Abstract:
The invention relates to a method, a system and a network element for controlling data transmission in a network environment comprising a first control function I-CSCF  206  and a second control function S-CSCF  203.  In order to optimise the network resource usage the second control function S-CSCF  203  is allocated by said first control function I-CSCF  206  dynamically when a network transaction is processed. Thus, network resources are saved as a permanent allocation of the second control function S-CSCF  203  is avoided.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates to a method, a system and a network element for controlling data transmission in a network environment such as in UMTS (Universal Mobile Telecommunications System) networks.  
         BACKGROUND OF THE INVENTION  
         [0002]    Internet Protocol Multimedia Subsystems (IMS) have been developed to support IP (Internet Protocol) Multimedia (IM) services in UMTS. Such IMSs have been developed with an attempt to conform to internet standards set out by the IETF (Internet Engineering Task Force), in order to achieve access independence and to maintain a smooth interoperation with wireless terminals across the internet. Therefore, interfaces specified conform as far as possible to IETF standards for those cases where an IETF protocol has been selected, e.g. the session initiation protocol (SIP) as defined in “SIP: Session Initiation Protocol”, IETF document RFC2543 by Handley/Schulzrinne/Schooler/Rosenberg.  
           [0003]    The session initiation protocol is used to establish multimedia sessions or calls, e.g. VoIP (Voice over Internet Protocol) sessions. The session initiation protocol is a request-response control (signalling) protocol for initiating, maintaining and terminating sessions with one or more participants or terminal devices. The session initiation protocol uses session initiation messages to negotiate between participants or terminal devices.  
           [0004]    The IMS enables operators of mobile networks to offer their subscribers multimedia services based on and build upon internet applications, services, and protocols. There are two possible scenarios to provide services, i.e. via the service platform in the home network or via an external service platform, e.g. a third party or visited network.  
           [0005]    Session control functions as call state control functions (CSCFs) are provided which can act as proxy CSCFs (P-CSCFs), serving CSCF (S-CSCFs) or interrogating CSCFs (I-CSCFs). The P-CSCF is the first contact point for a user or user equipment (UE) within the IMS. The S-CSCF actually handles the session states in the network. The I-CSCF is mainly the contact point within an operator&#39;s network for all connections destined to a subscriber of that network operator. A more detailed description of the IMS can be gathered from the 3GPP (Third generation partnership project) specification TS 23.228 “Technical Specification Group Services and System Aspects—IP Multimedia (IM) Subsystem—Stage 2”.  
           [0006]    In the conventional IMS the S-CSCF is allocated permanently in the registration of a user equipment. As this allocation is done for each user equipment a corresponding large amount of network resources is needed while the network resources are not used in an efficient manner.  
         SUMMARY OF THE INVENTION  
         [0007]    It is therefore an object of the present invention to improve the efficiency of network resource usage.  
           [0008]    This object is achieved by a method for controlling data transmission in a network environment by means of a first control function and a second control function, said method comprising the step of said first control function allocating said second control function dynamically when a network transaction is processed.  
           [0009]    Furthermore, the above object is achieved by a system for controlling data transmission in a network environment, comprising a first control function and a second control function, said first control function comprising allocating means for allocating said second control function dynamically when a network transaction is processed.  
           [0010]    Furthermore, the above object is achieved by a network element for controlling data transmission in a network environment and for use in such a method and/or in such a system, the network element comprising a control function, said control function comprising allocating means for allocating a second control function dynamically when a network transaction is processed.  
           [0011]    The present invention has realized that during data transmission in a network environment, such as during sessions between users or during user registration, the above mentioned second control function, e.g. a S-SCSF, is needed only under certain circumstances, namely when a network transaction is processed such as when any kind of transaction within the network, e.g. any kind of data exchange, in particular message exchange between users and/or network elements, is activated or terminated during a session, in particular when a session is setup or terminated for a user or when a user is registering.  
           [0012]    When a user is registering, the S-CSCF may be allocated in order to prepare for the incoming session setups, i.e. user originated or terminated session setup. The preparation of the S-CSCF means that e.g. the user profile information is downloaded to the S-CSCF. However, the preparation may also be performed dynamically when the session is setup.  
           [0013]    During the time when a user does not have any active sessions there is no need for that second control function (e.g. S-CSCF), and an allocation of that second control function by the first control function (e.g. I-CSCF) merely uses resources of the second control function (e.g. S-CSCF) and its database because the second control function stores subscriber specific information that is momentarily not needed. However, such a permanent allocation of the second control function (e.g. S-CSCF) does not have any reasonable advantage.  
           [0014]    Therefore, the invention proposes to allocate the second control function (e.g. S-CSCF) dynamically, i.e. only occasionally, namely only during such conditions when it is actually needed. Thus, a permanent allocation is avoided and network resources can be saved and hence the network resource usage is optimised as no second control function (e.g. S-CSCF) is reserved for inactive users that do not actually need the second control function (e.g. S-CSCF).  
           [0015]    A further advantage of the present invention is the fact that no data is lost as a result of second control function (e.g. S-CSCF) resets when such resets are performed at a time when this second control function has not been allocated.  
           [0016]    Preferably, the decision of whether or not to allocate said second control function is made  
           [0017]    either by or in said first control function or by or in a first network element comprising said first control function,  
           [0018]    or by or in a further control function or by or in a further network element comprising said further control function.  
           [0019]    Thereby the decision can be made on logical and/or on physical levels. Thus, as an example the further network element/further control function could decide that the second control function is allocated dynamically (or not) and the first control function makes the actual allocation of the second control function if decided by the further network element/further control function. Thereby, the control functions may be co-located, especially the first and the further control function may be co-located.  
           [0020]    Further advantageous developments of the present invention are defined in the dependent claims.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0021]    In the following, the present invention will be described in greater detail based on preferred embodiments with reference to the accompanying drawings, in which:  
         [0022]    [0022]FIG. 1 shows a signaling diagram indicating an IM subscriber registration according the prior art;  
         [0023]    [0023]FIG. 2 shows a signaling diagram indicating an IM subscriber registration according to a first embodiment of the present invention;  
         [0024]    [0024]FIG. 3 shows a signaling diagram indicating an mobile originated connection setup according to a second embodiment of the present invention; and  
         [0025]    [0025]FIG. 4 shows a signaling diagram indicating a mobile terminated connection setup according to a third embodiment of the present invention. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0026]    The preferred embodiments will be described hereinafter. However, a prior art registration procedure is described first with reference to FIG. 1 in order to ease the understanding of the present invention.  
         [0027]    When a subscriber, e.g. a corresponding user equipment  101 , roams to a visited network  102 , a session control function, namely a S-CSCF  103  is located in the home network  104  of the subscriber, while the visited network  102  supports another session control function, namely a P-CSCF  105 . The P-CSCF  105  enables the session control to be passed to the home network based S-CSCF  103  which provides the service control. Another session control function, namely an I-CSCF  106  can be used in the SIP signaling path to shield the internal structure of a network from other networks. An I-CSCF  106  is therefore located at the home network  104  location.  
         [0028]    Furthermore, at the home network  104  location a further control function, namely a home subscriber server (HSS)  107  is provided which substitutes a home location register (HLR) when the IMS is implemented. The HSS  107  is a master database for a given user and contains the subscription related information to support the network entities actually handling calls or sessions. Furthermore, subscriber related data may be stored in the S-CSCF  103  and the P-CSCF  105 .  
         [0029]    The above described network and network elements as well as their functions apply to the prior art system as well as to the system according to the present invention, in particular according to the preferred embodiments according ot FIGS.  2  to  4 .  
         [0030]    A conventional registration of the UE  101  will be described next with reference to FIG. 1.  
         [0031]    After the concerned UE  103  roaming in the visited network  102  has obtained a signaling channel through the access network, it can perform the IM registration as described in 3GPP specification TS 23.228. To do so, the UE  101  sends in a first step a SIP REGISTER message  108  comprising a register information flow (subscriber identity, home network&#39;s domain name) to the P-CSCF  105  which examines the home domain name to determine the entry point (i.e. I-CSCF  106 ) to the home network  104  based on a name-address resolution mechanism.  
         [0032]    Then, in a second step, the P-CSCF  105  sends a SIP REGISTER message  109  to the determined I-CSCF  106 . The SIP REGISTER message  109  comprises register information (P-CSCF&#39;s name in the contact header, subscriber identity, visited network&#39;s contact name). Having received the register information, the I-CSCF  106  employs a name-address resolution mechanism to determine (e.g. based on the subscriber identity and home domain name) the address of the HSS  107  to be contacted.  
         [0033]    Next, signaling flow shows the functional flows of the Cx interface. The Cx (interface between a CSCF and the HSS  107 ) registration signaling is performed in the next four steps. These steps are combined into two messages. All Cx steps may be combined between I-CSCF  106  and HSS  107  as well as between S-CSCF  103  and HSS  107 . Even though the steps may also be defined differently, basic functionality of the Cx interface as shown in the signaling flow remains the same.  
         [0034]    Firstly, a signalling is performed between the I-CSCF  106  and the HSS  107  to obtain the S-CSCF  103  capabilities at the I-CSCF  106 . Based on these S-CSCF  103  capabilities, the I-CSCF  103  derives the address of the S-CSCF  103  by using an S-CSCF selection function and a name-address resolution mechanism.  
         [0035]    To describe in detail, in the third step the I-CSCF  106  sends the Cx-Query information flow of a Cx-Query message  110  to the HSS  107 . The Cx-Query message  110  comprises information as P-CSCF  105  name, subscriber  101  identity, home domain name and visited network  102  contact name. The P-CSCF  105  name denotes the contact name that the operator wishes to use for future contact to P-CSCF  105 .  
         [0036]    Upon receipt of the Cx-Query message  110  the HSS  107  determines whether UE  101  is already registered. Furthermore, the HSS  107  indicates whether UE  101  is allowed to register in the visited network  102  taking into account the UE subscription and operator limitations or restrictions.  
         [0037]    In the fourth step HSS  107  sends a Cx-Query-response message  111  to the I-CSCF  106 . In case the HSS  107  indicated that UE  101  is not allowed to register in the visited network  102  the Cx-Query response message  111  comprises an information indicating a rejection of the registration attempt of the UE  101 .  
         [0038]    In the fifth step it is assumed that the authentication of the UE  101  has been successfully completed although this might have been determined earlier in the information flows. It is noted that the authentication is not shown in the signaling flow of FIG. 1. In FIG. 2 it is shown as a ‘box’.  
         [0039]    In this fifth step the I-CSCF  106  sends a Cx-select-pull message  112  to the HSS  107  to request information related to required S-CSCF  103  capabilities that are input into the S-CSCF  103  selection function. The Cx-Select-Pull message  112  comprises information about the serving network indication and the subscriber identity.  
         [0040]    In the sixth step HSS  107  returns a Cx-Select-Pull response message  113  indicating the required S-CSCF  103  capabilities to the I-CSCF  106 .  
         [0041]    Then, in the seventh step the I-CSCF  106  sends a SIP REGISTER message  114  to the selected S-CSCF  103  which performs a Cx registration signaling with the HSS  107  in steps  8  to  11  in order to obtain information required to access a platform used for service control while the UE  101  is registered at its S-CSCF  103 .  
         [0042]    In particular in the seventh step the I-CSCF  106 , using the name of the S-CSCF  103 , determines the address of the S-CSCF through a name-address resolution mechanism and then sends the register information flow to the selected S-SCSF  103 . The register information flow comprises the P-CSCF&#39;s name in the contact header, the subscriber identity and the visited network  102  contact name.  
         [0043]    In the eighth step the S-CSCF  103  sends a Cx-Put message  115  to the HSS  107  indicating the subscriber identity and the S-CSCF  103  name that is stored in the HSS  107  for that subscriber  101 .  
         [0044]    In the ninth step the HSS  107  sends a Cx-Put response message  116  to the I-CSCF  106  in order to acknowledge Cx-Put message  115 .  
         [0045]    In the tenth step S-CSCF  103  sends a Cx-Pull message  117  indicating the subscriber identity to the HSS  117  in order to enable S-CSCF  103  to download the relevant information from the subscriber profile. Then, S-CSCF  103  stores the P-CSCF&#39;s name that has been supplied by the visited network  102 . This P-CSCF&#39;s name represents the name that the home network  104  uses to forwarding subsequent terminating session signaling for the UE  101 .  
         [0046]    In the eleventh step HSS  107  returns a Cx-Pull response message  118  to the S-CSCF  103  indicating user information. This user information includes one ore more name/address information for use to access platforms used for service control, while the UE  101  is registered at this S-CSCF  103 . The S-CSCF  103  stores this information for the respective UE  101 . Furthermore, additional name/address information and security information can also be transmitted for use within the S-CSCF  103 .  
         [0047]    In the twelfth step S-CSCF  103  determines whether the home contact name is the S-CSCF name or an I-CSCF name. If an I-CSCF is chosen as the home contact name, it may be distinguished form the I-CSCF  106  that appears in the registration flow. This home contact name will be used by a P-CSCF  105  for forwarding signaling to the home network  104 . Hence, the S-CSCF  103  sends a 200 OK SIP message  119  comprising the serving network contact name and the S-CSCF name to the I-CSCF  106 .  
         [0048]    Then, in the thirteenth step the I-CSCF sends a 200 OK SIP message  120  indicating the serving network contact name to the P-CSCF  105 . The I-CSCF releases at this point all registration information after sending the 200 OK SIP message  120 .  
         [0049]    Then, P-CSCF  105  stores the serving network contact name and sends a 200 OK SIP message  120  to the UE  101 .  
         [0050]    According to the above described signaling the S-CSCF is allocated in the registration always or permanently. In the following preferred embodiments of the present invention are described, wherein the S-CSCF is allocated dynamically, i.e. from time to time, only when the S-CSCF is needed. However, in the preferred embodiments some parts of the above described prior art system are similar, including the network elements as well as parts of the signaling in between.  
         [0051]    [0051]FIG. 2 shows the registration of a UE  201  without allocating the S-CSCF  203 . UE  201 , P-CSCF  205 , I-CSCF  206 , HSS  207  and S-CSCF  203  respectively correspond to UE  101 , P-CSCF  105 , I-CSCF  106 , HSS  107  and S-CSCF as far as no deviations are described hereinafter.  
         [0052]    UE  201  roams in a visited network and sends in a first SIP REGISTER message  208  indicating in a “from” header field a private identifier and indicating in a “to” header field a public identifier as well as a contact name. However, these header fields may also contain any other identity allowed by SIP specifications. This SIP REGISTER message  208  is send to P-CSCF  205 . P-CSCF  205  forwards the content of the SIP REGISTER message  208  as a further SIP REGISTER message  209  to the I-CSCF  206 .  
         [0053]    Once the authentication is performed the I-CSCF  206  queries in a third step the HSS  207  whether the registration is allowed for the UE  201  by sending a Cx-AAA (AAA=Authentication, Authorization and Accounting) request message  210  comprising the above mentioned private identification. Thereby the Cx-AAA request message  210  actually starts the authentication if the HSS requires it.  
         [0054]    In the fourth step HSS  207  sends a Cx-AAA response message  211  indicating whether the registration is allowed for UE  201  or not, i.e. the registration attempt should be rejected or accepted.  
         [0055]    In the fifth step the I-CSCF  206  sends a Cx-Query message  212  to the HSS  207  comprising the identities, namely the private and the public identities of UE  201  as well as the P-CSCF  205  name indicating where the UE  201  is located at the moment. The P-CSCF  205  name is needed for routing the mobile terminated connection setups to the UE  201 . Furthermore, a registration timeout value is sent in the Cx-Query message  212  indicating an registration expiration time. The registration timeout value is needed for monitoring the lifetime of the registration. As in the previous case described with reference to FIG. 1, these and the following Cx messages may be combined.  
         [0056]    In the sixth step the HSS  207  returns a Cx-Query-Response message  213 .  
         [0057]    In the case the registration of the UE  201  is valid, the I-CSCF  206  may request information regarding the S-CSCF  203  capability requirements in the seventh step by sending a Cx-Select message  214 . A corresponding Cx-Select-Response message  215  is returned from HSS  207  to I-CSCF  206 . If the requirements are such that there is no need to allocate the S-CSCF  203 , the I-CSCF  206  decides that it does not allocate the S-CSCF  203 . Even though the I-CSCF  203  has requested the S-CSCF capability requirements information in the seventh step and has based its decision whether or not to allocate the S-CSCF (i.a.) on the information returned in step eight by Cx-Select-Response message  215 , it is noted that this message exchange according to step  7  and  8  is not needed in case there is no additional information that the I-CSCF  206  needs to perform the decision whether or not to allocate the S-CSCF  203 . Thus, step  7  and  8  and Cx-Select message  214  and Cx-Select-Response message  215  are just optional.  
         [0058]    Furthermore, it is noted that the I-CSCF  206  may perform additional notification to the HSS  207  that the S-CSCF  203  was not allocated.  
         [0059]    After the decision not to allocate the S-CSCF  203  the registration is acknowledged to the user  201 , i.e. in a ninth step a SIP 200 OK message  216  is send from the I-CSCF  206  to the P-CSCF  205  which is forwarded from the P-CSCF  205  to UE  201  in a tenth step by transmitting a 200 OK SIP message  217  from the P-CSCF  205  to UE  201 . Both 200 OK SIP messages  216  and  217  comprise the private identifier and the public identifier of the user  201 .  
         [0060]    Thus, I-CSCF has decided based on information received from HSS  207  or based on the decision of HSS whether or not to allocate S-CSCF in the registration. As I-CSCF  206  has decided in the case according to FIG. 2 that the S-CSCF  203  is not allocated, network resources have been saved.  
         [0061]    [0061]FIG. 3 shows a mobile originated connection setup procedure, i.e. a connection setup initiated from a mobile device, e.g. from UE  301 . UE  301 , P-CSCF  305 , I-CSCF  306 , HSS  307  and S-CSCF  303  correspond respectively to UE  201 , P-CSCF  205 , I-CSCF  206 , HSS  207  and S-CSCF  203  as described above with reference to FIG. 2.  
         [0062]    In a first step UE  301  sends a SIP INVITE message  308  to the P-CSCF  305  comprising a “from” header field indicating a public identifier of the calling party A, namely UE  301 , and a “to” header field indicating a public identifier of a called party B.  
         [0063]    In a second step the P-CSCF  305  forwards the message content from SIP INVITE message  308  as a further SIP INVITE message  309  to the I-CSCF  306 .  
         [0064]    In a third step the I-CSCF  306  sends a location query message Cx-Loc-Query  310  to the HSS  307  in order to locate the S-CSCF  303  for the user  301 . A corresponding response message Cx-Loc-Query-Response  311  is returned from HSS  307  to I-CSCF  306  in a fourth step. Note that the User  301  may be allocated in this point or the authentication may be started from the S-CSCF  303  after a message  314  is sent from I-CSCF  306  to the S-CSCF  303  as described hereinafter.  
         [0065]    In a fifth step a Cx-Select message  312  is sent from I-CSCF  306  to HSS  307  in order to request information regarding the S-CSCF capability requirements. The HSS  307  returns a corresponding Cx-Select-Response message  313  indicating information regarding the S-CSCF  303  capability requirements.  
         [0066]    Next, the I-CSCF  306  decides on the received information whether or not to allocate the S-CSCF  303 . However, it is also possible that the HSS  307  makes this decision and the I-CSCF  306  acts accordingly. In the example of FIG. 3, the I-CSCF  306  determines that no S-CSCF  303  has been allocated so far and based on the received information the I-CSCF  306  allocates the S-CSCF  303 , when it has the information based on which it can perform the allocation.  
         [0067]    In the seventh step the I-CSCF  306  sends a SIP INVITE message  314  to the S-CSCF  303  comprising the information contained in SIP INVITE message  309  that the I-CSCF  306  has received from P-CSCF  305 .  
         [0068]    In the eighth step the S-CSCF  303  may inform the location of the S-CSCF  303  to the HSS  307  by sending a Cx-Put message  315  to HSS  307 . It is noted that if it is allowed that for other sessions another S-CSCF may used then the S-CSCF does not need to inform the location of the S-CSCF to the HSS  307 . HSS  307  returns a corresponding Cx-Put-Response message  316  to S-CSCF  303  in order to acknowledge the sending of Cx-Put message  315  in a ninth step. In a tenth step the S-CSCF  303  requests subscriber information from the HSS  307  by sending a Cx-Pull message  317 . In step eleven a subscriber information is send from HSS  307  to S-CSCF  303  by a corresponding Cx-Pull-Response message  318 .  
         [0069]    After these operations a SIP INVITE message  319  is forwarded from the S-CSCF  303  to the next hop which may be an application server or the network of the called party B.  
         [0070]    It is noted that the I-CSCF  306  may also decide not to select any S-CSCF for the session if the I-CSCF  306  or HSS has determined that a S-CSCF is not needed for controlling the session. In this case signalling is directly performed from the I-CSCF  306  to the called user. Thus, network resources can be saved again.  
         [0071]    [0071]FIG. 4 shows a mobile terminated connection setup procedure, i.e. a connection setup that has been directed to a mobile device e.g. to UE  401 . UE  401 , P-CSCF  405 , I-CSCF  406 , HSS  407  and S-CSCF  403  correspond substantially to UE  201 , P-CSCF  205 , I-CSCF  206 , HSS  207  and S-CSCF  203 , respectively as being described above with reference to FIG. 2 or to UE  301 , P-CSCF  305 , I-CSCF  306 , HSS  307  and S-CSCF  303 , respectively, as described above with reference to FIG. 3.  
         [0072]    In a first step I-CSCF  406  receives a SIP INVITE message  408  from a calling party A (not shown). The SIP INVITE message  408  comprises a “from” header field indicating a public identifier of the calling party A and a “to” header field indicating a public identifier of a called party B, namely UE  401 .  
         [0073]    In a second step I-CSCF  406  sends a location query message Cx Loc-Query  409  to the HSS  407  in order to locate the S-CSCF for the user  401 . A corresponding response message Cx-Loc-Query-Response  410  is returned from HSS  407  to I-CSCF  406  in a fourth step indicating the P-CSCF  405  name where the user  401  is located.  
         [0074]    In a fourth step a Cx-Select message  411  is sent from I-CSCF  406  to HSS 407  in order to request information regarding the S-CSCF capability requirements. The HSS  407  returns a corresponding Cx-Select-Response message  412  indicating information regarding the S-CSCF  403  capability requirements.  
         [0075]    Next, I-CSCF  406  or HSS  407  decides whether or not to allocate the S-CSCF  403 , in particular it is determined whether the S-CSCF has already been allocated. In the example of FIG. 4 there is no S-CSCF allocated and the I-CSCF  406  allocates the S-CSCF  403  after step  5  when I-CSCF  406  has the information based on which it can perform an allocation.  
         [0076]    In step  6  I-CSCF  406  sends a SIP INVITE message  413  to S-CSCF  403  comprising the information contained in SIP INVITE message  408  that the I-CSCF  406  has received in step  1 . Thereby, also the P-CSCF  405  name is forwarded to the S-CSCF  403  for routing the setup. The P-CSCF name may be provided to the S-CSCF  403  from the HSS  407  when the S-CSCF  403  ‘pulls’ the subscriber information from the HSS  407 , i.e. in message  416  described hereinafter. Similarly as in the case described with reference to FIG. 3 the S-CSCF  403  may inform the location of the S-CSCF  403  to the HSS by sending a Cx-Put message  414  in step  7 . HSS  407  returns in a eighth step a corresponding Cx-Put-Response message  415  to S-CSCF  403  in order to acknowledge the sending of Cx-Put message  414 .  
         [0077]    In step  9  the S-CSCF  403  requests the subscriber information from the HSS  407  by sending a Cx-Pull message  416 . In step  10  the requested subscriber information is sent from HSS  407  to S-CSCF  403  by a corresponding Cx-Pull-Response message  417 .  
         [0078]    Next the setup is routed to the P-CSCF  405 , i.e. the S-CSCF  403  sends a SIP INVITE message  418  to P-CSCF  405  in step  11 , whereby this SIP INVITE message  418  comprises the information contained in SIP message  408  or  413 . Then, in step  12  a further SIP INVITE message  419  is forwarded from P-CSCF  405  to UE  401  comprising the information contained in SIP INVITE message  418 .  
         [0079]    It is noted that the I-CSCF  406  may not select any S-CSCF for the session if the I-CSCF  406  decides that no S-CSCF is needed for controlling the session. In this case the session signalling goes directly from the I-CSCF  406  to the called user UE  401 .  
         [0080]    The present invention allows due to the dynamical allocation of the S-CSCF on the basis of a decision made by the I-CSCF or the HSS a more efficient network resource usage as no S-CSCFs need to be reserved for inactive users that do not actually need a S-CSCF. Thus, network resources are saved as a permanent allocation of the S-CSCF is avoided.  
         [0081]    It is noted that the present invention is however not restricted to the preferred embodiments described above, in particular any other kind of SIP messages besides REGISTER and INVITE messages can be transmitted via the network elements. Furthermore, the present invention can be implemented in any fixed or wireless network environment using any kind of session initiation protocols in packet switched networks as well as in circuit switched networks as well as in combined packet switched an circuit switched networks, in particular 3GPP specified networks. The preferred embodiments may thus vary within the scope of the attached claims.