Abstract:
Disclosed is a transmission device that addresses the issue of alleviating the occurrence of delays in delivery, or the abandonment, of transmitted data in the transmission device when transmission capacity of the transmission device declines, as well as being easy to set up and maintain. The transmission device comprises a sorting means for assigning a first signal to either a first path or a second path on the basis of assignment information for assigning the first signal; a first transmitting means for transmitting the signal from among the first signal that is allocated to the first path upon the first path; and a second transmitting means for transmitting the signal from among the first signal that is allocated to the second path upon the second path.

Description:
TECHNICAL FIELD 
     The invention relates to a transmission device, a transmission method, and a transmission system, in particular a transmission device, a transmission method, and a transmission system which conduct transmission using a plurality of transmission paths. 
     BACKGROUND ART 
     A method of making a modulation scheme multiple-valued and a method of expanding a wireless band are known as a means for increasing a transmission capacity in a wireless transmission system. As an example of the method of making a modulation scheme multiple-valued, an adaptive modulation scheme is known. A wireless transmission device using the adaptive modulation scheme automatically changes a modulation scheme and a transmission rate depending on a condition of a wireless line. As a result, the wireless transmission device using the adaptive modulation scheme is capable of improving throughput of the wireless transmission system. However, in the adaptive modulation scheme, a modulation scheme having a small multiple-valued number is generally chosen, when a line condition is degraded. Accordingly, in the wireless transmission system employing the adaptive modulation scheme, the transmission capacity decreases as the line condition is degraded. As a result, in the wireless transmission system employing the adaptive modulation scheme, data is jammed on the transmission side, a delay time increases, and data overflowed from a buffer is lost. 
     As a technology for expanding the wireless band, MIMO (Multiple-Input Multiple—Output) and OFDM (Orthogonal Frequency Division Multiplexing) are known. However, since a high-level wireless control technology is required in order to realize these technologies, devices thereof become expensive. Link Aggregation defined in IEEE (Institute of Electrical and Electronics Engineers) 802.3ad is known as a technology of speeding up a transmission path by putting a plurality of links together. Since the technology, however, requires a protocol processing by Link Aggregation Control Protocol (LACP), the device is still expensive. 
     One of solutions to the problems described above is shown in a wireless transmission system in  FIG. 11 .  FIG. 11  is a diagram illustrating a configuration of the wireless transmission system related to the invention. 
     An external LAN (Local Area Network) signal  100  (MAC frame based on IEEE802.3) enters a Layer 2 switch (hereinafter referred to as “L2SW”)  50  from a user network  10 . The L2SW  50  allocates the external LAN signal  100  to each MAC frame and outputs branched LAN signals  110 - 1  to  110 - n  to wireless transmission devices  60 - 1  to  60 - n , respectively. Allocation of the frames may be conducted based on a value of a field defined in the MAC frame, like a MAC address, an IP address, and a VLAN ID (Virtual LAN Identifier). 
     The wireless transmission devices  60 - 1  to  60 - n  wireless-modulate inputted and branched LAN signals  110 - 1  to  110 - n , respectively. The wireless transmission devices  60 - 1  to  60 - n  output the modulated signals to opposite wireless transmission devices  70 - 1  to  70 - n  as wireless signals  201 - 1  to  201 - n , respectively. The wireless transmission devices  70 - 1  to  70 - n  demodulate received wireless signals  201 - 1  to  201 - n  to extract LAN signals, and output the extracted LAN signals to a L2SW  80  as branched LAN signals  510 - 1  to  510 - n . The L2SW  80  puts the branched LAN signals  510 - 1  to  510 - n  together and outputs them to a user network  40  as an external LAN signal  500 . A LAN signal is similarly transmitted from the user network  40  toward the user network  10 . In the wireless transmission system shown in  FIG. 11 , if the number of wireless transmission devices between the L2SW  50  and the L2SW  80  is increased, a transfer capacity may be increased. 
     The L2SW  50  and the L2SW  80  include a Link Aggregation function without the LACP (Link Aggregation Control Protocol) as descriptions in Patent document 1. The L2SW  50  and the L2SW  80  achieve the Link Aggregation function without the LACP requiring complicated protocol processing. As described in Patent document 2 and Patent document 3, the wireless transmission devices  60 - 1  to  60 - n ,  70 - 1  to  70 - n  includes a function for putting a port of the branched LAN signal of the opposite wireless transmission device into linkdown when a linkdown state is detected at a port of the branched LAN signal. 
     Detecting quality degradation of the wireless signal either in the direction from the present wireless transmission device to the opposite wireless transmission device or in the reverse direction, the wireless transmission device puts the port of the branched LAN signal of the present wireless transmission device and the opposite wireless transmission device into the linkdown. As a result, the wireless transmission devices are capable of informing L2SW  50  and L2SW  80  of abnormality and stopping signal transmission. 
     If the L2SW in Patent document 1 is combined with the wireless transmission devices in Patent documents 2 and 3, it becomes possible to increase the transmission capacity without expanding the wireless band. Further complex LACP processing required for the Link Aggregation may be eliminated. 
     However, when a wireless system is configured by combining inventions described in the above Patent documents, the L2SW device becomes necessary in addition to the wireless transmission device. Therefore, total system costs increase. In addition, since the L2SW device is different from the wireless transmission device, construction and maintenance thereof is cumbersome, and it is difficult to standardize a monitoring control network. 
     A combination different from the combination of above Patent documents is described with reference to  FIG. 12  and  FIG. 13 . 
       FIG. 12  is a diagram illustrating a configuration of a different wireless transmission system related to the invention. The wireless transmission system in  FIG. 12  includes L2SW  50 - 1  to  50 - n  and L2SW  80 - 1  to  80 - n  by which L2SW  50  and  80  in  FIG. 11  are replaced, and wireless transmission devices  60 - 1  to  60 - n  and  70 - 1  to  70 - n . The wireless transmission devices  60 - 1  to  60 - n  face the wireless transmission devices  70 - 1  to  70 - n  and connect therewith, respectively. 
     The LAN signal  100  from the user network  10  is branched into n-signals for each MAC frame by the L2SW  50 - 1  which is connected with the first stage wireless transmission device  60 - 1 . One LAN signal  110 - 1  is outputted to the wireless transmission device  60 - 1 , and the other n−1 LAN signals  110 - 2  to  110 - n  are outputted to the adjacent n−1 wireless transmission devices  60 - 2  to  60 - n . The L2SW  80 - 1  of the wireless transmission device  70 - 1  gathers the LAN signals  510 - 1  to  510 - n  which are transmitted through the wireless transmission devices  60 - 1  to  60 - n  and the wireless transmission devices  70 - 1  to  70 - n . The L2SW  80 - 1  outputs the gathered LAN signals  510 - 1  to  510 - n  to the user network  40  as the external LAN signal  500 . 
     As described, in the wireless transmission system in  FIG. 12 , the wireless transmission device connected with the user network connects with a plurality of wireless transmission devices adjacent thereto in a star arrangement. As a result, since the wireless transmission device connected with the user network gathers the other wireless transmission paths, extension of the transmission capacity is possible in proportion to the number of the adjacent wireless transmission devices. When a trouble occurs in the wireless transmission path, the wireless transmission device puts an internal LAN signal port connecting with the L2SW into linkdown and informs the L2SW of the trouble. The L2SW does not use the linkdown port for signal transmission based on the link aggregation function. As a result, signal transmission using only the wireless transmission device on the normal wireless transmission path becomes possible. 
     However, in the wireless transmission system in  FIG. 12 , connections from the adjacent plurality of wireless transmission devices concentrate on the wireless transmission device connecting with the user network. The wireless transmission device connecting with the user network requires interfaces, the number of which is equal to the number of the adjacent wireless transmission devices. As a result, in the wireless transmission system in  FIG. 12 , downsizing and price reduction of the device becomes difficult. The adjacent wireless transmission device connects only with the wireless transmission device connecting with the user network. Therefore, the adjacent wireless transmission device does not require the L2SW function. If the wireless transmission system in  FIG. 12  is configured using the wireless transmission device with the L2SW function, the adjacent wireless transmission device includes the L2SW function which is not used. In the wireless transmission system in  FIG. 12 , therefore, price reduction of the adjacent wireless transmission device becomes difficult. If the wireless transmission system in  FIG. 12  is configured by eliminating the L2SW function from the adjacent wireless transmission device, the wireless transmission device with the L2SW function and the wireless transmission device without the L2SW function exist in the wireless transmission system. In this case, construction and maintenance of the wireless transmission device becomes cumbersome. 
       FIG. 13  is a diagram illustrating another wireless transmission system related to the invention. The internal configuration of the wireless transmission system in  FIG. 13  is similar to that of the wireless transmission system in  FIG. 12 . The wireless transmission system in  FIG. 13  differs in connection between devices from the wireless transmission system in  FIG. 12 . 
     That is, in the wireless transmission system in  FIG. 13 , the wireless transmission device  60 - 1  connecting with the user network  10  connects with the adjacent wireless transmission device  60 - 2 , and the wireless transmission device  70 - 1  connects with the adjacent wireless transmission device  70 - 2 . In the wireless transmission system in  FIG. 13 , the wireless transmission device  60 - 2  connects with the adjacent wireless transmission device  60 - 3 , and the wireless transmission device  70 - 2  connects with the adjacent wireless transmission device  70 - 3 . In the wireless transmission system in  FIG. 13 , all the wireless transmission devices are similarly connected in cascade. 
     In the wireless transmission system in  FIG. 13 , when the adjacent wireless transmission devices are connected in cascade, the plurality of wireless transmission paths are put together and extension of the transmission capacity becomes possible depending on the number of the wireless transmission devices. In addition, the wireless transmission system in  FIG. 13  detects abnormality in the wireless transmission path or a wireless transmission circuit, puts the internal LAN signal port which is connection with the L2SW into linkdown, and informs the L2SW of the abnormality. The L2SW does not use the linkdown port for signal transmission based on the link aggregation function. Therefore, signal transmission only using the wireless transmission device on the normal wireless transmission path may continue. 
     In the wireless transmission system in  FIG. 13 , since all the wireless transmission devices include the link aggregation function-enabled L2SW, price reduction of the device is difficult. When a simple and low price L2SW without the link aggregation function is applied to the adjacent wireless transmission device, the wireless transmission device connecting with the user network and the adjacent wireless transmission device which are different from each other are used, and therefore construction and maintenance thereof becomes cumbersome. 
     PRIOR ART DOCUMENT 
     Patent Document 
     
         
         [Patent document 1] Tokukai 2004-349764 A (paragraph [0034]) 
         [Patent document 2] Tokukai 2005-217565 A (paragraph [0046]) 
         [Patent document 3] Tokukai 2006-067239 A (paragraph [0037]) 
       
    
     SUMMARY OF INVENTION 
     Technical Problem 
     As described above, a technology related to the invention includes the following problem. 
     That is, in the wireless transmission system employing the adaptive modulation scheme, data is jammed on the transmission side, a delay time increases, and data overflowed from a buffer is lost. 
     In addition, if a wireless system is realized by combining the inventions described in Patent documents 1 to 3, total system cost increases, construction and maintenance thereof is cumbersome. 
     Furthermore, in the wireless transmission system shown in  FIG. 12  and  FIG. 13 , construction and maintenance thereof is cumbersome. 
     An object of the present invention is to solve a problem of alleviating the occurrence of delays in delivery, or the abandonment, of transmitted data in the transmission device, and to provide a transmission device, a transmission method, and a transmission system, in which construction and maintenance thereof are easy. 
     Solution to Problem 
     A transmission device of the present invention includes a sorting means for assigning a first signal to either a first path or a second path on the basis of assignment information for assigning the first signal, a first transmitting means for transmitting the signal from among the first signal that is allocated to the first path upon the first path, and a second transmitting means for transmitting the signal from among the first signal that is allocated to the second path upon the second path. 
     A transmission method of the present invention includes assigning a first signal to either a first path or a second path on the basis of assignment information for assigning the first signal, transmitting the signal from among the first signal that is allocated to the first path upon the first path, and transmitting the signal from among the first signal that is allocated to the second path upon the second path. 
     Advantageous Effects of Invention 
     The present invention can alleviate the occurrence of delays in delivery, or the abandonment, of transmitted data in the transmission device, when the transmission capacity of the transmission device is reduced. The present invention can provide the transmission device, for which construction and maintenance are easy. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a first diagram illustrating a configuration of a wireless transmission system of a first exemplary embodiment of the present invention. 
         FIG. 2A  is a diagram illustrating an identification method of a MAC frame of the first exemplary embodiment of the present invention. 
         FIG. 2B  is a diagram illustrating an identification method of a MAC frame of the first exemplary embodiment of the present invention. 
         FIG. 3  is a second diagram illustrating a configuration of a wireless transmission system of the first exemplary embodiment of the present invention. 
         FIG. 4  is a diagram illustrating a configuration of a wireless transmission system of a second exemplary embodiment of the present invention. 
         FIG. 5  is a diagram illustrating a configuration of a wireless transmission system of a third exemplary embodiment of the present invention. 
         FIG. 6A  is a diagram illustrating an identification method of a MAC frame of the third exemplary embodiment of the present invention. 
         FIG. 6B  is a diagram illustrating an identification method of a MAC frame of the third exemplary embodiment of the present invention. 
         FIG. 7  is a diagram illustrating a configuration of a wireless transmission system of a fourth exemplary embodiment of the present invention. 
         FIG. 8A  is a diagram illustrating a MAC frame identification method of the fourth exemplary embodiment of the present invention. 
         FIG. 8B  is a diagram illustrating a MAC frame identification method of the fourth exemplary embodiment of the present invention. 
         FIG. 9  is a diagram illustrating a configuration of a communication quality control unit of the fourth exemplary embodiment of the present invention. 
         FIG. 10  is a diagram illustrating a configuration of a transmission device of a fifth exemplary embodiment of the present invention. 
         FIG. 11  is a diagram illustrating a configuration of a wireless transmission system related to the present invention. 
         FIG. 12  is a diagram illustrating a configuration of another wireless transmission system related to the present invention. 
         FIG. 13  is a diagram illustrating a configuration of another wireless transmission system related to the present invention. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     First Exemplary Embodiment 
     A first exemplary embodiment of the present invention is described below using  FIG. 1  to  FIG. 3 . 
       FIG. 1  is a first diagram illustrating a configuration of a wireless transmission system of a first exemplary embodiment of the invention. 
     The wireless transmission system described in  FIG. 1  includes user networks a 1  and b 1 , and wireless transmission devices a 2 , b 2 , c 2  and d 2 . The user network a 1  and the user network b 1  exchange data which is composed of a MAC (Media Access Control) frame through the wireless transmission devices each other. 
     A configuration of the wireless transmission devices a 2  is described below. A configuration of each of the wireless transmission devices b 2 , c 2  and d 2  is the same as that of the wireless transmission devices a 2 . Accordingly, descriptions on the configurations of the wireless transmission devices b 2 , c 2  and d 2  are omitted. 
     The wireless transmission device a 2  includes a frame sorting unit a 3 , a class A buffer a 4 , a class B buffer a 5 , and a wireless transmission unit a 6 . The wireless transmission device a 2  further includes a sorting system determination unit a 7 , an adaptive modulation determination unit a 8 , a wireless reception unit a 9  and a transmission buffer a 10 . 
     A transmission-reception antenna a 15  connects with the wireless transmission unit a 6  and the wireless reception unit a 9 . 
     The wireless transmission device a 2  includes a wireless modulation function of an adaptive modulation scheme. Operations of the adaptive modulation scheme in the wireless transmission device a 2  will be described afterwards. 
     The frame sorting unit a 3  receives an external input LAN signal a 100  from the user network a 1 . The frame sorting unit a 3  sorts the external input LAN signal a 100  received from the user network a 1  into a wireless direction and an adjacent station direction on the basis of a frame identification method designated by a sorting system notification signal a 701  from the sorting system determination unit a 7 . 
     Here, “wireless direction” means a direction of a path on which the MAC frame is transmitted from a present wireless transmission device as a wireless signal. “Adjacent station direction” means a direction of a path on which the MAC frame is transferred to an adjacent wireless transmission device (hereinafter referred to as “adjacent station”). 
     The frame sorting unit a 3  stores the MAC frame, which is sorted in the wireless direction, into the class A buffer a 4 , and stores the MAC frame, which is sorted in the adjacent station direction, into the class B buffer a 5 . 
     The class A buffer a 4  outputs the stored MAC frame to the wireless transmission unit a 6  as a wireless transmission LAN signal a 401 . The wireless transmission unit a 6  modulates the wireless transmission LAN signal a 401  on the basis of the modulation scheme designated by a modulation scheme notification signal a 801  received from the adaptive modulation determination unit a 8  and performs frequency conversion to generate a wireless signal a 601 . The transmission-reception antenna a 15  transmits a wireless signal a 601  to the wireless transmission device b 2  which is the wireless opposite station as a wireless signal a 602 . A transmission-reception antenna b 15  connecting with the wireless transmission device b 2  which is the wireless opposite station transmits the received wireless signal a 602  to the wireless transmission device b 2  as a wireless signal a 603 . In the first exemplary embodiment, a case in which the wireless transmission unit a 6  includes the transmission capacity of up to 150 Mbps is described as an example. Of course, the transmission capacity of the wireless transmission unit b 6  is not limited to the value. 
     The class B buffer a 5  outputs the stored MAC frame to the wireless transmission device c 2  as an adjacent transmission LAN signal a 501 . 
     On the other hand, the transmission-reception antenna a 15  receives the wireless signal b 602  transmitted by the transmission-reception antenna b 15  as a radio wave, and outputs it to the wireless reception unit a 9  as a wireless signal b 603 . The wireless reception unit a 9  frequency-converts and demodulates the wireless signal b 603  received by the transmission-reception antenna a 15 . 
     The modulation scheme used for modulation of the wireless signal b 603  is defined for each frame of the wireless signal b 603 . Regarding the wireless signal b 603 , information on the modulation scheme of the frame to be demodulated (hereinafter, referred to as “demodulation scheme information”) is described as a modulation scheme signal a 902  in the frame of the wireless signal b 603  which is received prior to the frame to be demodulated. The wireless reception unit a 9  extracts the modulation scheme signal a 902  from the received wireless signal b 603  and outputs it to the adaptive modulation determination unit a 8 . 
     The modulation scheme signal a 902  includes information related to a modulation scheme used hereafter in the wireless transmission unit a 6  (hereinafter, referred to as “modulation scheme information”) in addition to the above described demodulation scheme information. The modulation scheme information will be described afterwards. 
     The adaptive modulation determination unit a 8  generates a demodulation scheme notification signal a 802  on the basis of the demodulation scheme information included in the modulation scheme signal a 902  and outputs it to the wireless reception unit a 9 . The wireless reception unit a 9  demodulates the frame of the wireless signal b 603  using the demodulation scheme notification signal a 802 . 
     The wireless reception unit a 9  outputs the demodulated LAN signal to the transmission buffer a 10  as a wireless reception LAN signal a 901 . The transmission buffer a 10  outputs an output LAN signal c 1001  from the wireless transmission device c 2  which is the adjacent station and the wireless reception LAN signal a 901  to the user network a 1  as an external output LAN signal a 1001 . 
     A configuration of each part of the wireless transmission device b 2  is the same as that of the wireless transmission device a 2 . An explanation on each part of the wireless transmission device b 2  and illustration of each part thereof in  FIG. 1  are, therefore, omitted. Each part of the wireless transmission device b 2  corresponding to each part of the wireless transmission device a 2  is described by replacing “a” in the reference numerals in the wireless transmission device a 2  by “b”. 
     The transmission-reception antenna b 15  connected to the wireless transmission device b 2  receives the wireless signal a 602  transmitted as a radio wave from the wireless transmission device a 2  through the transmission-reception antenna a 15 , and outputs it as the wireless signal a 603 . The wireless reception unit b 9  frequency-converts the wireless signal a 603  received from the transmission-reception antenna b 15  and demodulates it according to the modulation scheme described in the received signal. Further, the wireless reception unit b 9  outputs the demodulated LAN signal to a transmission buffer b 10  as a wireless reception LAN signal b 901 . The transmission buffer b 10  outputs an output LAN signal d 1001  from the wireless transmission device d 2  which is the adjacent station and the wireless reception LAN signal b 901  to the user network b 1  as an external output LAN signal b 1001 . 
     Operations in the wireless transmission device a 2 , in which the modulation scheme used in the wireless transmission unit a 6  is determined by the adaptive modulation scheme, are described. The wireless transmission device b 2  determines the modulation scheme information which the wireless transmission device b 2  transmits by overlaying on the wireless signal b 603  on the basis of a reception result in the wireless reception unit b 9  of the wireless signal a 603  transmitted from the wireless transmission device a 2 . For example, if a reception condition of the wireless signal a 603  is degraded in the wireless transmission device b 2 , the wireless transmission device b 2  adds information indicating the modulation scheme with a smaller multi-value number to the modulation scheme signal a 902  and transmits them by overlaying on the wireless signal b 601 . The wireless reception unit a 9  extracts the modulation scheme signal a 902  from the received wireless signal a 603  and outputs it to the adaptive modulation determination unit a 8 . 
     The adaptive modulation determination unit a 8  generates the modulation scheme notification signal a 801  on the basis of the modulation scheme information included in the modulation scheme signal a 902  extracted from the wireless signal b 603 , and outputs it to the sorting system determination unit a 7  and the wireless transmission unit a 6 . The adaptive modulation determination unit a 8  generates the demodulation scheme notification signal a 802  demodulating the frame of the wireless signal b 603  continuously received on the basis of the modulation scheme information included in the modulation scheme signal a 902  and outputs it to the wireless reception unit a 9 . 
     Tolerance to degradation of the reception condition is generally improved, as the multi-value number of the modulation scheme becomes small. Accordingly, even if the condition of the wireless transmission path is degraded, transmission may be continued without fault by employing the modulation scheme with a smaller multi-value number. As the multi-value number of the modulation scheme becomes small, data volume which can be transmitted per one-time modulation decreases, and accordingly the transmission capacity of the wireless transmission path is reduced. 
     When the reception condition of the wireless signal a 603  is improved in the wireless transmission device b 2 , the wireless transmission device b 2  transmits the modulation scheme signal a 902  indicating the modulation scheme with the larger multi-value number by overlapping on the wireless signal b 601 . 
     As a result, the wireless transmission device a 2  is capable of performing transmission with the modulation scheme having the multi-value number suitable for the condition of the wireless transmission path. 
     The sorting scheme determination unit a 7  generates the sorting scheme notification signal a 701  designating the frame identification method on the basis of the modulation scheme notification signal a 801 , and informs the frame sorting unit a 3  of it. Details of the frame identification method will be described afterwards. 
     A sorting scheme of the MAC frame in the frame sorting unit a 3  is explained. Frame sorting is performed using a value of an arbitrary field defined in the MAC frame. As a type of the field, MAC DA (Destination Address), MAC SA (Source Address), VLAN CoS (Virtual LAN Class of Service), VLAN ID, and the like, are known. Further, as a type of the field, IPv4 (Internet Protocol version 4) SA, IPv4 DA, IPv6 SA, IPv6 DA, MPLS (Multiprotocol Label Switching) ID, and the like, are also known. A characteristic value may be extracted from the value of the field using a hash function, and sorting of the MAC frame may be performed using the characteristic value, on the basis of the number of paths to be allocated. Sorting of the MAC frame may be performed on the basis of the result of comparison of the value of the field with a threshold value, or the result of pattern-matching on the value of the field. 
     Identification in the VLAN CoS (Virtual LAN Class of Service) field is described below with reference to  FIG. 2A  and  FIG. 2B . 
       FIG. 2A  and  FIG. 2B  are diagrams illustrating the identification methods of the MAC frame. The identification method of the MAC frame in  FIG. 2A  and  FIG. 2B  shows a correspondence relation between the values of the CoS field of the MAC frame and buffer classes to be allocated (A or B). In  FIG. 2A  and  FIG. 2B , “CoS value” is the CoS value written in the MAC frame, and “allocation class” is a class of the buffer which is allocated to the value of the CoS field. 
     The MAC frame whose allocation class is the class A is stored in the class A buffer and outputted toward the wireless transmission unit a 6 . The MAC frame allocated to the class B is stored in the class B buffer and outputted toward the frame sorting unit c 3  of the wireless transmission device c 2 . 
       FIG. 2A  illustrates the identification method of the MAC frame with the wireless transmission capacity of the wireless transmission unit a 6  which ranges from greater than or equal to 100 Mbps to smaller than or equal to 150 Mbps. When the wireless transmission capacity is greater than or equal to 100 Mbps and smaller than or equal to 150 Mbps, the MAC frame with the CoS values of 1 to 4 is allocated to the class A (wireless direction) and the MAC frame with the CoS values of 5 to 8 is allocated to the class B (adjacent station direction). 
     On the other hand,  FIG. 2B  illustrates the identification method of the MAC frame with the wireless transmission capacity which is smaller than 100 Mbps.  FIG. 2B  illustrates that when the wireless transmission capacity is smaller than 100 Mbps, the MAC frame whose CoS value is 1 or 2 is allocated to the class A (wireless direction) and the MAC frame whose CoS value is 3 to 8 is allocated to the class B (adjacent station direction). 
     When the modulation scheme notification signal a 801  designates the modulation scheme in which the wireless transmission capacity is greater than or equal to 100 Mbps and smaller than or equal to 150 Mbps, the sorting scheme determination unit a 7  chooses the identification method corresponding to the wireless transmission capacity of greater than or equal to 100 Mbps and smaller than or equal to 150 Mbps. The sorting scheme determination unit a 7  transmits the sorting scheme notification signal a 701  to the frame sorting unit a 3  in order to apply the identification method corresponding to the wireless transmission capacity of greater than or equal to 100 Mbps and smaller than or equal to 150 Mbps to MAC frame allocation. 
     If the reception condition of the wireless signal a 603  is degraded in the wireless transmission device b 2 , modulation scheme information included in the modulation scheme signal a 902  changes. The adaptive modulation determination unit a 8  generates the modulation scheme notification signal a 801  on the basis of the modulation scheme information so that the modulation scheme in the wireless transmission unit a 6  is switched into the modulation scheme with the multi-value number smaller than that of the current modulation scheme. Accordingly, the wireless transmission capacity of the wireless signal a 601  of the wireless transmission device a 2  is reduced. 
     If the wireless transmission capacity becomes 80 Mbps due to change of the modulation scheme, the sorting scheme determination unit a 7  detects that the wireless transmission capacity becomes smaller than 100 Mbps, on the basis of the modulation scheme notification signal a 801 . The sorting scheme determination unit a 7  outputs the sorting scheme notification signal a 701  including information indicating the change of the wireless transmission capacity to the frame sorting unit a 3 . 
     If the wireless transmission capacity becomes 80 Mbps, the sorting scheme notification signal a 701  instructs the frame sorting unit a 3  to choose the identification method in which the wireless transmission capacity is smaller than 100 Mbps and to apply it to MAC frame allocation. Accordingly, the frame sorting unit a 3  sorts the MAC frame into the class A (wireless direction) only when the CoS value of the MAC frame is 1 or 2, according to the identification method shown in  FIG. 2B . When the CoS value takes the other value, the frame sorting unit a 3  sorts all the MAC frames into the class B (adjacent station direction). 
     As a result, if the wireless transmission capacity becomes 80 Mbps, MAC frames sorted into the class A buffer decreases compared with the case in which the wireless transmission capacity of the wireless signal a 601  is greater. Therefore, the MAC frames which are transmitted as the wireless signal a 601  through the wireless transmission unit a 6  decrease. When the wireless transmission capacity of the wireless signal a 601  is reduced, occurrence of MAC frame jam and packet loss due to overflow in the wireless transmission device a 2  are suppressed. 
     Next, operations of the wireless transmission devices c 2  and d 2  are described. 
     A configuration of the wireless transmission device c 2  is the same as that of the wireless transmission device a 2 . Each part of the wireless transmission devices c 2  and d 2  which includes the same function as that of each part of the wireless transmission device a 2  in  FIG. 1  is described by replacing “a” in the reference numerals of the wireless transmission device a 2  by “c” or “d”. Descriptions on the function are, therefore, omitted. 
     The frame sorting unit c 3  receives the adjacent transmission LAN signal a 501  from the class B buffer a 5  of the wireless transmission device a 2 . The frame sorting unit c 3  sorts the adjacent transmission LAN signal a 501  received from the wireless transmission device a 2  into the wireless direction and the adjacent station direction on the basis of the frame identification method designated by the sorting scheme notification signal a 701  from the sorting scheme determination unit c 7 . The frame sorting unit c 3  stores the MAC frame which is sorted into the wireless direction into the class A buffer c 4  and stores the MAC frame which is sorted into the adjacent station direction into the class B buffer c 5 . 
     The class A buffer c 4  outputs the stored MAC frame to the wireless transmission unit c 6  as a wireless transmission LAN signal c 401 . The wireless transmission unit c 6  modulates the wireless transmission LAN signal c 401  on the basis of the modulation scheme designated by the modulation scheme notification signal c 801  received from the adaptive modulation determination unit c 8 , generates the wireless signal c 601  through frequency conversion, and outputs it to the transmission-reception antenna. A transmission-reception antenna c 15  transmits the wireless signal c 601  to the wireless transmission device d 2  which is the wireless opposite station as a wireless signal c 602 . 
     The class B buffer c 5  outputs the stored MAC frame to an adjacent wireless transmission device which is not shown, as the adjacent transmission LAN signal c 501 . 
     If the wireless transmission device c 2  does not connect with any adjacent wireless transmission device, the wireless transmission device c 2  allocates all of the adjacent transmission LAN signals a 501  received from the wireless transmission device a 2  into the class A buffer. 
     The transmission-reception antenna c 15  receives a wireless signal d 602  transmitted by a transmission-reception antenna d 15 . A wireless reception unit c 9  frequency-converts a wireless signal d 603  outputted by the transmission-reception antenna c 15  and demodulates it by the modulation scheme described in the received signal. The wireless reception unit c 9  outputs the demodulated LAN signal to a transmission buffer c 10  as a wireless reception LAN signal c 901 . The transmission buffer c 10  outputs both an output LAN signal e 1001  from an adjacent wireless transmission device which is not shown and the wireless reception LAN signal c 901  to the transmission buffer a 10  of the wireless transmission device a 2  as the external output LAN signal c 1001 . 
     The wireless transmission LAN signal c 401  outputted from the class A buffer c 4  of the wireless transmission device c 2  is received by the wireless transmission device d 2 . The transmission-reception antenna d 15  of the wireless transmission device d 2  receives a wireless signal c 602  through the transmission-reception antenna c 15  and outputs the received wireless signal c 602  to a wireless reception unit d 9  as a wireless signal c 603 . The wireless reception unit d 9  frequency-converts the wireless signal c 603  and demodulates it by the modulation scheme described in the received signal. The wireless reception unit d 9  outputs the demodulated LAN signal to a transmission buffer d 10  as a wireless reception LAN signal d 901 . The transmission buffer d 10  outputs both an output LAN signal f 1001  from a wireless transmission device f 2  which is an adjacent station not shown and the wireless reception LAN signal d 901  to the transmission buffer b 10  of the wireless transmission device b 2  as an external output LAN signal d 1001 . 
     The wireless transmission device c 2  sorts the MAC frame by the same processing as the wireless transmission device a 2 , with respect to the adjacent transmission LAN signal a 501  outputted from the class B buffer of the wireless transmission device a 2 . The wireless transmission device c 2  modulates the wireless transmission LAN signal c 401  outputted from the class A buffer c 4  in the wireless transmission unit c 6  and generates the wireless signal c 601 . The transmission-reception antenna c 15  transmits the wireless signal c 601  as the wireless signal c 602 . 
     The wireless transmission device d 2  demodulates the wireless signal c 603  which is received from the wireless transmission device c 2  through the transmission-reception antennas c 15  and d 15 , and outputs the demodulated LAN signal to the transmission buffer b 10  of the wireless transmission device b 2  as the external output LAN signal d 1001 . The transmission buffer b 10  of the wireless transmission device b 2  outputs both the output LAN signal d 1001  from the wireless transmission device d 2  which is the adjacent station and the wireless reception LAN signal b 901  to the user network b 1  as the external output LAN signal b 1001 . 
     As described above, the wireless transmission system of the first exemplary embodiment changes the identification method of the MAC frame in conjunction with the change of the modulation scheme, even if the wireless transmission capacity of the wireless transmission device is reduced due to the adaptive modulation function. As a result, the wireless transmission system of the first exemplary embodiment can reduce MAC frame allocation amount in the wireless direction in the wireless transmission device, when the wireless transmission capacity of the wireless transmission device is reduced. In the wireless transmission system of the first exemplary embodiment, the adjacent station allocates again the MAC frame which is allocated in the adjacent station direction in the wireless transmission device prior to the station. 
     In each wireless transmission device, the MAC frame which is allocated in the wireless direction is modulated by each wireless transmission unit and transmitted to the opposite wireless transmission device as a wireless signal. 
     The wireless transmission device which receives the wireless signal demodulates the wireless signal, couples the demodulated wireless signal and the output LAN signal transmitted from the adjacent station, and transmits them toward a higher-ranked wireless transmission device or the user network. 
     As a result, the wireless transmission system of the first exemplary embodiment can suppress MAC frame jam in the wireless transmission device and occurrence of packet loss due to overflow of a high priority packet, in the wireless transmission system employing the adaptive nodulation scheme. 
     If the MAC frame is transmitted using the adjacent wireless transmission devices c 2  and d 2 , transmission delay between the wireless transmission devices a 2  and c 2 , and transmission delay between the wireless transmission devices d 2  and b 2  are added. That is, if the adjacent wireless transmission devices c 2  and d 2  are employed, transmission delay increases compared with the transmission by the wireless transmission device of the first stage. 
     Accordingly, when the condition of the wireless transmission path is improved and the wireless transmission capacity of the wireless transmission device a 2  increases, the wireless transmission device a 2  may automatically use the identification method corresponding to the increased wireless transmission capacity. 
     For example, if the wireless transmission capacity increases while the identification method for the MAC frame shown in  FIG. 2B  is used, the wireless transmission device a 2  may change the identification method for the MAC frame into the identification method shown in  FIG. 2A . As a result, since many high priority packets may be transmitted by the wireless transmission device a 2  close to the user network a 1 , increase of transmission delay in the wireless transmission system is suppressed. 
       FIG. 3  is a second diagram illustrating the wireless transmission system of the first exemplary embodiment. 
       FIG. 1  illustrates the configuration in which the wireless transmission devices c 2  and d 2  are arranged as the wireless transmission device adjacent to the wireless transmission devices a 2  and b 2 , respectively. On the contrary,  FIG. 3  illustrates a configuration in which wireless transmission devices e 2  and f 2  adjacent to the wireless transmission devices c 2  and d 2  are further connected in series, respectively. In the following, in the wireless transmission system of the first exemplary embodiment, operations, which are performed when the wireless transmission device is further connected in series, are described. 
     In  FIG. 3 , an inner configuration and a function of each of the wireless transmission devices e 2  and f 2  is the same as that of the wireless transmission devices a 2 , b 2 , c 2 , and d 2 . In addition, operations of the wireless transmission devices e 2  and f 2  are the same as those of the wireless transmission devices c 2  and d 2  described in the first exemplary embodiment. 
     That is, a frame sorting unit e 3  receives the adjacent transmission LAN signal c 501  from the class B buffer c 5  of the wireless transmission device c 2 . The frame sorting unit e 3  sorts the adjacent transmission LAN signal c 501  into the wireless direction and the adjacent station direction. The frame sorting unit e 3  stores the MAC frame sorted in the wireless direction into a class A buffer e 4  and stores the MAC frame sorted in the adjacent station direction into a class B buffer e 5 . 
     The class A buffer e 4  outputs the stored MAC frame to the wireless transmission device e 6  as a wireless transmission LAN signal e 401 . After modulating the wireless transmission LAN signal e 401  on the basis of the modulation scheme designated by the modulation scheme notification signal e 401 , the wireless transmission device e 6  generates a wireless signal e 601  through frequency conversion and outputs it to a transmission-reception antenna e 15 . The transmission-reception antenna e 15  transmits a wireless signal e 602  to a wireless transmission device f 2  which is a wireless opposite station as the wireless signal e 602 . 
     The class B buffer e 5  outputs the stored MAC frame to an adjacent wireless transmission device not shown, as an adjacent transmission LAN signal e 501 . 
     When the wireless transmission device e 2  connects with no adjacent wireless transmission device, the wireless transmission device e 2  allocates all the adjacent transmission LAN signals c 501  received from the wireless transmission device c 2  to the class A buffer. 
     Meanwhile, the transmission-reception antenna e 15  receives a wireless signal f 603  from the wireless transmission device f 2 . A wireless reception unit e 9  frequency-converts the wireless signal f 603  and demodulates it in the modulation scheme described in the reception signal. The wireless reception unit e 9  outputs the demodulated LAN signal to a transmission buffer e 10  as a wireless reception LAN signal e 901 . The transmission buffer e 10  outputs both the output LAN signal from an adjacent wireless transmission device not shown and the wireless reception LAN signal e 901  to the transmission buffer c 10  of the wireless transmission device c 2  as an external output LAN signal e 1001 . 
     A transmission-reception antenna f 15  of the wireless transmission device f 2  receives the wireless signal e 602  from the wireless transmission device e 2 . A wireless reception unit f 9  performs frequency conversion and demodulation with respect to the wireless signal e 602  received by the transmission-reception antenna f 15 . The wireless reception unit f 9  outputs the demodulated LAN signal to a transmission buffer f 10  as the wireless reception LAN signal e 901 . The transmission buffer f 10  outputs both the output LAN signal from an adjacent wireless transmission device not shown and a wireless reception LAN signal f 901  to the transmission buffer d 10  of the wireless transmission device d 2  as an external output LAN signal f 1001 . 
     As described, in the wireless transmission system shown in  FIG. 3 , by employing the wireless transmission devices e 2  and f 2 , the wireless transmission system in which the parallel number is further increased can be configured. In the wireless transmission system shown in  FIG. 3 , MAC frame jam in the wireless transmission device and occurrence of packet loss due to overflow of the high priority packet can be avoided in the wireless transmission system employing the adaptive nodulation system. 
     In the wireless transmission system shown in  FIG. 3 , when the condition of the wireless transmission path is improved and the wireless transmission capacity increases, the high priority packet may be transmitted by the wireless transmission device close to the user network. As a result, also in the wireless transmission system shown in  FIG. 3 , increase of transmission delay in the wireless transmission system can be suppressed. 
     Regarding the first exemplary embodiment, in the wireless transmission system described using  FIG. 1  and  FIG. 3 , if the MAC frames are transmitted in parallel by using a plurality of separate wireless transmission devices, the wireless transmission capacity may be increased. Consequently, only by changing the number of the wireless transmission devices, it is possible to respond to the request for increase and decrease of the transmission capacity of the whole of the wireless transmission system which occurs with expansion and reduction of the network. 
     In doing so, regardless of the number of the wireless transmission devices employed in the wireless transmission system, the wireless transmission devices with the same configuration are applicable. It is not necessary to arrange a high-price apparatus, like the L2SW, inside or outside the wireless transmission device. In the wireless transmission system of the first exemplary embodiment, system design, apparatus manufacturing, construction, and maintenance are easy, and the cost thereof may be suppressed. 
     Second Exemplary Embodiment 
       FIG. 4  is a diagram illustrating a configuration of a wireless transmission system of a second exemplary embodiment of the present invention. Wireless transmission devices a 21 , b 21 , c 21  and d 21  in the wireless transmission system of the second exemplary embodiment shown in  FIG. 4  differ in the wireless transmission unit and the wireless reception unit which are doubled from the wireless transmission devices a 2 , b 2 , c 2  and d 2  of the first exemplary embodiment in  FIG. 1 . 
     The wireless transmission device a 21  is described below as an example. Descriptions thereof are applicable to the wireless transmission devices b 21  to d 21 . The configuration and operations of the part other than the wireless transmission unit and the wireless reception unit are the same as those of the first exemplary embodiment. The inner configuration of the wireless transmission devices b 21 , c 21 , and d 21  are not shown. The configuration and operations of each of the wireless transmission devices b 21 , c 21 , and d 21  are the same as the wireless transmission device a 21 . Regarding the wireless transmission devices b 21 , c 21 , and d 21 , the same name as the wireless transmission device a 21  is used and the reference code “a” is replaced by “b”, “c”, or “d”. 
     The wireless transmission device a 21  shown in  FIG. 4  includes wireless transmission units a 6  and a 61 . The wireless transmission device a 21  includes wireless reception units a 9  and a 91 . 
     The wireless transmission LAN signal a 401  stored in the class A buffer enters the wireless transmission units a 6  and a 61 . The wireless transmission units a 6  and a 61  transmit the inputted wireless transmission LAN signal a 401  as wireless signals a 602  and a 6102  through transmission-reception antennas a 15  and a 151 , respectively. 
     Transmission-reception antennas b 15  and b 151  of the wireless transmission device b 21  which faces the wireless transmission device a 21  receive the wireless signals a 602  and a 6102 , respectively. The wireless reception units a 9  and a 91  output wireless signals a 603  and a 6103  received by the transmission-reception antennas b 15  and b 151  as wireless reception LAN signal b 901  and b 9101 , respectively. 
     The wireless transmission system shown in  FIG. 4  includes a redundant configuration of the wireless transmission units a 6  and a 61  and that of the wireless reception units b 9  and b 91 . That is, normally, transmission is performed on the path in which the wireless transmission unit a 6  transmits the wireless signal a 602 , and the wireless reception unit b 9  receives the wireless signal a 602 . 
     When the wireless transmission unit a 6 , the transmission-reception antennas a 15 , the wireless reception unit b 9  or transmission-reception antennas b 15  gets out of order, a path is switched to the one in which the wireless transmission unit a 61  works. The transmission-reception antenna a 151  transmits the wireless signal a 6101  generated by the wireless transmission unit a 61  due to the path switching. The transmission-reception antenna b 151  outputs the received wireless signal a 602  to the wireless reception unit b 91  as the wireless signal a 603 . 
     As described, the wireless transmission system of the second exemplary embodiment has an advantageous effect that, even when either the wireless transmission unit a 6  or the wireless reception unit b 9  goes out of order, transmission can be continued, by switching the path to the one including the wireless transmission unit a 61  and the wireless reception unit b 91 . In this case, when disappearance of the received signal or degradation of the reception condition in the wireless reception unit b 9  occurs, the transmission path may be switched to the path including the wireless transmission unit a 61  and the wireless reception unit b 91 . As a result of the switch, if continuation of the transmission is possible, the transmission on the path including the wireless transmission unit a 61  and the wireless reception unit b 91  may be continued. That is, the wireless transmission system of the second exemplary embodiment has an effect that it can reduce the probability that communication is interrupted due to a device failure, degradation of a line condition, or the like, by switching the path to the one including the wireless transmission unit a 61  and the wireless reception unit b 91  when a reception condition of a wireless signal is degraded. 
     In addition, in the second exemplary embodiment, the wireless transmission unit a 6  and the wireless reception unit a 9  connect with the transmission-reception antenna a 15 , and the wireless reception unit a 9  and the wireless reception unit a 91  connect with the transmission-reception antenna a 151 . However, a transmission-reception antenna may be configured as one system, and both the wireless transmission units a 6  and a 61  may connect with the transmission-reception antenna a 15 . 
     Further, the wireless transmission system of the second exemplary embodiment is applicable to configurations of the first exemplary embodiment and the modified example thereof. The first exemplary embodiment and the modified example thereof in which the wireless transmission unit and the wireless reception unit include the redundant configuration have effects of both the wireless transmission system of the first exemplary embodiment and the wireless transmission system of the second exemplary embodiment. 
     Third Exemplary Embodiment 
       FIG. 5  is a diagram illustrating a configuration of a wireless transmission system of a third exemplary embodiment of the present invention. Wireless transmission devices a 22  and b 22  in the wireless transmission system of a third exemplary embodiment of  FIG. 5  differ in configurations of the wireless transmission unit and the wireless reception unit from the wireless transmission devices a 21  and b 21  in the second exemplary embodiment in  FIG. 4 . 
     The wireless transmission device a 22  is described below as an example. An operation of each part of the wireless transmission device b 22  is the same as that of the wireless transmission device a 22 . A configuration and an operation of the part other than the wireless transmission unit and the wireless reception unit are the same as those of the first exemplary embodiment. 
     In the wireless transmission device a 22  shown in  FIG. 5 , the frame sorting unit a 3  receives the external input LAN signal a 100  from the user network a 1 . The frame sorting unit a 3  stores the area where the external input LAN signal a 100  to be stored in the class A buffer a 40  or the class B buffer a 50  which is the buffer in the wireless direction, or the class C buffer which is the buffer in the adjacent station direction, on the basis of the frame identification method designated by the sorting scheme notification signal a 701 . 
     The class A buffer a 40  outputs the stored MAC frame to the wireless transmission unit a 6  as the wireless transmission LAN signal a 401 . The wireless transmission unit a 6  modulates the wireless transmission LAN signal a 401  on the basis of the modulation scheme, used in the wireless transmission unit a 6 , designated by the modulation scheme notification signal a 801  received from the adaptive modulation determination unit a 8 . The wireless transmission unit a 6  frequency-converts the modulated wireless transmission LAN signal a 401  and outputs the wireless signal a 601 . The transmission-reception antenna a 15  transmits the frequency-converted wireless signal a 601  to the wireless transmission device b 22  which is the wireless opposite station as the wireless signal a 602 . 
     The class B buffer a 50  outputs the stored MAC frame to the wireless transmission unit a 61  as a wireless transmission LAN signal a 5001 . The wireless transmission unit a 61  modulates the wireless transmission LAN signal a 5001  on the basis of the modulation scheme, used in the wireless transmission unit a 61 , designated by the modulation scheme notification signal a 801  received from the adaptive modulation determination unit a 8 . The wireless transmission unit a 61  frequency-converts the modulated wireless transmission LAN signal a 5001  and outputs a wireless signal a 6101 . The transmission-reception antenna a 151  transmits the frequency-converted wireless signal a 5001  to the wireless transmission device b 22  which is the wireless opposite station as the wireless signal a 6101 . 
     In order to suppress interference between the wireless signal a 602  and the wireless signal a 6102  in the wireless transmission device b 22 , the wireless signal a 602  and the wireless signal a 6102  may be transmitted using carrier waves having different frequencies. Or, a transmission-reception antenna with high directionality is applicable to the transmission-reception antennas a 15 , a 151 , b 15 , and b 151 . In this case, the directionality is adjusted so that the transmission-reception antenna a 15  faces the transmission-reception antenna b 15 , and the transmission-reception antenna a 151  faces the transmission-reception antenna b 151 . Thereby, the transmission path between the transmission-reception antenna a 15  and the transmission-reception antenna b 15  may be spatially separated from the transmission path between the transmission-reception antenna a 151  and the transmission-reception antenna b 151 . Even though the wireless signal a 602  and the wireless signal a 6102  employ the carrier wave with the same frequency, interference between the wireless signal a 603  and the wireless signal a 6103  in the wireless reception unit b 9  or the wireless reception unit b 91  may be reduced. 
     The class C buffer a 60  outputs the stored MAC frame to the wireless transmission device c 2  which is the adjacent station as the adjacent transmission LAN signal a 6001 . 
     On the other hand, the transmission-reception antenna a 15  receives the wireless signal b 61  from the wireless transmission device b 22 . The wireless reception unit a 9  frequency-converts the wireless signal b 603  outputted by the transmission-reception antenna a 15  and demodulates it on the basis of the modulation scheme described in the reception signal. The wireless reception unit a 9  outputs the demodulated LAN signal to the transmission buffer a 10  as the wireless reception LAN signal a 901 . 
     The transmission-reception antenna a 151  receives a wireless signal b 6102  from the wireless transmission device b 22 . The wireless reception unit a 91  frequency-converts a wireless signal b 6103  outputted by the transmission-reception antenna a 151  and demodulates it on the basis of the modulation scheme described in the reception signal. The wireless reception unit a 91  outputs the demodulated LAN signal to the transmission buffer a 10  as a wireless reception LAN signal a 9101 . 
     By employing the configuration which is the same as the configuration reducing the interference in the wireless transmission device b 22  described above, interference between the wireless signal b 602  and the wireless signal b 6102  in the wireless transmission device a 22  may be suppressed. That is, the wireless signal b 601  and the wireless signal b 6101  may be transmitted using carrier waves with different frequency. A transmission-reception antenna with high directionality is applicable if the wireless transmission device a 22  and the wireless transmission device b 22  include the transmission-reception antennas a 15 , a 151 , b 15 , and b 151 , respectively. If the transmission-reception antenna with high directionality is employed, even though the wireless signal b 602  and the wireless signal b 6102  employ the carrier wave with the same frequency, interference between the wireless signal b 603  and the wireless signal b 6103  in the wireless reception unit a 9  or the wireless reception unit a 91  may be reduced. 
     The transmission buffer a 101  outputs the output LAN signal c 1001  from wireless transmission device c 22  which is the adjacent station and the wireless reception LAN signals a 901  and a 9101  to the user network a 1  as the external output LAN signal a 1001 . 
     Next, a sorting scheme of the MAC frame in the frame sorting unit a 3  is described. 
     As described above, the MAC frame stored in the class A buffer a 40  is transmitted on the path in which the MAC frame is transmitted from the transmission-reception antenna a 15  as the wireless signal a 602  and received by the transmission-reception antenna b 15  in the wireless transmission device b 22  (hereinafter, referred to as “path A”). The MAC frame stored in the class B buffer a 50  is transmitted on the path in which the MAC frame is transmitted from the transmission-reception antenna a 151  as the wireless signal a 6101  and reaches the wireless reception unit b 91  through the transmission-reception antenna b 151  in the wireless transmission device b 22  (hereinafter, referred to as “path B”). 
     The frame sorting unit a 3  sorts a frame using the value of the arbitrary field defined in the MAC frame. As the type of fields, MACDA, MAC SA, VLAN CoS (VLAN ID), IPv4 SA, IPv4 DA, IPv6 SA, IPv6 DA, MPLS ID, and the like are known. A characteristic value is extracted from the value of the field using the hash function on the basis of the number of paths to be allocated, and sorting of MAC frame may be performed on the basis of the value. Otherwise, the sorting of the MAC frame may be performed on the basis of the result of comparison of the value of the field with a threshold value or the result of pattern-matching on the value of the field. 
     Identification on the VLAN CoS (Virtual LAN Class of Service) field is described below. 
     In the third exemplary embodiment, the frame sorting unit a 3  stores the MAC frame with higher priority or higher importance of the MAC frame allocated in the wireless direction into the buffer connecting with the path with a good communication condition. That is, when the communication condition of the path A is better than the communication condition of the path B, the frame sorting unit a 3  stores the MAC frame with higher priority or higher importance of the MAC frame allocated in the wireless direction into the class A buffer. On the contrary, when the communication condition of the path B is better than the communication condition of the path A, the frame sorting unit a 3  stores the MAC frame with higher priority or higher importance of the MAC frame allocated in the wireless direction into the class B buffer. 
       FIG. 6A  and  FIG. 6B  are diagrams illustrating an identification method of the MAC frame in the third exemplary embodiment of the invention.  FIG. 6A  and  FIG. 6B  illustrates the correspondence relation between the values of the CoS field of the MAC frame and buffer classes to be allocated. In  FIG. 6A  and  FIG. 6B , “CoS value” is the CoS value written in the MAC frame, and “allocation class” is the class of the buffer which is allocated to the values of the CoS field (A, B, C). 
     In the identification method of the MAC frame shown in  FIG. 6A , the MAC frame with higher priority or higher importance is stored in the class A buffer. In the identification method of the MAC frame shown in  FIG. 6B , the MAC frame with higher priority or higher importance is stored in the class B buffer. 
     In  FIG. 6A , the MAC frame with the CoS value of 1 or 2, that is, the MAC frame with high priority, is stored in the class A buffer a 40  and transmitted on the path A. The MAC frame with the CoS value of 3 or 4, which is not allocated to the class A, among the MAC frames allocated in the wireless direction is stored the class B buffer a 50  and transmitted on the path B. 
     In  FIG. 6B , the MAC frame with the CoS value of 1 or 2, that is, the MAC frame with high priority, is stored in the class B buffer a 50  and transmitted on the path B. The MAC frame with the CoS value of 3 or 4, which is not allocated to the class B, among the MAC frames allocated in the wireless direction is stored the class A buffer a 40  and transmitted on the path A. 
     In both cases, a packet with CoS value of greater than or equal to 5 is allocated in the adjacent station direction as the class C. 
     The wireless reception unit a 9  extracts the modulation scheme signal a 902  from the wireless signal b 603  received by the transmission-reception antenna a 15  and outputs it to the adaptive modulation determination unit. The wireless reception unit a 91  extracts the modulation scheme signal a 903  from the wireless signal b 6101  received by the transmission-reception antenna a 151  and outputs it to the adaptive modulation determination unit. 
     The adaptive modulation determination unit a 8  generates the modulation scheme notification signal a 801  on the basis of information included in the modulation scheme signals a 902  and a 903  and outputs it to the sorting scheme determination unit a 7  and the wireless transmission unit a 6 . 
     The modulation scheme notification signal a 801  includes information indicating the modulation scheme to be employed with respect to the wireless transmission units a 6  and a 61 . 
     The sorting scheme determination unit a 7  generates the sorting scheme notification signal a 701  designating the frame identification method on the basis of information included in the modulation scheme notification signal a 801  and informs the frame sorting unit a 3  of it. The sorting scheme notification signal a 701  includes information designating the MAC frame identification method used by the frame sorting unit a 3 . 
     The sorting scheme determination unit a 7  compares transmission rates of the modulation schemes which the modulation scheme notification signal a 801  gives to the wireless transmission units a 6  and a 61 . If the transmission rate of the modulation scheme applied to the wireless transmission unit a 6  is larger than that applied to the wireless transmission unit a 61 , the sorting scheme determination unit a 7  determines that the communication condition of the path A is better than that of the path B. 
     In this case, the sorting scheme determination unit a 7  generates the sorting scheme notification signal a 701  designating the frame identification method shown in  FIG. 6A , and informs the frame sorting unit a 3  of it. The frame sorting unit a 3  stores the MAC frame with higher priority in the class A buffer a 40  using the frame identification method shown in  FIG. 6A . 
     Accordingly, the MAC frame with high priority is transmitted to the wireless transmission device b 22  through the path A whose communication condition is better than that of the path B. 
     Inversely, if the transmission rate of the modulation scheme applied to the wireless transmission unit a 61  is larger than that applied to the wireless transmission unit a 6 , the sorting scheme determination unit a 7  determines that the communication condition of the path B is better than that of the path A. In this case, the sorting scheme determination unit a 7  generates the sorting scheme notification signal a 701  designating the frame identification method shown in  FIG. 6B , and informs the frame sorting unit a 3  of it. The frame sorting unit a 3  stores the MAC frame with higher priority in the class B buffer a 50  using the frame identification method shown in  FIG. 6B . Accordingly, the MAC frame with high priority is transmitted to the wireless transmission device b 22  through the path B whose communication condition is better than that of the path A. 
     In the third exemplary embodiment, the wireless transmission device a 22  includes two buffers in the wireless direction, two wireless transmission units, and two wireless reception units. 
     However, the number of the buffers in the wireless direction, wireless transmission units and the wireless reception units is not necessarily two. For example, the wireless transmission device may include three buffers in the wireless direction, three wireless transmission units and three wireless reception units. In this case, the buffers in the wireless direction are the class A buffer to the class C buffer and a buffer in the adjacent direction is a class D buffer. The MAC frames stored in the buffers A to C are transmitted from the different wireless transmission units to the opposite wireless transmission devices. The frame sorting unit allocates the MAC frame to the buffers A to C in the wireless direction so that the wireless transmission unit of the better communication condition is used for higher priority indicated by the CoS value. As described above, when the modulation schemes of the wireless transmission units are compared on the basis of the modulation scheme notification signal outputted by the adaptive modulation determination unit, the communication condition of each of the wireless transmission unit may be obtained. If the three buffers in the wireless direction are arranged, the wireless transmission unit which transmits the MAC frame accurately in accordance with the priority of the MAC frame is chosen. 
     In the third exemplary embodiment, the wireless transmission unit a 6  and the wireless reception unit a 9  connect with the transmission-reception antenna a 15 , and the wireless reception unit a 9  and the wireless reception unit a 91  connect with the transmission-reception antenna a 151 . However, the transmission-reception antenna of one system may be configured, and the wireless transmission units a 6  and a 61  and the wireless reception units a 9  and a 91  may connect with the transmission-reception antenna a 15 . 
     As described in the first exemplary embodiment and modified example thereof, the third exemplary embodiment has the same effect as that of the first exemplary embodiment and modified example thereof if the adjacent wireless transmission device is dependently connected therewith. 
     If the configuration of the first exemplary embodiment and modified example thereof is applied to the wireless transmission system of the third exemplary embodiment, the wireless transmission system of the third exemplary embodiment has the effect that the request for increase and decrease of the transmission capacity of the whole wireless transmission system can be satisfied by only changing the number of the wireless transmission devices, and construction and maintenance are easy. 
     Fourth Exemplary Embodiment 
       FIG. 7  is a diagram illustrating a configuration of a wireless transmission system of a fourth exemplary embodiment of the invention. 
     In the wireless transmission system which performs transmission guaranteeing a transmission band of the MAC frame with priority, when the wireless transmission capacity is reduced due to the adaptive modulation function, a single wireless transmission path may not be able to provide the band to be guaranteed. 
       FIG. 8A  and  FIG. 8B  are diagrams illustrating a MAC frame identification method in the fourth exemplary embodiment. Operations that the wireless transmission system in  FIG. 7  performs band guarantee type priority control using the MAC frame identification method in  FIG. 8A  and  FIG. 8B  are described. 
       FIG. 8A  illustrates that the MAC frame with CoS value of 1 requires band guarantee of 100 Mbps in the external input LAN signal a 100 . 
     If the transmission capacity between the wireless transmission device a 23  and the wireless transmission device b 23  is 150 Mbps, the frame sorting unit a 3  allocates the external input LAN signal a 100  into the class A (wireless direction) and class B (adjacent station direction) using the identification method of the wireless transmission capacity of 100 Mbps to 150 Mbps shown in  FIG. 8A . 
     Communication quality control units a 41  and a 51  following the class A buffer and the class B buffer perform priority control and band control with respect to the MAC frame of the buffer of each class. 
       FIG. 9  is a diagram illustrating the configuration of the communication quality control unit a 41 . The communication quality control unit a 41  includes a frame analysis unit a 411 , a buffer a 412 , a priority control unit a 413  and a band control unit a 414 . The buffer a 412  is composed of a four independent buffers a 4121  to a 4124 . The MAC frames corresponding to allocation priority classes A 1  to A 4  shown in  FIG. 8A  are stored in the buffers a 4121  to a 4124 , respectively. 
     The frame analysis unit a 411  reads out the CoS value of the MAC frame received from the class A buffer. The frame analysis unit a 411  stores the MAC frame in any one of buffers a 4121  to a 4124  depending on the allocation priority classes A 1  to A 4 . 
     The priority control unit a 413  reads out the MAC frame stored in the buffer a 412  and outputs it to the band control unit a 414 . 
     For example, when the allocation class of the MAC frame is determined based on the identification method shown in  FIG. 8A , the MAC frames with four kinds of priority classes A 1  to A 4  are allocated to the class A buffer a 4 . The MAC frames with the allocation priorities of classes A 1  to A 4  are stored in the buffers a 4121  to a 4124  of the communication quality control unit a 41 , respectively. That is, the MAC frame with the allocation priority of class A 1  is stored in the buffer a 4121 . 
     When the identification method shown in  FIG. 8A  is employed, the wireless transmission device a 23  has to transmit the MAC frame with priority of the class A 1  at the band of 100 Mbps. 
     Accordingly, the priority control unit a 413  reads out the MAC frame stored in the buffer a 4121  so that the band is equal to or greater than 100 Mbps. 
     When enough transmission capacity still exists after the priority control unit a 413  reads out the MAC frame with priority of the class A 1  from the buffer a 4121 , the priority control unit a 413  reads out the MAC frames stored in the buffers a 4121  to a 4124  on the basis of the priorities of the classes A 2  to A 4 . 
     The priority control unit a 413  may read out the MAC frame from the buffer a 412  on the basis of the priority designated by a communication quality control signal a 702 . 
     The band control unit a 414  outputs the MAC frame read out by the priority control unit a 413  to the wireless transmission unit a 6  at the rate which does not exceed the transmission capacity designated by the communication quality control signal a 702 . 
     The wireless transmission device a 23  guarantees the transmission band of the MAC frame with the specific CoS value. 
     Meanwhile, if the wireless transmission capacity of the wireless transmission device a 23  is reduced to 80 Mbps due to the adaptive modulation function, the wireless transmission capacity falls below the bandwidth of 100 Mbps which has to be allocated to the frame with the CoS value of 1. In this case, if the current identification method is continued, the frame with the CoS value of 1 is liable to be abandoned in the buffer a 41  due to shortage of the wireless transmission capacity. In the wireless transmission system of the fourth exemplary embodiment, the identification method is changed based on the following operations. As a result, in the wireless transmission system of the fourth exemplary embodiment, abandonment of the frame with the CoS value of 1 in the buffer a 41  is avoidable. 
     The sorting scheme determination unit a 7  can automatically detect increase or decrease of the wireless transmission capacity on the basis of change of information, included in the modulation scheme notification signal a 801 , instructing the modulation scheme to the wireless transmission unit a 6 . 
     The sorting scheme determination unit a 7  instructs the frame sorting unit a 3  and the communication quality control units a 41  and a 51  to employ the identification method in which the wireless transmission capacity is less than 100 Mbps. The frame sorting unit a 3  sorts the MAC frame with the CoS value of 1 into the class A buffer on a priority basis. 
     The frame sorting unit a 3  controls the allocation volume of the MAC frame with the CoS value of 1 for the class A buffer so that the data volume of the MAC frame to be stored in the class A buffer a 4  does not exceed 80 Mbps in terms of the transmission capacity. As a result, the MAC frame with the CoS value of 1 is stored in the class A buffer a 4 . If the data volume of the MAC frame stored in the class A buffer a 4  exceeds 80 Mbps in terms of the transmission capacity, the frame sorting unit a 3  sorts the MAC frame with the CoS value of 1 into the class B buffer a 5 . Further, when the data volume of the MAC frame stored in the class A buffer a 4  falls below 80 Mbps in terms of the transmission capacity, the frame sorting unit a 3  stores the MAC frame with the CoS value of 1 into the class A buffer a 4  again. 
     The frame sorting unit a 3  sorts all the MAC frames with the CoS value other than 1 into the class B. 
     The communication quality control unit a 41  performs priority control to guarantee the bandwidth of 80 Mbps with respect to MAC frame stored in the class A buffer, and transmits it in the wireless direction. 
     The communication quality control unit a 51  performs priority control to guarantee the bandwidth of 20 Mbps (that is, bandwidth to be guaranteed (100 Mbps) minus wireless transmission capacity (80 Mbps)) with respect to the frame having CoS value of 1 of the frames sorted in the class B. The communication quality control unit a 51  performs priority control with respect to the MAC frame with the CoS value other than 1 depending on allocation priority, and transmits it to the adjacent station. 
     Operations of the communication quality control units a 41  and a 51  are the same as those of descriptions on  FIG. 9 . 
     The sorting scheme determination unit a 7  informs an adjacent sorting scheme determination unit in the wireless transmission device c 23 , which is not shown, of information including the current wireless transmission capacity and the bandwidth to be guaranteed, as a band guarantee notification signal a 703 . 
     The sorting scheme determination unit c 7  in the wireless transmission device c 23  recognizes that the MAC frame with the CoS value of 1 requires band guarantee of 20 Mbps by the band guarantee notification signal a 703  received from the wireless transmission device c 23 . The sorting scheme determination unit c 7  informs a communication quality control unit c 41  of the contents, as a priority allocation notification signal c 702 . The communication quality control unit c 41  of the wireless transmission device c 23  performs priority control of 20 Mbps band guarantee type with respect to the MAC frame in the wireless direction. 
     In the wireless transmission device c 23  which is the adjacent station, because of shortage of the band in the wireless direction, band guarantee of 20 Mbps in the wireless direction with respect to the MAC frame with the CoS value of 1 may be impossible. In such case, the wireless transmission device c 23  may further allocate the MAC frame with the CoS value of 1 into another adjacent station by the same operation as that of the wireless transmission device a 23  to compensate shortage of the band to be guaranteed. 
     As explained above, the wireless transmission system of the fourth exemplary embodiment may maintain the transmission capacity using the wireless transmission path of the adjacent station in the wireless transmission device providing band guarantee type priority control. As a result, the wireless transmission system of the fourth exemplary embodiment may continue the band guarantee type priority control even if the wireless transmission capacity is reduced due to the adaptive modulation function, in addition to the effect of the wireless transmission system of the first exemplary embodiment. 
     By applying the configuration of the modified example of the first exemplary embodiment to the configuration of the wireless transmission system of the fourth exemplary embodiment, the wireless transmission device c 23  may further connect with a plurality of wireless transmission devices in series. 
     On the basis of the configuration above, when the wireless transmission device c 23  cannot keep the bandwidth to be guaranteed by using available wireless transmission capacity, it is possible to extend the wireless transmission capacity to be guaranteed further using the wireless transmission path of adjacent wireless transmission device. 
     The configuration of the wireless transmission system of the fourth exemplary embodiment may be combined with the configuration of the second or the third exemplary embodiment. If the exemplary embodiments are combined, the wireless transmission system of the fourth exemplary embodiment obviously includes the effect of the second or the third exemplary embodiment. 
     In the first to the fourth exemplary embodiments, the configuration in which the wireless transmission device determines the modulation scheme of the wireless signal by the adaptive modulation scheme is described. In the first to the fourth exemplary embodiments, the wireless transmission unit modulates the wireless transmission LAN signal using the modulation scheme notification signal generated based on the modulation scheme signal included in the reception signal. 
     However, the present invention is applicable to a wireless transmission device which does not employ the adaptive modulation scheme for determination of the modulation scheme. That is, in the first to the fourth exemplary embodiments, the modulation scheme signal included in the reception signal may not be determined on the basis of the adaptive modulation scheme. The wireless transmission unit in each wireless transmission device may determine the modulation scheme without using the information included in the reception signal. In this case, the sorting scheme determination unit may choose the identification method of the MAC frame on the basis of the modulation scheme determined by the wireless transmission unit. In the first to the fourth exemplary embodiments, the present invention is applied to the wireless transmission device and the wireless transmission system. A target of the present invention is not limited to the wireless transmission. For example, the present invention is applicable to a wired transmission device using a coaxial cable or an optical fiber 
     A wireless transmission system targeted by the present invention is not limited to a system employing a radio wave. The present invention is applicable to a transmission device using an optical space transmission system, for example. 
     Fifth Exemplary Embodiment 
       FIG. 10  is a diagram illustrating a configuration of a transmission device of a fifth exemplary embodiment of the present invention. 
     A transmission device g 101  shown in  FIG. 10  includes a sorting unit g 102 , a transmission unit g 103 , a reception unit g 104 , and an output unit g 105 . The sorting unit g 102  allocates a transmission signal into a first path or a second path on the basis of information for allocation of the transmission signal. The transmission unit transmits the transmission signal allocated to the first path as a first signal at a predetermined transmission band. The reception unit g 104  outputs a reception signal and information corresponding to a modulation scheme applied to the transmission unit g 103 , from a received second signal. The output unit g 105  generates an output signal from the reception signal and other signal, and outputs the output signal. 
     In the configuration above, the sorting unit g 102  allocates the transmission signal into the first path or the second path on the basis of information of the transmission band corresponding to the modulation scheme for transmission of the first signal. The transmission signal allocated to the second path is outputted to the outside of the transmission device g 101 . 
     The sorting unit g 102  is capable of allocating the transmission signal to the first path without exceeding the transmission band of the first signal based on the configuration above. The sorting unit g 102  can allocate the transmission signal which exceeds the transmission band of the first signal into the second path. The transmission signal allocated to the second path is outputted to the outside of the transmission device g 101  to enter a different transmission device having the same configuration as the device, as the transmission signal. 
     In the different transmission device, based on information for allocation of the transmission signal, the path of the transmission signal may be allocated without exceeding the transmission band of the different transmission device. 
     The output unit g 105  generates the output signal from the reception signal received by the present transmission device and a signal outputted by an output unit of the different transmission device and outputs the output signal. Thereby, the transmission signal before allocation of the path during transmission is restored from the reception signal received by a plurality of transmission devices. 
     In the transmission device of the fifth exemplary embodiment, since the transmission devices with the same configuration are connected with each other in parallel, a signal jam in the transmission device and occurrence of signal abandonment due to overflow of the high priority signal may be suppressed and the transmission capacity may be extended. By changing the number of the wireless transmission devices, the transmission device of the fifth exemplary embodiment may respond to the request for increase and decrease of the transmission capacity of the whole of the system which occurs with expansion and reduction of the network. The transmission device with the same configuration is available, regardless of the number of the employed transmission devices. As a result, the transmission device of the fifth exemplary embodiment makes construction and maintenance of the transmission system easy. 
     While having described the present invention referring to the embodiments 1 to 5, the present invention is not limited to the above mentioned embodiments 1 to 5. It is to be understood that to the configurations and details of the present invention, various changes can be made within the scope of the present invention by those skilled in the art. 
     While the invention has been particularly shown and described with reference to exemplary embodiments thereof, the invention is not limited to these embodiments. It will be understood by those of ordinary skill in the art that various changes in form and details may be made therein without departing from the spirit and scope of the present invention as defined by the claims. 
     This application is based upon and claims the benefit of priority from Japanese patent application No. 2010-044348 filed on Mar. 1, 2010, the disclosure of which is incorporated herein in its entirety by reference. 
     REFERENCE SIGNS LIST 
     
         
         
           
               10 ,  40  user network 
               60 - 1 - 60 - n ,  70 - 1 - 70 - n  wireless transmission device 
               50 ,  80 ,  50 - 1 - 50 - n ,  80 - 1 - 50 - n  L2SW 
               110 - 1 - 110 - n ,  510 - 1 - 510 - n  branched LAN signal 
               201 - 1 - 201 - n  wireless signal 
               100 ,  500  external LAN signal 
             a 1 , b 1  user network 
             a 10 , a 101  transmission buffer 
             a 100 , b 100  external input LAN signal 
             a 15 , b 15 , c 15 , d 15 , e 15 , f 15  transmission-reception antenna 
             a 151 , b 151  transmission-reception antenna 
             a 2 , b 2 , c 2 , d 2 , e 2 , f 2  wireless transmission device 
             a 21 , b 21 , c 21 , d 21  wireless transmission device 
             a 22 , b 22  wireless transmission device 
             a 3  frame sorting unit 
             a 4 , a 40  class A buffer 
             a 41 , a 51  communication quality control unit 
             a 401 , a 4101 , a 5001 , c 401  wireless transmission LAN signal 
             a 411  frame analysis unit 
             a 412 , a 4121 , a 4122 , a 4123 , a 4124  buffer 
             a 413  priority control unit 
             a 414  band control unit 
             a 5 , a 50  class B buffer 
             a 51  communication quality control unit 
             a 5101 , b 5101  adjacent transmission LAN signal 
             a 501 , b 501 , c 501 , d 501  adjacent transmission LAN signal 
             a 6  wireless transmission unit 
             a 60  class C buffer 
             a 601 , a 6101 , b 601 , b 6101 , c 601  wireless signal 
             a 602 , a 6102 , b 602 , b 6102 , c 602  wireless signal 
             a 603 , a 6103 , b 603 , b 6103 , c 603  wireless signal 
             e 601 , e 602 , e 603  wireless signal 
             d 601 , d 602 , d 603  wireless signal 
             f 601 , f 602 , f 603  wireless signal 
             a 7 , c 7 , e 7  sorting scheme determination unit 
             a 701 , c 701 , e 701  sorting scheme notification unit 
             a 702  priority allocation notification signal 
             a 703  band guarantee notification signal 
             a 8  adaptive modulation determination unit 
             a 801 , c 801 , e 801  modulation scheme notification signal 
             a 802 , c 802 , e 802  demodulation scheme notification signal 
             a 9 , a 91 , c 9 , e 9  wireless reception unit 
             a 901 , a 9101 , c 901 , e 901  wireless reception LAN signal 
             a 902 , a 903 , c 902 , e 902  modulation scheme signal 
             a 1001 , b 1001 , c 1001 , d 1001 , e 1001 , f 1001  external output LAN signal 
             g 101  transmission device 
             g 102  frame sorting unit 
             g 103  transmission unit 
             g 104  reception unit 
             g 105  output unit