Abstract:
A user equipment (UE) implementing a message recovery method is disclosed, supporting messages of both the circuit switched (CS) domain and the packet switched (PS) domain. The UE roams in a wireless telecommunications system and performs a call setup procedure. When the UE receives an NAS message comprising a domain identifier and a protocol discriminator, the UE determines whether the domain identifier and the protocol discriminator are of the same domain. If the domain identifier and the protocol discriminator are of the same domain, the UE proceeds with processes corresponding to the NAS message. If the domain identifier and the protocol discriminator are of different domains, the NAS message is determined to be an error message, and the UE modifies the domain identifier of the NAS message to generate a recovered NAS message, and determines whether the recovered NAS message is compatible to the call setup procedure.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    A mobile telecommunications systems is disclosed, and in particular, to a message recovery method for a mobile device to increase call success rate. 
         [0003]    2. Description of the Related Art 
         [0004]    In a Universal Mobile Telecommunications System (UMTS) network, separation between the access stratum (AS) and the non-access stratum (NAS) is well defined by the Third Generation Partnership Project (3GPP) standard.  FIG. 1  shows a message structure according to the 3GPP standard, presenting a layered structure as defined by the open system interconnection (OSI) model. For brevity, only relevant fields are illustrated in the message structure. A non-access stratum (NAS) message  110  is embedded in the body of the access stratum (AS) message  100 , comprising certain header fields such as a core network (CN) domain identifier  112  and a protocol discriminator  114 , and a message body that may further be embedded with an Internet Protocol (IP) packet  120  or other types of payloads. For example, the IP packet  120  is typically used to contain a Transmission Control Protocol (TCP) packet  130  embedded with the Hypertext Transport Protocol (HTTP) packet  140 . 
         [0005]    According to the UMTS standard, the NAS message  110  is passed between a User Equipment (UE) and the core network (not shown) through Radio Resource Control (RRC) initial/uplink/down direct transfers. The CN domain identifier  112  in the NAS message  110  is designated to provide specific information for CN domain identification. Based on the CN domain identifier  112 , the UE is able to route the NAS message  110  to a proper message handler to handle the NAS message  110 . Based on 3GPP specification, the domain contains at least two types, the packet switched (PS) domain, and the circuit switched (CS) domain. An NAS message  110  of the PS domain is typically used to present internet data packets, in which the protocol discriminator  114  typically contain values of GMM(0x08), SM(0x0a), GTTP(0x04) or SMS(0x09). Meanwhile, an NAS message  110  of the CS domain is generally phone call related data packets, and the protocol discriminator  114  are filled with one of the values MM(0x05), CC(0x03), SS(0x0b) and SMS(0x09). 
         [0006]    Traffics for the PS domain and the CN domain are usually simultaneously processed if the UE supports both domains. For example, a mobile phone may be capable of surfing the internet concurrently while making a phone call. The interactions between the UE and the core network generally involve network entities such as Mobile Switching Centers (MSCs), Serving GPRS Support Nodes (SGSNs), Radio Network Controllers (RNCs) and Base Stations (BSs). Thus, sometimes the NAS message  110  may contain a CN domain identifier  112  with wrong domain values due to undeterminable reasons. The UE relies upon the CN domain identifier  112  to determine how to handle an NAS message  110  when receiving the NAS message  110 . If the UE receives an NAS message  110  containing a CN domain identifier  112  of invalid value, the UE may ignore the NAS message  110 . When such an error happens, a connection between the UE and the core network may fail or be disconnected, thereby degrading the service quality. 
         [0007]    Therefore, a more robust message control mechanism is desirable. 
       BRIEF SUMMARY OF THE INVENTION 
       [0008]    A user equipment (UE) is provided, supporting messages of both the circuit switched (CS) domain and the packet switched (PS) domain. The UE roams in the wireless telecommunications system and performs a call setup procedure. When the UE receives an NAS message comprising a domain identifier and a protocol discriminator, the UE determines whether the domain identifier and the protocol discriminator are of the same domain. If the domain identifier and the protocol discriminator are of the same domain, the UE proceeds with processes corresponding to the NAS message. If the domain identifier and the protocol discriminator indicate different domains, the NAS message is determined to be an error message, and the UE modifies the domain identifier of the NAS message to generate a recovered NAS message, and determines whether the recovered NAS message is compatible to the call setup procedure. If the recovered NAS message is compatible to the call setup procedure, the UE proceeds with the call setup procedure using the recovered NAS message. If the recovered NAS message is incompatible to the call setup procedure, the UE returns an error message to the source of the NAS message. 
         [0009]    A message recovery method used in a user equipment is provided, wherein the user equipment supports messages of both the circuit switched (CS) domain and the packet switched (PS) domain in a wireless telecommunications system. In the message recovery method, a Non-Access stratum (NAS) message comprising a domain identifier and a protocol discriminator while performing a call setup procedure is received, and whether the domain identifier and the protocol discriminator are of the same domain is determined. If the domain identifier and the protocol discriminator are of the same domain, the user equipment proceeds with processes corresponding to the NAS message, and if the domain identifier and the protocol discriminator indicate different domains, user equipment assesses the NAS message as an error message, and modifies the domain identifier of the NAS message to generate a recovered NAS message. 
         [0010]    A user equipment is provided, supporting messages of both a circuit switched (CS) domain and a packet switched (PS) domain in a wireless telecommunications system. In the user equipment, a first means receives a Non-Access stratum (NAS) message comprising a domain identifier and a protocol discriminator while performing a call setup procedure, and a second means determines whether the domain identifier and the protocol discriminator are of the same domain. A third means proceeds with processes corresponding to the NAS message if the domain identifier and the protocol discriminator are of the same domain, and a fourth means assesses the NAS message as an error message, and modifying the domain identifier of the NAS message to generate a recovered NAS message, if the domain identifier and the protocol discriminator indicate different domains. 
         [0011]    Message recovery methods may take the form of a program code embodied in a tangible media. When the program code is loaded into and executed by a machine, the machine becomes an apparatus for practicing the disclosed method. Message recovery methods may also take any form of hardware circuit. 
         [0012]    A message recovery method and a telecommunications system for the UE are also provided, and detailed descriptions are given with reference to the accompanying drawings in the following. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]    The disclosure can be more fully understood by reading the subsequent detailed description and examples with references made to the accompanying drawings, wherein: 
           [0014]      FIG. 1  shows a message structure according to the 3GPP standard; 
           [0015]      FIG. 2  shows an overall structure of a Universal Mobile Telecommunications System (UMTS). 
           [0016]      FIG. 3  shows a protocol diagram of an SRNS relocation procedure; 
           [0017]      FIG. 4  shows a protocol diagram of a call setup procedure when the SRNS relocation procedure is triggered; and 
           [0018]      FIG. 5  is a flowchart of an exemplary message recovery method. 
       
    
    
     DETAILED DESCRIPTION 
       [0019]      FIG. 2  shows an overall structure of a Universal Mobile Telecommunications System (UMTS). The UMTS is typically a distributed system that can provide services to a wide range of geometric areas. A user equipment (UE)  250  is interconnected to the core network  210  through the UMTS Terrestrial Radio Access Network (UTRAN)  220 . The core network  210  typically comprises one or more Message Switching Centers (MSCs)  212  and Serving GPRS Support Nodes (SGSNs)  214  distributed in different areas. The MSC  212  handles all call related traffics. The SGSN  214 , according to the UMTS standard, keeps track of the location of the UE  250  and performs security functions and access control. In the UTRAN  220 , a plurality of Radio Network Subsystems (RNSs) each provide transmission/reception for a group of radio cells distributed in various locations. For example, a first RNS  230  comprises a first RNC  232  and a plurality of BS nodes  234 , providing radio access services covering a first area, and a second RNS  240  comprising a second RNC  242  and a plurality of BS nodes  244  provides radio access services covering a second area. In the figure, for brevity of description, the UE  250  is presumed to be in the first area, using the first RNS  230  as an SRNS for proceeding with a call setup procedure. 
         [0020]    In  FIG. 2 , dialogs related to a call setup procedure and an SRNS relocation procedure are shown. Assume the UE  250  is moving from the coverage of the first RNS  230  to that of the second RNS  240 , an SRNS relocation procedure will be performed to hand over the SRNS from the first RNS  230  to the second RNS  240 . The related dialogs are labeled as  310  and  320 . The dialogs  310  and  320  are further introduced in  FIG. 3 . Meanwhile, the UE  250  may perform a call setup procedure while moving between the coverage areas. The call setup related dialogs are labeled as  410  and  420 , and detailed steps are described in  FIG. 4 . 
         [0021]      FIG. 3  shows a protocol diagram of an SRNS relocation procedure. When the UE  250  moves away from the coverage of first RNS  230 , the SNRS relocation procedure is triggered, for example by RNC  232 . Steps  310   a  and  310   b  show the dialog  310  as discussed in  FIG. 2 . In step  310   a , the first RNC  232  sends a UTRAN Mobility Information request to the UE  250 . In step  310   b , the UE  250  responds to the first RNC  232  with a UTRAN Mobility Information Confirm. Thereafter, the SRNS is handed over to the second RNS  240 , and further dialogs are processed with the second RNC  242 . Steps  320   a ,  320   b  and  320   c  show the dialog  320  as discussed in  FIG. 2 , which are parts of the SRNS relocation procedure. If the routing area is changed (connect to different SGSN), an Intra-SGSN Routing Area Update procedure is performed. In step  320   a , the UE  250  sends a Routing Area Update (RAU) request to the SGSN  214  via the second RNC  242 . The RAU request is typically an NAS message  110  with its CN domain identifier  112  set to “PS-domain”, hence it can be correctly routed to the SGSN  214 . In step  320   b , the SGSN  214  responds with a Routing Area Update Accept signal to the UE  250  via the second RNC  242 . In step  320   c , the UE  250  further sends a Routing Area Update Complete signal to the SGSN  214  to conclude the SRNS relocation procedure. 
         [0022]      FIG. 4  shows a protocol diagram of a call setup procedure when the SRNS relocation procedure is triggered. In the figure, a call setup procedure is performed while the SRNS procedure is ongoing. Steps  410   a  to  410   d  show the dialog  410  as discussed in  FIG. 2 . In step  410   a , the first RNC  232  initializes the call setup procedure by sending a mobile terminal (MT) paging signal to the UE  250 . Thereafter, in step  410   b , a series of handshakes between the UE  250  and MSC  212  via the first RNC  232 , such as a security mode procedure and a radio bearer (RB) setup procedure are performed. Upon completion of the handshakes, in step  410   c , a user picks up the phone, and consequently, the UE  250  initializes a call control (CC) connect signal to the MSC  212  as illustrated in step  410   d . In the figure, the CC connect signal is an NAS message  110  with a CN domain identifier  112  and a protocol discriminator  114 . For example, the value in the CN domain identifier  112  is “cs-domain”, and the value in the protocol discriminator  114  is “0x83c7”, a value consistent to the CS domain. Since the call setup procedure is performed while the UE  250  is moving, the SRNS relocation procedure may be performed simultaneously. For example, the dialog  310  as discussed in  FIG. 2  and  FIG. 3  is commenced right after step  410   d . Consequently, the successive dialogs are performed via the second RNC  242 . For example, in response to the CC connect signal delivered in step  410   d , the MSC  212  may respond with a CC connect acknowledge signal to the UE  250  via the second RNC  242  as shown in step  420   a . Thereby, completing the call setup procedure, wherein the UE  250  establishes a call session as shown in step  420   b.    
         [0023]    In step  420   a , the CC connect acknowledge signal is an NAS message  110 , comprising a CN domain identifier  112  of value “cs-domain”, and a protocol discriminator  114  of value “0x030F”. However, sometimes the CN domain identifier  112  of the CC connect acknowledge signal may be misplaced by a value “PS-domain”. Consequently, by examining the CN domain identifier  112 , the CC connect acknowledge signal is determined to be an invalid packet and dropped by the UE  250 . If the CC connect acknowledge signal is dropped in step  420   a , the call setup procedure will fail. 
         [0024]    The disclosure proposes an error handling mechanism to prevent such potential errors, and failures. During the call setup process, while SRNS relocation is ongoing, if the core network sends an NAS message of the wrong CN domain, a recovery mechanism is implemented in the UE  250  to check if the message can be recovered to fit a current state of the UE  250 . For example, if the NAS message is compatible to the current state of UE by replacing the wrong CN domain with a different one, the UE  250  would be able to proceed with the call setup procedure using the modified NAS message. Yet, if the modification of the wrong CN domain fails to generate a useful NAS message  110  compatible to the current states of the ongoing procedures, the UE  250  may implement an error notification mechanism that sends RRC Status to the core network with an error cause such as “NAS message not compatible with receiver state”. Thus, the core network may determine the error and perform appropriate recovery processes instead of simply waiting for a response until a time has run out. 
         [0025]      FIG. 5  is a flowchart of an exemplary message recovery method. The aforementioned descriptions can be summarized into the following steps. In step  501 , the telecommunications system as described in  FIG. 2  is initialized. In step  503 , a call setup procedure is initialized by the first RNC  232  sending a request to the UE  250 . In step  505 , the UE  250  sends a Call Control (CC) connect request to the MSC  212  upon determination of a call in the first area. In step  507 , the SRNS relocation procedure is triggered as the UE  250  reaches the coverage boundaries between the first RNS  230  and second RNS  240 . In step  509 , the SRNS is relocated to the second RNS  240  to serve the UE  250  in a second area, and the MSC  212  sends a CC connect acknowledge signal to the UE  250  via the second RNC  242  in response to the CC connect request signal shown in step  505 . In step  511 , the UE  250  determines whether a domain mismatch occurs between the domain identifier and the protocol discriminator of the CC connect acknowledge signal. If no error is determined, step  521  is processed, wherein a call session can be successfully established. Conversely, if the domain identifier and the protocol discriminator of the CC connect acknowledge signal indicate different domains, the UE  250  may perform a recovery procedure in steps  513 . For example, the domain identifier may have a problematic value “PS-domain”, and the UE  250  may modify it to “CS-domain” to generate a recovered NAS message. Thereafter, in step  515 , the UE  250  would determine whether the recovered NAS message is compatible to the call setup procedure. For example, according to step  420   a  in  FIG. 4 , the call setup procedure is at the step where a CC connect acknowledge signal of the CS-domain is expected. In this case, the recovered NAS message meets the expectation, thus step  517  is processed, wherein the UE  250  adapts the recovered NAS message as the CC connect acknowledge signal and proceeds to step  521 . However, if the recovered NAS message cannot be applied to any ongoing procedure in the UE  250 , step  519  is processed, wherein the UE  250  returns an error notification to the source of the NAS message, such as the MSC  212  in the core network  210 . 
         [0026]    The recovery method can be applied to any NAS message, wherein the core network abnormally placed a wrong value in the CN domain identifier  112 . The disclosure shows that the UE  250  is able to successfully complete the call setup procedure even if the NAS message is placed with a wrong CN domain value, thereby improving call success rate while SRNS relocation is ongoing. 
         [0027]    While various embodiments of the present invention have been described herein, it should be understood that they have been presented by way of example, and not limitation. It will be apparent to persons skilled in the relevant computer arts that various changes in form and detail can be made therein without departing from the scope of the invention. For example, software can enable, for example, the function, fabrication, modeling, simulation, description and/or testing of the apparatus and methods described herein. This can be accomplished through the use of general programming languages (e.g., C, C++), hardware description languages (HDL) including Verilog HDL, VHDL, and so on, or other available programs. Such software can be disposed in any known computer usable medium such as semiconductor, magnetic disk, or optical disc (e.g., CD-ROM, DVD-ROM, etc.). Embodiments of the apparatus and method described herein may be included in a semiconductor intellectual property core, such as a microprocessor core (e.g., embodied in HDL) and transformed to hardware in the production of integrated circuits. Additionally, the apparatus and methods described herein may be embodied as a combination of hardware and software. Thus, the present invention should not be limited by any of the herein-described exemplary embodiments, but should be defined only in accordance with the following claims and their equivalents. Specifically, the present invention may be implemented within a microprocessor device which may be used in a general purpose computer. 
         [0028]    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 and without departing from the spirit and scope. Accordingly, the above disclosure should be construed as limited only by the metes and bounds of the appended claims. Therefore, the scope of the appended claims should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements.