Abstract:
A method is provided of handing over a priority call with Single Radio Voice Call Continuity, SRVCC. The priority call is handed over from a User Equipment, UE, accessing an IP Multimedia Subsystem, IMS, network via a Circuit Switched, CS, access network to a target Packet Switched, PS, access network thereby establishing a priority IMS session. The priority call has a specified CS priority level. The method includes sending a handover request from the CS access network towards the PS access network and the IMS network to transfer the priority call. A PS priority level for the session in the PS network is determined based on the specified CS priority level. 
     Bearer resources for the IMS session are allocated. The bearer resources are based, at least in part, on the PS priority level. The call is transferred to the IMS session over the PS access network.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to methods and apparatus in a telecommunications network for enabling handover from a Circuit Switched (CS) to a Packet Switched (PS) access network for a priority call. More particularly, the invention relates to methods and apparatus for enabling handover of a priority call with Single Radio Voice Call Continuity, SRVCC. 
       BACKGROUND 
       [0002]    IP Multimedia services provide a dynamic combination of voice, video, messaging, data, etc, within the same session. This has lead to a growth in the numbers of basic applications and the media which it is possible to combine, leading to a growth in the number and variety of services offered to the end users—so-called “combinational IP Multimedia” services. 
         [0003]    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). 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). 
         [0004]      FIG. 1  illustrates schematically how the IMS fits into the mobile network architecture in the case of a General Packet Radio Service (GPRS) access network. As shown in  FIG. 1  control of communications occurs at three layers (or planes). The lowest layer is the Connectivity Layer  1 , also referred to as the bearer plane and through which signals are directed to/from user equipment (UE) accessing the network. The entities within the connectivity layer  1  that connect an IMS subscriber to IMS services form a network that is referred to as the IP-Connectivity Access Network, IP-CAN. The GPRS network includes various GPRS Support Nodes (GSNs). A gateway GPRS support node (GGSN)  2  acts as an interface between the GPRS backbone network and other networks (radio network and the IMS network). The middle layer is the Control Layer  4 , and at the top is the Application Layer  6 . 
         [0005]    The IMS  3  includes a core network  3   a,  which operates over the middle, Control Layer  4  and the Connectivity Layer  1 , and a Service Network  3   b.  The IMS core network  3   a  includes nodes that send/receive signals to/from the GPRS network via the GGSN  2  at the Connectivity Layer  1  and network nodes that include Call/Session Control Functions (CSCFs)  5 , which operate as SIP proxies within the IMS in the middle, Control Layer  4 . The top, Application Layer  6  includes the IMS service network  3   b.  Application Servers (ASs)  7  are provided for implementing IMS service functionality. 
         [0006]    As shown in  FIG. 1 , User Equipment (UE) can access the IMS by attaching to an access network and then over the Connectivity Layer  1 , which is part of a PS domain. In that case an IMS session can be set up by the UE using SIP signalling.  FIG. 2  illustrates schematically the main components of relevance to the present disclosure of a PS Evolved Packet Core (EPC) network in accordance with the 3GPP defined Systems Architecture Evolution (SAE), and shows a UE  20  accessing an IP network shown as the Internet  21 . The principal network entities shown include a Serving Gateway (SGW)  23 , a Public data network, PDN, Gateway (PGW)  24 , an evolved NodeB (eNodeB)  25 , a Mobility Management Entity (MME)  26  and the user&#39;s Home Subscriber Server (HSS)  27 . For the purposes of the following discussion the SGW  23  and PGW  24  will be grouped together as one entity SGW/PGW. 
         [0007]    Many existing access networks operate only using CS technology, but a UE may also access IMS services via a CS domain. Although the CS domain will not handle SIP, procedures are well established for dealing with the provision of media and services between the IMS and a UE using a CS access. There are many occasions when during a call/session it is required to transfer or hand over the call/session from one access network to another. There are a variety of factors that are used to determine when a call needs to be handed over to another access network, but these are not particularly relevant to the present discussion. All we need to know is that the CS access network determines, based on the cells for which the UE reports measurements, when the conditions arise that require a request to be made to the core network for the call to be handed over. Single Radio Voice Call Continuity (SRVCC) is described in 3GPP TS 23.237 and 3GPP TS 23.216, specifying procedures for handover of a voice call from a PS access to a CS access (e.g. transfer of a Voice-over-IP, VoIP, IMS session from an evolved Universal Terrestrial Radio Access Network, E-UTRAN, to a Universal Terrestrial Radio Access Network/GSM Edge Radio Access Network, UTRAN/GERAN). These technical specifications have also been extended to allow handover of a voice call from a CS access to a PS access. 
         [0008]    However, the procedures specified for CS to PS handover with SRVCC do not support the transfer of a priority call. In other words, under the current procedures, if a priority call is transferred from a CS to a PS access, the transferred call would no longer be a priority call. This problem is addressed by the following discussion. 
       SUMMARY 
       [0009]    A first aspect provides a method of handing over a priority call with Single Radio Voice Call Continuity, SRVCC. The priority call is handed over from a User Equipment, UE, accessing an IP Multimedia Subsystem, IMS, network via a Circuit Switched, CS, access network to a target Packet Switched, PS, access network thereby establishing a priority IMS session. The priority call has a specified CS priority level. The method includes sending a handover request from the CS access network towards the PS access network and the IMS network to transfer the priority call. A PS priority level for the session in the PS network is determined based on the specified CS priority level. Bearer resources for the IMS session are allocated. The bearer resources are based, at least in part, on the PS priority level. The call is transferred to the IMS session over the PS access network. 
         [0010]    A second aspect provides a network server configured for use in a telecommunications network in which a priority call from a User Equipment, UE, accessing an IP Multimedia Subsystem, IMS, network via a Circuit Switched, CS, access network is to be handed over with Single Radio Voice Call Continuity, SRVCC, to a target packet Switched, PS, access network and thereby establish an IMS priority session. The network server includes a priority mapping module configured to determine a PS priority level for the IMS session based on a specified CS priority level of the call. The network server also includes an input/output for receiving a signal with an indication of the specified CS priority level of the call, and for sending a signal that includes the determined PS priority level. 
         [0011]    A third aspect provides User Equipment, UE, configured to establish a priority call in a Circuit Switched, CS, access network, the priority call having a specified CS priority level. The UE is also configured to access an IP Multimedia Subsystem, IMS, network via the CS access network, and for the call to be handed over with Single Radio Voice Call Continuity, SRVCC, to a target packet Switched, PS, access network, thereby to establish an IMS priority session. The UE includes: an input/output for receiving and transmitting signals via the CS access network; an input/output for receiving and transmitting signals via the PS access network; a memory storing data and program instructions; and a processor. The processor is configured to process the program instructions to store in the memory an indication of an IMS priority level that corresponds to the specified CS priority level of the call, and to include the IMS priority level in IMS signalling sent to the IMS network to establish the IMS priority session with SRVCC. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]      FIG. 1  illustrates schematically an IMS network in association with a mobile network architecture of a General Packet Radio Service (GPRS) access network; 
           [0013]      FIG. 2  illustrates schematically the components of a PS access network in accordance with the 3GPP defined Systems Architecture Evolution (SAE); 
           [0014]      FIG. 3  illustrates schematically the interactions between network components involved in a handover from a CS access to a PS access with SRVCC; 
           [0015]      FIG. 4  is a signal diagram illustrating the procedure, as currently set out in 3GPP TS 23.216, for a CS to PS handover with SRVCC; 
           [0016]      FIG. 5  is a signal diagram illustrating a procedure for a CS to PS handover with SRVCC of a priority call; 
           [0017]      FIG. 6  is a flow diagram illustrating the principal method steps in a method of CS to PS handover with SRVCC of a priority call; 
           [0018]      FIG. 7  is a schematic block diagram of a network server; 
           [0019]      FIG. 8  is a schematic block diagram of a UE. 
       
    
    
     DETAILED DESCRIPTION 
       [0020]      FIG. 3  illustrates conceptually a handover of a UE  30 , which is shown in a first situation  30   a,  prior to the handover, having established a call. The call is carried by bearers represented by solid line  301  to a terminating entity (which is not shown) over a CS (UTRAN/GERAN) access network  31  and a Mobile Switching Centre (MSC) 
         [0021]    Server  32  in the CS domain. The UE  30   a  is also attached to the IMS  33  as represented by line  302 . After the handover, the UE  30  is shown in a second situation  30   b,  in which the call is carried by bearers represented by the dashed line  303  over a PS, E-UTRAN, access network  34  and Serving/Public data Network (PDN) gateways  35 . The UE  30   b  remains attached to the IMS, as shown by solid line  304  and uses SIP signalling to communicate with the IMS, as represented by the dashed arrow-headed line  305 . Also shown between the CS and PS access networks are a Serving GSN (SGSN)  36  a Mobile Management Entity (MME)  37  and the user&#39;s Home Subscriber Server (HSS)  38 . These entities communicate with other entities over the referenced network interfaces. Note that although both the CS and PS access networks are shown in  FIG. 3  communicating directly with the same SGSN  36  and MME  37 , this is a simplification and in general each access network will communicate with a separate SGSN and MME associated with that access network. Thus, in a real practical deployment there would be either a source SGSN or a source MME associated with the source (in this case the CS) access network, and the same for the target (PS) network. The SGSN and MME can be separate nodes or co-located. 
         [0022]      FIG. 4  shows the signalling involved in the handover process in accordance with TS 23.216 (version 11.2.0). The entities shown include a UE  40 , which initially has a priority call set up over a CS access network represented here by a Radio Network Controller/Base Station Controller, RNC/BSC  41 , that communicates with a MSC Server  43 . The UE  40  is also anchored in an IMS network  47 , which means that IMS sessions from and to the UE  40  are anchored at the Switching Control Centre (SCC) AS (not shown) in the UE subscriber&#39;s home IMS network so that the SCC AS sees all sessions and keeps track of them. Sessions are anchored at the SCC AS in the home IMS based on the subscriber&#39;s Initial Filter Criteria (iFC) provided by the HSS at registration. The handover procedure will require transferring the call to a PS access network, the entities of which include a NodeB (for Universal Mobile Telecommunications System, UMTS, network) or eNode B (for LTE/EPC network)  42  and a SGW/PGW  45 . Also shown are a Source MME/SGSN  44  (see  FIG. 3 ) with which the CS access network communicate, a Policy and Charging Rules Function (PCRF)  46 , which triggers modification of the filters for the IMS signalling bearer when this is changed caused by a change in priority level, and a Target MME/SGSN  46  that communicates with the PS access network to which the call will be transferred. 
         [0023]    Briefly, the procedure is as follows: 
         [0024]    The CS access network, represented here by the RNC/BSC  41 , sends a HO required signal  401  to the MSC Server  43  indicating that the call needs to be transferred to another access network. Signals  402  show the MSC Server  43  interacting with the IMS  47  according to standard procedures of TS 23.216. The MSC Server sends a SRVCC CS to PS HO request  403  to the Target MME according to standard procedures of TS 23.216. In signals  404  and  405  the target MME/SGSN  48  performs a context request if this is required according to standard procedures of TS 23.216. In signals  406 , the target MME/SGSN  48  allocates resources for all PS bearers in the PS access network (e.g. E-UTRAN or UTRAN (High Speed Packet Access—HSPA)). Signals  407 - 411  a are the standard procedure of TS 23.216, in which the handover instruction (CS to PS HO command  408   a,    408   b ) is sent back to the UE  40 , which then completes its attachment to the PS access network. The Target MME/SGSN  48  sends Modify Bearer Request  412  to the SGW/PGW  45 , to update PS bearer contexts according to the Inter Radio Access Technology handover (IRAT HO) procedure as specified in 3GPP TS 23.401. The target MME/SGSN  48  includes a CS to PS SRVCC indication, which is also provided to the PCRF  46  to trigger modification of the filters for the IMS signalling bearer. If the target MME/SGSN  48  has received a Context Request from the Source SGSN/MME  44  (signal  404 ), the target MME/SGSN  48  sends an Acknowledgment to the Context Response  413  to the Source SGSN/MME  44 . At step  414 , the UE  40  initiates the Session transfer procedures according to 3GPP TS 23.237. At step  415 , as a result of the Session transfer procedures, the setup of a dedicated bearer for the call is performed according to the dedicated bearer activation procedure as specified in TS 23.401. 
         [0025]    As previously explained, in the procedures specified and set out above, if the original call from the UE  40  was a priority call in the CS network, then this priority would be lost after the handover. Indeed, call priorities are handled and specified quite differently in CS and PS networks. 
         [0026]      FIG. 5  illustrates the signalling in a procedure for transferring a priority call from a CS to a PS access network. Note that a priority call is assigned a priority level in the CS domain. At the outset UE  40  has a priority call established in the CS domain and the UE  40  is anchored in the IMS  47 , such that both the MSC Server  43  and the IMS  47  have the understanding that it is a priority call. The MSC Server  43  stores a priority mapping between the CS priority levels and the IMS priority levels so that it can map the priority level of the call in the CS domain to the priority in the IMS  47 . Similarly, when access transfer is performed, the priority level used at the MSC Server  43  (CS or IMS) is used to map to the priority level of the PS domain (e.g. EPC priority level used by the Target MME  48 ). A mapping, which may be a unique mapping, is defined between CS and IMS priorities, and also between the CS/IMS priority levels the PS (e.g. EPC) priority levels—note that these systems may use different values for defining priority levels. 
         [0027]    The procedure shown in  FIG. 5  involves the same network entities as shown in  FIG. 4 , which carry the same reference numerals, and some of the same signalling steps. Signals  501  and  502  are the same as signals  401  and  402  of  FIG. 4 . The MSC Server  43  then sends a SRVCC CS to PS HO request  503  to the Target MME/SGSN  48  according to standard procedures of TS 23.216. Additionally, in signal  503 , the MSC Server  43  includes a priority indicator, including the CS (or IMS) priority level of the call. In this case, the Target MME/SGSN  48  maps the CS (or IMS) priority to the relevant priority level for the PS (EPC) network. Alternatively, the MSC Server  43  may map the priority level from the CS (or IMS) priority to the PS (EPC) priority level, and send the corresponding EPC priority to the Target MME/SGSN  48  directly. 
         [0028]    Signals  504 - 505  are the same as signals  404 - 405  of  FIG. 4 . In signals  506 , the Target MME/SGSN  48  allocates resources for all PS bearers in the PS access network by an exchange of messages with the NodeB or eNodeB, but in this case the resources are determined, at least in part, by the required priority level of the call. Optionally, the Target MME/SGSN  48  may change the priority of the IMS signalling bearer during this time by indicating the EPC priority level required for the IMS Signalling bearer as part of the resource allocation procedures. Alternatively, the priority of the IMS signalling bearer may be altered later (see step  512   a  below). 
         [0029]    Signals  507 - 511   a  are the same as  407 - 411   a  shown in  FIG. 4  and accord with the standard procedures of TS 23.216. However, in one alternative embodiment the HO command signals  508   a,    508   b  may include the IMS priority level to be used for the IMS signalling bearer after handover (see step  514  below). The Target MME/SGSN  48  then sends a Modify Bearer Request  512  to the SGW/PGW  45  to update the PS bearer contexts according to the IRAT HO procedure as specified in TS 23.401. The Target MME/SGSN  48  includes the CS to PS SRVCC indication, and if the required priority level of the bearer was not set in step  506 , it also includes the PS priority level required. At this stage, the CS to PS indication is also provided to the PCRF  46 , together with the EPC priority level if this was provided in signal  512 . This triggers modification of the filters for the IMS signalling bearer to change the EPC priority level of the signalling bearer so that it corresponds to the priority level previously used in the CS domain (and/or IMS). Signal  513  is the same as signal  413  in  FIG. 4 . 
         [0030]    At step  514 , the UE initiates the Session transfer procedures as specified in TS 23.237, and in addition the UE includes the corresponding IMS priority level in the IMS signalling. There are two ways that the UE may obtain the IMS priority level: 1) the IMS priority level is provided through the HO command signals  508   a/b  to the UE; or 2) the UE derives the IMS priority level itself from the CS priority level (which it knows), in which case the UE stores the same priority level mapping as the MSC server. When setting up the media bearer resources for the call, a P-CSCF (not shown) in the IMS network interacts with the PCRF  46  and uses the IMS priority level for authorizing resources. 
         [0031]    At step  515 , as a result of the Session transfer procedures, the setup of a dedicated bearer for the priority call is performed according to the dedicated bearer activation procedure as specified in TS 23.401. The dedicated bearer is allocated a priority level based on the received IMS priority level. 
         [0032]      FIG. 6  is a flow diagram illustrating the principal method steps of the procedure described above for a method of handing over, a priority call from a CS access network to a target PS access network with SRVCC. At step  601  the priority call is established over the CS access network with a specified CS priority level, and the call is anchored in the IMS. At step  602  a request for handover with SRVCC is sent from the CS access network towards the IMS network to transfer the priority call to a target PS access network. At step  603  a mapping is performed to determine a PS priority level for the session in the PS access network based on the specified CS priority level. At step  604  bearer resources for the session allocated, the bearer resources being based, at least in part, on the determined PS priority level. At step  605  the handover procedures are completed. Finally at step  606  the priority call is transferred to the IMS priority session over the PS access network. 
         [0033]      FIG. 7  is a schematic block diagram of a network server  700  configured for the implementation of the CS to PS handover procedure with SRVCC of a priority call. The network server  700  includes an input/output  702  for receiving and sending signals to other network entities, and a priority mapping module  704  that determines the PS priority level, based on a specified CS priority level of the call. The signals received at the input/output  702  include a signal that includes an indication of the specified CS priority level of the priority call. The signals sent via the input/output  702  include a signal that includes the determined PS priority level. 
         [0034]      FIG. 8  is a schematic block diagram of a UE  800  configured for the implementation of the CS to PS handover procedure with SRVCC of a priority call. The UE  800  includes an input/output  802  for receiving and transmitting signals via a CS access network, and an input/output  803  for receiving and transmitting signals via a PS access network. The UE  800  also includes a memory  806  storing data and program instructions. A processor  804  is configured to process the program instructions to store in the memory  806  an indication of an IMS priority level that corresponds to the specified CS priority level of the call. The processor  804  is also configured to include the IMS priority level in IMS signalling sent to the IMS network to transfer the call with SRVCC by establishing an IMS priority session. 
         [0035]    The procedures and associated configuration of the UE and network entities described above allow handover of a priority call from a CS access to a PS access with SRVCC such that the priority of the call in the CS domain is retained with an equivalent or corresponding priority in the PS domain.