Patent Publication Number: US-2016234089-A1

Title: Wireless communication system

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is based upon and claims the benefit of priority of the prior Japanese Patent Application No. 2015-021213, filed on Feb. 5, 2015, the entire contents of which are incorporated herein by reference. 
     FIELD 
     The embodiments discussed herein are related to a wireless communication system. 
     BACKGROUND 
     In recent years, with cost reduction for communication devices, the technical development about a machine to machine (M2M) system has been promoted that implements a service by devices connected to a network communicating data with each other without human intervention. As a network used for the M2M system, a wireless network of long term evolution (LTE) or the like, and a wired network of Ethernet (registered trademark) or the like are used. 
     A wireless network is often more unstable than a wired network. Therefore, if a wireless network is used as a network for the M2M system, it is preferable that when an abnormal condition occurs in the wireless communications of terminals (hereinafter, M2M devices) used in the M2M system, the condition is detected promptly. As a method of detecting an abnormality in communication with an M2M device, there is a method of periodically transmitting a packet to check connection from a device that communicates with an M2M device to the M2M device, and confirming a response thereto, for example. As a protocol for checking connection, for example, an Internet control message protocol (ICMP) and the like are available. The ICMP operates above Internet protocol layers. Prior art examples are disclosed in Japanese National Publication of International Patent Application No. 2013-511916, International Publication Pamphlet No. 2011/135964, Japanese Laid-open Patent Publication No. 2013-118556, and Japanese National Publication of International Patent Application No. 2014-502082. 
     In a wireless network of LTE and the like, a base station releases a bearer established in a wireless section to a terminal when a predetermined period of time has passed without user data communication with the terminal. When user data communication with the terminal takes place again, the base station establishes a bearer between itself and the terminal to perform communication of the user data. Thus, it is possible to avoid a radio resource from being occupied by a specific terminal, and to use the radio resource effectively. 
     However, if a packet to check connection is periodically transmitted to an M2M device to check connection in a wireless section with the M2M device by ICMP or the like, when the transmission interval of the packets is short, the bearer between the M2M device and the base station is not released. Thus, the radio resource is occupied by a specific M2M device, and the use efficiency of the radio resource is reduced. 
     Moreover, when the transmission interval of the packets to check connection is long, a bearer between the M2M device and the base station is released until user data communication occurs, or until the packet to check connection is transmitted, and therefore, the radio resource is used effectively. However, processing of establishing a bearer in the wireless section between the M2M device and the base station is to be performed each time the packet to check connection is transmitted. When a lot of M2M devices communicate through one base station, just the processing to establish a bearer between each M2M device and itself to transmit a packet to check connection causes a heavy processing load on the base station. 
     SUMMARY 
     According to an aspect of an embodiment, a wireless communication system includes a terminal and a base station that performs wireless communication with the terminal. The terminal includes a first transmitting unit. The first transmitting unit transmits, after a bearer between the terminal and the base station is released, a check signal to the base station every first period by wireless communication using a frequency that is used in the bearer. The base station includes a first determining unit and a first notifying unit. The first determining unit determines whether there is an abnormality in wireless connection to the terminal based on whether the check signal is received from the terminal within a second period after the bearer is released. The first notifying unit notifies, when the first determining unit determines that there is an abnormality in the wireless connection to the terminal, the abnormality to a predetermined notification destination. 
     The object and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the claims. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are not restrictive of the invention, as claimed. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  depicts one example of a wireless communication system; 
         FIG. 2  is a block diagram depicting one example of an M2M device; 
         FIG. 3  depicts one example of terminal setting information that is stored in a setting-information holding unit of the M2M device; 
         FIG. 4  is a block diagram depicting one example of a base station; 
         FIG. 5  depicts one example of base-station setting information that is stored in a setting-information holding unit of the base station; 
         FIG. 6  is a block diagram depicting one example of a Mobility Management Entity (MME); 
         FIG. 7  depicts one example of setting information that is acquired from a Home Subscriber Server (HSS); 
         FIG. 8  depicts one example of setting information that is stored in a setting-information holding unit of the MME; 
         FIG. 9  depicts one example of hardware of the M2M device; 
         FIG. 10  depicts one example of hardware of the base station; 
         FIG. 11  depicts one example of hardware of the MME; 
         FIG. 12  is a sequence diagram depicting one example of operation of the wireless communication system in attach processing; 
         FIG. 13  is a sequence diagram depicting one example of operation of the wireless communication system at the time of updating setting information; 
         FIG. 14  is a sequence diagram depicting one example of operation of the wireless communication system in a Data-Radio-Bearer (DRB) unconnected state; 
         FIG. 15  is a sequence diagram depicting one example of operation of the M2M device when an abnormality in wireless connection is detected; 
         FIG. 16  is a sequence diagram depicting one example of operation of the base station when an abnormality in wireless connection is detected; 
         FIG. 17  is a sequence diagram depicting one example of operation of the wireless communication system when the M2M device selects a different sector; 
         FIG. 18  is a sequence diagram depicting one example of operation of the base station when recovery of the wireless connection is detected; 
         FIG. 19  is a flowchart indicating one example of operation of the M2M device; 
         FIG. 20  is a flowchart indicating one example of operation of the M2M device; 
         FIG. 21  is a flowchart indicating one example of operation of the base station; and 
         FIG. 22  is a flowchart indicating one example of operation of the base station. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Preferred embodiments of the present invention will be explained with reference to accompanying drawings. Note that the disclosed technique is not limited to the following embodiments. Respective embodiments can be combined appropriately in a range not making processing inconsistent. 
     Wireless Communication System 
       FIG. 1  depicts one example of a wireless communication system  10 . The wireless communication system  10  includes, for example, multiple M2M devices  20 - 1  to  20 - n , multiple base stations  30 - 1  to  30 - n , and a core network  11 , as depicted in a  FIG. 1 . In the following, when the M2M devices  20 - 1  to  20 - n  are collectively referred without distinguishing each, it is described as M2M device  20 . Moreover, when the base stations  30 - 1  to  30 - n  are referred collectively without distinguishing each, it is described as base station  30 . The core network  11  is connected to an M2M server  90  through an Internet protocol (IP) network  80 . Each of the M2M device  20  is connected to the M2M server  90  through the base station  30  and the core network  11 . The wireless communication system  10  is constructed, for example, using a wireless network of LTE and the like. 
     The respective M2M device  20  and the respective base station  30  are connected by a wireless link. When the M2M device  20  communicates with the M2M server  90 , a part of radio resources that can be provided by the base station  30  is reserved for the M2M device, thereby enabling communication between the M2M device  20  and the M2M server  90  through the base station. 
     The M2M device  20  is one example of a terminal that performs wireless communication with the base station  30 . The M2M device  20  is, for example, a power usage meter having a wireless communication function, a vending machine having wireless communication function, and the like. The M2M device  20  acquires information about a power usage amount and the like based on information received from the M2M server  90  through the base station  30 , and transmits the acquired information to the M2M server  90  through base station  30  and the core network  11 . Moreover, the M2M server  90  controls the respective M2M devices  20  by transmitting control information to the respective M2M devices  20  through the core network  11  and the base station  30 . 
     The core network  11  includes an HSS  40 , an MME  50 , a serving gateway (SGW)  60 , and a packet data network gateway (PGW)  70 . The MME  50  is one example of a management unit. 
     The HSS  40  manages subscriber information (for example, terminal information, verification information, and the like) in the wireless communication system  10 , setting information described later, and the like. The MME  50  holds more than one base station  30 , and performs mobility management of terminals, such as the M2M devices  20 . The SGW  60  performs processing of packet transfer in the core network  11 , and the like. The PGW  70  performs processing of packet transfer between the core network  11  and an external network, and the like. 
     M2M Device 
       FIG. 2  is a block diagram depicting one example of the M2M device  20 . The M2M device  20  includes, for example, a setting-information holding unit  21 , a call control unit  22 , a determining unit  23 , a check-signal transmitting unit  24 , a wireless communication unit  25 , and an antenna  26 , as depicted in  FIG. 2 . 
     The wireless communication unit  25  performs processing such as encoding and modulation on a signal input from the call control unit  22  and the check-signal transmitting unit  24 , upconverts the signal subjected to the processing to a predetermined frequency, to transmit to the base station  30  through the antenna  26 . Moreover, the wireless communication unit  25  downconverts a signal received from the base station  30  through the antenna  26 , and performs processing such as demodulation and decoding, to output the signal subjected to the processing to the call control unit  22  and the check-signal transmitting unit  24 . 
     The call control unit  22  performs attach processing between itself and the base station  30  by selecting a sector of the base station  30  based on notification information transmitted from the sector of the base station  30 , and by transmitting and receiving a predetermined signal to and from the selected sector. Furthermore, the call control unit  22  receives terminal setting information from the base station  30  through a DRB that is established by the attach processing in a wireless section to the base station  30 , and stores the received terminal-setting information in the setting-information holding unit  21 . 
     The terminal setting information stored in the setting-information holding unit  21  includes information that is used for the processing (hereinafter, connection check processing) to check connection in the wireless section between the M2M device  20  and the base station  30  after the DRB between the M2M device  20  and the base station  30  is released.  FIG. 3  depicts one example of the terminal setting information that is stored in the setting-information holding unit  21  of the M2M device  20 . The terminal setting information stored in the setting-information holding unit  21  includes, for example, an execution flag  210 , a first timer duration  211 , a first threshold  212 , and a sector identifier  213 , as indicated in  FIG. 3 . 
     The execution flag  210  is information that controls whether to execute the connection check processing. When the execution flag  210  is set to “ON”, the connection check processing is performed, and when the execution flag  210  is set to “OFF”, the connection check processing is not performed. The first timer duration  211  indicates duration of a first timer that measures a cycle of transmitting a signal to check connection to the base station  30 . In the present embodiment, the duration of the first timer is, for example, about several hundred milliseconds. 
     The first threshold  212  indicates a threshold to determine whether there is an abnormality in wireless connection. When the number of times the connection check processing has not finished normally is equal to or more than the first threshold, the call control unit  22  performs sector selecting processing to search for a different sector. In the present embodiment, the first threshold is, for example, five. The sector identifier  213  is information to identify a sector of the base station  30  to be a destination of a signal to check wireless connection in the connection check processing. 
     Explanation is continued, referring back to  FIG. 2 . The call control unit  22  releases the DRB when user data communication has not occurred for a predetermined time since the DRB is established between the M2M device  20  and the base station  30 , and informs release of the DRB to the determining unit  23 . Moreover, when user data communication occurs between the M2M device  20  and the base station  30  after the DRB is released, the call control unit  22  performs processing to establish the DRB again between itself and the base station  30  by transmitting and receiving predetermined signals to and from the base station  30 . When the DRB is established, the call control unit  22  informs establishment of the DRB to the determining unit  23 . 
     Moreover, the call control unit  22  reselects a sector based on the quality of a signal transmitted from a sector of base station  30  when reselection of a sector is instructed by the determining unit  23 . The call control unit  22  performs, when a different sector having higher signal quality than the sector of a subject of the connection check processing is selected, the processing to establish a DRB between itself and the base station  30  through the different sector. The determining unit  23  stores terminal setting information that is received from the base station  30  through the different sector in the setting-information holding unit  21 . The call control unit  22  notifies the determining unit  23  of establishment of the DRB. On the other hand, when a different sector is not selected, the call control unit  22  notifies the determining unit  23  that a different sector is not selected. 
     The check-signal transmitting unit  24  reads a sector identifier from the terminal setting information in the setting-information holding unit  21  when transmission of a check signal is instructed by the determining unit  23 . The check-signal transmitting unit  24  transmits a check signal to check wireless connection with the base station  30  to the sector identified by the read sector identifier by wireless communications using a frequency used by the DRB between itself and the base station  30 . In the present embodiment, the check-signal transmitting unit  24  uses a signal that is used in a random access procedure in LTE as the check signal. Note that as long as it is a signal that enables to check wireless connection with the base station  30 , the check-signal transmitting unit  24  may use signals other than the signal that is used in the random access procedure in LTE as the check signal. 
     For example, the check-signal transmitting unit  24  transmits RA Preamble to the base station  30  that has the sector corresponding to the sector identifier set in the terminal setting information. When RA Response is received from base station  30  as a response to RA Preamble, the check-signal transmitting unit  24  transmits Scheduled Transmission that includes a terminal identifier to identify the M2M device  20 , and a connection-check identifier indicating that it is a connection check signal, to the base station  30 . RA Response is one example of the first signal. Scheduled Transmission including the terminal identifier and the connection-check identifier is one example of the check signal. 
     When Contention Resolution is received from the base station  30 , the check-signal transmitting unit  24  informs the determining unit  23  that a response signal is received, if the terminal identifier and the connection-check identifier included in Scheduled Transmission are included in received Contention Resolution. Contention Resolution including the terminal identifier and the connection-check identifier is one example of the response signal. 
     When informed of release of the DRB from the call control unit  22 , the determining unit  23  reads the terminal setting information in the setting-information holding unit  21 . Furthermore, when the execution flag is set to “ON” that indicates execution of the connection check processing, the determining unit  23  resets and starts the first timer, and instructs the check-signal transmitting unit  24  to transmit a check signal. 
     The determining unit  23  determines whether there is an abnormality in wireless connection between itself and the base station  30  based on whether a response signal is received from the base station  30  before the first timer times out. A period from start until timeout of the first timer from reset-start of the first timer is one example of a first period. 
     For example, the determining unit  23  resets the number of errors, which indicates the number of times that the connection check processing is not normally finished with the base station  30 , to 0 when informed that a response signal is received from the check-signal transmitting unit  24  before the first timer times out. The determining unit  23  then resets and starts the first timer. 
     On the other hand, the determining unit  23  increments the number of errors when not informed that a response signal is received from the check-signal transmitting unit  24  until the first timer times out. When the number of errors is equal to or more than the first threshold included in the terminal setting information, the determining unit  23  determines that there is an abnormality in wireless connection to the base station  30 . The determining unit  23  then instructs the call control unit  22  to reselect a sector. 
     Base Station 
       FIG. 4  is a block diagram depicting one example of the base station  30 . The base station  30  includes, for example, an antenna  31 , a wireless communication unit  32 , a call control unit  33 , a determining unit  34 , a notifying unit  35 , a setting-information holding unit  36 , and a wired communication unit  37 , as depicted in  FIG. 4 . 
     The wireless communication unit  32  performs processing such as encoding and modulation on a signal that is input from the call control unit  33 , and upconverts the signal subjected to the processing to a predetermined frequency, to transmit to the M2M device  20  through the antenna  31  of a sector. Furthermore, the wireless communication unit  32  downconverts a signal received from the M2M device  20  through the antenna  31  of the sector, and performs processing such as demodulation and decoding, to output the signal subjected to the processing to the call control unit  33 . 
     The wired communication unit  37  is connected to the core network  11 . The wired communication unit  37  transmits data input from the call control unit  33  and the notifying unit  35  to the core network  11 . Moreover, the wired communication unit  37  outputs data received from the core network  11  to the call control unit  33 . 
     The call control unit  33  transmits notification information per sector through the wireless communication unit  32 . Furthermore, the call control unit  33  performs attach processing by transmitting and receiving a predetermined signal among the M2M device  20 , the HSS  40 , the MME  50 , the SGW  60 , and the PGW 70 . Thus, a DRB is established in a wireless section between the base station  30  and the M2M device  20 , and an S 1  bearer is established in a wired section among the base station  30 , the MME  50 , and the SGW  60 . 
     The call control unit  33  receives setting information from the MME  50  through the S 1  bearer. The setting information includes terminal setting information and base-station setting information. The call control unit  33  stores the base-station setting information included in the setting information in the setting-information holding unit  36 . Moreover, the call control unit  33  adds an identifier of the sector that is used to establish the DRB to the terminal setting information included in the setting information. The call control unit  33  then transmits the terminal setting information to which the identifier of the sector is added to the M2M device  20  through the DRB. The call control unit  33  then informs the notifying unit  35  of establishment of a bearer together with a terminal identifier of the M2M device  20  for which the bear is established. 
     The base-station setting information stored in the setting-information holding unit  36  includes information that is used for the connection check processing performed after the DRB between the M2M device  20  and the base station  30  is released.  FIG. 5  depicts one example of the base-station setting information that is stored in the setting-information holding unit  36  of the base station  30 . The setting-information holding unit  36  holds, for example, setting information  361  associating with a terminal identifier  360  to identify each of the M2M devices  20  as depicted in  FIG. 5 . The setting information  361  includes an execution flag  362 , a second timer duration  363 , a second threshold  364 , a third timer duration  365 , a third threshold  366 , and a notification destination  367 . 
     The execution flag  362  is information to control whether to execute the connection check processing. The second timer duration  363  indicates duration of a second timer that measures a cycle of determining whether there is an abnormality in wireless connection in the connection check processing at the base station  30 . The duration of the second timer is longer than that of the first timer and, for example, about several ten seconds to several minutes. The second threshold  364  indicates a threshold to determine whether there is an abnormality in wireless connection in the base station  30 . When the number of times the connection check processing has not finished normally is equal to or more than the second threshold, the notifying unit  35  transmits a connection state notification that indicates that there is an abnormality in the wireless connection with the M2M device  20 , to a device that is specified by information indicated in the notification destination  367 . In the present embodiment, the second threshold is, for example, five. 
     The third timer duration  365  indicates duration of a third timer that measures a cycle of determining whether there is a recovery of connection in the connection check processing at the base station  30 . In the present embodiment, the duration of the third timer is, for example, about several seconds. The third threshold  366  indicates a threshold to determine whether there is a recovery of connection. When the number of times that the connection check processing has been finished normally is equal to or more than the third threshold, the notifying unit  35  transmits a connection state notification indicating that the wireless connection to the M2M device  20  has been recovered, to a device that is specified by information indicated in the notification destination  367 . In the present embodiment, the third threshold is, for example, 10. The notification destination  367  is information to identify a device to be a notification destination of the connection state notification, and is, for example, an IP address, or the like. 
     Explanation is continued referring back to  FIG. 4 . The call control unit  33  releases the S 1  bearer and the DRB when user data communication has not occurred for a predetermined time since the S 1  bearer and the DRB are established. The call control unit  33  then informs the determining unit  34  of the release of the bearers together with the terminal identifier of the M2M device  20  for which the bearers have been released. Subsequently, when receiving a check signal from the M2M device  20  for which the S 1  bearer and the DRB have been released, the call control unit  33  informs the determining unit  34  of reception of the check signal. 
     For example, after the S 1  bearer and the DRB are released, the call control unit  33  transmits, when RA Preamble is received from the M2M device  20 , RA Response to the M2M device  20  as a response to RA Preamble. Subsequently, when Scheduled Transmission is received from the M2M device, the call control unit  33  determines whether a connection check identifier indicating that it is a connection check signal is included in Scheduled Transmission. When a connection check identifier is included in Scheduled Transmission, the call control unit  33  informs the determining unit  34  of reception of the check signal. 
     Subsequently, the call control unit  33  acquires the terminal identifier and the connection check identifier from received Scheduled Transmission, and creates Contention Resolution holding the acquired terminal identifier and connection check identifier. The call control unit  33  transmits the created Contention Resolution to the M2M device  20 . 
     Moreover, when user data communication occurs between the base station  30  and the M2M device  20  after the S 1  bearer and the DRB are released, the call control unit  33  performs processing to establish the S 1  bearer and the DRB. When the S 1  bearer and the DRB are established, the call control unit  33  informs the determining unit  34  and the notifying unit  35  of establishment of the bearers together with the terminal identifier of the M2M device  20  for which the bearers have been established. 
     Furthermore, when a deletion instruction including terminal identifier of the M2M device  20  is received from the MME  50 , the call control unit  33  deletes base-station setting information that is associated with the terminal identifier included in the deletion instruction from the setting-information holding unit  36 . 
     When informed of the release of the bearer from by the call control unit  33 , the determining unit  34  reads the base-station setting information that is associated with the terminal identifier of the M2M device  20  for which the bearer has been released from the setting-information holding unit  36 . When the execution flag is set to “ON” indicating execution of the connection check processing, the determining unit  34  resets and starts the second timer having the duration set in the base-station setting information. 
     The determining unit  34  starts determination whether there is an abnormality in wireless link with the M2M device  20  based on whether a check signal is received from the M2M device  20  before the second timer times out. A period from reset and start until timeout of the second timer is one example of a second period. 
     For example, the determining unit  34  resets the number of errors, which indicates the number of times that the connection check processing is not normally finished, to 0 when informed that a check signal is received from the call control unit  33  before the second timer times out, and resets and starts the second timer. On the other hand, the determining unit  34  increments the number of errors when not informed that a check signal is received from the call control unit  33  before the second timer times out, and resets and starts the second timer. 
     When the number of errors is equal to or more than the second threshold included in the base-station setting information, the determining unit  34  determines that there is an abnormality in wireless connection between itself and the base station  30 , and informs the notifying unit  35  of abnormal connection together with the terminal identifier of the M2M device  20 . 
     Subsequently, the determining unit  34  starts determination whether the wireless connection of the wireless link with the M2M device  20  has been recovered based on whether a check signal is received from the M2M device  20  before the third timer times out. 
     For example, when not informed that a check signal is received from the call control unit  33  before the third timer times out, the determining unit  34  resets the number of successes, which indicates the number of times that the connection check processing is normally finished, to 0, and resets and starts the third timer again. On the other hand, when informed that a check signal is received from the call control unit  33  before the third timer times out, the determining unit  34  increments the number of successes, and resets and starts the third timer again. 
     When the number of successes is equal to or more than the third threshold included in the base-station setting information, the determining unit  34  determines that the wireless link to the base station  30  has been recovered from the abnormal connection, and informs the notifying unit  35  of the connection recovery together with the terminal identifier of the M2M device  20 . 
     When informed of the establishment of a bearer by the call control unit  33  together with the terminal identifier, the notifying unit  35  reads information that is associated with the terminal identifier from the setting-information holding unit  36 , and acquires information of a notification destination from the read base-station setting information. The notifying unit  35  then transmits a connection state notification that indicates that the bearer has been established, to a device that is identified by the acquired information of the notification destination through the wired communication unit  37 . 
     Moreover, when informed of the abnormal connection by the determining unit  34  together with the terminal identifier, the notifying unit  35  reads information that is associated with the terminal identifier from the setting-information holding unit  36 , and acquires information of a notification destination from the read base-station setting information. The notifying unit  35  then transmits a connection state notification that indicates the abnormal connection, to a device that is identified by the acquired information of the notification destination through the wired communication unit  37 . 
     Furthermore, when informed of connection recovery by the determining unit  34  together with the terminal identifier, the notifying unit  35  reads base-station setting information that is associated with the terminal identifier from the setting-information holding unit  36 , and acquires information of a notification destination from the read base-station setting information. The notifying unit  35  then transmits a connection state notification that indicates the connection recovery, to a device that is identified by the acquired information of the notification destination through the wired communication unit  37 . 
     MME 
       FIG. 6  is a block diagram depicting one example of the MME  50 . The MME  50  includes, for example, a setting-information holding unit  51 , a call control unit  52 , a setting-information notifying unit  53 , and a wired communication unit  54 , as depicted in  FIG. 6 . 
     The wired communication unit  54  is connected to the base station  30 , the HSS  40 , and the SGW  60 . The wired communication unit  54  transmits data input by the call control unit  52  and the setting-information notifying unit  53  to the base station  30 , the HSS  40 , and the SGW  60 . Moreover, the wired communication unit  54  outputs data received from the base station  30 , the HSS  40 , and the SGW  60  to the call control unit  52  and the setting-information notifying unit  53 . 
     The call control unit  52  performs processing such as attach, bearer establishment, and mobility management of terminals. Furthermore, when attach or bearer establishment processing is completed, the call control unit  52  informs the terminal identifier of the M2M device  20  for which the attach or bearer establishment processing has been performed, and the base station identifier of the base station  30 , and of completion of the attach or bearer establishment processing, to the setting-information notifying unit  53 . 
     When informed of completion of the attach processing by the call control unit  52 , the setting-information notifying unit  53  acquires setting information of the M2M device  20  for which the attach processing has been performed from the HSS  40 .  FIG. 7  depicts one example of setting information that is acquired from the HSS  40 . The setting information acquired from the HSS  40  includes, for example, an execution flag  400 , a first timer duration  401 , a first threshold  402 , a second timer duration  403 , a second threshold  404 , a third timer duration  405 , a third threshold  406 , and a notification destination  407 , as indicated in  FIG. 7 . The setting information from the execution flag  400  to the notification destination  407  indicated in  FIG. 7  are set in a database included in the HSS  40  by a terminal of an administrator of the HSS  40 , the M2M server  90 , or the like. 
     The execution flag  400  is information that controls whether to execute the connection check processing. The first timer duration  401  indicates duration of a first timer that is used in the connection check processing at the M2M device  20 . The first threshold  402  is a threshold that is used in the connection check processing at the M2M device  20 . The execution flag  400 , the first timer duration  401 , and the first threshold  402  are one example of the terminal setting information. 
     The second timer duration  403  is duration of a second timer that is used in the connection check processing at the base station  30 . The second threshold  404  is a threshold that is used in the connection check processing at the base station  30 . The third timer duration  405  is duration of a third timer that is used in the connection check processing at the base station. The third threshold is a threshold that is used in the connection check processing at the base station  30 . The notification destination  407  is information to identify a device to be a notification destination of a connection state notification. The execution flag  400 , the second timer duration  403 , the second threshold  404 , the third timer duration  405 , the third threshold  406 , and the notification destination  407  are one example of the base-station setting information. 
     Explanation is continued referring back to  FIG. 6 . The setting-information notifying unit  53  transmits the setting information acquired from the HSS  40  together with the terminal identifier of the M2M device  20  for which the attach processing has been performed, to the base station  30  for which the attach processing has been performed. The setting-information notifying unit  53  then adds the base-station identifier of the base station  30  for which the attach processing has been performed to the setting information acquired from the HSS  40 , as the base-station identifier of the base station  30  for which the connection check processing is to be performed. Subsequently, the setting-information notifying unit  53  stores the setting information to which the base-station identifier is added in the setting-information holding unit  51 , associating with the terminal identifier of the M2M device  20  for which the attach processing has been performed. 
       FIG. 8  depicts one example of setting information that is stored in the setting-information holding unit  51  of the MME  50 . The setting-information holding unit  51  holds setting information  511  associating with terminal identifiers  510  to identify the respective M2M devices  20 , for example, as depicted in  FIG. 8 . The setting information  511  includes an execution flag  512 , a first timer duration  513 , a first threshold  514 , a second timer duration  515  a second threshold  516 , a third timer duration  517 , a third threshold  518 , a notification destination  519 , and a base station identifier  520 . 
     In the setting information  511 , information from the execution flag  512  to the notification destination  519  are the same as the information of the execution flag  400  to the notification destination  407  in the setting information acquired from the HSS  40 , and therefore, explanation thereof is omitted. The base station identifier  520  is information to identify the base station  30  for which the M2M device  20  performs the connection check processing. 
     Explanation is continued referring back to  FIG. 6 . When informed of completion of bearer establishment by the call control unit  52 , the setting-information notifying unit  53  identifies setting information that is associated with the terminal identifier of the M2M device  20  informed by the call control unit  52  together with completion of bearer establishment, in the setting-information holding unit  51 . Subsequently, the setting-information notifying unit  53  determines whether the base station identifier of the base station  30  informed by the call control unit  52  together with completion of bearer establishment is included in the identified setting information. 
     When the base station identifier of the base station  30  informed by the call control unit  52  is not included in the setting information, the setting-information notifying unit  53  transmits a deletion instruction together with notification of completion of bearer establishment, to the base station  30  of the base station identifier in the setting information. In the deletion instruction, the terminal identifier of the M2M device  20  informed by the call control unit  52  is included. 
     The setting-information notifying unit  53  acquires setting information of the M2M device  20  that has performed the bearer establishment processing from the HSS  40 . The setting-information notifying unit  53  then transmits the setting information acquired from the HSS  40  to the base station  30  that has performed bearer establishment processing together with the terminal identifier of the M2M device  20  that has performed the bearer establishment processing. Subsequently, the setting-information notifying unit  53  adds the base station identifier of the base station  30  that has performed the bearer establishment processing to the setting information acquired from the HSS  40  as the base station identifier of the base station  30  that is to perform the connection check processing. The setting-information notifying unit  53  then stores the setting information to which the base station identifier of the base station  30  is added in the setting-information holding unit  51  associating with the terminal identifier of the M2M device that has performed the bearer establishment processing. 
     Hardware 
     Subsequently, hardware of the M2M device  20 , the base station  30 , and the MME  50  is explained.  FIG. 9  depicts one example of hardware of the M2M device  20 . The M2M device  20  includes a memory  200 , a processor  201 , a wireless communication module  202 , and the antenna  26 , for example, as depicted in  FIG. 9 . The call control unit  22 , the determining unit  23 , and the check-signal transmitting unit  24  are implemented by, for example, the processor  201  such as a central processing unit (CPU) and a digital signal processor (DSP). The wireless communication unit  25  is implemented by, for example, the processor  201  and the wireless communication module  202 . The setting-information holding unit  21  is implemented by, for example, the memory  200  such as a random access memory (RAM), a read only memory (ROM), and a flash memory. 
       FIG. 10  depicts one example of hardware of the base station  30 . The base station  30  includes the antenna  31 , a wireless communication module  301 , a memory  302 , a processor  303 , and a network interface unit  304 , for example, as depicted in  FIG. 10 . The call control unit  33 , the determining unit  34 , and the notifying unit  35  are implemented by, for example, the processor  303  such as a CPU and a DSP. The wireless communication unit  32  is implemented by, for example, the processor  303  and the wireless communication module  301 . The setting-information holding unit  36  is implemented by, for example, the memory  302  such as a RAM, a ROM, and a flash memory. The wired communication unit  37  is implemented by, for example the processor  303  and the network interface unit  304 . 
       FIG. 11  depicts one example of hardware of the MME  50 . The MME  50  includes a memory  500 , a processor  501 , and a network interface unit  502 , for example, as depicted in  FIG. 11 . The call control unit  52  and the setting-information notifying unit  53  are implemented by, for example the processor  501  such as a CPU and a DSP. The setting-information holding unit  51  is implemented by the memory  500  such as a RAM, a ROM, and a flash memory. The wired communication unit  54  is implemented by, for example, the processor  501  and the network interface unit  502 . 
     Operation of Wireless Communication System 
     Next, operation of the wireless communication system  10  is explained.  FIG. 12  is a sequence diagram depicting one example of operation of the wireless communication system  10  in attach processing. In the following sequence diagram, a case of a single unit of the M2M device  20  is explained. The sequence diagram starts in a state in which the DRB between the M2M device  20  and the base station  30 , the S 1  bearer among the base station  30 , the MME  50 , and the SGW  60 , and an S 5  bearer among the SGW  60 , the HSS  40 , and the PGW  70  are unconnected. 
     First, the call control unit  33  of the base station  30  transmits a notification information per sector (S 100 ). The call control unit  22  of the M2M device  20  selects a sector based on the quality of a signal of the notification information transmitted from the base station  30  (S 101 ). The call control unit  22  of the M2M device  20  and the call control unit  33  of the base station  30  execute an initial access procedure, according to LTE random access procedure (S 102 ). In the initial access procedure, the call control unit  22  of the M2M device  20  transmits RA Preamble to the selected sector. The call control unit  33  of the base station  30  transmits RA Response to the M2M device  20  as a response to RA Preamble. The call control unit  22  of the M2M device  20  transmits Scheduled Transmission including a radio-resource-control (RRC) connection-request message to the base station  30 . The call control unit  33  of the base station  30  transmits Contention Resolution including an RRC connection-setup message to the M2M device  20 . 
     Subsequently, the call control unit  22  of the M2M device  20  transmits an attach request to the MME  50  through the base station  30  (S 103 ). The attach processing is then performed among the M2M device  20 , the base station  30 , the MME  50 , the SGW  60 , the HSS  40 , and the PGW  70  (S 104 ). In the attach processing, processing such as verification, concealment, integrality control, position registration, and bearer establishment is performed. The call control unit  52  of the MME  50  transmits an attach acceptance to the M2M device  20  through the base station  30  (S 105 ). The call control unit  22  of the M2M device  20  transmits attach completion to the MME  50  through the base station  30  (S 106 ). Thus, the DRB is established between the M2M device  20  and the base station  30 , the S 1  bearer is established among the base station  30 , the MME  50 , and the SGW  60 , and the S 5  bearer is established between the SGW  60  and the PGW  70 . 
     Subsequently, the setting-information notifying unit  53  of the MME  50  acquires setting information of the M2M device  20  from the HSS  40  (S 107 ). The setting-information notifying unit  53  adds the base station identifier of the base station  30  that has performed the attach processing to the setting information acquired from the HSS  40 , and stores the setting information in the setting-information holding unit  51  associating with the terminal identifier of the M2M device  20  that has performed the attach processing (S 108 ). The setting-information notifying unit  53  transmits the setting information acquired from the HSS  40  to the base station  30  that has performed the attach processing, together with the terminal identifier of the M2M device  20  that has performed the attach processing (S 109 ). 
     The call control unit  33  of the base station  30  stores the base-station setting information that is included in the setting information received from the MME  50 , in the setting-information holding unit  36  (S 110 ). The call control unit  33  then adds the identifier of the sector that has been used to establish the DRB to the terminal setting information included in the setting information received from the MME  50 . The call control unit  33  transmits the terminal setting information to which the identifier of the sector is added, to the M2M device  20  through the DRB (S 111 ). 
     The call control unit  22  of the M2M device  20  stores the terminal setting information received from the base station  30  in the setting-information holding unit  21  (S 112 ). The call control unit  22  transmits a response indicating completion of storage of the terminal setting information to the base station  30  (S 113 ). The call control unit  33  of the base station  30  transmits a response indicating completion of storage of the base-station setting information to the MME  50  (S 114 ). The call control unit  33  then informs the notifying unit  35  of establishment of the bearer together with the terminal identifier of the M2M device  20 . The notifying unit  35  acquires information of a notification destination that is associated with the terminal identifier of the M2M device  20  from the setting-information holding unit  36 , and transmits a connection state notification indicating the establishment of the bearer to the device that is identified by the acquired information (the M2M server  90  in the present embodiment) (S 115 ). 
     When user data communication has not occurred for a predetermined period between the M2M device and the base station  30 , the DRB between the M2M device  20  and the base station  30  and the S 1  bearer among the base station  30 , the MME  50 , and the SGW  60  are released. Thus, the DRB and the S 1  bearer are brought into an unconnected state. Note that the S 5  bearer is remained connected. 
     Update of Setting Information 
     Next, operation of the wireless communication system  10  when setting information is updated is explained.  FIG. 13  is a sequence diagram depicting one example of operation of the wireless communication system  10  at the time of updating setting information. In the present sequence diagram, processing starts in a state in which the attach processing is completed, the DRB and the S 1  bearer are unconnected, and the S 5  bearer is connected. 
     First, the HSS  40  detects a change in the setting information held therein (S 120 ). The HSS  40  then transmits a change notification including the terminal identifier of the M2M device  20  for which the setting information has been changed (S 121 ). The setting-information notifying unit  53  of the MME  50  establishes a bearer between itself and the M2M device that is identified by the terminal identifier included in the change notification. 
     Specifically, the MME  50  performs paging for the M2M device  20  through the base station  30  (S 122 ). The M2M device  20  and the base station  30  then perform the initial access procedure explained in  FIG. 12  (S 102 ). The call control unit  22  of the M2M device  20  transmits a service request to the MME  50  through the base station  30  (S 123 ). The bearer establish processing is performed among the M2M device  20 , the base station  30 , the MME  50 , the SGW  60 , and the HSS  40  (S 124 ). In the bearer establishment processing, processing such as verification, concealment, and integrity control is also performed. Thus, the DRB is established between the M2M device  20  and the base station  30 , and the S 1  bearer is established among the base station  30 , the MME  50 , and the SGW  60 . 
     The M2M device  20 , the base station  30 , and the MME  50  perform processing from steps S 107  to S 115  explained in  FIG. 12 . When user data communication has not occurred for a predetermined period between the M2M device  20  and the base station  30 , the DRB and the S 1  bearer are released again. 
     Connection Check Processing 
     Next, operation of the wireless communication system  10  after the DRB between the M2M device  20  and the base station  30  is released is explained.  FIG. 14  is a sequence diagram depicting one example of operation of the wireless communication system  10  in a DRB unconnected state. In the present sequence diagram, processing starts in a state in in which the attach processing is completed, the DRB and the S 1  bearer are unconnected, and the S 5  bearer is connected. 
     First, the call control unit  33  of the base station  30  informs the determining unit  34  of release of the bearers. The determining unit  34  resets the number of errors to 0 (S 130 ), and resets and start the second timer (S 131 ). The call control unit  22  of the M2M device  20  informs the determining unit  23  of release of the bearers. The determining unit  23  resets the number of errors to 0 (S 132 ), and resets and starts the first timer (S 133 ). Subsequently, the determining unit  23  instructs the check-signal transmitting unit  24  to transmit a check signal. 
     The check-signal transmitting unit  24  transmits RA Preamble to the base station  30  (S 134 ). The call control unit  33  of the base station  30  transmits RA Response to the M2M device  20  as a response to RA Preamble (S 135 ). 
     Subsequently, the check-signal transmitting unit  24  of the M2M device  20  transmits Scheduled Transmission including the terminal identifier of the M2M device  20  and the connection check identifier to the base station  30  (S 136 ). When receiving Scheduled Transmission including the terminal identifier and the connection check identifier, the call control unit  33  of the base station  30  informs the determining unit  34  of reception of the check signal. The determining unit  34  resets the number of errors to 0 (S 137 ) when informed of reception of the check signal by the call control unit  33  before the second timer times out, and resets and starts the second timer again (S 138 ). 
     Scheduled Transmission including the terminal identifier and the connection check identifier is transmitted from the M2M device  20  as a response to RA Response transmitted from the base station  30 . By receiving Scheduled Transmission including the terminal identifier and the connection check identifier, the call control unit  33  can determine that communication is enabled in both a wireless link from the M2M device  20  to the base station  30  and a wireless link from the base station  30  to the M2M device  20 . 
     The call control unit  33  acquires the terminal identifier and the connection check identifier from received Scheduled Transmission. The call control unit  33  then transmits Contention Resolution that includes the terminal identifier and the connection check identifier acquired from Scheduled Transmission, to the M2M device  20  (S 139 ). 
     When receiving Contention Resolution including the terminal identifier and the connection check identifier identical to the terminal identifier and the connection check identifier included in the Schedule Transmission, the check-signal transmitting unit  24  of the M2M device  20  informs the determining unit  23  of reception of a response signal. The determining unit  23  resets the number of errors to 0 (S 140 ) when informed of reception of a response signal by the check-signal transmitting unit  24  before the first timer times out. When timeout of the first timer is detected (S 141 ), the determining unit  23  resets and starts the first timer again (S 133 ), and instructs the check-signal transmitting unit  24  to transmit a check signal. Thereafter, the processing at step S 134  and after is repeated. 
     Reception of Contention Resolution that includes the terminal identifier and the connection check identifier identical to the terminal identifier and the connection check identifier included in transmitted Scheduled Transmission means that Scheduled Transmission has been received by the base station  30  successfully. Therefore, based on reception of Contention Resolution that includes the terminal identifier and the connection check identifier identical to the terminal identifier and the connection check identifier included in Scheduled Transmission, the determining unit  23  can determine that the wireless link between the M2M device  20  and the base station  30  is normal. 
     Operation of M2M Device when Abnormality in Wireless Connection is Detected 
     Next, operation of the wireless communication system  10  when an abnormality in the wireless connection between the M2M device  20  and the base station  30  is detected is explained.  FIG. 15  is a sequence diagram depicting one example of operation of the M2M device  20  when an abnormality in wireless connection is detected. In the present sequence diagram, processing starts in a state in in which the attach processing is completed, the DRB and the S 1  bearer are unconnected, and the S 5  bearer is connected. 
     First, the call control unit  33  of the base station  30  informs the determining unit  34  of release of the bearers. The determining unit  34  resets the number of errors to 0 (S 130 ), and resets and starts the second timer (S 131 ). The call control unit  22  of the M2M device  20  informs the determining unit  23  of the release of the bearers. The determining unit  23  resets the number of errors to 0 (S 132 ), and resets and starts the first timer (S 133 ). The determining unit  23  then instructs the check-signal transmitting unit  24  to transmit a check signal. The check-signal transmitting unit  24  transmits RA Preamble to the base station  30  (S 134 ). 
     If a shield or an interference wave is present in a wireless section between the M2M device  20  and the base station  30 , it becomes difficult to connect the wireless link between the M2M device  20  and the base station  30 , and RA Preamble transmitted from the M2M device  20  is not received normally. Therefore, a response signal such as Contention Resolution is not transmitted from the base station  30 . Moreover, even if RA Preamble is normally received at the base station  30 , if there is an abnormality in the wireless link of the downlink from the base station  30  to the M2M device  20 , a response signal such as Contention Resolution transmitted from the base station  30  is not received by the call control unit  22 . Therefore, the determining unit  23  can determine whether communication is enabled in both the wireless link from the M2M device  20  to the base station  30  and the wireless link from the base station  30  to the M2M device  20 , by determine whether a response signal is received. 
     When detecting timeout of the first timer without being informed of reception of a response signal from the check-signal transmitting unit  24  (S 145 ), the determining unit  23  increments the number of errors, for example, by 1 (S 146 ). When the number of errors is less than the first threshold, the determining unit  23  resets and starts the first timer again (S 147 ), and instructs the check-signal transmitting unit  24  to transmit a check signal. The check-signal transmitting unit  24  transmits RA Preamble to the base station  30  again (S 148 ). 
     RA Preamble transmitted from the M2M device  20  does not reach the base station  30  due to an abnormality in the wireless section between the M2M device  20  and the base station  30 . Therefore, the determining unit  23  is not to be informed of reception of a response signal by the check-signal transmitting unit  24 , and detects timeout of the first timer again (S 149 ), and increments the number of errors, for example, by 1 (S 150 ). When detecting that the number of errors is equal to or more than N 1 , which is the first threshold (S 151 ), the determining unit  23  instructs selection of a sector to the call control unit  22  (S 152 ). 
     In the example depicted in  FIG. 15 , it is assumed that a different sector having a better signal quality than a cell being the transmission destination of RA Preamble at step S 134  is not detected. Because a different sector is not detected, the call control unit  22  selects the same sector (S 153 ). The call control unit  22  then informs the determining unit  23  that a different sector has not been selected. The determining unit  23  resets the number of errors to 0 (S 132 ), and the memory  200  repeats the processing at S 133  and after. 
     Operation of Base Station when Abnormality is Detected in Wireless Connection 
       FIG. 16  is a sequence diagram depicting one example of operation of the base station  30  when an abnormality in wireless connection is detected. In the present sequence diagram, processing starts in a state in which the attach processing is completed, the DRB and the S 1  bearer are unconnected, and the S 5  bearer is connected. 
     First, the call control unit  33  of the base station  30  informs the determining unit  34  of release of the bearers. The determining unit  34  resets the number of errors to 0 (S 160 ), and resets and starts the second timer (S 161 ). When there is an abnormality in the wireless link between the M2M device  20  and the base station  30 , a signal such as RA Preamble from the M2M device  20  is not normally received at the base station  30 . Therefore, the determining unit  34  detects timeout of the second timer without being informed of reception of a check signal by the call control unit  33  (S 162 ). The determining unit  34  increments the number of errors, for example, by 1 (S 163 ). When the number of errors is less than the second threshold, the determining unit  34  resets and starts the second timer again (S 164 ). 
     When timeout of the second timer is detected without being informed of reception of a check signal by the call control unit  33  (S 165 ), the determining unit  34  increments the number of errors, for example, by 1 (S 166 ). When the number of errors is equal to or more than N 2  that is the second threshold (S 167 ), the determining unit  34  informs the notifying unit  35  of abnormal connection together with the terminal identifier of the M2M device  20  being a subject of the connection check processing. The notifying unit  35  refers to the setting-information holding unit  36 , and identifies a notification destination of a connection state notification (S 168 ). Subsequently, the notifying unit  35  transmits the connection state notification that includes the terminal identifier and information indicating the abnormal connection, to the M2M server  90 , which is the identified notification destination (S 169 ). 
     Thus, the M2M server  90  can acquire information about the M2M device  20  for which an abnormality has occurred in the wireless link to the base station  30  promptly. The base station  30  can send the information about the M2M device  20  for which the abnormality has occurred in the wireless link to a predetermined notification destination promptly without performing processing to establish a DRB between itself and the M2M device  20 . Thus, it is possible to avoid performing the processing to establish the DRB to detect the M2M device  20  for which an abnormality has occurred in the wireless link, thereby reducing a processing load on the base station  30 . 
     Operation at Changing Sectors 
     Next, operation of the M2M device  20  when a different sector is selected as a result of sector selection performed by the M2M device  20  is explained.  FIG. 17  is a sequence diagram depicting one example of operation of the wireless communication system  10  when the M2M device  20  selects a different sector. In the present sequence diagram, processing starts in a state in which the attach processing is completed, the DRB and the S 1  bearer are unconnected, and the S 5  bearer is connected. Moreover, a base station  30 - 2  signifies the base station  30  having a different sector selected by the M2M device, and a base station  30 - 1  signifies the base station  30  having the sector that had been selected by the M2M device before the different sector is selected. 
     First, when the call control unit  22  of the M2M device  20  selects a different sector (S 170 ), the initial access procedure explained using  FIG. 12  is performed between the M2M device  20  and the base station  30 - 2  having the selected different sector (S 102 ). The call control unit  22  of the M2M device  20  then transmits a service request to the MME  50  through the base station  30 - 2  having the different sector (S 171 ). The bearer establishment processing is then performed among the M2M device  20 , the base station  30 - 2 , the MME  50 , the SGW  60 , and the HSS  40  (S 172 ). In the bearer establishment processing at step S 172 , processing such as verification, concealment, and integrity control is also performed. Thus, the DRB is established between the M2M device  20  and the base station  30 - 2 , and the S 1  bearer is established among the base station  30 - 2 , the MME  50 , and the SGW  60 . 
     Subsequently, the call control unit  52  of the MME  50  informs the terminal identifier of the M2M device  20  for which the bearer has been established and the base station identifier of the base station  30  to the setting-information notifying unit  53 . The setting-information notifying unit  53  refers to setting information that is associated with the terminal identifier of the M2M device  20  for which the bearer has been established, and determines whether the base station identifier included in the setting information is identical to the base station identifier informed by the setting-information notifying unit  53 . In the example depicted in  FIG. 17 , because the base station identifier of the base station  30 - 1  and the base station identifier of the base station  30 - 2  are different, the setting-information notifying unit  53  determines that the base station identifier included in the setting information and the base station identifier informed by the setting-information notifying unit  53  are different. 
     The setting-information notifying unit  53  then transmits a deletion instruction that includes the terminal identifier of the M2M device  20  informed by the call control unit  52 , to the base station  30 - 1  that is identified by the base station identifier included in the setting information (S 173 ). The call control unit  33  of the base station  30 - 1  deletes base-station setting information that is associated with the terminal identifier included in the deletion instruction that is received from the MME  50  from the setting-information holding unit  36  (S 174 ). Subsequently, the call control unit  33  transmits a response indicating completion of deletion of the base-station setting information to the MME  50  (S 175 ). The M2M device  20 , the base station  30 , and the MME  50  perform the processing from step S 107  to step S 115  explained in  FIG. 12 . When user data communication has not occurred for a predetermined time between the M2M device  20  and the base station  30 , the DRB and the S 1  bearer are released again. 
     Operation at Recovery of Wireless Connection 
     Next, operation of the wireless communication system  10  when recovery of wireless connection between the M2M device  20  and the base station  30  is detected is explained.  FIG. 18  is a sequence diagram depicting one example of operation of the base station  30  when recovery of the wireless connection is detected. Processing in the present sequence diagram is performed, for example, following the processing depicted in  FIG. 16 . 
     First, the determining unit  34  of the base station  30  resets the number of successes, which indicates the number of times that the connection check processing is normally finished (S 180 ), and resets and starts the third timer (S 181 ). When not informed of reception of a check signal by the call control unit  33  before the third timer times out (S 182 ), the determining unit  34  resets the number of successes to 0 (S 183 ), and resets and starts the third timer again (S 184 ). 
     Furthermore, when RA Preamble is received from the M2M device  20  (S 185 ), the call control unit  33  transmits RA Response to the M2M device  20  (S 186 ). When Scheduled Transmission that includes the terminal identifier of the M2M device  20  and the connection check identifier is received from the M2M device  20  (S 187 ), the call control unit  33  informs the determining unit  34  of reception of a check signal. When the check signal is received from the call control unit  33  before the third timer times out, the determining unit  34  increments the number of successes, for example, by 1 (S 188 ). The determining unit  34  then resets and starts the third timer again (S 189 ). 
     The call control unit  33  acquires the terminal identifier and the connection check identifier from received Scheduled Transmission. The call control unit  33  then transmits Contention Resolution that includes the terminal identifier and the connection check identifier acquired from Scheduled Transmission to the M2M device  20  (S 190 ). 
     When, as a result of incrementing the number of successes (S 191 ), the number of successes becomes equal to or more than N 3  that is the third threshold (S 192 ), the determining unit  34  informs the notifying unit  35  of connection recovery together with the terminal identifier of the M2M device  20  that is the subject of the connection check processing. The call control unit  33  acquires the terminal identifier and the connection check identifier from received Scheduled Transmission, and transmits Contention Resolution that includes the acquired terminal identifier and connection check identifier to the M2M device  20  (S 193 ). 
     The notifying unit  35  refers to the setting-information holding unit  36 , and identifies the notification destination of the connection state notification (S 194 ). Subsequently, the notifying unit  35  transmits the connection state notification that includes the terminal identifier and information indicating the connection recovery, to the M2M server  90  of the identified notification destination (S 195 ). 
     Thus, the M2M server  90  can acquire information about the M2M device  20  for which wireless connection is recovered from an abnormality that has occurred in the wireless link to the base station  30 , promptly. The base station  30  can inform information about the M2M device  20  for which the wireless link has been recovered from the abnormality to a predetermined notification destination promptly, without performing the processing to establish the DRB between itself and the M2M device  20 . Thus, it is possible to avoid performing the processing to establish the DRB to detect the M2M device  20  for which a wireless link has been recovered from an abnormality, thereby reducing a processing load on the base station  30 . 
     Operation of M2M Device 
       FIG. 19  and  FIG. 20  are flowcharts indicating one example of operation of the M2M device  20 . For example, upon power-up, the M2M device  20  starts the operation indicated in the present flowcharts. 
     First, the call control unit  22  selects a sector based on a notification information transmitted from a sector of the base station  30 . The call control unit  22  then performs the attach processing between itself and MME  50  through the base station  30  having the selected sector (S 200 ). The call control unit  22  receives terminal setting information from the base station  30  through the DRB established between itself and the base station  30 , and stores the received terminal setting information in the setting-information holding unit  21  (S 201 ). 
     Subsequently, the call control unit  22  determines whether the DRB is released (S 202 ). For example, when user data communication with the base station  30  has not occurred for a predetermined period, the call control unit  22  releases the DRB. When the DRB is released (S 202 : YES), the call control unit  22  informs the determining unit  23  of release of the DRB. The determining unit  23  refers to the terminal setting information in the setting-information holding unit  21 , and determines whether the execution flag is set to “ON”. 
     When the execution flag is set to “ON”, the determining unit  23  resets the number of errors to 0 (S 203 ). The determining unit  23  sets a normal finish flag that is set to “ON” when the connection check processing executed between itself and the base station  30  is normally finished to “OFF” (S 204 ). Subsequently, the determining unit  23  refers to the terminal setting information in the setting-information holding unit  21  to acquire the duration of the first timer, and resets and starts the first timer of the acquired duration (S 205 ). 
     The determining unit  23  then instructs the check-signal transmitting unit  24  to transmit a check signal (S 206 ). The check-signal transmitting unit  24  reads a sector identifier from the terminal setting information in the setting-information holding unit  21 . The check-signal transmitting unit  24  transmits RA Preamble to the sector that is identified by the read sector identifier by wireless communication using a frequency that is used in the DRB between itself and the base station  30 . When RA Response is received from the base station  30 , the check-signal transmitting unit  24  transmits Scheduled Transmission that includes the terminal identifier of the M2M device  20  and the connection check identifier to the base station  30  as a check signal. When Contention Resolution that includes the terminal identifier of the M2M device  20  and the connection check identifier is received from the base station  30 , the check-signal transmitting unit  24  informs the determining unit  23  of reception of a response signal. 
     Subsequently, the determining unit  23  determines whether it has been informed of reception of a response signal by the check-signal transmitting unit  24  (S 207 ). When informed of reception of a response signal (S 207 : YES), the determining unit  23  sets the normal finish flag to “ON” (S 208 ), and performs the processing indicated at step S 207  again. 
     On the other hand, when not informed of a reception of a response signal (S 207 : NO), the determining unit  23  determines whether the first timer has timed out (S 209 ). When the first timer has not timed out (S 209 : NO), the call control unit  22  determines whether the DRB is established due to occurrence of user data communication and the like (S 211 ). When the DRB is not established (S 211 : NO), the determining unit  23  performs the processing indicated at step S 207  again. On the other hand, when the DRB is established (S 211 : YES), the call control unit  22  informs the determining unit  23  of establishment of the DRB. The determining unit  23  stops the first timer (S 212 ). Subsequently, the call control unit  22  performs the processing indicated at step S 202  again. 
     When the first timer times out (S 209 : YES), the determining unit  23  determines whether the normal finish flag is set to “ON” (S 210 ). When the normal finish flag is set to “ON” (S 210 : YES), the determining unit  23  performs the processing indicated at step S 204  again. 
     On the other hand, when the normal finish flag is set to “OFF” (S 210 : NO), the determining unit  23  increments the number of errors by 1 (S 220  indicated in  FIG. 20 ). The determining unit  23  determines whether the number of errors is equal to or more than N 1  that is the first threshold (S 221 ). When the number of errors is less than the first threshold (S 221 : NO), the determining unit  23  resets and starts the first timer (S 227 ), and performs the processing indicated at step S 206  in  FIG. 19  again. 
     On the other hand, when the number of errors is equal to or more than the first threshold (S 221 : YES), the determining unit  23  instructs reselection of a sector to the call control unit  22  (S 222 ). Subsequently, the determining unit  23  determines whether a different sector is selected by the call control unit  22  (S 223 ). When a different sector is not selected by the call control unit (S 223 : NO), the determining unit  23  resets the number of errors to 0 (S 226 ), and performs the processing indicated at step S 227 . On the other hand, when a different sector is selected by the call control unit  22  (S 223 : YES), the determining unit  23  stops the first timer (S 224 ). The call control unit  22  then establishes a bearer between itself and the MME  50  through the base station  30  that has selected the different sector (S 225 ), and performs the processing indicated at step S 201  again. 
     Operation of Base Station 
       FIG. 21  and  FIG. 22  are flowcharts indicating one example of operation of the base station  30 . With predetermined processing such as power-up, the base station  30  starts the operation indicated in the present flowchart. In the present flowchart, processing of the base station  30  for a single unit of the M2M device  20  is indicated. When multiple units of the M2M devices  20  communicate through the base station  30 , the base station  30  performs the processing indicated in  FIG. 21  and  FIG. 22  for each of the M2M devices  20 . 
     First, the call control unit  33  determines whether an attach request is received from the M2M device  20  (S 300 ). When receiving an attach request (S 300 : YES), the call control unit  33  performs the attach processing among the M2M device  20 , the MME  50 , the SGW  60 , HSS  40 , and the PGW  70  (S 301 ). The attach processing includes processing such as verification, concealment, integrality control, position registration, and bearer establishment. The call control unit  33  performs the processing indicated at step S 304 . 
     When an attach request is not received (S 300 : NO), the call control unit  33  determines whether a service request is received (S 302 ). When a service request is not received (S 302 : NO), the call control unit  33  performs the processing indicated at step S 300  again. When a service request is received (S 302 : YES), the call control unit  33  performs the bear establishment processing among the M2M device  20 , the MME  50 , the SGW  60 , and the HSS  40  (S 303 ). At step S 303 , processing such as verification, concealment, and integrity control is also performed. 
     Subsequently, the call control unit  33  receives setting information from the MME  50  through the S 1  bearer. The call control unit  33  stores base-station setting information included in the received setting information, in the setting-information holding unit  36  (S 304 ). Subsequently, the call control unit  33  transmits terminal setting information that is included in the setting information received from the MME  50  to the M2M device  20  through the DRB (S 305 ). 
     The call control unit  33  informs the notifying unit  35  of establishment of the bearer together with the terminal identifier of the M2M device  20  for which the bearer is established. The notifying unit  35  refers to the setting-information holding unit  36 , and identifies base-station setting information that is associated with the terminal identifier informed by the call control unit  33 . The notifying unit  35  then transmits a connection state notification that indicates that the bearer is established to the device identified by the notification destination that is included in the identified base-station setting information through the wired communication unit  37  (S 306 ). 
     Subsequently, the call control unit  33  determines whether the DRB is released (S 307 ). When the DRB is released (S 307 : YES), the call control unit  33  informs the determining unit  34  of release of the bearer. The determining unit  34  reads the base-station setting information that is associated with the terminal identifier of the M2M device for which the bearer has been released, from the setting-information holding unit  36 . When the execution flag is set to “ON” indicating execution of the connection check processing, the determining unit  34  resets the number of errors to 0 (S 308 ). The determining unit  34  then resets and starts the second timer having duration set in the base-station setting information read from the setting-information holding unit  36  (S 309 ). 
     Subsequently, the determining unit  34  determines whether reception of a check signal is informed by the call control unit  33 , thereby determining whether a check signal is received (S 310 ). When a check signal is received (S 310 : YES), the determining unit  34  performs the processing indicated at step S 308  again. 
     On the other hand, when a check signal is not received (S 310 : NO), the determining unit  34  determines whether the second timer has timed out (S 311 ). When the second timer has not timed out (S 311 : NO), the call control unit  33  determines whether the DRB is established due to occurrence of user data communication and the like (S 315 ). When the DRB is not established (S 315 : NO), the determining unit  34  performs the processing indicated at step S 310  again. On the other hand, when the DRB is established (S 315 : YES), the call control unit  33  informs the determining unit  34  of establishment of the DRB. The determining unit  34  stops the second timer (S 316 ). The call control unit  33  performs the processing indicated at step S 307  again. 
     When the second timer has timed out (S 311 : YES), the determining unit  34  increments the number of errors by (S 312 ), and determines whether the number of errors is equal to or more than N 2  that is the second threshold (S 313 ). When the number of errors is less than the second threshold (S 313 : NO), the determining unit  34  performs the processing indicated at step S 309  again. 
     On the other hand, when the number of errors is equal to or more than the second threshold (S 313 : YES), the determining unit  34  informs abnormal connection to the notifying unit  35  together with the terminal identifier of the M2M device  20 . The notifying unit  35  refers to the setting-information holding unit  36 , and identifies base-station setting information that is associated with the terminal identifier informed by the determining unit  34 . Subsequently, the notifying unit  35  transmits a connection state notification indicating the abnormal connection to the device identified by the notification destination included in the identified base-station setting information through the wired communication unit  37  (S 314 ). 
     Subsequently, the determining unit  34  resets the number of successes to 0 (S 320  in  FIG. 22 ). Moreover, the determining unit  34  resets and starts the third timer having duration set in the base-station setting information read from the setting-information holding unit  36  (S 321 ). The determining unit  34  determines whether the third timer has timed out (S 322 ). When the third timer has timed out (S 322 : YES), the determining unit  34  performs the processing indicated at step S 320  again. 
     On the other hand, when the third timer has not timed out (S 322 : NO), the determining unit  34  determines whether a reception of a check signal is informed by the call control unit  33 , thereby determining whether a check signal is received (S 323 ). When a check signal is not received (S 323 : NO), the call control unit  33  determines whether the DRB is established due to occurrence of user data communication and the like (S 327 ). When the DRB is not established (S 327 : NO), the determining unit  34  performs the processing indicated at step S 322  again. On the other hand, when the DRB is established (S 327 : YES), the call control unit  33  informs the determining unit  34  of establishment of the DRB. The determining unit  34  stops the third timer (S 328 ). The call control unit  33  performs the processing indicated at step S 307  again. 
     On the other hand, when a check signal is received (S 323 : YES), the determining unit  34  increments the number of successes by 1 (S 324 ), and determines whether the number of successes is equal to or more than N 3  that is the third threshold (S 325 ). When the number of successes is less than the third threshold (S 325 : NO), the determining unit  34  performs the processing indicated at step S 321  again. On the other hand, when the number of successes is equal to or more than the third threshold (S 325 : YES), the determining unit  34  informs the notifying unit  35  of connection recovery together with the terminal identifier of the M2M device  20 . The notifying unit  35  refers to the setting-information holding unit  36 , and identifies base-station setting information that is associated with the terminal identifier informed by the determining unit  34 . Subsequently, the notifying unit  35  transmits a connection state notification indicating the connection recovery to the device that is identified by the notification destination included in the identified base-station setting information through the wired communication unit  37  (S 326 ). The determining unit  34  performs the processing indicated at step S 308  again. 
     The embodiments have been explained as above. As described above, the check-signal transmitting unit  24  transmits Scheduled Transmission to the base station  30  every first period, by wireless communication using a frequency used in a bearer after the bearer in a wireless section between itself and the base station  30  is released. Moreover, the determining unit  34  of the base station  30  determines whether there is an abnormality in wireless connection to the M2M device  20  based on whether Scheduled Transmission is received from the M2M device  20  within the second period. Furthermore, when it is determined that there is an abnormality in the wireless connection between itself and the M2M device  20  by the determining unit  34 , the notifying unit  35  informs a predetermined notification destination of the abnormality. Thus, the wireless communication system  10  of the present embodiment can detect the abnormality in wireless connection between the base station  30  and the M2M device  20  promptly, suppressing increase of a processing load on the base station. 
     Moreover, as described above, the call control unit  33  of the base station  30  transmits RA Response as a response to RA Preamble that is transmitted from the M2M device  20 . The call control unit  22  of the M2M device  20  transmits Scheduled Transmission as a response to RA Response transmitted from the base station  30 . Thus, the determining unit  34  can determine whether communication is enabled in both in a wireless link from the M2M device  20  to the base station  30  and a wireless link from the base station  30  to the M2M device  20 . 
     Furthermore, as described above, when the M2M device  20  and the base station  30  perform the attach processing, the setting-information notifying unit  53  of the MME  50  informs setting information that includes information indicating a notification destination to the base station  30  that has performed the attach processing. Moreover, when it is determined that there is an abnormality in a wireless connection between itself and the terminal by the determining unit  34 , the notifying unit  35  of the base station  30  notifies the notification destination indicated by the information included in the setting information informed by the MME  50 . Thus, the notifying unit  35  can identify the notification destination easily when it is determined that there is an abnormality in wireless connection. 
     Moreover, as described above, the call control unit  33  of the base station  30  adds an identifier of a sector of the base station  30  to terminal setting information that is included in the setting information received from the MME  50 , to transmit to the M2M device  20 . The check-signal transmitting unit  24  of the M2M device  20  transmits a check signal to the sector corresponding to the terminal setting information received from the base station  30 . Thus, the processing of selecting a sector before transmission of a check signal can be omitted, and a processing load on the M2M device  20  can be reduced. 
     Furthermore, as described above, the call control unit  33  of the base station  30  transmits Contention Resolution to the M2M device  20  as a response to Scheduled Transmission. The determining unit  23  of the M2M device  20  determines whether there is an abnormality in wireless connection to the base station  30  based on whether Contention Resolution is received from the base station  30  within the first period after transmission of the check signal by the check-signal transmitting unit  24 . When it is determined that there is an abnormality in the wireless connection to the base station by the determining unit  23 , the call control unit  22  of the M2M device  20  searches for the different base station  30 . The call control unit  22  then establishes a bearer between itself and the searched different base station  30 . The setting-information notifying unit  53  of the MME  50  informs base-station setting information associated with the M2M device  20  to the different base station  30  when the bearer is established between the M2M device  20  and the different base station  30 . The setting-information notifying unit  53  then instructs deletion of the base-station setting information associated with the M2M device  20 , to the base station  30  for which a bearer has been established by the M2M device  20  before the different base station  30 . Thus, when the M2M device  20  selects a sector of the different base station  30  after the DRB between the M2M device  20  and the base station  30  is released, the connection check processing can be performed succeedingly between itself and the different base station  30 . 
     Modification 
     The technique disclosed in the present application is not limited to each embodiment described above, but various modifications can be applied within a range of the gist. 
     For example, although explanation has been given with the M2M device  20  as one example of a terminal that performs wireless communication with the base station  30  in the embodiments described above, the disclosed technique is not limited thereto. For example, as a terminal that performs wireless communication with the base station  30 , the disclosed technique is applicable to a wireless communication system having a mobile phone, a smartphone, a tablet terminal, an in-car communication terminal, and the like instead of the M2M device  20 . 
     Furthermore, for example, when the M2M device  20  and the base station  30  have a carrier aggregation function, the terminal setting information may include information about a part of component carriers among multiple component carriers used in carrier aggregation. The M2M device  20  transmits a check signal by using a component carrier identified by information included in the terminal setting information, in the connection check processing after release of the DRB. The information about a component carrier that is used in the connection check processing is set, for example, by the base station  30  or a device at a higher level than the base station  30  (for example, the MME  50  and the like). 
     Thus, when more than one unit of the M2M device  20  transmits a check signal to a single unit of the base station  30 , it is possible to vary a component carrier that is used in transmission of the check signal among the M2M devices  20 . Accordingly, the component carriers used in transmission of the check signal can be distributed, and the possibility of collision of the check signals among the M2M devices  20  can be reduced. Moreover, by avoiding concentration of the check signals to a specific component carrier, an influence to other terminals that uses the specific component can be reduced. 
     Although setting information is stored in the HSS  40  and the setting-information notifying unit  53  of the MME  50  acquires the setting information from the HSS  40  at the time of the attach processing or the bearer establishment processing in the above embodiments, the disclosed technique is not limited thereto. The setting information may be stored in a device other than the HSS  40 , such as the MME  50  and the base station  30 . 
     Furthermore, although setting information is transmitted from the MME  50  to the base station when the attach processing or the bearer establishment processing are performed in the M2M device  20  and the base station  30 , and terminal setting information is transmitted from the base station  30  to the M2M device  20  in the above embodiments, the disclosed technique is not limited thereto. For example, setting information may be transmitted from the MME  50  to the base station  30  and terminal setting information may be transmitted from the base station  30  to the M2M device  20  through a bearer that is established when the M2M device  20  performs position registration processing. 
     Moreover, in the above embodiments, when the number of times that a check signal is not received from the M2M device  20  before the second timer times out after the DRB is released reaches the second threshold or more, the base station  30  transmits a notification indicating abnormal connection to a predetermined notification destination. However, the disclosed technique is not limited thereto. For example, in a device other than the base station  30  such as the MME  50  and the PGW  70  (hereinafter, a different device), information about a notification destination is stored associating with the terminal identifier of the M2M device  20 . When the number of times that a check signal is not received from the M2M device  20  before the second timer times out reaches the second threshold or more after release of the DRB, the base station  30  transmits the terminal identifier of the M2M device  20  and information indicating the abnormal connection to the different device. It may be configured such that the different device transmits a connection state notification indicating the abnormal connection to the notification destination associated with the terminal identifier. 
     Furthermore, respective functional blocks in the M2M device  20 , the base station  30 , and the MME  50  are ones classified per function according to the main processing, to facilitate understanding of functions of devices according to the present embodiment. Therefore, the disclosed technique is not to be limited by a classification method and names of the functional blocks. Moreover, the M2M device  20 , the base station  30 , and the MME  50  according to the present embodiments can be classified into more functional blocks, or can be classified such that one functional block performs more kinds of processing. Furthermore, each processing may be implemented as processing by software, or may be implemented by dedicated hardware such as an application specific integrated circuit (ASIC). 
     According to one embodiment, abnormality in wireless connection between a base station and a terminal can be promptly detected, suppressing the increase of a processing load on the base station. 
     All examples and conditional language recited herein are intended for pedagogical purposes of aiding the reader in understanding the invention and the concepts contributed by the inventor to further the art, and are not to be construed as limitations to such specifically recited examples and conditions, nor does the organization of such examples in the specification relate to a showing of the superiority and inferiority of the invention. Although the embodiment of the present invention has been described in detail, it should be understood that the various changes, substitutions, and alterations could be made hereto without departing from the spirit and scope of the invention.