Abstract:
Methods and apparatuses are provided for handling Internet protocol (IP) multimedia subsystem (IMS) and circuit switch fall back (CSFB) calls at a user equipment (UE) in a wireless network. A non-access stratum (NAS) of the UE receives an IMS indication when an IMS call is initiated at the UE. The NAS determines a state of the UE. The NAS performs an action based on the state of the UE to trigger one of the IMS call and a CSFB call.

Description:
PRIORITY 
       [0001]    This application claims priority under 35 U.S.C. §119(a) to Indian Provisional Patent Application Serial No. 6085/CHE/2015 (PS), which was filed in the Indian Intellectual Property Office on Nov. 11, 2015, and to Indian Complete Patent Application Serial No. 6085/CHE/2015 (CS), which was filed in the Indian Intellectual Property Office on Mar. 31, 2016, the contents of which are incorporated herein by reference. 
     
    
     BACKGROUND 
       [0002]    1. Field of the Disclosure 
         [0003]    The present disclosure relates generally to Internet protocol (IP) multimedia subsystem (IMS) call initiation, and more particularly, to methods and apparatuses for triggering an IMS call or a circuit switch fall back (CSFB) call based on a state of the user equipment (UE). 
         [0004]    2. Description of the Related Art 
         [0005]    The introduction of the IMS has significantly impacted wireless network equipment and their deployment. The IMS and the deployment of long term evolution (LTE) have had a significant effect on the operation of UEs. Further, the problems resulting from IMS deployment on UEs has not been completely explored from a non-access stratum (NAS) perspective. From the NAS perspective IMS call triggers in abnormal states must be considered, and include, for example, initiating by a tracking area update (TAU) request or a service request, the attempting-to-update in a UE, when timer(s) T3411/T3402/T3430/T3440 are running, and when TAU_INITIATED and TAU Reject with Temp causes network failure/congestion. 
         [0006]    According to an existing method and system, as shown in  FIGS. 1A and 1B ,  FIG. 1A  illustrates an activity diagram illustrating an IMS call drop scenario at a UE which incorporates a single subscriber identity module (SIM) and dual SIM, according to the prior art. 
         [0007]    In step  102   a,  once the LTE NAS layer is in a registered state with a network, the UE initiates a call request, such as, for example, a voice over LTE (VoLTE)/IMS call with the network. Further, in step  104   a,  the UE transmits session initiation protocol (SIP) data and uplink (UL) data packets to the network (i.e., NAS data plane). Thereafter, in step  106   a,  the UE transmits a request to NAS control plane, with the request, for example, switching from an NAS idle mode to NAS connected mode. In step  108   a,  the UE detects abnormal states in the NAS layer (i.e., NAS layer at the UE). The abnormal states include, for example, ATTEMPTING_TO_UPDATE the NAS, timer T3411 running, T340 running, and TAU_INITIATED and TAU reject with Temp causes like network failure, congestion, etc. Thus, due to this abnormal service state (i.e., evolved packet system (EPS) mobility management (EMM)) at the NAS layer, the UE sends no response message to the network. In step  110   a,  the UE may receive a drop/delay the IMS call/CSFB call indication from the network due to time out. 
         [0008]    Thus, the unwanted SIP packet transfer of queued data even after the IMS call has timed out may cause confusion in the network and lead to the call drop. 
         [0009]      FIG. 1B  illustrates another activity diagram illustrating an IMS call drop scenario at a UE which incorporates a multi SIM, according to the prior art. 
         [0010]    In step  102   b,  the UE triggers the recovery procedure, such as local area identity (LAI) using SIM- 2  before initiating the IMS call by a SIM- 1  of the UE. In step  104   b,  the SIM- 2  acquires a radio frequency (RF) stack to perform the recovery procedure (such as performing local area update (LAU)). In step  106   b,  the LTE NAS layer (i.e., associated with the SIM- 1 ) is in a registered state with a network, and the SIM- 1  of the UE initiates a call request, such as, for example, a voice over LTE (VoLTE)/IMS call with the network. Further, in step  108   b,  the UE transmits session initiation protocol (SIP) data and uplink (UL) data packets to the network (i.e., NAS data plane). Thereafter, in step  110   b,  the UE transmits a request to NAS control plane, with the request, for example, switching from an NAS idle mode to NAS connected mode. In step  112   b,  the UE transmits a connection establishment request (i.e., associated with the SIM- 1 ) to an LTE-AS. 
         [0011]    As the RF stack has been utilized by the SIM- 2  in performing recovery procedure, signaling, or other high priority activity, this may result in a poor user experience due to the lack of prioritization in a dual SIM dual standby (DSDS) UE and the unavailability of the RF for the SIM- 1  that is triggering the IMS Call. 
         [0012]    In step  114   b,  the UE may receive drop/delay the IMS call/CSFB call indication from the network due to time out. 
         [0013]    This problem applies to two-receiver-DSDS (2RX-DSDS) if the second stack is busy in high priority UL activity. Similar problems (e.g., call drop, delay in establishing a voice call, and the like) persist in a dual SIM dual active (DSDA) UE. 
         [0014]    In case of the DSDS UE, as since a VoLTE call is a packet switched (PS) call, and when circuit switched (CS) paging is received on a second SIM, there is a possibility that the VoLTE signaling may be interrupted since a modem may not be able to distinguish between a voice call and an ongoing video call. Hence, the VoLTE call may be disconnected and a default priority may be given to the CS call, continuing the poor user experience. 
       SUMMARY 
       [0015]    An aspect of the embodiments of the present disclosure provides a mechanism for handing an IMS call and a circuit switch fall back (CSFB) call at a UE in wireless network. 
         [0016]    According to an aspect of the present disclosure, a method is provided for handling IMS and CSFB calls at a UE in a wireless network. An NAS of the UE receives an IMS indication when an IMS call is initiated at the UE. The NAS determines a state of the UE. The NAS performs an action based on the state of the UE to trigger one of the IMS call and a CSFB call. According to an aspect of the present disclosure, a method is provided for handling IMS and CSFB calls at a UE in wireless network. An NAS of the UE receives an IMS indication when an IMS call is initiated at the UE. The NAS determines whether a connection to a network entity in the wireless network is successful. The IMS call is triggered when the connection to the network entity is successful. The CSFB call is triggered when the connection to the network entity is not successful. 
         [0017]    According to an aspect of the present disclosure, a method is provided for handling an IMS at a UE in a wireless network. An NAS of the UE receives an IMS indication when the IMS call is initiated from a first SIM at the UE. The NAS detects that the UE is in a connected mode and a second SIM at the UE is signaling data to the wireless network. The NAS blocks the signaling of data by the second SIM. The NAS triggers the IMS call from the first SIM. According to an aspect of the present disclosure, a UE is provided for handling IMS and CSFB calls in a wireless network. The UE includes a memory, a processor, and an NAS coupled to the processor and memory. The NAS is configured to receive an IMS indication when an IMS call is initiated at the UE, determine a state of the UE, and perform an action based on the state of the UE to trigger one of an IMS call and a CSFB call. 
         [0018]    According to an aspect of the present disclosure, a UE is provided for handing IMS and CSFB calls in a wireless network. The UE includes a memory, a processor, and an NAS, coupled to the processor and memory. The NAS is configured to receive an IMS indication when an IMS call is initiated at the UE, determine whether a connection to a network entity in the wireless network is successful, trigger the IMS call when the connection to the network entity is successful, and trigger the CSFB call when the connection to the network entity is not successful. 
         [0019]    According to an aspect of the present disclosure, a UE is provided for handing an IMS call in a wireless network. The UE includes a memory, a processor, and an NAS coupled to the processor and memory. The NAS is configured to receive an IMS indication when the IMS call is initiated from a first SIM at the UE, detect that the UE is in a connected mode and a second SIM at the UE is signaling data to the wireless network, block the signaling of data by the second SIM, and trigger the IMS call from the first SIM. 
         [0020]    According to an aspect of the present disclosure, a method is provided for handling calls at a UE in a wireless network. An NAS of the UE determines whether the UE is in a TAU initiated state upon initiating of an IMS call. The NAS determines whether the UE is attempting to update and one or more retransmission timers are running, when the UE is not in a TAU initiated state. The NAS triggers a CSFB call, when at least one of the UE is not attempting to update and the one or more retransmission timers are not running. The NAS triggers the CSFB call or the IMS call in accordance with whether a TAU accept message is received, when the UE is in a TAU initiated state or when the UE is attempting to update and one or more retransmission timers are running. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0021]    The above and other aspects, features, and advantages of the present disclosure will be more apparent from the following detailed description when taken in conjunction with the accompanying drawings, in which: 
           [0022]      FIG. 1A  is flow diagram illustrating an IMS call drop in a single SIM and a dual SIM scenario; 
           [0023]      FIG. 1B  is flow diagram illustrating an IMS call drop in a multi SIM scenario; 
           [0024]      FIG. 2  is a diagram illustrating components of a UE for handling an IMS and CSFB call in wireless network, according to an embodiment of the present disclosure; 
           [0025]      FIG. 3  is a flow diagram illustrating a method for handling an IMS and CSFB call at the UE in wireless network, according to an embodiment of the present disclosure; 
           [0026]      FIG. 4  is a flow diagram illustrating a method for handling an IMS and CSFB call at a UE that is attempting to update when a retransmission timer expires in wireless network, according to an embodiment of the present disclosure; 
           [0027]      FIG. 5  is a sequence diagram illustrating operations performed for handling an IMS and CSFB call when the UE is registered and attempting to update, according to an embodiment of the present disclosure; 
           [0028]      FIG. 6  is a sequence diagram illustrating operations performed for handling a pending IMS call in the case of TAU failure due to timer expiration, according to an embodiment of the present disclosure; 
           [0029]      FIG. 7  is a sequence diagram illustrating operations performed for handling an IMS voice/emergency call when the UE receives a lower layer failure for a TAU request message, according to an embodiment of the present disclosure; 
           [0030]      FIG. 8  is a sequence diagram illustrating operations performed for handling an IMS call when a UE receives a TAU reject, according to an embodiment of the present disclosure; 
           [0031]      FIG. 9  is a sequence diagram illustrating operations performed for handling an IMS call while a congestion timer is running, according to an embodiment of the present disclosure; 
           [0032]      FIG. 10  is a sequence diagram illustrating operations performed for handling CS paging during IMS call establishment, according to an embodiment of the present disclosure; 
           [0033]      FIG. 11  is a flow diagram illustrating a method for IMS call handling of an IMS call flow indicating success and failure to an upper layer, according to an embodiment of the present disclosure; 
           [0034]      FIG. 12  is a flow diagram illustrating a method for handling an IMS call flow in evolved packet system (EPS) mobility management (EMM) abnormal states, according to an embodiment of the present disclosure; 
           [0035]      FIG. 13  is a flow diagram illustrating a method for handling an IMS call and a CSFB call at UE supporting multi SIMs, according to an embodiment of the present disclosure; 
           [0036]      FIG. 14  is a flow diagram illustrating a method for handling an IMS call and a CSFB call at UE, during CS paging, supporting multi SIMs, according to an embodiment of the present disclosure; and 
           [0037]      FIG. 15  is a diagram illustrating a computing environment implementing the method for handling an IMS call and a CSFB call at UE in a wireless network, according to an embodiment of the present disclosure. 
       
    
    
     DETAILED DESCRIPTION 
       [0038]    Embodiments of the present disclosure are described in detail with reference to the accompanying drawings. The same or similar components may be designated by the same or similar reference numerals although they are illustrated in different drawings. Detailed descriptions of constructions or processes known in the art may be omitted to avoid obscuring the subject matter of the present disclosure. 
         [0039]    The term “or”, as used herein, refers to a non-exclusive “or”, unless otherwise indicated. The examples used herein are intended to facilitate an understanding of the ways in which the embodiments can be practiced and to further enable those skilled in the art to practice the embodiments. Accordingly, the examples should not be construed as limiting the scope of the embodiments described herein. 
         [0040]    Generally, LTE is a technology proposed by the 3 rd  generation partnership project (3GPP) which supports a high data rate (e.g., 50 Mbps uplink (UL) and 100 Mbps downlink (DL)) and is purely packet switched (PS) system. A PS system handles massive types of data. In order to improve IMS call procedures in a multimode device capable of LTE/3 rd  generation (3G)/2 nd  generation (2G), the 3GPP has provided one or more mechanisms to enhance signaling procedures to a modem. However, the one or more mechanisms provided by the 3GPP have not been accurate in handling an IMS call on an LTE network when a UE is in an abnormal state. 
         [0041]    The abnormal sate may include one or more of IMS calls triggered in an evolved universal terrestrial radio access network (EUTRAN) when the UE is attempting to update and one or more timers (e.g. T3411/T3402/T3346) are running, IMS calls triggered in the EUTRAN when the UE is in a service request initiated state and one or more timers are running, IMS calls triggered in EUTRAN when the UE is waiting for TAU accept/reject from a network entity and one or more timers are running, IMS calls triggered in the EUTRAN during a UE initiated TAU procedure with a TAU active flag set to true, IMS calls triggered in the EUTRAN when the UE has received TAU reject/lower layer failure for the TAU with the TAU active flag as true, and IMS calls triggered from the UE when reselection is performed from the LTE to global system for mobile communication (GSM)/universal mobile telecommunications system (UMTS) radio access technology (RAT). 
         [0042]    Unlike conventional systems and methods, as shown in  FIGS. 1A and 1B , embodiments of the present disclosure provide the UE, when, during a TAU retransmit timer (T3411/T3402/T3430/T3440) or upon a TAU initiation or service request and waiting for a response from the network, the UE initiates a VoLTE/IMS call, then high layers can provide a request to the NAS to check whether the UE is in a proper state to continue the call in the LTE, or whether it can move to a legacy radio access technology (RAT) to complete the call in the CS domain. When the NAS receives this indication, the UE can immediately trigger the TAU irrespective of its timer states. After sending to the TAU that its attempts failed or max attempts are reached, this failure is indicated to the IMS client/dialer so that the IMS client will not trigger any further data and pending data will be flushed. This will avoid unnecessary re-transmissions of the SIP data packets from the IMS client. In case of a failure from the NAS, the IMS client/dialer can initiate a CS voice call by moving to a legacy RAT to complete the user request. 
         [0043]    Unlike conventional systems and methods, embodiments of the present disclosure provide a unique evolved packet system (EPS) mobility management (EMM) that caters to IMS call requests to handle or prioritize the IMS mobile originated (MO)/MT call requests when the UE is in an EMM abnormal state, such as, for example, attempting update/attach and running one or more timers T3411/T3410/T3430. 
         [0044]    Unlike conventional systems and methods, the proposed mechanism increases the success rate of VoLTE/IMS calls in DSDS devices where an RF is shared between two stacks. 
         [0045]      FIG. 2  is a diagram illustrating components of a UE communicating with a network entity for handing an IMS call and a CSFB call in wireless network, according to an embodiment of the present disclosure. 
         [0046]    A UE  102  includes a processor  202 , a memory  204  coupled to the processor  202 , a NAS  206  coupled to the processor  202 , a communication unit  208 , and a modem  210 . The UE  102  may be embodied as a laptop, a desktop computer, a mobile phone, a mobile station, a mobile terminal, a smart phone, a personal digital assistant (PDA), a tablet, a phablet, or any other electronic device. 
         [0047]    In an embodiment, the NAS  206  can be configured to receive an IMS indication when the IMS call is initiated at the UE  102 . The IMS indication notifies the NAS  206  that the IMS call has been triggered by the UE  102 . 
         [0048]    Further, the NAS  206  can be configured to detect a state of the UE  102 . The state of the UE  102  can include the abnormal state, such as, for example, initiation of TAU, running or expiration of timers, one or more of timers T3411, T3346, T3402 or T3430, dropping a call (IMS/CSFB), delaying of a call, or the like. 
         [0049]    Call drops and call delays based on timers are shown below in Table 1. 
         [0000]    
       
         
               
             
               
               
             
               
               
               
             
               
               
             
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Call Delays/Drops Based on Timers 
               
             
          
           
               
                 Timer Name 
                 Timer Value 
               
               
                   
               
             
          
           
               
                 T3411(At TAU failure due to lower layer failure, 
                 10 
                 s 
               
               
                 T3430 timeout or TAU rejected) 
               
               
                 T3346(TAU reject received with a timer value for 
                 15-30 
                 mins 
               
               
                 T3346) 
               
             
          
           
               
                 T3402(TAU accept with EMM cause #16 or #17 and 
                 Default 12 mins 
               
               
                 the attempt counter is equal to 5 for CS/PS mode 2 
               
               
                 UE, or TAU accept with EMM cause #22) 
               
             
          
           
               
                 T3430(TAU request sent) 
                 15 
                 s 
               
               
                   
               
             
          
         
       
     
         [0050]    Further, the NAS  206  can be configured to perform an action based on the state of the UE  102  to trigger one of the IMS call and a CSFB call, as described in greater detail below with respect to  FIGS. 4-7 . 
         [0051]    The NAS  206  can be configured to detect whether a connection to a network entity  220  in the wireless network is successful or unsuccessful. The UE  102  can be connected to the network entity  220 . 
         [0052]    Further, the NAS  206  can be configured to perform triggering the IMS call when the connection to the network entity is successful, or triggering the CSFB call when the connection to the network entity is unsuccessful. 
         [0053]    The NAS  206  can be configured to receive an IMS indication when the IMS call is initiated from the first SIM at the UE  102 . Further, the NAS  206  can be configured to detect that the UE  102  is in a connected mode, and a second SIM at the UE  102  is signaling data to the wireless network. Further, the NAS  206  can be configured to block the signaling of data at the second SIM and trigger the IMS call from the first SIM. 
         [0054]    The modem  210  can be configured to detect that CS paging is received at the second SIM while the IMS call is ongoing on the first SIM, and ignore the CS paging at the second SIM. 
         [0055]    The memory  204  may include one or more computer-readable storage media. The memory  204  may include non-volatile storage elements. Examples of such non-volatile storage elements may include magnetic hard discs, optical discs, floppy discs, flash memories, or forms of electrically programmable read only memories (EPROMs) or electrically erasable and programmable read only memories (EEPROMs). In addition, the memory  204  may, in some examples, be considered a non-transitory storage medium. The term “non-transitory” may indicate that the storage medium is not embodied in a carrier wave or a propagated signal. However, the term “non-transitory” should not be interpreted that the memory  204  is non-movable. In some examples, the memory  204  can be configured to store larger amounts of information than the memory. In certain examples, a non-transitory storage medium may store data that can, over time, change (e.g., in random access memory (RAM) or cache). The communication unit  208  can be configured to communicate internally between the units and externally with the networks. 
         [0056]      FIG. 2  shows a limited overview of the units of the UE  102  but it is to be understood that other embodiments are not limited thereon. In other embodiments, the UE  102  may include fewer or more units. Further, the labels or names of the units are used only for illustrative purposes and do not limit the scope of the disclosure. One or more units can be combined together to perform same or substantially similar function in the UE  102 . 
         [0057]      FIG. 3  is flow diagram illustrating a method for handling an IMS call and a CSFB call at a UE, according to an embodiment of the present disclosure. The method is performed by an NAS in a UE. 
         [0058]    In step  302 , the method includes receiving an IMS indication when an IMS call is initiated at the UE. In an embodiment, the method allows the NAS  206  to receive the IMS indication when the IMS call is initiated at the UE  102 . 
         [0059]    In step  304 , the method includes detecting a state of the UE  102 . In an embodiment, the method allows the NAS  206  to detect the state of the UE  102 . 
         [0060]    In step  306 , the method includes performing an action based on the state of the UE  102  to trigger one of the IMS call and the CSFB call. In an embodiment, the method allows the NAS  206  to perform the action based on the state of the UE  102  to trigger one of the IMS call and the CSFB call. 
         [0061]    The various actions, acts, blocks, steps, or the like in the method may be performed in the order presented, in a different order, or simultaneously. Further, in some embodiments, some of the actions, acts, blocks, steps, or the like may be omitted, added, modified, skipped, or the like without departing from the scope of the disclosure. 
         [0062]      FIG. 4  is a flow diagram illustrating a method for handling an IMS call and a CSFB call at a UE serving an attempt to update when a retransmission timer expires in a wireless network, according to an embodiment of the present disclosure. The method is performed by an NAS in a UE. 
         [0063]    In step  402 , the method includes receiving, by the NAS  206 , an IMS indication when an IMS call is initiated at the UE  102 . 
         [0064]    In step  404 , the method includes determining whether the UE  102  is in a TAU initiated state. In an embodiment, the method allows the NAS  206  to detect the state of the UE  102 . 
         [0065]    If the NAS  206  detects that the UE  102  is in the TAU initiated state, the NAS  206  determines whether the UE  102  is serving the ATTEMPTING-TO-UPDATE and 1.0 whether one or more of retransmission timers T3411, T3346, T3402, and T3430 have expired, in step  406 . 
         [0066]    If the UE  102  is determined to be serving the ATTEMPTING-TO-UPDATE and the one or more of the retransmission timers T3411, T3346, T3402, and T3430 have expired, the method includes stopping the transmission timer and setting the TAU active flag to “true”, in step  408 , to indicate data pending to the network entity  220 . In an embodiment, the method allows the NAS  206  to stop the transmission timer and set the TAU Active flag. The method then proceeds to step  414  as described below. 
         [0067]    If the UE  102  is not serving the ATTEMPTING-TO-UPDATE and the one or more of the retransmission timers T3411, T3346, T3402, and T3430 are running, the method includes indicating a failure to an IMS client and deleting the pending data, in step  410 , thereby triggering the CSFB call. In an embodiment, the method allows the NAS  206  to indicate the failure to the IMS client and deleting the pending data. 
         [0068]    If the NAS  206  determines that the UE  102  is not in a TAU initiated state, the method includes waiting for a TAU accept/reject or unit one or more timers expire, in step  412 . In an embodiment, the method allows the NAS  206  to wait for the TAU accept/reject or until the one or more timers expire. 
         [0069]    Further, in step  414 , the method includes determining whether an area update accepts message is received from the network entity  220 . In an embodiment, the method allows the NAS  206  to make the determination. 
         [0070]    If the NAS  206  determines that the area update accepts message is received from the network entity  220 , the method includes triggering the IMS call, in step  416 . In an embodiment the method allows the NAS  206  to trigger the IMS call. 
         [0071]    If the NAS  206  determines that the area update accepts message is rejected from the network entity  220 , the method includes indicating that the TAU failed to an upper layer and that the UE  102  cannot send user data, in step  418 . In an embodiment, the method allows the NAS  206  to provide the indication. 
         [0072]    In step  420 , the method includes triggering the CSFB call. In an embodiment, the method allows the NAS  206  to trigger the CSFB call. 
         [0073]    The various actions, acts, blocks, steps, or the like in the method may be performed in the order presented, in a different order, or simultaneously. Further, in some embodiments, some of the actions, acts, blocks, steps, or the like may be omitted, added, modified, skipped, or the like without departing from the scope of the disclosure. 
         [0074]      FIG. 5  is a sequence diagram illustrating operations performed for handling the IMS call and the CSFB call in the UE in a registered and attempting to update state, according to an embodiment of the present disclosure. 
         [0075]    Unlike conventional systems and methods, the proposed mechanism allows the UE  102  to immediately perform T3411 timer expiry actions and set the active flag as “true” in the TAU request. If the TAU is completed successfully, the UE  102  can therefore proceed with IMS signaling procedures and continue the IMS call. Thus, the proposed method can be utilized to effectively handle the MO Voice Call when the UE  102  is in a registered, attempting to update, update-MM state. 
         [0076]    In an embodiment, the NAS  206  (i.e. the NAS  206  in the UE  102 ) receives a combined evolved packet system (EPS) attach accept (location area identity (LAI)) from the network entity  220 , in step  502 . The NAS  206  broadcasts the TAI change observed indication and the TAU requests message (combined TA LA) to the network entity  220 , in step  504 . Further, the network entity  220  broadcasts a lower layer failure indication, to the NAS  206 , in an RRC connection, in step  506 . The NAS  206  broadcasts the TAU request message to the network entity  220  by setting the active flag to “true”, in step  508 , thereby triggering the IMS call. 
         [0077]      FIG. 6  is a sequence diagram illustrating operations performed in handling a pending IMS call in case of a TAU failure due to expiration of one or more timers, according to an embodiment of the present disclosure. 
         [0078]    Unlike conventional methods and systems, the proposed mechanism can effectively handle the IMS call and the CSFB call during an MO voice/emergency call pending when the UE  102  initiates the TAU procedure with active flag set to “true” and during an RRC connection release request, received from the network entity  220 , with lower layer failure/normal before receiving the TAU accept/reject request message. 
         [0079]    According to an embodiment, the NAS  206  releases the RRC connection locally, in response to the request received from the network entity  220 , and switches to GSM enhanced data rates for GSM evolution (EDGE) radio access network (GERAN)/UTRAN to support the voice call through the CS. Also, the NAS  206  can send an indication to the IMS client so that pending data can be flushed at the modem  210  and a transmission control protocol-Internet protocol (TCP-IP). 
         [0080]    In an embodiment, the NAS  206  (i.e. the NAS  206  in the UE  102 ) receives the combined EPS attach accept (LAI) from the network entity  220 , in step  602 . The NAS  206  broadcasts the TAI change observed indication and the TAU requests message (combined TA LA) to the network entity  220 , in step  604 . Further, the network entity  220  broadcasts a lower layer failure indication, to the NAS  206 , in the RRC connection, in step  606 . The NAS  206  broadcasts the TAU request message to the network entity  220  by setting the active flag to “true”, in step  608 . 
         [0081]    The NAS  206  can be configured to release the RRC connection locally, in step  610 . Further, the NAS  206  can be configured to indicate the connection failure to the IMS client. Furthermore, the NAS  206  can be configured to flush the pending data due to the IMS call at modem  210  and the TCP-IP to prevent unwanted data/re-transmissions of the data packets after the call is ended or attempted in legacy RATs. 
         [0082]      FIG. 7  is a sequence diagram illustrating operations performed for handling an IMS voice/emergency call in case the UE receives a lower layer failure request message, from the network entity, in response to the received TAU request message from the NAS  206 , according to an embodiment of the present disclosure. 
         [0083]    In an embodiment, the NAS  206  receives the combined EPS attach accept (LAI) from the network entity  220 , in step  702 . The NAS  206  broadcasts the TAI change observed indication and the TAU requests message (combined TA LA) to the network entity  220 , in step  704 . Further, the network entity  220  broadcasts a lower layer failure indication, to the NAS  206 , in an RRC connection, in step  706 . The NAS  206  broadcasts the TAU request message to the network entity  220  by setting the active flag to “true”, in step  708 . 
         [0084]    If the NAS  206  receives the RRC connection failure request message from the network entity  220  before the TAU accept/reject request message, in step  710 . The NAS  206  can stop the timer T3430 and indicate the RRC connection failure to the IMS client. Further, the NAS  206  can be configured to flush the pending data due to the IMS call at the modem  210  and the TCP-IP to prevent unwanted data/re-transmissions of the data packets after the call is ended or attempted in legacy RATs. 
         [0085]      FIG. 8  is a sequence diagram illustrating handling of an IMS Call when the UE receives the TAU reject message, according to an embodiment of the present disclosure. 
         [0086]    In an embodiment, the NAS  206  receives the combined EPS attach accept (LAI) from the network entity  220 , in step  802 . The NAS  206  broadcasts the TAI change observed indication and the TAU requests message (combined TA LA) to the network entity  220 , in step  804 . Further, the network entity  220  broadcasts a lower layer failure indication, to the NAS  206 , in the RRC connection, in step  806 . The NAS  206 , thereon, broadcasts the TAU request message to the network entity  220  by setting the active flag to “true”, in step  808 . 
         [0087]    Further, the NAS  206  receives the TAU Reject message with temporary cause, such as, for example, NW failure/congestion/MSC not reachable, in step  810 . The NAS  206  releases the RRC connection locally and switches to the GERAN/UTRAN to support voice call through CS, in step  812 . Also, the NAS  206  can send the indication to the IMS client (i.e., IMS application at the UE  102 ) so that pending data can be flushed at the modem  210  and the TCP-IP. 
         [0088]      FIG. 9  is a sequence diagram illustrating operations performed for handling CS paging during IMS call establishment, according to an embodiment of the present disclosure. 
         [0089]    In an embodiment, the UE  102  broadcasts the TAU/service request (RRC establishment can be mentioned as the IMS call) to the network entity  220 , in step  902 . Further, the network entity  220 , in response to received TAU/service request, broadcasts the TAU/service request reject, in step  904 , to wait for RRC release of the connection locally, in step  906 . 
         [0090]    Therefore, the UE  102  indicates a connection failure to the IMS client. Further, the UE  102  can flush the pending data due to an IMS call at the modem  210  and the TCP-IP, to prevent unwanted data/re-transmissions after call is ended or attempted in legacy RATs. 
         [0091]      FIG. 10  is a sequence diagram illustrating operations performed for handling CS paging during the IMS call establishment, according to an embodiment of the present disclosure. 
         [0092]    The UE  102  is capable of supporting multi SIM, and if the VoLTE call is triggered from the IMS client, the modem  210  receives an indication from the IMS client for a VoLTE call, in step  1002 . The modem connection with the network entity  220  is established in response to an indication received from the UE  102 . The UE  102  therefore sends a response to the IMS client to start transmitting an SIP data. 
         [0093]    Further, the modem  210  detects that CS paging is received at the second SIM while the IMS call is ongoing on the first SIM, in step  1004 . Thus, the modem  210  can be configured to ignore the CS paging and continue the IMS call by ending the VoLTE call, and once the call is ended the IMS client can send an indication to the modem  210  that the call has ended. 
         [0094]    The UE  102  initiates the IMS call when the UE  102  is in registered and idle states and either a TAU with an active flag or a service request is triggered. CS paging is received from the network entity, but the modem  210  receives the indication indicating the VoLTE call establishment is in progress, therefore the modem  210  can ignore the CS paging and continue the VoLTE call. 
         [0095]    Unlike conventional systems and methods, the embodiments prioritize the IMS call request in the UE  102  during the abnormal states, occurring at the UE  102 , to maximize the chances of successful IMS call establishment. 
         [0096]      FIG. 11  is a flow diagram illustrating a method for handling an IMS Call in EMM abnormal states, according to an embodiment of the present disclosure. 
         [0097]    In step  1102 , a method includes receiving an IMS indication when an IMS call is initiated at the UE  102 . In an embodiment, the method allows the NAS  206  to receive the IMS indication. 
         [0098]    In step  1104 , the method includes connecting the NAS  206  to the network entity  220  and indicating an accept/reject request message to the upper layer. In an embodiment, the method allows the NAS  206  to provide the indication to the upper layer. 
         [0099]    In step  1106 , the method includes determining whether the connection to the network entity  220  in the wireless network is successful. In an embodiment, the method allows the NAS  206  to make the determination. 
         [0100]    If the connection to the network entity  220  in the wireless network is unsuccessful, the method includes sending the failure indication to the upper layer and triggering the CSFB call (move to 2G/3G to continue the call on the CS domain), in step  1108 . In an embodiment, the method allows the NAS  206  to send the failure indication and trigger the CSFB call. 
         [0101]    If the connection to the network entity  220  in the wireless network is successful, the method includes triggering the IMS call (continue call on LTE network using IMS procedures), in step  1110 . In an embodiment, the method allows the NAS  206  to trigger the IMS call. 
         [0102]    The various actions, acts, blocks, steps, or the like in the method may be performed in the order presented, in a different order, or simultaneously. Further, in some embodiments, some of the actions, acts, blocks, steps, or the like may be omitted, added, modified, skipped, or the like without departing from the scope of the disclosure. 
         [0103]      FIG. 12  is a flow diagram illustrating a method for handling an IMS call in EMM abnormal states, according to an embodiment of the present disclosure. 
         [0104]    In step  1202 , a method includes receiving the IMS indication when the IMS call is initiated at the UE  102 . In an embodiment, the method allows the NAS  206  to receive the IMS indication. 
         [0105]    In step  1204 , the method includes determining whether the state of the UE  102  is a TAU initiated state. In an embodiment, the method allows the NAS  206  to determine the state of the UE  102 . 
         [0106]    If the NAS  206  detects that the UE  102  is not in the TAU initiated state, the method includes triggering, by the NAS  206 , the CSFB call, by following the existing 3G protocols, in step  1206 . 
         [0107]    If the NAS  206  detects that the UE  102  is in the TAU initiated state, the method includes prioritizing by the NAS  206  (in the UE  102 ) the MO voice call in the RRC connection request, in step  1208 . In step  1210 , the method includes sending a request, by the NAS  206 , to the network entity  220  to prioritize IMS traffic over normal traffic. 
         [0108]    For example, when a user initiates the IMS call and the UE  102  is in the registered and idle state, either a TAU with an active flag or a service request will be triggered. Further, the connection request for such a message can be considered on priority by the network entity  220 , hence establishment of the IMS call in the RRC connection request is introduced, usage of such an establishment can make the network entity aware of the reason for the connection request so that the network entity  220  can prioritize and minimize problems such as congestion, network failures, or the like. 
         [0109]    The various actions, acts, blocks, steps, or the like in the method may be performed in the order presented, in a different order, or simultaneously. Further, in some embodiments, some of the actions, acts, blocks, steps, or the like may be omitted, added, modified, skipped, or the like without departing from the scope of the disclosure. 
         [0110]      FIG. 13  is a flow diagram illustrating a method for handling an IMS call and a CSFB call at UE supporting multi SIM, according to an embodiment of the present disclosure. 
         [0111]    In step  1302 , the method includes receiving an IMS indication when an IMS call is initiated from a first SIM at the UE  102 . In an embodiment, the method allows the NAS  206  to receive the IMS indication. 
         [0112]    In step  1304 , the method includes detecting that the UE  102  is in connected mode and that a second SIM at the UE  102  is signaling data to the wireless network. In an embodiment, the method allows the NAS  206  to perform the detections. 
         [0113]    In step  1306 , the method includes blocking the signaling of data at the second SIM. In an embodiment, the method allows the NAS  206  to block the signaling. 
         [0114]    In step  1308 , the method includes triggering the IMS call from the first SIM. In an embodiment, the method allows the NAS  206  to trigger the IMS call. 
         [0115]    Further, the UE  102  supporting the multi SIM (DSDA, DSDS) is described in greater detail below with respect to  FIG. 14 . 
         [0116]    The various actions, acts, blocks, steps, or the like in the method may be performed in the order presented, in a different order, or simultaneously. Further, in some embodiments, some of the actions, acts, blocks, steps, or the like may be omitted, added, modified, skipped, or the like without departing from the scope of the disclosure. 
         [0117]      FIG. 14  is a flow diagram illustrating a method for handling an IMS call and a CSFB call at UE, during CS paging, supporting multi SIM, according to an embodiment of the present disclosure. 
         [0118]    In step  1402 , the method includes receiving the IMS indication when the IMS call is initiated from the first SIM at the UE  102 . In an embodiment, the method allows the NAS  206  to receive the IMS indication. 
         [0119]    In step  1404 , the method includes detecting that the UE  102  is in connected mode and that the second SIM at the UE  102  is signaling data to the wireless network. In an embodiment, the method allows the NAS  206  perform the detections. 
         [0120]    In step  1406 , the method includes blocking the signaling of data at the second SIM. In an embodiment, the method allows the NAS  206  to block the signaling. 
         [0121]    In step  1408 , the method includes triggering the IMS call from the first SIM. In an embodiment, the method allows the NAS  206  to trigger the IMS call. 
         [0122]    In step  1410 , the method includes detecting that the CS paging is received at the second SIM while the IMS call is ongoing on the first SIM. In an embodiment, the method allows the modem  210  to detect that the CS paging is received at the second SIM. 
         [0123]    In step  1412 , the method includes ignoring the CS paging at the second SIM. In an embodiment, the method allows the modem  210  to ignore the CS paging at the second SIM and continue on the VoLTE call. 
         [0124]    The various actions, acts, blocks, steps, or the like in the method may be performed in the order presented, in a different order, or simultaneously. Further, in some embodiments, some of the actions, acts, blocks, steps, or the like may be omitted, added, modified, skipped, or the like without departing from the scope of the disclosure. 
         [0125]      FIG. 15  is a diagram illustrating a computing environment implementing the method for handing an IMS call and a CSFB call at the UE in wireless network, according to an embodiment of the present disclosure. 
         [0126]    A computing environment  1502  comprises at least one processing unit  1508 , which is equipped with a control unit  1504  and an arithmetic logic unit (ALU)  1506 , a memory  1510 , a storage unit  1512 , a plurality of networking devices  1516 , and a plurality of input/output (I/O) devices  1514 . The processing unit  1508  is responsible for processing the instructions of the schemes. The processing unit  1508  receives commands from the control unit  1504  in order to perform its processing. Further, any logical and arithmetic operations involved in the execution of the instructions are computed with the help of the ALU  1506 . 
         [0127]    The overall computing environment  1502  can be composed of multiple homogeneous or heterogeneous cores, multiple central processing units (CPUs) of different kinds, special media and other accelerators. The processing unit  1508  is responsible for processing the instructions of the schemes. Further, the plurality of processing units  1508  may be located on a single chip or over multiple chips. 
         [0128]    The scheme having instructions and codes required for the implementation are stored in the memory unit  1510 , the storage  1512 , or both. At the time of execution, the instructions may be fetched from the corresponding memory  1510  or storage  1512 , and executed by the processing unit  1508 . 
         [0129]    In case of any hardware implementations various networking devices  1516  or external I/O devices  1514  may be connected to the computing environment to support the implementation through the networking unit and the I/O device unit. 
         [0130]    The embodiments disclosed herein can be implemented through at least one software program running on at least one hardware device and performing network management functions to control the elements. The elements shown in the  FIGS. 1 through 15  include blocks which can be at least one of a hardware device, or a combination of hardware device and software units. 
         [0131]    The present disclosure may be utilized in conjunction with the manufacture of integrated circuits, chip sets, or system-on-chips (SoCs). One skilled in the art would know how to dice wafers and package die to produce integrated circuits. Integrated circuits so manufactured are considered part of this disclosure. 
         [0132]    While the disclosure has been shown and described with reference to certain embodiments thereof, it will be understood by those skilled in the art that various changes in form and detail may be made therein without departing from the spirit and scope of the disclosure as defined by the appended claims.