WO2004054302A1

WO2004054302A1 - Simultaneous registrations of a user in different service servers with different directory numbers

Info

Publication number: WO2004054302A1
Application number: PCT/EP2002/013953
Authority: WO
Inventors: Carolina Canales Valenzuela; Juan Antonio Sanchez Herrero
Original assignee: Telefonaktiebolaget Lm Ericsson (Publ)
Priority date: 2002-12-09
Filing date: 2002-12-09
Publication date: 2004-06-24
Also published as: AU2002356639A1

Abstract

The invention provides a system and a method, basically oriented to the support for multiple registrations of a user identified by a global identity and a number of subscriber directory identities in different Service-Control servers, each of these registrations in different Service-Control servers carried out with a different subscriber directory identity, or combinations thereof.

Description

SIMULTANEOUS REGISTRATIONS OF A USER IN DIFFERENT SERVICE SERVERS WITH DIFFERENT DIRECTORY NUMBERS

FIELD OF THE INVENTION

[0001] The present invention relates to telecommunication systems providing multimedia services to subscribers based on IP technology. More specifically, the present invention relates to the registration procedure followed to register a subscriber in more than one multimedia server.

BACKGROUND

[0002] An IP Multimedia system allows users to establish IP multimedia conferences and to make use of IP Multimedia services. The IP Multimedia systems can be implemented using several technologies such as the Session Initiation Protocol (generally known as SIP) .

[0003] The SIP technology specified by the IETF RFC 3261 is the base for the IP Multimedia Subsystem as defined by the 3^rd Generation Partnership Project (3GPP) "Technical Specification Group Services and System Aspects; IP Multimedia (IM) Subsystem (3GPP TS 23.228 version 5.5.0)" for providing the UMTS users with IP multimedia services.

[0004] In accordance with said 3GPP TS 23.228 version, there are various user identifiers that may be associated with a user of IP multimedia services. The general principle behind this multiplicity of user identifiers is that every subscriber of an IM Subsystem (IMS) shall have a private user identity (IMPI), namely a global identity that identifies and represents the subscription to the IM services within a telecommunication system, and shall have one or more public user identities (IMPU) , hereinafter referred to as subscriber directory identities in a broad sense, which are used by any user for communication with other users.

[0005] The above global identity is generally used for Registration, Authorisation, Administration and Accounting purposes but not for routing. This global identity is securely stored in a sort of Subscription Identity Module (SIM) embedded in a removable card which is traditionally used to configure a telecommunication equipment and have it ready for use with subscription data stored for the subscription that said global identity represents. In this respect, this global identity is generally assigned by the home network operator to identify the user from a network perspective and thus used to identify the user's information stored in a Home Subscriber Database within the network, for example authentication information. In short, this global identity identifies the user independently of other identifiers used for addressing purposes, and is contained in a Registration request passed from the User Equipment (UE) to the home network. Thus, the global identity is authenticated only during registration related procedures of the owner subscriber. An exemplary format for a global identity is a Network Access Identifier (NAI) which in particular may comprise an International Mobile Subscriber Number (IMSI) .

[0006] Apart from the above global identity, a user must have at least one subscriber directory identity associated to said global identity for communication with another users. In general, such subscriber directory identities, which are associated with a particular global identity, have been previously registered before being used for establishing any IM service. Such registration can be carried out for an individual subscriber directory identity following an explicit registration procedure, or for a plurality of subscriber directory identities following an implicit registration procedure, or for combinations thereof. As for the global identity, at least one subscriber directory identity is preferably stored in the above Subscription Identity Module (SIM) though not all the possibly assigned subscriber directory identities must be stored in said sort of SIM. Also, in a like manner as for the global identity, a subscriber directory identity assigned by the home network operator can be used to identify the user's information stored in a Home Subscriber Database within the network. For instance, queries performed for routing during session establishment. However, in contrast with the global identity, the subscriber directory identities are not authenticated by the network during registration procedures. Both Telecom numbering and Internet naming schemes can be used to address users depending on the subscriber directory identities that the users have. In particular, the subscriber directory identities may take the form of a SIP URL, or a "tel:"-URL format, and may be extended to other type of identities such as e-mail addresses.

[0007] Each particular subscriber directory identity is associated with a unique Service Profile that collects service and user related data, whereas a particular Service Profile may be associated with more than one subscriber directory identity. More specifically, a Service Profile includes sets of mandatory and optional capabilities required per user by each particular network operator, depending on the different capabilities that different Service-Control servers might have for serving a number of users. Generally speaking, a Service-Control server is in charge of handling IM SIP sessions status in the network. In particular, such Service-Control server may be a Serving Call Status Control Function (S-CSCF) as referred to by some 3GPP Technical Specifications. [0008] Thus, all the IMS subscription data per subscriber basis, including relationship between the global identity, an associated Service-Control server identifier, a number of subscriber directory identities, and a number of service profiles, reside in a Home Subscriber Database of the home network. In particular, such Home Subscriber Database may be a Home Subscriber Server (HSS) as referred to by some 3GPP Technical Specifications.

[0009] Currently, all the Service Profiles sharing the same global identity, and irrespective of the number of subscriber directory identities assigned with, are associated with the same Service-Control server. That is, a unique Service-Control server is assigned to a unique global identity representing a user subscription, as shown in Fig. 1.

[0010] The selection of said unique Service-Control server to be associated with a global identity representing a subscription is performed at a Multimedia Contact-Point server in the home network by requesting information from the Home Subscriber Database, such as required capabilities for subscriber services and operator preferences, and by obtaining other relevant information via non-standard procedures, such as topological network information and availability status related to the Service-Control server. In particular, such Multimedia Contact-Point server may be a Interrogating Call Status Control Function (I-CSCF) as referred to by some 3GPP Technical Specifications, said I- CSCF thus requesting information from a Home Subscriber Server (HSS) and assigning a unique Serving Call Status Control Function (S-CSCF) where at least one subscriber directory number is registered.

[0011] At present, when a User Equipment (UE) attaches and makes itself available for access to IMS services by explicitly registering in the IMS, an appropriate Service- Control server must be assigned to serve the UE, such UE being identified by a global identity and by a selected subscriber directory identity. To this end, a Multimedia Contact-Point server at the home network, having received such user registration request, queries the Home Subscriber Database to check the registration status. A check is then performed at the Home Subscriber Database to determine whether or not the user identified by a given global identity was already registered, and a Service-Control server was thus already assigned, for the given subscriber directory identity or for another subscriber directory identity sharing the same global identity. Provided that such user was already registered for the given or for another subscriber directory identity, the Home Subscriber Database returns toward the Multimedia Contact-Point server the name of the Service-Control server assigned to the global identity. Then, the Multimedia Contact-Point server communicates with the Service-Control server for performing the registration of the given subscriber directory identity. As commented above, there can be only a unique Service-Control server assigned per global identity basis at a time. In particular, Fig. 2 illustrates an exemplary mechanism where a Home Subscriber Server (HSS) is the Home Subscriber Database, an Interrogating Call Status Control Function (I-CSCF) acts as the above Multimedia Contact- Point server, and a Serving Call Status Control Function (S-CSCF) is the Service-Control server assigned to the global identity.

[0012] Nevertheless, any user may be given a plurality of subscriber directory identities. An advantage of having a plurality of subscriber directory identities is that each individual subscriber directory identity may be used for a specific multimedia service like, for example, voice applications, or videoconference applications, or e- commerce applications. These different multimedia services are ultimately carried out with help of dedicated Application Servers (AS) which are individually controlled by a unique Service-Control server in which the user was already registered.

[0013] Thus, an important disadvantage of the presently existing IMS is the definition and establishment of different sets of mandatory and optional capabilities required per user, appropriate for different multimedia services under each particular Service Profile, and by each particular network operator.

[0014] A further more significant disadvantage of the presently existing IMS is the lack of feasible load- balancing mechanisms. For example, a user might initially register with a first subscriber directory identity that is used for voice related services, voice services which are supported thanks to a dedicated Application Server (AS-1) . As this first registration takes place, the Home Subscriber Database provides a set of capabilities suitable for voice services and a Multimedia Contact-Point server receiving such capabilities selects an appropriate Service-Control server (SC-1) by using certain selection criteria on its own basis. The same user might register later with a second subscriber directory identity used for videoconference services which are supported by another dedicated Application Server (AS-2) . As a matter of fact, the same Service-Control server (SC-1) as selected for voice related services will be also used for controlling sessions related to videoconference services. That is, a high load in this Service-Control server (SC-1) due to voice related services may adversely affect other services such as videoconference services in this exemplary use case. This disadvantage may be even more adversely affected if any secondary registration is carried out for a subscriber directory identity used for services requiring a special security, such as e-commerce services for instance.

[0015] A still further disadvantage is that some capabilities required per user, appropriate for different multimedia services under each particular Service Profile, and by each particular network operator, have to be made optional only in order to make the controlling functions at a Service-Control server being compatible for the different services related sessions that a user might register at a time.

[0016] Furthermore, with the currently existing IMS nowadays, there is no way to provide a well dimensioned network where the most commonly used services are controlled by a plurality of Service-Control servers thus offering the required capacity for a huge amount of subscribers, whereas the less commonly used services are controlled by a reduced number of Service-Control servers thus offering the required capacity for a smaller amount of subscribers .

[0017] Thereby, an object of the present invention is to overcome the above disadvantages whilst providing a well dimensioned IMS network where the most commonly used services are controlled by a plurality of Service-Control servers whereas the less commonly used services are controlled by a reduced number of Service-Control servers.

[0018] A further object of the present invention is to achieve both subscription and user-registration related conditions where more capabilities in a Service Profile can be specifically marked as compulsory rather than optional to better carry out a Service-Control server selection.

SUMMARY OF THE INVENTION [0019] The above objects, among others, are accomplished in accordance with the invention by the provision of a telecommunication system and a method for providing multimedia services to subscribers of a network operator, each subscriber identified by a global identity (IMPI) and a number of subscriber directory identities (IMPU-1, IMPU- 2), and said global identity (IMPI) associated with a first Service-Control Server (S-CSCF-1) in which at least one first subscriber directory identity (IMPU-1) is registered.

[0020] The telecommunications system is thus arranged for allowing at least one second subscriber directory identity (IMPU-2), which shares said global identity (IMPI), to be registered in a second Service-Control Server (S-CSCF-2).

[0021] Therefore, telecommunication system includes a Home Subscriber Database (HSS) for maintaining subscriber registration data that comprises in accordance with the invention processing means and storage for different Service-Control Server (S-CSCF-1; S-CSCF-2) identifiers on a per subscriber directory identity (IMPU-1; IMPU-2) basis.

[0022] Moreover, the processing means at said Home Subscriber Server (HSS) includes:

- means for receiving from a Multimedia Contact-Point server (I-CSCF) a registration request for a subscriber with a global identity (IMPI) and a subscriber directory identity (IMPU-2);

- means for verifying that the received subscriber directory identity (IMPU-2) is not registered yet whereas another subscriber directory identity (IMPU-1) for the same global identity (IMPI) is already registered in a first Service-Control Server (S-CSCF-1) ; - means for returning to said Multimedia Contact-Point server (I-CSCF) service capabilities (SC-2) of a suitable Service-Control Server (S-CSCF-2) for serving the subscriber (IMPI) with the second subscriber directory identity (IMPU-2); and

- means for receiving an identifier of a second Service- Control Server (S-CSCF-2) to be stored for the subscriber (IMPI) with the second subscriber directory identity (IMPU-2) and along with a new registration status.

[0023] In respect of this new registration status, the second subscriber directory identity (IMPU-2), for which a second Service-Control Server (S-CSCF-2) has been selected, is marked authentication pending, pre-registration' . The Home Subscriber Database (HSS) participates in the new registration procedure by triggering a generation of a number of new authentication vectors to allow a suitable synchronisation in both authentication ends.

[0024] The Home Subscriber Database (HSS) has further means for downloading a number of Service Profiles (SP-1; SP-2) associated with respective subscriber directory identities (IMPU-1; IMPU-2) toward corresponding Service- Control Servers (S-CSCF-1; S-CSCF-2) respectively assigned to said subscriber directory identities. Moreover, the Home Subscriber Database (HSS) is configured to return toward a Multimedia Contact-Point server (I-CSCF), in addition to service capabilities (SC-2) of a suitable Service-Control Server (S-CSCF-2) for a subscriber directory identity (IMPU-2) still in a registration stage, identifiers of those Service-Control Servers (S-CSCF-1) where the user is registered in with other subscriber directory identities (IMPU-1) . [0025] A more advantageous system is obtained with a Home Subscriber Database (HSS) that may be configured for selecting the most appropriate Service-Control Server for a new subscriber directory identity still in a registration stage, by applying selective criteria among a number of Service-Control Servers already assigned to a number of subscriber directory identities registered in. More particularly, this selective criteria in the Home Subscriber Database (HSS) include means for comparing capabilities between different Service Profiles (SP-1, SP- 2) , and means for identifying an already assigned Service- Control Server (S-CSCF-1) that can provide acceptable capabilities for the new subscriber directory identity (IMPU-2) being registered. Further, this selective criteria in the Home Subscriber Database (HSS) may include means for weighting capabilities in Service Profiles and assigned Service-Control Server, thus allowing simpler comparisons and analysis.

[0026] For the purpose of the present invention, the registration of a user in this telecommunications system with a global identity and a particular subscriber directory identity, is understood as a result of carrying out an explicit registration procedure for said particular subscriber directory identity individually, or carrying out an implicit registration procedure for a group that includes said particular subscriber directory identity, or ^assumed' for active services that do not require a registration for having a Service-Control Server assigned.

[0027] A method of registering a subscriber identified by a global identity (IMPI) and a number of subscriber directory identities (IMPU-1, IMPU-2) in a plurality of Service-Control Servers is also provided by the present invention, the method applicable once the subscriber has at least one first subscriber directory identity (IMPU-1) already registered in a first Service-Control Server (S- CSCF-1) . The method, in accordance with the invention, comprising the steps of:

(a) receiving a request for registration of the subscriber (IMPI) with a second subscriber directory identity

(IMPU-2); and

(b) verifying that said subscriber (IMPI) is not registered with said second subscriber directory identity (IMPU- 2);

(c) obtaining service capabilities (SC-2) of a suitable Service-Control Server for serving the subscriber (IMPI) with the second subscriber directory identity (IMPU-2); and

(d) registering the subscriber (IMPI) with the second subscriber directory identity (IMPU-2) in a second Service-Control Server (S-CSCF-2) other than the first Service-Control Server (S-CSCF-1), said second Service- Control Server (S-CSCF-2) fitting the received service capabilities (SC-2) .

[0028] More specifically, the request for registration includes a step of querying from a Multimedia Contact-Point server (I-CSCF) a check at a Home Subscriber Database (HSS) about the registration status for the received subscriber identities (IMPI, IMPU-2).

[0029] One further aspect of the present invention is how to preclude authentication related synchronisation errors. Therefore, the above step of registering a subscriber in a second Service-Control Server (S-CSCF-2) comprises a step of triggering the generation of a number of new authentication vectors, generation that in particular may include an indication of generating a number of equal vectors just differing in a Sequence number (SQN) field. [0030] Moreover, the above step of obtaining service capabilities (SC-2) of a suitable Service-Control Server for serving a subscriber with a second subscriber directory identity (IMPU-2) includes a step of additionally selecting the most appropriate Service-Control Server for said second subscriber directory identity, by applying selective criteria among a number of Service-Control Servers already assigned to a number of subscriber directory identities registered in.

[0031] Still further, the method may comprise a step of downloading from a Home Subscriber Database (HSS) a number of Service Profiles (SP-1; SP-2) associated with respective subscriber directory identities (IMPU-1; IMPU-2) toward corresponding Service-Control Servers (S-CSCF-1; S-CSCF-2) respectively assigned to said subscriber directory identities .

BRIEF DESCRIPTION OF DRAWINGS

[0032] The features, objects and advantages of the invention will become apparent by reading this description in conjunction with the accompanying drawings, in which:

[0033] FIG. 1 illustrates a basic overview of a currently existing Home Subscriber Database structure where a unique Service-Control server is supported on a per subscriber global identity basis.

[0034] FI6. 2 presents a flow sequence currently followed at a Home Subscriber Database after having received a registration request for a user with a second subscriber directory identity.

[0035] FIG. 3 shows a simplified flow sequence to follow at a Home Subscriber Database for allowing a multiple registration of a user in different Service-Control servers with different subscriber directory identities.

[0036] Fig. 4 shows a basic overview of a Home Subscriber

Database structure appropriate for handling a number of Service-Control servers respectively assigned on a per subscriber directory identity basis.

[0037] FIG. 5 illustrates a simplified procedure that can be followed, by including the appropriate set of data, to support a multiple registration of a user in different Service-Control servers with different subscriber directory identities .

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0038] The following describes currently preferred embodiments of a system and method to provide and support multiple registrations of a user with a terminal (UE) in different Service-Control servers (S-CSCF-1; S-CSCF-2), the user identified by a global identity (IMPI) and a number of subscriber directory identities (IMPU-1, IMPU-2, IMPU-3) , each of these registrations in different Service-Control servers (S-CSCF-1; S-CSCF-2) carried out with a different subscriber directory identity or combinations thereof (IMPU-1; IMPU-2, IMPU-3).

[0039] Therefore, in accordance with an aspect of the present invention, there is provided a mechanism for storing and transmitting enough service information to perform an optimal selection between a central entity and the entity in charge of selecting an appropriate Service- Control server (S-CSCF-1; S-CSCF-2) . In particular, such central entity is aware of registration status of different IP multimedia service identities for a specific user, and such central entity is preferably a Home Subscriber Database (HSS) , whereas the selection of a Service-Control server is carried out by a Multimedia Contact-Point server. Moreover, said Multimedia Contact-Point server may be an Interrogating Call Status Control Function (I-CSCF) whereas said Service-Control server may be a Serving Call Status Control Function (S-CSCF) entity.

[0040] This service information is delivered independently of any previous registrations for other subscriber directory identities of the user, and includes service data relevant for a current registration with a subscriber directory identity other than those used for said previous registrations .

[0041] Therefore, the Home Subscriber Database (HSS) is thus arranged for associating specific Service-Control server data with each particular subscriber directory identity. That is, as shown in Fig. 4, subscription data for a user comprises in accordance with the invention a global identity (IMPI) identifying the user as a whole, a number of service profiles (SP-1; SP-2), each service profile associated with a number of subscriber directory identities (IMPU-1, IMPU-j, IMPU-k; IMPU-2, IMPU-m, IMPU- n) , and each subscriber directory identity being possibly assigned a particular Service-Control server (S-CSCF-1; S- CSCF-2; S-CSCF-3; S-CSCF-4).

[0042] The present invention presents several aspects in connection with different problems arising as a consequence of supporting multiple registrations of a user identified by a global identity (IMPI) and a number of subscriber directory identities (IMPU-1, IMPU-2) in different Service- Control servers (S-CSCF-1; S-CSCF-2) .

[0043] In accordance with a first aspect of the present invention, there is provided a new Registration procedure wherein the signalling flow between network entities, as illustrated in Fig. 5, is not affected enough to risk backward compatibility between such entities. There are, however, new information elements to be transferred between said entities, thus resulting in a new Registration procedure as described hereinafter under currently preferred embodiments with reference to the signalling diagram presented in Fig. 3.

[0044] Under a first embodiment, the ^ΛCx-Selection-Info' procedure, as described in 3GPP TS 29.229 standard, is upgraded in such a manner that, upon reception of a user registration with a subscriber directory identity (IMPU-2) and a global identity (IMPI) from a Multimedia Contact- Point server, a Home Subscriber Database (HSS) checks whether the user is already registered with said subscriber directory identity (IMPU-2) for the given global identity (IMPI). Provided that the user is already registered with said subscriber directory identity (IMPU-2) , the Home Subscriber Database (HSS) returns the current Service- Control server (S-CSCF) name for that identity and the process is finished according to currently existing procedures, where the Multimedia Contact-Point server (I- CSCF) can contact with said Service-Control server (S-CSCF) by using this information. This case corresponds to the re- registration of a given subscriber directory identity. On the other hand, if such user was not previously registered with any subscriber directory identity, the current one (IMPU-2) is assumed as the first registration and, according to existing procedures, the Home Subscriber Database (HSS) returns toward the Multimedia Contact-Point server (I-CSCF) an indication of this first registration along with a set of service capabilities needed for allocation of an appropriate Service-Control server (S- CSCF) . These services capabilities returned are preferably based on a particular Service Profile associated to the specific subscriber directory identity (IMPU-2) given for the current registration. [0045] However, provided that such user was not registered yet with said subscriber directory identity (IMPU-2), and even though the user were already registered with another subscriber directory identity (IMPU-1), the Home Subscriber Database (HSS) selects those service capabilities required for the subscriber directory identity (IMPU-2) given during this registration procedure for returning to the requester Multimedia Contact-Point server (I-CSCF) . In addition, the Home Subscriber Database (HSS) may also return names or identifiers of those Service-Control servers (S-CSCF-1) where the user has other subscriber directory identities (IMPU-1; IMPU-3) previously registered, or combinations thereof that can be used by the Multimedia Contact-Point server (I-CSCF) for selecting thereafter the most appropriate Service-Control servers (S-CSCF-2) . Further embodiments are provided for data exchanged to this end between the Home Subscriber Database (HSS) and the Multimedia Contact-Point server (I-CSCF) .

[0046] Then, and following the exemplary flow in Fig. 5, the Multimedia Contact-Point server (I-CSCF), by applying selective criteria with the received service capabilities returned for the new registered subscriber directory identity (IMPU-2) , selects a new Service-Control server (S-

CSCF-2), and registers such user (IMPI, IMPU-2) for said new Service-Control server assignment. The Home Subscriber

Database (HSS) during the ^ΛCx-Put' procedure, thus stores the new Service-Control server (S-CSCF-2) name for the given subscriber directory identity (IMPU-2), and marks it with an 'authentication-pending, pre-registered' state. In this state, an initially assigned Service-Control server

(S-CSCF-1) can continue with the rest of the registration procedures until a new authentication challenge procedure is successfully carried out between the user and the new assigned Service-Control server (S-CSCF-2) . [0047] Traditionally, a global identity (IMPI) identifying the subscription is the identifier used for authentication and a number of Authentication Vectors (AV) are thus prepared on a per subscriber global identity basis. In the present situation where, in accordance with the invention, a second Service-Control server (S-CSCF-2) is being assigned for a user (IMPI) with a second subscriber directory identity (IMPU-2), having a first Service-Control server (S-CSCF-1) already assigned for said user (IMPI) with a first subscriber directory identity (IMPU-1) , a new AV is sent toward the User Equipment (UE) within the authentication challenge procedure triggered from the second Service-Control server (S-CSCF-2). The reception of this AV in the terminal (UE) might produce an authentication synchronisation error since there may be more than one Service-Control server (S-CSCF-1, S-CSCF-2) handling AV s for the same user (IMPI). Then, it may happen that a first AV is received in the terminal (UE) later than a second AV more recently generated for the same user (IMPI, IMPU-1; IMPI, IMPU-2)) being both AV s dealt with by different Service-Control servers (S-CSCF-1, S-CSCF-2) . In such case the terminal may detect that a Sequence number (SQN) part of the firstly regenerated AV is out of range, and return a synchronisation error. The Sequence number is a field included in an AV for validating the AV freshness.

Thus, a simple solution is that the Service-Control server

(S-CSCF-1, S-CSCF-2) receiving such error requests new AV s and repeat the challenge procedure. Further embodiments are provided following this for introducing still more advantageous alternatives to this resynchronisation.

[0048] In this respect, the use of a unique SQN window in the above sort of SIM for all the subscriber directory identities (IMPU-1, IMPU-2, IMPU-3) associated with the same user (IMPI), namely with a particular global identity, can produce synchronisation errors. There are certain solutions to solve these contingencies but they additionally produce extra signalling in the network. One of these solutions is based on arranging the SIM for handling different SQN's depending on particular subscriber directory identities registered, however, this implies that AV s are provided having different SQN windows to each Service-Control server in charge of different subscriber directory identities. Under this approach, the SIM maintains a different SQN window for each particular subscriber directory identity. This solution thus results on that the authentication is performed independently for each subscriber directory identity.

[0049] There is provided in accordance with another aspect of the present invention an alternative mechanism by virtue of which a generation of AV s is triggered in the Home Subscriber Database (HSS) whenever a new authentication challenge procedure is required from a new assigned Service-Control server (S-CSCF-2) . Thereupon, said new authentication may be synchronised in both authentication ends (UE, S-CSCF) , thus allowing a simple control of the SQN used.

[0050] Once the authentication challenge procedure has been successfully performed, and the second Service-Control server (S-CSCF-2) is serving the user (IMPI) with a second subscriber directory identity (IMPU-2), in accordance with the invention, the second Service-Control server (S-CSCF-2) is enabled to download the corresponding Service Profile (SP-2) associated with said second subscriber directory identity (IMPU-2) from the Home Subscriber Database (HSS). Therefore, another 'Cx-Put' procedure along with a Cx- Pull' procedure are used as illustrated in Fig. 5, said ^ΛCx-Put' and ^ΛCx-Pull' procedures being triggered before or after having acknowledged the successful authentication toward the user (UE) . [0051] As anticipated above, there are further embodiments within the 'Cx-Selection-Info' procedure for data exchanged between the Home Subscriber Database (HSS) and the Multimedia Contact-Point server (I-CSCF) for an optimal selection algorithm of a second Service-Control server (S- CSCF-2) . In this respect, and given that the Home Subscriber Database (HSS) contains all the Service Profiles (SP-1; SP-2) information, such Home Subscriber Database is the appropriate entity to choose which information may be returned to the Multimedia Contact-Point server (I-CSCF) for an optimal selection of the second Service-Control server (S-CSCF-2) . Moreover, the Home Subscriber Database may even decide which Service-Control server is the most appropriate for the new given second subscriber directory identity (IMPU-2) still being in a registration stage, especially in the case where other subscriber directory identities (IMPU-1) were previously registered with another Service-Control server (S-CSCF-1) already assigned.

[0052] An advantageous embodiment is proposed wherein the Home Subscriber Database (HSS) may try to minimise the number of Service-Control servers (S-CSCF-1; S-CSCF-2) serving an specific user (IMPI) , thereby allowing the optimisation of the 'Cx-Pull' and the Cx-Push' procedures and updating at a same time the user profile for all those subscriber directory identities registered in a same

Service-Control server. In this respect, all the subscriber directory identities sharing a same service profile also share those data transported in the ^λCx-Pull' and ^ΛCx-Push' commands, for instance a User-Data Attribute Value Peer (AVP) . A further advantage resulting from minimising the number of Service-Control servers serving a specific user

(IMPI) is that potential authentication synchronisation errors are also minimised as well. [0053] Then, under this advantageous embodiment herein described with reference to Fig. 4, the Home Subscriber Database (HSS) compares the service profile (SP-2) of a subscriber directory identity (IMPU-2) being registered versus those service profiles (SP-1) associated with other subscriber directory identities (IMPU-1) previously registered. By virtue of this analysis the Home Subscriber Database (HSS) may identify if an already assigned Service- Control server (S-CSCF-1) exists for this user (IMPI) that might provide acceptable capabilities as required by the new subscriber directory identity (IMPU-2) being registered.

[0054] Provided that at least one previously registered subscriber directory identity (IMPU-m) shares the same service profile (SP-2) as the current subscriber directory identity (IMPU-2) being registered, the same Service- Control server (S-CSCF-4) assigned for said previously registered subscriber directory identity (IMPU-m) can be directly selected for the new subscriber directory identity (IMPU-2) being registered.

[0055] That is, in accordance with this additional embodiment, the Home Subscriber Database (HSS) returns toward the Multimedia Contact-Point server (I-CSCF) an indication of this first registration along with a set of service capabilities needed for allocation of an appropriate Service-Control server (S-CSCF-2; S-CSCF-4), and an identifier, preferably a name, of a suitable Service-Control server (S-CSCF-4) thus selected at the Home Subscriber Database (HSS) .

[0056] However, provided that the capabilities required by the new subscriber directory identity (IMPU-2) being registered do not appropriately match the capabilities of the previously registered subscriber directory identities (IMPU-m; IMPU-n) , the Home Subscriber Database (HSS) returns the capabilities (SC-2) required by the new subscriber directory identity (IMPU-2) being registered and, optionally, an identifier of a suitable Service- Control server (S-CSCF-1), preferably a name, where another subscriber directory identity (IMPU-1) with the most similar acceptable capabilities was registered.

[0057] Regarding different types of capabilities, network operators could mark via provisioning some capabilities as compulsory whereas others may be marked as optional. In this respect, the selection of the above most similar acceptable capabilities takes into consideration an accurate matching of these compulsory capabilities as a primary criterion. Moreover, network operators may assign via provisioning a weight to each particular capability that, compulsory or optional, is part of a service profile. As carrying out such comparison, these weights are used to evaluate differences among the capabilities, thus weighting up each capability accordingly with its relevance for the provision of a service to each specific subscriber directory identity.

[0058] Bearing all this in mind, the Home Subscriber Database (HSS) is arranged for sending the service capabilities for the subscriber directory identity (IMPU-2) being registered in accordance to an aspect of the present invention, or even those capabilities associated to other subscriber directory identities previously registered, depending on different selection criteria established via provisioning means. This information can be further used by a Multimedia Contact-Point server (I-CSCF) for carrying out a more sophisticated Service-Control server selection based on network topology or Service-Control server status. Taking into account the information obtained, the Multimedia Contact-Point server (I-CSCF) is enabled to select an appropriate Service-Control server (S-CSCF-2) for the subscriber directory identity (IMPU-2) being registered, wherein subscriber directory identities sharing a particular service profile preferably share a common Service-Control server.

[0059] There are services, for instance a voice mailbox, that may be active for a user (IMPI) without having a specific subscriber directory number (IMPU) registered to this end. Such a service may have a particular Service- Control server (S-CSCF) assigned for serving said user, and relevant data for said Service-Control server (S-CSCF) is stored in the Home Subscriber Database (HSS) indicating a corresponding registration status. In this respect, and for the purpose of the present invention, such Service-Control server (S-CSCF) is considered under previous and further embodiments as if a corresponding subscriber directory number (IMPU), namely an 'assumed' subscriber directory number were formally registered and associated with a valid service profile to this end.

[0060] In accordance with another aspect of the present invention, a subscriber directory identity being registered following an 'Implicit Registration' procedure is, for the purpose of the present invention, considered under previous and further embodiments as if said subscriber directory identity (IMPU) were formally registered, associated with a valid service profile, and getting an appropriate Service- Control server assigned to this end.

[0061] Thus, having a number of Service-Control servers (S-CSCF-1; S-CSCF-2) per user (IMPI), in accordance with the invention, each Service-Control server assigned to a specific subscriber directory identity (IMPU-1; IMPU-2) implicitly or explicitly registered or 'assumed' , in case that more than one subscriber directory identity has to be de-registered, the Home Subscriber Database (HSS) initiates as many De-registration procedures as Service-Control servers are assigned for that user. Moreover, in like manner as for the de-registration procedures, the Home Subscriber Database (HSS) may initiate when required as many User Data Update procedures as Service-Control servers are assigned for that user. Furthermore, in respect of User Data Access procedures, when registration status of a user is requested by an application, identifiers of those Service-Control servers (S-CSCF-1; S-CSCF-2) assigned per subscriber directory identity (IMPU-1; IMPU-2) are returned.

[0062] The invention is described above in respect of several embodiments in an illustrative and non-restrictive manner. Obviously, many modifications and variations of the present invention are possible in light of the above teachings. The scope of the invention is determined by the claims, and any modification of the embodiments that fall within the scope of these claims is intended to be included therein.

Claims

1. A telecommunications system arranged for providing multimedia services to subscribers of a network operator, each subscriber identified by a global identity (IMPI) and a number of subscriber directory identities (IMPU-1, IMPU-2) , and said global identity

(IMPI) associated with a first Service-Control Server

(S-CSCF-1) in which at least one first subscriber directory identity (IMPU-1) is registered the telecommunications system characterised in -that: it comprises a second Service-Control Server (S-CSCF-2) in which at least one second subscriber directory identity

(IMPU-2) sharing said global identity (IMPI) is registered in.

2. The telecommunications system of claim 1, wherein each Service-Control Server (S-CSCF-1; S-CSCF-2) is coupled to a Home Subscriber Database (HSS) maintaining subscriber registration data and comprising processing means and storage on a per subscriber directory identity (IMPU-1; IMPU-2) basis.

3. The telecommunications system of claim 2, wherein the processing means at a Home Subscriber Server (HSS) include :

- means for verifying that the received subscriber directory identity (IMPU-2) is not registered yet whereas another subscriber directory identity (IMPU- 1) for the same global identity (IMPI) is already registered in a first Service-Control Server (S- CSCF-1) ;

- means for returning to said Multimedia Contact-Point server (I-CSCF) service capabilities (SC-2) of a suitable Service-Control Server (S-CSCF-2) for serving the subscriber (IMPI) with the second subscriber directory identity (IMPU-2) ; and

- means for receiving an identifier of a second Service-Control Server (S-CSCF-2) to be stored for the subscriber (IMPI) with the second subscriber directory identity (IMPU-2) and along with a new registration status.

4. The telecommunications system of claim 3, wherein the second subscriber directory identity (IMPU-2) having been assigned said second Service-Control Server (S- CSCF-2) is marked 'authentication pending, pre- registration' .

5. The telecommunications system of claim 4, wherein the Home Subscriber Database (HSS) triggers generation of a number of new authentication vectors to allow a suitable synchronisation in both authentication ends.

6. The telecommunications system of claim 3, wherein the Home Subscriber Database (HSS) has further means for downloading a number of Service Profiles (SP-1; SP-2) associated with respective subscriber directory identities (IMPU-1; IMPU-2) toward corresponding Service-Control Servers (S-CSCF-1; S-CSCF-2) respectively assigned to said subscriber directory identities .

7. The telecommunications system of claim 3, wherein the Home Subscriber Database (HSS) is configured to return toward a Multimedia Contact-Point server (I-CSCF) , in addition to service capabilities (SC-2) of a suitable Service-Control Server (S-CSCF-2) for a subscriber directory identity (IMPU-2) still in a registration stage, identifiers of those Service-Control Servers (S- CSCF-1) where the user is registered in with other subscriber directory identities (IMPU-1) .

8. The telecommunications system of claim 7, wherein the Home Subscriber Database (HSS) is configured to select the most appropriate Service-Control Server for a new subscriber directory identity still in a registration stage, by applying selective criteria among a number of Service-Control Servers already assigned to a number of subscriber directory identities registered in.

9. The telecommunications system of claim 8, wherein the selective criteria in the Home Subscriber Database (HSS) include means for comparing capabilities between different service profiles (SP-1, SP-2), and means for identifying an already assigned Service-Control Server (S-CSCF-1) that can provide acceptable capabilities for the new subscriber directory identity (IMPU-2) being registered.

10. The telecommunications system of claim 8, wherein the selective criteria in the Home Subscriber Database (HSS) include means for weighting capabilities in Service Profiles and assigned Service-Control Server thus allowing comparisons and analysis.

11. The telecommunications system of any preceding claim, wherein registration of a user, with a global identity and a particular subscriber directory identity, is carried out following an explicit registration procedure for said particular subscriber directory identity individually, or following an implicit registration procedure for a group that includes said particular subscriber directory identity, or 'assumed' for active services that do not require a registration for having a Service-Control Server assigned.

12. A method of registering in a telecommunication system a subscriber identified by a global identity (IMPI) and a number of subscriber directory identities (IMPU-1, IMPU-2) , the subscriber having at least one first subscriber directory identity (IMPU-1) already registered in a first Service-Control Server (S-CSCF- 1), the method comprising the steps of:

(a) receiving a request for registration of the subscriber (IMPI) with a second subscriber directory identity (IMPU-2) ; and

(b) verifying that said subscriber (IMPI) is not registered with said second subscriber directory identity (IMPU-2);

the method characterised by including the steps of

(c) obtaining service capabilities (SC-2) of a suitable Service-Control Server for serving the subscriber

(IMPI) with the second subscriber directory identity (IMPU-2); and

(d) registering the subscriber (IMPI) with the second subscriber directory identity (IMPU-2) in a second Service-Control Server (S-CSCF-2) other than the first Service-Control Server (S-CSCF-1), said second Service-Control Server (S-CSCF-2) fitting the received service capabilities (SC-2) .

13. The method of claim 12, wherein the step a) of receiving a request for registration includes the step of querying from a Multimedia Contact-Point server (I- CSCF) a check at a Home Subscriber Database (HSS) about the registration status for the received subscriber identities (IMPI, IMPU-2) .

14. The method of claim 12, wherein the step d) of registering the subscriber in a second Service-Control

Server (S-CSCF-2) comprises the step of triggering generation of a number of new authentication vectors.

15. The method of claim 14, wherein the step of triggering generation of a number of new authentication vectors includes an indication of generating a number of equal vectors just differing in an SQN field.

16. The method of claim 12, wherein the step c) of obtaining service capabilities (SC-2) of a suitable Service-Control Server for serving a subscriber with a second subscriber directory identity (IMPU-2) includes the step of additionally selecting the most appropriate Service-Control Server for said second subscriber directory identity, by applying selective criteria among a number of Service-Control Servers already assigned to a number of subscriber directory identities registered in.

17. The method of claim 12, further comprising the step of downloading from a Home Subscriber Database (HSS) a number of Service Profiles (ΞP-1; SP-2) associated with respective subscriber directory identities (IMPU-1; IMPU-2) toward corresponding Service-Control Servers (S-CSCF-1; S-CSCF-2) respectively assigned to said subscriber directory identities.