Abstract:
This invention provides a wireless LAN base station capable of accommodating all of the client terminal stations if the number of client terminal stations wirelessly connected to the wireless LAN base station increases and capable of reducing power consumption if the number of wireless client terminal stations decreases. This wireless LAN base station includes at least two wireless LAN modules, each of which are capable of holding the wireless communication with at least one client terminal station, a section for detecting the number of client terminal stations that are being holding the wireless communication with the wireless LAN base station; and a section for changing the number of active wireless LAN modules according to the detected number of the client terminal stations.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to a wireless LAN (Local Area Network) base station that holds wireless communication with a client terminal station.  
           [0003]    2. Description of the Related Art  
           [0004]    Conventional LANs were wired LANs based on, for example, IEEE (Institute of Electrical and Electronics Engineers) 802.3 standard. Recently, wireless LANs based on IEEE 802.11 standard have spread.  
           [0005]    Prior art documents related to the present invention are as follows:  
           [0006]    Japanese Patent Application Laid-Open Publication No. 06-178352;  
           [0007]    Japanese Patent Application Laid-Open Publication No. 06-245260;  
           [0008]    Japanese Patent Application Laid-Open Publication No. 08-256162; and  
           [0009]    Japanese Patent Application Laid-Open Publication No. 2002-246969.  
           [0010]    If a wireless LAN is constructed in a company or in a home, the number of client terminal stations wirelessly connected to a wireless LAN base station does not change greatly. However, if a wireless LAN is constructed in, for example, an Internet Café, the number of client terminal stations wirelessly connected to the wireless LAN base station changes as the number of customers increases or decreases.  
           [0011]    If the number of wireless LAN modules provided at the wireless LAN base station is determined according to the maximum estimated number of client terminal stations wirelessly connected to the wireless LAN base station, it is possible to deal with the change of the number of client terminal stations. This, however, disadvantageously consumes power and the like if the number of clients terminal stations decreases.  
         SUMMARY OF THE INVENTION  
         [0012]    It is, therefore, an object of the present invention to provide a wireless LAN base station and a communication control method at the wireless LAN base station capable of accommodating all of the client terminal stations if the number of client terminal stations wirelessly connected to the wireless LAN base station increases and capable of reducing power consumption if the number of wireless client terminal stations decreases.  
           [0013]    According to a first aspect of the present invention, there is provided a wireless LAN base station which holds wireless communication with at least one client terminal station, the wireless LAN base station comprising: at least two wireless LAN modules, each of which is capable of holding the wireless communication with at least one client terminal station; means for detecting the number of client terminal stations which are being holding the wireless communication with the wireless LAN base station; and means for changing the number of active wireless LAN modules according to the detected number of the client terminal stations.  
           [0014]    The wireless LAN base station may comprise: a first wireless LAN module capable of holding the wireless communication with at least one client terminal station; a second wireless LAN module capable of holding the wireless communication with at least one client terminal station; determination means for determining whether the number of the client terminal stations which are holding the wireless communication with the wireless LAN base station is equal to or smaller than a predetermined number; first control means for controlling all of the client terminal stations which are holding the wireless communication with the wireless LAN base station to hold the wireless communication with the first wireless LAN module, controls the first wireless LAN module to be activated and controls the second wireless LAN module to be deactivated, if a determination result of the determination means is YES; and second control means for controlling a part of the client terminal stations which are holding the wireless communication with the wireless LAN base station to hold the wireless communication with the first wireless LAN module, controls the rest of the client terminal stations which are holding the wireless communication with the wireless LAN base station to hold the wireless communication with the second wireless LAN module and controls the first wireless LAN module and the second wireless LAN module to be activated, if the determination result is NO.  
           [0015]    In the wireless LAN base station, the first wireless LAN module may comprise a plurality of wireless communication sections based on different wireless communication systems from one another, the second wireless LAN module may comprise a plurality of wireless communication sections based on different wireless communication systems from one another, and the determination means, the first control means, and the second control means may operate according to each of the wireless communication systems.  
           [0016]    In the wireless LAN base station, the different wireless communication systems may be used for respective packet sizes.  
           [0017]    In the wireless LAN base station, the different wireless communication systems may be allocated for respective packet types.  
           [0018]    According to a second aspect of the present invention, there is provided a communication control method at a wireless LAN base station which holds wireless communication with at least one client terminal station, and which comprises at least two wireless LAN modules, each of which is capable of holding the wireless communication with at least one client terminal station, the control method comprising steps of: detecting the number of client terminal stations which are being holding the wireless communication with the wireless LAN base station; and changing the number of active wireless LAN modules according to the detected number of the client terminal stations.  
           [0019]    In the communication control method, the wireless LAN base station may comprise: a first wireless LAN module capable of holding the wireless communication with at least one client terminal station; and a second wireless LAN module capable of holding the wireless communication with at least one client terminal station, and wherein the communication control method may comprise: determination step of determining whether the number of the client terminal stations which are holding the wireless communication with the wireless LAN base station is equal to or smaller than a predetermined number; a first control step of controlling all of the client terminal stations which are holding the wireless communication with the wireless LAN base station to hold the wireless communication with the first wireless LAN module, controlling the first wireless LAN module to be activated and controlling the second wireless LAN module to be deactivated, if a determination result of the determination step is YES; and a second control step of controlling a part of the client terminal stations which are holding the wireless communication with the wireless LAN base station to hold the wireless communication with the first wireless LAN module, controlling the rest of the client terminal stations which are holding the wireless communication with the wireless LAN base station to hold the wireless communication with the second wireless LAN module and controlling the first wireless LAN module and the second wireless LAN module to be activated, if the determination result of the determination step is NO.  
           [0020]    In the communication control method, the first wireless LAN module may comprise a plurality of wireless communication sections based on different wireless communication systems from one another, the second wireless LAN module comprises a plurality of wireless communication sections based on different wireless communication systems from one another, and the determination step, the first control step, and the second control step are executed according to each of the wireless communication systems.  
           [0021]    In the communication control method, the different wireless communication systems may be allocated for respective packet sizes.  
           [0022]    In the communication control method, the different wireless communication systems may be allocated for respective packet types. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0023]    [0023]FIG. 1 is a block diagram illustrating the configuration of a wireless LAN base station in a first embodiment of the present invention;  
         [0024]    [0024]FIG. 2 is a flow chart which illustrates a number-of-antennas control method executed by a control section  109  of the wireless LAN base station in the first embodiment of the invention;  
         [0025]    [0025]FIG. 3 is a block diagram illustrating the configuration of a second IEEE802.11a+IEEE802.11g wireless LAN module of the wireless LAN base station in the first embodiment of the invention;  
         [0026]    [0026]FIG. 4 is a conceptual view illustrating a manner of connection between the wireless LAN base station and an upper base station in a second embodiment of the present invention;  
         [0027]    [0027]FIG. 5 is a conceptual view illustrating the configuration of a wireless LAN system in a third embodiment of the present invention;  
         [0028]    [0028]FIG. 6 is a block diagram illustrating the configuration of an upper base station  151  in the third embodiment of the invention;  
         [0029]    [0029]FIG. 7 is a block diagram illustrating the configuration of a wireless LAN base station  101  in the third embodiment of the invention;  
         [0030]    [0030]FIG. 8 is a block diagram illustrating the configuration of a client terminal station  161  in the third embodiment of the invention;  
         [0031]    [0031]FIG. 9 is a first format diagram for explaining encapsulation performed by an encapsulation section  183  in the third embodiment of the invention;  
         [0032]    [0032]FIG. 10 is a second format diagram for explaining the encapsulation performed by the encapsulation section  183  in the third embodiment of the invention;  
         [0033]    [0033]FIG. 11 is a first format diagram for explaining wireless LAN signal extraction performed by an extraction section  191  in the third embodiment of the invention;  
         [0034]    [0034]FIG. 12 is a second format diagram for explaining the wireless LAN signal extraction performed by the extraction section  191  in the third embodiment of the invention;  
         [0035]    [0035]FIG. 13 is a first format diagram for explaining encapsulation performed by an encapsulation section  188  in the third embodiment of the invention;  
         [0036]    [0036]FIG. 14 is a second format diagram for explaining the encapsulation performed by the encapsulation section  188  in the third embodiment of the invention;  
         [0037]    [0037]FIG. 15 is a first format diagram for explaining wireless LAN signal extraction performed by an extraction section  182  in the third embodiment of the invention;  
         [0038]    [0038]FIG. 16 is a second format diagram for explaining the wireless LAN signal extraction performed by the extraction section  182  in the third embodiment of the invention;  
         [0039]    [0039]FIG. 17 is a conceptual view illustrating a manner of connection between the wireless LAN base station and the client terminal station in the fourth embodiment of the present invention; and  
         [0040]    [0040]FIG. 18 is a conceptual view illustrating a manner of connection between the wireless LAN base station and the client terminal station in the fifth embodiment of the present invention. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0041]    Embodiments of the present invention will be described hereinafter in detail with reference to the drawings.  
         [0042]    [First Embodiment] 
         [0043]    [0043]FIG. 1 illustrates the configuration of the first embodiment of the present invention.  
         [0044]    Referring to FIG. 1, an antenna control method executed by a wireless LAN base station having three or more antennas is shown as the first embodiment of the present invention.  
         [0045]    In FIG. 1, the wireless LAN base station  101  includes a wired LAN interface  103 , a first IEEE802.11a+IEEE802.11g wireless LAN module  105 , a second IEEE802.11a+IEEE802.11g wireless LAN module  107 , a control section  109 , a storage section  111 , and a timer  113 .  
         [0046]    The wired LAN interface  103  is an interface between the wireless LAN base station  101  and an upper LAN.  
         [0047]    Each of the first and second IEEE802.11a+IEEE802.11g wireless LAN modules  105  and  107  is a double MAC (Media Access Control) wireless LAN module that can hold a communication according to IEEE802.11a and a communication according to IEEE802.11g simultaneously. If the two modules are combined, two channels for IEEE802.11a and two channels for IEEE802.11g, i.e., a total of four wireless channels can be simultaneously used.  
         [0048]    The control section  109  controls entirety of the wireless LAN base station  101 .  
         [0049]    The storage section  111  is a section that temporarily stores packets which the wired LAN interface  103 , the first IEEE802.11a+IEEE802.11g wireless LAN module  105 , and the second IEEE802.11a+IEEE802.11g wireless LAN module  107  transmit and receive.  
         [0050]    The timer  113  is a section which regularly generates an interrupt which will be applied to the control section  109 .  
         [0051]    When the wireless LAN base station  101  is turned on or reset, only the first IEEE802.11a+IEEE802.11g wireless LAN module  105  is activated, while the second IEEE802.11a+IEEE802.11g wireless LAN module  107  is kept inactive.  
         [0052]    If the timer  113  regularly generates an interrupt which will be applied to the control station  109 , the control station  109  checks the present number of client terminal stations. If the number exceeds a preset value, the control station  109  controls the second IEEE802.11a+IEEE802.11g wireless LAN module  107  to be activated and issues an instruction to roam some of the client terminal stations for the first IEEE802.11a+IEEE802.11g wireless LAN module  105  to the second IEEE802.11a+IEEE802.11g wireless LAN module  107 .  
         [0053]    The client terminal stations which receive this instruction are roamed to the second IEEE802.11a+IEEE802.11g wireless LAN module  107  in response to the instruction.  
         [0054]    Thereafter, the control section  109  similarly monitors the number of client terminal stations for the first IEEE802.11a+IEEE802.11g wireless LAN module  105  and those for the second IEEE802.11a+IEEE802.11g wireless LAN module  107 . If the sum of the numbers is below the preset value, the control section  109  issues an instruction to return some of the client terminal stations for the second IEEE802.11a+IEEE802.11g wireless LAN module  107  to the first IEEE802.11a+IEEE802.11g wireless LAN module  105 .  
         [0055]    The client terminal stations which receive this instruction are returned to the first IEEE802.11a+IEEE802.11g wireless LAN module  105  in response to the instruction.  
         [0056]    After confirming that all the client terminal stations of the second IEEE802.11a+IEEE802.11g wireless LAN module  107  are returned to the first IEEE802.11a+IEEE802.11g wireless LAN module  105 , the control section  109  controls the second IEEE802.11a+IEEE802.11g wireless LAN module  107  to be inactivated.  
         [0057]    The wireless LAN base station antenna control system having three or more antennas shown in FIG. 1 will next be described.  
         [0058]    In FIG. 1, when the wireless LAN base station  101  is activated, the control section  109  activates the first IEEE802.11a+IEEE802.11g wireless LAN module  105 , while the second IEEE802.11a+IEEE802.11g wireless LAN module  107  is kept inactive.  
         [0059]    The timer  113  is a timer which regularly generates an interrupt which will be applied to the control section  109 . Whenever the timer  113  generates an interrupt which will be applied to the control section  109 , the control section  109  executes a method as shown in FIG. 2.  
         [0060]    According to this method, the control section  109  activates/deactivates a common section  107 - 1  (shown in FIG. 3), an IEEE802.11a inherent section  107 - 2  (shown in FIG. 3), and an IEEE802.11g inherent section (shown in FIG. 3) of the second IEEE802.11a+IEEE802.11g wireless LAN module  107 .  
         [0061]    It is noted that the common section  107 - 1  is a section which is not inherent in IEEE802.11a and IEEE802.11g.  
         [0062]    The IEEE802.11a inherent section  107 - 2  is a section such as an RF section and an MAC section relevant only to IEEE802.11a.  
         [0063]    The IEEE802.11g inherent section  170 - 3  is a section such as an RF section and an MAC section relevant only to IEEE802.11g.  
         [0064]    The method shown in FIG. 2 will next be described.  
         [0065]    It is first determined whether at least one of a condition that the number of IEEE802.11a client terminal stations is greater than a first set value and a condition that the number of IEEE802.11g client terminal stations is greater than a second set value (in a step S 201 ). If the determination result of the step S 201  is YES, the processing goes to a step S 203 . If NO, the processing goes to a step S 229 . It is noted that the first set value may or may not be equal to the second set value.  
         [0066]    In the step S 203 , the common section  107 - 1  of the second IEEE802.11a+IEEE802.11g wireless LAN module  107  is activated. It is then determined whether the number of IEEE802.11a client terminal stations is greater than the first set value (in a step S 205 ). If the determination result of the step S 205  is YES, the processing goes to a step S 207 . If NO, the processing goes to a step S 211 .  
         [0067]    In the step S 207 , the IEEE802.11a inherent section  107 - 2  of the second IEEE802.11a+IEEE802.11g wireless LAN module  107  is activated. Next, an instruction to roam some of the IEEE802.11a client terminal stations from the first IEEE802.11a+IEEE802.11g wireless LAN module  104  to the second IEEE802.11a+IEEE802.11g wireless LAN module  107  is issued (in a step S 209 ), and the processing goes to a step S 217 .  
         [0068]    In the step S 211 , an instruction to return from the second IEEE802.11a+IEEE802.11g wireless LAN module  107  to the first IEEE802.11a+IEEE802.11g wireless LAN module  105  is issued to the IEEE802.11a client terminal stations connected to the second IEEE802.11a+IEEE802.11g wireless LAN module  107  (in a step S 211 ) Next, it is determined whether the number of the IEEE802.11a client terminal stations for the second IEEE802.11a+IEEE802.11g wireless LAN module  107  becomes zero (in a step S 213 ). If the determination result of the step S 213  is YES, the IEEE802.11a inherent section  107 - 2  of the second IEEE802.11a+IEEE802.11g wireless LAN module  107  is deactivated (in a step S 215 ), and the processing goes to a step S 217 . If the determination result of the step S 213  is NO, the processing directly goes to the step S 217 .  
         [0069]    In the S 217 , it is determined whether the number of the IEEE802.11g client terminal stations is greater than the second set value. If the determination result of the step S 217  is YES, the processing goes to a step S 219 . If NO, the processing goes to a step S 223 .  
         [0070]    In the step S 219 , the IEEE802.11g inherent section  107 - 3  of the second IEEE802.11a+IEEE802.11g wireless LAN module  107  is activated. Next, an instruction to roam from the first IEEE802.11a+IEEE802.11g wireless LAN module  105  to the second IEEE802.11a+IEEE802.11g wireless LAN module  107  is issued to some of the IEEE802.11g client terminal stations (in a step S 221 ), and the processing is finished.  
         [0071]    In the step S 223 , an instruction to return from the second IEEE802.11a+IEEE802.11g wireless LAN module  107  to the first IEEE802.11a+IEEE802.11g wireless LAN module  105  is issued to the IEEE802.11g client terminal stations connected to the second IEEE802.11a+IEEE802.11g wireless LAN module  107 . Next, it is determined whether the number of the IEEE802.11g client terminal stations of the second IEEE802.11a+IEEE802.11g wireless LAN module  107  becomes zero (in a step S 225 ). If the determination result of the step S 225  is YES, the IEEE802.11g inherent section  107 - 3  of the second IEEE802.11a+IEEE802.11g wireless LAN module  107  is deactivated (in a step S 227 ), and the processing is finished. If the determination result of the step S 225  is NO, the processing is finished without deactivating the IEEE802.11g inherent section  107 - 3 .  
         [0072]    In the step S 229 , an instruction to return from the second IEEE802.11a+IEEE802.11g wireless LAN module  107  to the first IEEE802.11a+IEEE802.11g wireless LAN module  105  is issued to the IEEE802.11a client terminal stations and the IEEE802.11g client terminal stations connected to the second IEEE802.11a+IEEE802.11g wireless LAN module  107  (in a step S 229 ). Next, it is determined whether the number of the IEEE802.11a client terminal stations and the IEEE802.11g client terminal stations connected to the second IEEE802.11a+IEEE802.11g wireless LAN module  107  becomes zero (in a step S 231 ). If the determination result of the step S 231  is YES, the IEEE802.11a inherent section  107 - 2  and the IEEE802.11g inherent section  107 - 3  of the second IEEE802.11a+IEEE802.11g wireless LAN module  107  are deactivated (in a step S 233 ), and the processing is finished.  
         [0073]    As can be seen, according to the present invention, the channels are activated or deactivated based on the number of client terminal stations that hold communication with the wireless LAN base station  101 . Therefore, it is possible to realize both high throughput and low power consumption by fully using the four antennas when communication demand is high and by decreasing the number of the antennas when the communication demand is low.  
         [0074]    [Second Embodiment] 
         [0075]    [0075]FIG. 4 illustrates the configuration of the second embodiment of the present invention.  
         [0076]    In the second embodiment, not the wired LAN interface  103  but one of the four antennas is used to hold communication with an upper base station  151 .  
         [0077]    The communication with the upper base station  151  is established in either an ad hoc mode or an infrastructure mode. In the infrastructure mode, only the relevant antenna operates as a terminal station, whereas the other three antennas operate as base stations.  
         [0078]    As a result, the number of antennas allocated to the client terminal stations decreases by one as compared with the first embodiment. Nevertheless, the second embodiment is free from a physical restriction of providing a wired LAN.  
         [0079]    [Third Embodiment] 
         [0080]    In order to introduce the second embodiment, it is necessary to perform encapsulation of an MAC address and to transship data between packets. The second embodiment provides a system to do so.  
         [0081]    [0081]FIG. 5 is a conceptual view illustrating the configuration of a wireless LAN system in the third embodiment of the present invention.  
         [0082]    Referring to FIG. 5, the wireless LAN system in the third embodiment includes the upper base station  151 , the wireless LAN base station  101 , and a client terminal  161 . The upper base station  151  can communicate with a plurality of wireless LAN terminals in a wireless LAN area  171 . Therefore, the upper base station  151  can also communicate with the wireless LAN base station  101 . The wireless LAN base station  101  also function as a wireless LAN base station and can communicate with a plurality of wireless LAN terminals in a wireless LAN area  172 . Therefore, the wireless LAN base station  101  can also communicate with the client terminal station  161 . It is noted that wireless channels in the wireless LAN area  171  differ from those in the wireless LAN area  172  so as to avoid collision between the wireless channels.  
         [0083]    Referring next to FIGS.  6  to  8 , configurations of the upper base station  151 , the wireless LAN base station  101 , and the client terminal station  161  will be described, respectively. While FIGS.  6  to  8  only illustrate constituent elements relevant to the present invention, the upper base station  151  has also a function of the conventional wireless LAN base station, not shown, and each of the wireless LAN base station  101  and the client terminal station  161  has also a function of the conventional wireless LAN base station.  
         [0084]    [0084]FIG. 6 is a block diagram illustrating the configuration of the upper base station  151 . Referring to FIG. 6, the upper base station  151  includes a reception section  181 , an extraction section  182 , an encapsulation section  183 , a transmission section  184 , and an MAC address table  185 . The reception section  181  receives a wireless LAN signal from the wireless LAN base station  101 . The extraction section  182  extracts a wireless LAN signal from the client terminal station  161  which signal is encapsulated into the received wireless LAN signal received from the wireless LAN base station  101 . The encapsulation section  183  encapsulates the wireless LAN signal to be transmitted to the client terminal station  161  into the wireless LAN signal to be transmitted to the wireless LAN base station  101 . The transmission section  184  transmits the wireless LAN signal, into which the wireless LAN signal to be transmitted to the client terminal station  161  is encapsulated, to the wireless LAN base station  101 . The MAC address table  185  has a configuration as shown in Table 1 below.  
                   TABLE 1                       First Row   Second Row                   MAC address of wireless LAN base   Null value       station 101       MAC addresses of wireless LAN   Null value       terminals directly connected to       upper base station 151       MAC address of client terminal   MAC address of wireless LAN       station 161   base station 101       MAC addresses of other wireless   MAC address of interposing       LAN terminals indirectly connected   wireless LAN base station       to upper base station 151                  
 
         [0085]    For the wireless LAN terminals directly connected to the upper base station  151 , a pair of the MAC address of each of the wireless LAN terminals and a null value (e.g., “0”) are stored in the MAC address table  185 . For the wireless LAN terminals indirectly connected to the upper base station  151 , a pair of the MAC address of each of the wireless LAN terminal stations and the MAC address of the wireless LAN base station that interposes between the wireless LAN terminals and the upper base station  151  are stored in the MAC address table  185 . Therefore, for the wireless LAN base station  101  directly connected to the upper base station  151 , a pair of the MAC address of the wireless LAN base station  101  and the null value are stored in the MAC address table  185 . In addition, for the client terminal station  161  connected to the upper base station  151  through the wireless LAN base station  101 , a pair of the MAC address of the client terminal station  161  and the MAC address of the wireless LAN base station  101  are stored in the MAC address table  185 .  
         [0086]    The extraction section  182  determines whether a special header (a header other than a conventional, standard header such as a header of an IP (Internet Protocol) packet, a header of an ICMP (Internet Control Message Protocol) packet and a header of an ARP (Address Resolution Protocol) header) is inserted into a top of a payload of a second OSI (Open Systems Interconnection) layer. The extraction section  182  can thereby know whether the wireless LAN signal from the client terminal station  161  which signal is encapsulated into the wireless LAN signal received by the reception section  181  is present. If the determination result as to whether the special header is inserted thereinto is YES, the extraction section  182  determines that the wireless LAN signal from the client terminal station  161  which signal is encapsulated into the wireless LAN signal received by the reception section  181  is present and extracts the encapsulated wireless signal. If the determination result is NO, the extraction section  182  outputs the wireless LAN signal received by the reception section  181  as it is.  
         [0087]    Further, the encapsulation section  183  searches a record, in which the destination MAC address in the header of the wireless LAN signal to be transmitted from the transmission section  184  is the MAC address in the first row, from the MAC address table  185 , and determines whether the value in the second row of the record is a null value. The encapsulation section  183  can thereby know whether to encapsulate the wireless LAN signal to be transmitted from the transmission section  814 . If the determination result as to whether the value is a null value is NO, the encapsulation section  183  determines that the wireless LAN signal to be transmitted from the transmission section  184  should be encapsulated and encapsulates the wireless LAN signal. If the determination result is YES, the encapsulation section  183  outputs the wireless LAN signal to be transmitted from the transmission section  184  without encapsulation.  
         [0088]    The client terminal station  161  transmits its MAC address to the wireless LAN base station  101  when transmitting an adscription request to wireless LAN base station functional sections of the wireless LAN base station  101 . If so, the wireless LAN base station  101  stores the MAC address of the client terminal station  161  in the MAC address table  193  and transfers a pair of the MAC address of the client terminal station  161  and the MAC address of the wireless LAN base station  101  to the upper base station  151 . The upper base station  151  stores a record, in which the MAC address of the client terminal station  161  is in the first row and the MAC address of the wireless LAN base station  101  is in the second row, in the MAC address table  185 .  
         [0089]    [0089]FIG. 7 is a block diagram illustrating the configuration of the wireless LAN base station  101 . Referring to FIG. 7, the wireless LAN base station  101  includes a first reception section  186 , an encapsulation section  188 , a first transmission section  189 , a second reception section  190 , an extraction section  191 , a second transmission section  192 , and an MAC address table  193 . The MAC address table  193  has a configuration as shown in Table 2 below and stores MAC addresses of all the wireless LAN terminals wirelessly connected to the wireless LAN base station functional sections of the wireless LAN base station  101 .  
               TABLE 2                           MAC address of client terminal station 161       MAC addresses of the other wireless LAN terminals connected to wireless       LAN base station 101                  
 
         [0090]    The encapsulation section  188  and the extraction section  191  are included in a bridge  187 . The first reception section  186  receives a wireless LAN signal from the client terminal station  161 . The encapsulation section  188  encapsulates the wireless LAN signal received from the client terminal station  161  and generates a new wireless LAN signal. The first transmission section  189  transmits the wireless LAN signal generated by the encapsulation section  188  to the upper base station  151 . The second reception section  190  receives a wireless LAN signal from the upper base station  151 . The extraction section  191  determines whether a special header is inserted into the top of the payload of the second OSI layer of the wireless LAN signal received by the second reception section  190 , whereby the extraction section  191  can know whether a wireless LAN signal is encapsulated in the wireless LAN signal received by the second reception section  190 . If the determination result as to whether the special header is inserted is YES, the extraction section  191  determines that the wireless LAN signal addressed to the client terminal station  161  and encapsulated into the wireless LAN signal received by the second reception section  190  is present and extracts the encapsulated wireless LAN signal.  
         [0091]    The second transmission section  192  transmits the wireless LAN signal, which is extracted by the extraction section  191  and addressed to the client terminal station  161 , to the client terminal station  161 . The MAC address table  193  holds MAC addresses of the wireless LAN terminals, such as the client terminal station  161 , connected to the upper base station  151  through the wireless LAN base station  101 .  
         [0092]    The client terminal station  161  transmits its MAC address to the wireless LAN base station  101  when transmitting the adscription request to the wireless LAN base station functional sections (including the first reception section  186  and the second transmission section  192 ) of the wireless LAN base station  101 .  
         [0093]    [0093]FIG. 8 is a block diagram illustrating the configuration of the client terminal station  161 . The client terminal station  161  includes a transmission section  195  and a reception section  196 . The transmission section  195  transmits a wireless LAN signal to the wireless LAN base station  101 . The reception section  196  receives a wireless LAN signal from the wireless LAN base station  101 . The transmission section  195  and the reception section  196  are connected to an upper layer (not shown).  
         [0094]    Referring next to FIGS. 9 and 10, encapsulation performed by the encapsulation section  183  of the upper base station  151  will be described. As shown in FIG. 9, normally, a destination MAC address and a sender MAC address are described in the header of a wireless LAN signal  301 . If inputting a wireless LAN signal  302  in which the destination MAC address is the MAC address of the client terminal  161 , the encapsulation section  183  adds the MAC address of the wireless LAN base station  101  as the destination MAC address MAC and the MAC address of the upper base station  151  as the sender MAC address to the input wireless LAN signal  302  and further adds thereto a special header  321 , thereby generating a wireless LAN signal  303  into which the wireless LAN signal  302  is encapsulated. The encapsulation section  183  may add the MAC address of the wireless LAN base station  101  as the destination MAC address and the MAC address of the upper base station  151  as the sender MAC address to a plurality of wireless LAN signals  304  and  305  addressed to the client terminal station  161  and may further add thereto the special header  321 , thereby generating a wireless LAN signal  306  into which the wireless LAN signals  304  and  305  are encapsulated. Further, the encapsulation section  183  may add the MAC address of the wireless LAN base station  101  as the destination MAC address and the MAC address of the upper base station  151  as the sender MAC address to a pair of a wireless LAN signal  307  addressed to the wireless LAN base station  101  and a wireless LAN signal  308  addressed to the client terminal station  161  and may further add thereto the special header  321 , thereby generating a wireless LAN signal  309  into which the wireless LAN signals  307  and  308  are encapsulated.  
         [0095]    Referring next to FIGS. 11 and 12, wireless LAN signal extraction performed by the extraction section  191  of the wireless LAN base station  101  will be described. When the wireless LAN signal  303 , into which the wireless LAN signal  302  addressed to the client terminal station  161  is encapsulated, is input to the wireless LAN base station  101 , the extraction section  191  extracts the wireless LAN signal  302  from the wireless LAN signal  302 . When the wireless LAN signal  306 , into which the wireless LAN signals  304  and  305  addressed to the client terminal station  161  are encapsulated, is input to the wireless LAN base station  101 , the extraction section  191  extracts the wireless LAN signals  304  and  305  from the wireless LAN signal  306 . Further, when the wireless LAN signal  309 , into which the wireless LAN signals (the wireless LAN signal  307  addressed to the wireless LAN base station  101  and the wireless LAN signal  308  addressed to the client terminal station  309 ) are encapsulated, is input to the wireless LAN base station  101 , the extraction section  191  extracts the wireless LAN signals  307  and  308  from the wireless LAN signal  309 .  
         [0096]    Referring next to FIGS. 13 and 14, encapsulation performed by the encapsulation section  188  of the wireless LAN base station  101  will be described. When the wireless LAN signal  312 , which indicates that a sender is the client terminal station  161 , is input to the wireless LAN base station  101 , the encapsulation section  188  adds the MAC address of the upper base station  151  as the destination MAC address and the MAC address of the wireless LAN base station  101  as the sender MAC address to the input wireless LAN signal  312  and further adds thereto the special header  321 , thereby generating a wireless LAN signal  313  into which the wireless LAN signal  312  is encapsulated. The encapsulation section  188  may add the MAC address of the upper base station  151  as the destination MAC address and the MAC address of the wireless LAN base station  101  as the sender MAC address to the wireless LAN signals  314  and  315 , each of which indicates that the sender is the client terminal station  161 , and may further add thereto the special header  321 , thereby generating a wireless LAN signal  316  into which the wireless LAN signals  314  and  315  are encapsulated. Further, the encapsulation section  188  may add the MAC address of the upper base station  151  as the destination MAC address and the MAC address of the wireless LAN base station  101  as the sender MAC address to a pair of the wireless LAN signal  317  which indicates that the sender is the wireless LAN base station  101  and the wireless LAN signal  318  which indicates that the sender is the client terminal station  161  and may further add thereto the special header  321 , thereby generating a wireless LAN signal  309  into which the wireless LAN signals  317  and  318  are encapsulated.  
         [0097]    Referring next to FIGS. 15 and 16, wireless LAN signal extraction performed by the extraction section  182  of the upper base station  151  will be described. When the wireless LAN signal  313 , into which the wireless LAN signal  312  indicating that the sender is the client terminal station  161  is encapsulated, is input to the upper base station  151 , the extraction section  182  extracts the wireless LAN signal  312  from the wireless LAN signal  313 . When the wireless LAN signal  316 , into which the wireless LAN signals  314  and  315  each indicating that the sender is the client terminal station  161  are encapsulated, is input to the upper base station  151 , the extraction section  182  extracts the wireless LAN signals  314  and  315  from the wireless LAN signal  316 . Further, when the wireless LAN signal  319 , into which the wireless LAN signals (the wireless LAN signal  317  indicating that the sender is the wireless LAN base station  101  and the wireless LAN signal  318  indicating that the sender is the client terminal station  161 ) are encapsulated, is input to the upper base station  151 , the extraction section  182  extracts the wireless LAN signals  317  and  318  from the wireless LAN signal  319 .  
         [0098]    [Fourth Embodiment] 
         [0099]    [0099]FIG. 17 illustrates the configuration of the fourth embodiment of the present invention.  
         [0100]    In the fourth embodiment, the client terminal station  161  which holds communication with the wireless LAN base station  101  includes an IEEE802.11a+IEEE802.11g wireless LAN module  163  similarly to the wireless LAN base station  101 . Communication between the base station and the terminal station is established using both of the IEEE802.11a standard and the IEEE802.11g standard.  
         [0101]    Based on which standard the communication is to be held, IEEE802.11a or IEEE802.11g, is set between the wireless LAN base station  101  and the client terminal station  161  in advance according to a packet size. For example, if the packet size is equal to or smaller than a set value, communication is set to be held according to the IEEE802.11a standard. If the packet size exceeds the set value, communication is set to be held according to the IEEE802.11g standard.  
         [0102]    Each of the wireless LAN base station  101  and the client terminal station  161  employs different antennas according to the packet size of a packet to be transmitted.  
         [0103]    It is thereby possible to segregate a short packet and a long packet from each other and improve the communication efficiency of the wireless LAN system.  
         [0104]    [Fifth Embodiment] 
         [0105]    [0105]FIG. 18 illustrates the configuration of the fifth embodiment of the present invention.  
         [0106]    The fifth embodiment is basically equal to the fourth embodiment except that communication path switching is performed based on not the packet size but a packet type.  
         [0107]    If each of the wireless LAN base station  101  and the client terminal station  161  makes a setting such that RTP (Real-Time Transport Protocol) packets are communicated therebetween according to the IEEE802.11a standard and the other packets are communicated therebetween according to the IEEE802.11g standard, the wireless LAN base station  101  and the client terminal station  161  can transmit and receive real-time data such as voices and roaming pictures transmitted by RTP&#39;s without damaging the data.  
         [0108]    In the embodiments described above, the IEEE802.11a+IEEE802.11g wireless LAN modules are employed. However, non-composite modules (e.g., two IEEE802.11a modules) can be similarly employed.  
         [0109]    In addition, the IEEE802.11a standard and the IEEE802.11g standard are used for facilitating understanding. However, standard types are irrelevant to the present invention.  
         [0110]    As described so far, the present invention can exhibit the following advantages.  
         [0111]    The first advantage is as follows. Channel activation and deactivation are performed based on the number of client terminal stations. Therefore, it is possible to realize both high throughput and low power consumption by fully using the four antennas when communication demand is high and by decreasing the number of antennas when the communication demand is low.  
         [0112]    The second advantage is that even if one of the modules is physically out of order, communication can be continuously held using the other module.  
         [0113]    The third advantage is that one of the modules can be exchanged without turning off the power of the base station.