in a gateway server (100) which handles user plane data of the call before the hand over,

- an input/output module (41 1 ) configured to receive a message to release the call, the message containing identification information allowing another control node (70, 41 ) which will control the call after hand over, to be identified, wherein the input/output module (41 1 ) is configured to transmit the identification information to the gateway server (100) allowing the other control node to be identified by the gateway server, wherein the control unit (410) is configured to release the control plane of the call connection between the mobile user entity and the control node.

12. A method to take over from another control node a call of a mobile user entity (10) in a hand over process between a packet switched attachment to an access node (200) and a circuit switched attachment to the access node (200), the method comprising the steps of:

- receiving a message containing a address of a gateway server (100) which handles the user plane of the call before the hand over,

- establishing a control connection to the gateway server (100) based on the received address,

- transmitting an acknowledgement message to an anchor control entity (43) which controls the anchoring of the user plane of the call, the message optionally containing a user plane address of the gateway server (100),

- updating the user plane of the call between the mobile user entity (10) and the gateway server (100) which handled the call before the hand over for the call after the hand over.

13. The method according to claim 12, wherein the received message contains a reference to the logical universal gateway instance.

14. The method according to claim 13, further comprising the step of transmitting the reference to the logical universal gateway access instance within the gateway server to the gateway server (100).

15. A control node (70, 41 ) configured to take over from another control node (41 , 70) a call of a mobile user entity (10) in a hand over process between a packet switched attachment to an access node (200) and a circuit switched attachment to the access node, the control node (70, 41 ) comprising:

- an input/output module (41 1 ) configured to receive a message containing a address of a gateway server (100) which handles the call before the hand over,

- a control unit (410) configured to establish control connection with the gateway server (100) based on the received address, wherein the input/output module is configured to transmit an acknowledgement message to an anchor control entity (43) which controls the anchoring of the user plane of the call, the message containing a user plane address of the gateway server (100), wherein the control unit (410) is configured to update the user plane between the mobile user entity (10) and the gateway server (100) which handled the call before the hand over for the call after the hand over.