Patent Publication Number: US-11653259-B2

Title: Method for controlling service in radio communication system

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of application Ser. No. 16/557,741, filed Aug. 30, 2019, which is a continuation of application Ser. No. 15/489,610, filed Apr. 17, 2017, now U.S. Pat. No. 10,405,227, which is a continuation of application Ser. No. 14/383,889, now U.S. Pat. No. 9,629,021, which is the National Stage of International Application No. PCT/KR2013/001892, filed Mar. 8, 2013, which claims the benefit of Provisional Application No. 61/614,470, filed Mar. 22, 2012, Provisional Application No. 61/612,484, filed Mar. 19, 2012, and Provisional Application No. 61/608,580, filed Mar. 8, 2012, the disclosures of which are herein incorporated by reference in their entirety. 
    
    
     BACKGROUND 
     1. Field 
     The present disclosure relates to a method and apparatus that control communication services in a wireless communication system without increasing network load. 
     2. Description of Related Art 
       FIG.  1    illustrates an architecture of the LTE mobile communication system. 
     As shown in  FIG.  1   , the radio access network (EUTRAN) of the LTE mobile communication system is composed of an evolved base station (Evolved Node B, ENB or Node B)  105 , Mobility Management Entity (MME)  110 , and Serving Gateway (S-GW)  115 . A user equipment (UE or terminal)  100  may connect to an external network through the ENB  110 , S-GW  115  and PDN Gateway (P-GW)  120 . 
     Application Function (AF)  130  is an entity that provides session related information to user applications. 
     The PCRF  125  is an entity for controlling policies related to user QoS. Policy and charging control (PCC) rules corresponding to a specific policy are sent to the P-GW  120  for enforcement. 
     The ENB  105  is a radio access network (RAN) node, which corresponds to the RNC of the UTRAN system or the BSC of the GERAN system. The ENB  105  is connected with the UE  100  through a wireless channel and functions similarly to the existing RNC or BSC. 
     In the LTE system, as all user traffic including real-time services like VoIP (Voice over IP) services is served by shared channels, it is necessary to perform scheduling on the basis of status information collected from UEs  100 . The ENB  105  performs this scheduling function. 
     The S-GW  115  provides data bearers, and creates and removes a data bearer under control of the MME  110 . 
     The MME  110  performs various control functions, and may be connected to multiple base stations. 
     Policy Charging and Rules Function (PCRF)  125  is an entity performing overall QoS and charging control functions for traffic. 
     For service provisioning in a wireless communication system described above, it is necessary to develop a method and apparatus that can provide services without increasing network load. 
     SUMMARY 
     Aspects of the present disclosure are to address the above mentioned problems. Accordingly, an aspect of the present disclosure is to provide a method and apparatus that can generate, when a request for a circuit switched voice network (CS) service is issued to a user having subscribed only to a packet switched data network (PS) service, a service restriction notification without increasing network load. 
     Another aspect of the present disclosure is to provide a method and apparatus that enable, when a need for an emergency call is generated in a user equipment handling a normal call, the user equipment to place an emergency call under required conditions without increasing network load. 
     Another aspect of the present disclosure is to provide a method and apparatus that can adjust, when the network is congested, services without aggravating network congestion. 
     In accordance with an aspect of the present disclosure, a method for signal transmission and reception in a user equipment of a wireless communication system is provided. The method may include: sending a request for a Circuit Switched voice network (CS) service to a Gateway Mobile Switching Center (GMSC) to send and receive data to and from a second user equipment; and receiving a call reject response corresponding to the service request from the GMSC, wherein the call reject response is sent by an entity in a core network of the second user equipment. 
     In accordance with another aspect of the present disclosure, a method for congestion control in a user equipment of a wireless communication system is provided. The method may include: sending a Non-Access-Stratum (NAS) request to a base station; receiving a NAS reject response corresponding to the request from a core network; and sending a Radio Resource Control (RRC) connection setup signal containing an indicator indicating necessity of release of an RRC connection between the user equipment and base station to the base station. 
     In accordance with another aspect of the present disclosure, a user equipment in a wireless communication system is provided. The user equipment may include: a transceiver unit to send a request for a Circuit Switched voice network (CS) service to a Gateway Mobile Switching Center (GMSC) to send and receive data to and from a second user equipment; and a control unit to control the transceiver unit to receive a call reject response corresponding to the service request from the GMSC, wherein the call reject response is sent by an entity in a core network of the second user equipment. 
     In accordance with another aspect of the present disclosure, a user equipment supporting congestion control is provided. The user equipment may include: a transceiver unit to send a Non-Access-Stratum (NAS) request to a base station and to receive a NAS reject response due to congestion, corresponding to the request, from a core network; and a control unit to control the transceiver unit to send a Radio Resource Control (RRC) connection setup signal containing an indicator indicating necessity of release of an RRC connection between the user equipment and base station to the base station. 
     According to one embodiment, it is possible to provide a suitable service to a sender terminal making a CS service request to a receiver terminal having subscribed only to a PS service without increasing network load. 
     According to another embodiment, even when a need for an emergency call is generated in a user equipment engaged in a normal call, it is possible to provide a service satisfying emergency call requirements. 
     According to another embodiment, when the network is congested, it is possible to provide services with reduced network load for congestion resolution. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG.  1    illustrates an architecture of the LTE mobile communication system. 
         FIG.  2    illustrates a procedure for signal exchange according to a first embodiment. 
         FIG.  3    illustrates another procedure for signal exchange according to the first embodiment. 
         FIG.  4    illustrates still another procedure for signal exchange according to the first embodiment. 
         FIG.  5    illustrates a procedure for signal exchange according to a second embodiment. 
         FIG.  6    illustrates another procedure for signal exchange according to the second embodiment. 
         FIG.  7    illustrates still another procedure for signal exchange according to the second embodiment. 
         FIG.  8    illustrates yet another procedure for signal exchange according to the second embodiment. 
         FIG.  9    illustrates a procedure for signal exchange according to a third embodiment. 
         FIG.  10    illustrates another procedure for signal exchange according to the third embodiment. 
         FIG.  11    illustrates still another procedure for signal exchange according to the third embodiment. 
         FIG.  12    illustrates yet another procedure for signal exchange according to the third embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     In the following description of embodiments of the present disclosure, detailed descriptions of well-known functions and structures incorporated herein may be omitted to avoid obscuring the subject matter of the present disclosure. 
     In the drawings, some elements are exaggerated, omitted, or outlined in brief, and thus may be not drawn to scale. The same reference symbols are used throughout the drawings to refer to the same or like parts. 
     The following description of embodiments of the present disclosure is focused on Third Generation Partnership Project (3GPP) LTE systems. However, it should be apparent to those skilled in the art that the subject matter of the present disclosure is also applicable to other communication/computer systems having a similar technical basis and system configuration without significant modification. For example, the description related to the LTE system may be applied to the UTRAN/GERAN system having similar system architecture. In this case, the ENB (RAN node) may be replaced with the RNC/BSC, the MME may be replaced with the SGSN, the S-GW may be omitted or be included in the SGSN, and the P-GW may correspond to the GGSN. Bearers in the LTE system may correspond to PDP context in the UTRAN/GERAN system. In various embodiments, each communication entity may include a transceiver unit to exchange signals with another entity, and a control unit to control the transceiver unit and perform operations on the basis of signals exchanged through the transceiver unit. In various embodiments, a user equipment may include a display unit to present a visual signal to a user. 
     First Embodiment 
     In most cases, users wishing to receive wireless communication services subscribe to both circuit switched voice network (CS) services and packet switched data network (PS) services. 
     In one embodiment, it may be assumed that a particular user has a subscription to only a PS service. A user having only a PS service subscription may receive an SMS service but may be not allowed to receive a CS service without a separate measure. That is, it may be assumed that a particular user is allowed to receive a PS data service and SMS service but is not allowed to receive a CS service such as a voice call (such a user is referred to as a PS-only-with-SMS user). 
     When a user equipment for a PS-only-with-SMS user is registered with an operator network, the operator network may provide a data service and SMS service to the PS-only-with-SMS user. In this case, a mobile terminating CS service request (e.g. a voice call) may be issued to the PS-only-with-SMS user. For example, a random call for advertisement or promotion may be placed without consideration of a recipient phone number. A voice call may be wrongly placed to a phone number that had been previously assigned to a voice call subscriber and has been reassigned to a PS-only-with-SMS subscriber. 
     As such, when a mobile terminating CS service request (voice call) is issued to a PS-only-with-SMS user, the network may have to perform signaling such as subscriber location identification and paging. However, as the PS-only-with-SMS user cannot receive a CS service like a voice call, such a signaling procedure is useless but merely increases network load. 
     To solve the above problem, a procedure is proposed as follows. When a mobile terminating CS service request is issued to a recipient having registered as a PS-only-with-SMS user, the operator network may determine that the user equipment is detached from a CS service (i.e. reject voice call or disallow CS service). More specifically, the following schemes may be used. 
       FIG.  2    illustrates a procedure for signal exchange according to the first embodiment, wherein Home Location Register (HLR) is responsible for determination. 
     At operation  210 , a first user equipment (UE1)  201  is registered as a PS-only-with-SMS terminal in at least one of the MME  202 , MSC  203  and HLR  204 . Here, the UE1  201  registered as a PS-only-with-SMS terminal may receive only a PS data service or a SMS service. PS-only-with-SMS may be given to a user equipment not allowed to receive a CS service. 
     At operation  215 , a second user equipment (UE2)  206 , being unaware that the UE1  201  is a PS-only-with-SMS terminal, makes a voice call to the UE1  201 . Here, in addition to a voice call, a request for any CS service may be issued. 
     At operation  220 , the UE2  206  sends a request for placing a voice call destined for the UE1  201  to Gateway Mobile Switching Center (GMSC)  205 . 
     Upon reception of the voice call request, at operation  225 , to find an MSC/VLR at which the corresponding subscriber is located, the GMSC  205  transmits a Send Routing Information (SRI) Request message to the HLR  204 . This message may include a service indicator indicating the requested service (voice call or SMS, or CS service other than SMS). 
     At operation  230 , the HLR  204  checks whether the request destined for the UE1  201  is SMS according to UE registration information and examines subscription information of the UE1  201 . If the request destined for the UE1  201  is SMS, the HLR  204  may forward the request to the MSC  203  in which the UE1  201  is registered. 
     If the request destined for the UE1  201  is non-SMS (CS service or voice call), the HLR  204  may regard the UE1  201  as being detached from the requested service. 
     To this end, at operation  235 , the HLR  204  sends a message indicating call rejection as a response to SRI to the GMSC  205 . The call reject message may include information notifying PS-only-with-SMS subscription. At operation  235 , a Send Routing Information Response message may be used. Another type message may also be used. 
     Upon reception of a negative response, at operation  240 , not to retry paging, the GMSC  205  stops the paging retry timer. 
     At operation  245 , the GMSC  205  sends a notification indicating failure of a voice call (CS service other than SMS) attempt to the sender network, which then notifies the sender (UE2  206 ) that the request for a CS service other than SMS (voice call) is denied because of recipient&#39;s PS-only-with-SMS subscription and an SMS service is available. The GMSC  205  may directly send such notification to the sender terminal  206 , or may send such notification to a corresponding to MSC of the sender network and the MSC may forward the notification to the sender terminal  206  via RNC (BSC). The sender terminal  206  may store the received information or notify the user of the same. The sender terminal  206  may notify the user of the received information by means of sound output or screen output. 
       FIG.  3    illustrates another procedure for signal exchange according to the first embodiment. 
     Referring to  FIG.  3   , at operation  310 , a first user equipment (UE1)  301  is registered as a PS-only-with-SMS terminal in at least one of the MME  302 , MSC  303  and HLR  304 . Here, the UE1  301  registered as a PS-only-with-SMS terminal may receive only a PS data service or a SMS service. 
     At operation  315 , a second user equipment (UE2)  306 , being unaware that the UE1  301  is a PS-only-with-SMS terminal, makes a voice call to the UE1  301 . 
     At operation  320 , the UE2  306  sends a voice call request to the GMSC  305 . At operation  325 , the GMSC  305  transmits an IAM message to the MSC  303  at which the UE1  301  is located. The IAM message may include a service indicator indicating the requested service (voice call or SMS, or CS service other than SMS). 
     At operation  330 , the MSC  303  checks whether the request destined for the UE1  301  is SMS. If the request destined for the UE1  301  is SMS, the MSC  303  sends a paging request to the MME  302  in which the UE1  301  is registered. If the request destined for the UE1  301  is a CS service (e.g. voice call), at operation  335 , the MSC  303  sends a message indicating that the UE1  301  is registered as a PS-only-with-SMS terminal to the GMSC  305 . Here, this message may be a call reject message, which may include a cause of rejection. At operation  335 , the MSC  303  may use an RCH message or another type message to notify that the UE1  301  is registered as PS-only-with-SMS. 
     After reception of a negative response, the GMSC  305  performs operation  340  and operation  345 , which are identical respectively to operation  240  and operation  245  in  FIG.  2   . 
       FIG.  4    illustrates another procedure for signal exchange according to the first embodiment. 
     At operation  410 , a first user equipment (UE1)  401  is registered as a PS-only-with-SMS terminal in at least one of the MME  402 , MSC  403  and HLR  404 . Here, the UE1  401  may receive only a PS data service or a SMS service. 
     At operation  415 , a second user equipment (UE2)  406 , being unaware that the UE1  401  has a PS-only-with-SMS subscription, makes a voice call to the UE1  401 . 
     At operation  420 , the UE2  406  sends a voice call request to the GMSC  405 . At operation  425 , the GMSC  405  transmits an IAM message to the MSC  403  at which the UE1  401  is located. The IAM message may include a service indicator indicating the requested service (voice call or SMS, or CS service other than SMS). 
     At operation  430 , the MSC  403  sends a paging request message to the MME  402 . Here, the paging request message may be a SGs_Paging_Req message, which may include an indicator to a service to be provided by paging. 
     At operation  435 , the MME  402  checks whether the request destined for the UE1  401  is SMS. If the request destined for the UE1  401  is SMS, the MME  402  may perform further processing. 
     If the request destined for the UE1  401  is a CS service other than SMS (e.g. voice call), at operation  440 , the MME  402  sends a message indicating that the UE1  401  is registered as a PS-only-with-SMS terminal to the MSC  403 . Here, this message may be a paging reject message, which may include a cause of rejection. The MME  402  may also use a SGs paging reject message or another type message. 
     Upon reception of a negative response such as paging reject, at operation  445 , not to retry paging, the MSC  403  stops the paging retry timer. 
     At operation  450 , the MSC  403  notifies the GMSC  405  of voice call rejection. Here, a call reject message including a cause of rejection may be sent to the GMSC  405  to notify the sender network of failure of a voice call attempt. 
     Thereafter, operation  455  and operation  460  in the sender network are performed in a manner identical respectively to operation  240  and operation  245  in  FIG.  2   . 
     Second Embodiment 
     When a user equipment wishes to receive an emergency service, generation of an emergency bearer rather than a normal bearer is needed. In most cases, a typical normal bearer cannot be converted to an emergency bearer. When a user requests an emergency service, the operator network must set up an emergency call within a time duration set by local or government regulations. 
     Meanwhile, the maximum number of simultaneously activatable bearers in a user equipment may be limited according to implementation. Typical user equipments may simultaneously activate up to one, three or five bearers. In a user equipment, the number of simultaneously active bearers may exceed the maximum number of bearers supportable by the user equipment when an emergency call is placed in a state wherein the number of normal bearers currently active for system attachment has reached the maximum number of supportable bearers, or when multiple bearers needed to provide an emergency service are simultaneously created (for example, while the maximum number of supportable bearers is three, the number of active normal bearers is two and the number of emergency bearers needed is two). In this case, for emergency service provisioning, it is necessary to release normal bearers and create emergency bearers. 
     In consideration of the maximum number of supportable bearers, the number of currently active normal bearers, and the number of bearers needed for an emergency call or service, the user equipment may perform normal bearer cleanup first and then perform emergency bearer setup, or may simultaneously issue an emergency bearer setup request and a normal bearer cleanup request. 
     That is, when an emergency service is needed, the user equipment may send a normal bearer deactivation request to the network, or may locally deactivate normal bearers and notify the core network of a bearer context state through TAU. In the event that the core network (e.g. MME) is aware of the maximum number of bearers supportable by a user equipment, when the user equipment issues an emergency bearer creation request, the core network may automatically perform normal bearer cleanup for the user equipment. More specifically, the following schemes may be used. 
       FIG.  5    illustrates a procedure for signal exchange according to the second embodiment. 
     Specifically,  FIG.  5    depicts emergency service provisioning through the attach procedure. 
     Referring to  FIG.  5   , in the second embodiment, a UE  501  may exchange signals with a network composed of the MME  502 , S-GW  503 , P-GW  504  and HSS  505 . The UE  501  may exchange signals with the network via a base station (ENB). 
     At operation  510 , the UE  501  detects a user request for an emergency service. 
     At operation  515 , the UE  501  identifies the number of currently active bearers. Here, the number of currently active bearers may be the number of normal bearers. The UE  501  may determine whether some or all of the currently active bearers are to be deactivated on the basis of the number of currently active bearers, the number of bearers to be used for the emergency service, and the number of simultaneously supportable bearers. The UE  501  may determine that the number of simultaneously active bearers fails to accommodate the number of bearers to be used for the emergency service. 
     If there is no need to deactivate some or all of the currently active bearers, the UE  501  may exchange signals with the MME  502  to establish a call for the emergency service. 
     If some or all of the currently active normal bearers are to be deactivated, at operation  520 , the UE  501  performs local detach. Here, the UE  501  may autonomously conduct the local detach procedure. 
     At operation  525 , the UE  501  performs a re-attach procedure for the emergency service in cooperation with the MME  502 . This procedure may be initiated by sending an attach request, and the attach request may include an indication to the service or call type. 
     Upon reception of the attach request from the UE  501 , at operation  530  and at operation  535 , the core network identifies emergency attach from the attach request, and performs cleanup of existing normal bearers and generation of emergency bearers by means of authentication and authorization, and bearer context setup. 
     As a result, at operation  540 , an IMS call is set up between the UE  501  and the core network. 
       FIG.  6    illustrates another procedure for signal exchange according to the second embodiment. 
     Specifically,  FIG.  6    depicts a scheme in which the user equipment explicitly makes a normal bearer cleanup request. 
     Referring to  FIG.  6   , a UE  601  may exchange signals with a network composed of the MME  602 , S-GW  603  and P-GW  604 . The UE  601  may exchange signals with the network via a base station (ENB). 
     At operation  610 , the UE  601  detects a user request for an emergency service. 
     At operation  615 , the UE  601  identifies the number of currently active bearers. Here, the number of currently active bearers may be the number of normal bearers. The UE  601  may determine whether some or all of the currently active bearers are to be deactivated on the basis of the number of currently active bearers, the number of bearers to be used for the emergency service, and the number of simultaneously supportable bearers. The UE  601  may determine that the number of simultaneously active bearers fails to accommodate the number of bearers to be used for the emergency service. 
     If there is no need to deactivate some or all of the currently active bearers, the UE  601  may exchange signals with the MME  602  to establish a call for the emergency service. 
     If it is not possible to accommodate the number of bearers to be used for the emergency service owing to the number of simultaneously active bearers, at operation  620 , the UE  601  sends a request for releasing a normal bearer or normal PDN connection to the core network. Here, this request may be sent to the MME  602  through a bearer resource modification request or PDN disconnection request. 
     Upon reception of the request, at operation  625 , the MME  602  forwards the received request to the S-GW  603  through a bearer resource command or a delete session request. 
     Upon reception of the request, at operation  630 , the S-GW  603  forwards the received request to the P-GW  605  through a bearer resource command or a delete session request. 
     At operations  635  to  645 , a response to the request is sent by the P-GW  604  via the S-GW  603  and the MME  602  to the UE  601 . 
     As a result, at operation  650 , the UE  601  may be detached. 
     At operation  655 , the UE  601  sends an Attach request or PDN connectivity request to the MME  602 . This request may include type information indicating “emergency”. At operation  655 , an emergency PDN connectivity request may be sent. In the event that all bearers have been released, as the UE  601  is detached, an emergency attach request may be sent. 
     At operation  660 , at least one of the emergency attach procedure and the emergency PDN connection procedure may be performed. At operation  665 , an IMS call is set up. 
     Meanwhile, in another embodiment of the present disclosure, for an emergency service, the UE  601  may locally deactivate existing bearers and notify the core network of this deactivation through a TAU procedure. More specifically, in response to a user request for an emergency service, when some or all of existing bearers are to be deactivated, the UE  601  selects bearers to be deactivated and sends a TAU request message containing information on the remaining active EPS bearers (except for the selected bearers) to the core network (active bits of the EPS bearer context status are set in the TAU request message). The UE  601  may also send a TAU request message containing bearer information (bearers to be deactivated are marked as inactive and bearers to be kept are marked as active) to the core network. Here, the UE  601  may notify the core network of the emergency service request by setting the EPS update type of the TAU request. The UE  601  may notify the core network of the emergency service request by using an additional update type of the TAU request. The UE  601  may also notify the core network of the emergency service request by using a separate emergency indicator of the TAU request. In addition, the UE  601  may notify the core network of S1/S5 setup for the following emergency service by setting the active flag of the TAU request. 
     Thereby, the MME  602  may be aware that the UE  601  has changed the bearer context for the emergency service. The MME  602  compares the pre-stored EPS bearer context status with the EPS bearer context status in the TAU request received from the UE  601  and may perform bearer cleanup if an EPS bearer to be deactivated is present. 
     When the UE  601  has sent a TAU request for an emergency service, to allow the UE  601  to promptly send a subsequent emergency service request (PDN connectivity request), the MME  602  may send a TAU accept response to the UE  601  before completion of bearer cleanup. 
     Upon reception of a TAU accept response, the UE  601  may be aware of completion of requested bearer context status update, and may perform a subsequent procedure for the emergency service. 
     As another embodiment, when bearer cleanup for an emergency service is needed during idle mode, the following scheme may be applied. 
     If some or all of existing bearers are to be deactivated for the emergency service, the UE  601  selects bearers to be deactivated and sends a TAU request containing information on the remaining active EPS bearers (except for the selected bearers) to the core network (active bits of the EPS bearer context status are set in the TAU request message). The UE  601  may also send a TAU request message containing bearer information (bearers to be deactivated are marked as inactive and bearers to be kept are marked as active) to the core network. To send a TAU request in idle mode, the UE  601  has to establish an RRC connection with the ENB. The UE  601  may send an RRC connection setup request whose establishment cause is set to “emergency”. 
     The ENB forwards the TAU request message contained in an RRC message received from the UE  601  through S1-AP Initial UE message. When the RRC establishment cause is set to “emergency”, the ENB may forward this together with the TAU request message. 
     Upon reception of the TAU request message with RRC establishment cause set to “emergency”, the MME  602  may be aware that the UE  601  has sent the TAU request for an emergency service. Thereafter, subsequent operations may be processed as described before. 
     In the above two embodiments, the UE  601  is depicted as notifying the network of information on locally deactivated bearers using a TAU message. The same procedure may be applied to 2G/3G networks using a Routing Area Update (RAU) message. Alternatively, the UE  601  may send an Extended Service Request (ESR) message having bearer status information instead of a TAU message having bearer status information. 
     The schemes described above may be readily applied without significant modification to existing systems and may be further enhanced so that requested emergency services can be provided to users as rapidly as possible. 
       FIG.  7    illustrates another procedure for signal exchange according to the second embodiment. 
     Specifically,  FIG.  7    depicts a signal exchange scheme in which cleanup of existing bearers and creation of new emergency bearers are simultaneously performed. 
     Referring to  FIG.  7   , a UE  701  may exchange signals with a network composed of the ENB  702 , MME  703 , S-GW  704  and P-GW  705 . 
     At operation  710 , the UE  701  detects a user request for an emergency service. 
     At operation  715 , the UE  701  identifies the number of currently active bearers. Here, the number of currently active bearers may be the number of normal bearers. The UE  701  may determine whether some or all of the currently active bearers are to be deactivated on the basis of the number of currently active bearers, the number of bearers to be used for the emergency service, and the number of simultaneously supportable bearers. The UE  701  may determine that the number of simultaneously active bearers fails to accommodate the number of bearers to be used for the emergency service. 
     If there is no need to deactivate some or all of the currently active bearers, the UE  701  may exchange signals with the MME  703  to establish a call for the emergency service. 
     If some or all of the currently active bearers are to be deactivated to provide the emergency service for the emergency call from the user, at operation  720 , the UE  701  selects bearers to be deactivated and sends a PDN connectivity request message containing information on the remaining active EPS bearers (except for the selected bearers) to the MME  703  (active bits of the EPS bearer context status are set in the PDN connectivity request message). The UE  701  may also send a PDN connectivity request message containing bearer information (bearers to be deactivated are marked as inactive and bearers to be kept are marked as active) to the MME  703 . The UE  701  may select bearers to be removed according to a criteria based on ARP, QCI, default/dedicated bearer (a dedicated bearer is removed), or inactivity duration. 
     Thereby, at operation  720 , the MME  703  may be aware that the UE  701  has changed the bearer context for the emergency service. The MME  703  compares the pre-stored EPS bearer context status with the EPS bearer context status in the PDN connectivity request received from the UE  701  and may perform bearer cleanup if an EPS bearer to be deactivated is present. The MME  602  may also perform bearer cleanup on the basis of information received at operation  715 . In addition, the MME  703  may perform a PDN connection creation procedure for the emergency service. 
     At operation  725 , the MME  703  sends a Create Session Request to the S-GW  704 . This Create Session Request may contain IDs of bearers to be removed. 
     At operation  730 , the S-GW  704  forwards the Create Session Request received at operation  725  to the P-GW  705 . 
     At operations  735  to  745 , each node sends or forwards a response message. At operation  750 , the ENB  702  may send an RRC connection Reconfiguration to the UE  701 . This RRC connection Reconfiguration may contain a DBR list. 
     At operation  755 , an IMS call is set up between the UE  701  and core network. 
       FIG.  8    illustrates another procedure for signal exchange according to the second embodiment. 
     Specifically, in  FIG.  8   , a UE  801  requests the network to clean up existing normal bearers and to create bearers for an emergency service. 
     Referring to  FIG.  8   , the UE  601  may exchange signals with a network composed of the ENB  802 , MME  803 , S-GW  804  and P-GW  805 . 
     Operation  810  and operation  815  are identical respectively to operation  710  and operation  715  in  FIG.  7   . 
     If it is not possible to accommodate the number of bearers to be used for the emergency call placed by the user owing to the number of simultaneously active bearers, at operation  820 , the UE  801  sends an Emergency PDN connectivity request containing a cleanup indicator to existing bearers. This request may also include information on the number of bearers simultaneously supportable by the UE  801 . 
     At operation  825 , in the case wherein the maximum number of simultaneously activatable bearers in the UE  801  is known to the core network in advance when the UE  801  has issued an emergency service request, the MME  803  may autonomously perform bearer cleanup. In the case wherein the number of supportable bearers in the UE  801  is not known to the core network when the UE  801  has issued a bearer cleanup request, the MME  803  may simply remove all normal bearers. 
     The MME  803  may be aware of the maximum number of simultaneously supportable bearers in the UE  801  according to user subscription information. The context table in the UE  801  may have a mapping to the maximum number of supportable bearers based on ISMI, or the maximum number of active bearers may be known according to the model of the UE  801  based on IMEISV. In this case, the MME  803  may have to store mappings for UE models based on IEMISV and the maximum number of active bearers in each UE model. 
     To notify the MME  803  of such information, the operator may update UE information in the HSS or may set information on the IMEISV-based model of the UE  801  and the maximum number of bearers corresponding to the model in the MME  803  through O&amp;M configuration or the like. 
     Upon reception of the emergency PDN connectivity request from the UE  801 , the MME  803  may determine that some or all of normal bearers are to be removed on the basis of information on the UE  801  and the maximum number of supportable bearers. When only some of normal bearers are to be removed, selection of bearers to be removed may be performed according to a criteria based on ARP, QCI, default/dedicated bearer (a dedicated bearer is removed), or inactivity duration. 
     At operations  830  and  835 , the core network removes corresponding active normal bearers (bearer cleanup from the S-GW  804  to the P-GW  805 ) and creates emergency bearers/sessions. Here, cleanup of existing bearers and creation of new emergency bearers may be simultaneously performed or separately performed in sequence. For simultaneous performance, a list of bearers to be removed or a cleanup indicator indicating removal of all normal bearers may be inserted in the Create session request message for emergency bearer creation. 
     The MME  803  notifies the ENB  802  of information on removed bearers and newly created bearers (bearer ID, bearer QoS, S5 TEID or the like). The ENB  802  updates data radio bearers related to the UE  801  accordingly. 
     When IDs of bearers to be removed are included in the emergency PDN connectivity request, the core network may remove the indicated active normal bearers (bearer cleanup from the S-GW  804  to the P-GW  805 ) and perform emergency bearer/session creation. 
     Here, cleanup of existing bearers and creation of new emergency bearers may be simultaneously performed or separately performed in sequence. For simultaneous performance, a list of bearers to be removed or a cleanup indicator indicating removal of all normal bearers may be inserted in the Create session request message for emergency bearer creation. 
     The MME  803  notifies the ENB  802  of information on removed bearers and newly created bearers (bearer ID, bearer QoS, S5 TEID or the like). The ENB  802  updates data radio bearers related to the UE  801  accordingly. Finally, this update is known to the UE  801 . 
     In another embodiment of the present disclosure, merging of the EMM process and the ESM process is proposed. That is, a TAU/RAU request message or ESR message containing an ESM message container is transmitted. Here, an ESM request message (e.g. PDN connectivity request) created by the UE  801  may be contained in the ESM message container. 
     More specifically, upon emergency service initiation, the UE  801  sends information on locally deactivated bearers through a TAU/RAU request message or an ESR message, whose ESM message container includes a PDN connectivity request. In other words, the UE  801  inserts information on locally deactivated bearers or information on the remaining active bearers after deactivation in the EPS bearer context status IE of a TAU request, RAU request or ESR message, inserts a PDN connectivity request for the emergency service in the ESM message container thereof, and sends the TAU request, RAU request or ESR message to the MME  803 . The MME  803  may not only perform bearer cleanup for the UE  801  according to the above embodiments but also handle a PDN connection creation request using the PDN connectivity request contained in the ESM message container. In the above-described schemes for emergency call provisioning according to the second embodiment, information elements contained in each signal may be used interchangeably. 
     Third Embodiment 
     When a core network node (e.g. MME or SGSN) becomes congested owing to heavy signaling load, the core network node may redirect user equipments being served so that the user equipments can be served by another core network node. 
     This process may be realized through a procedure performed by a base station node to newly select a core network node for a user equipment and through a procedure to register UE information in the core network node. However, if the previous core network node in congestion is reselected by the base station node performing the core network node selection procedure, congestion may be not resolved. In addition, when the core network node serving a user equipment is changed after a new core network node is selected by the base station node, it is necessary to register UE information in the new core network node. Here, to obtain information on the user equipment stored in the previous core network node, making an information request to the previous core network node may exacerbate conditions of the previous core network node already in congestion. 
     In this embodiment, a core network node in congestion may notify a user equipment of difficulty of service provisioning owing to congestion through NAS layer information. Upon reception of such notification, the user equipment does not provide information enabling identification of the previous core network node (e.g. S-TMSI, GUTI, GUMMEI, or P-TMSI) to the lower layer (AS layer) during reconfiguration of core network registration. Without information on the previous core network node, the AS layer of the user equipment cannot provide information thereon when an RRC connection is established with the base station, causing the base station to select a new core network node for the user equipment. A NAS request message (e.g. attach request) from the user equipment is delivered to the newly selected core network node via the base station. Here, to prevent exacerbation of congestion that may be caused by the new core network node trying to obtain UE context information from the previous core network node, the user equipment may provide the new core network node with information (or indicator) that directs the new core network node to directly communicate with the HSS to thereby configure UE context information. 
       FIG.  9    illustrates a procedure for signal exchange according to the third embodiment. 
     Referring to  FIG.  9   , a UE  901  may exchange signals with the MME/SGSN  903  or  904  via the RAN  902 . Here, in terms of congestion, the MME/SGSN  903  or  904  may be referred to as old MME/SGSN  903  (to which a connection has been made) or referred to as new MME/SGSN  904  (to which a connection will be made). However, it should be evident that change of the MME/SGSN may also be triggered by a cause other than congestion. 
     At operation  910 , congestion is generated in the old MME/SGSN  903 . Congestion may be caused by an increase in traffic, operational failure of some devices or the like. 
     As shown, at operation  915 , the UE  901  sends a NAS request message (e.g. TAU request or attach request) to a core network node in congestion (referred to as old core network node). At operation  920 , the old core network node sends an NAS response or reject message (e.g. TAU reject or attach reject) together with information indicating rejection of the request owing to congestion or indicating necessity of UE re-registration as loaded state to the UE  901 . Here, the S1-AP message (Downlink NAS Transport), which encapsulates the NAS message and is sent to the RAN  902  (base station), may further include information indicating necessity of releasing an RRC connection between the UE  901  and RAN  902  after delivery of the NAS message (e.g. immediate release required indicator). 
     At operation  925 , the RAN  902  sends an RRC message (DLInformationTransfer), which contains the message received from the core network node, to the UE  901 . The RRC message may contain a congestion notification and timer information. 
     Upon reception of a request for immediate release of the RRC connection, at operation  930 , the RRC connection is immediately released after message delivery. The RRC connection is released because, as the RAN  902  reselects a core network node for the UE  901  during RRC connection setup, if an existing RRC connection is reused when the UE  901  makes a NAS request, the old core network node may be reused. 
     As a scheme to obtain similar effects, a timer value may be inserted in a NAS response/reject message sent by the core network node to the UE  901  or in an RRC message (DLInformationTransfer) sent by the RAN  902  to the UE  901 . This timer value acts as a protection interval needed by the RAN  902  to release an existing RRC connection. That is, when a timer value is received through a core network node or an RRC message, the UE  901  may make a NAS request after expiration of the corresponding timer. 
     Thereafter, at operation  935 , when a NAS request (Attach request) is made, the NAS layer of the UE  901  prevents delivery of information enabling identification of the old core network node (e.g. S-TMSI, GUTI, GUMMEI or P-TMSI) to the lower layer. 
     At operation  940 , RRC connection request/setup are performed. At operation  945 , during RRC connection setup, the AS layer of the UE  901  does not send information enabling identification of the old core network node (e.g. MME routing information) to the RAN  902 . 
     At operation  945 , the RAN  902  receives an RRC connection setup complete message. At operation  950 , as information enabling identification of the old core network node is not present, the RAN  902  selects a new core network node. At operation  955 , the RAN  902  forwards the contained NAS request message to the new core network node. 
     Upon reception of the NAS request message for the UE  901 , the new core network node has to perform a procedure for obtaining context information of the UE  901 . Here, the new core network node may send a request for context information of the UE  901  to the old core network node if necessary by use of an identifier (GUTI, or old GUTI) inserted by the UE  901  in the NAS request message. However, when many such requests are concentrated in the old core network node in congestion, congestion may worsen. To solve this problem, 1) when congestion arise, the old core network node may notify the new core network node of the congestion state through message exchange between core network nodes such as overload indication, or 2) when the new core network node sends an identification request for context information, the old core network node may notify the new core network node of the congestion state through a response/rejection to the request. Upon reception of the notification, the new core network node may be aware that the old core network node is overloaded and may store the notification for later use. At operation  960 , the core network performs a procedure of identity request/response to obtain the IMSI of the UE  901 . At operation  965 , the core network receives the IMSI from the UE  901  and performs UE context setup and necessary registration using the received IMSI. 
       FIG.  10    illustrates another procedure for signal exchange according to the third embodiment. 
     Referring to  FIG.  10   , a UE  1001  may exchange signals with the MME/SGSN  1003  or  1004  via the RAN  1002 . Here, in terms of congestion, the MME/SGSN  1003  or  1004  may be referred to as old MME/SGSN  1003  (to which a connection has been made) or referred to as new MME/SGSN  1004  (to which a connection will be made). However, it should be evident that change of the MME/SGSN may also be triggered by a cause other than congestion. 
     Operations  1010  to  1040  correspond respectively to operations  910  to  940  in  FIG.  9   . 
     At operation  1045 , the UE  1001  generates a NAS request (attach request) containing an indication to overload of the old core network node or to preference for core network change at the NAS layer and sends the NAS request to the RAN  1002  (base station). At operation  1050 , the RAN  1002  selects a new core network node (MME/SGSN  1004 ) and sends an Initial UE message to the MME/SGSN  1004 . Here, the Initial UE message may contain an indication notifying re-attachment due to core network overload. Upon reception of the NAS request message for the UE  1001 , at operation  1060 , the new core network node has to perform a procedure for obtaining context information of the UE  1001 . When cause information (overload of the old core network node or preference for core network change) is included, the new core network node may be aware that the old core network node is overloaded and may store the notification for later use. If necessary, the new core network node may perform a procedure of identity request/response to obtain the IMSI of the UE  1001 . At operation  1065 , the core network receives the IMSI from the UE  1001  and performs UE context setup and necessary registration using the received IMSI. 
       FIG.  11    illustrates another procedure for signal exchange according to the third embodiment. 
     Referring to  FIG.  11   , a UE  1101  may exchange signals with the MME/SGSN  1103  or  1104  via the RAN  1102 . Here, in terms of congestion, the MME/SGSN  1103  or  1104  may be referred to as old MME/SGSN  1103  (to which a connection has been made) or referred to as new MME/SGSN  1104  (to which a connection will be made). However, it should be evident that change of the MME/SGSN may also be triggered by a cause other than congestion. 
     Operations  1110  to  1150  correspond respectively to operations  910  to  950  in  FIG.  9   . 
     The UE  1101  generates a NAS request (attach request) containing IMSI as UE ID instead of GUTI or old GUTI at the NAS layer when the present NAS request is to be generated owing to overload of the old core network node or preference for core network change, and sends the NAS request. Thereby, the new core network node does not send a request for UE context information to the old core network node and may perform UE context setup for the UE  1101  and necessary registration on the basis of the IMSI. 
     At operation  1155 , the RAN  1102  sends an Initial UE message containing the NAS request and IMSI of the UE  1101 . At operation  1165 , necessary registration is performed. 
       FIG.  12    illustrates another procedure for signal exchange according to the third embodiment. 
     Referring to  FIG.  12   , a UE  1201  may exchange signals with the MME/SGSN  1203  or  1204  via the RAN  1202 . Here, in terms of congestion, the MME/SGSN  1203  or  1204  may be referred to as old MME/SGSN  1203  (to which a connection has been made) or referred to as new MME/SGSN  1204  (to which a connection will be made). However, it should be evident that change of the MME/SGSN may also be triggered by a cause other than congestion. 
     While the embodiments described in  FIGS.  9  to  11    are related to a case wherein the UE  1201  explicitly sends a NAS request to a core network node, the embodiment described in  FIG.  12    may also be applied to a case wherein the UE  1201  does not explicitly send a NAS request. For example, upon determining that a core network node is overloaded, the UE  1201  is notified of necessity of core network node reconfiguration via the RAN  1202  and is registered through a new core network node using a procedure similar to those depicted in  FIGS.  9  to  11   . At operation  1210 , the UE  1201 , RAN  1202  and old MME/SGSN  1203  operate in connected mode. 
     At operation  1215 , a core network node (old MME/SGSN  1203 ) detects overload. At operation  1220 , the core network node sends a command message to the RAN  1202 . Here, the command message contains information indicating necessity of connection release as to the UE  1201  for load balancing. 
     Upon reception of the command message, at operation  1225 , the RAN  1202  sends an RRC connection release command indicating necessity of core network change for load balancing to the UE  1201 . Operations after RRC connection release are similar to those after RRC connection release described in  FIGS.  9  to  11   , and a detailed description thereof is omitted. 
     The above description with reference to the accompanying drawings is provided to assist in a comprehensive understanding of various embodiments of the present disclosure as defined by the claims and their equivalents. It includes various specific details to assist in that understanding but these are to be regarded as merely exemplary. Accordingly, those of ordinary skill in the art will recognize that various changes and modifications of the embodiments described herein can be made without departing from the scope and spirit of the present disclosure. 
     Hereinabove, embodiments of the present disclosure have been described with reference to the accompanying drawings. Specific terms or words used in the description should be construed in accordance with the spirit of the present disclosure without limiting the subject matter thereof. It should be understood that many variations and modifications of the basic inventive concept described herein will still fall within the spirit and scope of the present disclosure as defined in the appended claims and their equivalents.