HK1198676B - Method and device for handling handover of a communications service - Google Patents

Description

Method and apparatus for handling handover of communication services

Technical Field

Embodiments herein generally relate to a network node, a method in the network node, and more particularly, to enabling handover of communication services between a circuit switched (CS) network and a packet switched (PS) network.

Background Art

In a typical cellular network (also known as a wireless communication system), user equipment (UE) communicates with one or more core networks (CNs) via a radio access network (RAN).

A user equipment (UE) is a mobile terminal that a subscriber can use to access services provided by an operator's core network and services located outside the operator's network to which the operator's RAN and CN provide access. A UE may be a communication device such as a mobile phone, a cellular phone, or a laptop computer with wireless capabilities. A UE may be a portable, pocket-sized, handheld, computer-containing, or vehicle-mounted mobile device that enables voice and/or data communications with another entity (such as another mobile station or a server) via a radio access network.

Enables user equipment to communicate wirelessly in a cellular network. Communications may be performed between two user equipment, between a user equipment and a conventional phone, and or between a user equipment and a server, for example, via a radio access network and possibly one or more core networks contained within the cellular network.

A cellular network covers a geographic area divided into cell areas. Each cell area is served by a base station, such as a radio base station (RBS), which may sometimes be referred to as an evolved Node B (eNB), "eNodeB," "NodeB," "Node B," or BTS (Base Transceiver Station), depending on the technology and terminology used. The base station communicates with user equipment within range of the base station over an air interface operating on radio frequencies.

In a typical cellular system (also known as a wireless communication network), wireless terminals (also known as mobile stations and/or user equipment units) communicate with the core network via the Radio Access Network (RAN). A wireless terminal can be a mobile station or user equipment, such as a mobile phone (also known as a cellular phone), or a laptop computer with wireless capabilities, such as a mobile terminal, and thus can be a portable, pocket-sized, handheld, computer-containing, or vehicle-mounted mobile device that communicates voice and/or data with the RAN.

A radio access network covers a geographic area divided into cell areas, each of which is served by a base station, such as a radio base station (RBS), also known in some radio access networks as an eNodeB (eNB), NodeB, B-node, or base station. A cell is a geographic area where a radio base station located at a base station site provides radio coverage. Each cell is identified within the local radio area by an identification number that is broadcast within the cell. A base station communicates with user equipment within range of the base station over an air interface operating on radio frequencies.

The cellular network may be applicable to one or more radio access technologies such as Long Term Evolution (LTE), LTE-Advanced, Wideband Code Division Multiple Access (WCDMA), Global System for Mobile Communications (GSM), or any other 3rd Generation Partnership Project (3GPP) radio access technology.

In LTE, for example, users expect new networks to support all the services from legacy networks. To meet these requirements, inter-technology mobility is a key feature. In LTE, the voice service offered by LTE is Voice over Internet Protocol (VoIP) based on the Internet Protocol Multimedia Subsystem (IMS). LTE is a packet data network, and VoIP is used to support voice over packet networks.

Inter-technology mobility is also important for the introduction of new services. Inter-technology mobility enables new services to be rolled out across the network, even if the wireless broadband access technology that best and most efficiently supports the new service is deployed only in the highest traffic areas. Inter-technology mobility provides a bridge between the old and new access networks, enabling seamless service continuity for users over a wide area.

Inter-technology mobility can simplify the rollout of new LTE, where voice services are moved to IMS-based VoIP in conjunction with the deployment of the LTE access network by using inter-technology mobility together with functionality called Single Radio Voice Call Continuity (SRVCC). SRVCC is an LTE feature that allows VoIP/IMS calls in the LTE packet domain to be moved to the traditional circuit domain, such as GSM/UMTS or CDMA.

When a user device with an ongoing IMS voice call in LTE leaves LTE coverage, assuming the 2G/3G, or circuit-switched (CS) network, does not support VoIP, the user performs SRVCC to 2G/3G and continues the voice call in the CS network via the Mobile Switching Center (MSC). The MSC is a 3G core network element that controls the network switching subsystem elements. When the user device returns to LTE coverage, the operator may want to move the user device back to LTE for various reasons. This process is called CS-to-PS or return SRVCC (rSRVCC). Another use case for rSRVCC could be when the user device is camped on 2G/3G and initiates a CS voice call in 2G/3G via the MSC. After a period of time, the user device enters LTE coverage, triggering rSRVCC.

The handover of an ongoing voice call from LTE to a 3G or 2G network, or vice versa, is achieved using mechanisms related to handling dedicated bearers. Generally speaking, a bearer is a logical channel that carries certain information. A bearer can also be referred to as a radio resource. An EPS bearer is established when the user equipment 101 connects to a packet data network (PDN) and remains for the entire lifetime of the connection. It is called the default bearer. The default bearer is always provided over IP connectivity to the network. Any additional EPS bearer is called a dedicated bearer. A dedicated bearer context is established when a service in the network requests prioritization of IP packets belonging to a specific media flow between two IP addresses and TCP/UDP ports. A dedicated bearer is a bearer that carries traffic for IP flows that have been identified as requiring specific packet forwarding treatment. The user equipment requests a dedicated bearer to transmit data with a specific QoS.

A criterion for enabling rSRVCC may be that it should have minimal impact on the network. An alternative solution for enabling rSRVCC is for the user equipment to reselect to LTE and then initiate a session handover from CS to PS, including an IMS transfer of the session from CS to PS and radio bearer establishment of a dedicated voice bearer in the EPC. This alternative solution implies significant disruption of the user equipment's voice call during the handover, which can result in high bit rate errors and degraded quality of service for the user equipment.

Another solution (see Figure 3) involves a preparation phase in the source system before starting the handover to the target system, which implies a CS-to-PS request from the MSC to the SGSN. This alternative further implies the establishment of a dedicated voice bearer in the SGSN before the rSRVCC handover. This affects several nodes in the network, such as the SGSN, SGW and PGW, as well as the PCC, which increases both the complexity of the communication network and the signaling load in the network.

The disadvantage is that it reduces the user's service quality experience, such as QoS, call drop rate, and interruption time, to a minimum.

Another problem is that the MSC needs a procedure to find the source SGSN/MME, thereby implying a DNS procedure in the MSC.

Summary of the Invention

It is therefore an object of embodiments herein to obviate at least one of the above-mentioned disadvantages and to provide improved handling of communication service handovers.

According to a first aspect, the object is achieved by a method in a network node for enabling handover of a communication service from a circuit-switched network (CS) to a packet-switched network (PS). A user equipment is located in the CS network and has a communication service in the CS network. The network node receives a trigger instructing establishment of a dedicated bearer associated with a communication network in the PS network. Based on the trigger, the network node determines that establishment of the dedicated bearer in the PS network should be delayed. The network node delays establishment of the dedicated bearer. The network node receives information indicating that handover from the CS network to the PS network has been completed. Upon receiving the information indicating that handover from the CS network to the PS network has been completed, the network node establishes a dedicated bearer associated with the communication service.

According to a second aspect, the object is achieved by a network node for enabling handover of a communication service from a circuit-switched network, referred to as a CS, to a packet-switched network, referred to as a PS. A user equipment is located in a CS network and has a communication service in the CS network. The network node comprises a receiving unit configured to receive a trigger indicating establishment of a dedicated bearer associated with a communication network in the PS network. The network node comprises a determining unit configured to determine, based on the trigger, that establishment of the dedicated bearer in the PS network should be delayed. The network node comprises a delaying unit configured to delay establishment of the dedicated bearer. The receiving unit is further configured to receive information indicating that handover from the CS network to the PS network has been completed. The network node further comprises an establishing unit configured to establish a dedicated bearer associated with the communication service upon receiving information indicating that handover from the CS network to the PS network has been completed.

Embodiments herein provide a number of advantages, of which a non-exhaustive list of examples is listed below:

There is a requirement in the 3rd Generation Partnership Project (3GPP) to specify a return SRVCC functionality, rSRVCC. The embodiments herein are a simpler approach than any other approach discussed so far in 3GPP, as the approach relies more heavily on existing approaches – IRAT and TAU, the release of redirection from 3G, and the fact that in EPC/LTE, if there is no dedicated bearer for voice, there is a fully enabled default bearer that can be used to send voice until the dedicated bearer is established.

One advantage of the embodiments herein is that, while still meeting the 3GPP requirements for rSRVCC, the impact on the existing EPC network is minimal, affecting only one network node, while still having no further impact on other system nodes such as the MSC, IMS, RAN, or UE. The SGSN is completely unaffected.

An advantage of embodiments herein is that it is simple in the sense that the impact on the EPC is only in one node, such as PCRF, P-CSCF or PGW.

Yet another advantage is that embodiments herein mean that the voice quality of rSRVCC is improved without new requirements for SGSN or DNS queries in the MSC server.

Another advantage of the embodiments herein is that user quality of service experience, such as QoS, call drop rate, and interruption time, is improved and minimized.

The embodiments of the present invention are not limited to the features and advantages mentioned above. Those skilled in the art will recognize other features and advantages upon reading the following detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

The embodiments herein will now be further described in more detail in the following detailed description with reference to the accompanying drawings which illustrate the embodiments, in which:

FIG1 is a schematic block diagram illustrating an embodiment of a communication network.

FIG2 is a schematic block diagram illustrating an embodiment of a communication network.

Figure 3 is a combined flow chart and signalling diagram illustrating an embodiment of a method.

Figure 4 is a combined flow chart and signalling diagram illustrating an embodiment of a method.

Figure 5 is a combined flow chart and signalling diagram illustrating an embodiment of a method.

Figure 6 is a combined flow chart and signalling diagram illustrating an embodiment of a method.

FIG7 is a schematic block diagram illustrating an embodiment of a network node.

The drawings are not necessarily to scale and the dimensions of some features may be exaggerated for clarity of illustration. Emphasis is instead placed upon illustrating the principles of the embodiments herein.

DETAILED DESCRIPTION

Embodiments herein relate to reverse or CS to PS Single Radio Voice Call Continuity (SRVCC).

FIG1 illustrates a communication network 100 in which embodiments herein may be implemented. In some embodiments, the communication network 100 may be adapted for use with one or more radio access technologies, such as Long Term Evolution (LTE), LTE-Advanced, Wideband Code Division Multiple Access (WCDMA), Global System for Mobile Communications (GSM), or any other Third Generation Partnership Project (3GPP) radio access technology.

The communication network 100 includes a base station 103 serving a cell. The base station 103 may be a base station such as a NodeB, an eNodeB, or any other network element capable of communicating with a user equipment 101 via a radio carrier. In this example, the user equipment 101 is capable of communicating with a first network node 110 via a radio carrier.

User equipment 101 can be any suitable communication device or computing device capable of communicating with a base station via a radio channel, such as, but not limited to, a mobile phone, a smartphone, a personal digital assistant (PDA), a laptop computer, an MP3 player or portable DVD player (or similar media content device), a digital camera, or even a stationary device such as a PC. A PC can also be connected via a mobile station as an end station for broadcast/multicast media. User equipment 101 can also be, for example, an electronic photo frame, a cardiac monitoring device, an intrusion or other surveillance device, a weather data monitoring system, or an embedded communication device in a vehicle, car, or transportation communication device. User equipment 101 is referred to as a UE in some figures.

User equipment 101 may be located in an area with 2G/3G coverage, i.e., in a CS network 100a. User equipment 101 has an ongoing IMS 105 communication service in CS network 100a. IMS 105 is a framework for delivering IP multimedia services. At some point, user equipment 101 moves from CS network 100a to an area with LTE coverage, i.e., to PS network 100b. This is referred to as a CS-to-PS handover. For various reasons, the operator may also wish to move the communication service from CS network 100a to PS network 100b. CS network 100a is a technology used, for example, by two network nodes to establish a dedicated communication channel, i.e., a circuit, before the nodes can communicate. This circuit operates as if the nodes were physically connected, as in the case of electronic circuits. In PS network 100b, data is moved in separate small blocks, i.e., packets, based on the destination address in each packet. Upon receipt, the packets are reassembled in the proper order to form a message. The bit delay in the CS network 100a is constant during the connection relative to the PS network 100b where packet queues may cause varying packet transfer delays.

Figure 2 illustrates communications network 100 in more detail. User equipment 101 is handed off from CS network 100a (also known as UTRAN/GERAN) to PS network 100b (also known as target E-UTRAN). CS network 100a is connected to MSC server 203 via the Iu-cs/A interface, and further to IMS 105. As mentioned above, MSC server 203 controls network switching subsystem elements. CS network 100a is connected to Serving General Packet Radio Service Support Node (SGSN) 205 via the Iu-ps/GB interface. Serving General Packet Radio Service Support Node (SGSN) 205 is the node responsible for delivering data packets sent to and from user equipment 101 within its geographic service area. SGSN 205 is connected to MME 201, a key control node for LTE access network 100b, via the Gn/S3 interface. MME 201 is connected to Home Subscriber Server (HSS) 210 via the S6a interface. HSS 210 is the master subscriber database of the IMS network entity that supports actual call processing. It contains subscription-related information, performs authentication and authorization of user devices, and can provide information about the subscriber's location and IP address. PS network 100b is also connected to MME 201 via the S1-MME interface. PS network 100b is connected to Serving Packet Data Network (PDN) Gateway (PGW) 207 via the S1-U interface. PGW 207 is connected to MME 201 via the S11 interface. PGW 207 is further connected to Policy and Charging Rules Function (PCRF) 212 via the S7 interface. PCRF 212 is responsible for determining policy rules within the multimedia network. PGW 207 is connected to IMS 105 via the SGi interface. The solid lines in Figure 2 illustrate the bearer path before handover from CS network 100a to PS network 100b. The dashed lines illustrate the bearer path after handover, and the dash-dot lines illustrate the Session Initiation Protocol (SIP) signaling path before handover. SIP is a signaling protocol used to control multimedia communication sessions such as voice and video calls over IP.

The embodiments herein involve performing rSRVCC with minimal preparation before a handover is performed, including early IMS session transfer, deferred allocation of dedicated bearers, and temporary media streams on existing bearers in LTE. Deferred allocation means that a dedicated bearer is allocated after the user equipment 101 moves to LTE to provide voice quality bearer with a latency of a few tenths of a second. The embodiments herein are a compromise between complexity and voice quality, which does not imply new requirements for DNS queries in the MSC server 203 or the SGSN 205, and also means that the voice quality of rSRVCC is improved.

The prerequisite is that the user equipment is attached to the PS network 100a and has at least one PS bearer allocated in the PC/EPC. The user equipment 101 supporting S1 mode will keep the default PDP context activated during the validity period of the PDN connection. The user equipment 101 not supporting S1 mode should apply the same behavior.

The following use cases are supported in the embodiments herein. Note that for non-DTM Global System for Mobile communications (GSM) Enhanced Data rates for GSM Evolution (EDGE) Radio Access Network (GERAN), i.e., Cases 1 and 3, there are slightly different scenarios depending on whether the call originates in LTE or 2G/3G. In other cases, i.e., Cases 2 and 4 below, the use cases are the same regardless of whether the voice call originates in LTE or 2G/3G. DTM is a protocol based on the GSM standard that allows simultaneous transfer of CS voice and PS data over the same radio channel.

1. CS voice call initiated in GERAN without DTM:

a. Mobility to LTE is performed by TAU.

2. CS voice call initiated in GERAN/UTRAN with DTM:

a. Mobility to LTE is performed by TAU (idle or connected state) or IRAT/PS handover (connected state).

3. Voice call originating in LTE and SRVCC to GERAN without DTM support:

a. Mobility from GERAN to LTE can be performed using TAU (idle or connected state).

4. Voice call origination in LTE and SRVCC to GERAN/UTRAN with DTM:

a. Mobility from GERAN/UTRAN to LTE can be performed using TAU (idle or connected state) or IRAT/PS handover (connected state).

A method for handling handover of a communication service according to some embodiments will now be described with reference to the combined signalling diagram and flow chart shown in Figure 4. In Figure 4 the network node performing the suspension of the bearers is the PCRF 210.

Step 401

BSC/RNC 301 sends a Handover Request message to MSC Server 203. This message includes the target tracking area code. The Handover Request message includes an indication that this HO is for SRVCC. If MSC Server 203 is the target MSC, it forwards the Handover Request message to the anchor MSC server.

Step 403

In some embodiments, the MSC server 203 sends an access transfer notification, such as a SIP re-INVITE or INVITE message, to the access transfer control function (ATCF) 501 , which indicates to the ATCF 501 that it should prepare to transfer media to the PS 100b .

The method uses signaling from the MSC 203 towards the IMS system to initiate the session transfer from the CS 100a to the PS 100b. However, the signaling towards the IMS may be implemented in other suitable ways.

Step 404

In some embodiments, ATCF 501 retrieves the port/codec received from user device 101 during its IMS registration. MSC 203 can correlate the IMS registration performed by user device 101 with the IMS registration performed by MSC 203 on behalf of user device 101, for example, based on the C-MSISDN or an instance id derived from the IMEI used for both registrations. ATCF 501 allocates media ports on the ATGW and forwards the transfer prepare request to P-CSCF 305 (before including in that message the IP address/port to be used by user device 101 after rSRVCC and the IP address/port to which the ATGW is sending voice media, i.e., the SDP for both user device 101 and the ATGW can be included in the message).

Step 405

The IMS, ie, P-CSCF 401 initiates bearer processing in the EPC by sending a resource allocation request to PCRF 210 over the Rx interface to trigger the establishment of a guaranteed bit rate (GBR) dedicated PS bearer with quality of service (QoS) suitable for voice.

In some embodiments, the resource allocation request includes a parameter indicating that rSRVCC has been performed. This parameter enables PCRF 210 to distinguish between the case where the establishment of a dedicated bearer is to be delayed, i.e., rSRVCC, and a normal bearer resource request over Rx when the dedicated bearer is to be established immediately.

This is the trigger for the next step, step 406a for suspending the bearer establishment.

Step 406a

Because this bearer establishment is due to rSRVCC, PCRF 212 waits or suspends initiating the bearer establishment to P-GW 207 .

Normally, when PCRF 210 receives the resource allocation message from step 405, it will start resource allocation to PGW 207, which will trigger PGW 207 to send a create bearer request to SGW 207 and MME 201. However, due to contention conditions, i.e., user equipment 101 may not have performed handover to LTE at this point in time, the bearer allocation of the voice bearer needs to be delayed and performed only when user equipment 101 completes handover to LTE.

RCRF 210 needs to distinguish this case (rSRVCC) where the establishment of a dedicated bearer will be delayed from a normal bearer resource request via Rx where the dedicated bearer is established immediately. PCRF 210 is informed that rSRVCC has been performed through parameters contained in the message from the P-CSCF in step 405. In some embodiments, it may be possible to know from the configuration that a delayed allocation will be performed, even without an explicit indication via Rx. For example, if IMS voice is deployed only on LTE and not on 2G/3G, PCRF 210 can use radio access technology (RAT) information indicating which RAT the user equipment 101 is connecting to.

Step 412

In some embodiments, in the case of an ATCF 501 with media anchored in the ATGW, the MSC server 203 sends an access transfer prepare request, such as a SIP Re-Invite or PRACK message, to the ATCF 501 to trigger the ATCF/ATGW to switch the media path to the IP address/port of the user equipment 101 on the target access.

In the case that no media is anchored in the ATGW, the MSC server 203 sends an access transfer prepare request to the ATCF 501 and a media path between the ATCF/ATGW and the MSC server/MGW will be established.

Step 411

The MSC 203 sends a "Handover Command" to the BSC 301. The Handover Command can be considered as a confirmation of the handover requirement. The Handover Command can be sent via the target MSC. The MSC server 203 can include the Handover Command, the IP address/port for the ATGM, the MGW, or the remote end, as appropriate, and the selected codec.

The BSC 301 forwards the "Handover Command" to the user equipment 101, thereby instructing a CS to PS handover.

Step 413

Perform a Tracking Area Update (TAU) or Inter-Radio Access Technology (IRAT) PS handover. A TAU is a procedure initiated by user equipment 101 to update its registration status with the network. TAUs can occur for a variety of reasons, such as when user equipment 101 moves to a new Tracking Area (TA), when there is an inter-system change, or when user equipment 101 returns to LTE after CS fallback.

In some embodiments, in the case of an ATCF 501 with media anchored in the ATGW, the MSC server 203 sends an access transfer prepare request, such as a SIP Re-Invite or PRACK message, to the ATCF 501 to trigger the ATCF/ATGW to switch the media path to the IP address/port of the user device 101 on the target access.

In the absence of media anchored in the ATGW, the MSC server 203 may send an access transfer prepare request to the ATCF 501 and a media path between the ATCF/ATGW and the MSC server/MGW will be established.

The user equipment 101 may send a handover confirmation to the eNB 103. In other words, the handover to LTE is performed.

The eNB 103 may send a handover notification to the MME 201. In other words, a handover to LTE is performed.

Step 415

MME 201 sends a Modify Bearer Request to SGW 207 to first update the PS bearer context. SGW 207 forwards the Modify Bearer Request to PGW 207.

The MME 201 informs the PGW 207 and the SGW 207 that the user equipment 101 is now reachable via the eNB 103. As described below, a new dedicated bearer for voice is added in step 406b.

Step 416

A VoIP call or any communication service may be sent to the user equipment 101 via the default bearer in LTE.

Step 417

The PDN GW 207 informs the PCRF 212 about the change of, for example, the RAT type.

Step 406b

PCRF 212 continues the suspended voice bearer allocation and constructs the corresponding PCC rule and sends it to PGW 207.

The PCRF 210 will be informed through the existing RAT change signaling from the PGW 207 that the handover to LTE is complete, at which point the PCRF 210 can trigger the PGW 207 to start dedicated bearer allocation.

Step 418

The MME 201 establishes a dedicated bearer for the user equipment 101 .

Although the arrow symbolically represents step 418 as being between MME 201 and user equipment 101 , it may include multiple steps between PCRF 210 , PGW 207 , SGW 207 , MME 201 , and UE 101 .

Step 419

Until the point where a dedicated GBR bearer is allocated, i.e., after step 406b, voice call user data can be sent in the default bearer in LTE. The default bearer in the IMS APN can be a QCI=5 bearer optimized for signaling rather than voice media. However, for a short period after switching to LTE, this bearer can also be used for voice media. The TFT filter will then be set to allow voice user flows in addition to signaling packets. In the case of an "allow all" default bearer and the use of a firewall on the IMS APN, MGW services must also be allowed.

After step 406b, a dedicated bearer has been allocated in LTE, and the VoIP call will be moved to the dedicated bearer, thereby minimizing the impact on the user's quality of service experience, such as QoS, call drop rate, and interruption time.

A method for handling handover of a communication service according to some embodiments will now be described with reference to the combined signalling diagram and flow chart shown in Figure 5. In Figure 5 the network node performing the suspension of the bearer is the P-CSCF 401 .

Step 501

This step corresponds to step 401 in FIG. 4 .

BSC/RNC 301 sends a Handover Request message to MSC Server 203. This message includes the target tracking area code. The Handover Request message includes an indication that this HO is for SRVCC. If MSC Server 203 is the target MSC, it forwards the Handover Request message to the anchor MSC server.

Step 503

This step corresponds to step 403 in FIG. 4 .

In some embodiments, the MSC server 203 sends an access transfer notification, such as a SIP Re-Invite or Invite message, to the Access Transfer Control Function (ATCF) 501, which indicates to the ATCF 501 that it should prepare to transfer media to the PS 100b.

The method uses signaling from the MSC 203 to the IMS system to initiate the session transfer from the CS 100a to the PS 100b. However, other suitable ways may be used to implement the signaling to the IMS.

Step 504

This step corresponds to step 404 in FIG. 4 .

In some embodiments, the ATCF 501 retrieves the port/codec received from the user device 101 during its IMS registration. The MSC 203 can correlate the IMS registration made by the user device 101 with the IMS registration made by the MSC 203 on behalf of the user device 101, for example, based on the C-MSISDN or an instance id derived from the IMEI used for both registrations. The ATCF 501 allocates media ports on the ATGW and forwards a Transfer Prepare Request (TRP) to the P-CSCF 401 (before including in that message the IP address/port to be used by the user device 101 after rSRVCC and the IP address/port to which the ATGW is sending voice media, i.e., the SDP for both the user device 101 and the ATGW can be included in the message).

In some embodiments, the TRP includes a parameter indicating that rSRVCC has been performed. This parameter enables the P-CSCF 401 to distinguish the case where the establishment of a dedicated bearer is to be delayed, i.e., rSRVCC, from a normal bearer resource request over Rx where a dedicated bearer is to be established immediately.

This is the trigger to the next step, step 506a for suspending the bearer establishment.

Step 506a

This step corresponds to step 406a in FIG. 4 .

Because this bearer establishment is due to rSRVCC, P-CSCF 401 waits for or suspends initiating the bearer establishment to P-GW 207 .

Due to the contention condition, ie the user equipment 101 may not have performed handover to LTE at this point in time, the bearer allocation of the voice bearer needs to be delayed and performed only when the user equipment 101 completes handover to LTE.

P-CSCF 401 needs to distinguish between the case where the dedicated bearer establishment will be delayed (rSRVCC) and the normal bearer resource request via Rx when the dedicated bearer is established immediately. P-CSCF 401 is informed that rSRVCC has been performed through the parameters contained in the message from ATCF in step 504. In some embodiments, if delayed allocation is to be performed, this may be known from the configuration even without an explicit indication via Rx. For example, if IMS voice is deployed only on LTE and not on 2G/3G, P-CSCF 401 can use radio access technology (RAT) information to indicate which RAT the user equipment 101 is connecting to.

Step 512

This step corresponds to step 412 in FIG. 4 .

In some embodiments, in the case of an ATCF 501 with media anchored in the ATGW, the MSC server 203 sends an access transfer prepare request, such as a SIP Re-Invite or PRACK message, to the ATCF 501 to trigger the ATCF/ATGW to switch the media path to the IP address/port of the user device 101 on the target access.

In the absence of media anchored in the ATGW, the MSC server 203 sends an access transfer prepare request to the ATCF 501 and a media path between the ATCF/ATGW and the MSC server/MGW will be established.

Step 511

This step corresponds to step 411 in FIG. 4 .

The MSC 203 sends a "Handover Command" to the BSC 301. The Handover Command can be considered as a confirmation of the handover requirement. The Handover Command can be sent via the target MSC. The MSC server 203 can include the Handover Command, the IP address/port for the ATGM, the MGW, or the remote end, as appropriate, and the selected codec.

The BSC 301 forwards the "Handover Command" to the user equipment 101, thereby instructing a CS to PS handover.

Step 513

This step corresponds to step 413 in FIG. 4 .

Perform a Tracking Area Update (TAU) or Inter-Radio Access Technology (IRAT) PS handover. A TAU is a procedure initiated by user equipment 101 to update its registration status with the network. TAUs can occur for a variety of reasons, such as when user equipment 101 moves to a new Tracking Area (TA), when there is an inter-system change, or when user equipment 101 returns to LTE after CS fallback.

In some embodiments, in the case of an ATCF 501 with media anchored in the ATGW, the MSC server 203 sends an access transfer prepare request, such as a SIP Re-Invite or PRACK message, to the ATCF 501 to trigger the ATCF/ATGW to switch the media path to the IP address/port of the user device 101 on the target access.

In case there is no media anchored in the ATGW, then the MSC server 203 may send an access transfer prepare request to the ATCF 501 and a media path between the ATCF/ATGW and the MSC server/MGW will be established.

The user equipment 101 may send a handover confirmation to the eNB 103. In other words, the handover to LTE is performed.

The eNB 103 may send a handover notification to the MME 201. In other words, a handover to LTE is performed.

Step 515

This step corresponds to step 415 in FIG. 4 .

MME 201 sends a Modify Bearer Request to SGW 207 to first update the PS bearer context. SGW 207 forwards the Modify Bearer Request to PGW 207.

The MME 201 informs the PGW 207 and the SGW 207 that the user equipment 101 is now reachable via the eNB 103. As described below, a new dedicated bearer for voice is added in step 506b.

Step 516

This step corresponds to step 416 in FIG. 4 .

A VoIP call or any communication service may be sent to the user equipment 101 via the default bearer in LTE.

Step 517

This step corresponds to step 417 in FIG. 4 , except that this step is from PGW 207 to P-CSCF 401 , rather than from PGW 207 to PCRF 201 as in FIG. 4 .

The PDN GW 207 informs the P-CSCF 401 about the change of, for example, the RAT type.

Step 506b

This step corresponds to step 406b in FIG. 4 .

P-CSCF 401 continues the suspended voice bearer allocation and constructs the corresponding PCC rule and sends it to PGW 207 .

The P-CSCF 401 will be informed through the existing RAT change signaling from the PGW 207 that the handover to LTE is complete, at which point the P-CSCF 401 can trigger the PGW 207 to start dedicated bearer allocation.

Step 505

After continuing with the bearer establishment in step 506b, the IMS, i.e., P-CSCF 401, initiates bearer processing in the EPC by sending a resource allocation request to PCRF 210 over the Rx interface to trigger the establishment of a guaranteed bit rate (GBR) dedicated PS bearer with quality of service (QoS) suitable for voice.

Step 518

This step corresponds to step 418 in FIG. 4 .

The MME 201 establishes a dedicated bearer for the user equipment 101 .

Although the arrow symbolically represents step 518 as being between MME 201 and user equipment 101 , it may include multiple steps between PCRF 210 , PGW 207 , SGW 207 , MME 201 , and UE 101 .

Step 519

This step corresponds to step 419 in FIG. 4 .

Until the point where a dedicated GBR bearer is allocated, i.e., after step 506b, voice call user data can be sent in the default bearer in LTE. The default bearer in the IMS APN can be a QCI=5 bearer optimized for signaling rather than voice media. However, for a short period after switching to LTE, this bearer can also be used for voice media. The TFT filter will then be set to allow voice user traffic in addition to signaling packets. In the case of an "allow all" default bearer and the use of a firewall on the IMS APN, MGW services must also be allowed.

After step 506b, a dedicated bearer has been allocated in LTE, and the VoIP call will be moved to the dedicated bearer, thereby minimizing the impact on the user's quality of service experience, such as QoS, call drop rate, and interruption time.

A method for handling handover of a communication service according to some embodiments will now be described with reference to the combined signalling diagram and flow chart shown in Figure 6. In Figure 56 the network node performing the suspension of the bearer is the PGW207.

Step 601

This step corresponds to step 401 in FIG. 4 and step 501 in FIG. 5 .

BSC/RNC 301 sends a Handover Request message to MSC Server 203. This message includes the target tracking area code. The Handover Request message includes an indication that this HO is for SRVCC. If MSC Server 203 is the target MSC, it forwards the Handover Request message to the anchor MSC server.

Step 603

This step corresponds to step 403 in FIG. 4 and step 503 in FIG. 5 .

In some embodiments, the MSC server 203 sends an access transfer notification, such as a SIP Re-Invite or Invite message, to the Access Transfer Control Function (ATCF) 501, which indicates to the ATCF 501 that it should prepare to transfer media to the PS 100b.

The method uses signaling from the MSC 203 to the IMS system to initiate the session transfer from the CS 100a to the PS 100b. However, other suitable ways may be used to implement the signaling to the IMS.

Step 604

This step corresponds to step 404 in FIG. 4 and step 504 in FIG. 5 .

In some embodiments, the ATCF 501 retrieves the port/codec received from the user device 101 during its IMS registration. The MSC 203 can correlate the IMS registration made by the user device 101 with the IMS registration made by the MSC 203 on behalf of the user device 101, for example, based on the C-MSISDN or an instance id derived from the IMEI used for both registrations. The ATCF 501 allocates media ports on the ATGW and forwards a Transfer Prepare Request (TRP) to the P-CSCF 401 (before including in that message the IP address/port to be used by the user device 101 after rSRVCC and the IP address/port to which the ATGW is sending voice media, i.e., the SDP for both the user device 101 and the ATGW can be included in the message).

Step 605a

This step corresponds to step 405 in FIG. 4 .

The IMS, ie, P-CSCF 401 initiates bearer processing in the EPC by sending a resource allocation request to PCRF 210 over the Rx interface to trigger the establishment of a guaranteed bit rate (GBR) dedicated PS bearer with quality of service (QoS) suitable for voice.

Step 605b

PCRF 210 forwards the resource allocation request to PGW 207 .

In some embodiments, the resource allocation message includes a parameter indicating that rSRVCC has been performed. This parameter enables PGW 207 to distinguish the case where the establishment of a dedicated bearer is to be delayed, i.e., rSRVCC, from a normal bearer resource request over Rx when the dedicated bearer is to be established immediately.

This is the trigger for the next step, step 606a, which is used to suspend the bearer establishment.

Step 606a

This step corresponds to step 406a in FIG. 4 and step 506a in FIG. 5 .

Because this bearer establishment is due to rSRVCC, PGW 207 waits for or suspends the bearer establishment initiated to P-GW 207 .

Due to the contention condition, ie the user equipment 101 may not have performed handover to LTE at this point in time, the bearer allocation of the voice bearer needs to be delayed and performed only when the user equipment 101 completes handover to LTE.

PGW 207 needs to distinguish between the case where the establishment of a dedicated bearer is delayed, i.e., rSRVCC, and a normal bearer resource request via Rx when the dedicated bearer is established immediately. PGW 207 is informed that rSRVCC has been performed through the parameters contained in the message from ATCF in step 605b. In some embodiments, if delayed allocation is to be performed, this is possible even without an explicit indication via Rx, as long as it is known from the configuration. For example, if IMS voice is deployed only over LTE and not in 2G/3G, PGW 207 can use radio access technology (RAT) information indicating which RAT the user equipment 101 is connected to.

Step 612

This step corresponds to step 412 in FIG. 4 and step 512 in FIG. 5 .

In some embodiments, in the case of an ATCF 501 with media anchored in the ATGW, the MSC server 203 sends an access transfer prepare request, such as a SIP Re-Invite or PRACK message, to the ATCF 501 to trigger the ATCF/ATGW to switch the media path to the IP address/port of the user device 101 on the target access.

In the absence of media anchored in the ATGW, the MSC server 203 sends an access transfer prepare request to the ATCF 501 and a media path between the ATCF/ATGW and the MSC server/MGW will be established.

Step 611

This step corresponds to step 411 in FIG. 4 and step 511 in FIG. 5 .

The MSC 203 sends a "Handover Command" to the BSC 301. The Handover Command can be considered as a confirmation of the handover requirement. The Handover Command can be sent via the target MSC. The MSC server 203 can include the Handover Command, the IP address/port for the ATGM, the MGW, or the remote end, as appropriate, and the selected codec.

The BSC 301 forwards the "Handover Command" to the user equipment 101, thereby instructing a CS to PS handover.

Step 613

This step corresponds to step 413 in FIG. 4 and step 513 in FIG. 5 .

Perform a Tracking Area Update (TAU) or Inter-Radio Access Technology (IRAT) PS handover. A TAU is a procedure initiated by user equipment 101 to update its registration status with the network. TAUs can occur for a variety of reasons, such as when user equipment 101 moves to a new Tracking Area (TA), when there is an inter-system change, or when user equipment 101 returns to LTE after CS fallback.

In some embodiments, in the case of an ATCF 501 with media anchored in the ATGW, the MSC server 203 sends an access transfer prepare request, such as a SIP Re-Invite or PRACK message, to the ATCF 501 to trigger the ATCF/ATGW to switch the media path to the IP address/port of the user device 101 on the target access.

In the absence of media anchored in the ATGW, the MSC server 203 may send an access transfer prepare request to the ATCF 501 and a media path between the ATCF/ATGW and the MSC server/MGW will be established.

The user equipment 101 may send a handover confirmation to the eNB 103. In other words, the handover to LTE is performed.

The eNB 103 may send a handover notification to the MME 201. In other words, a handover to LTE is performed.

Step 615

This step corresponds to step 415 in FIG. 4 and step 515 in FIG. 5 .

MME 201 sends a Modify Bearer Request to SGW 207 to first update the PS bearer context. SGW 207 forwards the Modify Bearer Request to PGW 207.

The MME 201 informs the PGW 207 and the SGW 207 that the user equipment 101 is now reachable via the eNB 103. As described below, a new dedicated bearer for voice is added in step 506b.

Step 616

This step corresponds to step 416 in FIG. 4 and step 516 in FIG. 5 .

A VoIP call or any communication service may be sent to the user equipment 101 via the default bearer in LTE.

Step 517

This step corresponds to step 417 in FIG. 4 and step 517 in FIG. 5 .

The PGW 207 informs the PCRF 210 about the change of, for example, the RAT type.

Step 606b

This step corresponds to step 406b in FIG. 4 and step 506b in FIG. 5 .

PGW 207 continues the suspended voice bearer allocation and constructs the corresponding PCC rule.

The PGW 207 will be informed through the existing RAT change signaling from the PGW 207 that the handover to LTE is complete, at which point the PGW 207 can start dedicated bearer allocation.

Step 618

This step corresponds to step 418 in FIG. 4 and step 518 in FIG. 5 .

The MME 201 establishes a dedicated bearer for the user equipment 101 .

Although the arrow symbolically represents step 618 as being between MME 201 and user equipment 101 , it may include multiple steps between PCRF 210 , PGW 207 , SGW 207 , MME 201 , and UE 101 .

Step 619

This step corresponds to step 419 in FIG. 4 and step 519 in FIG. 5 .

Until the point where a dedicated GBR bearer is allocated, i.e., after step 606b, voice call user data can be sent in the default bearer in LTE. The default bearer in the IMS APN can be a QCI=5 bearer optimized for signaling rather than voice media. However, for a short period after switching to LTE, this bearer can also be used for voice media. The TFT filter will then be set to allow voice user flows in addition to signaling packets. In the case of an "allow all" default bearer and the use of a firewall on the IMS APN, MGW services must also be allowed.

After step 606b, a dedicated bearer has been allocated in LTE, and the VoIP call will be moved to the dedicated bearer, thereby minimizing the impact on the user's service quality experience, such as QoS, call drop rate, and interruption time.

The method described above will now be described from the perspective of a network node, which enables handover of communication services between a circuit-switched network 100a (referred to as CS) and a packet-switched (referred to as PS) network 100b, with communication services in the CS network 100a. A user equipment 101 is located in the CS network 100a and has communication services in the CS network 100a.

The method includes the following steps to be performed by MME 201:

The following description uses an IMS voice call as an example. However, any other type of communication service or multimedia service (such as a video call) is also applicable.

The method includes the following steps, which may also be performed in other suitable orders than those described below.

Step 701

This step corresponds to step 405 in FIG. 4 , step 504 in FIG. 5 , and step 605 b in FIG. 6 .

The network node receives a trigger instructing to establish a bearer associated with the communication network in the PS network 100b.

In some embodiments, the trigger is based on Return Single Radio Voice Call Continuity, referred to as rSRVCC.

In some embodiments, the bearer is a dedicated bearer associated with the communication service.

In some embodiments, the bearer is a guaranteed bit rate bearer, also known as a GBR bearer.

In some embodiments, the communication service is a voice service or a video service.

Step 701a

This is a subordinate step of step 701. This step corresponds to step 405 in FIG.

In some embodiments, the network node is a Policy and Charging Rules Function (also referred to as PCRF 210) node.

In some embodiments, the network node receives a first request message from a Proxy Call Session Control Function (also known as P-CSCF) node 401. The first request message contains a request to establish a bearer associated with a communication service in the PS network 100b.

Step 701b

This is a subordinate step of step 701 and is performed instead of step 701a. This step corresponds to step 505 in FIG.

In some embodiments, the network node is a Proxy Call Session Control Function (also referred to as P-CSCF 401 ) node.

In some embodiments, the network node receives a second request message from an Access Transfer Control Function (also referred to as ATCF 403) node. The second request message is a transfer prepare request including an address associated with the communication service.

Step 701b

This is a subordinate step of step 701 and a step performed instead of step 701a and instead of step 701b. This step corresponds to step 605b in Figure 6.

In some embodiments, the network node is a packet data network, referred to as PGW 207 .

In some embodiments, the network node receives a first request message from a Policy and Charging Rules Function (also referred to as PCRF 210) node, the first request message comprising a request to establish a bearer associated with a communication service in the PS network 100b.

Step 702

This step corresponds to step 406a in FIG. 4 , step 506a in FIG. 5 , and step 606a in FIG. 6 .

Based on the trigger, the network node determines that the establishment of the bearer in the PS network 100b should be suspended.

Step 702a

This is a subordinate step of step 702. This step corresponds to step 406a in FIG4, step 506a in FIG5, and step 606b in FIG6.

In some embodiments, the network node determines that a trigger is included in a Return Single Radio Voice Call Continuity (rSRVCC) message.

Step 702b

This is a subordinate step of step 702. This step corresponds to step 406a in FIG4, step 506a in FIG5, and step 606b in FIG6.

In some embodiments, the network node determines that the trigger contains a parameter indicating that establishment of a bearer associated with the communication service in the PS network 100b should be suspended.

Step 703

This step corresponds to step 406a in FIG. 4 , step 506a in FIG. 5 , and step 606a in FIG. 6 .

The network node suspends bearer establishment.

Step 704

This step corresponds to step 605 in FIG. 6 .

In some embodiments, the network node receives a third request message from a mobility management entity (referred to as MME 201). The third request message contains a request to modify a bearer in the PS network.

Step 705

This step corresponds to step 617 in FIG. 6 .

In some embodiments, the network node sends information to the PCRF 210 that the handover from the CS network 100a to the PS network 100b is completed.

Step 706

This step corresponds to step 619 in FIG. 6 .

In some embodiments, the network node transmits the communication service using a bearer.

Step 707

This step corresponds to step 417 in FIG. 4 , step 517 in FIG. 5 , and step 615 in FIG. 6 .

The network node receives information that the handover from the CS network 100a to the PS network 100b is completed.

In some embodiments, information is received 417 from a packet data network gateway (referred to as PGW 207 ) that the handover from the CS network ( 100 a ) to the PS network 100 b is completed.

Step 708

This step corresponds to step 406b in FIG. 4 , step 506b in FIG. 5 , and step 606b in FIG. 6 .

Upon receiving the information that the handover from the CS network 100a to the PS network 100b is completed, the network node establishes a bearer associated with the communication service.

Step 709

This step corresponds to step 505 in FIG. 5 .

In some embodiments, when the network node is a P-CSCF 401, the network node sends a first request message to a Policy and Charging Rules Function (referred to as PCRF 210). The first request message includes a request to establish a bearer associated with a communication service in the PS network 100b.

In order to perform the method steps shown in Figures 4, 5 and 6 for enabling handover of a communication service from a circuit-switched network 100a, referred to as CS, to a packet-switched (referred to as PS) network 100b, the network node comprises the arrangement shown in Figure 7. A user equipment 101 is located in the CS network 100a and has a communication service in the CS network 100a.

The network node includes a receiving unit 801 configured to receive a trigger indicating establishment of a bearer associated with a communication service in PS network 100b. Receiving unit 801 is further configured to receive information indicating that a handover from CS network 100a to PS network 100b has been completed. In some embodiments, the network node is a Policy and Charging Rules Function (PCRF 210) node. In some embodiments, receiving unit 801 is further configured to receive a first request message from a Proxy Call Session Control Function (P-CSCF) node 401. The first request message includes a request to establish a bearer associated with a communication service in PS network 100b. In some embodiments, the information indicating that a handover from CS network 100a to PS network 100b has been completed is received from a Packet Data Network Gateway (PGW 207).

In some embodiments, the network node is a Proxy Call Session Control Function (P-CSCF 401) node. In some embodiments, the receiving unit 801 is further configured to receive a second request message from an Access Transfer Control Function (ATCF 403) node. The second request message is a transfer preparation request including an address associated with the communication service.

In some embodiments, the network node is a packet data network, referred to as PGW 207. In some embodiments, the receiving unit 801 is further configured to receive a first request message from a policy and charging rules function (PCRF 210) node, the first request message including a request to establish a bearer associated with a communication service in PS network 100b. In some embodiments, the network node receives a third request message from a mobility management entity (MME 201), the third request message including a request to modify a bearer in the PS network.

In some embodiments, the trigger is based on Return Single Radio Voice Call Continuity, referred to as rSRVCC.

In some embodiments, the bearer is a dedicated bearer associated with a communication service.In some embodiments, the bearer is a guaranteed bit rate (also known as GBR) bearer.

In some embodiments, the communication service is a voice service or a video service, or any other media service.

The network node includes a determining unit 803 configured to determine, based on the trigger, that establishment of a bearer in the PS network 100b should be suspended. In some embodiments, the determining unit 803 is further configured to determine that a return single radio voice call continuity (rSRVCC) message contains a trigger. In some embodiments, the determining unit 803 is further configured to determine that the trigger contains a parameter indicating that establishment of a bearer associated with a communication service in the PS network 100b should be suspended.

The communication node comprises a suspending unit 805 configured to suspend the bearer establishment.

The network node further comprises an establishing unit 810 configured to establish a bearer associated with the communication service upon receiving information that the handover from the CS network 100a to the PS network 100b is completed.

In some embodiments, the network node further includes a sending unit 807 configured to send a first request message to a Policy and Charging Rules Function (also known as PCRF 210). In some embodiments, the first request message includes a request to establish a bearer associated with the communication service in PS network 100b. In some embodiments, the sending unit 807 is further configured to send information to PCRF 210 indicating that the handover from CS network 100a to PS network 100b has been completed. In some embodiments, the sending unit 807 is configured to send the communication service using the bearer.

The present mechanism for enabling the handover of communication services from a circuit-switched network (referred to as CS) 100a to a packet-switched (referred to as PS) network 100b can be implemented by one or more processors, such as processor unit 815, in the network node arrangement shown in FIG7 , together with computer program code for performing the functions of the embodiments herein. The processor can be, for example, a digital signal processor (DSP), an application-specific integrated circuit (ASIC) processor, a field-programmable gate array (FPGA) processor, or a microprocessor. The program code mentioned above can also be provided as a computer program product, for example, in the form of a data carrier carrying computer program code for executing the embodiments herein when loaded into a network node. One such carrier can take the form of a CD ROM, but other data carriers, such as memory sticks, are also feasible. The computer program code can also be provided as pure program code on a server and downloaded remotely to the network node.

The embodiments herein are not limited to the preferred embodiments described above. Various alternatives, modifications, and equivalents may be used. Therefore, the embodiments above should not be considered to limit the scope of the embodiments, which is defined by the appended claims.

It should be emphasized that the term "comprises/comprising" when used in this specification is understood to indicate the presence of the proposed features, integers, steps or components, but does not exclude the presence or addition of one or more other features, integers, steps, components or groups thereof. It should also be noted that the word "a" or "an" preceding an element does not exclude the presence of a plurality of such elements.

It should also be emphasized that the method steps defined in the appended claims may be performed in another order than the order in which they appear in the claims without departing from the embodiments herein.

Claims (18)

1. A method in a network node for enabling handover of a communication service from a circuit-switched network (100a) called CS to a packet-switched network (100b) called PS, wherein a user equipment (101) is located in the CS network (100a) and has a communication service in the CS network (100a), the method comprising: receiving (405, 504, 605b) a trigger indicating establishment of a dedicated bearer associated with a communication network in said PS network (100b); Based on the trigger, determining (406a, 506a, 606a) that establishment of the dedicated bearer in the PS network (100b) should be delayed; Delaying (406a, 506a, 606a) the bearer establishment; sending (416, 516, 616) media on a default bearer while establishment of the dedicated bearer is delayed; receiving (417, 517, 615) information indicating that handover from the CS network (100a) to the PS network (100b) has been completed; and When receiving information that the handover from the CS network (100a) to the PS network (100b) is completed, a dedicated bearer associated with the communication service is established (406b, 506b, 606b). 2. The method according to claim 1, wherein the network node is a Policy and Charging Rules Function node called PCRF (210), and The triggering of receiving (405, 504, 605b) an instruction to establish a dedicated bearer in the PS network (100b) includes: receiving (405, 701a) a first request message from a Proxy Call Session Control Function node (401), referred to as P-CSCF, the first request message comprising a request to establish a bearer associated with a communication service in the PS network (100b); and The information that the handover from the CS network (100a) to the PS network (100b) is completed is received (417) from a packet data network gateway called PGW (207). 3. The method according to claim 1, wherein the network node is a Proxy Call Session Control Function node, referred to as P-CSCF (401), and wherein receiving (405, 504, 605b) a trigger indicating establishment of a bearer in the PS network (100b) comprises: receiving (504) a second request message from an access transfer control function node called ATCF (403), said second request message being a transfer preparation request containing an address associated with said communication service; and The method further comprises: A first request message is sent (505) to a Policy and Charging Rules Function node called PCRF (210), the first request message comprising a request to establish a dedicated bearer associated with the communication service in the PS network (100b). 4. The method according to claim 1, wherein the network node is a Packet Data Network Gateway called PGW (207), wherein receiving (405, 504, 605b) a trigger indicating establishment of a dedicated bearer in the PS network (100b) comprises: receiving (605b) a first request message from a Policy and Charging Rules Function node called PCRF (210), said first request message comprising a request for establishing a dedicated bearer associated with said communication service in said PS network (100b); and The method further comprises: receiving (615) a third request message from a mobility management entity called MME (201), said third request message comprising a request to modify said dedicated bearer in said PS network; sending (617) information that the handover from the CS network (100a) to the PS network (100b) is completed to the PCRF (210); and The communication service is sent (619) using the dedicated bearer. 5. The method according to any of claims 1-4, wherein the triggering is based on CS to PS handover returning to single radio voice call continuity known as CS to PS rSRVCC. 6. The method according to any one of claims 1 to 5, wherein determining (406a, 506a, 606a) that establishment of the dedicated bearer associated with the communication service in the PS network (100b) should be delayed further comprises: It is determined (406a, 506a, 606b) that the trigger is contained in a Return Single Radio Voice Call Continuity message referred to as rSRVCC. 7. The method of any of claims 1-6, wherein determining (406a, 506a, 606a) that establishment of the dedicated bearer in the PS network (100b) should be delayed further comprises: It is determined (406a, 506a, 606b) that the trigger includes a parameter indicating that establishment of a bearer associated with the communication service in the PS network (100b) should be delayed. 8. The method according to any one of claims 1 to 7, wherein the dedicated bearer is provided with a guaranteed bit rate called GBR. 9. The method according to any one of claims 1 to 8, wherein the communication service is an Internet Protocol Multimedia Subsystem service (IMS) including a voice service or a video service. 10. A network node for enabling handover of a communication service from a circuit-switched network (100a) called CS to a packet-switched network (100b) called PS, wherein a user equipment (101) is located in the CS network (100a) and has a communication service in the CS network (100a), the network node comprising: A receiving unit (801) configured to receive a trigger indicating establishment of a dedicated bearer associated with the communication network in the PS network (100b); a determining unit (803), the determining unit being configured to determine, based on the trigger, that establishment of the dedicated bearer in the PS network (100b) should be delayed; a processing unit (815) configured to arrange for transfer of the media stream on a default bearer when establishment of the dedicated bearer is delayed; a delay unit (805) configured to delay the bearer establishment; wherein the receiving unit (801) is further configured to receive information indicating that the handover from the CS network (100a) to the PS network (100b) has been completed, and wherein the network node further comprises: An establishing unit 810 is configured to establish a dedicated bearer associated with the communication service upon receiving information indicating that the handover from the CS network (100a) to the PS network (100b) is completed. 11. The network node according to claim 10, wherein the network node is a Policy and Charging Rules Function node called PCRF (210), and The receiving unit (801) is further configured to receive a first request message from a proxy call session control function node (401) called P-CSCF, the first request message comprising a request to establish a dedicated bearer associated with the communication service in the PS network (100b); and information that a handover from the CS network (100a) to the PS network (100b) has been completed is received from a packet data network gateway called PGW (207). 12. The network node according to claim 10, wherein the network node is a Proxy Call Session Control Function node called P-CSCF (401), and wherein the receiving unit (801) is further configured to receive a second request message from an Access Transfer Control Function node called ATCF (403), the second request message being a transfer preparation request containing an address related to the communication service; and wherein the network node further comprises: A sending unit (807) is configured to send a first request message to a Policy and Charging Rules Function node called PCRF (210), the first request message containing a request to establish a bearer associated with the communication service in the PS network (100b). 13. The network node according to claim 12, wherein the network node is a packet data network gateway called PGW (207), wherein the receiving unit (801) is further configured to: receiving a first request message from a Policy and Charging Rules Function node called PCRF (210), said first request message comprising a request for establishing a dedicated bearer associated with said communication service in said PS network (100b); and receiving a third request message from a mobility management entity called MME (201), said third request message comprising a request to modify said dedicated bearer in said PS network; And wherein the sending unit (807) is further configured to: Sending information that the handover from the CS network (100a) to the PS network (100b) is completed to the PCRF (210); and The communication service is sent using the dedicated bearer. 14. The network node according to any of claims 10-13, wherein the trigger is based on CS to PS handover returning to Single Radio Voice Call Continuity, referred to as CS to PSrSRVCC. 15. The network node according to any of claims 10-14, wherein the determining unit (803) is further configured to determine that the trigger is contained in a Return Single Radio Voice Call Continuity message, referred to as rSRVCC. 16. The network node according to any one of claims 10-15, the determining unit (803) being further configured to determine that the trigger includes a parameter indicating that establishment of a dedicated bearer associated with the communication service in the PS network (100b) should be delayed. 17. The network node according to any of claims 10-16, wherein the dedicated bearer is provided with a guaranteed bit rate called GBR. 18. The network node according to any one of claims 10 to 17, wherein the communication service is an Internet Protocol Multimedia Subsystem service (IMS) including a voice service or a video service.