Patent Publication Number: US-2023155750-A1

Title: Communication device and communication system

Description:
TECHNICAL FIELD 
     The present disclosure relates to a communication device and a communication system. 
     BACKGROUND ART 
     In recent years, in order to achieve an automated vehicle driving technology, various sensors such as image sensors including a CIS (CMOS (Complementary Metal Oxide Semiconductor) Image Sensor), or a distance sensor utilizing TOF (Time of Flight) are mounted on a vehicle. In addition, it has been considered to utilize CIS (Camera Serial Interface)-2 standard for communication between these sensors and an application processor (AP: Application Processor). 
     In addition, PTL1 proposes a system that is able to reduce the number of data buses when coupling a processing device and multiple image sensors together by utilizing the CIS-2 standard. 
     CITATION LIST 
     Patent Literature 
     
         
         PTL 1: Japanese Unexamined Patent Application Publication No. 2017-211864 
       
    
     SUMMARY OF THE INVENTION 
     Incidentally, in a case where retransmission of sensor data frequently occurs due to deterioration in transmission path characteristics in communication between various sensors and an AP, there is a possibility that retransmission exceeding a transmission start timing of a next frame may occur. In such a case, there is a concern that a delay may spread to the next frame and thereafter, making it unable to maintain a desired frame rate. It is therefore desirable to provide a communication device and communication system that make it possible to maintain a desired frame rate. 
     A communication device according to a first aspect of the present disclosure includes: a determination section that determines propriety of a retransmission request on the basis of priority specified for each area within a frame in retransmission control; and a transmission section that generates retransmission request data, and makes transmission to another communication device, in a case where the retransmission request is permitted in the determination section. 
     In the communication device according to the first aspect of the present disclosure, propriety of a retransmission request is determined on the basis of priority specified for each area within a frame in retransmission control, and retransmission request data is generated and transmitted to another communication device in a case where the retransmission request is permitted. This makes it possible to prevent a delay from spreading to the next frame and thereafter. 
     A communication device according to a second aspect of the present disclosure includes: a reception section that receives retransmission request data in retransmission control; a transmission section that transmits data to another communication device; a holding section that holds, as transmitted data, the data transmitted from the transmission section; and a control section that performs retransmission control of the transmitted data on the basis of the retransmission request data. 
     In the communication device according to the second aspect of the present disclosure, upon reception of retransmission request data in retransmission control, retransmission control of transmitted data held by a holding section is performed on the basis of the retransmission request data. This makes it possible to prevent a delay from spreading to the next frame and thereafter. 
     A communication device according to a third aspect of the present disclosure includes: a determination section that determines propriety of retransmission of a transmitted packet on the basis of priority specified for each area within a frame in retransmission control; and a transmission section that transmits the transmitted packet to another communication device in a case where the retransmission of the transmitted packet is permitted in the determination section. 
     In the communication device according to the third aspect of the present disclosure, propriety of retransmission of a transmitted packet is determined on the basis of priority specified for each area within a frame in retransmission control, and the transmitted packet is transmitted to another communication device in a case where the retransmission of the transmitted packet is permitted. This makes it possible to prevent a delay from spreading to the next frame and thereafter. 
     A communication system according to a fourth aspect of the present disclosure includes a transmission device and a reception device, and the transmission device and the reception device communicate with each other. In this communication system, the reception device includes: a determination section that determines propriety of a retransmission request on the basis of priority specified for each area within a frame in retransmission control; and a first transmission section that generates retransmission request data, and makes transmission to the transmission device, in a case where the retransmission request is permitted in the determination section. In this communication system, the transmission device includes: a reception section that receives the retransmission request data; a second transmission section that transmits data to the reception device; a holding section that holds, as transmitted data, the data transmitted from the second transmission section; and a control section that performs retransmission control of the transmitted data on the basis of the retransmission request data. 
     In the communication system according to the fourth aspect of the present disclosure, propriety of a retransmission request is determined on the basis of priority specified for each area within a frame in retransmission control, and retransmission request data is generated and transmitted from a transmission device to a reception device in a case where the retransmission request is permitted. This makes it possible to prevent a delay from spreading to the next frame and thereafter. 
     A communication system according to a fifth aspect of the present disclosure includes a transmission device and a reception device, and the transmission device and the reception device communicate with each other. In this communication system, the reception device includes a first transmission section that generates retransmission request data, and makes transmission to the transmission device in retransmission control. In this communication system, the transmission device includes: a reception section that receives the retransmission request data; a second transmission section that transmits data to the reception device; a holding section that holds; as transmitted data; the data transmitted from the second transmission section; and a determination section that determines propriety of retransmission of the transmitted data on the basis of the retransmission request data and priority specified for each area within a frame. The second transmission section transmits the transmitted packet to the reception device in a case where retransmission of the transmitted packet is permitted in the determination section. 
     In the communication system according to the fifth aspect of the present disclosure, propriety of retransmission of transmitted packet is determined on the basis of priority specified for each area within a frame in retransmission control, and the transmitted packet is transmitted from a transmission device to a reception device in a case where the retransmission of the transmitted packet is permitted or not permitted. This makes it possible to prevent a delay from spreading to the next frame and thereafter. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    illustrates a schematic configuration example of a communication system according to an embodiment of the present disclosure. 
         FIG.  2    illustrates an example of a frame format and a packet format to be used in a communication pursuant to an A-PHY standard in the communication system in  FIG.  1   . 
         FIG.  3    illustrates an example of functional blocks of communication devices in a sensor unit and a control unit. 
         FIG.  4    illustrates an example of a retransmission procedure at the time when an error occurs in the communication system in  FIG.  1   . 
         FIG.  5    illustrates an example of the retransmission procedure at the time when an errors occurs in the communication system in  FIG.  1   . 
         FIG.  6    illustrates an example of the retransmission procedure at the time when an errors occurs in the communication system in  FIG.  1   . 
         FIG.  7    illustrates a functional block example of a retransmission controller in  FIG.  3   . 
         FIG.  8    illustrates a functional block example of a transmitted buffer in  FIG.  3   . 
         FIG.  9    illustrates a functional block example of a transmission scheduler in  FIG.  3   . 
         FIG.  10    illustrates an example of priority in retransmission within one frame. 
         FIG.  11    illustrates an example of settings of a register. 
         FIG.  12    illustrates an example of priority in retransmission within one frame. 
         FIG.  13    illustrates an example of settings of the register. 
         FIG.  14    illustrates an example of priority in retransmission within one frame. 
         FIG.  15    illustrates an example of settings of the register. 
         FIG.  16    illustrates an example of retransmission control in the communication system in  FIG.  1   . 
         FIG.  17    illustrates an example of the retransmission control in the communication system in  FIG.  1   . 
         FIG.  18    illustrates a modification example of the functional block of the retransmission controller in  FIG.  3   . 
         FIG.  19    illustrates a modification example of the functional block of the transmitted buffer in  FIG.  3   . 
         FIG.  20    illustrates an example of a schematic configuration of an imaging system. 
         FIG.  21    is a block diagram depicting an example of schematic configuration of a vehicle control system. 
         FIG.  22    is a diagram of assistance in explaining an example of installation positions of an outside-vehicle information detecting section and an imaging section. 
     
    
    
     MODES FOR CARRYING OUT THE INVENTION 
     Hereinafter, description is given in detail of modes for carrying out the present disclosure with reference to the drawings. The following description is given of specific examples of the present disclosure, and the present disclosure is not limited to the following embodiments. 
     &lt;Configuration Example of Communication System&gt; 
       FIG.  1    illustrates a schematic configuration example of a communication system  1  according to an embodiment of the present disclosure. The communication system  1  is mounted on a vehicle such as an automobile, for example, and includes a sensor unit  10  and a control unit  20 . The sensor unit  10  and the control unit  20  are coupled to each other via a bus  30 . A-PHY, which is standard of a physical layer developed by MIPI (Mobile Industry Processor Interface) alliance, is used for signal transmission between the sensor unit  10  and the control unit  20  via the bus  30 . The A-PHY is assumed to correspond to a transmission distance of up to 15 m, for example. 
     The sensor unit  10  includes, for example, an image sensor  11  (CIS) and a communication device  12 . The image sensor  11  is coupled to the communication device  12  via a bus pursuant to an I3C standard. The control unit  20  includes, for example, a communication device  21  and an application processor  22  (AP). The application processor  22  is coupled to the communication device  21  via a bus pursuant to the I3C standard. The communication device  12  and the communication device  21  communicate with each other pursuant to the A-PHY standard via the bus  30 . 
       FIG.  2    illustrates an example of a frame format and a packet format to be used in the communication pursuant to the A-PHY standard in the communication system  1 . 
     For example, in a frame format to transmit one frame of an image, for each line of the image, a packet storing data of the line is generated; or, for each data resulting from multiple division of the line, a packet storing the data is generated, between frame start (FS: Frame Start) indicating the start of a frame and frame end (FE: Frame End) indicating the end of the frame. In addition, for example, in a frame format to transmit one frame of an image, for every multiple pixels included in an image, a packet storing data of the multiple pixels is generated, between the frame start (FS) indicating the start of a frame and the frame end (FE) indicating the end of the frame. 
     In the packet format in the A-PHY, a packet header is disposed in the head, and a packet tail is disposed at the tail. Then, a payload storing data is disposed between the packet header and the packet tail. The packet tail includes, for example, PHY CRC-32. The packet header includes, for example, Adaptation Descriptor, Service Descriptor, Placement Descriptor, PHY2, Target Address, PHY3, Payload Length, and PHY Header CRC. The Adaptation Descriptor includes, for example, Adaptation Type Value. Examples of the Adaptation Type Value include I 2 C, I3C, GPIO, and the like. 
     Further, the packet header includes, for example, a message count (MC: Message Count) to discriminate a packet. As the message count, a transmission number is used that is incremented every time a packet is transmitted. The packet header includes, in addition to the message count, for example, various types of information required to apply the communication system  1  to a vehicle such as an automobile. The communication system  1  utilizes the message count included in the packet header to discriminate a packet individually, thereby achieving retransmission for each packet. 
     &lt;Functional Blocks of Communication Devices  12  and  21 &gt; 
       FIG.  3    illustrates an example of functional blocks of the communication devices  12  and  21 . 
     In the communication system  1 , for example, the communication device  12  and the communication device  21  transmit and receive control data to and from each other via a low-speed and bidirectional control channel. In addition, in the communication system  1 , the communication device  12  transmits, for example, sensor data such as an image to the communication device  21  via a high-speed and unidirectional data channel. 
     The communication device  12  includes, for example, a transmitter  51 , a receiver  52 , an error detection code calculator  53 , an error detector  54 , a control data discriminator  55 , a transmitted buffer  56 , a transmission scheduler  57 , an error detection code calculator  58 , and a transmitter  59 . Each block disposed on an upper side of a broken line illustrated in the communication device  12  is used for processing of transmission and reception of control data, and each block disposed on a lower side of the broken line is used for processing of transmission of sensor data. 
     The communication device  21  includes, for example, a transmitter  61 , a receiver  62 , an error detection code calculator  63 , an error detector  64 , a transmission/reception scheduler  65 , a receiver  66 , an error detector  67 , a retransmission detector  68 , and a retransmission controller  69 . Each block disposed on an upper side of a broken line illustrated in the communication device  21  is used for processing of transmission and reception of the control data, and each block disposed on a lower side of the broken line is used for processing of reception of the sensor data. 
     The transmitters  51  transmit control data supplied from the error detection code calculator  53  to the communication device  21  via the control channel. The receiver  52  receives the control data transmitted from the communication device  21  via the control channel, and supplies the received control data to the error detector  54 . The error detection code calculator  53  calculates a parity bit or CRC (Cyclic Redundancy Check), for example, as an error detection code for the control data supplied via the control data discriminator  55 . Then, the error detection code calculator  53  adds the calculated error detection code to the control data, and supplies it to the transmitter  51 . 
     The error detector  54  detects whether or not an error has occurred in the control data by using the error detection code added to the control data supplied from the receiver  52 . Then, in a case where no error has occurred in the control data, the error detector  54  supplies the control data as it is to the control data discriminator  55 , in accordance with a result of the error detection in the control data. Meanwhile, in a case where an error has occurred in the control data, the error detector  54  notifies that an error has occurred in the control data and supplies the control data to the control data discriminator  55 , in accordance with the result of the error detection in the control data. 
     For example, the control data discriminator  55  receives an input of the control data and outputs it from and to a control circuit (unillustrated) that controls the image sensor  11 , and discriminates the control data transmitted from the communication device  21 . That is, the control data discriminator  55  supplies the error detection code calculator  53  with the control data inputted from the control circuit, and outputs to the control circuit the control data supplied from the error detector  54 ; at that time, the control data discriminator  55  discriminates the control data. For example, in a case where retransmission request data described later is transmitted, as the control data supplied from the error detector  54 , from the communication device  21 , the control data discriminator  55  detects the retransmission request data, and supplies the detected retransmission request data to the transmitted buffer  56 . 
     A transmission packet to be transmitted from the communication device  12  to the communication device  21  is supplied to the transmitted buffer  56  from the transmission scheduler  57 . Then, the transmitted buffer  56  temporarily holds, as a transmitted packet (transmitted transmission data), the transmission packet (transmission data) supplied from the transmission scheduler  57 . In addition, when the retransmission request data is supplied from the control data discriminator  55 , the transmitted buffer  56  appropriately supplies, as a retransmission packet (retransmission data), a transmitted packet (transmitted transmission data) for which retransmission is requested by the retransmission request data, from among the held transmitted packets (transmitted transmission data), to the transmission scheduler  57 . It is to be noted that sensor data stored in a payload of the transmission packet is hereinafter referred to also as transmission data appropriately. The sensor data stored in the payload of the retransmission packet is hereinafter referred to also as the retransmission data appropriately. 
     Sensor data is inputted to the transmission scheduler  57  from an imaging element (unillustrated) of the image sensor  11 . Here, a packet storing, in a payload, sensor data newly inputted from the image sensor  11  to the transmission scheduler  57 , i.e., a packet storing, in the payload, sensor data which is not the retransmission data is hereinafter referred to as a new packet appropriately. In addition, the transmission scheduler  57  exchanges retransmission data transmission request and retransmission data transmission permission with the transmitted buffer  56 , as described later, and the retransmission packet is supplied from the transmitted buffer  56 . Then, the transmission scheduler  57  adjusts respective transmission timings of the new packets or the retransmission packets, and sequentially supplies the packets, as transmission packets to be transmitted from the communication device  12  to the communication device  21 , to the transmitted buffer  56  and the error detection code calculator  58 . 
     The error detection code calculator  58  calculates a parity bit or CRC, for example, as a code for error detection in the transmission packet supplied from the transmission scheduler  57 , adds the code for error detection to the packet, and supplies it to the transmitter  59 . The transmitter  59  transmits the transmission packet supplied from the error detection code calculator  58  to the communication device  21  via the data channel. 
     The transmitter  61 , the receiver  62 , the error detection code calculator  63 , and the error detector  64  are configured similarly to the transmitter  51 , the receiver  52 , the error detection code calculator  53 , and the error detector  54 , respectively. 
     For example, the transmission/reception scheduler  65  receives an input of the control data and outputs it from and to a control circuit (unillustrated) that controls the application processor  22 , and manages a schedule of the control data to be transmitted and received to and from the communication device  12 . That is, the transmission/reception scheduler  65  outputs the control data supplied from the error detector  64  to the control circuit, and supplies the control data inputted from the control circuit to the error detection code calculator  63  at a predetermined timing, for transmission to the communication device  12 . Further, when the retransmission request data is supplied from the retransmission controller  69 , the transmission/reception scheduler  65  supplies, as the control data, the retransmission request data to the error detection code calculator  63 , for transmission to the communication device  12 . 
     The receiver  66  receives the transmission packet transmitted from the communication device  12  via the data channel, and supplies the received transmission packet to the error detector  67 . 
     The error detector  67  uses the error detection code added to the transmission packet supplied from the receiver  66  to detect whether or not an error has occurred in the transmission packet. Then, the error detector  67  uses the error detection code added to the transmission packet to detect whether or not an error has occurred in the transmission packet, and outputs the sensor data stored in the payload of the transmission packet to a processing circuit in a subsequent stage. 
     Further, the error detector  67  supplies the retransmission detector  68  with an error detection result indicating a result of detection of an error having occurred in the transmission packet. For example, in a case where a detection has been made that a header error or a payload error has occurred in the transmission packet received by the communication device  21 , the error detector  67  supplies the retransmission detector  68  with an error detection result indicating that a header error or a payload error has been detected. In addition, the error detector  67  supplies the retransmission detector  68  with a message count included in an additional packet header of the transmission packet received by the communication device  21 . 
     The retransmission detector  68  performs retransmission detection processing to detect a message count of the transmission packet which fails to be received by the communication device  21  and for which retransmission is required, on the basis of the error detection result and the message count supplied from the error detector  67 . Then, the retransmission detector  68  detects the message count detected by the retransmission detection processing as a retransmission candidate message count which is a candidate to request the retransmission, and supplies the detected message count to the retransmission controller  69 . It is to be noted that the retransmission detection processing to be performed by the retransmission detector  68  is described later. 
     On the basis of the retransmission candidate message count supplied from the retransmission detector  68  and various types of information transmitted from the communication device  12 , the retransmission controller  69  performs retransmission request processing to generate the retransmission request data requesting transmission of the retransmission packet and to supply the generated retransmission request data to the transmission/reception scheduler  65 . It is to be noted that the retransmission request processing to be performed by the retransmission detector  68  is described later. 
     Here, the communication system  1  embeds various types of information required for the retransmission controller  69  to perform the retransmission request processing, in a header of the transmission packet storing the sensor data in the payload, and transmits the embedded various types of information from the communication device  12  to the communication device  21 . For example, the communication device  12  utilizes a user-defined region of the packet header specified by the CSI-2 to transmit the various types of information required for the retransmission controller  69  to perform the retransmission request processing and to supply the information to the retransmission controller  69  via the error detector  67 . 
     In accordance with the above-described configurations of the communication device  12  and the communication device  21 , utilizing the message count included in the packet header specified in the A-PHY allows for achievement of retransmission for each packet. That is, the communication device  12  and the communication device  21  perform retransmission control of MIPIA-PHY. 
     Next, schematic description is given, with reference to a flowchart illustrated in  FIG.  4   , of packet retransmission processing to be executed in the communication system  1 .  FIG.  4    illustrates a flowchart of the packet retransmission processing to be executed in the communication system  1 . 
     In step S 101 , the retransmission detector  68  of the communication device  21  performs retransmission detection processing. For example, on the basis of the error detection result and the message count supplied from the error detector  67 , the retransmission detector  68  detects a message count of the transmission packet which fails to be received by the communication device  21  and for which retransmission is required. 
     In step S 102 , the retransmission controller  69  of the communication device  21  performs the retransmission request processing. For example, the retransmission controller  69  creates a retransmission list that registers a message count of the packet for which retransmission is requested, while performing filtering by the oldest message count held by the transmitted buffer  56  from among retransmission candidate message counts supplied from the retransmission detector  68 . This allows the retransmission list to register a newer message count than the oldest message count held by the transmitted buffer  56 , among the retransmission candidate message counts. Then, the retransmission controller  69  determines propriety of a retransmission request on the basis of priority specified for each area within a frame, and generates the retransmission request data in a case where the retransmission request is permitted. Specifically, on the basis of priority specified for each area within the frame, a possible retransmission frequency specified for each area, and the number of retransmissions counted for each area, the retransmission controller  69  determines propriety of the retransmission request, and generates the retransmission request data in a case where the retransmission request is permitted. 
     In step S 103 , the control data discriminator  55  and the transmitted buffer  56  of the communication device  12  perform retransmission acceptance processing. For example, the control data discriminator  55  discriminates the retransmission request data generated by the retransmission controller  69  from among control data supplied from the error detector  54 , and supplies a packet of the retransmission request to the transmitted buffer  56 . Then, when using a message count specified by the retransmission request data to search for the transmitted packet being held, and acquires, as a search result, a transmitted packet for which retransmission is requested, the transmitted buffer  56  secures the transmitted packet as a retransmission packet. Meanwhile, in a case of failing to acquire, as a search result, the transmitted packet for which retransmission is requested, the transmitted buffer  56  discards the retransmission request data used for the search. 
     In step S 104 , the transmission scheduler  57  of the communication device  12  performs retransmission data transmission processing. For example, the transmission scheduler  57  adjusts timings of transmitting the retransmission packet secured by the transmitted buffer  56  and a new packet storing newly inputted sensor data in a payload, and transmits, as the transmission packet, the new packet or the retransmission packet appropriately. 
     As described above, it is possible for the communication system  1  to request retransmission for each transmission packet transmitted from the communication device  12  to the communication device  21  and to execute transmission of a retransmission packet for which retransmission has been requested. 
     &lt;Processing Example of Retransmission Detection Processing&gt; 
     Description is given, with reference to  FIGS.  5  and  6   , of the retransmission detection processing in the communication system  1 .  FIGS.  5  and  6    each illustrate an example of the retransmission detection processing at the time when a payload error has occurred in the communication pursuant to the A-PHY standard in the communication system  1 .  FIGS.  5  and  6    each illustrate the retransmission detection processing triggered by reception of a message count included in the header, and numbers (1 to 6) surrounded by rectangular frames illustrated in  FIG.  6    indicate message counts.  FIGS.  5  and  6    each illustrate a state in which payload errors have occurred in transmission packets of message counts  2 ,  3 , and  5 . 
     When receiving a transmission packet of the message count  2 , the communication device  21  checks a header of the transmission packet to thereby acquire the message count  2  included in the header. At this time, in a case where a payload error has occurred in the transmitted packet, the communication device  21  performs predetermined error detection processing to thereby recognize that a payload error has occurred in the transmitted packet of the message count  2 . 
     The communication device  21  determines propriety of a retransmission request on the basis of priority specified for each area within a frame, and generates retransmission request data in a case where the retransmission request is permitted. Specifically, on the basis of priority specified for each area within the frame, a possible retransmission frequency specified for each area, and the number of retransmissions counted for each area, the communication device  21  determines propriety of the retransmission request, and generates the retransmission request data in a case where the retransmission request is permitted. The communication device  21  transmits the generated retransmission request data to the communication device  12 . When receiving the retransmission request data from the communication device  21 , the communication device  12  secures, as a retransmission packet, transmitted packet corresponding to the message count  2  specified by the received retransmission request data. The communication device  12  transmits, as the transmission packet, the secured retransmission packet to the communication device  21  at a predetermined timing. Also when a payload error occurs in a transmission packet of the message count  3  or the message count  5 , processing similar to that described above is performed. In addition, also in a case where a header error has occurred in the transmission packet, processing similar to that described above is performed. 
     &lt;Configuration Example of Retransmission Controller  69  and Processing Example of Retransmission Request Processing&gt; 
     Next, description is given of a configuration example of the retransmission controller  69  and a processing example of the retransmission request processing. 
       FIG.  7    illustrates a functional block example of the retransmission controller  69 . The retransmission controller  69  includes, for example, a retransmission list holding section  71 , a retransmission frequency holding section  72 , a timing control section  73 , and a retransmission request data generation section  74 , as illustrated in  FIG.  7   . 
     Here, the retransmission controller  69  is supplied, for example, with various types of information transmitted while being embedded in the header of the transmission packet. Example of this information include information indicating the oldest message count of a transmitted packet buffer  84  ( FIG.  10   ) of the transmitted buffer  56 . 
     The retransmission list holding section  71  holds a retransmission list that registers a message count of a packet for which retransmission is requested, and manages the retransmission list. For example, the retransmission list holding section  71  manages the retransmission list to register only a newer message count than the oldest message count held by the transmitted buffer  56  among retransmission candidate message counts detected by the retransmission detector  68 . 
     For example, when message counts  98  to  102  are registered in the retransmission list in accordance with the retransmission candidate message counts, it is assumed that the message count  100  is newly supplied as the oldest message count of the transmitted packet buffer  84  of the transmitted buffer  56 . In this case, the retransmission list holding section  71  removes the message counts  98  and  99 , which are older than the message count  100 , from retransmission list to thereby cause only the message counts  100  to  102 , which are newer than the message count  100 , to be registered in the retransmission list. 
     The retransmission frequency holding section  72  includes a register ( FIG.  9   ) in which data corresponding to the concept illustrated in  FIG.  8    is stored.  FIG.  8    exemplifies a state in which priority is set for each predetermined area within a frame (one image data). In the frame (one image data) in  FIG.  8   , priority is set for each predetermined row number from the top to the bottom. For example, priority “Low” is set for each line (first area) for a row of N 1  from the top; priority “Middle” is set for each line (second area) for a row of N 2  from the line where the priority “Low” is set; priority “High” is set for each line (third area) for a row of N 3  from the line where the priority “Middle” is set; and priority “Middle” is set for each line (fourth area) for a row of N 4  from the line where the priority “High” is set. As illustrated in  FIG.  9   , for example, in the register, a possible retransmission frequency RN corresponding to the priority is set to N 5  for the first area; the possible retransmission frequency RN corresponding to the priority is set to N 6  for the second area; the possible retransmission frequency RN corresponding to the priority is set to N 7  for the third area; and the possible retransmission frequency RN corresponding to the priority is set to N 8  for the fourth area. N 5 +N 6 +N 7 +N 8  is equivalent to a possible retransmission frequency Nmax per frame (one image data). 
     From the viewpoint of the A-PHY standard, a packet is required to be sent to allow a logical layer not to exceed 97.5% of an effective transfer rate of the physical layer. Therefore, from the viewpoint of the A-PHY standard, the possible retransmission frequency Nmax is preferably set to a frequency that enables a packet to be sent within a range to allow the logical layer not to exceed 97.5% of the effective transfer rate of the physical layer. 
     In the register, various set values such as a row number LN, the possible retransmission frequency RN, an own area carry-over propriety flag CA, and a carry-over propriety flag M are specified for each area. In the register, set values for respective areas are recorded in order. For example, in the register, a set value for the first area, a set value for the second area, a set value for the third area, and a set value for the fourth area are recorded in this order. 
     Here, the row number LN is the number of pixel rows included in a relevant area. The possible retransmission frequency RN is the upper limit value of a frequency of a possible retransmission request in the relevant area. The own area carry-over propriety flag CA is a flag that, in a case where a frequency of the retransmission request (RNa) actually made in the relevant area is less than the possible retransmission frequency RN set in the register in the relevant area, sets whether or not to add a value of subtraction of RNa from RN (RN−RNa) to the possible retransmission frequency RN in the next area. The frequency (RNa) for each area of the retransmission request actually made is recorded in the register. 
     In a case where the own area carry-over propriety flag CA is “Disable”, RN−RNa is not added to the possible retransmission frequency RN in the next area. In a case where the own area carry-over propriety flag CA is “Enable”, RN−RNa is added to the possible retransmission frequency RN in the next area. In a case where RN−RNa is added to the possible retransmission frequency RN, the possible retransmission frequency RN in that area is the number greater, by RN−Rna, than the possible retransmission frequency RN set in the register. 
     The carry-over propriety flag M is a flag that sets whether or not a possible retransmission frequency (remaining frequency RNb) remaining unused is added to the possible retransmission frequency RN in the next area, regardless of whether or not RN−Rna in an area prior to the own area is added to the possible retransmission frequency RN of the own area. In a case where the carry-over propriety flag M is “Disable”, the remaining frequency RNb is not added to the possible retransmission frequency RN in the next area. In a case where the carry-over propriety flag M is “Enable”, the remaining frequency RNb is added to the possible retransmission frequency RN in the next area. 
     On the basis of the priority set for each area within the frame (one image data), the timing control section  73  performs control to retransmit, as the retransmission packet, a transmitted packet (hereinafter referred to as a “transmitted packet X”) corresponding to a message count registered in the retransmission list of the retransmission list holding section  71 . Specifically, on the basis of various settings set in the register for an area (hereinafter, referred to as an “area a”) corresponding to the message count registered in the retransmission list of the retransmission list holding section  71  as well as the frequency of the retransmission request (RNa) actually made in the area a, the timing control section  73  determines whether or not to request the transmission/reception scheduler  65  to transmit a retransmission request that causes the transmitted packet X to be retransmitted as the retransmission packet. 
     In a case where the frequency of the retransmission request (RNa) actually made in the area a does not exceed the possible retransmission frequency RN set in the register for the area a, for example, the timing control section  73  determines that the transmission request for the retransmission request is permitted, and requests the transmission/reception scheduler  65  to transmit the retransmission request that causes the transmitted packet X to be retransmitted as the retransmission packet. However, on an occasion where the carry-over propriety flag M for the area a is set to “Enable” in the register, and in a case where the frequency of the retransmission request (RNa) actually made in the area a does not exceed a frequency of addition of the remaining frequency RNb to the possible retransmission frequency RN set in the register for the area a, for example, the timing control section  73  determines that the transmission request for the retransmission request is permitted, and requests the transmission/reception scheduler  65  to transmit the retransmission request that causes the transmitted packet X to be retransmitted as the retransmission packet. Further, in a case where the frequency of the retransmission request actually made within the frame (one image data) does not exceed the possible retransmission frequency Nmax specified for the frame (one image data), for example, the timing control section  73  determines that the transmission request for the retransmission request is permitted, and requests the transmission/reception scheduler  65  to transmit the retransmission request that causes the transmitted packet X to be retransmitted as the retransmission packet. 
     In a case where a transmission permission is obtained from the transmission/reception scheduler  65  for the transmission request issued by the timing control section  73 , the retransmission request data generation section  74  generates a retransmission request data requesting retransmission of a transmitted packet corresponding to the message count read from the retransmission list of the retransmission list holding section  71 , and supplies the generated retransmission request data to the transmission/reception scheduler  65 . 
     &lt;Configuration Example of Transmitted Buffer  56  and Processing Example of Retransmission Acceptance Processing&gt; 
     Next, description is given of a configuration example of the transmitted buffer  56  and a processing example of the retransmission acceptance processing. 
       FIG.  10    illustrates a functional block example of the transmitted buffer  56 . The transmitted buffer  56  includes, for example, a write control section  81 , a transfer control section  82 , a read control section  83 , a transmitted packet buffer  84 , and a retransmission packet buffer  85 , as illustrated in  FIG.  10   . 
     The write control section  81  performs write control to write the transmitted packet supplied from the transmission scheduler  57  into the transmitted packet buffer  84 . 
     The transfer control section  82  searches for a transmitted packet of a message count specified by the retransmission request data supplied from the control data discriminator  55 , from among the transmitted packets held by the transmitted packet buffer  84 . Then, in a case of being able to acquire, as a search result, the transmitted packet of the message count specified by the retransmission request data, the transfer control section  82  performs transfer control to transfer the transmitted packet to the retransmission packet buffer  85 . It is to be noted that, in a case of not being able to acquire, as a search result, the transmitted packet of the message count specified by the retransmission request data, the transfer control section  82  discards the retransmission request data. 
     The read control section  83  manages an address of the retransmission packet held by the retransmission packet buffer  85 , and issues a retransmission data transmission request requesting the transmission scheduler  57  to transmit the retransmission packet held by the retransmission packet buffer  85 . Then, when the retransmission data transmission permission is obtained from the transmission scheduler  57 , the read control section  83  performs read control to read the retransmission packet from the retransmission packet buffer  85  and supply the read retransmission packet to the transmission scheduler  57 . The transmitted packet buffer  84  holds the transmitted packet, and the retransmission packet buffer  85  holds the retransmission packet. 
     &lt;Configuration Example of Transmission Scheduler  57 &gt; 
     Next, description is given of a configuration example of the transmission scheduler  57 . 
       FIG.  11    illustrates a functional block example of the transmission scheduler  57 . The transmission scheduler  57  includes, for example, a sensor data buffer  91 , a switching section  92 , and a control section  93 , as illustrated in  FIG.  11   . 
     The sensor data buffer  91  temporarily stores a new packet storing, in a payload, sensor data newly inputted from an imaging element (unillustrated) of the image sensor  11 , for example. For example, the sensor data buffer  91  is required because it is assumed that the new packet storing sensor data in the payload is inputted during an output of the retransmission packet supplied from the transmitted buffer  56 . 
     Under the control of the control section  93 , the switching section  92  switches between the new packet stored in the sensor data buffer  91  and the retransmission packet supplied from the transmitted buffer  56 , and outputs the packet from the transmission scheduler  57 . When acquiring the retransmission data transmission request issued from the transmitted buffer  56 , the control section  93  controls the switching of the output by the switching section  92 . When acquiring the retransmission data transmission request issued from the transmitted buffer  56 , for example, the control section  93  switches the output by the switching section  92  to side of the retransmission data. 
     Effects 
     Next, description is given of effects of the communication system  1  according to the present embodiment. 
     In the present embodiment, the propriety of the retransmission request is determined on the basis of priority specified for each area within the frame in the retransmission control, and, in a case where the retransmission request is permitted, the retransmission request data is generated and transmitted to another communication device. This makes it possible to prevent a delay from spreading to the next frame and thereafter. As a result, it is possible to maintain a desired frame rate. 
     In the present embodiment, in the register provided in the retransmission frequency holding section  72 , a possible retransmission frequency corresponding to the priority is specified for each area within the frame. This allows the propriety of the retransmission request to be determined on the basis of the possible retransmission frequency specified for each area within the frame; in a case where the retransmission request is permitted, the retransmission request data is generated and transmitted to another communication device. This makes it possible to prevent a delay from spreading to the next frame and thereafter. As a result, it is possible to maintain a desired frame rate. 
     In the present embodiment, in a case where the frequency of the retransmission requested actually made in each area does not exceed the possible retransmission frequency, determination is made that the retransmission request is permitted. This makes it possible to prevent a delay from spreading to the next frame and thereafter. As a result, it is possible to maintain a desired frame rate. 
     In the present embodiment, an area is set for each predetermined row number in the frame. Here, for example, it is assumed that the image sensor  11  is provided in a mobile body such as an automobile and that image data is obtained by imaging of the front of the mobile body by the image sensor  11 . At this time, there is a possibility that: a lower region in the image data may reflect a hood of the automobile; a middle region in the image data may reflect a road ahead of the automobile; and an upper region in the image data may reflect a sky ahead of the automobile. In this case, the importance of the middle region in the image data is higher than the importance of other regions, and thus setting an area within the frame for each predetermined row number in the frame makes it possible to secure retransmission of a region of high importance in the image data while maintaining a desired frame rate. 
     MODIFICATION EXAMPLES 
     Modification Example A 
     In the foregoing embodiment, the priority is set for each predetermined row number from the top to the bottom within the frame (one image data). However, in the foregoing embodiment, for example, the priority may be set for each predetermined column number from the left to the right within the frame (one image data), as illustrated in  FIG.  12   . In this case, in a frame format to transmit one frame of an image, for each column of the image, a packet storing data of the column is generated; or, for each data resulting from multiple division of the column, a packet storing the data is generated, between the frame start (FS) indicating the start of a frame and the frame end (FE) indicating the end of the frame. 
     In the frame (one image data) in  FIG.  12   , priority is set for each predetermined column number from the left. For example, priority “Low” is set for each column (first area) for a column of N 1  from the left; priority “High” is set for each column (second area) for a column of N 2  from the column where the priority “Low” is set; priority “Middle” is set for each column (third area) for a column of N 3  from the column where the priority “High” is set; and priority “Low” is set for each column (fourth area) for a column of N 4  from the column where the priority “Middle” is set. For example, the possible retransmission frequency RN is set to N 5  for the first area; the possible retransmission frequency RN is set to N 6  for the second area; the possible retransmission frequency RN is set to N 7  for the third area; and the possible retransmission frequency RN is set to N 8  for the fourth area. N 5 +N 6 +N 7 +N 8  is equivalent to the possible retransmission frequency Nmax per frame (one image data). 
     In the register, for example, various set values such as a column number CN, the possible retransmission frequency RN, the own area carry-over propriety flag CA, and the carry-over propriety flag M are specified for each area, as illustrated in  FIG.  13   . In the register, set values for respective areas are recorded in order. For example, in the register, a set value for the first area, a set value for the second area, a set value for the third area, and a set value for the fourth area are recorded in this order. The column number CN is the number of pixel columns included in a relevant area. 
     In the present modification example, an area is set for each predetermined column number in a frame. Here, for example, it is assumed that the image sensor  11  is provided in a mobile body such as an automobile and that image data is obtained by imaging of the left front of the mobile body by the image sensor  11 . At this time, there is a possibility that: a right end region in the image data may reflect a body and a pillar of the automobile; a middle region in the image data may reflect a road ahead of the automobile on the left; and a left end region in the image data may reflect a road shoulder or a guardrail of the road. In this case, the importance of the middle region in the image data is higher than the importance of other regions, and thus setting an area within the frame for each predetermined column number in the frame makes it possible to secure retransmission of a region of high importance in the image data while maintaining a desired frame rate. 
     Modification Example B 
     In the foregoing embodiment, the priority is set for each predetermined row number from the top to the bottom within the frame (one image data). However, in the foregoing embodiment, for example, the priority may be set for every predetermined multiple pixels in the frame (one image data), as illustrated in  FIG.  14   . In this case, in a frame format to transmit one frame of an image, for every multiple pixels included in the image, a packet storing data of the multiple pixels is generated, between the frame start (FS) indicating the start of a frame and the frame end (FE) indicating the end of the frame. 
     In the frame (one image data) in  FIG.  14   , priority is set for every predetermined multiple pixels. For example, priority “High” is set for an inverted V-shaped area (first area); priority “Middle” is set for an area (second area) adjacent to a lower portion of the area (first area) where the priority “High” is set; priority “Low” is set for each area (third area) adjacent to an upper right portion and an upper left portion of the area (first area) where the priority “High” is set; and priority “Ultra Low” is set for an area (fourth area) adjacent to an upper portion of each area (third area) where the priority “Low” is set and for an area (fourth area) adjacent to a lower portion of the area (second area) where the priority “Middle” is set. For example, the possible retransmission frequency RN is set to N 8  for the first area; the possible retransmission frequency RN is set to N 7  for the second area; the possible retransmission frequency RN is set to N 6  for the third area; and the possible retransmission frequency RN is set to N 5  for the fourth area. N 5 +N 6 +N 7 +N 8  is equivalent to the possible retransmission frequency Nmax per frame (one image data). 
     In the register, for example, various set values such as the possible retransmission frequency RN, the own area carry-over propriety flag CA, and the carry-over propriety flag M are specified for each area, as illustrated in  FIG.  15   . In the register, set values for respective areas are recorded in order. For example, in the register, a set value for the first area, a set value for the second area, a set value for the third area, and a set value for the fourth area are recorded in this order. In the register, for example, coordinates to partition each area are further specified, as illustrated in  FIG.  15   . 
     In the present modification example, an area is set for every predetermined multiple pixels in the frame. Here, for example, it is assumed that the image sensor  11  is provided in a mobile body such as an automobile and that image data is obtained by imaging of the front of the mobile body by the image sensor  11 . At this time, there is a possibility that: the inverted V-shaped region in the image data may reflect a road extending far and straight ahead of the automobile; the region adjacent to the lower portion of the inverted V-shaped region may reflect a hood of the automobile; and the region adjacent to the upper left portion and the upper right portion of the inverted V-shaped region in the image data may reflect a road shoulder or a guardrail. In this case, the importance of the inverted V-shaped region in the image data is higher than the importance of other regions, and thus setting an area within the frame for every predetermined multiple pixels in the frame makes it possible to secure retransmission of a region of high importance in the image data while maintaining a desired frame rate. 
     Modification Example C 
     In the foregoing embodiment and modification examples thereof, for example, not only the possible retransmission frequency Nmax per frame (one image data) but also a possible retransmission frequency for every multiple frames may be specified, as illustrated in  FIG.  16   . In this case, the concept of a block is introduced for every multiple frames, and the upper limit value of the retransmission frequency is specified for each block size BS. This makes it possible to prevent a delay from spreading to the next frame and thereafter. As a result, it is possible to maintain a desired frame rate. 
     Modification Example D 
     In the foregoing embodiment and modification examples thereof, for example, in a case where the frequency of the retransmission request (RNa) actually made in the relevant area exceeds the possible retransmission frequency RN set in the register in the relevant area, the retransmission controller  69  may transmit, to the transmission/reception scheduler  65 , an error flag indicating that the RNa exceeds the possible retransmission frequency RN, as illustrated in  FIGS.  16  and  17   . In this case, when the error flag is supplied from the retransmission controller  69 , the transmission/reception scheduler  65  supplies the error flag as control data to the error detection code calculator  63 , for transmission to the communication device  12 . 
     When the communication device  12  receives the error flag as the control data from the communication device  21 , the control data discriminator  55  supplies the error flag to the transmitted buffer  56 . When acquiring the error flag, the transmitted buffer  56  stops supplying the retransmission packet to the transmission scheduler  57 . When receiving the error flag, the communication device  12  further adds, to the frame end, an NG flag indicating that retransmission is not permitted. 
     In this manner, in the present modification example, in a case where RNa exceeds the possible retransmission frequency RN, an error flag indicating that RNa exceeds the possible retransmission frequency RN is transmitted to the communication device  12 . This allows transmission of the retransmission packet to be stopped, thus making it possible to prevent a delay from spreading to the next frame and thereafter. As a result, it is possible to maintain a desired frame rate. 
     Modification Example E 
     In the foregoing embodiment and modification examples thereof, for example, the retransmission frequency holding section  72  may be provided in the transmitted buffer  56  instead of being provided in the retransmission controller  69 , as illustrated in  FIGS.  18  and  19   . 
     In this case, the timing control section  73  performs control to retransmit, as the retransmission packet, a transmitted packet (hereinafter referred to as the “transmitted packet X”) corresponding to a message count registered in the retransmission list of the retransmission list holding section  71 . Specifically, the timing control section  73  requests the transmission/reception scheduler  65  to transmit a retransmission request that causes the transmitted packet X to be retransmitted as the retransmission packet. 
     On the basis of the priority set for each area within the frame (one image data), the transfer control section  82  performs control to retransmit, as the retransmission packet, a transmitted packet (hereinafter referred to as the “transmitted packet X”) corresponding to a message count specified by the retransmission request data supplied from the control data discriminator  55 . Specifically, on the basis of various settings set in the register for an area (hereinafter, referred to as the “area a”) corresponding to the message count specified by the retransmission request data supplied from the control data discriminator  55  as well as the frequency of the retransmission request (RNa) actually made in the area a, the transfer control section  82  determines whether or not to retransmit the transmitted packet X 
     In a case where the frequency of the retransmission request (RNa) actually made in the area a does not exceed the possible retransmission frequency RN set in the register for the area a, for example, the transfer control section  82  determines that retransmission of the transmitted packet X is permitted, and performs transfer control to transfer the transmitted packet X to the retransmission packet buffer  85 . However, on an occasion where the carry-over propriety flag M for the area a is set to “Enable” in the register, and in a case where the frequency of the retransmission request (RNa) actually made in the area a does not exceed a frequency of addition of the remaining frequency RNb to the possible retransmission frequency RN set in the register for the area a, for example, the transfer control section  82  determines that retransmission of the transmitted packet X is permitted, and performs transfer control to transfer the transmitted packet X to the retransmission packet buffer  85 . Further, in a case where the frequency of the retransmission request actually made within the frame (one image data) does not exceed the possible retransmission frequency Nmax set for the frame (one image data), for example, the transfer control section  82  determines that retransmission of the transmitted packet X is permitted, and performs transfer control to transfer the transmitted packet X to the retransmission packet buffer  85 . 
     In a case where the retransmission of the transmitted packet X is permitted, the transfer control section  82  searches for the transmitted packet X from among the transmitted packets held by the transmitted packet buffer  84 . Then, in a case of being able to acquire the transmitted packet X as a search result, the transfer control section  82  performs transfer control to transfer the transmitted packet X to the retransmission packet buffer  85 . It is to be noted that, in a case of not being able to acquire, as a search result, the transmitted packet of the message count specified by the retransmission request data, the transfer control section  82  discards the retransmission request data. 
     In the present modification example, the retransmission control using the retransmission frequency holding section  72  is performed in the communication device  12  (transmitted buffer  56 ). Also in such a case, it is possible to prevent a delay from spreading to the next frame and thereafter. As a result, it is possible to maintain a desired frame rate. 
     3. Application Example 
       FIG.  20    illustrates an example of a schematic configuration of an imaging system  2  including the communication system  1  according to any of the foregoing embodiment and modification examples thereof. The imaging system  2  includes, for example, an optical system  210 , a shutter device  220 , the communication system  1 , a signal processing circuit  230 , and a display unit  240 . 
     The optical system  210  forms an image of image light (incident light) from a subject on an imaging surface of the communication system  1  (image sensor  11 ). The shutter device  220  is disposed between the optical system  210  and the imaging system  2 , and controls periods of light irradiation and light blocking with respect to the communication system  1  (image sensor  11 ). The communication system  1  receives the image light (incident light) incident from the outside by the image sensor  11 , and outputs a pixel signal corresponding to the received image light (incident light) to the signal processing circuit  230 . The signal processing circuit  230  processes the pixel signal inputted from the communication system  1 , and generates picture data. The signal processing circuit  230  further generates a picture signal corresponding to the generated picture data, and outputs the generated picture signal to the display unit  240 . The display unit  240  displays a picture based on the picture signal inputted from the signal processing circuit  230 . 
     In the present application example, the communication system  1  according to any of the foregoing embodiment and modification examples thereof is applied to the imaging system  2 . This makes it possible to communicate appropriately in accordance with the capacity and speed of data to be transmitted, for example, thus making it possible to provide the imaging system  2  with high-quality captured images. 
     &lt;Example of Practical Application&gt; 
     The technology according to an embodiment of the present disclosure (present technology) is applicable to various products. For example, the technology according to an embodiment of the present disclosure may be achieved in the form of an apparatus to be mounted to a mobile body of any kind. Non-limiting examples of the mobile body may include an automobile, an electric vehicle, a hybrid electric vehicle, a motorcycle, a bicycle, any personal mobility device, an airplane, an unmanned aerial vehicle (drone), a vessel, and a robot. 
       FIG.  21    is a block diagram depicting an example of schematic configuration of a vehicle control system as an example of a mobile body control system to which the technology according to an embodiment of the present disclosure can be applied. 
     The vehicle control system  12000  includes a plurality of electronic control units connected to each other via a communication network  12001 . In the example depicted in  FIG.  21   , the vehicle control system  12000  includes a driving system control unit  12010 , a body system control unit  12020 , an outside-vehicle information detecting unit  12030 , an in-vehicle information detecting unit  12040 , and an integrated control unit  12050 . In addition, a microcomputer  12051 , a sound/image output section  12052 , and a vehicle-mounted network interface (I/F)  12053  are illustrated as a functional configuration of the integrated control unit  12050 . 
     The driving system control unit  12010  controls the operation of devices related to the driving system of the vehicle in accordance with various kinds of programs. For example, the driving system control unit  12010  functions as a control device for a driving force generating device for generating the driving force of the vehicle, such as an internal combustion engine, a driving motor, or the like, a driving force transmitting mechanism for transmitting the driving force to wheels, a steering mechanism for adjusting the steering angle of the vehicle, a braking device for generating the braking force of the vehicle, and the like. 
     The body system control unit  12020  controls the operation of various kinds of devices provided to a vehicle body in accordance with various kinds of programs. For example, the body system control unit  12020  functions as a control device for a keyless entry system, a smart key system, a power window device, or various kinds of lamps such as a headlamp, a backup lamp, a brake lamp, a turn signal, a fog lamp, or the like. In this case, radio waves transmitted from a mobile device as an alternative to a key or signals of various kinds of switches can be input to the body system control unit  12020 . The body system control unit  12020  receives these input radio waves or signals, and controls a door lock device, the power window device, the lamps, or the like of the vehicle. 
     The outside-vehicle information detecting unit  12030  detects information about the outside of the vehicle including the vehicle control system  12000 . For example, the outside-vehicle information detecting unit  12030  is connected with an imaging section  12031 . The outside-vehicle information detecting unit  12030  makes the imaging section  12031  image an image of the outside of the vehicle, and receives the imaged image. On the basis of the received image, the outside-vehicle information detecting unit  12030  may perform processing of detecting an object such as a human, a vehicle, an obstacle, a sign, a character on a road surface, or the like, or processing of detecting a distance thereto. 
     The imaging section  12031  is an optical sensor that receives light, and which outputs an electric signal corresponding to a received light amount of the light. The imaging section  12031  can output the electric signal as an image, or can output the electric signal as information about a measured distance. In addition, the light received by the imaging section  12031  may be visible light, or may be invisible light such as infrared rays or the like. 
     The in-vehicle information detecting unit  12040  detects information about the inside of the vehicle. The in-vehicle information detecting unit  12040  is, for example, connected with a driver state detecting section  12041  that detects the state of a driver. The driver state detecting section  12041 , for example, includes a camera that images the driver. On the basis of detection information input from the driver state detecting section  12041 , the in-vehicle information detecting unit  12040  may calculate a degree of fatigue of the driver or a degree of concentration of the driver, or may determine whether the driver is dozing. 
     The microcomputer  12051  can calculate a control target value for the driving force generating device, the steering mechanism, or the braking device on the basis of the information about the inside or outside of the vehicle which information is obtained by the outside-vehicle information detecting unit  12030  or the in-vehicle information detecting unit  12040 , and output a control command to the driving system control unit  12010 . For example, the microcomputer  12051  can perform cooperative control intended to implement functions of an advanced driver assistance system (ADAS) which functions include collision avoidance or shock mitigation for the vehicle, following driving based on a following distance, vehicle speed maintaining driving, a warning of collision of the vehicle, a warning of deviation of the vehicle from a lane, or the like. 
     In addition, the microcomputer  12051  can perform cooperative control intended for automatic driving, which makes the vehicle to travel automatedly without depending on the operation of the driver, or the like, by controlling the driving force generating device, the steering mechanism, the braking device, or the like on the basis of the information about the outside or inside of the vehicle which information is obtained by the outside-vehicle information detecting unit  12030  or the in-vehicle information detecting unit  12040 . 
     In addition, the microcomputer  12051  can output a control command to the body system control unit  12020  on the basis of the information about the outside of the vehicle which information is obtained by the outside-vehicle information detecting unit  12030 . For example, the microcomputer  12051  can perform cooperative control intended to prevent a glare by controlling the headlamp so as to change from a high beam to a low beam, for example, in accordance with the position of a preceding vehicle or an oncoming vehicle detected by the outside-vehicle information detecting unit  12030 . 
     The sound/image output section  12052  transmits an output signal of at least one of a sound and an image to an output device capable of visually or auditorily notifying information to an occupant of the vehicle or the outside of the vehicle. In the example of  FIG.  21   , an audio speaker  12061 , a display section  12062 , and an instrument panel  12063  are illustrated as the output device. The display section  12062  may, for example, include at least one of an on-board display and a head-up display. 
       FIG.  22    is a diagram depicting an example of the installation position of the imaging section  12031 . 
     In  FIG.  22   , the imaging section  12031  includes imaging sections  12101 ,  12102 ,  12103 ,  12104 , and  12105 . 
     The imaging sections  12101 ,  12102 ,  12103 ,  12104 , and  12105  are, for example, disposed at positions on a front nose, sideview mirrors, a rear bumper, and a back door of the vehicle  12100  as well as a position on an upper portion of a windshield within the interior of the vehicle. The imaging section  12101  provided to the front nose and the imaging section  12105  provided to the upper portion of the windshield within the interior of the vehicle obtain mainly an image of the front of the vehicle  12100 . The imaging sections  12102  and  12103  provided to the sideview mirrors obtain mainly an image of the sides of the vehicle  12100 . The imaging section  12104  provided to the rear bumper or the back door obtains mainly an image of the rear of the vehicle  12100 . The imaging section  12105  provided to the upper portion of the windshield within the interior of the vehicle is used mainly to detect a preceding vehicle, a pedestrian, an obstacle, a signal, a traffic sign, a lane, or the like. 
     Incidentally,  FIG.  22    depicts an example of photographing ranges of the imaging sections  12101  to  12104 . An imaging range  12111  represents the imaging range of the imaging section  12101  provided to the front nose. Imaging ranges  12112  and  12113  respectively represent the imaging ranges of the imaging sections  12102  and  12103  provided to the sideview mirrors. An imaging range  12114  represents the imaging range of the imaging section  12104  provided to the rear bumper or the back door. A bird&#39;s-eye image of the vehicle  12100  as viewed from above is obtained by superimposing image data imaged by the imaging sections  12101  to  12104 , for example. 
     At least one of the imaging sections  12101  to  12104  may have a function of obtaining distance information. For example, at least one of the imaging sections  12101  to  12104  may be a stereo camera constituted of a plurality of imaging elements, or may be an imaging element having pixels for phase difference detection. 
     For example, the microcomputer  12051  can determine a distance to each three-dimensional object within the imaging ranges  12111  to  12114  and a temporal change in the distance (relative speed with respect to the vehicle  12100 ) on the basis of the distance information obtained from the imaging sections  12101  to  12104 , and thereby extract, as a preceding vehicle, a nearest three-dimensional object in particular that is present on a traveling path of the vehicle  12100  and which travels in substantially the same direction as the vehicle  12100  at a predetermined speed (for example, equal to or more than 0 km/hour). Further, the microcomputer  12051  can set a following distance to be maintained in front of a preceding vehicle in advance, and perform automatic brake control (including following stop control), automatic acceleration control (including following start control), or the like. It is thus possible to perform cooperative control intended for automatic driving that makes the vehicle travel automatedly without depending on the operation of the driver or the like. 
     For example, the microcomputer  12051  can classify three-dimensional object data on three-dimensional objects into three-dimensional object data of a two-wheeled vehicle, a standard-sized vehicle, a large-sized vehicle, a pedestrian, a utility pole, and other three-dimensional objects on the basis of the distance information obtained from the imaging sections  12101  to  12104 , extract the classified three-dimensional object data, and use the extracted three-dimensional object data for automatic avoidance of an obstacle. For example, the microcomputer  12051  identifies obstacles around the vehicle  12100  as obstacles that the driver of the vehicle  12100  can recognize visually and obstacles that are difficult for the driver of the vehicle  12100  to recognize visually. Then, the microcomputer  12051  determines a collision risk indicating a risk of collision with each obstacle. In a situation in which the collision risk is equal to or higher than a set value and there is thus a possibility of collision, the microcomputer  12051  outputs a warning to the driver via the audio speaker  12061  or the display section  12062 , and performs forced deceleration or avoidance steering via the driving system control unit  12010 . The microcomputer  12051  can thereby assist in driving to avoid collision. 
     At least one of the imaging sections  12101  to  12104  may be an infrared camera that detects infrared rays. The microcomputer  12051  can, for example, recognize a pedestrian by determining whether or not there is a pedestrian in imaged images of the imaging sections  12101  to  12104 . Such recognition of a pedestrian is, for example, performed by a procedure of extracting characteristic points in the imaged images of the imaging sections  12101  to  12104  as infrared cameras and a procedure of determining whether or not it is the pedestrian by performing pattern matching processing on a series of characteristic points representing the contour of the object. When the microcomputer  12051  determines that there is a pedestrian in the imaged images of the imaging sections  12101  to  12104 , and thus recognizes the pedestrian, the sound/image output section  12052  controls the display section  12062  so that a square contour line for emphasis is displayed so as to be superimposed on the recognized pedestrian. The sound/image output section  12052  may also control the display section  12062  so that an icon or the like representing the pedestrian is displayed at a desired position. 
     The description has been given hereinabove of one example of the mobile body control system, to which the technology according to an embodiment of the present disclosure may be applied. The technology according to an embodiment of the present disclosure may be applied to the imaging section  12031  among components of the configuration described above. Specifically, the imaging system  2  is applicable to the imaging section  12031 . The application of the technology according to an embodiment of the present disclosure to the imaging section  12031  allows for a high-quality captured image, thus making it possible to perform highly accurate control utilizing the captured image in the mobile body control system. 
     Although the description has been given hereinabove of the present disclosure with reference to the embodiment, the modification examples, and the application example, the present disclosure is not limited to the foregoing embodiment, etc., and may be modified in a wide variety of ways. It is to be noted that the effects described herein are merely illustrative. The effects of the present disclosure are not limited to those described herein. The present disclosure may have other effects than those described herein. 
     In addition, the present disclosure may also have the following configurations. 
     (1) 
     A communication device including: 
     a determination section that determines propriety of a retransmission request on a basis of priority specified for each area within a frame in retransmission control; and 
     a transmission section that generates retransmission request data, and makes transmission to another communication device, in a case where the retransmission request is permitted in the determination section. 
     (2) 
     The communication device according to (1), further including a register in which a possible retransmission frequency corresponding to the priority is set for each area. 
     (3) 
     The communication device according to (2), in which the determination section determines that the retransmission request is permitted in a case where a frequency of the retransmission request actually made in each area does not exceed the possible retransmission frequency. 
     (4) 
     The communication device according to (2), in which the determination section determines that the retransmission request is permitted in a case where a frequency of the retransmission request actually made within the frame does not exceed the possible retransmission frequency specified for the frame. 
     (5) 
     The communication device according to any one of (1) to (4), in which the area is set for each predetermined row number in the frame. 
     (6) 
     The communication device according to any one of (1) to (4), in which the area is set for each predetermined column number in the frame. 
     (7) 
     The communication device according to any one of (1) to (4), in which the area is set for every predetermined multiple pixels in the frame. 
     (8) 
     The communication device according to any one of (1) to (7), in which the communication device performs retransmission control of MIPIA-PHY. 
     (9) 
     A communication device including: 
     a reception section that receives retransmission request data in retransmission control; 
     a transmission section that transmits data to another communication device; 
     a holding section that holds, as transmitted data, the data transmitted from the transmission section; and 
     a control section that performs retransmission control of the transmitted data on a basis of the retransmission request data. 
     (10) 
     A communication device including: 
     a determination section that determines propriety of retransmission of a transmitted packet on a basis of priority specified for each area within a frame in retransmission control; and 
     a transmission section that transmits the transmitted packet to another communication device in a case where the retransmission of the transmitted packet is permitted in the determination section. 
     (11) 
     The communication device according to (10), in which the determination section determines that the retransmission of the transmitted packet is permitted in a case where a frequency of a retransmission request for each area actually made does not exceed a possible retransmission frequency specified for each area. 
     (12) 
     The communication device according to (10) or (11), in which the determination section determines that the retransmission of the transmitted packet is permitted in a case where a frequency of a retransmission request actually made within the frame does not exceed a possible retransmission frequency specified for the frame. 
     (13) 
     A communication system including: 
     a transmission device; and 
     a reception device, the transmission device and the reception device communicating with each other, 
     the reception device including
         a determination section that determines propriety of a retransmission request on a basis of priority specified for each area within a frame in retransmission control, and   a first transmission section that generates retransmission request data, and makes transmission to the transmission device, in a case where the retransmission request is permitted in the determination section, and       

     the transmission device including
         a reception section that receives the retransmission request data,   a second transmission section that transmits data to the reception device,   a holding section that holds, as transmitted data, the data transmitted from the second transmission section, and   a control section that performs retransmission control of the transmitted data on a basis of the retransmission request data.
 
(14)
       

     A communication system including: 
     a transmission device; and 
     a reception device, the transmission device and the reception device communicating with each other, 
     the reception device including
         a first transmission section that generates retransmission request data, and makes transmission to the transmission device in retransmission control, and       

     the transmission device including
         a reception section that receives the retransmission request data,   a second transmission section that transmits data to the reception device,   a holding section that holds, as transmitted data, the data transmitted from the second transmission section, and   a determination section that determines propriety of retransmission of the transmitted data on a basis of the retransmission request data and priority specified for each area within a frame, in which       

     the second transmission section transmits the transmitted packet to the reception device in a case where retransmission of the transmitted packet is permitted in the determination section. 
     According to the communication device of a first aspect of the present disclosure, propriety of a retransmission request is determined on the basis of priority specified for each area within a frame in retransmission control, and retransmission request data is generated and transmitted to another communication device in a case where the retransmission request is permitted, thus making it possible to prevent a delay from spreading to the next frame and thereafter. As a result, it is possible to maintain a desired frame rate. 
     According to a communication device of a second aspect of the present disclosure, upon reception of retransmission request data in retransmission control, retransmission control of transmitted data held by a holding section is performed on the basis of the retransmission request data, thus making it possible to prevent a delay from spreading to the next frame and thereafter. As a result, it is possible to maintain a desired frame rate. 
     According to a communication device of a third aspect of the present disclosure, propriety of retransmission of a transmitted packet is determined on the basis of priority specified for each area within a frame in retransmission control, and the transmitted packet is transmitted to another communication device in a case where the retransmission of the transmitted packet is permitted, thus making it possible to prevent a delay from spreading to the next frame and thereafter. As a result, it is possible to maintain a desired frame rate. 
     According to a communication system of a fourth aspect of the present disclosure, propriety of a retransmission request is determined on the basis of priority specified for each area within a frame in retransmission control, and retransmission request data is generated and transmitted from a transmission device to a reception device in a case where the retransmission request is permitted, thus making it possible to prevent a delay from spreading to the next frame and thereafter. As a result, it is possible to maintain a desired frame rate. 
     According to a communication system of a fifth aspect of the present disclosure, propriety of retransmission of transmitted packet is determined on the basis of priority specified for each area within a frame in retransmission control, and the transmitted packet is transmitted from a transmission device to a reception device in a case where the retransmission of the transmitted packet is permitted or not permitted, thus making it possible to prevent a delay from spreading to the next frame and thereafter. As a result, it is possible to maintain a desired frame rate. 
     This application claims the benefit of Japanese Priority Patent Application JP2020-056057 filed with the Japan Patent Office on Mar. 26, 2020, the entire contents of which are incorporated herein by reference. 
     It should be understood by those skilled in the art that various modifications, combinations, sub-combinations, and alterations may occur depending on design requirements and other factors insofar as they are within the scope of the appended claims or the equivalents thereof.