Abstract:
Disclosed is an information distribution system capable of distributing high quality data without lowering throughput of a network even in a communication area where many movable bodies could be traveling. The information distribution system comprises a radio transmission unit for transmitting data signals at a transmission rate being variable; a detection unit for detecting speed control information for a mobile object having a receiver for receiving data signals transmitted without wires from the radio transmission unit; and a transmission rate setting unit for changing the transmission rate of the data signals according to the speed control information.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to an information distribution system for distributing information through a radio transmission channel, and to related technology thereof.  
         [0003]     2. Description of the Related Art  
         [0004]     As a system for distributing information to a mobile object or movable body, such as a vehicle, via a radio transmission line, VICS (Vehicle Information and Communication System), which distributes traffic information including road conditions to the vehicle of a user using radio beacons, light beacons or FM multiple broadcasting, is known. Also along with the spread of cellular phones, the Internet and wireless LAN (Local Area Network), systems for distributing information to many unspecified movable bodies in a relatively narrow communication area using a wireless LAN conforming to IEEE 802.11 (Institute of Electrical and Electronics Engineers standard 802.11) have been proposed. This type of system is disclosed in Japanese Patent Kokai No. 2003-317187 and Japanese Patent Kokai No. 2001-184595.  
         [0005]     Compared with an unmoving receive terminal, the quality of distribution information received by a receive terminal when moving generally tends to deteriorate more easily, and particularly when a transmission terminal distributes data to a moving receive terminal at a high transmission rate, the probability of the occurrence of receive errors is high. So in an auto fall back system, the receive terminal replies with an ACK (Acknowledgment) signal when receiving data from the transmission terminal succeeds. The transmission terminal transmits data at a relatively high transmission rate when ACK signals are being received from the receive terminal, and retransmits data at a low transmission rate if the ACK signal is not received after a predetermined time elapses so as to assure the reliability of information distribution.  
         [0006]     Broadcasting data is appropriate to distribute data to many unspecified receive terminals. However if an ACK signal is demanded for all the receive terminals to assure the reliability of the broadcast communication, the throughput of the network tends to drop because of the increase in the retry count. If an ACK signal is not demanded for the receive terminals, however, the transmission terminal cannot resend the data at a low transmission rate appropriate for the transmission line, even if a receive error occurs, so the reliability of the information distribution cannot be completely assured. This problem in particular tends to occur when broadcast communication is performed in a communication area where many receive terminals are moving. This is also true for the case when data is multicasted to a plurality of receive terminals, that is when the same data is transmitted specifying the addresses of a plurality of receive terminals.  
       SUMMARY OF THE INVENTION  
       [0007]     In view of the foregoing, it is an object of the present invention to provide an information distribution system for enabling high quality data distribution without dropping the throughput of the network even in a communication area where a mobile object would be traveling.  
         [0008]     According to one aspect of the present invention, an information distribution system is provided. This information distribution system comprises: a radio transmission unit for transmitting data signals at a transmission rate being variable; a detection unit for detecting speed control information for a mobile object having a receiver for receiving data signals transmitted without wires from the radio transmission unit; and a transmission rate setting unit for changing the transmission rate of the data signals according to the speed control information.  
         [0009]     According to another aspect of the present invention, an information distribution system is provided. This information distribution system comprises: a radio transmission unit for transmitting data signals at a transmission rate being variable; a detection unit for detecting traffic congestion information for a mobile object having a receiver for receiving data signals transmitted without wires from the radio transmission unit; and a transmission rate setting unit for setting a transmission rate of the data signals according to the traffic congestion information.  
         [0010]     Further features of the invention, its nature and various advantages will be more apparent from the accompanying drawings and the following detailed description of the preferred embodiments. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]      FIG. 1  is a block diagram depicting a general configuration of the information distribution system according to the first embodiment of the present invention;  
         [0012]      FIG. 2  is a table showing the combination of the transmission rate, modulation method and encoding rate;  
         [0013]      FIG. 3  is a diagram depicting a general structure of a frame signal according to the IEEE 802.11 standard;  
         [0014]      FIG. 4A  is a table showing the transmission rate according to the color of the traffic light, and  FIG. 4B  is a table showing the transmission rate according to the level of traffic congestion;  
         [0015]      FIG. 5  is a flow chart depicting an example of the processing procedure by the controller of the data distribution device of the first embodiment;  
         [0016]      FIG. 6  is a block diagram depicting a general configuration of the information distribution system according to a variant form of the first embodiment;  
         [0017]      FIG. 7  is a block diagram depicting a general configuration of the information distribution system according to another variant form of the first embodiment;  
         [0018]      FIG. 8  is a block diagram depicting a general configuration of the information distribution system according to the second embodiment of the present invention;  
         [0019]      FIG. 9  is a timing chart to indicate the color of the traffic light;  
         [0020]      FIG. 10  is a timing chart to indicate the color of the traffic light;  
         [0021]      FIG. 11  is a flow chart depicting an example of the processing procedure by the controller of the data distribution device according to the second embodiment;  
         [0022]      FIG. 12  is a flow chart depicting an example of the processing procedure by the controller of the data distribution device according to the second embodiment;  
         [0023]      FIG. 13  is a block diagram depicting a general configuration of the information distribution system according to the third embodiment of the present invention;  
         [0024]      FIG. 14  is a flow chart depicting an example of the processing procedure by the controller of the data distribution device according to the third embodiment;  
         [0025]      FIG. 15  is a block diagram depicting a general configuration of the information distribution system according to the fourth embodiment of the present invention; and  
         [0026]      FIG. 16  is a flow chart depicting an example of the processing procedure by the controller of the data distribution device according to the fourth embodiment. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0027]     Embodiments of the present invention will now be described with reference to the drawings.  
       1. First Embodiment  
       [0028]      FIG. 1  is a block diagram depicting a general configuration of the information distribution system  1 A according to the first embodiment of the present invention. The information distribution system  1 A comprises a data distribution device  2 A, a traffic control unit  12  and an information distribution server (information distribution source)  13 . The data distribution device  2 A further comprises a controller  11 A, a radio transmission unit (access point)  9 , communication interfaces (I/F)  6  and  7  and a distribution data storage unit  8 , and the controller  11 A further comprises a communication control unit  20 , a transmission rate setting unit  21  and a detection unit  22 A.  
         [0029]     The controller  11 A is an integrated circuit comprising a microprocessor, a non-volatile memory for storing programs, a RAM (Random Access Memory), an internal bus and an input/output interface. The communication control unit  20 , the transmission rate setting unit  21  and the detection unit  22 A may be constructed by a program or a series of instructions executed by the microprocessor, or may be built-in to the controller  11 A as hardware.  
         [0030]     The traffic control unit  12  generates the speed control information (or speed regulation information) according to a predetermined time schedule, supplies this to the traffic light  15 , and remote-control the traffic light  15 . The traffic light  15  has a control box  18 , and the control box  18  controls the ON/OFF of the lights of the traffic light  15  based on the speed control information supplied by the traffic control unit  12 . The controller  11 A and the traffic control unit  12  are inter-connected via an electric communication line (not illustrated), such as a packet communication network. The traffic light  15  is positioned in the vicinity of the traveling path  17 , and is traffic control means (or traffic regulation means) for controlling the speed of the movable bodies (vehicles)  10   1 ,  10   2 , . . . ,  10   N  (N is an integer of 2 or higher) on the traveling path  17 . The traffic light  15 , which comprises a red light source  16 R, a yellow light source  16 Y and a blue light source  16 B, turns ON or OFF the red light source  16 R, yellow light source  16 Y and blue light source  16 B individually according to the speed control information provided by the traffic control unit  12 . ON of the red light source  16 R indicates “Stop”, ON of the yellow light source  16 Y indicates “Go with caution”, and ON of the blue light source  16 B indicates “Go”. The blue light source  16 B may include an arrow shaped area which indicates “Go”. Or the blue light source  16 B may have areas to indicate “Right turn possible” or “Left turn possible”, in addition to an area to indicate “Go”. For example, in some cases, an area to indicate “Right turn possible” of the blue light source  16 B is turned ON, and an area to indicate “Go” is turned OFF at the same time with ON of the red light source  16 R.  
         [0031]     In the present embodiment, the traffic light for vehicles  15  is used, but the present invention is not limited to the traffic light for vehicles  15 , and a traffic light or a bar for controlling the traveling speed of a pedestrian or a train, or a speaker which emits sound to control the traveling of the pedestrian may be used.  
         [0032]     The movable bodies  10   1 , . . . ,  10   N  have the receivers (stations) ST 1 , . . . , ST N  respectively. The radio transmission unit  9  has a function to perform radio communication with the receivers ST 1 , . . . , ST N . The communication area is a relatively narrow area with a radius of several m-several hundred m. The radio communication standard to be used would be a known wireless LAN standard, such as IEEE 802.11 (Institute of Electrical and Electronics Engineers Standard 802.11), Bluetooth®, HIPERLAN/2 (HIgh PErformance Radio Local Area Network Type 2) or HiSWAN (High Speed Wireless Access System). IEEE 802.11 is a standard which includes IEEE 802.11a, IEEE 802.11b and IEEE 802.11g. For the communication medium, visible light, infrared (IR) or electro-magnetic waves on a radio frequency band can be used. In the present embodiment, the movable bodies are automobiles  10   1 , . . . ,  10   N , but the present invention is not limited to this, and a mobile object may be a train, a bicycle or a pedestrian.  
         [0033]     The information distribution system  1 A of the present embodiment distributes information to a plurality of movable bodies  10   1 ,  10   2 , . . . ,  10   N , but the information distribution system according to the present invention is not limited to a plurality of movable bodies but may distribute information to a single mobile object. This is the same for the later mentioned other embodiments.  
         [0034]     The information distribution server  13  supplies distribution information, such as traffic information on an area around the control point (or regulation point) where the speed control of the movable bodies  10   1 , . . . ,  10   N  is performed, that is an area around the location  17 P where the traffic light  15  is positioned, and destination guide information, advertisement information or news information, to the controller  11 A. The controller  11 A and the information distribution server  13  are inter-connected via an electric communication line (not illustrated), such as a packet communication network, and the controller  11 A stores the distribution information received by the communication interface  7  to the distribution data storage unit  8 . The traffic information is, for example, real-time road traffic information required by drivers of the movable bodies  10   1 , . . . ,  10   N  such as traffic congestion statuses, congestion statuses, block traveling times, construction control points and traffic accidents or problem statuses of vehicles. The destination guide information is, for example, store information, such as restaurants, hospitals or banks, or sightseeing information on amusement parks and tourist resorts.  
         [0035]     The communication control unit  20  of the controller  11 A reads the distribution information from the distribution data storage unit  8 , transfers it to the radio transmission unit  9 , and controls the radio transmission unit  9  so as to broadcast or multicast the same distribution information at predetermined timings. The radio transmission unit  9  has a function to transmit the data signals at a variable transmission rate by switching the digital modulation method and encoding rate of the data signals to be transmitted without wires. If the radio transmission unit  9  conforms to the IEEE 802.11a standard, the data signals can be transmitted by one of the combinations of the transmission rate (bit rate), modulation method and encoding rate shown in  FIG. 2 . The encoding rate is defined by the ratio of the bit count of the original data to the bit count after encoding. According to  FIG. 2 , the radio transmission unit  9  performs digital modulation on the data signals by one of BPSK (Binary Phase Shift Keying), QPSK (Quadrature Phase Shift Keying), 16-QAM (16-Quadrature Amplitude Modulation) and 64-QAM (64-Quadrature Amplitude Modulation), and transmission data can be acquired by sampling the modulation signals acquired by this result at one of the encoding rates  ½, 3/4  or ⅔.  FIG. 3  is a diagram depicting a general structure of a frame signal according to the IEEE 802.11a standard. According to  FIG. 3 , the frame signal  30  to be transmitted from the radio transmission unit  9  is comprised of a preamble section  31 , header section  32  and data section  33 . The header section  32  is transmitted at a fixed transmission rate of 6 Mbps, and the transmission rate of the data section  33  and data length are included in the header section  32 . The data section  33  constituting the transmission data is transmitted at a transmission rate specified by the header section  32 . Therefore the receivers (stations) ST 1 , . . . , ST N  can receive the data section  33  by reading the header section  32  of the received frame signal  30 .  
         [0036]     The transmission rate setting unit  21  has a function to change the transmission rate (bit rate) of the data signals to be transmitted without wires according to the later mentioned speed control information and traffic congestion information. The transmission rate setting unit  21  has a predetermined transmission rate table, as shown in  FIG. 4 , and can change the transmission rate of the data signals referring to this transmission rate table. According to the transmission rate table in  FIG. 4A , low speed transmission rate, 6 Mbps or 9 Mbps, is selected when the color of the traffic light  15  ( FIG. 1 ) is blue, mid-transmission rate 12 Mbps is selected when the traffic light color is yellow, and high transmission rate 18 Mbps-45 Mbps is selected when the traffic light color is red. According to the transmission rate table in  FIG. 4B , the low transmission rate, 6 Mbps or 9 Mbps, is selected when the level of traffic congestion of the traveling path  17 , that is the level of traffic volume of the movable bodies  10   1 , . . . ,  10   N  per unit time, is low, the mid-transmission rate 12 Mbps is selected when the level of traffic congestion is medium, and the high transmission rate, 18 Mbps-45 Mbps, is selected when the level of traffic congestion is high.  
         [0037]     In the transmission rate table shown in  FIG. 4A , three levels of transmission rates corresponding to the respective colors of the traffic light  15 , blue, yellow and red respectively are set, but instead a transmission rate table where two levels of transmission rates corresponding to red and another color of the traffic light  15  or two levels of transmission rates corresponding to blue and another color of the traffic light  15  are set, may be used.  
         [0038]     The detection unit  22 A has a function to detect the speed control information for the movable bodies  10   1 , . . . ,  10   N  having the receivers ST 1 , . . . , ST N  respectively. Specifically, responding to the transmission request from the data distribution device  2 A, the traffic control unit  12  transmits information the same as the speed control information to be supplied to the traffic light  15  to the data distribution device  2 A via an electric communication line, and the detection unit  22 A detects the speed control information received by the communication interface  6 , and sends this to the transmission rate setting unit  21 . The traffic control unit  12  also has a function to transmit [information] periodically or according to a predetermined time schedule without waiting for the transmission request from the information distribution unit  2 A. As described above, the transmission rate setting unit  21  determines the transmission rate of the data signals according to the speed control information.  
         [0039]     Operations of the information distribution system  1 A having the above configuration will be described below.  FIG. 5  is a flow chart depicting an example of the processing procedure by the controller  11 A of the information description device  2 A.  
         [0040]     In  FIG. 5  the detection unit  22 A first requests the speed control information to the traffic control unit  12  according to a predetermined time schedule (step S 1 ). Then the detection unit  22 A judges whether the speed control information transmitted from the traffic control unit  12  was detected (step S 2 ). After transmitting the transmission request to the traffic control unit  12  (step S 1 ), the detection unit  22 A stands by until the speed control information transmitted from the traffic control unit  12  is detected. When the speed control information cannot be detected, even if a predetermined time has elapsed, the detection unit  22 A may repeatedly request the speed control information to the traffic control unit  12 .  
         [0041]     When the traffic control unit  12  transmits the speed control information responding to the transmission request and the detection unit  22 A detects the speed control information received by the communication interface  6 , the detection unit  22 A sends the speed control information to the transmission rate setting unit  21 , and the transmission rate setting unit  21  sets the transmission rate according to the speed control information referring to the transmission rate table, and stores this in a register of the communication control unit  20  (steps S 4 -S 7 B). In other words, the speed control information includes the information on the colors of the traffic light  15 , the transmission rate setting unit  21  judges whether the color of the traffic light  15  is blue, yellow or red (step S 4 ), refers to the transmission rate table shown in  FIG. 4A  (steps S 4 , S 6 , S 8 ), and sets the transmission rate of the data signals to the bit rate according to the determined traffic light color (steps S 5 , S 7 , S 9 ). If the color of the traffic light  15  is blue, the transmission rate setting unit  21  judges that the movable bodies  10   1 , . . . ,  10   N  are moving at a relatively high speed, and sets the transmission rate to the low bit rate (6 Mbps or 9 Mbps). If the traffic light color is yellow, the transmission rate setting unit  21  judges that the movable bodies  10   1 , . . . ,  10   N  are moving at a relatively slow speed, and sets the transmission rate to the mid-bit rate (12 Mbps). If the traffic light color is red, the transmission rate setting unit  21  judges that the movable bodies  10   1 , . . . ,  10   N  are stopping, and sets the transmission rate to a high bit rate (one of 18 Mbps-54 Mbps).  
         [0042]     After the transmission rate is set in steps S 5 , S 7  and S 9 , the processing after step S 1  is repeatedly executed. The communication control unit  20  controls the radio transmission unit  9  so as to transmit the data signals at the transmission rate stored in a register. If the transmission rate stored in a register is updated, the communication control unit  20  controls the radio transmission unit  9  so as to transmit the data signals at a new transmission rate. The radio transmission unit  9  performs digital modulation on the distribution information stored in the distribution data storage unit  8  according to a specified modulation method, and multicasts or broadcasts it at a specified encoding rate.  
         [0043]     As described above, the information distribution system  1 A enables high quality data distribution without dropping the throughput of a wireless LAN, since data is transmitted at a transmission rate appropriate for the traveling speed of the movable bodies  10   1 , . . . ,  10   N  based on the speed control information supplied from the traffic control unit  12 . Real-time transmission rate control is also possible by a relatively simple algorithm. Therefore even in a relatively narrow communication area of a wireless LAN, highly reliable information distribution is possible.  
         [0044]     Now a variant form of the information distribution system  1 A will be described.  FIG. 6  and  FIG. 7  are block diagrams depicting a general configuration of the information distribution systems  1 B and  1 C according to the first and second variant forms respectively. In each variant form, a composing element denoted with a same reference numeral as a composing element of the information distribution system  1 A has a same function as this composing element, therefore detailed description is omitted.  
         [0045]     In the information distribution system  1 B of the first variant form ( FIG. 6 ), speed control information to the data distribution device  2 B is not supplied from the traffic control unit  12 , but from the control box  18  of the traffic light  15 . The detection unit  22 B of the controller  11 B detects the speed control information received by the communication interface  6 , and sends it to the transmission rate setting unit  21 . The control box  18  of the traffic light  15  and the communication interface  6  may be inter-connected via a cable, or the control box  18  and the communication interface  6  may be inter-connected via a communication line using Internet technology by linking an IP address (Internet Protocol address) conforming to IPv4 (Internet Protocol version 4) or IPv6 (Internet Protocol version 6) to the control box  18 .  
         [0046]     In the present embodiment, the data distribution device  2 B does not receive speed control information from a remote traffic control unit  12  via an electric communication line, but receives speed control information from a traffic light  15  at a control point, so the detection unit  22 B can acquire information on an ON and OFF of the traffic light  15  with more certainty. For example, even when the traffic light  15  autonomously operates without being controlled by the traffic control unit  12 , the transmission rate can be changed according to the traveling speed of the movable bodies  10   1 , . . . ,  10   N . And even if an abnormality occurs to the electric communication line between the traffic control unit  12  and the data distribution device  2 B, the transmission rate can be controlled regardless the abnormality.  
         [0047]     In the case of the information distribution system  1 C of the second variant form ( FIG. 7 ), the data distribution device  2 C receives traffic congestion information instead of speed control information from the traffic control unit  12  via an electric communication line, and the detection unit  22 C of the controller  11 C detects the traffic congestion information received by the communication interface  6 , and sends this to the transmission rate setting unit  21 . The transmission rate setting unit  21  sets a transmission rate according to the traffic congestion information referring to the transmission rate table shown in  FIG. 4B , and stores this in a register of the communication control unit  20 . The transmission rate setting unit  21  sets the transmission rate to the high bit rate if the level of traffic congestion is high, assuming that the movable bodies  10   1 , . . . ,  10   N  are moving at low speed or are stopping, sets the transmission rate to the mid-bit rate if the level of traffic congestion is medium, assuming that the movable bodies  10   1 , . . . ,  10   N  are moving at mid-speed, and sets the transmission rate to the low bit rate if the level of traffic congestion is low, assuming that the movable bodies  10   1  . . . ,  10   N  are moving at a relatively high speed. As a result, the radio transmission unit  9  transmits the data signals at a transmission rate according to the traffic congestion status of the traveling path  17 . The traffic control unit  12  is, for example, a known VICS (Vehicle Information and Communication System).  
       2. Second Embodiment  
       [0048]     Now the second embodiment according to the present invention will be described.  FIG. 8  is a block diagram depicting a general configuration of the information distribution system  1 D according to the second embodiment. In  FIG. 8 , a composing element denoted with a same reference numeral as a composing element of the information distribution system  1 A has a same function as this composing element, therefore detailed description is omitted.  
         [0049]     The information distribution system  1 D comprises a data distribution device  2 D, a traffic control unit  12  and an information distribution server  13 . The data distribution device  2 D further comprises a controller  1 D, a radio transmission unit  9 , communication interfaces  6  and  7  and a distribution data storage unit  8 , and the controller  11 D further comprises a communication control unit  20 , a transmission rate setting unit  21 , and a detection unit  22 D and timer  34 . The controller  11 D is an integrated circuit comprising a microprocessor, a non-volatile memory for storing programs, a RAM, an internal bus and an input/output interface, just like the controller  11 A of the first embodiment.  
         [0050]     The transmission rate setting unit  21  has a function to change the transmission rate (bit rate) of the data signals to be transmitted without wires according to the speed control information and traffic congestion information, just like the transmission rate setting unit  21  of the first embodiment, and can change the transmission rate of the data signals referring to the transmission rate table shown in  FIG. 4 .  
         [0051]     The detection unit  22 D has a function to detect the speed control information for the movable bodies  10   1 , . . ,  10   N  having the receivers ST 1 , . . . , ST N  respectively, just like the detection unit  22 A of the first embodiment. In the present embodiment, the timer  34  can measure the start time and end time of the speed control for the movable bodies  10   1  , . . . ,  10   N  based on the speed control information. The detection unit  22 D has a function to acquire the time schedule of executing speed control for the movable bodies  10   1 , . . . ,  10   N  using the timer  34 , and storing this.  
         [0052]     As the timing chart in  FIG. 9  shows, the color of the traffic light  15  periodically changes in the sequence of R (red), B (blue), Y (yellow), R, B, Y, . . . according to the speed control information. The red light ON period T R , blue light ON period T B  and yellow light ON period T Y  could change according to the traffic congestion status of the traveling path  17 , the time zone and the day of the week, but normally the light ON periods T R , T B  and T Y  are not changed over a short time period, such as ten and several seconds. The detection unit  22 D acquires the time schedule by measuring the light ON periods T R , T B  and T Y  using the timer  34  in the first period (measurement period) T 1 , and stores this. In the second period (autonomous period) T 2  subsequent to the measurement period T 1 , the detection unit  22 D itself generates the speed control information according to the stored time schedule without depending on the speed control information supplied from the outside, and sends this to the transmission rate setting unit  21 . Therefore in the autonomous period T 2 , the transmission rate setting unit  21  sets the transmission rate according to the above mentioned time schedule. As  FIG. 10  shows, the measurement period T 1  and the autonomous period T 2  appear periodically.  
         [0053]     Now the operation of the information distribution system  1 D having the above configuration will be described.  FIG. 11  and  FIG. 12  are flow charts depicting an example of the processing procedure by the controller  11 D of the data distribution device  2 D. The flow chart in  FIG. 11  and the flow chart in  FIG. 12  are continuous via the connectors C 1  and C 2 .  
         [0054]     According to  FIG. 11 , the timer  34  is started by the detection unit  22 D, and the measurement of elapsed time in measurement period T 1  is started (step S 10 ). Then the detection unit  22 D requests speed control information to the traffic control unit  12  (step S 11 ). Then the detection unit  22 D judges whether the speed control information transmitted from the traffic control unit  12  was detected (step S 12 ). The detection unit  22 D transmits a transmission request to the traffic control unit  12  (step S 11 ), and then stands by until the speed control information transmitted from the traffic control unit  12  is detected. When the speed control information is not detected even if a predetermined time elapses, the detection unit  22 D may repeatedly request speed control information to the traffic control unit  12 .  
         [0055]     When the traffic control unit  12  transmits the speed control information responding to the transmission request, and the detection unit  22 D detects the speed control information received by the communication interface  6 , the detection unit  22 D judges whether the speed control information has changed or not, that is whether the speed control information previously received is different from that received this time (step S 13 ). For example, if the previous color of the traffic light  15  is red and the traffic light color at this time is also red, then the detection unit  22 D returns the processing back to step S 11  judging that the speed control information is unchanged. If the color of the traffic light  15  changed from red to blue, on the other hand, the detection unit  22 D moves the processing to the next step, S 14 , judging that the speed control information changed. If the speed control information changes, the detection unit  22 D records the light information in the internal memory (step S 14 ). Specifically, if the color of the traffic light  15  changes from red to blue, the red light ON period T R  and the information that the traffic light color changed from red to blue are recorded.  
         [0056]     Then the transmission rate setting unit  21  judges whether the color of the traffic light  15  is blue, yellow or red (step S 15 ), refers to the transmission rate table shown in  FIG. 4A  (steps S 16 , S 18 , S 20 ), and sets the transmission rate of the data signals to the bit rate according to the judged traffic light color (steps S 17 , S 19 , S 21 ). If the traffic light color is yellow or red, the transmission rate is set to the mid-bit rate or the high bit rate (steps S 19 , S 21 ), then the processing after step S 11  is repeatedly executed. If the traffic light color is blue, on the other hand, the transmission rate setting unit  21  sets the transmission rate to the low bit rate (step S 17 ), then judges whether measurement period T 1  has elapsed based on the time data supplied from the timer  34  (step S 22 ), and returns processing back to step S 11  if it is judged that measurement period T 1  has not elapsed. If it is judged that measurement period T 1  has elapsed in step S 22 , on the other hand, the detection unit  22 D moves processing to step S 23  and later, as shown in  FIG. 12 .  
         [0057]     According to  FIG. 12 , in step S 23 , the timer  34  is started by the detection unit  22 D, and measurement of elapsed time in autonomous period T 2  is started (step S 23 ). Then the detection unit  22 D judges whether the blue light ON period T B  has elapsed, and stands by until the blue light ON period T B  elapses (step S 24 ). When the blue light ON period T B  elapses, the color of the traffic light  15  changes from blue to yellow. In the next step, S 25 , the detection unit  22 D reads the time schedule recorded in measurement period T 1 , generates the speed control information that the traffic light color is yellow, and sends this to the transmission rate setting unit  21 . The transmission rate setting unit  21  refers to the transmission rate table shown in  FIG. 4A , and sets the transmission rate to the mid-bit rate.  
         [0058]     Then the detection unit  22 D judges whether the yellow light ON period T Y  has elapsed, and stands by until the yellow light ON period T Y  elapses (step S 26 ). When the yellow light ON period T Y  elapses, the color of the traffic light  15  changes from yellow to red. In the next step, S 27 , the detection unit  22 D reads the above mentioned time schedule, generates the speed control information that the traffic light color is red, and sends this to the transmission rate setting unit  21 . The transmission rate setting unit  21  refers to the transmission rate table shown in  FIG. 4A , and sets the transmission rate to the high bit rate.  
         [0059]     Then the detection unit  22 D judges whether the red light ON period T R  has elapsed, and stands by until the red light ON period T R  elapses (step S 28 ). When the red light ON period T R  elapses, the color of the traffic light  15  changes from red to blue. In the next step, S 29 , the detection unit  22 D reads the above mentioned time schedule, generates the speed control information that the traffic light color is blue, and sends this to the transmission rate setting unit  21 . The transmission rate setting unit  21  refers to the transmission rate table shown in  FIG. 4A , and sets the transmission rate to the low bit rate. Then the detection unit  22 D judges whether autonomous period T 2  has elapsed based on the time data supplied from the timer  34  (step S 30 ). If the detection unit  22 D judged that autonomous period T 2  has not elapsed, the processing after step S 24  is repeatedly executed, and if the detection unit  22 D judged that autonomous period T 2  has elapsed, the processing in measurement period T 1  in step S 10  and later is repeatedly executed.  
         [0060]     As described above, the information distribution system  1 D enables high quality data distribution without dropping the throughput of the wireless LAN, just like the information distribution system  1 A of the first embodiment. Real-time transmission rate control is also possible by a relatively simple algorithm. Also in the case of the present embodiment, the number of times of transmitting speed control information transmission requests to the traffic control unit  12  decreases, so the processing load on the traffic control unit  12  can be decreased.  
       3. Third Embodiment  
       [0061]     The third embodiment according to the present invention will now be described.  FIG. 13  is a block diagram depicting a general configuration of the information distribution system  1 E according to the third embodiment. In  FIG. 13 a  composing element denoted with a same reference numeral as a composing element of the information distribution system  1 A has a same function as this composing element, therefore detailed description is omitted.  
         [0062]     The information distribution system  1 E comprises a data distribution device  2 E and an information distribution server  13 . The data distribution device  2 E further comprises a controller  11 E, a distribution data storage unit  8 , a radio transmission unit  9 , a communication interface  7 , an imaging device (image pickup device)  37  and an input interface  35 , and the controller  11 E further comprises a communication control unit  20 , a transmission rate setting unit  21 , a detection unit  22 E and an image processing unit  36 . The controller  11 E is an integrated circuit comprising a microprocessor, a non-volatile memory for storing programs, a RAM, an internal bus and an input/output interface, just like the controller  11 A of the first embodiment.  
         [0063]     The imaging device  37  further comprises a solid-state image sensing device comprised of a CCD sensor and CMOS sensor, and an optical system and signal processing system, and is positioned in an area near the control point where the speed control for the movable bodies  10   1 , . . . ,  10   N  is performed, and has a visual field which allows capturing an image of the area near the control point. In the present embodiment, the imaging device  37  captures images of the red light source  16 R, blue light source  16 B and yellow light source  16 Y of the traffic light  15 , generates moving image signals, and outputs them to the input interface  35 . The input interface  35  removes noise from the moving image signals, adjusts the gain of the moving image signals to an optimum level, and supplies the moving image signals to the image processing unit  36  after A/D conversion. The image processing unit  36  performs image processing on the moving image data to be input from the input interface  35 , then sends [the processed data] to the detection unit  22 E.  
         [0064]     The detection unit  22 E has a function to detect the speed control information based on the image data captured by the imaging device  37 . Specifically, the image processing unit  36  converts each image frame into plane sequential data comprised of R frames having only red components, Y frames having only yellow components and B frames having only blue components, calculates the average luminance of the R frames, Y frames and B frames respectively, and supplies these to the detection unit  22 E. The detection unit  22 E judges whether each average luminance is a predetermined threshold or less, so as to detect the ON or OFF of the red light source  16 R, yellow light source  16 Y and blue light source  16 B respectively, generates the speed control information, and sends it to the transmission rate setting unit  21 . Another analysis processing that may be used is that image data including only the image area of the red light source  16 R, image data including only the image area of the yellow light source  16 Y and image data including only the image area of the blue light source  16 B are extracted, and the average luminance of each image data is calculated.  
         [0065]     Operation of the information distribution system  1 E having the above configuration will now be described.  FIG. 14  is a flow chart depicting an example of the processing procedure by the controller  11 E of the data distribution device  2 E.  
         [0066]     According to  FIG. 14 , the image processing unit  36  samples the moving image data supplied from the input interface  35 , and acquires this (step S 40 ). Then the image processing unit  36  processes the moving image data (step S 41 ), the detection unit  22 E detects the speed control information based on the processing data which is input from the image processing unit  36 , and sends this to the transmission rate setting unit  21  (step S 42 ).  
         [0067]     The transmission rate setting unit  21  judges whether the color of the traffic light  15  is blue, yellow or red (step S 43 ), refers to the transmission rate table shown in  FIG. 4A  (steps S 44 , S 46 , S 48 ), and sets the transmission rate of the data signal to the bit rate according to the judged traffic light color (steps S 45 , S 47 , S 49 ). If the traffic light color is blue, the transmission rate is set to the low bit rate (step S 45 ), if the traffic light color is yellow, the transmission rate is set to the mid-bit rate (step S 47 ), and if the traffic light color is red, the transmission rate is set to the high bit rate (step S 49 ). Then the processing after step S 40  is repeatedly-executed.  
         [0068]     As described above, the information distribution system  1 E of the present embodiment enables high quality data distribution without dropping the throughput of the wireless LAN, since data is transmitted at a transmission rate appropriate for the traveling speed of the movable bodies  10   1 , . . . ,  10   N  based on the moving images captured by the imaging device  37 . Real-time transmission rate control is also possible by a relatively simple algorithm. Compared with the first and second embodiments, it is unnecessary to acquire speed control information from the outside via an electric communication line or cable, so stable transmission rate control is possible.  
       4. Fourth Embodiment  
       [0069]     The fourth embodiment according to the present invention will now be described.  FIG. 15  is a block diagram depicting a general configuration of the information distribution system  1 F according to the fourth embodiment. In  FIG. 15 , a composing element denoted with a same composing element as the information distribution system  1 A has a same function as this composing element, therefore detailed description is omitted.  
         [0070]     The information distribution system  1 F comprises a data distribution device  2 F and an information distribution server  13 . The data distribution device  2 F comprises a controller  11 F, a distribution data storage unit  8 , a radio transmission unit  9 , a communication interface  6 , a directional microphone  42  and an input interface  40 , and the controller  11 F further comprises a communication control unit  20 , a transmission rate setting unit  21 , a detection unit  22 F and a sound recognition unit  41 . The controller  11 F is an integrated circuit comprising a microprocessor, a non-volatile memory for storing programs, a RAM, an internal bus and an input/output interface, just like the controller  11 A of the first embodiment.  
         [0071]     The traffic control unit  12  generates the speed control information according to a predetermined time schedule, supplies this to the traffic light  43 , and remote-controls the traffic light  43 . The traffic light  43  comprises a red light source  48 R, blue light source  48 B and a speaker  45 , and is positioned in the vicinity of an intersection (control point)  46 . The traffic light  43  has a control box  44 , and the control box  44  controls the ON/OFF of the red light source  48 R and the blue light source  48 B respectively based on the speed control information supplied from the traffic control unit  12 . The control box  44  and the traffic control unit  12  are inter-connected via an electric communication line (not illustrated), such as a packet communication network. ON of the red light source  48 R indicates “Stop”, and ON of the blue light source  48 B indicates “Go”. The control box  44  also controls the speaker  45  so as to transmit control sound to the pedestrians  47   1  and  47   2  according to the ON and OFF of the red light source  48 R and the blue light source  48 B. For example, when the red light source  48 R is ON, the speaker  45  transmits an alarm sound, and when the blue light source  48 B is ON, the speaker  45  transmits a guiding sound.  
         [0072]     The directional microphone  42  is positioned near the control point where the speed control for the movable bodies (pedestrians)  47   1  and  47   2  is performed, and detects sounds in the area near the control point. The input interface  40  removes noise from the sound signals which are input from the directional microphone  42 , and A/D converts the sound signals and then outputs the converted sound signals to the sound recognition unit  41 . The sound recognition unit  41  extracts the characteristics of the cepstrum area of the sound data which is input from the input interface  40 , and outputs it to the detection unit  22 F. The detection unit  22 F detects the speed control information by comparing the sound patterns which are provided in advance, and this characteristic quantity.  
         [0073]     Operation of the information distribution system  1 F having the above configuration will now be described below.  FIG. 16  is a flow chart depicting an example of the processing procedure by the controller  11 F of the data distribution device  2 F.  
         [0074]     According to  FIG. 16 , the sound recognition unit  41  samples and acquires the sound data supplied from the input interface  40  (step S 50 ). Then the sound recognition unit  41  calculates the characteristic quantity by performing sound recognition processing on the sound data, and outputs this to the detection unit  22 F (step S 51 ). The detection unit  22 F detects the speed control information based on the characteristic quantity which is input from the sound recognition unit  41  (the characteristic quantity is hereinafter the “detected sound”), and supplies this to the transmission rate setting unit  21  (step S 52 ).  
         [0075]     The transmission rate setting unit  21  judges whether the detected sound is a sound indicating either “Go” or “Stop” (step S 53 ), refers to the transmission rate table (steps S 54 , S 56 ), and sets the transmission rate of the data signals to a bit rate according to the judged sound (steps S 55 , S 57 ). If the detected sound indicates “Go”, the transmission rate is set to the low bit rate (step S 55 ), and if the detected sound indicates “warning”, the transmission rate is set to the high bit rate (step S 57 ). Then the processing after step S 50  is repeatedly executed.  
         [0076]     As described above, the information distribution system  1 F of the present embodiment enables high quality data distribution without dropping the throughput of the wireless LAN, since the data is transmitted at a transmission rate appropriate for the moving speed of the pedestrians  471  and  472  based on the sounds detected by the directional microphones  42 .  
         [0077]     It is understood that the foregoing description and accompanying drawings set forth the preferred embodiments of the invention at the present time. Various modification, additions and alternatives will, of course, become apparent to those skilled in the art in light of the foregoing teachings without departing from the spirit and scope of the disclosed invention. Thus it should be appreciated that the invention is not limited to the disclosed embodiments, but may be practiced within the full scope of the appended Claims.  
         [0078]     This application is based on Japanese Patent Application No. 2004-122697, which is hereby incorporated by reference.