WO2010045960A1

WO2010045960A1 - Handling set up of a session in a gsm-ims overlay network

Info

Publication number: WO2010045960A1
Application number: PCT/EP2008/008987
Authority: WO
Inventors: Rogier August Caspar Joseph Noldus
Original assignee: Telefonaktiebolaget Lm Ericsson (Publ)
Priority date: 2008-10-21
Filing date: 2008-10-21
Publication date: 2010-04-29

Abstract

The invention relates to a method of handling set-up of a session for providing a telecommunications service to a user in a telecommunications system. The user is registered in a packet switched domain of said telecommunications system for receiving services provided from said packet switched domain, and uses a terminal unit which is operatively connected with a circuit switched domain for accessing said telecommunications system. A network node receives a session establishment message and sends a query towards a subscriber database node for receiving routing data. The query is intercepted by a signalling relay function node. Prior to sending the query, the network node determines whether the session establishment message was processed by the packet switched domain. Dependent on the result, the query comprises either a first type of request for receiving circuit switched domain routing data, or a second type of request for receiving packet switched domain routing data. In addition, the present invention is directed to a network node and signalling relay function node for use in such methods, and methods of operating these nodes, as well as a telecommunications service provided using any of the methods described above.

Description

Title:

HANDLING SET UP OF A SESSION IN A GSM-IMS OVERLAY NETWORK

DESCRIPTION

Technical field

The present invention relates generally to a method of handling set up of a session for providing a telecommunications service to a user in a telecommunications system, wherein the user is registered in a packet switched domain of said telecommunications system for receiving services provided from said packet switched domain, and wherein said user uses a terminal unit which is operatively connected with a circuit switched domain for accessing said telecommunications system.

In addition, the present invention is directed to a method of operating a network node in a circuit switched domain for a telecommunications system for handling session set up for providing a telecommunications service to a user of said telecommunications system as referred to hereinabove. The present invention is further directed to a method of operating a signalling relay function node in a telecommunications system for handling session set up for providing a telecommunications service to a user of said telecommunications system using the method as described above.

In addition, the present invention is directed to a network node and signalling relay function node for use in such methods, as well as a telecommunications service provided using any of the methods described above.

Background

In telecommunications industry, operators are currently integrating the global system for mobile communications (GSM) networks and internet protocol (IP) multimedia subsystem (IMS) networks. Some existing methods of integration comprise multi access extension (MAE) and GSM - IMS overlay.

MAE entails that a GSM subscriber, accessing the telecommunications system through the GSM network, is registered in the IMS network for using services thereof. Calls to and from the MAE subscriber are routed through IMS and may trigger IMS services. GSM - IMS overlay entails that calls to the GSM subscriber (i.e. the GSM - IMS overlay subscriber) are routed through IMS and may trigger IMS service(s). The service network may be defined as the network where service triggering takes place and where services are executed. For the MAE subscriber or the GSM - IMS overlay subscriber, the IMS network constitutes the service network. The access network may be defined as the network that provides access from the subscriber's terminal towards the service network. For the MAE subscriber or the GSM IMS overlay subscriber, the GSM network constitutes the access network. In the remainder of the present document, the MAE subscriber and the GSM - IMS overlay subscriber are jointly referred to as GSM-IMS subscriber.

When a gateway mobile switching centre (GMSC) in a GSM network is processing an incoming call to a GSM-IMS subscriber for the use of telecommunications services, this call must be routed by the GMSC to the IMS network, since IMS network is the service network for this subscriber. Once the IMS network has processed a call for a GSM-IMS subscriber and the call is routed back to the GSM network for delivering the call to the GSM-IMS subscriber, the GMSC in the GSM network needs to route the call to the visited mobile switching centre (VMSC) of this GSM-IMS subscriber.

The standard way for the GMSC to process a call is to send a mobile application part (MAP) send routing information (SRI) query to the home location register (HLR). Since the circuit switched network is developed to handle calls completely within the circuit switched network, the HLR and GMSC are not geared to routing calls occasionally to a packet switched domain and occasionally within the circuit switched domain for a same subscriber. In order to provide this functionality anyway, operators often presently 'abuse' the terminating IN category key (TICK) functionality for achieving this goal. The TICK parameter is intended to be used for invoking IN services to users. Using the TICK parameter for the purpose of integrating circuit switched networks and IMS networks may be implemented as follows.

When a GMSC in a GSM network is processing an incoming call for a GSM-IMS subscriber, it sends a MAP SRI message to the HLR. The HLR returns a TICK parameter to the GMSC. The TICK parameter is used by the GMSC to route the call to the IMS. When, however, a GMSC in a GSM network is processing a call for a GSM-IMS subscriber, which call has already been processed by the IMS network, it sends a MAP SRI message to the HLR and indicates "suppress TICK' in the MAP SRI message. The HLR will now return the mobile station roaming number (MSRN) to the GMSC, so the call can be routed to the destination subscriber.

Although this method provides currently a working approach, it has its limitations. One limitation is for example that it is difficult or even impossible to invoke an IN Service from the GMSC when the GMSC is routing the call to the destination VMSC. This is because the 'suppress TICK' provided by the GMSC in the MAP SRI message prevents invoking such services. An alternative solution is therefore desired.

Summary It is an object to obviate the above mentioned disadvantage and provide an improved method and means for integrating circuit switched networks with IP multimedia subsystem (IMS) networks.

The present invention achieves the above mentioned object by providing a method of handling set up of a session for providing a telecommunications service to a user in a telecommunications system. Within this telecommunications system, the user is registered in a packet switched domain of the telecommunications system for receiving services, which services are provided from the packet switched domain. The user uses a terminal unit which is operatively connected with a circuit switched domain of the telecommunications system. The circuit switched domain is used for accessing the telecommunications system. For performing the session set up, the method comprises the steps indicated below. A network node receives a session establishment message and sends a query towards a subscriber database node for receiving routing data, for forwarding the session establishment message for a setting up of the session. A signalling relay function node intercepts the query. The method is characterized in that it comprises the following further steps. The network node determines whether the received session establishment message has been forwarded from the packet switched domain prior to receiving the session establishment message by the network node. Dependent on the result of the step of determining, the query forwarded towards the subscriber database node comprises either a first type of request or a second type of request. A first type of request is a request for receiving circuit switched domain routing data for forwarding the session establishment message within the circuit switched domain. The second type of request is a request for receiving packet switched domain routing data for forwarding the session establishment message within the packet switched domain.

The above mentioned method provides the advantage that by analysing and determining whether the received session establishment message is forwarded from the packet switched domain, the network node is able to distinguish whether the session establishment message is to be routed within the packet switched domain, or whether it is to be routed within the circuit switched domain. By having this information, the network node is able to specifically request the required routing data to either the packet switched domain or the circuit switched domain. This allows the SS7 signalling network to provide an accurate response for routing the session establishment message. Most important, dynamic routing may be performed without preventing the use of intelligent network services to be invoked for the terminating call.

Practically the subscriber database will be the subscriber database node in the circuit switched network. However, the skilled person will appreciate that it may be any suitable subscriber network to which the query is sent by the network node. The phrase 'the user is registered in the packet switched domain' is to be interpreted in that the user is allowed to receiving services from the packet switched domain and should as such be known within the premisses of the packet switched domain, e.g. registered. The term 'to register' may also refer to the actual act registration (i.e. identifying a user terminal as active and ready to receive services from the network, e.g. upon switching on the terminal), also known as 'provisioning', which is required in order to be able to receive services from the network at all.

Hereinbelow, the embodiments relating to the dependent claim of the present application will be discussed. It is specifically indicated that the elements of the dependent claims are optional for the invention. In a specific embodiment, the signalling relay function node responds to the query by returning routing data for routing to the packet switched domain if the query comprises the second type of request.

The signalling relay function (SRF) node is an entity in the signalling system 7 (SS7) network). It takes care of among others routing the MAP SRI message to the right HLR. One rationale of deploying an SRF is that the mobile station international subscriber directory number (MSISDN), that is used in the MAP- SRI message to identify the destination subscriber, cannot be mapped to a specific HLR. In addition, mobile number portability has the effect that it cannot be derived from an MSISDN to which network it actually belongs. Therefore, the SRF intercepts the MAP SRI query message and routes it to the right HLR. The SRF uses a subscriber database. The database contains an entry for each subscriber of the network. If the specified MSISDN does not belong to the network wherein the GMSC is operating, then the SRF returns a network routing number (NRN) to the GMSC. The GMSC uses the NRN to route the call to the correct public land mobile network (PLMN), i.e. the PLMN wherein the called subscriber is provisioned as GSM subscriber.

The signalling relay function (SRF) node allows for adding additional intelligence and functionality to the SS7 signalling performed in the circuit switched network. The signalling relay function (SRF) node in the present embodiment intercepts the query provided by the network node, i.e. the MAP SRI query message, and based on the contents of this query it responds to the query by returning routing data to the packet switched domain. The SRF simply determines whether the query comprises a second type of request as is mentioned above. According to a further embodiment of the invention, the signalling relay function (SRF) node relays the query to the correct subscriber database node within the circuit switched domain if the query comprises a first type of request. In this embodiment, when the query comprises a first type of request, the signalling relay function simply leaves responding to the query to the HLR. The intercepted query is forwarded to the right HLR selected by the SRF, and may be left unaltered by the SRF.

In a preferred embodiment of the invention, the step of determining is performed by the network node by detecting the presence or absence of a prefix in the received session establishment message. The media gateway control function (MGCF) in the IMS network applies, as per current method in operation, a prefix in the called party number in the integrated services digital network (ISDN) user part (ISUP) initial address message (IAM) when routing a call to the GMSC for delivering that call to the GSM-IMS subscriber. The prefix is based on the destination address in the SIP Invite message received from the IMS network. By detecting the presence or absence of a prefix in the received session establishment message, the network node within the circuit switched domain is able to determine whether or not the incoming session establishment message has been forwarded by the packet switched domain. If the prefix is present, the session establishment message is coming from the packet switched domain. As a result of the nature of service provisioning to GSM-IMS subscribers, service logic already has been applied by the service network. The session establishment message, received by the network node in the circuit switched domain, has been forwarded there because the call is a terminating call. This requires forwarding the session establishment message within the circuit switched network. Upon receiving the session establishment message including a prefix, the network node may thus transmit a query towards the subscriber database node comprising a first type of request, for receiving circuit switched domain routing data. The benefit of the above described method is that it may be implemented by adding a (software or hardware) facility to the network nodes that determines the presence of such a prefix and controls the type of query being sent by the network node. Moreover this message no longer requires 'abuse' of the terminating IN category key (TICK). As a result, this method enables the option to invoke terminating IN services, which is not possible using the prior art methods described above.

The packet switched domain may comprise an internet protocol (IP) multimedia subsystem (IMS) network, or any other suitable type of packet switched network cooperating with the circuit switched network as described above. The circuit switched network may be any regular type of Integrated Service Digital Network (ISDN) or another suitable circuit switched type of network.

In addition, the network node may comprise at least one element of a group comprising mobile switching centre (MSC), service control point (SCP), short message service centre (SMSC), and service switching point (SSP). According to a second aspect, the invention is directed to a method of operating a network node in a circuit switched domain of a telecommunication system for handling session set-up for providing a telecommunications service to a user of said telecommunications system, using a method as described above, wherein said user is registered in a packet switched domain of said telecommunications system for receiving services provided from said packet switched domain, and wherein said user uses a terminal unit which is operatively connected with said circuit switched domain for accessing said telecommunications system, said method comprising a step of: said network node receiving a session establishment message and forwarding a query to a subscriber database node for receiving routing data for forwarding said session establishment message for setting up said session; characterized in that, said method further comprises the step of said network node determining whether said received session establishment message has been forwarded from said packet switched domain prior to receiving of said session establishment message by said network node; and said network node dependent on a result of said step of determining, including in said query either a first type of request for receiving circuit switched domain routing data for forwarding said session establishment message within said circuit switched domain, or a second type of request for receiving packet switched domain routing data for forwarding said session establishment message within said packet switched domain.

According to a third aspect the present invention provides a method of operating a signalling relay function node in a telecommunications system for handling session set-up for providing a telecommunications service to a user of said telecommunications system, using a method according to the first aspect of the invention, wherein said user is registered in a packet switched domain of said telecommunications system for receiving services provided from said packet switched domain, and wherein said user uses a terminal unit which is operatively connected with a circuit switched domain for accessing said telecommunications system, said method comprising a step of: said signalling relay function node intercepting a query sent from a network node towards a subscriber database node for receiving routing data for forwarding a session establishment message by said network node for setting up said session; characterized in that, said method further comprises the steps of: said signalling relay function analysing said query for determining whether said query comprises either a first type of request for receiving circuit switched domain routing data for forwarding said session establishment message within said circuit switched domain, or a second type of request for receiving packet switched domain routing data for forwarding said session establishment message within said packet switched domain; said signalling relay function relaying said query to said subscriber database node if said query comprises said first type of request, or said signalling relay function providing to said network node in response to said query said routing data for routing said session establishment message to said packet switched domain if said query comprises said second type of request. According to a fourth aspect, the present invention is directed to a network node for use in a method according to the first aspect of the invention, wherein said network node is arranged for handling set-up of a session within a circuit switched domain of a telecommunications system for providing a telecommunications service to a user of said telecommunications system, wherein said network node comprises an input-output unit operatively connected with said telecommunications system for receiving and forwarding a session establishment message, and a querying unit for composing a query for receiving routing data for forwarding said session establishment message, and a communication unit for forwarding said query towards a subscriber database node of said telecommunications system; characterized in that, said network node further comprises processing unit for determining whether a received session establishment message has been forwarded from a packet switched domain of said telecommunication system prior to receipt of said session establishment message by said network node; and wherein said querying means are arranged for including in said query, dependent on a result of said step of determining, either a first type of request for receiving circuit switched domain routing data for forwarding said session establishment message within said circuit switched domain, or a second type of request for receiving packet switched domain routing data for forwarding said session establishment message within said packet switched domain. According to a fifth aspect the present invention is directed to a signalling relay function node for use in a method according to the first aspect, wherein said signalling relay function node comprises an input-output unit operatively connected with a circuit switched domain of a telecommunications system, and an interception unit for intercepting a query sent from a network node to a subscriber database node for receiving routing data for forwarding a session establishment message by said network node for setting up a session for providing a telecommunications service to a user of said telecommunications system; characterized in that, said signalling relay function node further comprises an analysis unit for analysing said query for determining whether said query comprises either a first type of request for receiving circuit switched domain routing data for forwarding said session establishment message within said circuit switched domain, or a second type of request for receiving packet switched domain routing data for forwarding said session establishment message within a packet switched domain of said telecommunications system; a relay unit for relaying said query to said subscriber database node if said query comprises said first type of request; and a response unit for providing to said network node in response to said query said routing data for routing said session establishment message to said packet switched domain if said query comprises said second type of request. According to a sixth aspect the present invention is directed to a telecommunications service provider using any method according to the first, second or third aspect of the invention.

Brief description of the drawings

Hereinbelow the invention is further elucidated by means of some specific examples and embodiments thereof, with reference to the enclosed drawings, wherein: figures 1 and 2 are schematic representations of a telecommunications system wherein the invention is applied; figure 3 is a schematic illustration of the method of the present invention, in particular disclosing communication activity between various parts of a telecommunications system; figure 4 schematically illustrates a network node in accordance with an embodiment of the invention; figure 5 schematically illustrates a signalling relay function node in accordance with an embodiment of the invention; figure 6 schematically illustrates a method of operating a network node in accordance with the invention; figure 7 schematically illustrates a method of operating a signalling relay function node in accordance with the invention.

Detailed description In figure 1 there is illustrated a telecommunications system generally indicated with reference numeral 1. The telecommunications system 1 comprises a circuit switched domain 3, e.g. a Public land mobile network (PLMN) based on the Integrated services digital network (ISDN) signalling methodology, and a packet switched domain 5, e.g. an IP multimedia subsystem (IMS) network. Such ISDN based network 3 falls in the category of Circuit switched (CS) based networks, i.e. belongs to the CS domain. The mobile network of the circuit switched domain 3 comprises a mobile network enabling a plurality of users to access the telecommunications system and make use of telecommunications services. Some users of the telecommunications service will be completely served within the circuit switched domain 3, receiving only intelligent network (IN) based services. Other users will be served completely within the packet switched domain 5 of the telecommunications system 1. The present invention, however, is directed to the ongoing efforts of the telecommunications industry to integrate the circuit switched domain 3 with the packet switched domain 5 of telecommunications system 1. The invention is therefore focussed on users that will access the telecommunications system 1 through the circuit switched domain 3, which users are served with services from the packet switched domain 5. For these users, a circuit switched part 3 of the telecommunications system 1 can be seen as the access network, while the packet switched domain 5 of the telecommunications system can be seen as the service network. Hereinbelow use of the terms "access network" and "service network" will refer to the circuit switched domain 3 and the packet switched domain 5 respectively.

In one application of the invention, as already hinted at above, the circuit switched domain will be a global system for mobile communications (GSM) based network, while the packet switched domain 5 will be an IP multimedia subsystem (IMS) network. In this context, the description may refer to "GSM-IMS subscribers" for indicating these type of users. More specifically, such users may be multi access extension (MAE) subscribers, GSM-IMS overlay subscribers, or any other type of subscriber that uses the circuit switched domain as an access network and the packet switched domain as a service network. A skilled person will understand that the invention may be applied to any telecommunications system wherein a circuit switched domain and a packet switched domain are integrated, i.e. providing a circuit switched based access network and a packet switched based service network. The telecommunications system of figure 1 is schematically also illustrated in figure 2. Figure 2 is however a functional illustration of the network that enables to understand the interactivity between different network nodes in the mobile network and the IMS network. A dashed line in the middle of figure 2 illustrates the separation of the circuit switched domain from the packet switched domain of the network. Elements that are illustrated in figure 1 , which have an equivalent counterpart in figure 2, are indicated by the same reference numbers for the purpose of intelligibility, and in order to enable direct comparison of figure 2 to figure 1. In figure 1 user equipment unit 13 is operatively connected with a wireless connection 14 and through base station antenna 8 with (eventually) a gateway mobile switching centre (GMSC) residing in the circuit switched domain 3. In between base station (BS) antenna 8, and GMSC 9 one or more mobile switching centres (MSCs) 6 (as illustrated in figure 2) may facilitate communication between antenna 8 and GMSC 9. GMSC 9 is part of a telecommunications network comprising many different network nodes, and owned by one or more providers having interconnections. This is schematically illustrated by network 7.

The circuit switched domain 3 further comprises a home location register (HLR) 10 functioning as a subscriber database comprising subscriber/user information of subscribers/users of the circuit switched domain 3 of the telecommunications system 1. The circuit switched domain 3 further comprises an enhanced signalling relay function node 28, the functioning of which will be explained in further detail below.

In order to enable communication with the packet switched network, an operative connection is present in between GMSC 9 and media gateway control function (MGCF) 25 in the packet switched domain 5 (the IMS network). The MGCF 25 enables interconnectivity between the IMS network and the network of the circuit switched domain 3 of telecommunications system 1. MGCF 25 is operatively connected with call session control function (CSCF's) such as CSCF 17 in packet switched domain 5. CSCF 17 functions as a registrar and as a service node in the IMS network, and may be connected to a home subscriber server (HSS) 23 and a plurality of application servers (AS) 19, 20 and 21. HSS 23 in packet switched domain 5 can be seen as the equivalent of HLR 10 in circuit switched domain 3: it contains a subscriber database of subscribers to the IMS network. Reference is made to figure 2. When user equipment unit 13 invokes a call for receiving telecommunications services from the telecommunications system, the incoming call is received at antenna 8. User equipment unit 13 belongs to a GSM-IMS subscriber, for which the circuit switched domain 3 is the access network, and the packet switched domain 5 is the service network. The base station associated with antenna 8 forwards the call to MSC 6, and upon receiving the call, a integrated services digital network (ISDN) user part (ISUP) initial address message (IAM), or briefly ISUP-IAM message is forwarded from MSC 6 to GMSC 9. This ISUP-IAM message may, for the purpose of explaining the invention, be interpreted as the session establishment message in the implementation described in relation to figure 2. It will be understood that a session establishment message may broadly be interpreted a signalling message to be forwarded through the telecommunications system for the setting up of a session for the provisioning of one or more services. The GMSC 9 being a service node in the CS network associated with the called party needs to decide where to forward the ISUP-IAM message to. In order to determine where the ISUP-IAM is to be forwarded to, the GMSC 9 sends a mobile application part (MAP) send routing information (SRI) query towards the home location register (HLR) 10 in the circuit switched network. Prior to sending the MAP-SRI query, GMSC 9 determines whether the ISUP-IAM message relates to a call that has already been processed as terminating call in the IMS network of the packet switched domain 5. One way of performing this task, by the GMSC 9, is to investigate whether the ISUP-IAM message contains a prefix contained in ISUP-IAM message when that message results from terminating call processing in the IMS network of the packet switched domain 5. This will be explained in more detail further below. The outcome of this task is reflected by the type of query included in the MAP SRI query.

Enhanced signalling relay function node (E-SRF) 28 intercepts the MAP SRI message in order to determine whether further processing is required. In particular, it determines whether the MAP SRI message relates to a call that has already been processed as terminating call in the IMS network of the packet switched domain 5. This is performed by analysing the type of query in the MAP SRI message.

If the ISUP-IAM message has been forwarded by the packet switched domain 5 of the telecommunications system 1 , after the call has been processed as terminating call in the IMS network, the GMSC determines that further handling of the call is to be performed within the circuit switched domain 3 of the telecommunications system 1. In that case, the MAP SRI message, sent from GMSC 9 to HLR 10 and reflecting that this MAP SRI message relates to a call that has been processed as terminating call in the IMS network, s relayed by the E-SRF 28 towards the appropriate subscriber database, for example HLR 10. The skilled person will appreciate that the destination of the call, i.e. the terminating party, is decisive for determining to which HLR the MAP SRI message is to be relayed, in the case that multiple HLRs are deployed in the network. For the purpose of deciding to which HLR to forward the MAP SRI message, the E-SRF is equipped with or is functionally connected to subscriber database, as known to the skilled person. In the present example, it is assumed that the call is to be terminated with the same operator network as from where it was originated, and therefor the ISUP-IAM message in the present example is sent from the IMS network in the PS domain to GMSC 9 in the CS domain.

On the other hand, if the E-SRF 28 determines, from inspecting the MAP SRI message, that the MAP SRI message relates to a call that has not been processed as a terminating call in the IMS network, but instead has been forwarded from within the circuit switched domain 3 of the telecommunications system 1 , then the E-SRF 28 will not relay the MAP SRI message to the HLR 10. Instead, E-SRF 28 provides a response to GMSC 9 containing routing data for routing the ISUP-IAM message to the packet switched domain 5 of the telecommunications system. GMSC 9 makes the decision, whether or not the call has been processed as a terminating call in the IMS network, based on the presence or absence of a prefix in the ISUP- IAM message. The outcome of this decision by GMSC 9 is reflected in the MAP SRI message sent from GMSC 9 towards HLR 10. If the prefix is absent in the ISUP IAM, then the ISUP-IAM message is apparently forwarded by the circuit switched domain 3 of the telecommunications system 1. In that case, the ISUP-IAM message is to be forwarded, using routing data received from E-SRF, to the service network of the called subscriber, i.e. the packet switched domain 5 formed by the IMS network.

On the packet switched domain side 5 of the telecommunications system 1 the MGCF 25 takes action by composing the required session initiation protocol (SIP) messages to be forwarded to the serving call session control function (S-CSCF) 17 that will coordinate service provisioning within the IMS network for this subscriber. The S-CSCF 17, for coordinating the service provisioning, is operatively connected a home subscriber server (HSS) 23, containing one or more databases comprising subscriber details and data required for service provisioning, such as the locations and specifics of the application servers 19, 20 and 21 facilitating the services. When all necessary action is taken within the IMS network for setting up a requested service, i.e. offering the call to the called party, the S-CSCF 17 eventually provides a SIP message to the contact address previously registered in that S-CSCF 17, said contact address being registered in S-CSCF 17 for the purpose of offering a terminating call or other IMS service to a device associated with that contact address. In the present example, the contact address is an address associated with MGCF 25, for the purpose of offering a call to a device in the CS domain, such as GSM terminal. In addition, the contact address registered in S-CSCF 17 and associated with MGCF 25, contains a designated prefix. The MGCF 25, upon receiving this SIP message, including the prefix, generates an ISUP-IAM message and routes the ISUP-IAM message back to the circuit switched network domain 3, in accordance with regular call routing methods as known to the skilled person.

Figure 3 provides a schematic illustration the communication taking place between the different network entities in the packet switched and circuit switched domains of the telecommunications system. User equipment (UE) unit 13, in step 30 places a call within the access network for this subscriber, i.e. the circuit switched domain 3 in figure 1. The call is received at GMSC 9 as an ISUP-IAM message composed by one of the mobile switching centres (MSCs) in between user equipment unit 13 and GMSC 9 (not illustrated in figure 3 for reasons of clarity of the figure). Upon receiving the ISUP-IAM message, in step 32, GMSC 9 determines whether or not the ISUP-IAM message has been forwarded by the packet switched domain 5 (as shown in figure 1 ) of the telecommunications system 1 , and sends a query towards HLR 10 containing either a first type of request or a second type of request for routing data. A first type of request is a request for receiving routing data within the circuit switched domain 3 of telecommunications system 1 , while a second type of request is a request for receiving routing data to the packet switched domain 5 of the telecommunications system 1. In the present case, illustrated in figure 3, the ISUP-IAM message is originated within the access network, i.e. the circuit switched domain 3, and the GMSC 9 composes a query containing a second type of request. The query will be sent towards the HLR 10 as a MAP-SRI message.

On its way to the HLR however, the MAP-SRI message will be intercepted in step 34 by E-SRF 28. E-SRF 28 verifies the type of request contained within the query. Since the query contains a second type of request, E-SRF 28 will retrieve routing data to the packet switched domain 5 of the telecommunications system 1 from a database functionally connected to, or contained in E-SRF 28. E- SRF 28 composes a response to the query based on the routing data retrieved, and sends this response back to GMSC 9. Therefore, the original MAP-SRI query will not reach its original destination, HLR 10. After interception by E-SRF 28, it will be processed locally by E-SRF 28. GMSC 9 receives the response from E-SRF 28 in step 36. In step 38, using the routing data received, GMSC 9 forwards the ISUP-IAM message to MGCF 25.

In step 40, MGCF 25 composes the required SIP message(s) upon receipt of the ISUP-IAM message forwarded by GMSC 9. This SIP message is forwarded by MGCF 25 to S-CSCF 17 for enabling service provisioning from the IMS network. In step 42, S-CSCF 17 commences service invocation based on the received SIP message. The required course of action performed within the IMS network for setting up the requested services will be known to the skilled person, and it is not necessary to discuss this in more detail here. This course of action is in figure 3 summarized as step 45, illustrated as a dashed-lined box.

Once all required action is taken within the IMS network for setting up the service, in step 48 the S-CSCF 17 sends a SIP message back to the MGCF 25 indicating that the call has been processed in the IMS network. MGCF 25, in step 50 receives the SIP message. In step 52, MGCF 25 generates an ISUP-IAM message. The ISUP-IAM message contains a prefix to the called party number of the destination terminal of the call. This prefix enables routing of the ISUP-IAM message back to the circuit switched domain 3 of telecommunications system 1 , without the ISUP-IAM subsequently being sent back to the IMS domain.

In the circuit switched domain 3, GMSC 9 receives the ISUP-IAM message in step 54, checks whether the ISUP-IAM message is forwarded by the MGCF 25, and finds the designated prefix in the ISUP-IAM message. GMSC 9 thereby composes a MAP-SRI query containing a first type of request for receiving routing data for routing the ISUP-IAM message within the circuit switched network. This MAP-SRI query is sent towards HLR 10, however is again intercepted in step 56 by E-SRF 28. E-SRF 28 determines once again, whether the MAP-SRI query contains a first type or a second type of request. In the present case, since the ISUP-IAM message relates to a call that has already been processed as a terminating call in the IMS network, the query contains a first type of request. E-SRF 28 therefore relays the MAP-SRI query message and forwards it to HLR 10. HLR 10 in step 58 provides the required routing data for routing the ISUP-IAM message within the circuit switched network. In step 62, GMSC 9 receives the response from HLR 10 containing the routing data, and also in step 62, the call is forwarded to the terminating party where it is received in step 64. Figure 4 schematically illustrates a network node in accordance with the invention: GMSC 68. GMSC 68 comprises an input/output unit 71 which enables operatively connecting GMSC 68 with a circuit switched network. Amongst others, GMSC 68 comprises a communication unit 70 enabling communication between GMSC 68 and other network entities or user equipment units. GMSC 68 further comprises a processing unit 75 and a query unit 76. The processing unit 75 is, amongst others, arranged for determining whether a received ISUP-IAM message is forwarded, after having been processed as terminating call, by a packet switched domain, such as an IMS network, or is forwarded, without having been processed as terminating call, by a circuit switched domain. One way of performing this task is to verify whether the ISUP-IAM message contains a designated prefix in the called party number, said prefix enabling routing from the packet switched domain to the circuit switched domain, without subsequently routing back to the IMS domain. The processing unit 75 is further arranged for instructing the query unit 76 to compose a MAP-SRI query containing a first type of request for routing data within the circuit switched domain, or a second type of request for routing data to the packet switched domain. Query unit 76 is arranged for composing a MAP-SRI query in response to instructions from the processing unit 75. It is arranged for composing the MAP-SRI query either a first type of request or a second type of request. The first type of request is a request for receiving routing data for routing a received ISUP-IAM message within a circuit switched domain (said routing constituting routing the call to the called subscriber's access network), and the second type of request is a request for routing data for routing the received ISUP-IAM message to the packet switched domain (said routing constituting routing the call to the called subscriber's service network). The query unit instructs the communication unit to forward the MAP-SRI message towards a home location register HLR, enabling interception of the MAP SRI message by an enhanced signalling relay function (E-SRF). Alternatively, the query unit may also instruct the communication unit to forward the MAP-SRI directly to the E-SRF, making interception of the MAP-SRI message by the E-SRF superfluous.

Figure 5 schematically illustrates an enhanced signalling relay function node (E-SRF) 79, in accordance with the present invention. E-SRF 79 of figure 5 comprises an input/output unit 80 enabling connection of the E-SRF 79 within the circuit switched network. E-SRF 79 further comprises a communication unit 81 , an interception unit 82, a relay unit 84, a response unit 86 and a memory 85 for an internal database.

Communication unit 81 enables communication between E-SRF 79 and various network entities within a Circuit switched (CS) network, such as a PLMN. Interception unit 82 is arranged for recognizing MAP-SRI query messages that are forwarded between the GMSC and an HLR in the CS mobile network, and for intercepting these messages. Upon interception, interception unit 82 determines whether the MAP SRI message contains a first type of request for receiving routing data within the circuit switched domain, or a second type of request for receiving routing data for routing to a packet switched domain of the telecommunications system. Dependent on the type of request, the interception unit 82 either instructs relay unit 84 to relay the MAP-SRI message to the appropriate HLR within the circuit switched domain, or instructs the response unit 86 to compose a response to the received MAP-SRI message for providing routing data to the packet switched domain. If the received MAP-SRI message contains a first type request relay unit 84 may either decide to relay the MAP-SRI message to the HLR indicated in the message or may consult internal database 85 for retrieving routing data to the apporiate HLR. Response unit 86, in the case that the MAP-SRI message contains a second type of request, consults memory 85 for retrieving routing data to the IMS network, and composes a response to the MAP-SRI message containing the routing data. Relayed MAP-SRI messages from relay unit 84 and responses from response unit 86 are provided back to communication unit 81 for forwarding, by the GMSC, the ISUP IAM message in the mobile network in the CS domain or forwarding the ISUP IAM message to the IMS domain. Figure 6 discloses a method of operating a network node, in accordance with the present invention. In figure 6, element 89 schematically illustrates an ISUP-IAM message, which is received by the network node in step 90. The network node may be a GMSC, or any other network node that has been assigned the task of handling ISUP-IAM messages related to calls destinated for GSM-IMS subscribers. Therefore this network node may also be a mobile switching centre (MSC) acting as GMSC at the same time or a service switching point (SSP). A skilled person will appreciate that such a network node should comprise the same or similar units as GMSC 68 of figure 4 for implementing the invention. Upon receipt of the ISUP-IAM message 89 in step 90, the network node in step 92 determines whether the ISUP-IAM message relates to a call that has been handled as a terminating call in an IMS network within a packet switched domain of a telecommunications system. In the present example, this is implemented by determining whether the ISUP-IAM message received in step 90 contains a designated prefix. If the designated prefix is found it is determined that a first type of request for routing data within the circuit switched network is required in step 93. If however the designated prefix is not found in the ISUP-IAM message, in step 94 it is determined that instead a second type of request retrieving routing data to the packet switched domain is required. Based on whether either a first type or second type of request is required, in step 95 the network node composes a MAP- SRI query to be forwarded to the appropriate home location register (HLR) 98. In step 97 the MAP-SRI message is forwarded to HLR 98. As mentioned above the MAP-SRI message will be intercepted by an enhanced SRF in accordance with the invention. Therefore, alternatively, instead of sending the MAP-SRI query message towards HLR 98, the network node may also directly provide the MAP-SRI query message to the enhanced-SRF for processing it.

Figure 7 schematically illustrates a method of operating an enhanced signalling relay function node (E-SRF) in accordance with the present invention. For explaining this method of operation, it is assumed here as an example that GMSC 124, upon receiving an ISUP IAM message, sends a MAP SRI message 123 towards HLR 122, which MAP SRI message 123 is intercepted by the E-SRF. This situation is schematically indicated in figure 7, generally referred to by reference numeral 101. In step 100, the MAP SRI message is intercepted by the E- SRF. The MAP-SRI query message is analysed in step 104, and in step 105 it is determined whether the analysed query of step 104 contains either a first type of request or a second type of request. When the MAP-SRI message contains a first type of request, in step 108 the MAP-SRI message is relayed to a HLR in the CS domain. This may be the HLR which is indicated in the original MAP-SRI message, or another suitable HLR on which routing data can be retrieved from the internal memory 120 of the E-SRF. In step 109, the message is sent on to the HLR 122, as is generally indicated by reference numeral 110.

If however in step 105 it is determined that the MAP-SRI query message contains a second type of request, in step 113 it is determined that a response message is to be composed by the E-SRI. In step 115 a response to the MAP-SRI query message is composed, containing routing data to the packet switched network, which routing data is retrieved from internal memory 120 of the E- SRF. This response is sent back to GMSC 124 in step 116, as is generally indicated by reference numeral 118. The invention may be implemented differently than specifically described herein, and the invention is only limited by the scope of the appended claims.

Claims

1. Method of handling set-up of a session for providing a telecommunications service to a user in a telecommunications system, wherein said user is registered in a packet switched domain of said telecommunications system for receiving services provided from said packet switched domain, and wherein said user uses a terminal unit which is operatively connected with a circuit switched domain for accessing said telecommunications system, wherein for performing said session set-up said method comprises the steps of: a network node receiving a session establishment message and forwarding a query towards a subscriber database node for receiving routing data for forwarding said session establishment message for setting up said session; and a signalling relay function node intercepting said query; characterized in that, said method further comprises the step of said network node determining whether said received session establishment message has been forwarded from said packet switched domain prior to receipt of said session establishment message by said network node; wherein dependent on a result of said step of determining, said query comprises either a first type of request for receiving circuit switched domain routing data for forwarding said session establishment message within said circuit switched domain, or a second type of request for receiving packet switched domain routing data for forwarding said session establishment message within said packet switched domain.

2. Method according to claim 1 , wherein said signalling relay function node responds to said query by returning routing data for routing to said packet switched domain if said query comprises said second type of request.

3. Method according to any of the previous claims, wherein said signalling relay function node relays said query to said subscriber database within said circuit switched domain if said query comprises said first type of request.

4. Method according to any of the previous claims, wherein said step of determining is performed by said network node by detecting a presence or absence of a prefix in said received session establishment message.

5. Method according to any of the previous claims, wherein said packet switched domain comprises an internet protocol multimedia subsystem network.

6. Method according to any of the previous claims, wherein said network node comprises at least one element of a group comprising mobile switching centre, gateway mobile switching centre, service control point, short message service centre, and service switching point.

7. Method of operating a network node in a circuit switched domain of a telecommunication system for handling session set-up for providing a telecommunications service to a user of said telecommunications system, using a method according to any of the claims 1-6, wherein said user is registered in a packet switched domain of said telecommunications system for receiving services provided from said packet switched domain, and wherein said user uses a terminal unit which is operatively connected with said circuit switched domain for accessing said telecommunications system, said method comprising a step of: said network node receiving a session establishment message and forwarding a query towards a subscriber database node for receiving routing data for forwarding said session establishment message for setting up said session; characterized in that, said method further comprises the step of said network node determining whether said received session establishment message has been forwarded from said packet switched domain prior to receipt of said session establishment message by said network node; and said network node dependent on a result of said step of determining, including in said query either a first type of request for receiving circuit switched domain routing data for forwarding said session establishment message within said circuit switched domain, or a second type of request for receiving packet switched domain routing data for forwarding said session establishment message within said packet switched domain.

8. Method according to claim 7, wherein said step of determining is performed by said network node by detecting a presence or absence of a prefix in said received session establishment message.

9. Method according to claim 7 or 8, wherein said network node comprises at least one element of a group comprising mobile switching centre, gateway mobile switching centre service control point, short message service centre, and service switching point.

10. Method according to any of the claims 7-9, further comprising a step of said network node receiving routing data in response to said query, and routing said session establishment message within said telecommunications system based on said routing data received.

11. Method of operating a signalling relay function node in a telecommunications system for handling session set-up for providing a telecommunications service to a user of said telecommunications system, using a method according to any of the claims 1-6, wherein said user is registered in a packet switched domain of said telecommunications system for receiving services provided from said packet switched domain, and wherein said user uses a terminal unit which is operatively connected with a circuit switched domain for accessing said telecommunications system, said method comprising a step of: said signalling relay function node intercepting a query sent from a network node towards a subscriber database node for receiving routing data for forwarding a session establishment message by said network node for setting up said session; characterized in that, said method further comprises the steps of: said signalling relay function analysing said query for determining whether said query comprises either a first type of request for receiving circuit switched domain routing data for forwarding said session establishment message within said circuit switched domain, or a second type of request for receiving packet switched domain routing data for forwarding said session establishment message within said packet switched domain; said signalling relay function relaying said query to said subscriber database node if said query comprises said first type of request, or said signalling relay function providing to said network node in response to said query said routing data for routing said session establishment message to said packet switched domain if said query comprises said second type of request.

12. Network node for use in a method according to any of the claims 1- 6, wherein said network node is arranged for handling set-up of a session within a circuit switched domain of a telecommunications system for providing a telecommunications service to a user of said telecommunications system, wherein said network node comprises an input-output unit operatively connected with said telecommunications system for receiving and forwarding a session establishment message, and a querying unit for composing a query for receiving routing data for forwarding said session establishment message, and a communication unit for forwarding said query to a subscriber database node of said telecommunications system; characterized in that, said network node further comprises processing unit for determining whether a received session establishment message has been forwarded from a packet switched domain of said telecommunication system prior to receipt of said query by said network node; and wherein said querying means are arranged for including in said query, dependent on a result of said step of determining, either a first type of request for receiving circuit switched domain routing data for forwarding said session establishment message within said circuit switched domain, or a second type of request for receiving packet switched domain routing data for forwarding said session establishment message within said packet switched domain.

13. Network node according to claim 12, wherein said processing unit is arranged for detecting a presence or absence of a prefix in said received session establishment message for determining whether said received session establishment message has been forwarded from said packet switched domain.

14. Signalling relay function node for use in a method according to any of the claims 1-6, wherein said signalling relay function node comprises an input- output unit operatively connected with a circuit switched domain of a telecommunications system, and an interception unit for intercepting a query sent from a network node towards a subscriber database node for receiving routing data for forwarding a session establishment message by said network node for setting up a session for providing a telecommunications service to a user of said telecommunications system; characterized in that, said signalling relay function node further comprises an analysis unit for analysing said query for determining whether said query comprises either a first type of request for receiving circuit switched domain routing data for forwarding said session establishment message within said circuit switched domain, or a second type of request for receiving packet switched domain routing data for forwarding said session establishment message within a packet switched domain of said telecommunications system; a relay unit for relaying said query to said subscriber database node if said query comprises said first type of request; and a response unit for providing to said network node in response to said query said routing data for routing said session establishment message to said packet switched domain if said query comprises said second type of request.

15. Signalling relay function node according to claim 14, further comprising a memory unit for maintaining a routing database containing said routing data, wherein said routing data comprises content for relaying said session establishment message to said subscriber database node, and content for providing to said response unit said routing data for routing said session establishment message to said packet switched domain.

16. Telecommunications service provided using a method according to any of the claims 1-11.