WO2005109936A1

WO2005109936A1 - An arrangement and a method for operator assignment in a shared network environment

Info

Publication number: WO2005109936A1
Application number: PCT/EP2004/004922
Authority: WO
Inventors: Hans RÖNNEKE; Gunnar Rydnell
Original assignee: Telefonaktiebolaget L M Ericsson (Publ)
Priority date: 2004-05-07
Filing date: 2004-05-07
Publication date: 2005-11-17

Abstract

The present invention relates to an arrangement for assigning core network operator to a mobile user station (UE), e.g. a roaming mobile user station not supporting a functionality of extended broadcast information for shared networks, in a shared network environment wherein different core network operators share a radio network. Each core network comprises a number of core network nodes and supports a redirection functionality for redirecting mobile user stations to find the appropriate core network operator. it comprises a caching arrangement in or associated with a node. In said caching arrangement identification information, e.g. IMSI or MSISDN or IMEI, of a mobile user station, requesting to be registered, is stored. Further information is stored about to which operators an attach request has been sent, such that for subsequent attach requests from the same mobile user station during a mobile user station retry period, said information about identification information and operators is used at subsequent selections of operators during the mobile user station retry period to prevent unnecessary reselection of an already tried operator or core network node.

Description

Title:

AN ARRANGEMENT AND A METHOD FOR OPERATOR ASSIGNMENT IN A SHARED NETWORK ENVIRONMENT

FIELD OF THE INVENTION

The present invention relates to an arrangement for assigning core network operator to a mobile user station which e.g. is roaming, particularly a mobile user station not supporting a functionality of extended broadcast information for shared networks, in a shared network environment in which a number of core networks have different core network operators and share a radio network with a number of radio network control means. Each core network comprises a number core network nodes and supports a redirection functionality for redirecting mobile, e.g. roaming user stations, in order to assign such mobile user stations to the appropriate core network operator. The invention also relates to a method for assigning core network operator to a mobile user station in a shared network environment. Still further the invention relates to a node in a communication system in which a number of different operators, each controlling respective core network nodes, share a radio network comprising shared radio network control means and in which a redirection functionality is supported for redirecting mobile user stations, e.g. roaming mobile user stations, in order to assign such user stations to the appropriate core network operator. STATE OF THE ART

Network sharing has become popular in order to allow for the operators to have their own core networks but still sriare radio network comprising e.g. Radio Network Controllers (RNCs), Radio Base Stations or Base Station Controllers (BSCs), Base Transceiver Stations (BTSs), etc.

When a mobile user station, for example a User Equipment (UE) , wants to register, or to get attached or connected to the network, it has to be connected to the appropriate core network operator. For mobile user stations or UEs fulfilling the requirements of 3GPP release 6, this is not a problem, since then the RNCs via broadcast provide information about which operators that are available (share the radio network, particularly the RNC) and the mobile user station is then capable of informing the RNC about to which core network node or to which operator it is to be connected. However, for mobile user stations not fulfilling such requirements, here denoted extended broadcast information for shared networks, the situation is different. Such a mobile user station which previously has been registered or attached to one of the core network operators sharing for example an RNC, has a TMSI/P-TMSI which is valid in one of the CN nodes. Using the well known Iu- flex functionality, a mobile station should indicate the NRI (Network Resource Indicator) , which is part of TMSI/P-TMSI to the RNC. The Iu-flex functionality in the RNC then selects the correct core network node, and hence the appropriate core network operator for the registration request by the mobile user station. The core network operators have to coordinate their NRIs for example by letting the most significant bit or bits indicate the operator. The Iu-flex functionality makes redirection unnecessary for UEs that have been previously registered.

Redirection is a mechanism which is used as a part of the assignment of core network operator in some situations in a Multi Operator Core Network (MOCN) configuration where the Radio Network Control node, e.g. RNC, does not know what core network operator should be assigned to a requesting mobile user station, i.e. the mobile user station has not been previously registered.

Today there are two main alternatives for performing redirection in a shared network, denoted a RAN Centric approach and a CN Centric approach. The redirection mechanism is implemented for example when a visiting roaming subscriber requests a very first attach to a new network. The RAN Centric approach is for example described in 3GPP Technical Report TR 23.851 V6.0.0. In the RAN Centric approach it is the radio network control node that is responsible for the redirection mechanism, whereas in the CN Centric approach it is a core network node that is responsible for the redirection mechanism.

Fig. 1 is a block diagram schematically illustrating redirection in a RAN Centric solution. For a mobile user station UE 1₀ which has not previously been attached to any of the core network operators, here core network operator A and core network operator B, and consequently does not have a valid TSMI/P-TMSI for any one of the CN nodes (an MSC or an SGSN) ; here MSC/SGSN 6A₀ and MSC/SGSN 6B₀, the RNC 2o, upon reception of an initial registration request from the mobile station, which does have any valid TMSI/P-TMSI (no valid NRI) for CN operator A or B, selects a CN node 6 o; 6B₀ randomly or based on an invalid NRI. In both those situations the CN node may be able to attach the mobile user station successfully, but often it has to request the RNC 2₀ to perform a redirection. Generally the RNC performs a random selection if no NRI is present or performs an Iu-flex based selection if NRI, valid or invalid, is present. The redirection is performed based on IMSI (International Mobile Subscriber Identity) as provided by, in this case, operator A CN node 6A₀, if the mobile user station is a home subscriber, or based on random selection among remaining CN operators (here the CN operator B) (there may of course be more operators sharing the RNC) , if it is a visiting subscriber. The amount of successful attachments will depend, for home subscribers, on the number of CN operators in the MOCN, and for visiting subscribers it will depend on the roaming agreement of the CN operators. For home subscribers, where, through the IMSI, the CN operator can be identified in the RNC, the attach request will always be successful on the second try if it was not successfully already on the first try, whereas for visiting subscribers the number of tries depends on the number of CN operators and what roaming agreements they have. In this implementation, as illustrated in Fig. 1, when the operator A node has been selected for example randomly, IMSI is checked in MSC/SGSN 6A₀ and it becomes clear that this subscriber, i.e. the mobile user station UE lo, is not allowed. A normal reject message with additional redirect information attached is then returned to the RNC 2₀, which redirects the request to the operator B CN node 6B₀. An operator of a CN node may reject an attach request from the mobile user e.g. because it has no roaming agreement with the home PLMN of the mobile user station subscriber or because of there being a roaming restriction in the HLR associated with the CN node. If despite trying several core network nodes to redirect to after having- a received a so called Reroute Command message from a CN node, the RNC may compare the cause code from all the Reroute Command messages it has received from different core network nodes for the concerned UE. Generally a cause code ranking is performed and the "softest" cause code is selected and an attach reject message is returned to the mobile user station.

During the whole redirect procedure the RNC keeps a timer which corresponds to the mobile user station (UE) timer of releasing the Radio Resource (RR) connection (20 seconds), i.e. the connection RNC-UE. If the RNC upon reception of the Reroute Command message finds that there is not enough time for another redirection, the whole procedure will be aborted and all messages and stored information will be silently discarded, i.e. without providing any information as to that effect or storing any information whatsoever. The mobile user station will however repeat its attach request four times, each time waiting 15 seconds before it re-establishes the RR connection for another try. Since all information was discarded at the previous try or tries, there is however a risk that for example a random selection after four retries would still have missed a CN node, the operator that would have accepted the mobile station. It is possible that if for example the same CN nodes or operators are selected the second time as the first time and after all tries of the mobile user station to connect, if the appropriates operator CN node has not been found, the user station will disconnect. The problem is not serious if there are only two networks, the problem becomes more serious the more networks there are which share one and the same radio network control node, since the mobile station only has a certain time available before it disconnects. If a mobile station again connects and starts a new attach request procedure comprising a number of retries, there is still a slight possibility that again the appropriate core node network node will not be found. It is also a waste of time and messaging attempting attach to the same CN nodes (erroneously) several times .

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to suggest an arrangement, as initially referred to, through which the number of redirections can be reduced, and particularly through which the number of useless redirections, during the retry period of a mobile user station, to CN nodes to which a redirection has been done already, can be reduced. Particularly it is an object of the invention to provide an arrangement through which the probability can be increased that the appropriate operator or core network node can be found for a mobile user station (in particular for a visiting mobile user station) within the retry period of the mobile user station, i.e. before it disconnects. Even more particularly it is an object of the invention to provide an arrangement through which it gets possible to try as many as possible, or even all, CN operators (CN nodes) sharing radio network control node within the mobile user station retry period. Still further it is an object of the invention to provide an arrangement through which attach to the appropriate CN operator, or CN node, for a roaming or visiting mobile user station can be speeded up and through which the necessary messaging between radio network control node and core network node can be reduced.

It is also an object of the present invention to provide a node as initially referred to, through which one or more of the above mentioned objects can be achieved. Still further it is an object of the invention to suggest a method as initially referred to, through which one or more of the above mentioned objects can be achieved.

Particularly it is an object of the invention to provide an arrangement, a node and a method, respectively, that can be implemented in a so called RAN centric approach. It is also an object of the invention to suggest an arrangement, a node and a method that can be implemented in a CN centric approach.

Therefore an arrangement as initially referred to is provided, which comprises a caching arrangement. The caching arrangement is provided in a node or associated with or in communication with a node. In said caching arrangement identification information of a mobile user station, particularly a visiting or a roaming user station requesting to be attached or registered, is stored. Said identification information may comprise IMSI or MSISDN for the mobile user station or any other relevant identification information. In one implementation for example the IMSI or MSISDN is revealed by the node (e.g. RNC or BSC) , in other implementations it is received from a first CN node to which the attach request is forwarded.

If IMSI or MSISDN (Mobile Station Integrated Services Digital Newtork number) or IMEI (International Mobile Equipment Identity) (in a RAN centric solution) is somehow established by the node (RNC; BSC) itself, any reselection of operator or core network node is not needed for the purpose of finding out the IMSI or

MSISDN (reselection may still be needed for roaming UEs in order to find the correct CN Operator to attach to) . If, however,

IMSI/MSISDN/IMEI can only be provided by a core network node to the node (RNC/BSC) , forwarding of an attach request to a core network node (which may already have been tried) may be necessitated to find IMSI or MSISDN.

Further, connected to or associated with identification information, information will be stored about to which operators or core network nodes an attach request has been sent by said node, such that for subsequent attach requests from the same mobile user station during a mobile user station retry period comprising a number of mobile user station attach requests, said information, i.e. the identification information and information about the operator (s) or core network node(s) that have been tried, i.e. been attempted to attach to, is used at subsequent selections of operators or core network nodes (CN nodes) during the mobile user station retry period to prevent unnecessary reselection of an already tried operator or core network node. This means that, if during a first attach request this has been directed or forwarded to a particular CN node, information thereon will be stored such that upon redirection, another CN node will be selected (unless IMSI/MSISDN/IMEI is needed) , in which case there may be a reselection and if said attempt also is not successful, also information about this CN node will be stored such that for a subsequent try, a third CN node etc. will be tried.

In a particular implementation the caching arrangement is provided in a radio network control node, e.g. an RNC or a BSC, shared by a plurality of the core network operators. Alternatively the caching arrangement is associated with, or in communication with, a radio network control node, e.g. an RNC or a BSC, shared by a plurality of core node operators. If the caching arrangement is provided in a radio network control node (or associated with or in communication with such a node) the concept is implemented in a so called RAN centric approach.

In an alternative implementation, concerning a so called CN Centric approach, the caching arrangement will be provided in a core network node run by a given operator or alternatively the caching arrangement may be associated with or in communication with a core network node. The core network node holding a cache arrangement or in communication with the caching arrangement etc. may comprise an MSC or an SGSN or a CGSN for circuit and packet switched communication respectively.

The identification information, particularly in a RAN centric solution but also in a CN centric solution, advantageously comprises the IMSI or the MSISDN of the IMEI of the mobile user station, which particularly comprises a so called User Equipment (UE) . Preferably the caching arrangement comprises a timer. The timeout or expiration period of the timer is preferably set to have a length which is such that it encompasses the mobile user station retry period which is the period during which the mobile user station performs attach requests, i.e. before the mobile user station disconnects or in other words, the period as from the first try until the last retry of the mobile user station. In one implementation the timeout or timer expiration period may be reconfigurable. Alternatively it is fixed. Particularly the timeout period or timer expiration period is set to about 5 minutes. In one embodiment the information in the caching arrangement (identity and tried operators or core nodes) is discarded when said timer has timeout or expires. Alternatively the information is discarded when there has been a successful attach, but at the latest when there is timeout for the timer. Particularly there are two operators sharing the same radio network, but even more particularly there are more than two operators sharing the same radio network or the same radio control means, e.g. three, four or five or even more network operators. A network node as initially referred to is therefore also provided whereby a caching arrangement is provided in or associated with or communication with the said node. The caching arrangement is used for storing identification information of non-supporting, e.g. roaming mobile user stations requesting to be attached or registered and further for, associated with the identification information of such a mobile user station, storing information about to which operators, or operator core network nodes, an attach request has been sent, such that information about identification and about tried operators or core network nodes is used by the radio network control node such that for subsequent attach requests from the same (non-supporting), e.g. roaming mobile user station during mobile user station retry period comprising a number of attach requests, operators or call network nodes which have been previously selected during the same mobile user station retry period will not be unnecessarily selected. By a non-supporting mobile user station is meant a user station not-supporting a functionality of extended broadcasting for shared networks, e.g. a functionality as disclosed in 3GPP Release 6. The node particularly comprises a radio network control node, e.g. an RNC or a BSC shared by a plurality of core network operators. Alternatively it comprises a core network node controlled by a given operator, in which case it may comprise an MSC or an SGSN or a CGSN. Irrespectively of which is the node, the caching arrangement advantageously comprises a timer for which the timeout or expiration period is set to have a length such as to encompass or cover the mobile user station retry period during which the mobile user station performs subsequent attach requests without disconnecting in between. The timeout or expiration period may be reconfigurable or fixed. In a particular implementation the timeout or expiration period is set to about 5 minutes. Particularly the identification information comprises the IMSI or MSISDN or IMEI of the mobile user station. Other alternatives are also possible.

Therefore, according to the invention, a method as initially referred to is also suggested which, when receiving an attach request from e.g. a roaming or a so called non-supporting mobile user station in a shared network control node (or an attach request forwarded from a shared network control node in a core network node) , storing, in a caching arrangement, identity information of the mobile user station. The caching arrangement is, in a RAN Centric solution, provided in a radio network control node, whereas in a CN centric solution it is provided in (in communication with) a core network node. The method further comprises the steps of storing information about with which operators or core network nodes an attach has been unsuccessfully attempted, which information is stored and connected to said identity in ormation; and of sending subsequent attach request to operators or core network nodes during a retry period of said mobile user station comprising a number of attach requests, while using said in the caching arrangement, stored information to prevent unnecessary reselection of an already tried operator or core network node during the retry period of said mobile user station. Preferably the method further comprises the steps of setting a timer associated with the caching arrangement substantially at the same time as the mobile station identification information is stored into the caching arrangement or when the mobile user station identity information is obtained; and discarding the mobile user station identity information and the information about attempted operators or operator core network nodes at the latest upon expiry of said time, or, if earlier, upon establishment of a successful attempt. Particularly the timeout or expiration time of said timer covers the retry period of the mobile user station whereby the retry period here is defined as the period as from the first attach request including attach request retries without the mobile user station disconnecting in between. In a particular implementation the timeout or expiration time is about 5 minutes. As referred to earlier, in the caching arrangement may be provided in or associated with the shared radio network node, which may be an RNC or a BSC or in a core network node. The user station identity information preferably comprises IMSI of a mobile user station. Alternatively it may comprise the MSISDN or IMEI of the mobile user station. According to the invention it is particularly guaranteed that all core network operators are really tried during the retry period of the UE, i.e. during the retries a UE does .

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will in the following be further described, in a non-limiting manner, and with reference to accompanying drawings, in which:

Fig. 2 is a schematical block diagram illustrating the implementation of the inventive concept in a RAN-Centric solution, Fig. 3 is a schematical block diagram illustrating the implementation in a CN-Centric solution,

Fig. 4A is a sequence diagram illustrating the procedure when a visiting mobile user station (subscriber) makes a very first attempt to connect to a network, which fails,

Fig. 4B is a sequence diagram illustrating the procedure when the mobile user station of Fig. 4A, during the same retry period, makes a second connection attempt, which is successful, and

Fig. 5 is a further sequence diagram illustrating the inventive procedure according to one particular embodiment in which three retries are needed.

Fig. 6 is a flow diagram illustrating the inventive procedure according to one particular embodiment.

DETAILED DESCRIPTION OF THE INVENTION

Fig. 2 is a block diagram of an embodiment of the present invention as implemented in a RAN Centric solution. The block diagram illustrates a mobile user station UE 1 which is supposed to be a so called non-supporting UE, i.e. a mobile user station which does not support an extended broadcast functionality for shared networks as for example disclosed in 3GPP Release 6, and it is also supposed that the UE 1 has no valid TSMI/PTSMI and thus no valid NRI for any of the core network operators CN A, CN B, CN C, CN D. The figure also illustrates an RNC 2 which comprises a caching arrangement, cache 3, and means 4 for CN selection and redirection, which are not shown in a detailed manner since they may be of any known kind. However, the cache 3 is a caching arrangement according to the inventive concept in which information is stored about the IMSI (or MSISDN, or IMEI) of the requesting UE 1 (as well as of other requesting UEs as illustrated through XXX and YYY in the figure, as well as a list of those operators or core network nodes that have already been tried during the retry period of the concerned mobile user station, i.e. for a mobile user station with IMSI XXX it is supposed that core network operators A and B have been tried, whereas for mobile user station with IMSI YYY CN operators C,D,A have been tried. This list is built up successively as CN nodes (operators) are tried. The cache 3, also called dedicated cache, is in this embodiment associated with a timer 5 which in one embodiment is set substantially when for example UE 1 sends a first initial registration request to the RNC 2 and which expires after a given expiration period which advantageously covers the entire period during which the user station UE 1 retries to get connected to the network without disconnecting in between, generally about 5 minutes. The inventive concept is of course not limited to 5 minutes, it could be less as well as more. The caching arrangement can also be implemented in other manners, e.g. as a fixed circular memory (without timer), wherein the information data is stored until the memory is full.

In an alternative embodiment the timer 5 is set, or started, when the identification information of UE 1 is obtained or has been retrieved. The timer is of course set for each mobile user station sending an initial registration request. The timer 5 is not to be confused with a conventionally used timer which also is set e.g. upon reception of an initial registration request from a mobile user station and which expires after a given time period corresponding to a mobile user station timer indicating release of the Radio Resource Connection (RRC) which generally is about 20 seconds and which indicates that the UE considers the network to have failed or lost the attach request. For the RNC redirect function this timer can also be used as an indication that no more redirect attach requests should be done by the RNC operator until a subsequent registration request is received from the mobile user station, UE 1, within the retry period. This timer is known per se and may be associated with additional (not shown) information holding means in which it is indicated which CN operators or core network nodes that are tried during one (and only one) registration request such that during this one try it is not redirected to the same CN node for example if a randomized selection is used and the information in which is discarded after each individual registration request from the mobile user station even within the retry period of the mobile user station. Preferably the cache 3 is separate from said known information holding means (not shown) and an information transfer is performed e.g. as the known timer expires. In one embodiment the information in the cache 3 cannot be collected until after the first attach request has been sent to a CN node, in this particular example MSC/SGSN 6A, which provides the IMSI to the CN selection and redirection means 4 (or RNC) , which then provides the IMSI of the UE 1 to the cache 3 which stores it together with information that CN operator A has been tried. In this example it is supposed that four operators, CN operator A to CN operator D, share RNC 2 and each comprise a CN node MSC/SGSN 6A, 6B, 6C, 6D respectively, each of which communicating with a respective HLR 7A,7B,7C,7D. In the embodiment illustrated in Fig. 2 it is supposed that the first attach request (initial registration request) is sent to CN node 6A (which may be selected according to any appropriate given criteria or algorithm) or randomly. As UE 1 did not have any valid NRI for any of the CN operators CN A,B,C,D, the CN selection and redirection means 4 may perform a random selection for example if no NRI is present, or an Iu- flex based selection if an NRI is present which may be valid or invalid.

It is here supposed that MSC/SGSN 6A after checking IMSI of UE 1, establishes that this subscriber is not allowed. A normal reject message with additional redirect information attached is then returned in a Reroute Command to CN selection and redirection means 4. A redirection is then made based on IMSI as provided by, in this case operator A CN node 6A, if it is a home subscriber, or based on random selection (or some other criteria) among remaining CN nodes (operators) if it is a visiting subscriber. Identity information (e.g. IMSI, MSISDN or IMEI) and information that CN node 6A (or operator A) has been tried is stored into the cache 3.

It is here supposed that the subsequent (redirected) attach request is sent to CN node 6C which, in this case, accepts and sends an attach acceptance to CN selection and redirection means 4 of RNC 2, which in turn sends an Attach Complete message to the UE 1. The timer 5 expires upon expiry of the set timeout period, e.g. 5 minutes, preferably covering the entire retry period of the UE 1. Alternatively the information relating to UE 1 is discarded when the timer 5 expires or when an Attach Complete is received, whichever is the earlier.

In this case it is supposed that both CN operator A and CN operator C could be tried without expiry of the per se known timer (not shown) . However, the cache 3 would store the information relating to UE 1 also during subsequent requests, if such were required. How cache 3 is implemented is implementation specific. It could be entirely separated from existing node information holding or caching means as referred to above or provided as forming a part of said node information holding means holding information about tried CN operators during one attempt by e.g. the RNC e.g. including redirection.

Cache 3 may be in communication with said node information holding means in which case information transfer is performed upon expiry of the known timer or the information may be provided both to the known node information holding means and to the separate cache 3. Any variations are in principle possible. It should be clear that the number of CN operators is not limited to four, it could be two, three, four, five or six or more.

Fig. 3 is a block diagram similar to that of Fig. 2 but for a CN Centric solution. Similar reference numerals are used for similar means or elements with the difference that they are provided with an index. Thus, it is here supposed that UE 1' sends an Initial Registration request to RNC 2'. Again it is supposed that UE 1' has no valid TMSI/P-TMSI (no valid NRI) for CN operators A,B,C,D. In the CN selection means 4A¹ in RNC 2' a random selection (if no NRI is present) is performed or alternatively an Iu-flex based selection is performed if an NRI is present which may be valid or invalid. Then RNC 2' sends the attach request, or tries, MSC/SGSN 6A' . MSC/SGSN 6A' communicates with a caching arrangement 3' like the embodiment of Fig. 2 where it is RNC that comprises said cache arrangement and handles redirection (means 4B') • A timer 5' is used as in the embodiment of Fig. 2 and the functioning is similar to that explained with reference to this embodiment and can be valid as described in the description as a whole. It should be clear that in this embodiment the caching arrangement is illustrated as located outside MSC/SGSN 6A' but in communication therewith. Of course it might be provided in MSC/SGSN, as well as in an embodiment of e.g. Fig. 2 the caching arrangement might have been provided externally of and in communication with RNC 2'. However, after IMSI (or MSISDN or IMEI etc.) of UE 1' has been checked in MSC/SGSN 6A' , it is established that the subscriber is not allowed. MSC/SGSN 6A¹ then tries to find (selects) a new CN operator and forwards the initial registration to that CN operator, in this case to CN operator B. Any subsequent signalling between the new MSC/SGSN (in this case 6B') and the UE 1' is also forwarded (in both directions) . CN operator B in this case returns a Reroute Command and using the caching list information as described when reference to Fig. 2, CN operator C is selected, i.e. MSC/SGSN 6C, which accepts. In other accepts the functioning is similar to that described above and with reference to the embodiments to follow. In this case it is the cache 3' associated with MSC/SGSN 6A' that is used (since that node was selected first) . Caches are particularly also provided in or in communication with MSC/SGSN 6B',6C and 6D'.

Fig. 4A is a sequence diagram describing, in a RAN Centric solution according to the present invention, the very first attach request from a visiting mobile station UE, i.e. the very first attempt to connect or attach to the network during a retry period of the UE. Thus, first a RRC connection (Radio Resource Control) is established between UE and RNC, 1. Subsequently the RNC receives an Initial Direct Transfer from UE, 2. In the RNC the IMSI (if available) of the UE is cached in a caching arrangement and a timer TO is started as is well known in the art and it is determined according to any given criteria, e.g. randomly, that MSC/SGSN A or operator A is to be tried first. If an IMSI (or MSISDN, or IMEI) is available in RNC, a cache 3 as in Fig. 2, is established and a timer Tl according to the inventive concept is started. (If IMSI (or MSISDN, or IMEI) is not available at this point, the Cache 3 is established when RNC receives a Reroute Command) . The RNC is configured to work in a shared RAN MOCN and it forwards the NAS (Non Access Stratum) message in an Initial UE request with an additional redirect attempt flag set to the selected MSC/SGSN A, 3. Alternatively the RNC may use other ways than a flag to indicate to MSC/SGSN that redirect shall be initiated if the attach is not successful, e.g. presence or non-presence of certain protocol information elements. The Initial UE is received in the selected MSC/SGSN A with the redirect attempt flag set. MSC/SGSN A then knows that it shall answer with a Reroute Command or a Reroute Complete message. MSC/SGSN A needs the IMSI of the UE, it may e.g. be retrieved either from old MSC/old SGSN or from the UE as in this example, 4. (Alternatively is thus the IMSI of the UE stored into the caching arrangement at this stage; the timer (s) Tl and/or TO may also be started at this stage instead.) By comparing the IMSI with the roaming agreements of the CN operator, the MSC/SGSN A discovers that roaming is not allowed and the attach procedure is aborted. A message Reroute Command, 5, is then returned to the

RNC with two NAS messages, the attach reject message and the original attach request message received from the UE (alternatively the original NAS message may be stored in the RNC) . The IMSI is also included in the message in addition to a cause code indication to the RNC. The message (5) may for example be a new RANAP (Radio Access Network Application Protocol) message, Reroute Command, or an extended Direct Transfer message. The RNC releases the signalling connection with MSC/SGSN A and the RNC then compares the received IMSI in the Reroute Command message with configured IMSI ranges it has of the CN operators of the shared RAN. If a match is found, the UE is a home subscriber of one of the CN operators and the RNC then knows the correct MSC/SGSN to redirect to in the subsequent step. If no match is found, as in this example, the UE is a visiting UE and the RNC has to select an MSC/SGSN, e.g. randomly, in the next step. Information about the already tried MSC/SGSN A is stored in the caching arrangement, which thus is updated during the redirect procedure so that the same node will not selected twice during the first attach request of the retry period. (This information is not discarded from conventional holding means until expiry of the timer TO in a known manner.) However, according to the present invention the cache list of the caching arrangement is updated and it is indicated that MSC/SGSN A has been tried (and failed) and, in this example it is supposed that, according to any criteria, MSC/SGSN B is selected for the subsequent attach request. Other information may also be stored in the cache for each tried CN node, for example, cause code for rejection, NAS reject message etc.

RNC then sends a new Initial UE, 6, to the next selected MSC/SGSN B with the original NAS attach request message. The redirect attempt flag is set and IMSI may also be included to avoid a second IMSI retrieval from UE or from old MSC/SGSN, β. MSC/SGSN B then starts its attach procedure. In this case it is supposed that MSC/SGSN B supports roaming for the HPLMN of the IMSI in concern and thus request Authentication Vectors from HLR, 7. MSC/SGSN B authenticates the UE, 8, by sending a RAND request to the user the equipment, 8, and receives an authentication response (SRES) from the UE, 9. (SRES is a signed response). Subsequently the MSC/SGSN B verifies authentication results and starts RAN ciphering, Security Mode Command (key) is sent to the UE which returns security mode complete to MSC/SGSN B, 9. Then MSC/SGSN B updates the HLR and receives subscriber data from HLR, 10. Then, however, it is established that the subscription data do not allow roaming, e.g. the user station might not be allowed to use 3G without paying etc. MSC/SGSN B then sends a Reroute

Command message to RNC, 11. The Reroute Command message includes the attach reject message, the original attach request message (which alternatively may be stored in the RNC) and the IMSI (MSC/SGSN does not know that RNC may have already got the IMSI so it may be included, or is always included although this is not necessarily the case), and finally the sequence number (for MSC only; i.e. on the circuit switched side) and authentication vectors are included. The RNC then releases the signalling connection with MSC/SGSN B and, according to the inventive concept the information about CN operator in the caching arrangement is updated indicating operator B or MSC/SGSN B as tried (failed). The timer TO (e.g. 20 seconds timeout) expires and the RNC stops trying further core network nodes. However, timer Tl still runs. This figure (4A) described the first attempt made by UE during its retry period.

In Fig. 4B the second try (first retry) is illustrated, i.e. the second connection or attachment attempt made by the UE . It is thus supposed that an RRC is established between UE and RNC (cf. 1) above) . IMSI of the UE (if available by RNC already at this stage, cf. also Fig. 5) is checked in the list in the cache arrangement and it can be seen that operators A and B failed, during the first try, which means that in this case operator C is to be tried. Timer TO is again started. MSC/SGSN C receives an Initial UE (with the original NAS attach included, cf . 2) above) . MSC/SGSN C updates the HLR and receives subscriber data from HLR, 14. In this case it is supposed that the attach procedure can be completed and a Reroute Complete message, 15, is sent to RNC with a NAS attach accept message included. (A Direct Transfer message with a new Reroute Complete flag may alternatively be chosen.) The RNC then notices that the redirect procedure is finished (and stored redirect data may be discarded from the conventional holding means or it may be waited until timer TO expires. The Attach Accept (Direct Transfer) is forwarded to the UE, 16. The UE may store the TMSI/P-TMSI with the Iu-flex NRI to be used for future signalling, even after power off. This is however an existing functionality not necessary for the functioning of the inventive concept. The UE then responds with an Attach Complete message, 17, to MSC/SGSN C. Timer Tl expires and information associated to UE is discarded in the cache. The discarding may be done when Tl expires or more or less immediately upon reception of attach (Reroute) Complete. In any case, the information relating to UE is not stored in the caching arrangement any longer then the expiration time period set for timer Tl . This is preferably about 5 minutes, but it may be shorter (e.g. 1 minute to about 2 minutes) or longer, e.g. up to about 10 minutes or more, among other depending on storing capacity. The main thing is that at least, with a high (or 100%) probability, the information is kept during the whole retry period of the UE.

It is here however supposed that the cache arrangement has a timeout in order of 5 minutes, i.e. extending over the whole retry period of the UE. Given the assumption that there is always time for at least one redirect, i.e. trial of two core network nodes during one try and that the RNC has the IMSI at the first Reroute Command message (i.e. cached information available at the first Reroute Command) , it will be possible to try all core network operators with substantially 100% probability in an MOCN with five sharing CN operators . The usage of cached redirect information between retries is a trade off between added implementation complexity and not reaching the 100% probability.

Fig. 5 is still a further sequence diagram illustrating the procedure when a UE (a roaming so called non-supporting UE) sends an attach request to a shared RNC. In this case it is supposed that there are four operators A,B,C,D with core network nodes MSC/SGSN A,B,C,D, . TO indicates, as in the preceding figures 4A, B the known timer in the RNC, whereas Tl indicates the timer according to the inventive concept. It is here supposed that UE sends an attach request, which is the first try by the UE, to RNC, 1. The attach request is in this case forwarded according to some given criteria, e.g. randomly to MSC/SGSN A, 2. MSC/SGSN A establishes that it is not the correct operator, and sends a Reroute Command with among others IMSI, MSISDN or IMEI, 3. Timer TO was e.g. set already at the reception of the attach request, 1, in RNC. After receiving the Reroute Command, 3, containing e.g. IMSI, operator A or core network node A information (failed) is sent (with IMSI, MSISDN or IMEI) to the cache list of the cache arrangement and in this case Tl is started at this stage. RNC for example then selects MSC/SGSN B for a subsequent attach request according to the redirection mechanism, 4. It is supposed that also MSC/SGSN B establishes that it is not the appropriate operator and returns a Reroute Command (with IMSI) to RNC, 5. Information that B has been tried (and found not to be the correct operator) is introduced into the cache list, which thus is updated. Other information such as cause code and NAS reject message may also be introduced into the cache list. At this stage timer TO tells RNC that there is no time for additional redirects in this try (UE will timeout) and TO and its "context information" is deleted. If additional CN operators remains to be tried no NAS reject message is forwarded to UE. (If no more CN operators had remained, a NAS reject message should have been forwarded to UE to indicate to UE that this network can not accept requests from UE, i.e. no need for any (more) retries.) Tl continues to run, i.e. it has not expired, and UE sends a subsequent attach request (the first retry) to RNC, 6. Then an attach request is sent, e.g. randomly, to, here, MSC/SGSN A, 7. (It could as well have been MSC/SGSN B,C or D) . The selection of MSC/SGSN may be done according to any criteria, such as always the first (second etc.), randomly, or by using a Round Robin algorithm. In the received Reroute Command, 8, also IMSI is returned to RNC. Upon reception of the attach request in the RNC, timer TO has again been started. Since RNC has been informed about IMSI in the Reroute Command (8) it can fetch information from the cache list, and hence it establishes that core network nodes A,B, i.e. operators A,B, had already been tried and selects a new core network node using this information. Here it selects MSC/SGSN C and thus sends a new attach request, 9, to MSC/SGSN C. However, also MSC/SGSN C establishes that it is not the correct operator, and sends a Reroute Command (with IMSI) to RNC, 10. Information that C was not the correct operator or that MSC/SGSN C was not the correct core network node is introduced into the cache list (i.e. that MSC/SGSN C has been tried) associated with the IMSI of UE. Timer TO again tells RNC that there is no time for additional redirects in this try (UE will timeout) and TO and its "context information" is deleted whereas timer Tl continues to run.

Generally UE can be found in the cache list as soon as for example IMSI or any other information identity is known. In this case, when receiving Reroute Command (8) with IMSI, RNC can see that A and B already have been tried. This can also be done in other manners, for example are implementations possible where RNC identifies that it is a subsequent try by an UE and thus immediately can establish the identity from the cache list, and it will now be aware of the fact that A and B have been unsuccessfully tried or RNC evaluates the attach request from UE . Also other implementations are possible.

Subsequently it is supposed that UE performs a second retry, and sends an attach request, 11, to RNC. The RNC forwards, also in this case, the attach request to MSC/SGSN A, 12, and receives a Reroute Command with IMSI. If assuming that the attach request is randomly performed, it might of course also have been sent to MSC/SGSN B or C in which case it would also have received a Reroute Command with IMSI and could then fetch the information that operators A, B and C had been tried and thus that the attach request should be sent to MSC/SGSN D, but if it (e.g. randomly) had sent the attach request to MSC/SGSN D, it would of course instead have received a Reroute Complete message and a redirection had been superfluous.

In any case it is here supposed that the request was again sent to MSC/SGSN A and a Reroute Command with IMSI returned, 13, whereupon RNC fetches information from the cache list, using IMSI, and finds that A and B and C have been tried, and then sends an attach request, 14, to MSC/SGSN D, which establishes that it is the correct operator and sends a Reroute Complete message, 15, to RNC. If the information that is stored into the cache arrangement is based on that it should indicate which operators or core network nodes that have been tried, information about D is provided to the cache list. In another implementation, only when there has been a rejected attach request, i.e. a Reroute Command, information as to the concerned core network node is introduced into the cache list. Various alternatives are possible. As can be seen from the figure also after step 8 and 13 information is introduced into the cache list that A was not the correct operator, which however is superfluous information. Subsequently the timer Tl expires after a given time period or when a Reroute Complete message has been received in RNC, which then has to inform the caching arrangement and/or the associated timer Tl. The information in the cache list associated to the IMSI for which the Tl timer expired, will then be deleted from the cache list.

It should be clear that this merely shows a specific implementation of the inventive concept, and that the inventive concept can be varied in a number of ways within the scope of the appended claims. The caching arrangement may e.g. be implemented in different manners, with or without a timer (Tl) ; the length of the timeout period of Tl (if provided) may vary, the number of core network operators may very, the identity information can be of different kinds, the identity information can be obtained in different manners, through evaluation or "opening" the NAS attach request in RNC (BSC) or MSC/SGSN (CGSN) (the latter for a CN centric approach) or it may be retrieved from a CN node. Also in other aspects the invention is not limited to the specifically illustrated embodiments.

Claims

1. An arrangement for assigning core network operator to a mobile user station (UE) , e.g. a roaming mobile user station not supporting a functionality of extended broadcast information for shared networks, in a shared network environment in which a number of core networks have different core network operators (CN A, CN B, ... , CN N) and share a radio network with a number of radio network control means (RNC;BSC) , each core network comprising a number of core network nodes (MSC; SGSN; CGSN) and supporting a redirection functionality for redirecting mobile, e.g. roaming, user stations in order to assign such mobile user stations to the appropriate core network (CN) operator, c h a r a c t e r i z e d i n that it comprises a caching arrangement in or associated with or communicating with a node, that in said caching arrangement identification information, e.g. IMSI or MSISDN or IMEI, of a mobile e.g. a roaming user station, requesting to be attached or registered, is stored, that further, associated with said identification information, information is stored about to which operators or core network nodes an attach request has been sent, such that for subsequent attach requests from the same mobile user station during a mobile user station retry period comprising a number of mobile user station attach requests, said information about identification information and operators or core network nodes is used at subsequent selections of operators or core network nodes during the mobile user station retry period to prevent unnecessary reselection of an already tried operator or core network node.

2. An arrangement according to claim 1, c h a r a c t e r i z e d i n that the caching arrangement is provided in a radio network control node (RNC; BSC) shared by a plurality of core network operators.

3. An arrangement according to claim 1, c h a r a c t e r i z e d i n that the caching arrangement is associated with, or in communication with, a radio network control node (RNC; BSC) shared by a plurality of core node operators.

4. An arrangement according to claim 1, c h a r a c t e r i z e d i n that the caching arrangement is provided in a core network node run by a given operator.

5. An arrangement according to claim 1, c h a r a c t e r i z e d i n that the caching arrangement is associated with or communicates with a core network node run by a given operator.

6. An arrangement according to claim 4 or 5, c h a r a c t e r i z e d i n that said core network node comprises an MSC.

7. An arrangement according to claim 4 or 5, c h a r a c t e r i z e d i n that said core network node comprises an SGSN or a CGSN.

An arrangement according to any one of the preceding claims, c h a r a c t e r i z e d i n that the identification information comprises the IMSI or the

MSISDN or the IMEI of the mobile user station.

9. An arrangement according to any one of the preceding claims, c h a r a c t e r i z e d i n that the caching arrangement comprises a timer (Tl) .

10. An arrangement according to claim 9, c h a r a c t e r i z e d i n that the timeout, or timer expiration period of the timer (Tl) is set to have a length such as to encompass the retry period during which the mobile user station performs attach requests, i.e. before the mobile user station disconnects.

11. An arrangement according to claim 10, c h a r a c t e r i z e d i n that the timeout or timer expiration period of the timer (Tl) of the caching arrangement is reconfigurable.

12. An arrangement according to claim 10 or 11, c h a r a c t e r i z e d i n that the timeout or timer expiration period of the timer (Tl) is set to about 5 minutes.

13. An arrangement according to any one of the preceding claims, c h a r a c t e r i z e d i n that more than two operators share the same radio network.

14. An arrangement at least according to claim 9, c h a r a c t e r i z e d i n that upon timeout of the timer (Tl) , the information relating to a requesting mobile user station, i.e. identity information and associated operator or core network node, is discarded.

15. A node in a communications system in which a number of different operators, each controlling respective core network nodes share a radio network comprising shared radio network control means (RNCs;BSCs) and in which a redirection functionality is supported for redirecting mobile user stations (UEs) e.g. roaming mobile user stations not supporting a functionality of extended broadcasting for shared networks, in order to assign such mobile user stations to the appropriate core network operator, c h a r a c t e r i z e d i n that a caching arrangement is provided in or associated with or in communication with the node, said caching arrangement being used for storing identification information of non-supporting, mobile, e.g. roaming, user stations requesting to be attached/registered and further for, associated with the identification information of such a mobile user station, storing information about to which operators or operator core network nodes an attach request has been sent, said information about identification and about operators or core network nodes being used, such that for subsequent attach requests from the same mobile user station during a mobile user station retry period comprising a number of attach requests, operators or core network nodes will not be unnecessarily reselected during the same mobile user station retry period.

16. A node according to claim 15 , c h a r a c t e r i z e d i n that it comprises a radio network control node e.g. an RNC or a BSC shared by a plurality of core network operators.

17. A node according to claim 15, c h a r a c t e r i z e d i n that it comprises a core network node run by a given operator.

18. A node according to claim 17, c h a r a c t e r i z e d i n that it comprises an MSC.

19. A node according to claim 17, c h a r a c t e r i z e d i n that it comprises an SGSN or a CGSN.

20. A node according to any one of claims 15-19, c h a r a c t e r i z e d i n that the caching arrangement comprises a timer (Tl) .

21. A node according to claim 20, c h a r a c t e r i z e d i n that the timeout or expiration period of the timer (Tl) is set to have a length such as to encompass the mobile user station retry period during which the mobile user station performs subsequent attach requests without disconnecting.

22. A node according to claim 21, c h a r a c t e r i z e d i n that the timeout or expiration period of the timer (Tl) of the caching arrangement is reconfigurable.

23. A node according to claim 21 or 22, c h a r a c t e r i z e d i n that the timeout or expiration period of the timer (Tl) is set to about 5 minutes.

24. A node according to any one of claims 15-23, c h a r a c t e r i z e d i n that the identification information comprises the IMSI or MSISDN or IMEI of the mobile user station.

25. A method for assigning core network operator to a mobile e.g. roaming, user station (UE) e.g. a mobile user station not supporting a functionality of extended broadcasting for shared networks, in a shared network environment in which a number of core networks have different core network operators sharing a radio network with a number of radio network control nodes e.g. RNCs or BSCs, and wherein a redirection functionality is supported for redirecting mobile, e.g. roaming, user stations to the appropriate core network operator or core network node, comprising the steps of, when a mobile user station requests to be attached: receiving an attach request from the mobile user station in the shared network control node, c h a r a c t e r i z e d i n that it further comprises the steps of: storing, in a caching arrangement, identity information of the mobile user station, storing information about with which operators or core network nodes attach has been attempted, in connection with said identity information, sending subsequent attach requests to operators or core network nodes during a retry period of said mobile user station comprising a number of attach requests, while using said, in the caching arrangement stored information, to prevent unnecessary reselection of an already tried operator or core network node.

26. A method according to claim 25, c h a r a c t e r i z e d i n that it comprises the steps of: setting a timer (Tl) associated with said caching arrangement substantially at the same time as the mobile station identification information is stored into the caching arrangement or when the identification information is obtained, discarding the mobile user station identity information and the information about attempted operators or operator core network nodes upon expiry of said timer (Tl) or upon establishment of a successful attempt.

27. A method according to claim 26, c h a r a c t e r i z e d i n that the timeout or expiration time of said timer (Tl) covers the retry period of the mobile user station.

28. A method according to claim 27, c h a r a c t e r i z e d i n that the timeout or expiration time of said timer (Tl) is about 5 minutes .

29. A method according to anyone of claims 24-27, c h a r a c t e r i z e d i n that the caching arrangement is provided in, or associated with, or in communication with, the shared radio network node (RNC; BSC) .

30. A method according to any one of claims 25-28, c h a r a c t e r i z e d i n that the caching arrangement is provided in, or associated with or in communication with, a core network node (MSC; SGSN; CGSN) .

31. A method according to any one of claims 25-30, c h a r a c t e r i z e d i n that the mobile user station identity information comprises IMSI or MSISDN or IMEI.