Abstract:
A wireless resource allocation control system for allocating and releasing wireless resource used for communication between vehicles in a predetermined area is provided. The control system includes a unit mounted on a vehicle, and the unit includes a transmitter for transmitting first information associated with the vehicle. A station receives information from the unit and sends information to the unit. The station calculates a priority level for each of the units on the basis of the first information transmitted from each of the units, ranks the units in the order of the priority level, allocates wireless resources to the units in descending of the priority levels, and releases the wireless resource previously allocated to the unit in the case of the unit unworthy of current allocation of the wireless resource.

Description:
BACKGROUND 
     1. Field 
     The present invention relates to a wireless resource allocation control system, road-side unit, wireless resource allocation control method and wireless resource allocation control program in which a road-side unit placed on a road allocates and releases a wireless resource to and from vehicle-mounted units, each of which is mounted in a vehicle, in a predetermined area in which a vehicle-mounted unit and the other vehicle-mounted unit use a wireless resource to communicate. 
     2. Description of the Related Art 
     Hitherto, in order to implement services for aiming improvement of security, improvement of efficiency of transportation and improvement of comfort, an Intelligent Transport System (ITS) has been known in which roads and vehicles are integrated. This system attempts to implement the services through road-to-vehicle communication, which is performed between a base station (road-side unit) placed on a road and a mobile station (or vehicle-mounted unit) mounted in a vehicle, and a vehicle-to-vehicle communication, which is performed between mobile stations, (refer to Japanese Laid-open Patent Publication No. 11-306490). 
     Those services adopt a synchronous time-division communication method performing road-to-vehicle communication through spot communication (ad hoc communication) only within a road-to-vehicle area, which is limited as a communication area, performing full-duplex communication using different frequencies between communication from a base station to a mobile station and communication from the mobile station to the base station. Further in this the full-duplex communication, a communication frame comprises time-dived fixed lengths called slot. The communication is performed between one base station and multiple mobile stations (refer to Japanese Unexamined Laid-open Patent Publication No. 2001-112059 and No. 2005-100231). 
     The communication employing the synchronous time-division communication method requires the allocation of a slot, which is a wireless resource, to a vehicle-mounted unit by a road-side unit. For example, as a method for allocating slots, a technology has been known in which a base station allocates a time slot to mobile stations in order of the time that they request communication (refer to Japanese Laid-open Patent Publication No. 2001-223660). 
     Since a base station allocates time slots to mobile stations in order of the time they request communication in the conventional technology, the allocation of time slots to mobile stations in order of the time they request communication may result in the problems of the cases where communication is disabled since no time slot is allocated to a mobile station that highly requires information within a service area or, conversely, a time slot is allocated to a mobile station that less requires the information within a service area, which waists wireless resources. 
     SUMMARY 
     The object of the present is directed to allow a vehicle-mounted unit having a high requirement for information to perform communication may be allowed a more secure communication than that by a vehicle-mounted unit having a lower requirement and effectively use wireless resources by reducing the waste of wireless resources. 
     According to an aspect of the invention, there is a wireless resource allocation control system in which a road-side unit placed on a road or the vicinity allocates and releases wireless resources to and from vehicle-mounted units, each of which is mounted in a vehicle, within a predetermined area in which a vehicle-mounted unit and the other vehicle mounted unit communicate by using wireless resources, wherein the vehicle-mounted unit includes own-vehicle information transmitting unit for transmitting own-vehicle information on a vehicle having the vehicle-mounted unit to the road-side unit, and the road-side unit includes priority level calculating unit for calculating the priority level of each of the vehicle-mounted units based on the own-vehicle information, which is transmitted by the own-vehicle information transmitting unit, ranking unit for ranking the vehicle-mounted units according to the priority levels calculated by the priority level calculating unit, wireless resource allocating unit for allocating wireless resources in order from a vehicle-mounted vehicle with a highest priority level among the vehicle-mounted units ranked by the ranking unit and wireless resource releasing unit for releasing a wireless resource of a vehicle-mounted unit with a low priority level to which no wireless resources have been allocated by the wireless resource allocating unit. 
     Since the disclosed system allocates a wireless resource to a vehicle-mounted unit that highly requires information and forcibly releases the wireless resource from the vehicle-mounted unit that less requires information, the system provides effects that the vehicle-mounted unit that highly requires information can perform communication securely and that wireless resources can be effectively used as a result of the release of wireless resources of vehicle-mounted units that less require information to reduce the waste of wireless resources. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a schematic diagram according to a first embodiment for explain an overview and a feature; 
         FIG. 2  shows a schematic configuration of a road-side unit according to a first embodiment; 
         FIG. 3  shows an example of a management table stored in a memory unit shown in  FIG. 2 . 
         FIG. 4  shows an example of a priority table; 
         FIG. 5  shows an example of a priority table; 
         FIG. 6  shows an example of a priority table; 
         FIG. 7  shows an example of a priority table; 
         FIG. 8  shows an example of an expression to calculate a level of priority; 
         FIG. 9  shows a schematic diagram of a vehicle mounted unit shown in  FIG. 1 ; 
         FIG. 10  shows an example of a frame configuration; 
         FIG. 11  shows an example of a sequence diagram of time slot allocation process in a wireless resource allocation control system according to the first embodiment; 
         FIG. 12  shows a schematic configuration of a road-side unit according to a second embodiment; 
         FIG. 13  shows an example of a priority table; 
         FIG. 14  shows an example of an expression to calculate a level of priority; 
         FIG. 15  shows an example of a sequence diagram of time slot allocation process in a wireless resource allocation control system according to the second embodiment; 
         FIG. 16  shows a schematic diagram of a vehicle mounted unit according a third embodiment; 
         FIG. 17  shows an example of a sequence diagram of time slot allocation process in a wireless resource allocation control system according to the third embodiment; 
         FIG. 18  shows a schematic diagram of a vehicle mounted unit according to a fourth third embodiment; and 
         FIG. 19  shows an example of a sequence diagram of time slot allocation process in a wireless resource allocation control system according to the fourth embodiment; 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     With reference to attached drawings, embodiments of a wireless resource allocation control system, road-side unit, wireless resource allocation control method and wireless resource allocation control program according to the present embodiment will be described in detail below. 
     [First Embodiment] 
     A general outline and characteristics of a wireless resource allocation control system according to a first embodiment, a configuration and processing flow of the wireless resource allocation control system will be described step by step, and finally effects of the first embodiment will be described. Notably, a case that the embodiment is applied to the control on the wireless resource allocation will be described below. 
     [eneral Outline and Characteristics of Wireless Allocation Control System According to First Embodiment] 
     First of all, with reference to  FIG. 1 , a general outline and characteristics of a wireless resource allocation control system according to a first embodiment will be described.  FIG. 1  is a diagram for explaining a general outline and characteristics of a wireless resource allocation control system according to a first embodiment. 
     According to a general outline of a wireless resource allocation control system  1  of a first embodiment, a road-side unit placed on a road (a station preferably located by a road or at a vicinity of the road at which the station may communicated with vehicle each other) allocates and releases wireless resources to and from vehicle-mounted units, each of which is mounted in a vehicle, within a predetermined area in which a vehicle-mounted unit and the other vehicle-mounted unit use a wireless resource to communicate. The wireless resource allocation control system  1  has a main characteristic that wireless resources can be effectively used by allowing a vehicle-mounted unit that highly requires information within a service area to securely communicate and reducing the waste of wireless resources. 
     Describing the main characteristic more specifically, vehicle-mounted units  20   a  to  20   k  of the wireless resource allocation control system  1  transmit own-vehicle information (or vehicle information) on vehicles having them to a road-side unit  10 . Then, the road-side unit  10  receives the own-vehicle information from the vehicle-mounted units  20   a  to  20   k  (refer to ( 1 ) in  FIG. 1 ). More specifically, the vehicle-mounted units  20   a  to  20   k  obtain own-vehicle information from the outside of the vehicle-mounted units. Here, examples of the own-vehicle information may include speed information, which is obtained from the number of revolutions of the vehicle engine (or probe information), positional information, which is obtained from a GPS receiver, vehicle&#39;s direction-of-travel information, which is obtained from speed information and positional information, planned driving route of a vehicle, which may be obtained from a navigation system, and/or destination information of a vehicle, for example. All or some of those own-vehicle information pieces are notified to a road-side unit periodically. 
     Then, the road-side unit  10  calculates the priority levels of the vehicle-mounted units  20   a  to  20   k  based on the own-vehicle information transmitted from the vehicle-mounted units  20   a  to  20   k  (refer to ( 2 ) in  FIG. 1 ). More specifically, the road-side unit  10  calculates the priority levels of all of the vehicle-mounted units  20   a  to  20   k  that have transmitted own-vehicle information based on the own-vehicle information notified from the vehicle-mounted units  20   a  to  20   k , traffic information obtained from the own vehicle information of all vehicle-mounted units present within a service area and the determination on whether the subject vehicle-mounted units are within the service area or not. In the example in  FIG. 1 , the priority level “ 5 ” is calculated for the vehicle-mounted unit  20   a , and the priority level “ 0 ” is calculated for the vehicle-mounted unit  20   g  outside of the service area. 
     Then, the road-side unit  10  ranks the vehicle-mounted units  20   a  to  20   k  according to the calculated priority levels (refer to ( 3 ) in  FIG. 1 ) and allocates wireless resources to the vehicle-mounted units in the order of priority (refer to ( 4 ) in  FIG. 1 ). In other words, time slots are allocated in order from the vehicle-mounted unit with a highest priority level, and, if the number of vehicle-mounted units within a service area is lower than the number of time slots, time slots are allocated to all vehicle-mounted units within the service area in decreasing order of priority levels. 
     On the other hand, the wireless resources are released from vehicle-mounted units with lower priority levels to which no wireless resources have been allocated (refer to ( 5 ) in  FIG. 1 ). In other words, if the number of vehicle-mounted units within a service area is higher than the number of time slots, the road-side unit  10  allocates the time slots to vehicle-mounted units in decreasing order of priority levels, and the time slots are forcibly released from vehicle-mounted units with low priority levels to which no time slots have been allocated. Referring to the example in  FIG. 1 , the road-side unit  10  gives priority to the vehicle-mounted unit  20   a  with a higher priority level to allocate a wireless resource and forcibly releases the wireless resource from the vehicle-mounted unit  20   b  with a lower priority level. 
     In this way, the wireless resource allocation control system  1  allocates wireless resources to vehicle-mounted units that highly require information and forcibly releases the wireless resources from vehicle-mounted units that less require information. Therefore, the vehicle-mounted units that highly require information can perform communication securely, and the waste of wireless resources can be reduced by releasing the wireless resources from vehicle-mounted units that less require information. As a result, as in the main characteristic, the wireless resources can be used effectively. 
     [Configuration of Road-Side Unit] 
     Next, with reference to  FIG. 2 , the configuration of the road-side unit  10  shown in  FIG. 1  will be described.  FIG. 2  is a block diagram showing a configuration of the road-side unit  10  according to the first embodiment. As shown in  FIG. 2 , the road-side unit  10  includes a network interface unit  11 , a Media Access Control (MAC) processing unit  12 , a Physical Layer (PHY) processing unit  13 , a Radio Frequency (RF) unit  14 , a Global Positioning System (GPS) receiver  15  and a control device  16 . The processing by those components will be described below. Though the communication system may adopt either Time Divisional Multiple Access (TDMA) or Orthogonal Frequency Division Multiplex (A) (OFDM(A)), the case that OFDM(A) is adapted according to this embodiment will be described. 
     The network interface unit  11  has an interface function of inputting traffic information (such as nearest signal information the nearest traffic light, nearest signal state transition information and traffic accident information) of the inside of a service area network to the road-side unit  10  and transmitting it to the MAC processing unit  12 . The MAC processing unit  12  has an MAC layer function of performing encoding and/or error correction on transmit data. The RF unit  14  has a transmitting/receiving function of converting a base band signal to a wireless frequency or converting a wireless frequency to a base band signal and transmits a health check signal that checks the communication state of a vehicle-mounted unit to each vehicle-mounted unit periodically. The GPS receiver  15  has a function of generating a reference time for synchronizing the road-side unit  10  and the vehicle-mounted unit  20  and an internal timing signal. 
     The PHY processing unit  13  includes transmitting functions and receiving functions. The transmitting functions include a Preamble signal generating section  13   a  that generates a Preamble signal, a Broadcast signal generating section  13   b  that generates a Broadcast signal, a DL_Burst signal generating section  13   c  that generates a down link Burst signal that vehicles transmit data, a Modulation section  13   d  that performs the modulation processing instructed from the MAC processing unit  12 , a multiplexing processing section  13   e  that performs processing of multiplexing a signal, and an IFFT section  13   f.    
     The receiving functions of the PHY processing unit  13  include an FFT section  13   g  that performs FFT processing on a signal at a base band level, a Ranging receiving processing section  13   h  that detects a Ranging signal from a receive signal, a UL_Burst receiving processing section  13   i  that performs processing of receiving a UL_Burst signal in the area subject to the UL_MAP instruction from the MAC processing unit  12  and a vehicle information processing section  13   j  that performs processing of receiving vehicle information from the vehicle-mounted unit  20 . 
     The control device  16  has an internal memory for storing a program that defines a processing routine and required data and executes various processing by using them. The control device  16  includes an interface unit  16   a , a memory unit  16   b  and a priority processing unit  16   c , which closely relate to the present invention. The priority processing unit  16   c  corresponds to “priority level calculating means”, “ranking means”, “wireless resource allocating means” and “wireless resource releasing means”. 
     The memory unit  16   b  stores a “management table” (refer to  FIG. 3 ) that manages time stamps, which are allocated wireless resources, and a “priority table” (refer to  FIGS. 4 to 7 ) having correspondence between parameters of own-vehicle information and the values of priority levels. More specifically, the management table  600  stores, as illustrated in  FIG. 3 , a “vehicle ID”  602  that uniquely identifies a vehicle, such as number plate information, vehicle unique information and an IP address of a vehicle-mounted unit, a “priority level transition information”  604  that indicates the transition in priority levels and an “allocation TS”  606  that indicates the allocated time stamp in connection. 
       FIGS. 4 to 7  show priority tables  610 ,  620 ,  630 , and  640  respectively. The priority table is a table that stores a priority level according to a speed, a priority level according to a position, a priority level according a position inside or outside of a service area and a priority level according to congestion information, as illustrated in  FIGS. 4 to 7 . From the parameters (such as the shown “Level(v)”  612 , “Level(p)”  622 , “Level(a)”  632  and “Level (j)”)  642  of the priority level, the priority levels of all vehicle-mounted units are calculated (refer to the shown “Total Priority Level” in  FIG. 8 ). 
     In the example of the priority table  610  shown in  FIG. 4 , vehicle-mounted units are ranked according to the speeds, and it is set that the necessity of information by a vehicle-mounted unit and the priority level increase as the speed of the vehicle-mounted unit increases. In other words, since a slower vehicle may be stopped or be delayed, the vehicle has a lower necessity for obtaining information and has a later opportunity to obtain the information. Therefore, the vehicle is set to rank lower. On the other hand, since a faster vehicle may highly require traffic information for driving the vehicle, the vehicle is set to rank higher to prioritize.  FIG. 4  illustrates an example of the case of the road with a speed limit of 60 km/h. 
     In the example of the priority table  620  shown in  FIG. 5 , vehicle-mounted units are ranked according to the positions of the vehicle-mounted units. In the example in  FIG. 5 , the priority table  620  sets that a vehicle-mounted unit that has just entered into a service area, positions at an end of the service area and therefore highly requires information is ranked higher while a vehicle-mounted unit that positions at the center of the service area and therefore less requires information is ranked lower. In other words, since a vehicle-mounted unit that has just entered into a service area has a higher possibility that it has not received any service such as obtaining information, it is set to rank to prioritize. A vehicle-mounted unit positioned at the center of a service area has a higher possibility that it has been serviced already, it is set to rank lower. 
     In the example of the priority table shown in  FIG. 6 , vehicle-mounted units are ranked according to the positions inside or outside of a service area. In the example in  FIG. 6 , the priority table ranks a vehicle mounted unit inside of a service area as “1” and ranks a vehicle-mounted unit outside of the service area as “0”. The priority level of a vehicle-mounted unit outside of a service area is always ranked “0” in the priority level calculating method example in  FIG. 8 , as will be described later with reference to  FIG. 8 . 
     In the example of the priority table shown in  FIG. 7 , vehicle-mounted units are ranked according to the congestion information. In the example in  FIG. 7 , a delayed vehicle-mounted unit is set to rank lower since the delayed vehicle-mounted unit less requires information. Conversely, a vehicle-mounted unit having nothing to do with congestion is set to rank higher since the vehicle-mounted unit highly requires information. In other words, a delayed vehicle is set to rank lower since it may have a lower necessity for obtaining information and have a later opportunity for obtaining information. On the other hand, a vehicle having nothing to do with congestion is set to rank for priority since it may have a higher necessity for traffic information, for example, for driving the vehicle. 
     The priority processing unit  16   c  calculates the priority levels of the vehicle-mounted units  20   a  to  20   k  based on the own-vehicle information transmitted from the vehicle-mounted units  20   a  to  20   k  and ranks the vehicle-mounted units  20   a  to  20   k  according to the calculated priority levels. The priority processing unit  16   c  further has a function of inputting the result to the MAC processing unit  12 . 
     More specifically, the priority processing unit  16   c  in the road-side unit  10  calculates the priority levels of all of the vehicle-mounted units  20   a  to  20   k  that have transmitted the own-vehicle information based on the own-vehicle information notified by the vehicle-mounted units  20   a  to  20   k , traffic information obtained from the own-vehicle information of all of the vehicle-mounted units present within a service area and the determination on whether each of the vehicle-mounted units exists within the service area or not. 
     Here, describing an example of the method for calculating a priority level with reference to  FIG. 8 , the parameters “Level(v)”  612 , “Level(p)”  622 , and “Level(j)”  642  of the priority levels are added, and the added value is multiplied by “Level(a)”  632 . Thus, the priority level (which is “Total Priority Level” in  FIG. 8 ) is calculated. By using such a calculation method, the priority levels of all vehicle-mounted units are calculated. 
     [Configuration of Vehicle-Mounted Unit] 
     Next, with reference to  FIG. 9 , the configuration of the vehicle-mounted unit  20  shown in  FIG. 1  will be described.  FIG. 9  is a block diagram showing a configuration of the vehicle-mounted unit  20  according to the first embodiment. As shown in  FIG. 9 , the vehicle-mounted unit  20  includes an external sensor interface unit  21 , a MAC processing unit  22 , a PHY processing unit  23 , an RF unit  24 , a GPS receiver  25  and a control device  26 . The processing by those components will be described below. 
     The external sensor interface unit  21  for a vehicle has functions of receiving speed information and vehicle information from the outside of the vehicle-mounted unit and notifying it to the MAC processing unit  22  and a function of notifying information from the road-side unit  10 , which is notified from the MAC, to the outside of the vehicle-mounted unit. The MAC processing unit  22  has a MAC layer function of performing encoding and/or error correction on transmit data. 
     The RF unit  24  has a transmitting/receiving function of converting a base band signal to a signal in a wireless frequency or converting a signal in a wireless frequency to a base band signal and transmits a response signal to a health check signal from the road side unit  10  as a signal indicating that the own vehicle is within a predetermined area. The GPS receiver  25  has a function of generating a reference time for synchronizing the road-side unit  10  and the vehicle-mounted unit  20  and an internal timing signal. The control device  26  has a function of performing control based on received information. 
     The PHY processing unit  23  includes transmitting functions and receiving functions. The transmitting functions include a Ranging signal generating section  23   a , a UL-Burst generating section  23   b  that generates an uplink Burst signal for road-to-vehicle communication, a DL_Burst signal generating section  23   c  that generates a downlink Burst signal for vehicle-to-vehicle communication, a vehicle information signal generating section  23   d  that generates a vehicle information signal describing information on the own vehicle, a Modulation section  23   e  that performs the modulation processing instructed from the MAC processing unit  22 , a multiplexing processing section  23   f  that multiplexes signals and an IFFT section  23   g.    
     The receiving functions of the PHY processing unit  23  include an FFT section  23   h  that performs FFT processing on a signal at a base band level, a Preamble receiving processing section  23   i  that detects a Preamble signal from a receive signal and a DL_Burst receiving processing section  23   k  that performs processing of receiving a DL_Burst signal in an area subject to a DL_MAP instruction from the MAC processing unit  22 . 
     Now, with reference to  FIG. 10 , a frame configuration will be described. As illustrated in  FIG. 10 , this embodiment has a frame configuration time-divided into a frame period for performing road-to-vehicle communication and a frame period, which functions as a time slot for performing vehicle-to-vehicle communication. The modulation method adopts an orthogonal frequency division multiple access (OFDMA), and each data is divided in used frequencies based on the times and sub-carrier. The frame includes a Preamble, a Broadcast signal containing an Flame Control Header (FCH), downlink allocation information (DL_MAP) and uplink allocation information (UL_MAP), a vehicle ID, which is transmitted from a road-side unit periodically, multiple downlink Bursts and uplink Bursts that carry transmit data containing own-vehicle information and information on an allocated time slot and multiple slots. 
     [Processing by Wireless Resource Allocation Control System] 
     Next, with reference to  FIG. 11 , processing by the wireless resource allocation control system  1  according to the first embodiment will be described.  FIG. 11  is a sequence diagram showing a flow of time slot allocation control processing by a wireless resource allocation control system according to the first embodiment. 
     As shown in  FIG. 11 , each vehicle-mounted unit  20  of the wireless resource allocation control system  1  obtains own-vehicle information from the outside of the vehicle-mounted unit (steps S 101  to  103 ). Then, the own-vehicle information is notified to a road-side unit periodically (steps S 104  to  106 ). 
     Then, the road-side unit  10  calculates the priority levels of all of the vehicle-mounted units  20 , which have transmitted own-vehicle information, based on the own-vehicle information notified from the vehicle-mounted units  20 , traffic information obtained from the own-vehicle information of all vehicle-mounted units present within a service area and the determination on whether each of the subject vehicle-mounted units is within the service area or not (step S 107 ). 
     Next, the road-side unit  10  ranks the vehicle-mounted units according to the calculated priority levels (step S 108 ), performs processing of allocating wireless resources in order from a vehicle-mounted unit with the highest priority level. If the area of the time slot is determined, the time slot allocated area for notifying the area is notified to the vehicle-mounted unit  20  (step S 109 ). The road-side unit  10  releases the wireless resource of a vehicle-mounted unit with a lower priority level, which has not allocated any wireless resource (step S 110 ). 
     After that, the vehicle-mounted unit  20 , which has allocated a time slot, recognizes the time slot area (steps S 111  and  113 ), and the vehicle-mounted unit  20  from which the wireless resource has been released recognizes the forced release of the time slot (step S 112 ). Then, each of the vehicle-mounted units  20  transmits the response indicating the reception of the notification to the road side unit  10  (steps S 114  to S 116 ). Then, the vehicle-mounted units  20 , which have allocated time slots, use the time slots allocated from the road-side unit  10  to perform the vehicle-to-vehicle communication (steps S 117  to  120 ). 
     As described above, the wireless resource allocation control system  1  allocates a wireless resource to a vehicle-mounted unit, which highly requires information, and forcibly releases the wireless resource from a vehicle-mounted unit, which less requires information. Thus, a vehicle-mounted unit, which highly requires information, can perform communication securely. Furthermore, as a result of the reduction of the waste of wireless resources by releasing the wireless resource from a vehicle-mounted unit, which less requires information, the wireless resources can be effectively used, as in the main characteristic. 
     [Second Embodiment] 
     Having described the case that priority levels are calculated based on the own-vehicle information of vehicle-mounted units according to the first embodiment, the present invention is not limited thereto. Priority levels may be calculated based on traffic information of the inside of a service area, which is obtained from the inside of the network, in addition to own-vehicle information. 
     Now, with reference to  FIGS. 12 to 15 , the configuration and processing in a wireless resource allocation control system la according to a second embodiment will be described in a case that a road-side unit obtains traffic information from the inside of a network and priority levels are calculated based on own-vehicle information and the traffic information.  FIG. 12  is a block diagram showing a configuration of a road-side unit  10   a  according to the second embodiment.  FIG. 13  is a diagram for describing an example of the priority table  660 .  FIG. 14  is a diagram for explaining an example of the priority level calculating method.  FIG. 15  is a sequence diagram showing a flow of time slot allocation control processing by a wireless resource allocation control system according to the second embodiment. 
     First of all, a configuration of a road-side unit  10   a  according to the second embodiment will be described with reference to  FIG. 12 . As shown in  FIG. 12 , the road-side unit  10   a  according to the second embodiment is different from the road-side unit  10  shown in  FIG. 2  in that it further includes a function of obtaining traffic information from the inside of a network. In the road-side unit  10   a , a network interface unit  11  obtains traffic information from the inside of a service area network. 
     Then, unlike the control device  16  in the first embodiment, a memory unit  16   b  of a control device  16  according to the second embodiment further stores a priority table  660  storing priority levels according to traffic information parameters from the inside of a service area network, as shown in  FIG. 13 . More specifically, if the nearest signal state of traffic light is blue, it is set to rank high since the necessity for information is high. On the other hand, if it is red, it is set to rank low since the necessity for information is low. 
     A priority processing unit  16   c  calculates priority levels of vehicle-mounted units  20   a  to  20   k  based on own-vehicle information transmitted from the vehicle-mounted units  20   a  to  20   k  and the traffic information obtained from the inside of a service area network and ranks the vehicle-mounted units  20   a  to  20   k  according to the calculated priority levels. 
     Describing an example of the priority level calculation method with reference to  FIG. 14 , the parameter “Level(v)”  612 , “Level(p)”  622 , “Level(j)”  642  of the priority level and the parameter “Level(s)”  662  of the priority level from the traffic information are added, and the added value is multiplied by “Level(a)”  632  to calculate the priority level (“Total Priority Level” in  FIG. 14 ). This calculation method is used to calculate the priority levels of all vehicle-mounted units. 
     Next, time slot allocation control processing by a wireless resource allocation control system according to the second embodiment will be described with reference to  FIG. 15 . The access control processing of the second embodiment is different from the access control processing of the first embodiment shown in  FIG. 11  in that traffic information is obtained from the inside of a service area network. 
     That is, as shown in  FIG. 15 , the road-side unit  10   a  receives own-vehicle information from vehicle-mounted units  20  periodically (steps S 201  to  206 ) and obtains traffic information from the inside of a service area network (step S 207 ). Then, based on the own-vehicle information transmitted from the vehicle-mounted units  20   a  to  20   k  and the traffic information obtained from the inside of the service area, the priority levels of the vehicle-mounted units are calculated (step S 208 ), and the vehicle-mounted units  20   a  to  20   k  are ranked according to the calculated priority levels (step S 209 ). After that, the road-side unit  10   a  performs time slot allocation releasing processing (steps S 210  and  211 ) like the first embodiment. 
     In this way, according to the second embodiment, more accurate priority levels of vehicle-mounted units can be calculated since the priority levels are calculated based on traffic information obtained from the inside of an area network in addition to the own-vehicle information. 
     [Third Embodiment] 
     According to the present embodiment, a time slot may be reallocated to a vehicle-mounted unit, which has been forced to release the time slot, if the necessity for information increases later. 
     Now, with reference to  FIGS. 16 to 18 , the configuration and processing in a wireless resource allocation control system  1   b  according to a third embodiment will be described in a case that a road-side unit, which has been forced to release the time slot, increases the necessity for information later, and is reallocated a time slot.  FIG. 16  is a block diagram showing a configuration of a vehicle-mounted unit  20   a  according to the third embodiment, and  FIG. 17  is a sequence diagram showing a flow of time slot allocation processing by a wireless resource allocation control system according to the third embodiment. 
     First of all, a configuration of the vehicle-mounted unit  20   a  according to the third embodiment will be described with reference to  FIG. 16 . As shown in  FIG. 16 , the vehicle-mounted unit  20   a  according to the third embodiment is different from the vehicle-mounted unit  20  shown in  FIG. 9  in that the control device  26  includes an interface unit  26   a , a memory  26   b  and a priority processing unit  26   c . In the vehicle-mounted unit  20   a , the control device  26  calculates the priority levels of vehicle-mounted units based on own-vehicle information and ranks the vehicle-mounted units  20   a  to  20   k  according to the calculated priority levels. 
     The interface unit  26   a  exchanges data with the MAC processing unit  22 . More specifically, the interface unit  26   a  receives a reallocation request permitting level, which is notified from a road-side unit  10 , and traffic information. The memory  26   b  stores a priority table as illustrated in  FIGS. 4 to 7  and  FIG. 13 . 
     The priority processing unit  26   c  calculates a priority level (Total Priority Level) from own-vehicle information and road-to-vehicle information, compares the priority level and a reallocation request permitting level and, if the priority level reaches the reallocation request permitting Level, requests the reallocation to the road-side unit  10 . If the priority level does not reach the reallocation request permitting level on the other hand, the reallocation is not requested. 
     Next, with reference to  FIG. 17 , processing by the wireless resource allocation control system  1   b  according to the third embodiment will be described.  FIG. 17  is a sequence diagram showing a flow of time slot allocation control processing by a wireless resource allocation control system according to the third embodiment. 
     As shown in  FIG. 17 , the road-side unit  10  releases the time slot of a vehicle-mounted unit with a low Total Priority Level (step S 301 ). At the same time, the road-side unit  10  notifies the Total Priority Level at the time of the release of the vehicle-mounted unit and a reallocation request permitting level to the vehicle-mounted unit  20   a , which has been forced to release by the road-side unit  10  (step S 302 ). Then, the vehicle-mounted unit  20   a , which has been forced to release, obtains own-vehicle information from the outside of the vehicle-mounted unit as required after that (steps S 303  and  304 ). The road-side unit  10  further obtains traffic information from the inside of a service area network (step S 305 ). 
     Then, the road-side unit  10  notifies the traffic information as road-to-vehicle information to the vehicle-mounted unit  20   a  (step S 306 ). The vehicle-mounted unit  20   a , which has been notified the road-to-vehicle information, calculates a Total Priority Level from the own-vehicle information and the road-to-vehicle information (steps S 307  and  308 ), compares the Total Priority Level and the reallocation request permitting level (steps S 309  and  310 ) and, if the Total Priority Level reaches the reallocation request permitting level (step S 311 ), requests the reallocation (step S 313 ). If the Total Priority Level does not reach the reallocation request permitting level on the other hand (step S 312 ), the reallocation is not requested (step S 314 ). Then, if the reallocation is requested from the vehicle-mounted unit  20   a , the road-side unit  10  recalculates the priority levels of the vehicle-mounted units and ranks them (steps S 315 ). 
     In this way, according to the third embodiment, a time slot can be reallocated when a vehicle-mounted unit, which has been forced to release the wireless resource, increases the necessity for information. In addition, wireless resources can be used effectively since the reallocation is not requested if the priority level for determining the necessity for information does not reach the reallocation permitting level notified from a road-side unit. 
     [Fourth Embodiment] 
     Having described the case that a vehicle-mounted unit, which has been forced to release the time slot, is allowed to request the reallocation of a time slot and to be reallocated the time slot according to the third embodiment, the present invention is not limited thereto. The request for reallocation by a predetermined vehicle-mounted unit may be restricted for a predetermined period of time. 
     Now, with reference to  FIGS. 18 and 19 , the configuration and processing of a wireless resource allocation control system  1   c  according to a fourth embodiment will be described in a case where a vehicle-mounted unit with a low priority level (such as the value “0” of the priority level) is inhibited to request the reallocation within a timer period.  FIG. 18  is a block diagram showing a configuration of a vehicle-mounted unit  20   b  according to the fourth embodiment, and  FIG. 19  is a sequence diagram showing a flow of time slot allocation control processing by a wireless resource allocation control system according to the fourth embodiment. 
     First of all, with reference to  FIG. 18 , a configuration of a vehicle-mounted unit  20   b  according to the fourth embodiment will be described. As shown in  FIG. 18 , the vehicle-mounted unit  20   b  according to the fourth embodiment is different from the vehicle-mounted unit  20   a  shown in  FIG. 16  in that it further includes a priority level monitoring section  27 . 
     The priority level monitoring section  27  monitors the priority level notified from a road-side unit  10 . More specifically, the priority level monitoring section  27  monitors the priority level notified from a road-side unit  10  and, if the value “0” is notified as the priority level from the road-side unit (or if it is determined by the road-side unit  10  that it is outside of a service area), starts a timer, not shown. Then, within a timer period, the reallocation request is controlled to inhibit. 
     Next, with reference to  FIG. 19 , the processing by the wireless resource allocation control system  1   c  according to the fourth embodiment will be described.  FIG. 19  is a sequence diagram showing a flow of time slot allocation control processing by a wireless resource allocation control system according to the fourth embodiment. 
     As shown in  FIG. 19 , the vehicle-mounted unit  20   b  if notified from the road-side unit  10  the value “0” as the Total Priority Level at the time when the time slot has been forced to release (step S 501 ) starts a timer (steps S 502  and  503 ), controls to inhibit reallocation request within a timer period (steps S 504  and  505 ) and allows transmission of a reallocation request after a lapse of a predetermined period measured by the timer. 
     In this way, since a vehicle-mounted unit with a low priority level is inhibited to perform reallocation request within a timer period, the congestion of communication can be prevented, and wireless resources can be effectively used. 
     [Other Embodiments] 
     Having described the embodiments of the present invention, the present invention may be implemented in various different forms excluding the above-described embodiments. Other embodiments included in the present invention will be described below. 
     (1) System Configurations and Others 
     The components of the shown devices are functionally conceptual and are not necessarily required to physically configure as shown. In other words, the specific forms of distribution and unity of the devices are not limited to the shown ones, but all or a part of them may be configured to distribute or unit functionally or physically in arbitrary units according to the various loads and usages. All or an arbitrary part of the processing functions to be performed by the devices may be implemented by a CPU and a program to be analyzed and executed by the CPU or may be implemented as hardware based on wired logics. 
     All or a part of the processes described as ones to be performed automatically among the processes described according to the embodiments may be performed manually, or all or a part of the processes described as ones to be performed manually may be performed automatically by a publicly known method. Alternatively, the processing routines, control routines, specific names and information including data and parameters described in the documents and illustrated in the drawings can be changed arbitrarily except for the cases mentioned specifically. 
     (2) Programs 
     The wireless resource allocation control methods described with reference to embodiments can be implemented by executing a prepared program by a computer such as a personal computer and a workstation. The program can be distributed over a network such as the Internet. The program may be recorded on a computer-readable recording medium such as a hard disk, a flexible disk (FD), a CD-ROM, an MO and a DVD and can be executed by being read out from the recording medium by a computer. 
     As described above, the wireless resource allocation control system, road side unit, wireless resource allocation control method and wireless resource allocation control program according to the present embodiment are effective for a case that a road-side unit placed on a road allocates and releases a wireless resource to and from vehicle-mounted units within a predetermined area in which vehicle-mounted units, each of which is mounted in a vehicle, and other vehicle-mounted units communicate by using wireless resources and particularly allow a vehicle-mounted unit, which highly requires information within a service area, to securely communicate and reduces the waste of wireless resources to effectively use the wireless resources.