Abstract:
The invention relates to method of executing a service in a Media Gateway Control Function for establishing a call between a calling party&#39;s terminal served by a Mobile Switching Centre, and an IP Multimedia Subsystem Service. The Media Gateway Control Function is arranged to query a Service Control Point on which an overlay process is loaded. The correct SCP is identified by the MGCF using a correlation identifier originally published by the SCP itself after having received an IN service trigger from an MSC or GMSC. The MGCF will receive call related data from the SCP, such as the original dialled number. This data is used by the MGCF to populate information elements in a SIP Invite message. The invention gives a solution for providing GSM specific information, e.g. dialled number, location information, to an IMS service. This information may not be available to the IMS service through present methods.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to a method of routing a call to an IMS service. 
       BACKGROUND 
       [0002]    Registration Surrogate (RS) is a technique that enables an operator to register GSM subscribers or ISDN subscribers in an Internet Protocol. Multimedia Subsystem (IMS) network. The registered IMS subscribers can use GSM access, but have their services reside in the IMS network. The RS facilitates triggering of one or more IMS services when a call to/from such a GSM subscriber is routed from a GSM network to an IMS network. The routing of a call from GSM to IMS may be performed using the well known Customized Application of Mobile Network Enhanced Logic (CAMEL) “Route Home” service. The routing of a GSM call through IMS, for the purpose of executing one or more 1MS services in the IMS network, is known as “GSM-IMS overlay”. 
         [0003]    One example of an IMS service for which it is desirable to be accessible in the above mentioned manner is IP-Centrex. IP-Centrex is an IMS service offering Wireless Office features. One of the aspects of IP-Centrex is the possibility to access an IMS service with GSM terminals, as well as with Session Initiation Protocol (SIP) phones. 
         [0004]    Such IMS service is commonly referred to as Multi Access IMS service. The remainder of the description takes IP-Centrex as example IMS service. The invention is, however, equally applicable to any other IMS service that may be accessed through GSM, such as Call Completion or Incoming Call Screening. Furthermore, the invention is equally applicable to calls in fixed telecommunication networks, as in mobile networks such as GSM networks or UMTS networks. 
         [0005]    When applying CAMEL for the overlay between GSM and IMS, the called party number (CdPN), as dialled by the calling subscriber in the case of calls originating in the GSM network, can not always be maintained in the ISDN User Part (ISUP) signalling between a GSM network and an IMS network. This has mainly three reasons. 
         [0006]    Firstly, the CdPN that is carried in the ISUP Initial Address Message (IAM) is limited to 15 address digits, by ITU-T recommendation E.164. But 15 digits may not be sufficient to address a Media Gateway Control Function (MGCF) and also includes the original dialled number, especially when the GSM call is established from abroad or from a network of a different provider. Secondly, the number frame of the CdPN in ISUP contains a Nature of Address (NoA). When an MGCF address, required for routing the call to the MGCF, is added to the CdPN, the NoA in the frame of the ISUP 
         [0007]    CdPN has to be set to national or international, depending on whether the GSM call is established in the home country or from abroad or from a different provider. As a result, the NoA of the original dialled number gets lost. A third reason is that the user may include operator-specific service selection codes (* and #) in the dialled number. These codes can not be conveyed, through ISUP, to the IMS service. 
         [0008]    Also, when ISUP signalling traverses (inter)national network boundary, the Calling party number (CgPN) may get corrupted or lost. The CgPN is however needed to identify the calling subscriber and to trigger the IMS service. Hence, when the CgPN is not available or when it is corrupted, there is no IMS triggering possible and hence, the call can not be established through IMS overlay. 
         [0009]    Finally, when the IMS service is triggered, vital information such as Location Information is not available to the LMS service. 
       SUMMARY OF THE INVENTION 
       [0010]    A goal of the present invention is to provide a method for establishing a call between a calling party&#39;s terminal (e.g. fixed phone in a PSTN or ISDN, or a mobile phone in a PLMN) served by a Switching node and an IMS service that overcomes one or more of the problems mentioned above. 
         [0011]    This goal is achieved by providing a method of executing a service in a Media Gateway Control Function for establishing a call between a calling party&#39;s terminal served by a Switching Node, and an IP Multimedia Subsystem service. the method comprising:
   receiving a call initiation message from the Switching Node, the call initiation message comprising a called party number;   analysing the called party number to sec whether the call set up is requested for a connection to the IP Multimedia Subsystem service;   in the case that the call is a connection to said IP Multimedia Subsystem service, deriving from the called party number which Service Control Point is in charge;   querying said Service Control Point for data associated with the call;   processing the data in order to create a Session Initiation Protocol Invite message;   sending the Session Initiation Protocol Invite message to a Call Session Control Function.   
 
         [0018]    The Call Session Control Function will then further execute the call establishment process. 
         [0019]    In an embodiment, the called party number comprises:
   a country code of an operator of said IP Multimedia Subsystem Service;   a national destination code for said operator,   an address of said Media Gateway Control Function;   a correlation identifier identifying an overlay process on the Service Control Point.   
 
         [0024]    The data comprises one or more of the following:
   an original dialled number dialled at said calling party&#39;s terminal;   a calling party number associated with said call;   location information of said calling party;   a called party number associated with said called party;   location information of a called party associated with said called party number;   an International Mobile Subscriber identification;   an International Mobile Equipment Identity;   a subscriber state associated with said called party.   
 
         [0033]    According to an aspect of the invention, there is provided a Media Gateway Control Function for use in a telecommunication network, which Media Gateway Control Function is arranged to perform the above mentioned method. 
         [0034]    According to another aspect, there is provided a telecommunication network comprising at least one Media Gateway Control Function as described above. 
         [0035]    According to yet another aspect, there is provided a method of executing a service in a Service Control Point for establishing a call between a calling party&#39;s terminal served by a Switching Node and an IP Multimedia Service, the method comprising:
       receiving an IN service trigger from the Switching Node. the IN service trigger comprising data associated with the call;       storing the data;   starting up an overlay process for the call;   allocating a correlation identifier identifying the overlay process;   sending a destination subscriber number to the Switching Node;   on receiving a request from a Media Gateway Control Function, providing the data to the Media Gateway Control Function.   
 
         [0042]    The invention also relates to a Service Control Point for use in a telecommunication network, the Service Control Point being arranged to perform the method described above. 
         [0043]    Finally, the invention relates to a telecommunication network comprising at least a Service Control Point as described above. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0044]    The present invention will be discussed in more detail below, using a number of exemplary embodiments, with reference to the attached drawings, in which: 
           [0045]      FIG. 1  shows a network diagram which depicts GSM-IMS overlay for a mobile originated call according to the state of the art; 
           [0046]      FIG. 2  shows a network diagram which depicts GSM-IMS overlay for a mobile originated call according to an embodiment of the present invention; 
           [0047]      FIG. 3  depicts a network diagram which depicts GSM-IMS overlay for a mobile terminating call according an embodiment; 
           [0048]      FIG. 4  depicts a network diagram of the network of  FIG. 2  in which a call is established using an URI instead of a number; 
           [0049]      FIG. 5  is an example of a correlation identifier; 
           [0050]      FIG. 6  is a flow chart of actions taken by an MGCF according to an embodiment in order to establish a call to a GSM MAE subscriber; 
           [0051]      FIG. 7  is a flow chart of actions taken by an SCP according to an embodiment. 
       
    
    
     DETAILED DESCRIPTION 
       [0052]      FIG. 1  depicts a GSM network  9  together with (part of) an IMS network  11  according to the state of the art. In  FIG. 1 , functional blocks are depicted, each of which may be incorporated is a separate network node, but some of them may as well be combined into a single network node. The GSM network  9  comprises a serving Mobile Switching Centre (MSC)  2  and a Home Location Register (HLR)  4  which communicates with the serving MSC  2  via e.g. a SS7 network (not shown). Furthermore a Radio Access Network (RAN)  6  is shown via which a calling party may set up a call through the GSM network  9  using a mobile station, such as a mobile phone  8 . The serving MSC  2  is also communicating with a SCP  10  using CAMEL Application Part (CAP) as will be known by the skilled person. In the example of  FIG. 1 , the GSM network  9  is a Visited Public Land Mobile Network (VPLMN)  9 . The GSM network  9  is in communication with the IMS network  11  which comprises a MGCF  12 , a P-CSCF  14 , a S-CSCF  16  and an IMS service  20 . The P-CSCF  14  is the first contact point in the IMS network and it provides authorization of bearer resources. It also forwards a SIP register request received from a SIP User Agent (not shown) to an I-CSCF (not shown) determined using the home domain name, as provided by the IMS subscriber. In the case of Registration Surrogate (RS) the P-CSCF  14  receives the SIP register request from the RS. In the opposite direction it forwards the SIP request or response to a SIP Phone (not shown). In the case of RS, it forwards the SIP request IS or response to the MGCF  12 . Moreover the P-CSCF  14  forwards SIP messages received from the MGCF  12  to the S-CSCF  16 . 
         [0053]    Now, a typical call flow for a Mobile originated call establishment from a GSM subscriber having access to the IMS is described according to the state of the art. This GSM subscriber is also referred to as a GSM Multi Access Extension (MAE) subscriber. The subscriber registers in the MSC  2  in the VPLMN  9 , see  FIG. 1 . The subscriber has a profile in the HLR  4  for Centrex functionality. This profile consists of an IN service trigger named Originating CAMEL Subscription Information (O-CSI). The HLR  4  sends the O-CSI to the MSC  2 . The GSM MAE Subscriber initiates a Mobile Originated call. The subscriber may dial a short code (e.g. 4500) or a public number (e.g. number in international format +31 161 24 9911; or a number in unknown format 00 31 161 24 9911; 0161 24 9911). Since the GSM MAE subscriber has O-CSI, the MSC  2  triggers an IN service in the SCP  10 , i.e. the MSC  2  sends a CAMEL Application Part (CAP) Initial Detection Point (IDP) Operation to the SCP  10 . 
         [0054]    A CAP dialogue is established between the MSC  2  and the SCP  10 . The SCP  10  resides in the home network of the GSM subscriber. The operator that operates this home network, known as Home Public Land Mobile Network (HPLMN), may also be the operator that operates the IMS network. The address of the SCP  10  is derived from the O-CSI. Dynamic load sharing may be applied to the IN service triggering. 
         [0055]    The IN service may be the CAMEL Route Home service. It is noted that “CAMEL Route Home” is not official GSM terminology, but is commonly used for a CAMEL service that routes calls from a GSM subscriber to an entity in the home network. For the purpose of GSM-IMS overlay, the CAMEL Route Home service routes the call to the IMS network  11 , so the service is also referred to as “CAMEL Route to IMS” service. The IN service inserts a prefix in front of the original dialled number from the subscriber. The prefix results in routing the call to the MGCF  12  of the IMS network  11 . It should be noted that the CAMEL Route to IMS service may be deployed on two or more SCPs. An SS7 network (not shown) connecting the MSC  2  and the SCP  10  will route the service invocation to the SCP  10 . 
         [0056]    The SCP  10  determines that the subscriber is a GSM MAE subscriber (by virtue of a service key in the CAP IDP) and gives the MSC  2  an instruction to route the call to the MGCF  12  in the IMS network  11 . The IMS network II contains one or more MGCFs. The number of MGCFs is determined by, amongst others, the number of subscribers in the IMS network  11  and by the geographical size of the IMS network  11 . The SCP  10  will determine which MGCF to route the call to, e.g. based on the service key. 
         [0057]    The Called Party Binary Coded Decimal (BCD) Number (CdPBN) is sent from the mobile phone  8  to the MSC  2 , over the RAN  6 . It contains the number as dialled by the subscriber (i.e. entered on the keypad of the GSM phone). The MSC  2  converts the CdPBN to a Called Party Number (CdPN). Further call handling in the GSM network  9 , i.e. routing the call to the destination of this call (called party), is done with this CdPN. The SCP  10  will provide a different CdPN for this call, i.e. it overrides the CdPN that the MSC  2  had derived from the CdPBN, by a different number referred to as ‘destination subscriber number’. The SCP  10  provides this destination subscriber number to the MSC  2  in a CAP Connect operation. For the purpose of GSM-IMS overlay, the destination subscriber number has the following format: 
         [0000]      &lt;cc&gt;&lt;ndc&gt;&lt;mgcf address&gt;&lt;original dialled number&gt; 
         [0058]    whereby 
         [0059]    &lt;cc&gt;: country code of the IMS operator 
         [0060]    &lt;ndc&gt;: national destination code for the IMS operator 
         [0061]    &lt;mgcf address&gt;: address of the MGCF  12   
         [0062]    &lt;original dialled number&gt;: sequence of numbers dialled by the calling party 
         [0063]    The &lt;mgcf address&gt;consists of a number of digits to identify the MGCF  12  within the IMS network  11 . The concatenation of &lt;cc&gt;, &lt;ndc&gt;and &lt;mgcf address&gt;constitutes the aforementioned “prefix”. The SCP  10  inserts the prefix in front of the original dialled number and returns the result to the MSC  2 . 
         [0064]    The MSC  2  routes the call to the MGCF  12  of the IMS network  11 . The routing of the call to the MGCF  12  is done in accordance with standard ISUP routing methods. The Called Party Number (CdPN) in the ISUP IAM contains the destination subscriber number that was provided by the SCP  10  in the CAP Connect operation. The MGCF  12  will then receive the ISUP IAM and applies ISUP to SIP conversion. The MGCF  12  generates a SIP Invite message and sends the SIP Invite to the P-CSCF  14 . By using a designated IP port address, the SIP Invite at the P-CSCF  14  will be interpreted as User-to-Network Interface (UNI) signalling. Alternatively, this SIP Invite may be sent to a dedicated P-CSCF used for GSM-IMS overlay. 
         [0065]    The GSM MAE subscriber is registered in the P-CSCF  14 . The GSM MAE subscriber is also registered in the S-CSCF  16 . The registration in the P-CSCF  14  and S-CSCF  16  is performed by the Registration Surrogate. The P-CSCF  14  forwards the SIP Invite to the S-CSCF  16  where this subscriber is registered. The S-CSCF  14  has so-called Initial Filter Criteria (IFC) stored for this subscriber. It will use the IFC to trigger the IP-Centrex service. The Request Universal Resource Identifier (Request URI, R-URI) in the SIP Invite contains the original dialled number. The IMS service handles the call by this subscriber, depending on the subscriber&#39;s profile in the IMS service. 
         [0066]    When applying overlay between GSM and IMS, the original dialled number can not always be maintained in the ISDN User Part (ISUP) signalling between the GSM network  9  and the IMS network  11 . This is caused by the fact that the CdPN in the ISUP IAM is limited to a predetermined number of address digits (e.g. in ITU-T recommendation E.164, the number is  15 ). According to an embodiment of the present invention, this problem is solved by letting the SCP  10  and the MGCF  12  communicate with each other so that the MGCF  12  can retrieve the original dialled number from the SCP  10  during call establishment, see  FIG. 2 .  FIG. 2  depicts a typical call flow for a Mobile Originated call establishment from the GSM network  9 , including an enhancement described in the present invention. 
         [0067]    In  FIG. 2  corresponding reference numbers arc used for corresponding functional blocks of  FIG. 1 . According to this embodiment, the SCP  10  is arranged to store the information it has received from the MSC  2  in the CAP IDP. This information may include, the CdPBN, the CgPN, an International Mobile Subscriber Identification (IMSI), an International Mobile Equipment Identity (IMEI) and Location Information. 
         [0068]    In a particular embodiment of the present invent ibn, the SCP  10  and the MGCF  12  are arranged to communicate the CgPN via an IP based network using the SOAP protocol to communicate XML messages (referred to as SOAP/XML query), see  FIG. 2 . If the CgPN gets lost in the ISUP signalling from the MSC  2  (in the VPLMN  9 ) to the MGCF  12  (in the IMS network  11 ), then the CgPN will be restored through a SOAP/XML query from the MGCF. In an embodiment, the SCP  10  is arranged to provide a destination subscriber number in a CAP Connect (CON) operation which has the following format: 
         [0000]      &lt;cc&gt;&lt;ndc&gt;&lt;mgcf address&gt;&lt;correlation identifier&gt; 
         [0069]    whereby 
         [0070]    &lt;cc&gt;: country code of the IMS operator 
         [0071]    &lt;ndc&gt;: national destination code for the IMS operator 
         [0072]    &lt;mgcf address&gt;: address of the MGCF  12   
         [0073]    &lt;correlation identifier&gt;: a process identifier of a GSM-IMS overlay process on the SCP  10 . 
         [0074]    Please note that the original dialled number is not part of this destination subscriber number as was the situation in the SCP according to the state of the art. The correlation identifier is allocated by the SCP  10 . The SCP  10  has a pool (i.e. a list) of correlation identifiers. If there are multiple SCPs for this CAMEL Route to IMS service, then each SCP  10  has a dedicated range of correlation identifiers. 
         [0075]    The correlation identifier may contain over-decadic digits. The use of over-decadic digits prevents that GSM users (or PSTN users) can manually dial a correlation identifier. 
         [0076]    Once the SCP  10  has connected the call, it will release the CAP dialogue with the MSC  2 . The MSC  2  routes the call to the MGCF  12  in the IMS network  11 . The routing of the call to the MGCF  12  is done in accordance with standard ISUP routing method. The MSC  2  sends an ISUP IAM to the MGCF  12 . The IAM contains the parameter CdPN. The MSC  2  copies the content of the destination subscriber number from CON into the parameter CdPN in the ISUP IAM. Hence, the CdPN in the ISUP IAM now contains the prefix plus the correlation identifier. The combination of the mgcf address and the correlation identifier indicates to the MGCF  12  that this call is a GSM multi access call. The MGCF  12  is arranged to store a list comprising possible combinations of the mgcf address and the correlation identifier. Said list may consist of one or more number ranges; each number range may consist of the mgcf address plus a range of correlation numbers. When an ISUP IAM is received, the MGCF  12  will look in the list and if it finds a matching combination, it will conclude that this is a call from a GSM MAE subscriber. Now, the MGCF  12  shall in response contact the SCP  10 . The SCP address is derived from the correlation identifier. Since the SCP  10  and the MGCF  12  arc operated by the same operator, the SCP address(es) can be configured in the MGCF  12 . After having derived the SCP address, the MGCF  12  uses SOAP/XML, to query the SCP  10 . The MGCF  12  includes the correlation identifier in the SOAP/XML query. In an embodiment, the MGCF  12  is arranged to obtain data from the SCP  10 , using SOAP to do a remote procedure call towards the SCP  10 . The query towards the SCP  10  (e.g. ‘provide call data’), the parameter for this remote call (e.g. ‘correlation identifier’) and the return parameters (e.g. ‘original dialled number’, ‘calling party number’ etc.) arc represented in XML. 
         [0077]    The SCP  10  uses the correlation identifier to retrieve the stored call information. The SCP  10  returns the data to the MGCF  12 . After that, the SCP  10  purges the stored data and returns the correlation identifier to the pool. The correlation identifier may now be re-used. 
         [0078]    The correlation identifier is used only for the duration of call routing between MSC  2  and MGCF  12  and the interrogation from the MGCF  12  to the SCP  10 . Hence, a small number of correlation identifiers may suffice. 
         [0079]    The SCP  10  will also supply the address of the P-CSCF  14  to the MGCF  12 . The GSM MAE subscriber is registered in one of a number of P-CSCFs. The SCP  10  may be configured with the P-CSCF address for the subscriber, or the P-CSCF address may be derived from the service key. The SCP  10  may have received the P-CSCF address from the Registration Surrogate (RS) after the RS had registered the subscriber in the IMS network  11 . 
         [0080]    Next, the MGCF  12  applies an ISUP to SIP conversion. The MGCF  12  generates a SIP Invite and populates the SIP Invite with the information that it has received in the SOAP/XML response, such as original dialled number and CgPN. The MGCF  12  sends the SIP Invite to the P-CSCF  14 . By using a designated IP port address, the SIP Invite at the P-CSCF  14  will be interpreted as User-to-Network Interface (UNI) signalling. The IMS network  11  then handles the call as was described above with reference to  FIG. 1 . 
         [0081]    Thanks to the communication between the MGCF  12  and the SCP  10 , there will be no loss of information in the CgPN. 
         [0082]    The signalling link between User and P-CSCF may be regarded as UNI, for the purpose of connecting the GSM subscriber to the IMS network. Once the call has arrived at the S-CSCF  16  of the IMS network  11 , the call will be handled in accordance with the user&#39;s profile. 
         [0083]    In an embodiment, the triggering of the CAMEL route to IMS service may pass through a Service Capability Integration Manager (SCIM) situated between the MSC  2  and the SCP  10 . Likewise, the ISC interface between S-CSCF  16  and the IP-Centrex may pass through a SCIM (not shown). The SCIM is arranged to call and integrate a plurality of services using a single trigger from the GSM network  9  or from the IMS network  11 . 
         [0084]    The CAMEL Route to IMS service and the IMS service have no correlation with one another. Hence, the IMS service is not dependent on access to the SCP  10  in the GSM network  9 . The required information is available in the SIP Invite coming from the S-CSCF  16 . In this manner, other IMS services may use the Location Information and IMSI and other GSM-specific data in the SIP Invite. 
         [0085]    Table I provides an overview of the GSM access specific information that is placed in the SIP Invite sent by the MGCF  12  to the IMS network  11  according to an embodiment. 
         [0000]    
       
         
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 GSM element 
                 SIP Invite header 
               
               
                   
               
             
             
               
                 Calling Party Number 
                 P-Asserted-Identity 
               
               
                 Called Party BCD Number 
                 Request-URI 
               
               
                 Location Information 
                 P-Access-Network-Info; see 3GPP TS 24.229 for mapping 
               
               
                   
                 between Location Information and P-Access-Network-Info. 
               
               
                 IMSI 
                 Pivate user identity; 3GPP TS 23.003 specifies how the Pivate 
               
               
                   
                 user identity may be derived from IMSI. 
               
               
                 Time and Time zone 
                 Timestamp; RFC 3261 defines how Time and Time zone from 
               
               
                   
                 the Serving MSC may be converted to the Date format and the 
               
               
                   
                 Timestamp formal for SIP. 
               
               
                 MCC + MNC 
                 P-Visited-Network-ID; the SCP may apply mapping between 
               
               
                   
                 the MCC + MNC, as received in CAP IDP, to a text string 
               
               
                   
                 representing the GSM operator. 
               
               
                 MSC Address 
                 P-Access-Network-Info; see RFC 3455 for a description of this 
               
               
                   
                 SIP header. It may be included in SIP Register, SIP Invite and 
               
               
                   
                 other SIP methods. 
               
               
                 Call Reference Number 
                 Call-Id; the Call-Id identifies a single call. Copying the GSM 
               
               
                   
                 Call Reference Number into this field has the advantage that 
               
               
                   
                 CDRs that are generated in the GSM network may be 
               
               
                   
                 correlated with charging function in the IMS network. 
               
               
                 IMEI 
                 The IMEI of the calling subscriber is reported to the SCP in 
               
               
                   
                 CAP IDP. This parameter is mapped on a SIP header. 
               
               
                 MS Classmark 
                 The MS Classmark of the calling subscriber is reported to the 
               
               
                   
                 SCP in CAP IDP. This parameter is mapped on a SIP header. 
               
               
                   
               
             
          
         
       
     
         [0086]    Now, an example of the sequence of events for a Mobile Terminated call establishment to a GSM MAE subscriber will be discussed with reference to  FIG. 3 . A call arrives at a Gateway MSC (GMSC)  30  for the GSM MAE subscriber. The routing of the call to the GMSC  30  follows GSM principles as will be known to the skilled person. The GSM MAE subscriber may e.g. be addressed with MSISDN. 
         [0087]    The GMSC  30  contacts the HLR  34  to obtain routing information for this call. The subscriber is registered as GSM MAE subscriber in the HLR  34 , i.e. the HLR  34  has terminating IN trigger information for the subscriber. This information consists of CAMEL subscription information (T-CSI). An optional part of terminating call handling in the HLR  34  for subscribers with CAMEL information is the obtaining of the subscriber&#39;s location information and state. Hereto, the HLR  34  contacts a Visitor Location Register (VLR) where the subscriber is currently registered. 
         [0088]    The HLR  34  returns the IN service subscription information (T-CSI) and the subscriber&#39;s location and state information, if obtained from the VLR, to the GMSC  30 . In this example, the GMSC  30  uses the T-CSI to invoke a CAMEL Route to IMS service on an SCP  36 . Hereto, the GMSC  30  sends a CAP IDP to the SCP  36 . The CAP IDP contains, amongst others, the Called Party Number, Calling Party Number (if available), IMSI and Location Information (if available). 
         [0089]    The SCP  36  is arranged to store the information received in the CAP IDP and allocate a correlation identifier. For a description and use of the correlation identifier, reference is made to the above description of the Mobile originated call, see  FIG. 2 . 
         [0090]    The SCP  36  routes the call to an MGCF  38  of an IMS network  40 , by sending CAP Connect to the GMSC  30 . The CAP Connect contains the address of the MGCF  38  and the correlation identifier. The GMSC  30  routes the call to the MGCF  38 , using the information received in the CAP Connect operation. The MGCF  38  deduces from the &lt;mgcladdress&gt;and the &lt;correlation identifier&gt;in the Called Party Number in the ISUP IAM that this call is a call to a GSM MAE subscriber. Therefore, the MGCF  38  uses SOAP/XML to query the SCP  36 . The address of the SCP  36  is derived from the correlation identifier. 
         [0091]    The SCP  36  uses the correlation identifier to retrieve the stored data. The SCP  36  sends the original called number to the MGCF  38 . In addition, the SCP  36  provides the called party&#39;s location information to the MGCF  38 . The calling party number was also stored in the SCP  36 , but this number will normally remain in the ISUP signalling from the GMSC  30  to the MGCF  38 , because the GMSC  30  and the MGCF  38  are located in the same network. So, the calling party number does not need to be restored. However, if the GMSC  30  is located in another network than the MGCF  38 , the calling party number may get lost in the ISUP signalling between the GMSC  30  and the MGCF  38 . Therefore, according to an embodiment, the SCP  36  is arranged to also provide the calling party number to the MGCF  38 . 
         [0092]    In an embodiment, a designated SIP header is defined for carrying the Location Information of the called subscriber. The P-Access-Network-Info strictly applies to the calling party. In addition, the SCP  36  may provide the following information related to the called party:
       IMSI, and   Subscriber state.       
 
         [0095]    For these parameters designated headers in a SIP Invite message may be defined. The MGCF  38  will now have all information to decide that the call is destined for a GSM MAE subscriber. The MGCF  38  can therefore generate a SIP Invite message and send the SIP Invite message to a predefined I-CSCF  42 . The MGCF  38  includes the called party&#39;s location information, and other information elements such as IMSI and Subscriber state, in the SIP Invite message. 
         [0096]    The I-CSCF  42  interrogates a Home Subscriber System (HSS)  44  to obtain the called subscriber address of a S-CSCF  46 . The subscriber is registered in the IMS network  40 , hence the HSS  44  has the S-CSCF address for this subscriber. 
         [0097]    When the I-CSCE has received the S-CSCF address from the HSS  44 , it routes the call to the S-CSCF  46 , i.e. sends the SIP Invite to the S-CSCF  46 . The subscriber is registered in the S-CSCF  46 , hence the S-CSCF  46  has Initial Filter Criteria (IFC) available for this subscriber. The S-CSCF  46  will now invoke a corresponding IMS service  50 . In the current example, the IMS service  50  is a multi access IMS service. 
         [0098]    The multi access IMS service handles the call to this subscriber, depending on the subscriber&#39;s profile in the multi access service. The triggering of the CAMEL Route to IMS service as well as the IMS service triggering may pass through a SCIM (not shown) as was described with reference to  FIG. 2 . The multi access IMS service is not dependent on access to the SCP  36  in the GSM network. The required information, notably the called party&#39;s location, is available in the SIP Invite message coming from the MGCF  38 . In this manner, other IMS services, such as charging, Voice Call Continuity and Multimedia Telephony, may also use the Location Information (and other information elements such as IMSI) in the SIP Invite message. This enables the IMS service  50  to perform e.g. terminating call service functionality that needs a called party&#39;s location. 
         [0099]    It is noted that the invention is not limited to the use of mobile networks, and that the invention also applies to fixed telephone networks, such as PSTN or ISDN. In that case the MSC  2  will be replaced by a Switching Node of a fixed telecommunication network, and the callings party&#39;s mobile station  8  by a calling party&#39;s fixed terminal. 
         [0100]      FIG. 4  shows the communication network of  FIG. 2 , in which corresponding reference numbers refer to corresponding functional blocks. In  FIG. 4 , with a call establishment by a GSM subscriber according to an embodiment is depicted using a Universal Resource Identifier (URI). In  FIG. 4 , the URI “matsson@telia.se” is used as the CdPBN. In this embodiment, the concept of having the MGCF  12  obtain the original dialled number from the SCP  10 , is used in combination with a method by means of which a GSM subscriber establishes a mobile originated call, using a URI instead of a number. E.g. set up a call to matsson@telia.se. 
         [0101]    in an embodiment, the URI that is used by the calling GSM subscriber to establish the call is provided by the SCP  10  to the MGCF  12 . In that manner, the GSM subscriber can use SIP URI calling, even when the GSM call is routed to the IMS network  11  for further processing. 
         [0102]    In  FIG. 5 , an example is given of a correlation identifier identifying the GSM-IMS-overlay process on the SCP  10 . The correlation identifier comprises 5 digits, four of which define the GSM-IMS overlay process on the SCP  10 . The first digit is used to define the address of the SCP. In this example, the SCP  10  having an address “A” deals with the call. And process “4687” is in charge of the routing of the call with the MGCF  12 . 
         [0103]      FIG. 6  is a flow chart of actions taken by the MGCF  12  according to an embodiment in order to establish a call from a GSM MAE subscriber. In a first step  61 , the MGCF  12  receives a call initiation message from the MSC  2  comprising a called party number. This may be an ISUP IAM as :described above. The CdPN in the ISUP IAM is interpreted by the MGCF  12  in a step  62 . In a step  63 , it is tested whether the call is a call to an IMS service. This testing is done by looking at the combination of the mgcf address and the correlation identifier as described above. lithe conclusion is NO, then normal call handling is continued, sec step  68 . If the conclusion is YES it is a call from a GSM MAE subscriber, a step  64  follows in which the MGCD  12  derives from the CdPN which SCP is in charge. In other words, which SCP has allocated the correlation identifier and runs an overlay process for this call. Since the destination subscriber number is contained in the CdPN, the MGCF  12  is able to derive the correlation identifier. The correlation identifier is then used to query the SCP  10  using for example SOAP/XML. Alternatively, the MGCF  12  may use Lightweight Directory Access Protocol (LDAP) to query the SCP  10  or any other suitable protocol known to the skilled person. The SOAP protocol may be used in order to communicate the SOAP/XML query to the SCP  10  and to communicate a SOAP/XML response to the MGCF. The SCP  10  is queried for data associated with the call, see step  65 . This data was stored by the SCP  10  when it received the IN trigger from the MSC  2 . Next in a step  66 , a SIP Invite message is created using the data received from the SCP  10 . Finally, the SIP Invite message is sent to the CSCF  14 , see step  67 . 
         [0104]      FIG. 7  is a flow chart of actions taken by the SCP  10  according to an embodiment. In a first step  71 , the SCP  10  receives an IN trigger from the MSC  2 . The IN trigger comprises data such as the original dialled number, which is stored by the SCP  10  in a memory of the network node on which the SCP  10  functions, see step  72 . It is noted that the data may be stored on several SCPs, as will be appreciated by the skilled person. Next, in a step  73 , an overlay process is started for the call. This overlay process will run on the SCP  10  until the MGCF  12  has received the call related data from the SCP  10 . The SCP  10  will also allocate a correlation identifier for the call, see step  74 . The correlation identifier identifies the overlay process as was discussed with reference to  FIG. 5 . Next, the SCP  10  sends a destination subscriber number to the MSC  2 . This is done in the CAP Connect operation. lf a request is received from an MGCF  12 , sec step  76 , the overlay process running on the SCP  10  will deal with the request. The overlay process will access the memory on which the call related data was stored. This data is then provided to the requesting MGCF  12 , which will use it to restore the information it received from the MSC  2 . 
         [0105]    It is noted that the SCP  10 ,  36  may constitute a service execution platform embodied as a single node or comprised as a functional entity within another node. 
         [0106]    The present invention has been explained above with reference to a number of exemplary embodiments. As will be apparent to the person skilled in the art, various modifications and amendments can be made without departing from the scope of the present invention, as defined in the appended claims. Instead of using GSM, UMTS may be used as will be appreciated by the skilled person. Also it is noted that other IN services may be used and that the invention is not limited to the use of CAMEL. Furthermore, it is noted that the invention can also be used in a call establishment from a fixed telecommunication network to an IMS service. 
       LIST OF ABBREVIATIONS 
       [0107]    CAMEL Customized Application of Mobile Network. Enhanced Logic 
         [0108]    CAP CAMEL Application Part 
         [0109]    CdPBN Called Party Binary coded decimal Number 
         [0110]    CdPN Called Party Number 
         [0111]    CDR Call Detail Record 
         [0112]    CgPN Calling Party Number 
         [0113]    CSCF Call Session Control Function 
         [0114]    CSI Combination of CS and IMS services or CAMEL Subscription Information 
         [0115]    GMSC Gateway MSC 
         [0116]    HPLMN Home PLMN 
         [0117]    HSS Home Subscriber System 
         [0118]    IAM Initial Address Message 
         [0119]    IMS IP Multimedia Subsystem 
         [0120]    IP Internet Protocol 
         [0121]    IPLMN Interrogating PLMN 
         [0122]    ISC IMS Service Control interface 
         [0123]    ISDN Integrated Services Digital Network ISUP ISDN User Part I-CSCF Interrogating CSCF 
         [0124]    MGCF Media Gateway Control Function 
         [0125]    MS Mobile Station 
         [0126]    MSC Mobile Switching Centre 
         [0127]    O-CSI Originating CAMEL Subscription Information 
         [0128]    PLMN Public Land Mobile Network 
         [0129]    PS Packet Switched 
         [0130]    PSTN Public Switched Telephone/Telecommunications Network 
         [0131]    P-CSCF Proxy CSCF 
         [0132]    RAN Radio Access Network 
         [0133]    R-URI request URI 
         [0134]    SCIM Service Capability Integration Manager 
         [0135]    SCP Service Control Point 
         [0136]    SIP Session Initiation Protocol 
         [0137]    SIP-URI SIP Universal Resource Identifier 
         [0138]    SOAP Simple Object Access Protocol 
         [0139]    S-CSCF Serving CSCF 
         [0140]    SR Surrogate Registration 
         [0141]    T-CSI Terminating CAMEL Subscription Information 
         [0142]    UNI User-to-Network Interface 
         [0143]    URI Universal Resource Identifier 
         [0144]    UTRAN Universal Terrestrial Radio Access Network 
         [0145]    VPLMN Visited PLMN 
         [0146]    XML Extended Mark-up Language