Abstract:
Method and system of establishing a connection between a calling terminal ( 1 ) and a called terminal ( 2 ) which allows the called terminal ( 2 ) to receive calls trough two different networks ( 3, 4 ), for example a circuit switched network and a Internet protocol Multimedia Subsystem, using a single number, thanks to a control element which determines the network to be used to reach the called terminal ( 2 ), and its number in said network to be used.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention has its application within the telecommunications sector and, especially, in the field of communication networks. 
       BACKGROUND OF THE INVENTION 
       [0002]    Many telecommunications operators are migrating from their traditional circuit switched (CS) networks, such as the Public Switched Telephone Network (PSTN) or the Public land mobile network (PLMN), to Next Generation Networking, such as 
         [0003]    Internet Protocol Multimedia Subsystem (IMS). In this scenario, both technologies are required to coexist and interwork, as the migration process may span several years. 
         [0004]    The way in which CS and IMS networks must interwork has been standardized, dealing with the problem of routing incoming calls to the proper domain. Traditional solutions rely on dial plans (also called “numbering plans”), which indicate which telephone numbers (also called MSISDN, “Mobile Station International Subscriber Directory Number”) correspond to each network. Thanks to the hierarchical nature of the dial plans, and its geographical basis, most calls can be routed based only on their prefix, although there are some exceptions, like intelligent network services with non-geographical numbers. 
         [0005]    Both CS and IMS networks use these plans when routing calls to other networks. If a calls is originated in a IMS network and the called number is connected to a CS network, a Breakout Gateway Control Function (BGCF) determines which Media Gateway Control Function (MGCF) the call should go through to reach the desired network, according to the dial plan. The BGCF is thus a Session Initiation Protocol (SIP) server which includes routing functionalities based on telephone numbers. 
         [0006]    On the other hand, If a calls is originated in a CS network and the called number is connected to a IMS network, the call is routed to the phone operator&#39;s Gateway Mobile Switching Centre (MSC), which relies on the dial plans to connect to the IMS network. 
         [0007]    In both scenarios, if a user migrates to another network, an exception needs to be indicated in the original dial plan to allow the user to keep his or her telephone number and still be reached in the network to which he or she is transferred. 
         [0008]    Portability issues have been tackled, for example in WO 2007/130323 A1, which presents an apparatus and method to perform a static number portability in a IMS network; and EP 1,761,077 B1, which presents a method to allow portability between two IMS networks. 
         [0009]    However, in this state of the art, if a user desires to be reachable through both CS and IMS networks, said user needs two different telephone numbers, and cannot choose the network through which to receive calls at a given time, as it is determined by the dialled number. Thus, there is the need of methods and systems that allow users whose terminals are able to work in both CS and IMS networks to dynamically choose through which networks to receive calls, using a single telephone number for both of the networks. 
       SUMMARY OF THE INVENTION 
       [0010]    The current invention solves the aforementioned problems by disclosing a method and system that allows a user to receive calls through any of two available communication networks according to his or her preferences using a single number as identification. 
         [0011]    In a first aspect of the present invention, a method of establishing a connection between two terminals (calling terminal and called terminal) is disclosed. In the scenario in which the present invention applies, the calling terminal is capable of connecting to a first network, preferably a CS network, while the called terminal is capable of connecting to both the CS network and a second network, preferably an IMS network. It should be noted that additional connections to other networks can be established without interfering with the disclosed invention, and thus, the calling terminal and the called terminal may have additional connection capabilities as long as they verify the stated requirements. 
         [0012]    In this scenario, if the calling terminal is connected to the first network and it uses said first network to send a request to establish a connection with the called terminal, said request is received by a gateway of the first network, which responds by requesting the necessary routing information to establish the connection. In a CS network, said request is preferably handled by a Service Control Point (SCP). According to the method of the invention, the SCP (or another control element) receives the request from the gateway and responds with a message that comprises an indication of the network that is to be used to reach the called terminal, as well as an identifier (such as a telephone number) of the called terminal in said network. Preferably, the network to be used is determined according to preferences of the user of the called terminal. These preferences may be established as unconditional priorities as well as conditions according to the state of the networks and the terminal. 
         [0013]    With the information provided by the control element, the gateway of the first network is able to establish the connection through the network indicated in the message (the first network or the second network). As the decision of which network to use is performed by the control element, which also provides the necessary routing information (that is, the aforementioned identifier in the network through which the connection is finally established), the calling terminal only needs to know a single identifier of the called terminal, which is thus able to use both networks without the need of using independent identifiers for each one. 
         [0014]    Preferably, the method comprises a previous registration step in which the control element receives a register request from the called terminal. In response to this register request, the control element assigns a dynamic identifier in the second network to the called terminal, for example, from a pool of numbers previously assigned to that purpose. Finally, in the registration step the preferences and conditions that determine which network is to be used to reach the called terminal in each situation are established. 
         [0015]    In another aspect of the present invention, a system of establishing a connection between two terminals is disclosed. The system comprises a control element (such as the SCP in a CS network) that performs the described method, thus allowing the called terminal to be reached through two different networks using a single identifier. 
         [0016]    These and other advantages will be apparent in the light of the detailed description of the invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]    For the purpose of aiding the understanding of the characteristics of the invention, according to a preferred practical embodiment thereof and in order to complement this description, the following figures are attached as an integral part thereof, having an illustrative and non-limiting character: 
           [0018]      FIG. 1  shows a schematic representation of a particular embodiment of the system of the invention, as well as some other elements of a CS network and an IMS network that take part in the communication process. 
           [0019]      FIG. 2  presents a flow diagram of a registration request. 
           [0020]      FIG. 3  shows a flow diagram of a de-registration request. 
           [0021]      FIG. 4  presents an example of connection establishment in which the called terminal is only connected to a CS network. 
           [0022]      FIG. 5  shows an example of a flow diagram of a connection establishment according a particular embodiment to the method of the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0023]    The matters defined in this detailed description are provided to assist in a comprehensive understanding of the invention. Accordingly, those of ordinary skill in the art will recognize that variations, changes and modifications of the embodiments described herein can be made without departing from the scope and spirit of the invention. 
         [0024]    Note that in this text, the term “comprises” and its derivations (such as “comprising”, etc.) should not be understood in an excluding sense, that is, these terms should not be interpreted as excluding the possibility that what is described and defined may include further elements, steps, etc. 
         [0025]    The terms “called terminal” and “called party” are used indistinctively in this text, and they refer to a device which can receive calls with the present method and system, even when there is no active connection, for example, during the registration process. 
         [0026]      FIG. 1  shows some elements of a CS network  3  and an IMS network  4  involved in the communication between a calling terminal  1  and a called terminal  2 . The calling terminal  1  is connected to the CS network  3 , while the called terminal  2  is connected to the CS network  3  (with a CS number  19 ) and to the IMS network  4  (with a IMS telephone number  20 ). It should be noted that, although the called terminal  2  has a telephone number in each network, the system and method hereby disclosed allow the calling terminal  1  to reach the called terminal  2  through both networks using only one of the numbers. 
         [0027]    CS network  3  comprises at least a first Mobile Media Gateway (M-MGW)  5  which provides service to the calling terminal  1  and a second M-MGW  5 ′ which provides service to the called terminal  2 . Each terminal also has an associated Mobile Switching Center (MSC), that is, calling terminal MSC  6  and called terminal MSC  6 ′. Also, the CS network  3  comprises a Session Description Protocol (SDP)  7  and a Home Location Register (HLR)  8  in the user&#39;s data plane and a SCP  9  in the application plane. Finally, the CS network  3  is connected to the IMS network  4  by a call handling Application Program Interface (API)  10 . 
         [0028]    IMS network  4  comprises IP core  11 , Media Gateway (MGW)  12 , Call Sesion Control Function (CSCF)  13 , BGCF  14  and SGW  15  in the signalling plane. The IMS user&#39;s data plane comprises Home Subscriber Service (HSS)  6  and Domain Naming System/Enumeration (DNS/ENUM)  7 . Finally, according to a preferred embodiment of the present invention, the IMS network  4  comprises in its application plane an Application Service (AS)  18  which is responsible of configuring the SCP  9  of the CS network  3 . The AS  18  has a subscription to the Reg event state, and has a pool of IMS phone number (that is, E.164 addresses) available to be assigned. Any number in this pool is unconditionally routed by the CS network to the IMS network. 
         [0029]      FIG. 2  presents a scheme of a registration process requested by the called terminal  2 . The called terminal sends a register request  101  to the CSCF  13 , which responds with an acknowledgement message  102  (ACK), and also sends register request info  103  to the AS  18 . The AS  18  gets the CS phone number  19  and the user preferences, either from the register request info  103  or from its own database, HSS  16 . The AS also chooses a free IMS phone number from the number pool and associates this dynamic IMS phone number with the CS phone number  19  of the user, and updates  104  the DNS/ENUM  17  with this association. Then, the AS  18  sends the SCP  9  the instructions  105  determining how to handle future calls to the called user according to his or her preferences and to different possible scenarios, like the following: 
         [0030]    Called Party is not reachable. 
         [0031]    Called Party not answer. 
         [0032]    Called Party is busy. 
         [0033]    After the registration process, the CS number  19  is re-routed to the IMS number  20  conditioned by the above cases and user&#39;s preferences. The interface between the AS  18  and SCP  9  can be connected directly using a SCP protocol or through the Call Handling API  10  interface. 
         [0034]    Figure shows an example of a de-registration process started by the called terminal  2 . Called terminal  2  sends a de-registration request  201  to the CSCF  13 , which responds with an acknowledgement message  202  (ACK), and also sends de-register request info  203  to the AS  18 . AS  18  sends an update message  204  to the DNS/ENUM  7 , thus freeing the dynamic IMS telephone number previously assigned to the called terminal. AS  18  additionally sends a reconfiguration message  205  to SCP  9 , after which SCP  9  stops rerouting calls to the IMS telephone number  20 . 
         [0035]      FIG. 3  presents an example of communication establishment in which the called terminal  2  is registered in the AS  18  but is nevertheless only connected to CS network  3 . Calling terminal  1  sends a ringing message  301  to the calling terminal MSC  6 , which sends a request  302  for information to HLR/VLR  13 . HLR/VLR  13  sends the requested information  303  to the calling terminal MSC  6 , which allows calling terminal MSC  6  to send a request for routing information  304  to SCP  9 . SCP  9  sends the requested routing information  305  (including the network to be used and the number of the called terminal in said network, which in this case is the original CS telephone number  19  used by the calling terminal  1  to start the communication), and calling terminal MSC  6  connects  306  to called terminal MSC  6 ′, which establishes a connection  307  with the called terminal  2  through its CS number  19 . Called terminal  2  is thus able to answer  308  the call through the CS network  3 . 
         [0036]      FIG. 4  shows another example in which the called terminal  2  is connected to both CS network  3  and IMS network  4 , and is thus reachable through both networks. In this particular example, the preferences of the user are such that incoming calls are to be routed to the CS telephone number  19  and then to the IMS network  4  if said CS telephone number  19  is not reachable or does not answer. 
         [0037]    For this reason, the first steps are performed as in the previous example: calling terminal  1  sends a ringing message  401  to the calling terminal MSC  6  addressed to the called terminal CS number  19 . Calling terminal  6  then sends a request  402  for information to HLR/VLR  13 . HLR/VLR  13  sends the requested information  403  to the calling terminal MSC  6 , which sends a request for routing information  404  to SCP  9 . SCP  9  sends the requested routing information  405 , which includes the calling terminal dynamic IMS number  20  and the indication of first trying the CS network. Calling terminal MSC  6  connects  406  to called terminal MSC  6 ′, which tries to establish a connection  407  with the called terminal CS number  19 . In this example, CS number  19  cannot be reached, and calling terminal MSC  6  is thus informed of this event  408 . As a result, calling terminal MSC  6  tries to establish a connection through the IMS network  4 , and thus sends a call  409  to SGW  15  addressed to the called terminal dynamic IMS telephone number  20  (previously provided by SCP  9 ). SGW  15  checks  410  the DNS/ENUM  17 , receiving a response  411  from said DSN/ENUM. This response  411  allows the SGW  15  to ring  412  the CSCF  13 , which transmits the ringing  413  to the called terminal  2  through its IMS number  20 . Answer  414  is thus sent to the SGW  15 , which finally establishes the connection  415  with calling terminal  1 . 
         [0038]    Thanks to the disclosed method and system, the called terminal can be reached trough both CS and IMS networks, prioritizing any of them according to its user&#39;s preferences , and the calling terminal only needs a single telephone number to request the connection through said networks.