Abstract:
A method for initiating handover of a circuit switched service using a circuit switched bearer of a mobile station from a circuit switched to a packet switched domain in a mobile communications network, the mobile communications network comprising a radio network and a core network, the radio network comprising a controlling node, the mobile station being o allocated an IP address and access port, said method comprising the controlling node communicating to the core network a relocation/handover required message; said relocation/handover required message comprising an indication of the IP address and access port allocated to the mobile station.

Description:
CLAIM OF PRIORITY 
     This application is a 371 of PCT/EP2007/055679 filed on Jun. 8, 2007, the disclosure of which is fully incorporated herein by reference. 
     TECHNICAL FIELD 
     The invention relates to a handover control mechanism for a mobile communication system. In particular, the invention relates to a method (and apparatus) for triggering the seamless handover of an established connection from a circuit switched domain to circuit switched service over packet switched domain, for example a Voice-over-IP call. 
     BACKGROUND 
     IP Multimedia (IPMM) services provide a dynamic combination of voice, video, messaging, data, etc, within the same session. By growing the numbers of basic applications and the media which it is possible to combine, the number of services offered to the end users will grow, and the inter-personal communication experience will be enriched. This will lead to a new generation of personalised, rich multimedia communication services, including so-called “combinational IP Multimedia” services. 
     IP Multimedia Subsystem (IMS) is the technology defined by the Third Generation Partnership Project (3GPP) to provide IP Multimedia services over mobile communication networks. IMS provides key features to enrich the end-user person-to-person communication experience through the integration and interaction of services. IMS allows new rich person-to-person (client-to-client) as well as person-to-content (client-to-server) communications over an IP-based network. The IMS makes use of the Session Initiation Protocol (SIP) to set up and control calls or sessions between user terminals (or user terminals and application servers). The Session Description Protocol (SDP), carried by SIP signalling, is used to describe and negotiate the media components of the session. Whilst SIP was created as a user-to-user protocol, IMS allows operators and service providers to control user access to services and to charge users accordingly. Other protocols are used for media transmission and control, such as Real-time Transport Protocol and Real-time Transport Control Protocol (RTP/RTCP), Message Session Relay Protocol (MSRP), and Hyper Text Transfer Protocol (HTTP). IMS requires an IP based access network which for example could be a 3GPP Packet Switched (PS) network, or some other access network such a fixed broadband or WiFi network. 
     A fundamental requirement for real-time service provision is the seamless handover of services for subscribers roaming across cell boundaries of the radio access network (RAN). Traditional circuit switched (CS) based call services have been designed to meet this requirement. In the case of 2G and currently implemented 3G networks, PS real time handover with low latency is not provided for although service continuity is achieved at the terminal side by ordering a session to be moved from one cell to another, i.e. there is no prepare phase to shorten latency when moving cell. 
     Real time PS handover is standardized in 3GPP for 3G networks, but the feature has not yet been deployed. It is expected that when High-Speed Downlink Packet Access (HSDPA) is deployed, or shortly thereafter, the mechanisms needed for fast PS handover will be also be deployed. In the initial implementation stage, roll-out of this feature across 3G networks will inevitably be patchy. For 2G networks, fast and efficient PS handover procedures in the packet switched (PS) domain within the 2G network (and between 2G and 3G networks) have only recently been standardized in 3GPP TS 43.129 for GSM/EDGE networks but are not yet deployed. Support for PS handover in 2G networks is never likely to be comprehensive (if implemented at all), yet handover of PS calls would be desirable as 2G networks will continue to provide a fallback network for 3G subscribers in the case of limited 3G network coverage. It can also be expected that the next generation radio and core network which are currently being specified under the name LTE (Long Term Evolution) and SAE (System Architecture Evolution) in 3GPP will also have limited coverage, and that these networks will also require fallback to 3G and 2G networks. 
     It is expected that in the future a major user of PS services will be Voice-over-IP (VoIP) applications. VoIP calls will be particularly sensitive to even relatively minor service interruptions caused by inter-cell handovers. As long as a terminal engaged in a VoIP call can perform PS handover to another cell (the “target cell”), the interruption can be kept short enough to avoid any noticeable drop in perceived quality. However, if either the current cell or the target cell do not support PS handover, a noticeable interruption is likely to occur as packets will be lost during the transition period. Consequently, until all RAN cells support PS handover, the provision of IMS services such as voice and video calls utilising the PS domain are likely to result in users receiving a reduced quality of service when crossing cell boundaries. 
     Mobile CS services based on GSM and WCDMA radio access are a world-wide success and allow obtaining telecommunication services with a single subscription in almost all countries of the world. Also today, the number of CS subscribers is still growing rapidly, boosted by the role out of mobile CS services in dense population countries such as India and China. This success story is furthermore extended by the evolution of the classical MSC architecture into a softswitch solution which allows using packet transport infrastructure for mobile CS services. 
     Recently the 3GPP work item “Evolved UTRA and UTRAN” (started in summer 2006) defined a Long-Term Evolution (LTE) concept that assures competitiveness of 3GPP-based access technology. It was preceded by an extensive evaluation phase of possible features and techniques in the RAN workgroups that concluded that the agreed system concepts can meet most of the requirements and no significant issue was identified in terms of feasibility. 
     LTE will use Orthogonal Frequency Division Multiplexing (OFDM) radio technology in the downlink and Single-Carrier Frequency Division Multiple Access (SC-FDMA) for the uplink, allowing at least 100 Mbps peak data rate for downlink data rate and 50 Mbps for uplink data rate. LTE radio can operate in different frequency bands and is therefore very flexible for deployment in different regions of the world. 
     In parallel to the RAN standardization 3GPP also drives a System Architecture Evolution (SAE) work item to develop an evolved core network (CN).  FIG. 1  illustrates schematically the System Architecture Evolution (SAE) and LTE interfaces. The SAE core network is made up of core nodes, which are further, split into Control Plane (Mobility Management Entity (MME)  21 ) and User Plane Gateway  22  (Serving Gateway and PDN Gateway) nodes. In the context of the present invention, the terms Access Gateway (AGW) and SAE GW are used to depict both the Serving Gateway and the PDN Gateway nodes and functions. In the terminology currently used AGW contains both User Plane Entity (UPE) and Inter-Access Anchor (IASA) functionality. The MME  21  is connected to the eNodeB  23 ,  23 ′ via the S1-MME interface and the AGW  22  is connected to the eNodeB  23 ,  23 ′ via the S1-U interface. 
     Common to both LTE and SAE is that only a Packet Switched (PS) domain will be specified, i.e. all services are to be supported via this domain. GSM (GPRS) and WCDMA however provide both PS and Circuit Switched (CS) access simultaneously. 
     Hence, if telephony services shall be deployed over LTE radio access and SAE core networks, an IMS based service engine (or similar) is needed. It has been recently investigated how to use LTE/SAE as access technology to the existing Mobile Switching Subsystem (MSS) infrastructure. The investigated solutions are called “CS over LTE/SAE”, or briefly just “CS over LTE” (CSoLTE). 
     The basic CSoLTE architecture for these solutions is shown in  FIG. 2 . The Packet Mobile Switching Center (PMSC)  24  can be serving both traditional 2G and 3G RANs and the new CS over LTE based solutions. Packet MSC  24  contains two new logical functions called Packet CS Controller (PCSC)  27  and Interworking Unit (IWU)  28  that are further described in relation to  FIG. 3 . 
     Referring now to  FIG. 3 , the communication between the terminal  31  and the PMSC  24  is based on the standard Gi interface which is also called as a SGi interface in the SAE terminology. This means that all direct communication between the terminal  31  and the PCSC  27  and the IWU  28  in the PMSC  24  is based on Internet Protocol (IP) and that the terminal  31  is visible and reachable using an IP-address via the Access Gateway (AGW)  22 . This communication between the terminal  31  and the PMSC  24  is divided into two different interfaces, U8c for the control plane and U8u for the user plane. The U8c is terminated in the PCSC  27  and the PCSC  27  has also an Rx interface to the Policy and Charging Rule Function (PCRF)  33  for authorising of LTE/SAE bearers. The U8u is terminated in the IWU  28 . 
     An example solution for providing CS services over the LTE radio access is called “CS Fallback” and means that the terminal is performing SAE MM procedures towards the MME  21  while camping on LTE access. The MME  21  registers the terminal in the MSC-S  29  for CS based services. When a page for CS services is received in the MSC-S  29  it is forwarded to the terminal  31  via the MME  21  and then the terminal  31  performs fallback to the 2G or 3G RANs  41 . Similar behavior applies for Mobile originated CS services and when these are triggered and the terminal  31  is camping on LTE access, it will fallback to 2G or 3G RANs and trigger the initiation of the CS service there. 
     The CSoLTE control plane protocol architecture between the terminal  31  and the PMSC  24  (i.e. the U8c interface) is shown in  FIG. 4 . Interposed between the two is the eNodeB  23  and the AGW  22 . This architecture is based on IP protocols (IP, TCP, UDP) and an additional tunneling protocol named as U8-Circuit Switched Resources (U8-CSR). This protocol carries the Mobility Management (MM) and all the protocol layers above MM transparently between the terminal  31  and the PMSC  24 . 
     The CSoLTE user plane protocols between the terminal and the PMSC  24  (i.e. the U8u interface) are shown in  FIG. 5 . EnodeB  23  and AGW  22  are arranged between the two. This architecture is based on IP protocols (IP, UDP, RTP) that are used to transmit the necessary voice and data communicating (e.g. AMR coded voice) between the terminal  31  and the PMSC  24 . 
     No known solutions exist for Handover from traditional CS domain to the CSoLTE based solutions. 
     It is an object of the present invention to obviate at least some of the above disadvantages and to provide a method and apparatus for triggering the seamless handover of an established connection from a circuit switched domain to circuit switched service over packet switched domain. 
     SUMMARY 
     According to a first aspect of the present invention, there is provided a method for initiating handover of a circuit switched service using a circuit switched bearer of a mobile station from a circuit switched to a packet switched domain in a mobile communications network. The mobile communications network comprises a radio network and a core network. The radio network comprises a controlling node. The mobile station has been being allocated an IP address and access port. The controlling node communicates to the core network a relocation/handover required message. The relocation/handover required message comprises an indication of the IP address and access port allocated to the mobile station. 
     According to a second aspect of the present invention, there is provided a controlling node adapted for assisting in initiating handover of a circuit switched service using a circuit switched bearer of a mobile station from a circuit switched to a packet switched domain in a mobile communications network. The mobile communications network comprises a radio network and a core network. The radio network comprises the controlling node. The mobile station has been allocated an IP address and access port. The controlling node comprises means for communicating to the core network a relocation/handover required message. The relocation/handover required message comprises an indication of the IP address and access (UDP) port allocated to the mobile station. 
     According to a third aspect of the present invention, there is provided a packet switching node adapted for assisting in initiating handover of a circuit switched service using a circuit switched bearer of a mobile station from a circuit switched to a packet switched domain in a mobile communications network. The mobile communications network comprises a radio network and a core network. The radio network comprises a controlling node. The mobile station has been allocated an IP address and access port. The packet switching node comprises means for receiving from the controlling node a relocation/handover required message. The relocation/handover required message comprises an indication of the IP address and access port allocated to the mobile station. 
     The terminals engaged in CS calls may be moved to the LTE/SAE access. This means that the terminals can start using the LTE/SAE broadband access when moving to LTE coverage area even in the case the CS services were initiated in 2G/3G RAN. 
     Moreover, no changes are needed to the existing CS CN infrastructure (i.e. the other MSCs other than the PMSC). 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates schematically SAE and LTE interfaces. 
         FIG. 2  illustrates schematically a CSoLTE architecture. 
         FIG. 3  illustrates schematically a CSoLTE reference architecture. 
         FIG. 4  illustrates schematically the CSoLTE control plane protocol architecture. 
         FIG. 5  illustrates schematically the CSoLTE user plane protocol architecture. 
         FIG. 6  illustrates schematically the Handover from CS to CSoLTE before execution—option 1. 
         FIG. 7  illustrates schematically the handover from CS to CSoLTE after execution—option 1. 
         FIG. 8  illustrates schematically the Handover from CS to CSoLTE before execution—option 2. 
         FIG. 9  illustrates schematically the Handover from CS to CSoLTE after Execution—option 2. 
         FIG. 10  is a Sequence Diagram for Handover from UTRAN CS to CSoLTE—option 1. 
         FIG. 11 . is a Sequence Diagram for Handover from UTRAN CS to CSoLTE—option 2. 
         FIG. 12  is a Sequence Diagram for Handover from GERAN CS to CSoLTE. 
     
    
    
     DETAILED DESCRIPTION 
     The present invention relates to Handover from the traditional CS domain (i.e. a 2G and/or a 3G RAN) to the CSoLTE based solutions. It applies for the case when a terminal in CS dedicated state moves from the 2G/3G RAN to an area where LTE coverage exists and when it is preferred that the call is handed over to the LTE/SAE networks. 
     This also means that the present invention may be applied for the CSoLTE-I and CSoLTE-D solutions as these solutions contain the usage of CSoLTE principles to transfer user plane and CS signalling connections. 
     The basic concept of the invention is that the Handover from traditional CS domain to CSoLTE is triggered from the 2G/3G RAN. This handover consists of two parallel handover requests, one request similar to the existing CS Handover request and the second one similar to the existing PS handover request. The CS Handover request is also expanded with mechanisms to communicate the needed terminal and PMSC IP-address information. 
     The situation that the present invention addresses is the following: 
     The terminal is initially:
     1. in CS dedicated state with reserved CS resources in the 2G/3G RAN and between the CS CN and 2G/3G RAN,   2. GPRS/PS domain attached via the current 2G/3G RAN, and   3. has a default Packet Data Protocol (PDP) context so that the terminal holds an IP-address in the GGSN/AGW.   

       FIG. 6  illustrates schematically the Handover from CS to CSoLTE: before execution under option 1.
     Note1: the CS domain may consist of MSC-S and MGW or may alternatively consist of classical MSC/VLR nodes. This is of no relevance for the present invention.   Note2: The word “option” is used in this document to describe different deployment alternatives and options.
 
Option 1:
   

     The first option assumes that the serving MSC  34  does not have the capability to act as a PMSC, i.e. does not have access to LTE/SAE network. This may be the case in the early introduction of LTE/SAE, but it can be assumed that at a later deployment phase all existing MSC will have also PMSC capability. 
     The following actions need to take place when handover from traditional CS domain (i.e. 2G/3G RAN) takes place to a CSoLTE solution:
     1. The path in the CN between MSC  34  and PMSC  24  needs to be established. The current MSC  34  will act as an anchor point at this handover. For this action the target LTE cell needs to be identified either as GERAN or UTRAN cell to avoid changes in the existing MSCs serving 2G/3G RANs.   2. The LTE/SAE bearers needed for CSoLTE need to be established for this terminal.   3. The PMSC  24  and the terminal  31  need to become aware of the IP-addresses and UDP-ports used for the control plane communication.   4. The PMSC  24  and the terminal  31  need to become aware of the IP-addresses and UDP-ports used for the user plane communication.   

       FIG. 7  illustrates schematically the handover from CS to CSoLTE after execution under option 1 
     Option 2: 
     In the second option, the serving MSC is also capable of functioning as a PMSC. 
     In this case, instead of handling the handover to LTE/SAE as an inter-MSC handover case, this can be handled as intra-MSC, inter-system handover. This is also possible today between GSM and WCDMA RANs. The handover procedures are very similar to option 1, the only difference being that no non-anchor MSC is needed and therefore also no user plane connection between MSCs. The option 2 is further shown in  FIGS. 8 and 9 . 
       FIG. 8  illustrates schematically the handover from CS to CSoLTE: before execution under option 2. 
       FIG. 9  illustrates schematically the handover from CS to CSoLTE: after execution under option 2. 
     The procedure for Handover from UTRAN CS to CSoLTE 
       FIG. 10  shows the relevant steps that are needed when a terminal occupied in a CS call is moving from 3G RAN to CSoLTE for the option 1 case when the PMSC  24  is not the serving MSC. 
     Initial State: The MS is engaged in a CS call in an UTRAN/WCDMA cell. The mobile station  31  is configured to perform measurements of neighbouring cells and at least one of the cells to be measured is a LTE cell. The MOBILE STATION  31  moves to the coverage area of the LTE cell and detects that cell. 
     Step  1 : The mobile station  31  reports the measurements it has performed for the detected LTE cell. The exact details of this are not standardized yet, but it can be assumed that the LTE cells are measured and reported as IRAT-cells. 
     Step  2 : In this step the RNC retrieves the IP-address of the mobile station  31  and also an UDP port to be used for the CSoLTE user plane. This is an optional step and enables that the PMSC may start sending downlink packets to the terminal already after the CS Handover procedure. 
     Another possibility would be that the PMSC  24  waits for the first uplink user payload packet from the terminal in step  10  and so learns the IP-address and UDP-port used by the mobile station  31  and enables the downlink direction first after this. 
     Step  3 : The RNC decides to perform Handover to the CSoLTE solution. This decision is based on the knowledge of the CS bearer/RAB being allocated for the mobile station  31  and that the reported target cell is a LTE cell. The following description is divided to two different parts, the CS handover and PS handover parts that are both triggered for the Handover from CS to CSoLTE procedure. The CS handover is shown as Steps  3   a - 7   a  and the PS handover is partly shown as steps  3   b - 7   b.    
     Steps  3   b - 7   b : The RNC triggers the PS handover procedure. As this procedure is performed as normally, the steps between steps  3   b  and  7   b  are not described. At step  7   b , the RNC waits for the completion of both CS and PS handover procedures until it continues to step  8 . 
     Step  3   a : The RNC triggers the CS Handover procedure by sending the (RANAP) Relocation Required message to the MSC. As mentioned earlier, the target LTE-cell is also associated with either an UTRAN or a GERAN cell. This cell information (e.g. CGI for GERAN cells or RNC-ID and CI for UTRAN cells) is used as the target cell in the handover request and so decides which PMSC is to handle the CSoLTE service for the specified LTE-cell. The reason for using GERAN and/or UTRAN cell information for this purpose is to avoid any unnecessary changes in the source MSC for this handover case. 
     If the optional step  2  above was performed, then the RNC includes the mobile station  31  IP-address and UDP-port information in the transparent information elements that are normally transported between the source BSC/RNC and the target BSC/RNC. If the target cell is identified as an UTRAN cell, then the Source RNC To Target RNC Transparent Container IE can be used and if the target cell is identified as GERAN cell, then the Old BSS To New BSS Information IE can be used to send transparently information from the source RNC to the PMSC  24  through the source MSC in the CS handover preparation request. 
     Step  4   a : The MSC uses the target cell identifier received in the Relocation Required message to identify the target MSC for this handover request. In this case, the analysis points to the PMSC  24  and the relevant MAP signalling (MAP-Prep-Handover-Request) is triggered towards the PMSC. The transparent container (e.g. Source RNC To Target RNC Transparent Container IE) is included in the MAP signalling and if the mobile station  31  IP-address and UPD-port were retrieved in step  2 , then this is the mechanism to pass that information to the PMSC  24 . 
     Step  5   a : The PMSC  24  reserves the needed resources for the CSoLTE call. This means that an IP-termination with an IP-address and UDP-port are reserved for the user plane. In the same way, an IP-termination with an IP-address and UDP-port may be reserved for the control plane. Then MAP signalling is used to communicate towards the source MSC (MAP-Prep-Handover-Response) and the IP-addresses and UDP ports received for the CSoLTE call can be included in the Target RNC To Source RNC Transparent Container IE. 
     Step  6   a : In this step the needed connectivity is established between the MSC and the PMSC  24  using standard CS call control signalling. 
     Step  7   a : The MSC returns the (RANAP) Relocation Command to the RNC as an indication that the CS handover is prepared and that the RNC can command the terminal to move to the target cell. As this specific handover is about handover to LTE and CSoLTE, the RNC needs to wait for both the steps  7   a  and  7   b  to happen before it can command the mobile station  31  to move to the target LTE cell. 
     Step  8   a : The RNC builds a Handover from UTRAN Command message and sends this message to the terminal. This message contains some specific information for the CSoLTE handover case as it is a combination of the information retrieved as part of the performed PS and CS handover preparations. First, the radio level information for the target LTE cell was received as part of the PS handover. Secondly, the needed PMSC address information is also included in the message. 
     Step  9 : The mobile station  31  accesses the target LTE cell using the mechanisms specified for normal PS handover. Note that the PS handover mechanism is still under standardisation. 
     Step  10 : The mobile station  31  and the PMSC  24  may communicate using the default best effort LTE/SAE bearers established as part of the PS handover scenario. The mobile station  31  knows the user plane IP-address and UDP-port for the reserved IP termination in the IWU/MGW and starts sending uplink user plane packets towards this termination. In parallel, the mobile station  31  establishes the control plane connection towards the PMSC  24  and the IP/UDIP address/port information needed for this was also signalled to the mobile station  31  in step  8 . 
     Depending on whether step  2  was performed, the PMSC  24  may start sending downlink user plane packets towards the mobile station  31  already after step  4   a . If step  2  was not performed, then the PMSC  24  will wait for the first uplink user plane packet from the mobile station  31  on the reserved IP-termination and then use the source IP-address and UDP-ports from these uplink packets as the destination IP-address and UDP-port for the downlink user plane packets. 
     Step  11 : The mobile station  31  signals to the PMSC  24  that the handover from CS to CSoLTE was successful by sending the U8c-HANDOVER COMPLETE message. 
     Step  12 : Based on either step  10  (receiving of uplink user plane traffic from the mobile station  31 ) or step  11  the PMSC  24  now knows that the mobile station  31  has arrived in the new cell. The PMSC  24  triggers the authorization and creation of the needed dedicated LTE/SAE bearer for the CSoLTE application by sending the AA-Request (AAR) to the Policy and Charging Rules Function (PCRF) over the Rx-interface. 
     Step  13 : The PCRF accepts the requested bearer and signals the need to create the dedicated LTE/SAE bearer for the CSoLTE by sending the Re-Auth-Request (RAR) message to the SAE-GW. 
     Step  14 : Standard signalling is performed between the SAE/LTE and the mobile station  31  to activate the dedicated LTE/SAE bearer for the CSoLTE. 
     Step  15 : The SAE-GW sends the Re-Auth-Accept (RAA) message to the PCRF to indicate that the bearer has been established. 
     Step  16 : The PCRF sends the AA-Accept (AAA) message to the PMSC  24  to indicate that the bearer has been established. 
     Step  17 : The CSoLTE call is using optimised LTE/SAE bearers. 
     Steps  18 - 20 : The PMSC  24  indicates the handover completion to the source MSC which requests the RNC to release the previously used resources in the source cell. All performed steps are not shown in  FIG. 8 , as these are the normal procedures performed after handover. 
       FIG. 11  is a Sequence Diagram for Handover from UTRAN CS to CSoLTE—option 2 It shows the relevant steps that are needed when a terminal occupied in a CS call is moving from  3 G RAN to CSoLTE for the option 2 case when the serving MSC is also functioning as the PMSC. The description of the  FIG. 10  applies here also expect that the steps  4 - 6  and  18 - 19  are omitted. 
     The procedure for Handover from GSM CS to CSoLTE 
       FIG. 12  is a Sequence Diagram for Handover from GERAN CS to CSoLTE showing the relevant steps that are needed when a terminal occupied in a CS call is moving from GERAN to CSoLTE. 
     Initial State: The mobile station  31  is engaged in a CS call in an GERAN/GSM cell. The mobile station  31  is configured to perform measurements of neighbouring cells and at least one of the cells to be measured is a LTE cell. The mobile station  31  moves to the coverage area of the LTE cell and detects that cell. 
     Step  1 : The mobile station  31  reports the measurements it has performed for the detected LTE cell. The exact details of this are not standardized yet, but it can be assumed that the LTE cells are measured and reported as IRAT-cells. 
     Step  2 : In this step the BSC retrieves the IP-address of the mobile station  31  and also an UDP port to be used for the CSoLTE user plane. This is an optional step and enables that the PMSC  24  may start sending downlink packets to the terminal already after the CS Handover procedure. 
     Another possibility would be that the PMSC  24  waits for the first uplink user payload packet from the terminal in step  10  and so learns the IP-address and UDP-port used by the mobile station  31  and enables the downlink direction first after this. 
     Step  3 : The BSC decides to perform Handover to the CSoLTE solution. This decision is based on the knowledge of the CS bearer being allocated for the mobile station  31  and that the reported target cell is a LTE cell. The following description is divided to two different parts, the CS handover and PS handover parts that are both triggered for the Handover from CS to CSoLTE procedure. The CS handover is shown as Steps  3   a - 7   a  and the PS handover is partly shown as steps  3   b - 7   b.    
     Steps  3   b - 7   b : The BSC  32 ′ triggers the PS handover procedure. As this procedure is performed as normally, the steps between steps  3   b  and  7   b  are not described. The only difference towards normal PS handover procedures is that if DTM was not supported by the mobile station  31  or in the current cell, then there might be a need for an extra indication in step  3   b  from the BSC to the SGSN to indicate that also non-active PDP contexts need to be transferred towards the target RAT (i.e. the MME and UPE and eNodeB and AGW). At step  7   b , the BSC waits for the completion of both CS and PS handover procedures until it continues to step  8 . 
     Step  3   a : The BSC triggers the CS Handover procedure by sending the (BSSMAP) Handover Required message to the MSC. As mentioned earlier, the target LTE-cell is also associated with either an UTRAN or a GERAN cell. This cell information (e.g. CGI for GERAN cells or RNC-ID and CI for UTRAN cells) is used as the target cell in the handover request and so decides which PMSC is to handle the CSoLTE service for the specified LTE-cell. The reason for using GERAN and/or UTRAN cell information for this purpose is to avoid any unnecessary changes in the source MSC for this handover case. 
     If the optional step  2  above was performed, then the BSC includes the mobile station  31  IP-address and UDP-port information in the transparent information elements that are normally transported between the source BSC/RNC and the target BSC/RNC. If the target cell is identified as an UTRAN cell, then the Source RNC To Target RNC Transparent Container IE can be used and if the target cell is identified as GERAN cell, then the Old BSS To New BSS Information IE can be used to send transparently information from the source BSC to the PMSC  24  through the source MSC in the CS handover preparation request. 
     Step  4   a : The MSC uses the target cell identifier received in the Handover Required message to identify the target MSC for this handover request. In this case, the analysis points to the PMSC  24  and the relevant MAP signalling (MAP-Prep-Handover-Request) is triggered towards the PMSC  24 . The transparent container (e.g. Source RNC To Target RNC Transparent Container IE) is included in the MAP signalling and if the mobile station  31  IP-address and UPD-port were retrieved in step  2 , then this is the mechanism to pass that information to the PMSC  24 . 
     Step  5   a : The PMSC  24  reserves the needed resources for the CSoLTE call. This means that an IP-termination with an IP-address and UDP-port are reserved for the user plane. In the same way, an IP-termination with an IP-address and UDP-port may be reserved for the control plane. Then MAP signalling is used to communicate towards the MSC (MAP-Prep-Handover-Response) and the IP-addresses and UDP ports received for the CSoLTE call can be included in the Target RNC To Source RNC Transparent Container IE. 
     Step  6   a : In this step the needed connectivity is established between the MSC and the PMSC  24  using standard CS call control signalling. 
     Step  7   a : The MSC returns the (BSSMAP) Handover Required Acknowledge message to the BSC as an indication that the CS handover is prepared and that the BSC can command the terminal to move to the target cell. As this specific handover is about handover to LTE and CSoLTE, the BSC needs to wait for both the steps  7   a  and  7   b  to happen before it can command the mobile station  31  to move to the target LTE cell. 
     Step  8   a : The BSC builds a Handover Command message and sends this message to the terminal. This message contains some specific information for the CSoLTE handover case as it is a combination of the information retrieved as part of the performed PS and CS handover preparations. First, the radio level information for the target LTE cell was received as part of the PS handover. Secondly, the needed PMSC address information is also included in the message. 
     Step  9 : The mobile station  31  accesses the target LTE cell using the mechanisms specified for normal PS handover. Note that the PS handover mechanism is still under standardisation. 
     Step  10 : The mobile station  31  and the PMSC  24  may communicate using the default best effort LTE/SAE bearers established as part of the PS handover scenario. The mobile station  31  knows the user plane IP-address and UDP-port for the reserved IP termination in the IWU/MGW and starts sending uplink user plane packets towards this termination. In parallel, the mobile station  31  establishes the control plane connection towards the PMSC  24  and the IP/UDIP address/port information needed for this was also signalled to the mobile station  31  in step  8 . 
     Depending on if the step  2  was performed, the PMSC  24  may start sending downlink user plane packets towards the mobile station  31  already after step  4   a . If step  2  was not performed, then the PMSC  24  will wait for the first uplink user plane packet from the mobile station  31  on the reserved IP-termination and then use the source IP-address and UDP-ports from these uplink packets as the destination IP-address and UDP-port for the downlink user plane packets. 
     Step  11 : The mobile station  31  signals to the PMSC  24  that the handover from CS to CSoLTE was successful by sending the U8c-HANDOVER COMPLETE message. 
     Step  12 : Based on either step  10  (receiving of uplink user plane traffic from the mobile station  31 ) or step  11  the PMSC  24  now knows that the mobile station  31  has arrived in the new cell. The PMSC  24  triggers the authorization and creation of the needed dedicated LTE/SAE bearer for the CSoLTE application by sending the AA-Request (AAR) to the Policy and Charging Rules Function (PCRF) over the Rx-interface. 
     Step  13 : The PCRF accepts the requested bearer and signals the need to create the dedicated LTE/SAE bearer for the CSoLTE by sending the Re-Auth-Request (RAR) message to the SAE-GW. 
     Step  14 : Standard signalling is performed between the SAE/LTE and the mobile station  31  to activate the dedicated LTE/SAE bearer for the CSoLTE. 
     Step  15 : The SAE-GW sends the Re-Auth-Accept (RAA) message to the PCRF to indicate that the bearer has been established. 
     Step  16 : The PCRF sends the AA-Accept (AAA) message to the PMSC  24  to indicate that the bearer has been established. 
     Step  17 : The CSoLTE call is using optimised LTE/SAE bearers. 
     Steps  18 - 26 : The PMSC  24  indicates the handover completion to the source MSC which requests the BSC to release the previously used resources in the source cell. All performed steps are not shown in  FIG. 8  as these are the normal procedures performed after handover. 
     Similar steps are needed when a terminal occupied in a CS call is moving from GSM to CSoLTE for the option 2 case when the serving MSC is also functioning as the PMSC. The description of the  FIG. 12  applies for that case also expect that the steps  4   a - 5   a  and  18 - 19  are omitted. 
     No doubt many other effective alternatives will occur to the skilled person. It will be understood that the invention is not limited to the described embodiments and encompasses modifications apparent to those skilled in the art lying within the spirit and scope of the claims appended hereto.