Abstract:
A method of handing an emergency PDN connection when a back-off timer is running for a core network in a wireless communication system is disclosed. The method comprises starting a mobile reachable timer when a mobile device enters an idle mode, wherein the mobile reachable timer has a first timer value; accepting the emergency PDN connection; stopping the mobile reachable timer when the mobile device enters a connected mode to establish the emergency PDN connection; resetting the mobile reachable timer to a second timer value when a remaining time of the back-off timer is longer than a periodic area update timer; and restarting the mobile reachable timer according to the second timer value when the emergency PDN is disconnected.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of U.S. Provisional Application No. 61/452,174, filed on Mar. 14, 2011 and entitled “Method to handle periodic timer, mobile reachability timer, and MM backoff timer” and the benefit of U.S. Provisional Application No. 61/475,681, filed on Apr. 15, 2011 and entitled “Method to handle periodic timer, mobile reachability timer, and MM backoff timer”, and the benefit of U.S. Provisional Application No. 61/442,465, filed on Feb. 14, 2011 and entitled “Method to handle mobile reachable timer for emergency PDN connection during running MM backoff timer”, and the benefit of U.S. Provisional Application No. 61/483,761, filed on May 9, 2011 and entitled “Method to handle mobile reachable timer for emergency PDN connection during running MM backoff timer”, the contents of which are incorporated herein in their entirety. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The application relates to a method and a related communication device used in a wireless communication system and related communication device, and more particularly, to a timer configuration method for an emergency packet data network connection in a wireless communication system. 
         [0004]    2. Description of the Prior Art 
         [0005]    A long-term evolution (LTE) system supporting the 3GPP Rel-8 standard and/or the 3GPP Rel-9 standard is developed by the 3rd Generation Partnership Project (3GPP) as a successor of a universal mobile telecommunications system (UMTS), for further enhancing performance of the UMTS to satisfy users&#39; increasing needs. The LTE system includes a new radio interface and radio network architecture that provides a high data rate, low latency, packet optimization, and improved system capacity and coverage. In the LTE system, a radio access network known as an evolved UTRAN (E-UTRAN) includes multiple evolved NBs (eNBs) for communicating with multiple user equipments (UEs), and communicates with a core network including a mobility management entity (MME), serving gateway, etc for Non Access Stratum (NAS) control. In UMTS system, a Serving GPRS Support Node (SGSN) is responsible for the delivery of data packets to the mobile devices back and forth within its geographical service area, including packet routing and transfer, mobility management (attach/detach and location management), logical link management, and authentication and charging functions. The MME also provides the control plane function for mobility between LTE and 2G/3G access networks with the S3 interface terminating at the MME from the SGSN. 
         [0006]    An access point name (APN) is used in the LTE system and the LTE-A system for establishing packet data connections, and is managed by the MME. Further, the APN is used for identifying a packet data network (PDN) or a service type (e.g. multimedia messaging service (MMS)), wherein the PDN or the service type is carried on the packet data connections. 
         [0007]    The core network manages the mobile reachability by using mobile reachable timer, paging proceed factor (PPF), and implicit detach timer. The core network supervises the periodic tracking/routing/location area updating procedure of the UE by means of the mobile reachable timer. By default, the mobile reachable timer is set slightly larger than the periodic TAU/RAU/LAU timer. Upon expiry of the mobile reachable timer the network shall clear the PPF flag and start the implicit detach timer. The value of the implicit detach timer is network dependent. Due to the clear of the PPF, the core network cannot page the UE during the period of the implicit detach timer. If the implicit detach timer expires before the UE contacts the network, the network shall implicitly detach the UE. The implicit detach timer shall be stopped when a NAS signaling connection is established for the UE. 
         [0008]    NAS level congestion control contains the functions: “APN based congestion control” and “General NAS level Mobility Management control”. The use of the APN based congestion control is for avoiding and handling of EMM and ESM signalling congestion associated with UEs with a particular APN. With General NAS level Mobility Management control, the MME may also use the reject of NAS level Mobility Management signaling requests under general congestion conditions. 
         [0009]    When the NAS level mobility management congestion control is applied due to network overload or congestion, the MME rejects an attach request, a tracking area update (TAU), a routing area update (RAU) or a service request, and the MME replies the UE with a Mobility Management back-off timer. 
         [0010]    When congestion control is active, the MME/SGSN stores the Mobility Management back-off timer and the Session Management back-off timer. Then the MME/SGSN enforces the stored back-off time by immediately rejecting any subsequent request from the UE targeting at connecting to the APN before the stored back-off time is expired. 
         [0011]    However, the UE is allowed to initiate a Mobility Management procedure or a Session Management procedure with the APN for a high priority service or an emergency service, even when the Mobility Management back-off timer or the Session Management back-off timer is running. In this situation, since the APN is still in the congestion and has no resource for providing the high priority service or the emergency service, the MME rejects the high priority service or the emergency service, and a delay to access the high priority service or the emergency service is caused. Therefore, disaster may happen due to the delay of the high priority service or the emergency service. 
         [0012]    When MME/SGSN performs NAS level congestion control, the MME/SGSN stores a Mobility Management back-off time and a Session Management back-off time. Then the MME/SGSN enforces the stored back-off time by immediately rejecting any subsequent MM/SM request from the UE and allocating a MM/SM back-off timer. However the MME/SGSN does not stores allocated MM/SM back-off timer in the EMM/ESM (EPS Mobility Management, EMM/EPS Session Management, ESM) context per UE basis. 
         [0013]    For a normal attached UE, if it gets rejection with a MM back-off timer due to tracking area update reject or service reject. The UE is normal attached (EMM-Registered state) but enters ECM-IDLE mode when the MM back-off timer is running. When MM back-off timer is running, the back-off UE is still allowed to make emergency call by PDN connection request with request type of emergency to an emergency APN. 
         [0014]    When the NAS signaling connection is built due to emergency PDN connection request and the UE returns from ECM-IDLE mode to ECM-Connected mode, the network stops the mobile reachable timer. 
         [0015]    After the UE disconnects emergency PDN connection and NAS signaling connection is released, the mobile reachable timer is reset and started with its initial value. The issue occurs when the MM back-off timer is not expired yet and the remaining time of the MM back-off timer is larger than periodic TAU timer. If the network resets the mobile reachable timer to its initial value, the mobile reachable timer is not able to reflect the remaining time of the MM back-off timer on the UE. Thus the network may lose the track of the UE activity. 
         [0016]    In addition, for a normal attached UE with one non-emergency PDN connection and one emergency PDN connection, if it gets rejection with a MM back-off timer due to tracking area update reject or service reject from the non-emergency PDN connection. 
         [0017]    After the UE disconnects emergency PDN connection and NAS signaling connection is released, the mobile reachable timer is reset and started with its initial value. The issue occurs when the MM back-off timer is not expired yet and the remaining time of the MM back-off timer is larger than periodic TAU timer. If the network resets the mobile reachable timer to its initial value, the mobile reachable timer is not able to reflect the remaining time of the MM back-off timer on the UE. Thus the network may lose the track of the UE activity. 
       SUMMARY OF THE INVENTION 
       [0018]    A method of handing an emergency PDN connection when a back-off timer is running for a core network in a wireless communication system is provided. 
         [0019]    A method of handing an emergency PDN connection when a back-off timer is running for a core network in a wireless communication system is disclosed. The method comprises starting a mobile reachable timer when a mobile device enters an idle mode, wherein the mobile reachable timer has a first timer value; accepting the emergency PDN connection; stopping the mobile reachable timer when the mobile device enters a connected mode to establish the emergency PDN connection; resetting the mobile reachable timer to a second timer value when a remaining time of the back-off timer is longer than a periodic area update timer; and resetting the mobile reachable timer according to the second timer value when the emergency PDN connection is disconnected and a remaining time of the mobile reachable timer is longer than a periodic area update timer. 
         [0020]    A method of handing an emergency PDN connection when a back-off timer is running for a core network in a wireless communication system is disclosed. The method comprises starting a mobile reachable timer when a mobile device enters an idle mode; accepting the emergency PDN connection; and not stopping the mobile reachable timer or an implicit detach timer when the mobile device enters a connected mode to establish the emergency PDN connection. 
         [0021]    A method of handing an emergency PDN connection when a back-off timer is running for a core network in a wireless communication system is disclosed. The method comprises starting a mobile reachable timer when a mobile device enters an idle mode; accepting the emergency PDN connection; stopping the mobile reachable timer when the mobile device enters a connected mode to establish the emergency PDN connection; and determining whether to reset the mobile reachable timer according to the mobile reachable timer and a periodic area update timer. 
         [0022]    A method of handling an emergency PDN connection when a back-off timer is running for a mobile device in a wireless communication system is disclosed. The method comprises disconnecting the emergency PDN connection when the MM back-off timer is running; and sending a remaining time of the back-off timer in a PDN disconnect request message or a deactivate bearer accept message when a remaining time of the back-off timer is greater than a periodic area update timer when the mobile device disconnects the emergency PDN connection when a remaining time of the back-off timer is larger than a periodic area update timer. 
         [0023]    A method of handing an emergency PDN connection when a back-off timer is running for a core network in a wireless communication system is disclosed. The method comprises setting a mobile reachable timer and an implicit detach timer according to a summation of a periodic area update timer and the implicit detach timer; starting the mobile reachable timer and the implicit timer when the emergency PDN connection is disconnected. 
         [0024]    These and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0025]      FIG. 1  is a schematic diagram of an exemplary wireless communication system. 
           [0026]      FIG. 2  is a schematic diagram of an exemplary communication device. 
           [0027]      FIGS. 3-5  are flowcharts of exemplary processes. 
           [0028]      FIG. 5B  is a flowchart of exemplary process. 
           [0029]      FIGS. 6-9  are flowcharts of exemplary processes. 
       
    
    
     DETAILED DESCRIPTION 
       [0030]    Please refer to  FIG. 1 , which illustrates a block diagram of a wireless communication system according to an embodiment of the invention. In the wireless communication system  100 , the user equipment (UE)  110  is wirelessly connected to the service network  120  for obtaining wireless services. The service network  120  comprises an access network (or called radio access network)  121  and a core network  122 . The service network  120  may be a network in compliance with the Global System for Mobile Communication (GSM), Universal Mobile Telecommunications System (UMTS), or Evolved Packet System (EPS) technology. The access network  121  may be a GSM Edge Radio Access Network (GERAN) in the GSM technology, a UMTS Terrestrial Radio Access Network (UTRAN) in the WCDMA/HSPA technology, or an Evolved-UTRAN (E-UTRAN) in the LTE/LTE-Advanced technology, and the core network  122  may be a GSM core or General Packet Radio Service (GPRS) core in the GSM/UMTS system or an Evolved Packet Core (EPC) in the EPS system. 
         [0031]    In  FIG. 1 , the access network  121  and the UE  110  are simply utilized for illustrating the structure of the wireless communication system  100 . Practically, the access network of the LTE system comprises a plurality of base stations such as an E-UTRAN (evolved-UTAN), which comprises a plurality of evolved Node-Bs (eNBs). In EPS, the core network includes a mobility management entity (MME). The MME is responsible for the delivery of data packets to the mobile devices back and forth within its geographical service area, including packet routing and transfer, mobility management (attach/detach and tracking area management), session management (PDN connection establishment/disconnect), logical link management, and authentication and charging functions. The MME can also serve as a local mobility anchor for inter-working with other RATs (e.g. GSM and UMTS). The access network of the UMTS system comprises a plurality of base stations such as an UTRAN, which comprises a plurality of Node-Bs (NBs). In UMTS, the core network includes a Serving GPRS Support Node (SGSN) is responsible for the delivery of data packets to the mobile devices back and forth within its geographical service area, including packet routing and transfer, mobility management (attach/detach and tracking area management), session management (PDP Context establishment/disconnect), logical link management, and authentication and charging functions. The access network of the GSM system comprises a plurality of base stations such as a GERAN, which comprises a plurality of Base stations (BSSs). In GSM, the core network includes a Mobile Switching Center (MSC) is responsible for the delivery of voice in circuit switch manner to the mobile devices back and forth within its geographical service area, including mobility management (attach/detach and location area management), logical link management, and authentication and charging functions. The UE can be a device such as a mobile phone, a computer system, etc. Besides, the access network  121  and the UE  110  can be seen as a transmitter or receiver according to transmission direction, e.g., for uplink, the UE  110  is the transmitter and the network  121  is the receiver, and for downlink, the network  121  is the transmitter and the UE is the receiver. 
         [0032]    In the following embodiments, the service network  120  may be a GSM/UMTS/EPS network belonging to a specific PLMN, and the UE  110  may be a Mobile Station (MS). The UMTS/EPS network is in compliance with the 3GPP TS 23.060/TS 23.401 specifications, 3GPP TS 24.008/TS 24.301 specifications, and/or other related UMTS/EPS specifications. Based on the technology used, the messages of Mobility Management (MM) operations may correspond to the GSM/UMTS/EPS technology, respectively, e.g. MM message for Location Area Update, GMM message for Routing Area Update, and EPS Mobility Management (EMM) message for Tracking Area Update. Further, the TAU/RAU procedure may be a combined TA/LA or combined RA/LA procedure in which the TAU/RAU REQUEST message indicates that the UE  110  wishes to perform TA/RA and LA update for a Packet-Switched domain and a Circuit-Switched (CS) domain, respectively. 
         [0033]    Please note that this invention can be applied to the UE  110  and the core network  122  when the UE  110  is rejected by not only the TAU, RAU requests but also a service request and a location area update request, such variation are also included in the scope of the present invention. The TAU request corresponds to an EPS system; the RAU request corresponds to Packet Switched domain in a GSM/GPRS system; the location area update corresponds to Circuit Switched domain in a GSM/GPRS system. 
         [0034]    Please refer to  FIG. 2 , which is a schematic diagram of an exemplary communication device  20 . The communication device  20  can be the UE or the core network  122  shown in  FIG. 1 , but is not limited herein. The communication device  20  may include a processor  200  such as a microprocessor or an Application Specific Integrated Circuit (ASIC), a storage unit  210  and a communication interfacing unit  220 . The storage unit  210  may be any data storage device that can store a program code  214 , accessed by the processor  200 . Examples of the storage unit  210  include but are not limited to a subscriber identity module (SIM), read-only memory (ROM), flash memory, random-access memory (RAM), CD-ROM/DVD-ROM, hard disk, and optical data storage device. The communication interfacing unit  220  is preferably a transceiver and can handle timer configuration according to processing results of the processor  200 . 
         [0035]    Periodic tracking/routing area updating (TAU/RAU) is used to periodically notify the availability of the UE to the network. The procedure is controlled in the UE by the periodic tracking/routing area update timer (timer T 3412 /T 3312 ). The value of timer T 3412 /T 3312  is sent by the core network  122  to the UE in the ATTACH ACCEPT message and can be sent in the TRACKING/ROUTING AREA UPDATE ACCEPT message. The UE shall apply this value in all tracking/routing areas of the list of tracking/routing areas assigned to the UE, until a new value is received. The two separate timers (timer T 3412 /T 3312 ) run in the UE for notifying availabilities to MME/SGSN/MSC and updating MME/SGSN independently. When APN (access point node) based congestion or general NAS level mobility management congestion happens, the core network  122  (e.g. MME/SGSN) rejects an attach request, a tracking area update (TAU), a routing area update (RAU) or a service request and the MME replies the UE with a Mobility Management (MM) back-off timer. 
         [0036]    When a normal attached UE (EMM-Registered state) gets rejection with a MM back-off timer due to tracking area update reject or service reject. The UE is normal attached but enters ECM-IDLE mode when the MM back-off timer is running. When the MM back-off timer is running, the back-off UE is still allowed to make emergency call by PDN connectivity request with request type of emergency to an MME. 
         [0037]    Please refer to  FIG. 3 , which is a flowchart of an exemplary process  30 . The process  30  is used for handling an emergency PDN connection for the core network  122  (e.g. MME/SGSN) in the wireless communication system  100 . The core network  122  receives a TAU/RAU request from the UE and sends a TAU/RAU update reject message including a back-off timer value TB of a MM back-off timer when the core network  122  rejects the TAU/RAU request. The process  30  may be compiled into the program code  214  and includes the following steps: 
         [0038]    Step  300 : Start. 
         [0039]    Step  302 : The core network  122  rejects the TAU/RAU request with the back-off timer value TB of the MM back-off timer to a normal attached UE and the core network  122  sets the mobile reachable timer to be greater than TB. The core network  122  starts a mobile reachable timer when the UE enters idle mode, wherein the mobile reachable timer has a timer value M 1 . 
         [0040]    Step  304 : The core network  122  accepts the emergency PDN connection request from the normal attached UE when the MM back-off timer is running. 
         [0041]    Step  306 : The core network  122  stops the mobile reachable timer when the UE enters a connected mode due to established emergency PDN connection. 
         [0042]    Step  308 : The core network  122  resets the mobile reachable timer to a timer value M 2  when the emergency PDN is disconnected and the UE enters an idle mode and if a remaining time of the mobile reachable timer is longer than a periodic tracking/routing area update timer (e.g. T 3412 , T 3412   ext , T 3312 , T 3312  ext) of the UE. 
         [0043]    Step  310 : The core network  122  restarts the mobile reachable timer according to the timer value M 2  when the emergency PDN is disconnected and the UE enters an idle mode. 
         [0044]    Step  312 : End. 
         [0045]    According to the process  30 , the core network  122  (e.g. MME/SGSN) starts the mobile reachable timer with the timer value M 1  when the UE enters idle mode. When the MM back-off timer is running, the core network  122  receives the emergency PDN connection request made by the UE. The core network  122  accepts the request for the emergency PDN connection from the normal attached UE. The core network  122  therefore stops the mobile reachable timer. When the emergency PDN is disconnected and the UE enters idle mode, if the remaining time of the MM back-off timer is longer than the periodic TAU/RAU timer, i.e. the periodic TAU/RAU timer expires before the MM back-off timer, the core network  122  resets the mobile reachable timer to the timer value M 2 . The timer value M 2  is greater than the remaining time of the back-off timer. Preferably, the timer value M 2  is four minutes longer than the remaining time of the back-off timer. Then the core network  122  restarts the mobile reachable timer according to the timer value M 2 . For example, the core network  122  stores the MM back-off timer in the EMM context per UE basis when the network allocates the MM back-off timer to the UE. With this stored value, the core network  122  can obtain the remaining time of the back-off timer when needed. 
         [0046]    Please refer to  FIG. 4 , which is a flowchart of an exemplary process  40 . The process  40  is used for handling an emergency PDN connection for the core network  122  (e.g. MME/SGSN) in the wireless communication system  100 . The core network  122  receives a TAU/RAU request from the UE and sends a TAU/RAU reject message including a back-off timer value TB of a MM back-off timer when the core network  122  rejects the TAU/RAU request. The process  40  may be compiled into the program code  214  and includes the following steps: 
         [0047]    Step  400 : Start. 
         [0048]    Step  402 : The core network  122  rejects the TAU/RAU request with the back-off timer value TB of the MM back-off timer to a normal attached UE and the core network  122  sets the mobile reachable timer to be greater than TB. The core network  122  starts a mobile reachable timer when the UE enters idle mode, wherein the mobile reachable timer has the timer value M 1 . 
         [0049]    Step  404 : The core network  122  accepts the emergency PDN connection request from the normal attached UE when the MM back-off timer is running. 
         [0050]    Step  406 : The core network  122  does not stop the mobile reachable timer when the UE enters connected mode due to established emergency PDN connection. 
         [0051]    Step  408 : End. 
         [0052]    According to the process  40 , the core network  122  (e.g. MME/SGSN) starts the mobile reachable timer with the timer value M 1  when the UE enters idle mode. When the MM back-off timer is running, the core network  122  receives the emergency PDN connection request made by the UE. The core network  122  accepts the request for the emergency PDN connection from the normal attached UE. However, the core network  122  does not stop the mobile reachable timer or implicit timer for the UE when the UE enters connected mode due to established emergency PDN connection. When the mobile reachable timer or implicit detach timer is expired and there exists an emergency PDN connection on the UE, the core network  122  does not detach the UE. On the other hand, the UE does not detach locally when there is MM back-off timer running. 
         [0053]    When the core network  122  allocates the MM back-off timer to the UE, the core network  122  sets the MM back-off indicator as active, which shows the back-off status of the UE, in the EMM context per UE basis. The MM back-off indicator is set inactive when the MM back-off timer or mobile reachable timer is expired or the UE contacts the network. With the back-off indicator, the core network  122  determines whether or not stop mobile reachable timer or implicit detach timer when the UE enters connected mode due to established emergency PDN connection. 
         [0054]    Please refer to  FIG. 5 , which is a flowchart of an exemplary process  50 . The process  50  is used for handling an emergency PDN connection for the core network  122  (e.g. MME/SGSN) in the wireless communication system  100 . The core network  122  receives a TAU/RAU request from the UE and sends a TAU/RAU reject message including a back-off timer value TB of a MM back-off timer when the core network  122  rejects the TAU/RAU request. The process  50  may be compiled into the program code  214  and includes the following steps: 
         [0055]    Step  500 : Start. 
         [0056]    Step  502 : The core network  122  rejects the TAU/RAU request with a back-off timer value TB of the MM back-off timer to the normal attached UE and the core network  122  sets the mobile reachable timer to be greater than TB. The core network  122  starts a mobile reachable timer when the UE enters idle mode, wherein the mobile reachable timer has a timer value M 1 . 
         [0057]    Step  504 : The core network  122  accepts the emergency PDN connection request from the normal attached UE when the MM back-off timer is running. 
         [0058]    Step  506 : The core network  122  stops the mobile reachable timer when the UE enters connected mode due to established emergency PDN connection. 
         [0059]    Step  508 : The core network  122  determines whether to reset the mobile reachable timer according to the mobile reachable timer value M 1  and the periodic TAU/RAU timer value U of the periodic TAU/RAU timer (e.g. T 3412 , T 3412   ext , T 3312 , T 3312  ext). 
         [0060]    Step  510 : End. 
         [0061]    According to the process  50 , the core network  122  (e.g. MME/SGSN) starts the mobile reachable timer with the timer value M 1  when the UE enters idle mode. When the MM back-off timer is running, the core network  122  receives the emergency PDN connection request made by the UE. The core network  122  accepts the request for the emergency PDN connection from the normal attached UE. The core network  122  therefore stops the mobile reachable timer. When the emergency PDN connection is disconnected and the UE enters idle mode, the core network  122  determines whether to reset the mobile reachable timer according to the mobile reachable timer value M 1  and the periodic TAU/RAU timer value U of the periodic TAU/RAU timer. 
         [0062]    For example, the core network  122  has a remaining time T 1  of the mobile reachable timer and an elapsed time caused by the emergency PDN connection denotes as T. When the emergency PDN connection is disconnected and the UE enters idle mode, the core network  122  sets the mobile reachable timer if ((T 1 −4)−T)&gt;0 and ((T 1 −4)−T)&gt;U, i.e. the remaining time of the MM back-off timer is greater than periodic TAU/RAU timer value. Then the core network  122  starts the mobile reachable timer with the value (T 1 −T). If (T 1 −4)=T or (T 1 −4)&lt;T, the core network  122  starts the mobile reachable timer with the initial value of the periodic TAU/RAU timer. In this example, the core network  122  needs to handle T 1  and T based on the back-off status indicator set when it assigns a back-off timer to a UE in the NAS reject message. 
         [0063]    Please refer to  FIG. 5B , which is a flowchart of an exemplary process  50 B. The process  50 B is used for handling an emergency PDN connection for the UE for in the wireless communication system  100 . The core network  122  receives a TAU/RAU request from the UE and sends a TAU/RAU reject message including a back-off timer value TB of a MM back-off timer when the core network  122  rejects the TAU/RAU request. The process  50 B may be compiled into the program code  214  and includes the following steps: 
         [0064]    Step  500 B: Start. 
         [0065]    Step  502 B: The UE receives the MM back-off timer from the core network  122 . 
         [0066]    Step  504 B: The UE disconnects the emergency PDN connection when the MM back-off timer is running. 
         [0067]    Step  506 B: The UE sends a remaining time of the MM back-off timer in a PDN disconnect request message or a deactivate bearer accept message when the UE having the MM back-off timer running disconnects emergency PDN connection if remaining time of the MM back-off timer is larger than periodic TAU/RAU timer. 
         [0068]    Step  508 B: End. 
         [0069]    According to the process  50 B, when the UE having the MM back-off timer running disconnects emergency PDN connection, if the remaining time of the MM back-off timer is larger than periodic TAU/RAU timer, the UE sends the remaining time of the MM back-off timer t in the PDN disconnect request message or Deactivate bearer context accept message. As a result, the network resets mobile reachable timer to a new value which is slightly larger than the received remaining time of the MM back-off timer. 
         [0070]    Please refer to  FIG. 6 , which is a flowchart of an exemplary process  60 . The process  60  is used for handling an emergency PDN connection for the core network  122  (e.g. MME/SGSN) in the wireless communication system  100 . The core network  122  receives a TAU/RAU request from the UE and sends a TAU/RAU update reject message including a back-off timer value TB of a MM back-off timer when the core network  122  rejects the TAU/RAU request. The process  60  may be compiled into the program code  214  and includes the following steps: 
         [0071]    Step  600 : Start. 
         [0072]    Step  602 : The core network  122  rejects the TAU/RAU request with the back-off timer value TB of the MM back-off timer to a normal attached UE. 
         [0073]    Step  604 : The core network  122  accepts the emergency PDN connection request from the normal attached UE when the MM back-off timer is running. 
         [0074]    Step  606 : The core network  122  sets the mobile reachable timer to a timer value M 3  and an implicit detach timer to a timer value N 2  when NAS signaling connection of the UE is release after successful emergency PDN disconnection if the remaining time of the MM back-off timer is larger than a summation of the periodic TAU/RAU timer vale U and an implicit detach timer value N 1 . The core network  122  sets M 3  and/or N 2  to make the summation of the timer values M 3  and/or the timer value N 2  is greater than the remaining time of the back-off timer. 
         [0075]    Step  608 : The core network  122  starts the mobile reachable timer and/or the implicit detach timer according to the timer value M 3  and the timer value N 2  when the emergency PDN connection is disconnected. 
         [0076]    Step  610 : End. 
         [0077]    According to the process  60 , the core network  122  (e.g. MME/SGSN) starts the mobile reachable timer with the timer value M 1  when the UE enters idle mode. When the MM back-off timer is running, the core network  122  receives the emergency PDN connection request made by the UE. The core network  122  accepts the request for the emergency PDN connection from the normal attached UE. The core network  122  therefore stops the mobile reachable timer or implicit detach timer. When the emergency PDN is disconnected and the UE enters idle mode, if a remaining time of the MM back-off timer is larger than the summation of the periodic TAU/RAU timer vale U and the implicit detach timer value N 1 . The summation of the timer values M 3  and/or the timer value N 2  is greater than the remaining time of the back-off timer. The core network  122  starts the mobile reachable timer and/or the implicit detach timer according to the timer value M 3  and the timer value N 2  when the emergency PDN connection is disconnected. For example, the core network  122  stores the MM back-off timer in the EMM context per UE basis when the network allocates the MM back-off timer to the UE. With this stored value, the core network  122  can obtain the remaining time of the back-off timer when needed. 
         [0078]    Please refer to  FIG. 7 , which is a flowchart of an exemplary process  70 . The process  70  is used for handling an emergency PDN connection for the UE for in the wireless communication system  100 . The core network  122  receives a TAU/RAU request from the UE and sends a TAU/RAU reject message including a back-off timer value TB of a MM back-off timer when the core network  122  rejects the TAU/RAU request. The process  70  may be compiled into the program code  214  and includes the following steps: 
         [0079]    Step  700 : Start. 
         [0080]    Step  702 : The UE receives the MM back-off timer from the core network  122 . 
         [0081]    Step  704 : The UE disconnects the emergency PDN connection when the MM back-off timer is running. 
         [0082]    Step  706 : The UE sends a remaining time of the MM back-off timer in a PDN disconnect request message or a deactivate bearer accept message for the emergency PDN connection when the UE having the MM back-off timer running disconnects emergency PDN connection if remaining time of the MM back-off timer is larger than periodic TAU/RAU timer. 
         [0083]    Step  708 : End. 
         [0084]    According to the process  70 , when the UE having the MM back-off timer running disconnects emergency PDN connection, if the remaining time of the MM back-off timer is larger than the periodic TAU/RAU timer, the UE sends the remaining time of the MM back-off timer t in the PDN disconnect request message or Deactivate bearer context accept message. As a result, if the remaining time of the MM back-off timer is larger than the summation of the periodic TAU/RAU timer and the implicit detach timer, the network resets the mobile reachable timer and/or the implicit detach timer to new values, so that the summation of the mobile reachable timer and the implicit detach timer is slightly larger than the remaining time of the MM back-off timer. 
         [0085]    Please refer to  FIG. 8 , which is a flowchart of an exemplary process  80 . The process  80  is used for handling an emergency PDN connection for the core network  122  (e.g. MME/SGSN) in the wireless communication system  100 . The core network  122  receives a TAU/RAU request from the UE and sends a TAU/RAU reject message including a back-off timer value TB of a MM back-off timer when the core network  122  rejects the TAU/RAU request. The process  50  may be compiled into the program code  214  and includes the following steps: 
         [0086]    Step  800 : Start. 
         [0087]    Step  802 : The core network  122  rejects the TAU/RAU request with a back-off timer value TB of the MM back-off timer to the normal attached UE. 
         [0088]    Step  804 : The core network  122  accepts the emergency PDN connection request from the normal attached UE when the MM back-off timer is running. 
         [0089]    Step  806 : The core network  122  stops the mobile reachable timer or implicit detach timer when the UE enters connected mode due to established emergency PDN connection. 
         [0090]    Step  808 : The core network  122  determines whether to reset the mobile reachable timer or implicit detach timer according to the remaining time of the MM back-off timer, implicit detach timer value N 1  of the implicit detach timer and the periodic TAU/RAU timer value U of the periodic tracking/routing area update timer (e.g. T 3412 , T 3412   ext , T 3312 , T 3312  ext). 
         [0091]    Step  810 : End. 
         [0092]    According to the process  80 , the core network  122  (e.g. MME/SGSN) starts the mobile reachable timer with the timer value M 1  when the UE enters idle mode. When the MM back-off timer is running, the core network  122  receives the emergency PDN connection request made by the UE. The core network  122  accepts the request for the emergency PDN connection from the normal attached UE. The core network  122  therefore stops the mobile reachable timer or implicit detach timer. When the emergency PDN connection is disconnected and the UE enters idle mode, the core network  122  determines whether to reset the mobile reachable timer according to the periodic TAU/RAU timer value U of the periodic tracking/routing area update timer and the implicit detach timer value N 1  of the implicit detach timer. 
         [0093]    For example, a remaining time for the summation of the mobile reachable timer and the implicit detach timer is denoted as T 2  and still the elapsed time caused by the emergency PDN connection denotes as T. The core network  122  resets both of the mobile reachable timer and the implicit detach timer when (T 2 −T)&gt;(periodic area update timer value U plus implicit detach timer value N 1 ) so that the summation of the mobile reachable timer and the implicit detach timer is greater than the value (T 2 −T). In this example, the core network  122  needs to handle T 2  and T based on the back-off status indicator set when it assigns a back-off timer to a UE in the NAS reject message. 
         [0094]    The normal attached UE (EMM-Registered state) may have one non-emergency PDN connection and one emergency PDN connection. When the normal attached UE with one non-emergency PDN connection and one emergency PDN connection, the normal attached UE may get rejection with a MM back-off timer due to tracking area update rejector service reject for the non-emergency PDN connection. Therefore, the aforementioned processes  80  can be modified as follows: 
         [0095]    Please refer to  FIG. 9 , which is a flowchart of an exemplary process  90 . The process  90  is used for handling an emergency PDN connection for the core network  122  (e.g. MME/SGSN) and the UE in the wireless communication system  100 . The UE is normal attached and has one emergency PDN connection and one non-emergency PDN connection. The core network  122  receives a TAU/RAU or service request from the UE and sends a TAU/RAU reject message including a back-off timer value of a MM back-off timer when the core network  122  rejects the TAU/RAU or service request to the UE for the non-emergency PDN connection. The process  90  may be compiled into the program code  214  and includes the following steps: 
         [0096]    Step  900 : Start. 
         [0097]    Step  902 : The core network  122  establishes the emergency PDN connection with the normal attached UE. 
         [0098]    Step  904 : The core network  122  rejects TAU/RAU request with a back-off timer value of a MM back-off timer TB for a normal attached UE with the emergency PDN connection and a non-emergency PDN connection. 
         [0099]    Step  906 : The core network  122  determines whether and how to reset the mobile reachable timer according to the mobile reachable timer value M 1  and a summation of the implicit detach timer value N 1  of the implicit detach timer and a TAU/RAU periodic area update timer value U of a periodic tracking/routing area update timer (e.g. T 3412 , T 3412   ext , T 3312 , T 3312  ext). 
         [0100]    Step  908 : The core network  122  starts the mobile reachable timer when the UE enters idle mode due to emergency PDN disconnection. 
         [0101]    Step  910 : End. 
         [0102]    According to the process  80 , the core network  122  receives the emergency PDN connection request made by the normal attached UE. The core network  122  accepts the request and the UE has both of the Emergency PDN connection and the non-emergency PDN connection. If the core network  122  rejects the TAU/RAU or service request with a MM back-off timer to the UE due to NAS level congestion control, the core network  122  stores the value of the MM back-off timer and calculates elapsed time T before the emergency PDN connection is disconnected. The UE starts the MM back-off timer if the UE receives the MM back-off timer. When the emergency PDN connection is disconnected and the UE enters idle mode, the core network  122  resets the mobile reachable timer or implicit detach timer according to the mobile reachable timer value M 1  and the summation of the implicit detach timer value N 1  of the implicit detach timer and a TAU/RAU periodic area update timer value U of a periodic tracking/routing area update timer 
         [0103]    For example, when back-off timer value TB is larger than elapsed time T and the value (TB-T) is larger than the summation of the periodic TAU/RAU timer (e.g. T 3412 ) and the implicit detach timer, the core network  122  resets the mobile reachable timer and/or the implicit detach timer in a way that the summation of the mobile reachable timer and the implicit detach timer is larger than the summation of the remaining time of the MM back-off timer (TB-T). To be specific, the core network  122  can reset the mobile reachable timer and/or implicit detach timer when the duration of back-off timer value TB minus elapsed time T is larger than the summation of periodic TAU/RAU area update timer value U plus the implicit detach timer). 
         [0104]    In addition, the core network  122  can receive the remaining time of the back-off timer in a PDN disconnect request message or a deactivate bearer context accept message sent by a mobile device. As a result, the core network  122  can reset mobile reachable timer and/or implicit detach timer accordingly. 
         [0105]    The abovementioned steps of the processes about mobile reachability management can be adapted to apply to the UE and MSC in the GSM network. Those skilled in the art can readily make necessary modifications and alterations to the circuit switch system to achieve this adaptation while retaining the teachings of this invention. 
         [0106]    Please note that, the abovementioned steps of the processes including suggested steps can be realized by means that could be a hardware, a firmware known as a combination of a hardware device and computer instructions and data that reside as read-only software on the hardware device, or an electronic system. Examples of hardware can include analog, digital and mixed circuits known as microcircuit, microchip, or silicon chip. Examples of the electronic system can include a system on chip (SOC), system in package (SiP), a computer on module (COM), and the communication device  20 . 
         [0107]    To sum up, a normal attached (EMM-Registered state) UE gets rejection with a MM back-off timer due to tracking area update reject or service reject. The UE enters ECM-IDLE mode when the MM back-off timer is running. When MM back-off timer is running, the UE is still allowed to make emergency call by PDN connection request with request type of emergency to an emergency APN. When the NAS signaling connection is built due to emergency PDN connection request and the UE returns from ECM-IDLE mode to ECM-Connected mode, the core network (e.g. MME/SGSN) stops the mobile reachable timer. The core network resets the mobile reachable timer to a second timer value when a remaining time of the back-off timer is longer than a remaining time of a periodic area update timer. After the UE disconnects emergency PDN connection and NAS signaling connection is released, the mobile reachable timer is reset and started with a new timer value. In addition, the back-off UE establishes an emergency PDN connection while the MM back-off timer is running. When the back-off UE with non-emergency PDN connection and emergency PDN connection in connected mode moves to anew tracking area, before handover procedure the back-off UE requests TAU when it enters a new tracking area that is not in the TA list. The back-off UE sends a remaining time of the MM back-off timer/a back-off indicator in the TAU request/complete message. The target MME/SGSN can decide how to handle the UE with MM back-off timer running according to the TAU request/complete message. 
         [0108]    Those skilled in the art will readily observe that numerous modifications and alterations of the device and method may be made while retaining the teachings of the invention. Accordingly, the above disclosure should be construed as limited only by the metes and bounds of the appended claims.