Patent Publication Number: US-2016233987-A1

Title: Initiator terminal, target terminal, initiator terminal error-processing method, and target terminal error-processing method

Description:
TECHNICAL FIELD 
     The present disclosure relates to a technique for realizing transaction process to access a storage medium that an accessible external terminal includes, similarly to a storage medium that an own terminal includes, by using a plurality of wireless communication methods. 
     BACKGROUND ART 
     Various data, for example, video data is generally transmitted and received by not only wired communication using conventional wired connection but also wireless communication, by improvement of wireless techniques in recent years. 
     PTL 1 discloses a technique with which serial number information indicating a generating order of a packet is added to each packet, and a packet loss is determined by detecting the serial number information in packets, when video data or the like is transmitted by wireless communication,. 
     CITATION LIST 
     Patent Literature 
     PTL 1: International Publication No. 2005/060262 
     SUMMARY 
     An initiator terminal of the present disclosure includes a first wireless unit configured to connect with a target terminal via wireless communication with a first wireless communication method, a second wireless unit configured to connect with the target terminal via wireless communication with a second wireless communication method different from the first wireless communication method, and an access processor configured to execute an access protocol process for remote access to a storage medium that the target terminal includes. If an error occurs during accessing to the storage medium, the access processor switches the access protocol process for remote access to the storage medium between in the wireless communication in the first wireless unit and in the wireless communication in the second wireless unit. 
     Further, a target terminal of the present disclosure includes a storage medium, a first wireless unit configured to connect with an initiator terminal via wireless communication with a first wireless communication method, a second wireless unit configured to connect with the initiator terminal via wireless communication with a second wireless communication method different from the first wireless communication method, and an access processor configured to permit the initiator terminal to access remotely to a storage medium and to execute an access protocol process for remote access to the storage medium. If an error occurs during accessing to the storage medium from the initiator terminal, the access processor switches the access protocol process for remote access to the storage medium between in the wireless communication in the first wireless unit and in the wireless communication in the second wireless unit. 
     The present disclosure enables to efficiently execute a retransmitting process for transmitting data and receiving data if an error occurs. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a diagram illustrating a configuration of a wireless communication system according to a first exemplary embodiment. 
         FIG. 2  is a block diagram illustrating a configuration of an initiator terminal and a target terminal according to the first exemplary embodiment. 
         FIG. 3  is a diagram describing a procedure for obtaining an access control right to a storage medium and a procedure for releasing the access control right, via wireless communication according to the first exemplary embodiment. 
         FIG. 4  is a diagram illustrating a logical configuration of a wireless packet according to the first exemplary embodiment. 
         FIG. 5  is a diagram illustrating a logical configuration in a case that encapsulates data of an upper layer in a wireless packet according to the first exemplary embodiment. 
         FIG. 6  is a diagram describing a format of a Command (CMD) packet for accessing according to the first exemplary embodiment. 
         FIG. 7  is a diagram describing a format of a command response (CMD Response) packet for accessing according to the first exemplary embodiment. 
         FIG. 8  is a diagram describing a format of a data (DATA) packet for accessing according to the first exemplary embodiment. 
         FIG. 9  is a diagram describing a format of an Acknowledgment (ACK) packet for accessing according to the first exemplary embodiment. 
         FIG. 10  is a diagram describing a format of a Check-Status packet according to the first exemplary embodiment. 
         FIG. 11  is a diagram describing a format of a Check-Status Response packet according to the first exemplary embodiment. 
         FIG. 12  is a diagram describing a format of a Clear Bus Handle Request packet according to the first exemplary embodiment. 
         FIG. 13  is a diagram describing a format of a Clear Bus Handle Response packet according to the first exemplary embodiment. 
         FIG. 14  is a diagram describing a format of an Interrupt (INT) packet according to the first exemplary embodiment. 
         FIG. 15  is a diagram describing a sequence where an initiator terminal reads data from a storage medium in a target terminal according to the first exemplary embodiment. 
         FIG. 16  is a diagram describing a sequence if a data loss occurs during reading data from the storage medium in the target terminal by the initiator terminal according to the first exemplary embodiment. 
         FIG. 17  is a diagram describing another sequence if a data loss occurs during reading data from the storage medium in the target terminal by the initiator terminal according to the first exemplary embodiment. 
         FIG. 18  is a diagram describing a sequence where the initiator terminal writes data into the storage medium in the target terminal according to a second exemplary embodiment. 
         FIG. 19  is a diagram describing a sequence if a data loss occurs during writing data into the storage medium in the target terminal by the initiator terminal. 
         FIG. 20  is a diagram describing another sequence if a data loss occurs during writing data into the storage medium in the target terminal by the initiator terminal. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Exemplary embodiments are described in detail below suitably with reference to the drawings. Description that is precise beyond necessity is occasionally omitted. For example, detailed description about items already well-known, and overlapped description about substantially the same configurations are occasionally omitted. This is because the following description is prevented from being unnecessarily redundant and is made to be easily understood by a person skilled in the art. 
     The accompanying drawings and the following description are provided for a person skilled in the art to fully understand the present disclosure, and are not intended to limit the subject matter described in Claims. 
     First Exemplary Embodiment 
     In general, a terminal including a plurality of wireless communication methods as well as one wireless communication method exists. For example, a terminal with wireless Local Area Network (LAN) functions includes two wireless communication methods including a wireless communication method using a frequency band of 2.5 GHz, and a wireless communication method using a frequency band of 5 GHz. 
     In such terminal, if an error such as a data loss occurs during transmitting data and receiving data, executing the same recovering process regardless of the wireless communication methods might not take advantage of the characteristic of each wireless communication method. 
     The present disclosure to provide an initiator terminal and a target terminal that include a plurality of wireless communication methods and efficiently execute a recovering process for transmitting data and receiving data if an error occurs. 
     1-1. Configuration 
       FIG. 1  is a diagram illustrating a configuration of a wireless communication system according to an exemplary embodiment. In  FIG. 1 , wireless communication system  100  is configured by initiator terminal  101  and target terminal  102 . Target terminal  102  includes storage medium  103 . 
     Storage medium  103  includes a storage device for recording data. Storage medium  103  is preferably a non-volatile storage device for holding recorded data even if an electric power that is supplied to storage medium  103  is interrupted. Storage medium  103  may be a bridge medium such as an SD memory card detachable from target terminal  102 , or may be a hard disc or a flash memory built in target terminal  102 . 
     Initiator terminal  101  and target terminal  102  are connected communicable with each other by using two wireless communication methods. Initiator terminal  101  establishes wireless communication with target terminal  102  using any one of the two wireless communication methods. 
     In the exemplary embodiment, for example, Wireless Fidelity (WiFi) is used as a first wireless communication method and Wireless Gigabit (WiGig) is used as a second wireless communication method. A communication speed of WiFi is slower than a communication speed of WiGig. 
     This exemplary embodiment is not limited to a combination of WiFi and WiGig, and thus any combination of the first wireless communication method and the second wireless communication method may be used as long as the wireless communication methods are different from each other. For example, there are combinations that communication speeds, frequency bands, modulation systems, and wireless protocols are different from each other, during wireless communication. 
     Further, initiator terminal  101  can access remotely to storage medium  103  that target terminal  102  includes similarly to a storage medium that initiator terminal  101  includes, via the wireless communication. When initiator terminal  101  accesses to storage medium  103 , target terminal  102  releases storage medium  103  to initiator terminal  101 . Such an access system is called transparent access. 
       FIG. 2  is a block diagram illustrating a detailed configuration of initiator terminal  101  and target terminal  102 . Initiator terminal  101  includes application unit  201 , access processor  202 , first wireless unit  203 , and second wireless unit  204 . 
     Application unit  201  includes a software program or the like for remote access to storage medium  103  in target terminal  102 . 
     Access processor  202  controls whole initiator terminal  101 . Further, access processor  202  executes a process as a host that initiator terminal  101  accesses remotely to storage medium  103 . Access processor  202  executes a process for transmitting a command with payload of the wireless communication method so as to access to storage medium  103  by using first wireless unit  203  or second wireless unit  204  according to a request from application unit  201 , and executes an access protocol process for issuing a command so as to access to storage medium  103  as an initiator. Access processor  202  further mediates an exclusive access right to storage medium  103  between initiator terminal  101  and target terminal  102  and executes also a data process and an error process in the wireless communication. 
     First wireless unit  203  executes a wireless communication protocol process for communication using the first wireless communication method according to an instruction from access processor  202 . 
     Second wireless unit  204  executes a wireless communication protocol process for communication using the second wireless communication method different from the first wireless communication method according to an instruction from access processor  202 . 
     Target terminal  102  includes application unit  205 , access processor  206 , first wireless unit  207 , and second wireless unit  208 . 
     Application unit  205  includes a software program or the like for access to storage medium  103  in target terminal  102 . 
     Access processor  206  controls whole target terminal  102 . Access processor  206  establishes wireless communication with initiator terminal  101  using first wireless unit  207  or second wireless unit  208 . Access processor  206  executes a process for writing data into storage medium  103  or reading data from storage medium  103  according to an instruction from initiator terminal  101  or an instruction from application unit  205 . Access processor  206  controls application unit  205  and initiator terminal  101  so that accessing exclusively to storage medium  103  realizes, namely, switches a control between a local access and a remote access to storage medium  103 . Further, access processor  206  executes an access protocol process for accessing to storage medium  103  as a target. 
     First wireless unit  207  executes a wireless communication protocol process for establishing the wireless communication with first wireless unit  203  in initiator terminal  101  using the first wireless communication method. 
     Second wireless unit  208  executes a wireless communication protocol process for establishing the wireless communication with second wireless unit  204  in initiator terminal  101  using the second wireless communication method. 
     In  FIG. 2 , as described with reference to  FIG. 1 , WiFi is used as the first wireless communication method using by first wireless units  203  and  207 , and WiGig is used as the second wireless communication method using second wireless units  204  and  208 . 
     1-2. Procedure for Obtaining and Releasing Access Control Right 
     A procedure for obtaining the access control right to storage medium  103  from initiator terminal  101  until accessible, and a procedure for releasing the access control right are described below.  FIG. 3  is a diagram describing a procedure for obtaining the access control right to the storage medium and a procedure for releasing the access control right, via wireless communication. 
     Target terminal  102  can access to storage medium  103  until storage medium  103  is released for initiator terminal  101 . That is to say, application unit  205  in target terminal  102  recognizes storage medium  103 , and thus target terminal  102  can write data to storage medium  103 , read data stored in storage medium  103 , and transmit a command to storage medium  103 . This state is called a local accessible state. 
     (Step S 100 ) Initiator terminal  101  and target terminal  102  connect with each other by wireless communication so that initiator terminal  101  accesses to storage medium  103 . As a connection for wireless communication, first wireless unit  203  in initiator terminal  101  and first wireless unit  207  in target terminal  102  may establish a wireless communication with the first wireless communication method, or second wireless unit  204  in initiator terminal  101  and second wireless unit  208  in target terminal  102  may establish a wireless communication with the second wireless communication method. As to a choice of the wireless communication method, for example, a wireless communication method with better communication channel information such as large communication channel capacity is selected among the two wireless communication methods. As to this connection for the wireless communication, a procedure does not matter as long as initiator terminal  101  and target terminal  102  establish data communication with the first wireless communication method or the second wireless communication method. 
     A procedure in which initiator terminal  101  obtains the access control right to storage medium  103  in target terminal  102  is described below. 
     (Step S 101 ) Application unit  201  in initiator terminal  101  requests processor  202  to access to storage medium  103 . 
     (Step S 102 ) Access processor  202  in initiator terminal  101  controls first wireless unit  203  or second wireless unit  204  based on the established wireless communication method, and instructs target terminal  102  to transmit a wireless packet using an access protocol. That is to say, if the wireless communication is established by the first wireless communication method, access processor  202  transmits a wireless packet to target terminal  102  through first wireless unit  203 . If the wireless communication is established by the second wireless communication method, access processor  202  transmits a wireless packet to target terminal  102  through second wireless unit  204 . 
     (Step S 103 ) A wireless packet to be transmitted from first wireless unit  203  or second wireless unit  204  in initiator terminal  101  to first wireless unit  207  or second wireless unit  208  in target terminal  102  is a Session-Start Request packet that initiator terminal  101  instructs storage medium  103  to prepare for a transparent access. 
     (Step S 104 ) First wireless unit  207  or second wireless unit  208  in target terminal  102  notifies access processor  206  of the received Session-Start Request packet. 
     (Step S 105 ) Access processor  206  receives an acknowledgment of receiving the Session-Start Request packet from initiator terminal  101 . If target terminal  102  can accept the transparent access, target terminal  102  prepares for transferring the access control right of storage medium  103  to initiator terminal  101 . An example of this preparation is a process for releasing the access control right to application unit  205  in target terminal  102 , if application unit  205  possesses the access control right. Access processor  206  instructs first wireless unit  207  or second wireless unit  208  in target terminal  102  to transmit a response packet upon normally receiving the Session-Start Request packet. 
     (Step S 106 ) First wireless unit  207  or second wireless unit  208  in target terminal  102  wirelessly transmits a Session-Start Response packet with respect to the Session-Start Request packet to initiator terminal  101  based on an instruction from access processor  206 . 
     (Step S 107 ) Receiving the Session-Start Response packet from first wireless unit  207  or second wireless unit  208  in target terminal  102 , first wireless unit  203  or second wireless unit  204  in initiator terminal  101  notifies access processor  202  of receiving the Session-Start Response packet. As a result, a session is established between initiator terminal  101  and target terminal  102 . 
     A procedure in which initiator terminal  101  obtains information about storage medium  103  is described below. 
     (Step S 108 ) The session is established and access processor  202  requests first wireless unit  203  or second wireless unit  204  in initiator terminal  101  to transmit a wireless packet for obtaining information about storage medium  103 . 
     (Step S 109 ) First wireless unit  203  or second wireless unit  204  in initiator terminal  101  wirelessly transmits a Capability Exchange Request packet to target terminal  102 . 
     (Step S 110 ) Receiving the Capability Exchange Request packet, first wireless unit  207  or second wireless unit  208  in target terminal  102  notifies access processor  206  of receiving the wireless packet for obtaining the information about storage medium  103 . 
     (Step S 111 ) Access processor  206  receives the wireless packet for obtaining the information about storage medium  103  from first wireless unit  207  or second wireless unit  208  in target terminal  102 , access processor  206  obtains the information about storage medium  103  including a type of bus interface, a type of device of storage medium  103 , a relative address allocated to storage medium  103  such as Relative Card Address (RCA), a status of storage medium  103  such as a write-protect status and a writable status, if storage medium  103  is, for example, an SD memory card. As for types of bus interfaces, there are a standard speed, a high speed, Ultra-High Speed I (UHS-I), and Ultra-High Speed II (UHS-II). As for types of devices of storage medium  103 , there are a memory device, an IO device, and a device including two functions both the memory device and the IO device. Access processor  206  instructs first wireless unit  207  or second wireless unit  208  to transmit a wireless packet in response to the Capability Exchange Request. 
     (Step S 112 ) Receiving an instruction of transmitting the wireless packet in response to the Capability Exchange Request packet from access processor  206 , first wireless unit  207  or second wireless unit  208  wirelessly transmits a Capability Exchange Response packet to initiator terminal  101 . The Capability Exchange Response packet includes the information about storage medium  103 . 
     (Step S 113 ) Receiving the Capability Exchange Response packet from target terminal  102 , first wireless unit  203  or second wireless unit  204  notifies access processor  202  of receiving the information about storage medium  103 . 
     A procedure in which initiator terminal  101  obtains the access control right to storage medium  103  for target terminal  102  is described below. 
     (Step S 114 ) Receiving the information about storage medium  103 , access processor  202  determines whether accessing to storage medium  103  based on the information. For example, if a type of a bus interface to be used in storage medium  103  is not compatible with access processor  202  or application unit  201 , access processor  202  determines that it is unable to access to storage medium  103 . If the type of the bus interface is compatible, access processor  202  determines that it is able to access to storage medium  103 . 
     Access processor  202  determines based on not limited only the type of the bus interface. Access processor  202  may determine based on other information obtained from storage medium  103 . 
     If access processor  202  determines that it is able to access to storage medium  103 , access processor  202  instructs first wireless unit  203  or second wireless unit  204  to transmit a wireless packet for obtaining the access control right. 
     (Step S 115 ) First wireless unit  203  or second wireless unit  204  transmits a Get Bus Handle Request packet for obtaining the access control right to storage medium  103 , to target terminal  102  according to an instruction from access processor  202 . 
     (Step S 116 ) Receiving the Get Bus Handle Request packet from initiator terminal  101 , first wireless unit  207  or second wireless unit  208  notifies access processor  206  of receiving the packet for obtaining the access control right to storage medium  103  from initiator terminal  101 . 
     (Step S 117 ) Recognizing for receiving of obtaining the access control right from initiator terminal  101 , access processor  206  deletes the access control right to storage medium  103  allocated to application unit  205 , from application unit  205 . Access processor  206  deletes the access control right allocated to application unit  205  and then application unit  205  cannot access to storage medium  103 . Concretely, even if access processor  206  receives a request for accessing to storage medium  103  from application unit  205 , access processor  206  cannot accept the request. 
     The above procedure disables application unit  205  to access to storage medium  103 . This state is called a local inaccessible state. 
     (Step S 118 ) Access processor  206  succeeds in deleting the access control right allocated to application unit  205  and then access processor  206  instructs first wireless unit  207  or second wireless unit  208  to transmit a wireless packet for notifying a change in the control right for transferring the access control right to initiator terminal  101 . 
     (Step S 119 ) First wireless unit  207  or second wireless unit  208  transmits a Get Bus Handle Response packet indicating the change in the access control right as a response to the Get Bus Handle Request packet to initiator terminal  101  according to the instruction from access processor  206 . 
     (Step S 120 ) Receiving the Get Bus Handle Response packet from target terminal  102 , first wireless unit  203  or second wireless unit  204  notifies access processor  202  of receiving this packet. 
     (Step S 121 ) Access processor  202  allocates the access control right to application unit  201 . Access processor  202  notifies application unit  201  of enabling to write data into storage medium  103  or to read data from storage medium  103 . 
     If application unit  201  receives an acknowledgment for enabling to access to storage medium  103  from access processor  202 , initiator terminal  101  can access to storage medium  103 . 
     By the above procedure, application unit  201  enables to access transparently to storage medium  103  similarly to a storage medium that is connected or inserted to an own terminal via wireless communication. This state is called a remote accessible state. 
     The above is the procedure for obtaining the access control right. 
     A procedure for releasing the access control right obtained by initiator terminal  101 , and returning the access control right to target terminal  102  is described below. 
     (Step S 122 ) Application unit  201  does not need to access to storage medium  103  and application unit  201  notifies access processor  202  that accessing to storage medium  103  to access processor  202  is no longer needed. 
     (Step S 123 ) Access processor  202  deletes the access control right to storage medium  103  allocated to application unit  201 . Access processor  202  instructs first wireless unit  203  or second wireless unit  204  to transmit a wireless packet for releasing the access control right to target terminal  102 . 
     (Step S 124 ) Receiving the packet for releasing the access control right from access processor  202 , first wireless unit  203  or second wireless unit  204  wirelessly transmits a Release Bus Handle Request packet indicating that the access control right is released for target terminal  102 . 
     (Step S 125 ) Receiving the Release Bus Handle Request packet from initiator terminal  101 , first wireless unit  207  or second wireless unit  208  notifies access processor  206  of receiving the packet for releasing the access control right to storage medium  103 . 
     (Step S 126 ) Receiving the an acknowledgement for releasing the access control right from initiator terminal  101 , the access processor  206  allocates the access control right to storage medium  103  to application unit  205 . Access processor  206  notifies application unit  205  of obtaining the access control right to storage medium  103 . As a result, application unit  205  can access to storage medium  103 . Application unit  205  is into a local accessible state again. 
     (Step S 127 ) Access processor  206  instructs first wireless unit  207  or second wireless unit  208  to transmit a wireless packet for notifying initiator terminal  101  of retuning the access control right to storage medium  103  to application unit  205  in target terminal  102 . 
     (Step S 128 ) First wireless unit  207  or second wireless unit  208  wirelessly transmits a Release Bus Handle Response packet indicating that the access control right is returned to target terminal  102 , to initiator terminal  101  according to an instruction from access processor  206 . 
     (Step S 129 ) Receiving the Release Bus Handle Response packet from target terminal  102 , first wireless unit  203  or second wireless unit  204  notifies access processor  202  of receiving this wireless packet. 
     The above procedure enables application unit  205  to access to storage medium  103 . As a result, application unit  205  returns to be into the local accessible state again. 
     The above is the procedure for releasing the access control right. 
     A data format of the wireless packet described in the procedures for obtaining and releasing the access control right is described below. 
       FIG. 4  is a diagram illustrating a logical configuration of the wireless packet. In  FIG. 4 , the wireless packet includes wireless packet header  41  and wireless packet payload  42 . 
     At the time that first wireless unit  203  and first wireless unit  207  establish wireless communication, or at the time second wireless unit  204  and second wireless unit  208  establish wireless communication, necessary predetermined information is stored in wireless packet header  41 . This information includes, for example, identification information for identifying a terminal as a transmission source of a wireless packet, identification information for identifying a terminal as a transmission destination of a wireless packet, and a size of wireless packet payload  42 . 
     Wireless packet payload  42  stores information for transmitting and receiving between initiator terminal  101  and target terminal  102 . Wireless packet payload  42  includes identification information for identifying the above-mentioned wireless packets such as a Session-Start Request, a Session-Start Response, a Capability Exchange Request, a Capability Exchange Response, a Get Bus Handle Request, a Get Bus Handle Response, a Release Bus Handle Request, and a Release Bus Handle Response. 
     As described above, the access control right is managed between initiator terminal  101  and target terminal  102 . As a result, initiator terminal  101  and target terminal  102  are prevented from accessing to storage medium  103  at the same time. Only a terminal possessing the access control right can access to storage medium  103 , so that a mechanism of an exclusive access to storage medium  103  can be established. 
     Furthermore, the access control right is transmitted and received between access processor  202  in initiator terminal  101  and access processor  206  in target terminal  102 . The access control right is transmitted and received among access processor  202 , first wireless unit  203 , second wireless unit  204 , access processor  206 , first wireless unit  207  and second wireless unit  208 , that is, among terminals through function blocks on a comparatively lower layer. Application units  201  and  205  on a comparatively upper layer do not directly transmit and receive the access control right. Accessing to storage medium  103  by application units  201  and  205  is managed without awareness of the access control right. This means that access processors  202  and  206  receive a request for accessing to storage medium  103  from application units  201  and  205 , and then access processors  202  and  206  control to access to storage medium  103  based on the request and information about existence or non-existence of the access control right. For this reason, application units  201  and  205  do not have to include a function for directly transmitting and receiving the access control right. 
     Particularly, application unit  201  in initiator terminal  101  can access to storage medium  103  in target terminal  102  as if application unit  201  accesses to a storage medium in a own terminal. 
     In order that application unit  201  in initiator terminal  101  may access to storage medium  103 , it is necessary to devise how to handle data to be transmitted to or received from storage medium  103  along with management for the access control right. 
       FIG. 5  is a diagram illustrating a logical configuration in a case that encapsulates data of an upper layer in a wireless packet. That is to say,  FIG. 5  is a diagram illustrating a logical configuration of a wireless packet to be transmitted and received between first wireless units  203  and  207  or between second wireless units  204  and  208 . The wireless packet includes wireless packet header  51  and wireless packet payload  52 . 
     Since wireless packet header  51  is basically the same as wireless packet header  41  described with reference to  FIG. 4 , description of wireless packet header  51  is omitted. 
     Wireless packet payload  52  includes specific information transmitted and received between the two terminals. Wireless packet payload  52  stores directly data formats to be used for instruction and notification from application unit  201  to access processor  202 , or data formats to be used for instruction and notification from application unit  205  to access processor  206 . 
     Access processor  202 , first wireless unit  203 , second wireless unit  204 , access processor  206 , first wireless unit  207  or second wireless unit  208  incorporate directly data formats on the upper layer, for example, application unit  201  and application unit  205  into the payload of the wireless packet. And then data or the like on the upper layer is transmitted by a communication mean on a lower layer. This is called encapsulation. 
     The wireless packet transmitted and received between first wireless units  203  and  207  or the wireless packet transmitted and received second wireless units  204  and  208  is obtained by packetizing and encapsulating data from application units  201  and  205 . The data from application unit  201  is encapsulated into a wireless packet by access processor  202  and first wireless unit  203  or second wireless unit  204  and is transmitted to target terminal  102 . Access processor  206  received this wireless packet extracts only a data portion output by application unit  201  in wireless packet payload  52  in the received wireless packet. Access processor  206  outputs the data portion extracted to storage medium  103 . 
     In this way, application unit  201  in initiator terminal  101  enables to transmit data to storage medium  103  in target terminal  102  that is another external terminal, as if storage medium  103  is connected directly to application unit  201 . 
     That is to say, in case where data from an upper layer in a terminal is transmitted and received by using a communication method on a lower layer between two terminals, the data is encapsulated and transmitted to another terminal, and the other terminal received the data extracts the encapsulated data and outputs the extracted data to storage medium  103  on an upper layer. As a result, a transparent access to the storage medium in the other terminal is enabled. 
     In  FIG. 5 , wireless packet payload  52  may be composed of only data from the upper layer, or data from the upper layer may be encapsulated into a part of wireless packet payload  52 . In this case, wireless packet payload  52  can add additional data besides data from the upper layer. 
     1-3. Data Formats of Packets for Accessing 
     Data formats of packets for accessing to be used after initiator terminal  101  is into a remote accessible state to storage medium  103  in target terminal  102  is described below. Since each packet for accessing is a wireless packet, this packet includes a wireless packet header. Further, a wireless packet payload of the wireless packet is encapsulated. 
       FIG. 6  is a diagram describing a format of a Command (CMD) packet for accessing. The CMD packet includes wireless packet header  61  and wireless packet payload  62 . Wireless packet header  61  is similar to wireless packet header  41  in  FIG. 4 . Wireless packet payload  62  includes control data  63  and CMD  64 . 
     Control data  63  stores predetermined information to be transmitted and received between access processors  202  and  206 . Packet Sequence Number (PSN)  65  indicating a sequence number of data packets is stored in control data  63 . The PSN starts from 0 and is incremented every time when a data packet is generated. 
     CMD  64  stores a content of a command for accessing. If initiator terminal  101  accesses to storage medium  103  in target terminal  102  for reading, a Read Command is stored in CMD  64 . If initiator terminal  101  accesses to storage medium  103  in target terminal  102  for writing, a Write Command is stored in CMD  64 . 
       FIG. 7  is a diagram describing a format of a Command Response (CMD Response) packet for accessing. The CMD Response packet is used for a response to a CMD packet. The CMD Response packet includes wireless packet header  71  and wireless packet payload  72 . Wireless packet header  71  is the same as wireless packet header  41  described with reference to  FIG. 4 . Wireless packet payload  72  includes control data  73  and CMD Response  74 . PSN  75  is stored in control data  73 . Control data  73  is the same as control data  63  and PSN  75  is the same as PSN  65 , described with reference to  FIG. 6 . Response content is stored in CMD Response  74 . 
       FIG. 8  is a diagram describing a format of a data (DATA) packet for accessing. The DATA packet is used for transmitting data to be transferred between application unit  201  and storage medium  103 . The DATA packet includes wireless packet header  81  and wireless packet payload  82 . Wireless packet header  81  is the same as wireless packet header  41  described with reference to  FIG. 4 . Wireless packet payload  82  includes control data  83  and DATA  84 . PSN  85  is stored in control data  83 . Control data  83  is the same as control data  63  and PSN  85  is the same as PSN  65 , described with reference to  FIG. 6 . 
     Data to be recorded from application unit  201  to storage medium  103  or data to be read from storage medium  103  to application unit  201  are stored in DATA  84 . 
       FIG. 9  is a diagram describing a format of an Acknowledgment (ACK) packet for accessing. The ACK packet is used for notifying a terminal of a transmission source that the DATA packet reaches. The ACK packet includes wireless packet header  91  and wireless packet payload  92 . Wireless packet header  91  is the same as wireless packet header  41  described with reference to  FIG. 4 . Wireless packet payload  92  includes control data  93  and ACK  94 . PSN  95  is stored in control data  93 . Control data  93  is the same as control data  63  and PSN  95  is the same as PSN  65 , described with reference to  FIG. 6 . 
     ACK  94  includes information indicating a response that the DATA packet has been received. ACK  94  includes information of Remaining Size  96  indicating an amount of remaining data to be transmitted. 0 is stored in Remaining Size  96  if the process is completed. 
       FIG. 10  is a diagram describing a format of a Check-Status packet for accessing. The Check-Status packet is used for notifying a transmission source of a data arrival state. The Check-Status packet includes wireless packet header  1001  and wireless packet payload  1002 . Wireless packet header  1001  is the same as wireless packet header  41  described with reference to  FIG. 4 . Wireless packet payload  1002  includes control data  1003  and Check-Status  1004 . PSN  1005  is stored in control data  1003 . Control data  1003  is the same as control data  63  and PSN  1005  is the same as PSN  65 , described with reference to  FIG. 6 . 
     Check-Status  1004  includes Remaining Size  1006  and Next PSN  1007  as information for inquiring a terminal of a transmission source about a state of transmitting and receiving data. 
     Remaining Size  1006  is the same as Remaining Size  96  described with reference to  FIG. 9 . 
     A sequence number of a packet to be expected next is stored in Next PSN  1007 . 
       FIG. 11  is a diagram describing a Check-Status Response packet for accessing. The Check-Status Response packet is used for a response to the Check-Status packet. The Check-Status Response packet includes wireless packet header  1101  and wireless packet payload  1102 . Wireless packet header  1101  is the same as wireless packet header  41  described with reference to  FIG. 4 . Wireless packet payload  1102  includes control data  1103  and Check-Status Response  1104 . PSN  1105  is stored in control data  1103 . Control data  1103  is the same as control data  63  described with reference to  FIG. 6 , and PSN  1105  is the same as PSN  65 . Check-Status  1104  includes Remaining Size  1106  and Next PSN  1107  as information for a response to a terminal of a transmission destination about a state of transmitting and receiving data. 
     Remaining Size  1106  is the same as Remaining Size  96  described with reference to  FIG. 9 . 
     A sequence number of a packet to be expected next is stored in Next PSN  1107 . 
       FIG. 12  is a diagram describing a format of a Clear Bus Handle Request packet for accessing. The Clear Bus Handle Request packet is used for interrupting a command process currently executing. The Clear Bus Handle Request packet includes wireless packet header  1201  and wireless packet payload  1202 . Wireless packet header  1201  is the same as wireless packet header  41  described with reference to  FIG. 4 . Wireless packet payload  1202  includes control data  1203  and Clear Bus Handle Request  1204 . PSN  1205  is stored in control data  1203 . Control data  1203  is the same as control data  63  and PSN  1205  is the same as PSN  65 , described with reference to  FIG. 6 . 
       FIG. 13  is a diagram describing a format of a Clear Bus Handle Response packet for accessing. The Clear Bus Handle Response packet is used for responding to the Clear Bus Handle Request packet. The Clear Bus Handle Response packet includes wireless packet header  1301  and wireless packet payload  1302 . Wireless packet header  1301  is the same as wireless packet header  41  described with reference to  FIG. 4 . Wireless packet payload  1302  includes control data  1303  and Clear Bus Handle Response  1304 . PSN  1305  is stored in control data  1303 . Control data  1303  is the same as control data  63  and PSN  1305  is the same as PSN  65 , described with reference to  FIG. 6 . 
       FIG. 14  is a diagram describing a format of an Interrupt (INT) packet for accessing. The INT packet is used for notifying initiator terminal  101  of an interruption of an event from target terminal  102 . The INT packet includes wireless packet header  1401  and wireless packet payload  1402 . Wireless packet header  1401  is the same as wireless packet header  41  described with reference to  FIG. 4 . Wireless packet payload  1402  includes control data  1403  and Interrupt  1404 . PSN  1405  is stored in control data  1403 . Control data  1403  is the same as control data  63  and PSN  1405  is the same as PSN  65 , described with reference to  FIG. 6 . 
     A content of notifying another terminal of an interruption of an event is stored in Interrupt  1404 . Interrupt  1404  includes Event Type 1406 , and a type of the preset event to be interrupted is stored in Interrupt  1404 . For example, in a case of a PSN error caused by a packet loss, a value of an event type corresponding to the PSN error, such as 8, is stored. 
     1-4. Normal Operation for Reading Data 
     A sequence in which initiator terminal  101  accesses to storage medium  103  for reading is described below.  FIG. 15  is a diagram describing a sequence where an initiator terminal reads data from a storage medium in a target terminal. 
     (Step S 100 ) As already described, initiator terminal  101  and target terminal  102  connect with each other by wireless communication so that initiator terminal  101  accesses to storage medium  103 . 
     (Step S 200 ) Further, as described above, initiator terminal  101  executes steps from step S 101  to step S 121  that obtain an access control right t to storage medium  103  in target terminal  102 . 
     If initiator terminal  101  obtains the access control right to storage medium  103 , application unit  201  in initiator terminal  101  enables to access to storage medium  103  in target terminal  102 . 
     Since target terminal  102  requests application unit  205  to release the access control right to storage medium  103 , application unit  205  in target terminal  102  cannot access to storage medium  103 . And then target terminal  102  is into a local inaccessible state. 
     (Step S 201 ) Application unit  201  in initiator terminal  101  notifies access processor  202  of a Data Read Command (Read CMD). It is assumed that an amount of data to be read from storage medium  103  is 45 Kbyte. 
     (Step S 202 ) Receiving an instruction for reading data of 45 Kbyte from application unit  201 , access processor  202  generates a CMD packet. Information indicating a Read CMD (Read Command) and information indicating 45 Kbyte as size information of data to be read are stored in CMD  64 . Since this is a packet for accessing that is generated first, 0 is stored as a value to be stored in PSN  65 . 
     (Step S 203 ) First wireless unit  203  or second wireless unit  204  wirelessly transmits the wireless packet generated by access processor  202  to target terminal  102 . 
     (Step S 204 ) First wireless unit  207  or second wireless unit  208  notifies access processor  206  of the received wireless packet. 
     (Step S 205 ) Access processor  206  interprets the received wireless packet, and notifies storage medium  103  of the Read CMD. 
     (Step S 206 ) Receiving the Read CMD, storage medium  103  responds RES (response) to the Read CMD to access processor  206 . 
     (Step S 207 ) Receiving RES from storage medium  103 , access processor  206  generates a CMD Response packet that is a wireless packet for responding to the CMD packet, and outputs the generated packet to first wireless unit  207  or second wireless unit  208 . Information about RES is stored in CMD Response  74 , and 0 is stored in PSN  75 . 
     (Step S 208 ) First wireless unit  207  or second wireless unit  208  transmits the wireless packet generated by access processor  206  to initiator terminal  101 . 
     (Step S 209 ) Receiving the CMD Response packet, first wireless unit  203  or second wireless unit  204  in initiator terminal  101  notifies access processor  202  of the CMD Response packet. 
     (Step S 210 ) Access processor  202  interprets the received CMD Response packet, and outputs a CMD Response to application unit  201 . Receiving the CMD Response, application unit  201  confirms that the Read CMD is notified to storage medium  103 . 
     (Step S 211 ) Since target terminal  102  accepts the Read CMD, target terminal  102  reads DATA specified by the Read CMD from storage medium  103 , and outputs the DATA to access processor  206 . It is assumed that an amount of data to be transmitted at one time is  7 . 5  Kbyte. 
     (Step S 212 ) Access processor  206  stores the DATA read from storage medium  103  in DATA  84  of the DATA packet. 0 is incremented and 1 is stored in PSN  85 . Access processor  206  outputs the DATA packet to first wireless unit  207  or second wireless unit  208 . 
     Further, access processor  206  holds the DATA read from storage medium  103  in an internal memory, not shown in the figure, in target terminal  102 . 
     (Step S 213 ) First wireless unit  207  or second wireless unit  208  transmits the DATA packet generated by access processor  206  to first wireless unit  203  or second wireless unit  204 . 
     (Step S 214 ) First wireless unit  203  or second wireless unit  204  receives the DATA packet transmitted from target terminal  102  and outputs the DATA packet to access processor  202 . 
     (Step S 215 ) Access processor  202  extracts DATA from the DATA packet and outputs the DATA to application unit  201 . Access processor  202  subtracts a size of the received DATA from a data size requested to storage medium  103 , and holds a remaining data size. That is to say, 7.5 Kbyte that is the size of the received DATA is subtracted from 45 Kbyte and 37.5 Kbyte that is the remaining data size is held. 
     The process at steps S 211  to S 215  is repeated until all data are read, namely, the remaining data size becomes 0 at step S 215 . The process at step S 211  to S 215  is executed six times in  FIG. 15 . In the DATA packet transmitted from access processor  206  to initiator terminal  101 , PSN  85  is incremented at every time of transmitting. 
     (Step S 216 ) While initiator terminal  101  reads data from storage medium  103 , access processor  202  replies to access processor  206  in target terminal  102  about a data acknowledgment (DATA-ACK) indicating that access processor  206  properly reads the data. In  FIG. 15 , access processor  202  replies to access processor  206  about the data acknowledgment after the fourth DATA is read from storage medium  103 . Access processor  202  generates an ACK packet. Access processor  202  inserts information indicating DATA-ACK into ACK  94 . Since access processor  202  receives the DATA four times, a size of the received DATA is 30 Kbyte, 15 Kbyte as a remaining size is stored in Remaining Size  96 . 
     Further, in  FIG. 15 , access processor  202  also replies to access processor  206  about DATA-ACK after the sixth DATA is read from storage medium  103 . Since access processor  202  receives the DATA six times, and a size of the received DATA is 45 Kbyte, 0 Kbyte as a remaining size is stored in Remaining Size  96 . 
     Access processor  202  outputs the generated ACK packet to first wireless unit  203  or second wireless unit  204 . 
     The process at steps S 217  to S 219  is executed with respect to step S 216  which replies the data acknowledgment after the fourth DATA and the sixth DATA are read from storage medium  103 . 
     (Step S 217 ) First wireless unit  203  or second wireless unit  204  wirelessly transmits the ACK packet requested by access processor  202  to first wireless unit  207  or second wireless unit  208 . 
     (Step S 218 ) First wireless unit  207  or second wireless unit  208  notifies access processor  206  of the ACK packet received from initiator terminal  101 . 
     (Step S 219 ) Access processor  206  interprets the received ACK packet, and confirms that the data transmitted from target terminal  102  to initiator terminal  101  is normally received by initiator terminal  101 . After this confirmation, access processor  206  discards the transmitted data held in the internal memory, not shown in the figure, in target terminal  102 . 
     A timing replying about a DATA-ACK may be determined the number of DATA-ACK in advance, or may be any timing, while initiator terminal  101  obtains the access control right. 
     (Step S 220 ) Initiator terminal  101  received desired data from storage medium  103 , executes a process for releasing the access control right to storage medium  103 . As already described, the process at steps S 122  to S 129  are executed in which initiator terminal  101  releases the access control right to storage medium  103  in target terminal  102 . 
     If the process for releasing the access control right is terminated, the access control right to storage medium  103  returns to target terminal  102 , and target terminal  102  is into a local accessible state. 
     As a result, initiator terminal  101  can read data from storage medium  103  in target terminal  102 . Since accessing to storage medium  103  is managed by switching the access control right, reading data from storage medium  103  can be exclusively controlled. 
     Further, a command transmitted from application unit  201  to storage medium  103 , and data read from storage medium  103  to application unit  201  are encapsulated and are transmitted via wireless communication. Therefore, the command can be transmitted from application unit  201  and the data can be read from storage medium  103  regardless of a halfway route of the wireless communication. As a result, even if application unit  201  and storage medium  103  are in different terminals, the transparent access is enabled as if they are in the same terminal. 
     The above description is a sequence in which initiator terminal  101  accesses to read from storage medium  103  without an error. 
     1-5. Error Recovery Operation If A Data Loss Occurs 
     An error recovery process if a data loss occurs during reading data is described below.  FIG. 16  is a diagram describing a sequence if a data loss occurs during reading data from the storage medium in the target terminal by the initiator terminal.  FIG. 17  is a diagram describing another sequence if a data loss occurs during reading data from the storage medium in the target terminal by the initiator terminal.  FIG. 16  illustrates a sequence that the wireless communication method between the initiator terminal and the target terminal is the first wireless communication method.  FIG. 17  illustrates a sequence that the wireless communication method between the initiator terminal and the target terminal is the second wireless communication method. 
     In  FIGS. 16 and 17 , a process for connecting a wireless communication at step S 100 , a process for obtaining access control right at step S 200 , a process at steps S 201  to S 210 , a process for transmitting the first DATA at steps S 211  to S 215  are as described above. 
     (Step S 301 ) The second DATA is read from storage medium  103  to access processor  206 . 
     (Step S 302 ) Access processor  206  stores the DATA read from storage medium  103  in DATA  84  of the DATA packet. In PSN  85 , 1 is incremented and 2 is stored. Access processor  206  outputs the DATA packet to first wireless unit  207  or second wireless unit  208 . 
     Further, access processor  206  holds the DATA read from storage medium  103  in the internal memory, not shown in the figure, in target terminal  102 . 
     (Step S 303 ) First wireless unit  207  or second wireless unit  208  transmits the DATA packet generated by access processor  206  to initiator terminal  101 , but the DATA packet is not transmitted and a data loss occurs. Therefore, the process at steps S 214  and S 215  is not executed. 
     (Step S 304 ) The third DATA is read from storage medium  103  to access processor  206 . 
     (Step S 305 ) Access processor  206  stores the DATA read from storage medium  103  in DATA  84  of the DATA packet. In PSN  85 , 2 is incremented and 3 is stored. Access processor  206  outputs the DATA packet to first wireless unit  207  or second wireless unit  208 . 
     Further, access processor  206  holds the DATA read from storage medium  103  in the internal memory, not shown in the figure, in target terminal  102 . 
     (Step S 306 ) First wireless unit  207  or second wireless unit  208  transmits the DATA packet generated by access processor  206  to first wireless unit  203  or second wireless unit  204 . 
     (Step S 307 ) First wireless unit  203  or second wireless unit  204  receives the DATA packet transmitted from target terminal  102 , and outputs the DATA packet to access processor  202 . Access processor  202  interprets the DATA packet. Since PSN  85  is 3, access processor  202  detects that 2 in PSN  85  is not received. 
     (Step S 308 ) Access processor  202  determines whether the wireless communication method between initiator terminal  101  and target terminal  102  is the wireless communication using the first wireless communication method or the wireless communication using the second wireless communication method. For example, at the time that the process of connecting wireless communication is executed, whether which a wireless communication system is used for the connection, is stored in a memory, not shown in the figure, in initiator terminal  101 . And access processor  202  confirms a using wireless communication method from information in the memory. 
       FIG. 16  illustrates a process if initiator terminal  101  connects with target terminal  102  via wireless communication with the first wireless communication method. 
     (Step S 401 ) Access processor  202  instructs first wireless unit  203  to transmit a Check-Status packet to first wireless unit  207  in order to confirm if an error is a data loss. 2 in PSN to be expected next is stored in Next PSN  1007  in the Check-Status packet. Further, 37.5 Kbyte that is the remaining size is stored in Remaining Size  1006 . 
     (Step S 402 ) First wireless unit  203  transmits the Check-Status packet to first wireless unit  207 . 
     (Step S 403 ) First wireless unit  207  notifies access processor  206  of the Check-Status packet. Access processor  206  interprets the Check-Status packet, and detects that the second DATA packet cannot be transmitted to initiator terminal  101 . 
     (Step S 404 ) Access processor  206  instructs first wireless unit  207  to transmit the Check-Status Response packet in response to the Check-Status Request packet. 
     4 in PSN of the DATA to be transmitted next by target terminal  102  is stored in Next PSN  1107  of the Check-Status Response packet. Since the third DATA has been transmitted, remaining 22.5 Kbyte is stored in Remaining Size  1106 . 
     (Step S 405 ) First wireless unit  207  transmits the Check-Status Response packet to first wireless unit  203 . 
     (Step S 406 ) Receiving the Check-Status Response packet, first wireless unit  203  notifies access processor  202  of the Check-Status Response packet. 
     Access processor  202  interprets the Check-Status Response packet, and confirms where a data loss occurs. In  FIG. 16 , access processor  202  confirms that the second DATA is lost. 
     Target terminal  102  restarts to transmit the second DATA, and repeats the process at steps S 211  to S 215  until all the data are read, namely, the remaining data size becomes 0. The second and third DATA are not executed a process at step S 211 , and are executed a process at step S 212  by using the DATA held in the internal memory. 
     As described above, if a data loss occurs during transmitting data and receiving data using the first wireless unit to connect with a target terminal via wireless communication with the first wireless communication method, the initiator terminal sends a packet for confirming if an error is a data loss during accessing to the storage medium, and the target terminal sends a packet indicating a data loss. As a result, if an error occurs, it is possible to avoid the error and to prepare for retransmitting data. Further, the initiator terminal retransmits data from the lost data to the target terminal. As a result, retransmitting data can be realized. 
     Further, retransmitting data is not read from storage medium  103 , the retransmitting data is obtained from the memory that holds the read data, and the retransmitting data can be more quickly. 
       FIG. 17  illustrates a process if initiator terminal  101  connects with target terminal  102  via wireless communication with the second wireless communication method. 
     (Step S 501 ) Access processor  202  instructs second wireless unit  204  to transmit the Clear Bus Handle Request packet. In the Clear Bus Handle Request packet, 0 is stored in PSN  1205 . 
     (Step S 502 ) Second wireless unit  204  transmits the Clear Bus Handle Request packet to second wireless unit  208 . 
     (Step S 503 ) Second wireless unit  208  notifies access processor  206  of the Clear Bus Handle Request packet. 
     (Step S 504 ) Receiving the Clear Bus Handle Request packet, access processor  206  instructs second wireless unit  204  to transmit a Clear Bus Handle Response packet as a response. 
     If the data read from storage medium  103  is held in the memory, access processor  206  may clear the memory or may hold the read data. 
     (Step S 505 ) Second wireless unit  208  transmits the Clear Bus Handle Response packet to second wireless unit  204 . 
     (Step S 506 ) Second wireless unit  204  notifies access processor  202  of the Clear Bus Handle Response packet. 
     (Step S 507 ) Access processor  202  interprets the Clear Bus Handle Response packet, and notifies application unit  201  of a failure of the communication with target terminal  102 . 
     (Step S 508 ) Receiving the failure of the communication, application unit  201  notifies access processor  202  of the Read CMD again. Therefore, the process at steps S 201  to S 219  is executed again. 
     As described above, if a data loss occurs during transmitting data and receiving data using the second wireless unit to connect with the target terminal via wireless communication with the second wireless communication method, the initiator terminal interrupts accessing to the storage medium, and the target terminal interrupts accessing to the recording medium. As a result, if an error occurs, it is possible to avoid the error. Further, the initiator terminal executes to read the data from the beginning. As a result, retransmitting data can be realized. 
     1-6. Effect 
     In the exemplary embodiment, initiator terminal  101  includes first wireless unit  203  configured to connect with target terminal  102  via the wireless communication with the first wireless communication method, second wireless unit  204  configured to connect with target terminal  102  via the wireless communication with the second wireless communication method different from the first wireless communication method, and access processor  202  configured to execute the access protocol process for remote access to storage medium  103  that target terminal  102  includes. If an error occurs during accessing to storage medium  103 , access processor  202  switches the access protocol process for remote access to storage medium  103  between in the wireless communication in first wireless unit  203  and in the wireless communication in the second wireless unit  204 . Further, target terminal  102  includes storage medium  103 , first wireless unit  207  configured to connect with initiator terminal  101  via the wireless communication with the first wireless communication method, second wireless unit  208  configured to connect with initiator terminal  101  via the wireless communication with the second wireless communication method different from the first wireless communication method, and access processor  206  configured to permit initiator terminal  101  to access remotely to storage medium  103  and to execute an access protocol process for the remote access to storage medium  103 . If an error occurs during accessing to storage medium  103 , access processor  206  switches the access protocol process for remote access to storage medium  103  between in the wireless communication in first wireless unit  207  and in the wireless communication in second wireless unit  208 . 
     As a result, a recovering process for reading data can be executed suitably according to a using wireless communication method if an error occurs. 
     Further, in initiator terminal  101  according to this exemplary embodiment, as the access protocol process for the error, access processor  202  transmits a packet for checking whether the error during accessing to storage medium  103  is a data loss in the wireless communication in first wireless unit  203  and access processor  202  interrupts the access protocol process to storage medium  103  in wireless communication in second wireless. In target terminal  102 , as the access protocol process for the error, access processor  206  transmits a packet indicating that an error during accessing to storage medium  103  is a data loss in the wireless communication in first wireless unit  207 , and access processor  206  interrupts the access protocol process for remote access to storage medium  103  in the wireless communication in second wireless unit  208 . 
     As a result, if an error occurs, it is possible to avoid the error according to a using wireless communication method. 
     Further, in initiator terminal  101  according to this exemplary embodiment, as the access protocol process for the error, access processor  202  retransmits data from data that the error has occurred in the wireless communication in first wireless unit  203 , and access processor  202  retransmits data from data before the error has occurred in the wireless communication in second wireless unit  204 . In target terminal  102 , as the access protocol process for the error, access processor  206  retransmits data from data that the error has occurred in the wireless communication in first wireless unit  207 , and access processor  206  retransmits data from data before the error has occurred in the wireless communication in second wireless unit  208 . 
     As a result, retransmitting data for a reading process can be realized according to a using wireless communication method. 
     Further, in first wireless unit  203  of initiator terminal  101  according to this exemplary embodiment, a communication speed is slower than a communication speed of second wireless unit  204 . In first wireless unit  207  of target terminal  102 , a communication speed is slower than a communication speed of second wireless unit  208 . 
     As a result, a process for an error can be realized according to a communication speed of a using wireless communication method. 
     Further, in initiator terminal  101  according to this exemplary embodiment, in the access protocol process for the error in access processor  202 , an amount of data retransmitted in the wireless communication in first wireless unit  203  is smaller than an amount of data retransmitted in the wireless communication in second wireless unit  204 . In target terminal  102 , in the access protocol process for the error in access processor  206 , an amount of data retransmitted in the wireless communication in first wireless unit  207  is smaller than an amount of data retransmitted in the wireless communication in second wireless unit  208 . 
     As a result, an amount of data retransmitted in a process for an error can be switched according to a communication speed of a using wireless communication method. 
     Further, in initiator terminal  101  according to this exemplary embodiment, the first wireless communication method is WiFi, and the second wireless communication method is WiGig. In target terminal  102 , the first wireless communication method is WiFi, and the second wireless communication method is WiGig. 
     As a result, a process for an error can be suitably executed according to the using wireless communication method, namely, WiFi or WiGig. 
     In this exemplary embodiment, in the case of the first wireless communication method that is comparatively slower like WiFi, if a data loss occurs, data is read from the lost data again. In the case of the second wireless communication method that is comparatively faster like WiGig, if a data loss occurs, data is read from the beginning. 
     In the case of the wireless communication method that is comparatively slower, the process for reading data is started from the beginning, a comparatively longer time a process for retransmitting is required. On the contrary, in the case of the wireless communication method that is comparatively faster, the process for retransmitting is started from a lost data, the control may be complicated. 
     Therefore, in this exemplary embodiment, a process for an error is switched according to wireless communication methods, and retransmitting data can be realized suitably according to each of wireless communication methods. 
     Further, since a transparent access is enabled between application unit  201  in initiator terminal  101  and storage medium  103  in target terminal  102 , retransmitting data to an upper layer such as application unit  201  is enabled without awareness of connecting. 
     By the above, an initiator terminal and a target terminal that include a plurality of wireless communication methods, can efficiently execute a recovering process for transmitting data and receiving data if an error occurs. 
     Second Exemplary Embodiment 
     An exemplary embodiment describes a case where an initiator terminal writes data to a storage medium in a target terminal. Since a configuration of a wireless communication system, procedures for obtaining and releasing an access control right and a data format of packets for accessing are similar to the first exemplary embodiment with reference to  FIGS. 1 to 14 . 
     2-1. Normal Operation for Writing Data 
       FIG. 18  is a diagram describing a sequence where the initiator terminal writes data in the storage medium in the target terminal. 
     (Step S 100 ) As described in the first exemplary embodiment, initiator terminal  101  and target terminal  102  connect with each other by wireless communication so that initiator terminal  101  accesses to storage medium  103 . 
     (Step S 200 ) Further, as described in the first exemplary embodiment, initiator terminal  101  executes steps from step S 101  to step S 121  that obtain an access control right to storage medium  103  in target terminal  102 . 
     If initiator terminal  101  obtains the access control right to storage medium  103 , application unit  201  in initiator terminal  101  enables to access to storage medium  103  in target terminal  102 . 
     Since target terminal  102  requests application unit  205  to release the access control right to storage medium  103 , application unit  205  in target terminal  102  cannot access to storage medium  103 . And then target terminal  102  is into a local inaccessible state. 
     (Step S 601 ) Application unit  201  in initiator terminal  101  outputs a Data Write Command (Write CMD) to access processor  202 . It is assumed that an amount of data to be written in storage medium  103  is 45 Kbyte. 
     (Step S 602 ) Receiving an instruction for writing data from application unit  201 , access processor  202  generates a CMD packet. Information indicating the Write CMD (Data Write Command) and information indicating 45 Kbyte as size information of data to be written are stored in CMD  64 . Since this is a packet for accessing that is generated first, 0 is stored as a value to be stored in PSN  65 . 
     (Step S 603 ) First wireless unit  203  or second wireless unit  204  wirelessly transmits the CMD packet to target terminal  102 . 
     (Step S 604 ) First wireless unit  207  or second wireless unit  208  notifies access processor  206  of the received wireless packet. 
     (Step S 605 ) Access processor  206  interprets the received wireless packet, and notifies storage medium  103  of the Write CMD. 
     (Step S 606 ) Receiving the Write CMD, storage medium  103  responds RES (response) to the Write CMD to access processor  206 . 
     (Step S 607 ) Receiving RES from storage medium  103 , access processor  206  generates a CMD Response packet that is a wireless packet for responding to the CMD packet, and outputs the generated packet to first wireless unit  207  or second wireless unit  208 . Information about RES is stored in CMD Response  74 , and 0 is stored in PSN  75 . 
     (Step S 608 ) First wireless unit  207  or second wireless unit  208  transmits the wireless packet generated by access processor  206  to initiator terminal  101 . 
     (Step S 609 ) Receiving the CMD Response packet, first wireless unit  203  or second wireless unit  204  in initiator terminal  101  notifies access processor  202  of the CMD Response packet. 
     (Step S 610 ) Access processor  202  interprets the CMD Response packet, and outputs the CMD Response packet to application unit  201 . Receiving the CMD Response packet, application unit  201  confirms that the Write CMD is notified to storage medium  103 . 
     (Step S 611 ) Application unit  201  divides data (DATA) written in storage medium  103  into a predetermined size, here 7.5 Kbyte and outputs the divided DATA to access processor  202 . 
     (Step S 612 ) Access processor  202  stores the DATA received from application unit  201  in DATA  84  of the DATA packet, and 0 is incremented and 1 is stored in PSN  85 . Access processor  202  outputs the DATA packet generated to first wireless unit  203  or second wireless unit  204 . 
     Access processor  202  holds the DATA received from application unit  201  in the internal memory, not shown in the figure, in initiator terminal  101 . 
     (Step S 613 ) First wireless unit  203  or second wireless unit  204  transmits the DATA packet generated by access processor  202  to first wireless unit  207  or second wireless unit  208 . 
     (Step S 614 ) First wireless unit  207  or second wireless unit  208  receives the DATA packet transmitted from initiator terminal  101  and outputs the DATA packet to access processor  206 . 
     (Step S 615 ) Access processor  206  extracts DATA from the DATA packet and outputs the DATA to storage medium  103  and writes the DATA in storage medium  103 . 
     Access processor  206  subtracts a size of the received DATA from a data size requested to be written in storage medium  103 , and holds a remaining data size. That is to say, 7.5 Kbyte that is the size of the received DATA is subtracted from 45 Kbyte and 37.5 Kbyte that is the remaining data size is held. 
     The process at steps S 611  to S 615  is repeated until all data are written, namely, the remaining data size becomes 0 at step S 615 . The process at steps S 611  to S 615  is executed six times in  FIG. 18 . In the DATA packet transmitted from access processor  202  to target terminal  102 , PSN  85  is incremented every time of transmitting. 
     (Step S 616 ) While initiator terminal  101  writes data in storage medium  103 , access processor  206  replies to access processor  202  in initiator terminal  101  about a data acknowledgment (DATA-ACK) indicating that access processor  206  properly transmits the data. In  FIG. 18 , access processor  202  replies to access processor  206  about the data acknowledgment after fourth DATA is read. Access processor  206  generates an ACK packet. Access processor  206  inserts information indicating DATA-ACK into ACK  94 . Since access processor  202  receives the DATA four times, a size of the received DATA is 30 Kbyte, 15 Kbyte as a remaining size is stored in Remaining Size  96 . 
     Further, in  FIG. 18 , access processor  202  also replies to access processor  206  about DATA-ACK after sixth DATA is read. Since access processor  202  receives the DATA six times, and a size of the received DATA is 45 Kbyte, 0 Kbyte as a remaining size is stored in Remaining Size  96 . 
     Access processor  206  outputs the ACK packet to first wireless unit  207  or second wireless unit  208 . 
     The process at steps S 617  to S 619  is executed with respect to step S 616  which replies the data acknowledgement after the fourth DATA and the sixth DATA are read. 
     (Step S 617 ) First wireless unit  207  or second wireless unit  208  wirelessly transmits the ACK packet requested by access processor  206  to first wireless unit  203  or second wireless unit  204 . 
     (Step S 618 ) First wireless unit  203  or second wireless unit  204  notifies access processor  202  of the ACK packet received from target terminal  102 . 
     (Step S 619 ) Access processor  202  interprets the received ACK packet, and confirms that the data transmitted to target terminal  102  is normally received by target terminal  102 . After this confirmation, access processor  202  discards the transmitted data held in the internal memory, not shown in the figure, in initiator terminal  101 . 
     A timing replying about a DATA-ACK may be determined the number of DATA-ACK in advance, or may be any timing. 
     (Step S 220 ) Initiator terminal  101  transmitted desired data to storage medium  103 , executes a process for releasing the access control right to storage medium  103 . As described in the first exemplary embodiment, the process at steps S 122  to S 129  are executed in which initiator terminal  101  releases the access control right to storage medium  103  in target terminal  102 . 
     By the above, initiator terminal  101  can write data in storage medium  103  in target terminal  102 . Since accessing to storage medium  103  is managed by switching the access control right, writing data in storage medium  103  can be exclusively controlled. 
     Further, a data write command transmitted from application unit  201  to storage medium  103 , and data written from storage medium  103  are encapsulated and are transmitted via wireless communication. Therefore, the command and the data can be transmitted to application unit  201  or storage medium  103  and received from application unit  201  or storage medium  103  regardless of a halfway route of the wireless communication. As a result, even if application unit  201  and storage medium  103  are in different terminals, the transparent access is enabled as if they are in the same terminal. 
     The above description is a sequence in which initiator terminal  101  accesses to write to storage medium  103 without an error. 
     2-2. Error Recovery Operation If A Data Loss Occurs 
     An error recovery process if a data loss occurs during writing data is described below.  FIG. 19  is a diagram describing a sequence if a data loss occurs during writing data in the storage medium in the target terminal by the initiator terminal.  FIG. 20  is a diagram describing another sequence if a data loss occurs during writing data in the storage medium in the target terminal by the initiator terminal.  FIG. 19  illustrates a sequence that the wireless communication method between the initiator terminal and the target terminal is the first wireless communication method.  FIG. 20  illustrates a sequence that the wireless communication method between the initiator terminal and the target terminal is the second wireless communication method. 
     In  FIGS. 19 and 20 , a process for connecting a wireless communication at step S 100 , a process for obtaining the access control right at step S 200 , a process at steps S 601  to S 610 , and a process for transmitting the first DATA at steps S 611  to S 615  are as described above. 
     (Step S 701 ) The second DATA is transmitted from application unit  201  to access processor  202 . 
     (Step S 702 ) Access processor  202  stores the DATA transmitted from application unit  201  in DATA  84  of the DATA packet. In PSN  85 , 1 is incremented, and 2 is stored. Access processor  202  outputs the DATA packet to first wireless unit  203  or second wireless unit  204 . 
     Further, access processor  202  holds the DATA transmitted from application unit  201  in the internal memory, not shown in the figure, in initiator terminal  101 . 
     (Step S 703 ) First wireless unit  203  or second wireless unit  204  transmits the DATA packet generated by access processor  202  to target terminal  102 , but the DATA packet is not transmitted and a data loss occurs. Therefore, the process at steps S 614  and S 615  is not executed. 
     (Step S 704 ) The third DATA is transmitted from application unit  201  to access processor  202 . 
     (Step S 705 ) Access processor  202  stores the DATA transmitted from application unit  201  in DATA  84  of the DATA packet. In PSN  85 , 2 is incremented and 3 is stored. Access processor  202  outputs a DATA packet to first wireless unit  203  or second wireless unit  204 . 
     Further, access processor  202  holds the DATA transmitted from application unit  201  in the internal memory, not shown in the figure, in initiator terminal  101 . 
     (Step S 706 ) First wireless unit  203  or second wireless unit  204  transmits the DATA packet generated by access processor  202  to first wireless unit  207  or second wireless unit  208 . 
     (Step S 707 ) First wireless unit  207  or second wireless unit  208  receives the DATA packet transmitted from initiator terminal  101 , and outputs the DATA packet to access processor  206 . Access processor  206  interprets the DATA packet. Since PSN  85  is 3, access processor  202  detects that 2 in PSN  85  is not received. 
     (Step S 708 ) Detecting the received data loss, access processor  206  instructs initiator terminal  101  to transmit an INT packet. 0 is stored in PSN  1405  of the INT packet, and information indicating the received data loss is stored in Event Type  1406 . 
     (Step S 709 ) First wireless unit  207  or second wireless unit  208  wirelessly transmits the INT packet to initiator terminal  101 . 
     (Step S 710 ) First wireless unit  203  or second wireless unit  204  notifies access processor  202  of the received INT packet. 
     (Step S 711 ) Access processor  202  determines whether the wireless communication method between initiator terminal  101  and target terminal  102  is the wireless communication using the first wireless communication method or the wireless communication using the second wireless communication method. For example, at the time that the process of connecting wireless communication is executed, whether which a wireless communication method is used for the connection, is stored in a memory, not shown in the figure, in initiator terminal  101 . And access processor  202  confirms the wireless communication method used for the connection in the memory. 
       FIG. 19  illustrates a process if initiator terminal  101  connects with target terminal  102  via wireless communication with the first wireless communication method. 
     (Step S 801 ) Access processor  202  instructs first wireless unit  203  to transmit a Check-Status packet to first wireless unit  207  in order to confirm if an error is a writing data loss. 4 in PSN to be transmitted next is stored in Next PSN  1007  in the Check-Status packet. Further, 22.5 Kbyte that is the remaining size is stored in Remaining Size  1006 . 
     (Step S 802 ) First wireless unit  203  transmits the Check-Status packet to first wireless unit  207 . 
     (Step S 803 ) First wireless unit  207  notifies access processor  206  of the Check-Status packet. Access processor  206  interprets the Check-Status packet, and detects that a second DATA packet cannot be received from initiator terminal  101 . 
     (Step S 804 ) Access processor  206  generates a Check-Status Response packet for initiator terminal  101 . 
     0 is stored in PSN  1105 . Access processor  206  stores 2 that is PSN expected to be received next in Next PSN  1107 . Since the first DATA is received, remaining 37.5 Kbyte is stored in Remaining Size  1106 . 
     Access processor  206  notifies first wireless unit  207  of the generated Check-Status Response packet. 
     (Step S 805 ) First wireless unit  207  wirelessly transmits the Check-Status Response packet to first wireless unit  203 . 
     (Step S 806 ) First wireless unit  203  notifies access processor  202  of the received Check-Status Response packet. 
     Access processor  202  interprets the Check-Status Response packet, and confirms where a data loss occurs. In  FIG. 19 , access processor  202  confirms that the second DATA is lost. 
     Target terminal  102  restarts to transmit the second DATA, and repeats the process at steps S 611  to S 615  until all the data are read, namely, the remaining data size becomes 0. The second and third DATA are not executed a process at step S 611 , and are executed a process at step S 612  by using the DATA held in the internal memory. 
     As described above, if a data loss occurs during transmitting data and receiving data using the first wireless unit to connect with the target terminal via wireless communication with the first wireless communication method, the initiator terminal transmits a packet for confirming if an error is a data loss during accessing to a storage medium, and the target terminal transmits a packet indicating a data loss. As a result, if an error occurs, it is possible to avoid the error and to prepare for retransmitting data. Further, the target terminal retransmits data to the initiator terminal from the lost data. As a result, retransmitting data can be realized. 
       FIG. 20  illustrates a process if initiator terminal  101  connects with target terminal  102  via wireless communication with the second wireless communication method. 
     (Step S 901 ) Access processor  202  instructs second wireless unit  204  to transmit a Clear Bus Handle Request packet. In the Clear Bus Handle Request packet, 0 is stored in PSN  1205 . 
     (Step S 902 ) Second wireless unit  204  transmits the received Clear Bus Handle Request packet to second wireless unit  208 . 
     (Step S 903 ) Second wireless unit  208  notifies access processor  206  of the Clear Bus Handle Request packet. 
     (Step S 904 ) Receiving the Clear Bus Handle Request packet, access processor  206  interrupts writing data in storage medium  103 . At the same time, access processor  206  may delete data written in storage medium  103 . Access processor  206  instructs second wireless unit  208  to transmit a Clear Bus Handle Response packet. 
     (Step S 905 ) Second wireless unit  208  transmits the Clear Bus Handle Response packet to second wireless unit  204 . 
     (Step S 906 ) Second wireless unit  204  notifies access processor  202  of the Clear Bus Handle Response packet. 
     (Step S 907 ) Access processor  202  interprets the Clear Bus Handle Response packet, and notifies application unit  201  of a failure of the communication with target terminal  102 . 
     (Step S 908 ) Receiving the failure of the communication, application unit  201  outputs the Write CMD again. As a result, the process at steps S 601  to S 619  is executed again. 
     As described above, if a data loss occurs during transmitting data and receiving data using the second wireless unit connect with a target terminal via wireless communication with the second wireless communication method, an initiator terminal interrupts accessing to the storage medium, and the target terminal interrupts accessing to the recording medium. As a result, if an error occurs, it is possible to avoid the error. Further, the initiator terminal executes to write the data from the beginning. As a result, retransmitting data can be realized. 
     2-3. Effects 
     In the exemplary embodiment, initiator terminal  101  includes first wireless unit  203  configured to connect with target terminal  102  via the wireless communication with the first wireless communication method, second wireless unit  204  configured to connect with target terminal  102  via the wireless communication with the second wireless communication method different from the first wireless communication method, and access processor  202  configured to execute the access protocol process for remote access to storage medium  103  that target terminal  102  includes. If an error occurs during accessing to storage medium  103 , access processor  202  switches the access protocol process for remote access to storage medium  103  between in the wireless communication in first wireless unit  203  and in the wireless communication in second wireless unit  204 . Further, target terminal  102  includes storage medium  103 , first wireless unit  207  configured to connect with initiator terminal  101  via the wireless communication with the first wireless communication method, second wireless unit  208  configured to connect with initiator terminal  101  via the wireless communication of second wireless communication method different from the first wireless communication method, and access processor  206  configured to permit initiator terminal  103  to access remotely to storage medium  103  and to execute an access protocol process for remote access to storage medium  103 . If an error occurs during accessing to storage medium  103 , access processor  206  switches the access protocol process for remote access to storage medium  103  between in the wireless communication in first wireless unit  207  and inthe wireless communication in second wireless unit  208 . 
     As a result, a writing process can be executed suitably according to a using wireless communication method if an error occurs. 
     Further, in initiator terminal  101  according to this exemplary embodiment, as the access protocol process for the error, access processor  202  transmits a packet for checking whether the error during accessing to storage medium  103  is a data loss in the wireless communication in first wireless unit  203  and access processor  202  interrupts the access protocol process to storage medium  103  in wireless communication in second wireless unit  204 . In target terminal  102 , as the access protocol process for the error, access processor  206  transmits a packet indicating that the error during accessing to storage medium  103  is a data loss in the wireless communication in first wireless unit  207 , and access processor  206  interrupts the access protocol process for remote access to storage medium  103  in the wireless communication in second wireless unit  208 . 
     As a result, if an error occurs, it is possible to avoid the error according to a using wireless communication method. 
     Further, in initiator terminal  101  according to this exemplary embodiment, as the access protocol process for an error, access processor  202  retransmits data from data that the error has occurred in the wireless communication in first wireless unit  203 , and access processor  202  retransmits data from data before the error has occurred in the wireless communication in second wireless unit  204 . In target terminal  102 , as the access protocol process for the error, access processor  206  retransmits data from data that the error has occurred in the wireless communication in first wireless unit  207 , and access processor  206  retransmits data from data before the error has occurred in the wireless communication in second wireless unit  208 . 
     As a result, retransmitting data for a writing process can be realized according to a using wireless communication method. 
     Further, in initiator terminal  101  according to this exemplary embodiment, a communication speed of first wireless unit  203  is slower than a communication speed of second wireless unit  204 . In target terminal  102 , a communication speed of first wireless unit  207  is slower than a communication speed of second wireless unit  208 . 
     As a result, the process for an error can be realized according to a communication speed of a using wireless communication method. 
     Further, in initiator terminal  101  according to this exemplary embodiment, in the access protocol process for the error in access processor  202 , an amount of data retransmitted in the wireless communication in first wireless unit  203  is smaller than an amount of data retransmitted in the wireless communication in second wireless unit  204 . In target terminal  102 , in the access protocol process for the error in access processor  206 , an amount of data retransmitted in the wireless communication in first wireless unit  207  is smaller than an amount of data retransmitted in the case of the wireless communication in second wireless unit  208 . 
     As a result, an amount of data retransmitted in a process for an error can be switched according to a communication speed of a using wireless communication method. 
     Further, in initiator terminal  101  according to this exemplary embodiment, the first wireless communication method is WiFi, and the second wireless communication method is WiGig. In target terminal  102 , the first wireless communication method is WiFi, and the second wireless communication method is WiGig. 
     As a result, a process for an error can be suitably executed according to the using wireless communication method, namely, WiFi or WiGig. 
     In this exemplary embodiment, in the case of the first wireless communication method that is comparatively slower like WiFi, if a data loss occurs, data is written from the lost data again. In the case of the second wireless communication method that is comparatively faster like WiGig, if a data loss occurs, data is written from the beginning. 
     In the case of the wireless communication method that is comparatively slower, the process for writing data is started from the beginning, a comparatively longer time a process for retransmitting is required. On the contrary, in the case of the wireless communication method that is comparatively faster, the process for retransmitting is started from a lost data, the control may be complicated. 
     Therefore, in this exemplary embodiment, a process for an error is switched according to wireless communication methods, and retransmitting data can be realized suitably according to each of wireless communication methods. 
     Further, since a transparent access is enabled between application unit  201  in initiator terminal  101  and storage medium  103  in target terminal  102 , retransmitting data to an upper layer such as application unit  201  is enabled without awareness of connecting. 
     By the above, an initiator terminal and a target terminal that include a plurality of wireless communication methods, can efficiently execute a recovering process for transmitting data and receiving data if an error occurs. 
     Another Exemplary Embodiment 
     The first and second exemplary embodiments describe the case where initiator terminal  101  and target terminal  102  can communicate with each other by the two wireless communication methods, but the present disclosure is not limited to it. Three or more different wireless communication methods may be provided. 
     Further, the present disclosure may be realized as a software program for controlling hardware provided to an initiator terminal and a target terminal. In this case, the software may be executed on a CPU, or may be realized on an integrated circuit. 
     INDUSTRIAL APPLICABILITY 
     The present disclosure is applicable to an apparatus or a system for realizing to access a storage medium that an accessible external terminal includes, similarly to a storage medium that an own terminal includes, by using a plurality of wireless communication methods. 
     REFERENCE MARKS IN THE DRAWINGS 
     
         
           100  wireless communication system 
           101  initiator terminal 
           102  target terminal 
           103  storage medium 
           201 ,  205  application unit 
           202 ,  206  access processor 
           203 ,  207  first wireless unit 
           204 ,  208  second wireless unit 
           41 ,  51 ,  61 ,  71 ,  81 ,  91 ,  1001 ,  1101 ,  1201 ,  1301 ,  1401  wireless packet header 
           42 ,  52 ,  62 ,  72 ,  82 ,  92 ,  1002 ,  1102 ,  1202 ,  1302 ,  1402  wireless packet payload 
           63 ,  73 ,  83 ,  93 ,  1003 ,  1103 ,  1203 ,  1303 ,  1403  control data  64  CMD 
           65 ,  75 ,  85 ,  95 ,  1005 ,  1105 ,  1205 ,  1305 ,  1405  PSN 
           74  CMD Response 
           84  DATA 
           94  ACK 
           96 ,  1006 ,  1106  Remaining Size 
           1004  Check-Status 
           1007 ,  1107  Next PSN 
           1104  Check-Status Response 
           1204  Clear Bus Handle Request 
           1304  Clear Bus Handle Response 
           1404  Interrupt 
           1406  Event Type