Abstract:
A method includes measuring a first congestion level of the first access point, determining to change a coupling destination of a terminal device from a first access point to a second access point, transmitting a request from the first access point to the plurality of access points, the request from the first access point requesting the plurality of access points other than the second access point not to send a response to a request from the terminal device, the request from the terminal device requesting the plurality of access points to send the response to the terminal device, cutting off a coupling between the first access point and the terminal device, transmitting the request from the terminal device to the plurality of access points, sending the response to the request from the terminal device, and establishing a coupling between the second access point and the terminal device.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application is based upon and claims the benefit of priority of the prior Japanese Patent Application No. 2015-106395, filed on May 26, 2015, the entire contents of which are incorporated herein by reference. 
       FIELD 
       [0002]    The embodiments discussed herein are related to a method, a system, and an access point. 
       BACKGROUND 
       [0003]    In most cases, a wireless communication terminal, such as a smartphone, or a tablet terminal, includes an interface for various wireless communication. The interfaces for wireless communication that are included in the wireless communication terminal include, for example, interfaces such as local area network (LAN), 3rd Generation (3G: a third mobile communication system), and Long Term Evolution (LTE). For example, in the case of the wireless LAN, the wireless communication terminal performs coupling to an access point (AP) in the vicinity, and thus can perform coupling to the Internet or a local network. The wireless communication terminal is simply referred to as a terminal.” 
         [0004]    Because a reach range of an electromagnetic wave of the AP is several tens of meters, in a case where coverage in a broader range is desired, in most cases, APs are installed at distances of 20 to 30 meters. In a place where a lot of people gather locally such as a sports stadium or a hotel lobby, although the APs are installed at given distances, coupling by terminals is concentrated on a specific AP, and communication speed decreases. 
         [0005]    This is due to specifications for terminals in the wireless LAN, which state that a coupling to an AP that has the highest received signal strength indicator (RSSI) is made. The received signal strength indicator is also called a received electromagnetic wave strength. For example, in a case where electromagnetic output strengths of all the APs within the network are the same, the shorter the distance to the terminal is, the stronger the received electromagnetic wave strength of the output electromagnetic wave from the AP becomes. For this reason, a terminal performs coupling to the AP that is positioned the closest to the terminal. Therefore, in a case where a lot of people gather locally, the coupling by the terminals is concentrated on the AP that is positioned the closest to the place where they gather. As a result, an electromagnetic wave occupancy rate per one terminal decreases, and communication speed decreases. 
         [0006]    As a method of suppressing the concentration of accesses by terminals to a specific AP, for example, a method is known in which a terminal is caused to acquire a congestion situation of an AP in the vicinity and to perform coupling to an AP that is not congested. Furthermore, in addition, for example, a method is known in which a wireless LAN controller that controls the AP monitors the congestion situation of each AP, coupling by multiple terminals to the AP that is in congestion is released, and thus a load on the AP that is in congestion is alleviated. Moreover, the expression “An AP is in congestion” means that the electromagnetic wave occupancy rate per one terminal that performs coupling to the AP is below a threshold at which the communication speed is maintained to such an extent that a user of the terminal does not feel discomfort. That is, the expression “An AP is in congestion” means that the number of the terminals that perform coupling to the AP or the electromagnetic wave occupancy rate of the AP is a threshold or above. 
         [0007]    As examples of the related art, Japanese Laid-open Patent Application Publication No. 2004-140614, Japanese Laid-open Patent Application Publication No. 2014-192899, and Japanese Laid-open Patent Application Publication No. 2009-111768 are known. 
       SUMMARY 
       [0008]    According to an aspect of the invention, a method using a plurality of access points including a first access point and a second access point includes measuring a first congestion level of the first access point coupled to a terminal device, determining, based on the measured first congestion level, to change a coupling destination of the terminal device from the first access point to the second access point, transmitting a first request from the first access point to the plurality of access points, the first request from the first access point requesting the plurality of access points other than the second access point not to send a response to a second request from the terminal device, the second request from the terminal device requesting the plurality of access points to send the response to the terminal device, cutting off a coupling between the first access point and the terminal device, transmitting the second request from the terminal device to the plurality of access points, sending, from the second access point, the response to the second request from the terminal device, and establishing a coupling between the second access point and the terminal device. 
         [0009]    The object and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the claims. 
         [0010]    It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are not restrictive of the invention, as claimed. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0011]      FIG. 1  is a diagram illustrating an example of a flow of all processing up to and including processing that establishes a coupling between a terminal and an AP on a wireless LAN; 
           [0012]      FIG. 2  is a diagram illustrating an example of terminal coupling control processing by an AP according to a first embodiment; 
           [0013]      FIG. 3  is a diagram illustrating an example of a configuration of a wireless communication network system according to the first embodiment; 
           [0014]      FIG. 4A  is a diagram illustrating an example of a flow of first-phase processing of the terminal coupling control processing; 
           [0015]      FIG. 4B  is a diagram illustrating an example of a flow of second-phase processing of the terminal coupling control processing; 
           [0016]      FIG. 5  is a diagram illustrating a hardware configuration of the AP; 
           [0017]      FIG. 6  is a diagram illustrating an example of functional constituents of the AP; 
           [0018]      FIG. 7  is a diagram illustrating an example of a coupling terminal management table; 
           [0019]      FIG. 8  is a diagram illustrating an example of a neighboring AP management table; 
           [0020]      FIG. 9  is a flowchart illustrating an example of processing by an AP control unit in the first-phase processing of the terminal coupling control processing; 
           [0021]      FIG. 10  is a flowchart illustrating an example of the processing by the AP control unit in the second-phase processing of the terminal coupling control processing; 
           [0022]      FIG. 11A  is a flowchart illustrating an example of the processing that is performed by the AP control unit when a terminal wireless communication IF of the host AP does not detect congestion; 
           [0023]      FIG. 11B  is a flowchart of an example of the processing that is performed by the AP control unit when the terminal wireless communication IF of the host AP does not detect the congestion; 
           [0024]      FIG. 12  is a diagram illustrating an example of a sequence of terminal coupling control processing by the wireless communication network system; 
           [0025]      FIG. 13  is a diagram illustrating an example of a coupling terminal management table according to a modification example of the first embodiment; 
           [0026]      FIG. 14  is a diagram illustrating an example of a flow of first-phase processing of terminal coupling control processing according to a second embodiment; 
           [0027]      FIG. 15  is a diagram illustrating an example of an RSSI change history management table; 
           [0028]      FIG. 16  is a flowchart illustrating an example of the processing by the AP control unit in a first phase of the terminal coupling control processing according to the second embodiment; 
           [0029]      FIG. 17  is a flowchart illustrating an example of processing by a coupling destination determination unit, which determines a next coupling destination of a target terminal; 
           [0030]      FIG. 18  is a diagram illustrating an example of a sequence of the terminal coupling control processing according to the second embodiment; 
           [0031]      FIG. 19  is a diagram illustrating an example of functional constituents of an AP according to a third embodiment; 
           [0032]      FIG. 20  is a diagram illustrating an example of an average congestion level history management table; 
           [0033]      FIG. 21  is a diagram illustrating an example of a congestion level that is exchanged between an AP # 1 , an AP # 2 , and an AP # 3  for a period of time from 13:00 to 13:30 on Tuesday; 
           [0034]      FIG. 22  is a flowchart illustrating an example of processing by a notification congestion level determination unit, which determines a notification congestion level; 
           [0035]      FIG. 23  is a flowchart illustrating an example of processing by an AP control unit according to the third embodiment, which is performed when congestion is detected; 
           [0036]      FIG. 24  is a flowchart of an example of the processing that, according to the third embodiment, is performed by the AP control unit when the terminal wireless communication IF does not detect the congestion; 
           [0037]      FIG. 25  is a diagram illustrating an example of a sequence of terminal coupling control processing according to the third embodiment; and 
           [0038]      FIG. 26  is a diagram illustrating an example of a system configuration of a wireless communication network system according to a modification example of each of the first to third embodiments. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0039]    A problem with the method in which the terminal is caused to acquire the congestion situation of the AP in the vicinity is as follows. For example, when the terminal does not perform coupling to the AP, the terminal has difficulty in acquiring the congestion situation of the AP in the vicinity. In a case where the terminal does not perform coupling to any AP, first, the terminal performs coupling to the AP, accesses to which are concentrated. Furthermore, for example, the terminal has to be equipped with a function for switching the AP that is a coupling destination, and this is not for a general purpose. 
         [0040]    A problem with the method in which the wireless LAN controller controls the number of the terminals that perform coupling to the AP that is in congestion is as follows. For example, the wireless LAN controller does not recognize the received electromagnetic wave strength in the AP in the vicinity of a terminal of an electromagnetic wave transmitted from the terminal that is performing coupling to the AP that is in congestion. For this reason, in a case where a terminal, in the vicinity of which a different coupling-available AP other than the AP that is in congestion is not present, is selected as a terminal that releases the coupling to the AP that is in congestion, there is a likelihood that the terminal will again perform coupling to the same AP after the coupling is released. In this case, an excessive load on the AP that is in congestion is difficult to alleviate. 
         [0041]    Moreover, the problem that accesses by the terminals are concentrated on a specific wireless base station is a problem that also occurs in a different wireless communication network, such as the 3G or the LTE, without being limited to the wireless LAN. 
         [0042]    Embodiments of the present disclosure will be described in detail below referring to the drawings. Configurations of the embodiments described below are examples, and the present disclosure is not limited to the configurations of the embodiments. 
       First Embodiment 
       [0043]      FIG. 1  is a diagram illustrating an example of a flow of any processing up to and including processing that establishes a coupling between a terminal or an AP on the wireless LAN. In an example that is illustrated in  FIG. 1 , an AP # 1 , an AP # 2 , and an AP # 3  are devices under the control of the same manager. The AP # 1 , the AP # 2 , and the AP # 3  use the same wireless network name. In the wireless LAN, the wireless network name is also referred to as a service set identifier (SSID) or an extended SSID (ESSID). In the present specification, for description convenience, the wireless network names are hereinafter collectively referred to as the SSID. 
         [0044]    (1) Each AP transmits a beacon with a given period. The SSID, information on a utilization channel, and the like are included in the beacon. A period of beacon transmission is, for example, approximately 100 milliseconds. Because the beacon is transmitted by broadcasting, the beacon reaches all devices within the reach of an electromagnetic wave. 
         [0045]    (2) Because a terminal # 1  receives a beacon of the same SSID from the AP # 1 , the AP # 2 , and the AP # 3 , the terminal # 1  transmits a probe request with respect to every utilization channel for each AP. The probe request is a request that is made to the AP to check whether or not an SSID that is set for the AP that is connected to the terminal # 1  and an SSID of the AP are the same. The probe request is transmitted using a broadcast address. In  FIG. 1 , in the AP # 1 , the AP # 2 , and the AP # 3 , in order to avoid electromagnetic wave interference, the terminal # 1  transmits the probe request three times with respect to the utilization channels because the utilization channels are different. 
         [0046]    (3) If the SSIDs are the same, each AP transmits a probe response to the probe request from the terminal # 1 . The terminal # 1  receives the probe response from each AP and thus recognizes the presence of the AP # 1 , the AP # 2 , and the AP # 3 . 
         [0047]    (4) Because the terminal # 1  performs coupling to one AP, the terminal # 1  selects an AP that has the greatest received electromagnetic wave strength from among the AP # 1 , the AP # 2 , and the AP # 3  from which the probe responses are received. In  FIG. 1 , the terminal # 1  is assumed to be present at a location near the AP # 1 , and to select the AP # 1 . (5) Authentication is performed between the terminal # 1  and the AP # 1 , and thereafter a coupling is established. 
         [0048]      FIG. 2  is a diagram illustrating an example of the terminal coupling control processing by the AP according to a first embodiment. According to the first embodiment, the APs exchange congestion levels indicating congestion situations with one another, and know the congestion situations mutually. In a case where the AP has a congestion level that is higher than a congestion threshold, the AP detects congestion. When the AP detects the congestion, the AP attempts to switch a coupling destination of a terminal that is performing coupling to the AP to a different AP and to alleviate the congestion. Specifically, in a case where the congestion is detected, the AP selects a coupling destination switching target terminal and a different AP, as the next coupling destination of the terminal, that has no congestion. The AP that has a high congestion level is hereinafter referred to as an in-congestion AP. The congestion level is an example of the “congestion situation”. 
         [0049]    When it comes to the in-congestion AP, because the coupling destination switching target terminal is caused to be connected to an AP that is selected as the next coupling destination, an AP that is not selected is instructed to temporarily stop the response to the probe request. Accordingly, although the coupling destination switching target terminal is disconnected by the in-congestion AP and then that terminal transmits the probe request, the probe response is not transmitted from the AP that is not selected as the next coupling destination. Therefore, the presence of the AP that is not selected is hidden from the coupling destination switching target terminal, and the selected AP is selected, as the coupling destination, for that terminal. Moreover, the in-congestion AP itself temporarily stops the probe response. 
         [0050]    In  FIG. 2 , the AP # 1  is the in-congestion AP. Furthermore, the terminal # 1  is the coupling destination switching target terminal. In a case where the AP # 1  selects the AP # 2  as the next coupling destination of the terminal # 1 , the AP # 2  replies to the probe request from the terminal # 1 . On the other hand, the AP # 1  and the AP # 3  do not reply to the probe request from the terminal # 1 . Accordingly, the terminal # 1  recognizes the presence of the AP # 2 , but does not recognize the presence of the AP # 1  and the AP # 3 , leading to performing coupling to the AP # 2 . The probe request is an example of a “signal from a wireless terminal for checking for the coupling destination”. 
         [0051]      FIG. 3  is a diagram illustrating an example of a configuration of a wireless communication network system  100  according to the first embodiment. The wireless communication network system  100  includes multiple APs  1  and multiple terminals  2 . In  FIG. 3 , for convenience, three APs  1  and four terminals  2  that perform coupling to one of the three APs  1  are illustrated. In a case where the APs and the terminals are not distinguished, the APs and the terminals are collectively expressed as the AP  1  and the terminal  2 , respectively. In a case where each of the APs and each of the terminals are specified, as illustrated in the drawings, each of the APs and each of the terminals  2  are individually expressed as an AP #X and a terminal #X (X is a number), respectively. 
         [0052]    An AP  1  is performing coupling to a wired LAN  50 . The wired LAN  50  is a network for communication between each of the APs  1 . The AP  1  has two wireless LAN interfaces, that is, an interface for communication with the terminal  2  and an interface for measuring an electromagnetic wave on the utilization channel for the in-congestion AP. The in-congestion AP instructs the interface of a different AP  1  for measuring the electromagnetic wave to measure a received electromagnetic wave strength, and, based on a result of the measurement of the received electromagnetic wave strength from each AP  1 , selects the next coupling destination of that terminal. 
         [0053]    Terminal coupling control processing by the AP  1  according to the first embodiment, which is described above, is performed in two phases that result from broad categorization. In the first phase, processing is performed that selects the coupling destination switching target terminal and the AP  1  which is the next coupling destination of that terminal. In the second phase, processing is performed for switching the coupling destination of the coupling destination switching target terminal from the in-congestion AP to the AP  1  that is the next coupling destination. 
         [0054]      FIG. 4A  is a diagram illustrating an example of a flow of the first-phase processing of the terminal coupling control processing. In  FIG. 4A , the wireless communication network system  100  in  FIG. 3  is illustrated, and it is assumed that the terminal # 1  is present at a location where the terminal # 1  is able to receive the beacon from any one of the AP # 1 , the AP # 2 , and the AP # 3 . It is assumed that the terminal # 1  is performing coupling to the AP # 1 . The terminal # 1  and the AP # 1  are assumed to use a channel # 1 . In  FIG. 4A , the AP # 1  is the in-congestion AP, which detects that the in-congestion AP itself has become congested. 
         [0055]    (1) The congestion AP # 1  transmits an instruction to the AP # 2  and the AP # 3  in such a manner that the AP # 2  and the AP # 3  start to measure the received electromagnetic wave strength with a channel for the electromagnetic wave measurement interface being adjusted to a channel # 1  for the communication interface of the in-congestion AP # 1 . This instruction is transmitted over a wired network  50 . 
         [0056]    (2) When receiving the instruction from the congestion AP # 1 , the AP # 2  and the AP # 3  each adjust the channel for the electromagnetic wave measurement interface to a utilization channel # 1  for the in-congestion AP # 1 . With the electromagnetic wave measurement interface, the AP # 2  and the AP # 3  measure the received electromagnetic wave strength on the utilization channel # 1  for the in-congestion AP # 1 . Accordingly, the AP # 2  and the AP # 3  detect the terminal  2  that performs coupling to the in-congestion AP # 1 . 
         [0057]    (3) The AP # 2  and the AP # 3  transmit to the in-congestion AP # 1  an identifier of a terminal that is detected with the measurement of the received electromagnetic wave strength on the channel # 1  and the received electromagnetic wave strength (RSSI). For the identifier of the terminal, for example, any identification information that is uniquely allocated to each terminal, such as an MAC address or a terminal identification number, is used. 
         [0058]    (4) The in-congestion AP # 1  selects the coupling destination switching target terminal based on the RSSIs of each terminal that are received from the AP # 2  and the AP # 3 . Furthermore, the in-congestion AP # 1  selects the AP  1  that is the next coupling destination of the coupling destination switching target terminal, based on the congestion level for each of the AP # 2  and the AP # 3 . For example, the AP  1  that has the lowest congestion level is selected as the next coupling destination of the coupling destination switching target terminal, and based on a result of the measurement by the selected AP  1 , the terminal  2  indicating the greatest RSSI is selected as the coupling destination switching target terminal. Moreover, the congestion levels are exchanged between each of the APs  1 . 
         [0059]      FIG. 4B  is a diagram illustrating an example of a flow of the second-phase processing of the terminal coupling control processing. In the first phase, the in-congestion AP # 1  is assumed to select the terminal # 1  as the coupling destination switching target terminal and the AP # 3  as the next coupling destination. 
         [0060]    (5) The in-congestion AP # 1  transmits information including an identifier of the coupling destination switching target terminal # 1  and an identifier of the AP # 3  that is the next coupling destination to the AP # 2  and the AP # 3  over the wired LAN  50 . Switch terminal information is also a request for acceptance by the coupling destination switching target terminal of the coupling switching from the in-congestion AP to the AP  1  that is the next coupling destination. 
         [0061]    (6) When receiving the switch terminal information, the AP # 2  and the AP # 3  perform setting of the probe response according to the switch terminal information. The AP # 2  is not the AP that is the next coupling destination, and an identifier of the AP # 2  is not included in the switch terminal information. For this reason, the AP # 2  stops the probe response. Because the AP # 3  is the AP that is the next coupling destination, the identifier of the AP # 3  is included in the switch terminal information. For this reason, the AP # 3  performs setting in such a manner that the probe response to the probe request from the coupling destination switching target terminal # 1  is transmitted, and that the probe response to the probe request from a different terminal is not transmitted. 
         [0062]    (7) The in-congestion AP # 1  releases a coupling to the coupling destination switching target terminal # 1 . (8) When the coupling is released, the coupling destination switching target terminal # 1  transmits the probe request. The probe requests are transmitted over the utilization channel # 1  for the AP # 1 , a utilization channel # 6  for the AP # 2 , and a utilization channel # 11  for the AP # 3 , respectively. The utilization channel for each AP  1  is acquired from the beacon that is transmitted with a given period from each AP  1 . 
         [0063]    (9) The probe request from the coupling destination switching target terminal # 1  reaches any one of the AP # 1 , the AP # 2 , and the AP # 3 . However, the AP # that is the next coupling destination transmits the AP # 3  and the AP # 1  and the AP # 2  do not transmit the probe response. Accordingly, the coupling destination switching target terminal # 1  recognizes the presence of the AP # 3 , but does not recognize the presence of the AP # 1  and the AP # 2 . For this reason, the coupling destination switching target terminal # 1  performs processing that performs coupling to the AP # 3 . 
         [0064]    As described above, according to the first embodiment, the in-congestion AP  1  switches the coupling destination of the terminal  2  that is performing coupling to the in-congestion AP  1  itself to a different AP  1  that is not congested, and thus the coupling by the terminals  2  is distributed. Furthermore, the in-congestion AP  1  selects the AP  1  that is the next coupling destination of the coupling destination switching target terminal  2 , from among the APs  1  that are not congested. The AP  1  that is the next coupling destination replies to the probe request from the coupling destination switching target terminal  2 . However, because the other APs  1  stop the probe response, they do not transmit the probe response. Accordingly, the coupling destination switching target terminal  2  does not recognize the presence of the APs  1  other than the AP  1  that is the next coupling destination, and the coupling destination switching target terminal  2  is guided in such a manner to perform coupling to the AP  1  that is the next coupling destination. 
         [0065]    &lt;Device Configuration&gt; 
         [0066]      FIG. 5  is a diagram illustrating an example of a hardware configuration of the AP  1 . The AP  1  is an example of a “wireless base station”. The AP  1  includes a central processing unit (CPU)  11 , a primary storage device  12 , a secondary storage device  13 , wireless communication interfaces  15 A and  15 B, and a wired network interface  16 , and these are electrically connected to one another with a bus  19 . 
         [0067]    The CPU  11  performs loading of an OS or various application programs that are retained in the secondary storage device  13  onto the primary storage device  12 , and thus performs various processing operations. The CPU  11  is not limited to one CPU and multiple CPUs  11  may be included. The CPUs  11  are an example of a “control unit”. 
         [0068]    The primary storage device  12  is a volatile storage medium that provides the CPU  11  with a storage area or a working area onto which a program which is stored in the secondary storage device  13  is loaded and that is used as a buffer. The primary storage device  12  is, for example, a so-called random access memory (RAM), such as a dynamic RAM (DRAM), a static RAM (SRAM), or a synchronous DRAM (SDRAM). 
         [0069]    Stored in the secondary storage device  13  are various programs, or pieces of data that the CPU  11  uses when executing each program. The secondary storage device  13  is, for example, a nonvolatile storage medium, such as an erasable programmable ROM (EPROM), or a hard disk drive. The secondary storage device  13  retains, for example, an operating system (OS), a terminal coupling control program  13 P, and various application programs. The terminal coupling control program  13 P is a program for performing the terminal coupling control processing described above. Furthermore, a coupling terminal management table  132  and a neighboring AP management table  133  are retained in the secondary storage device  13 . The coupling terminal management table  132  and the neighboring AP management table  133  will be described in detail below. 
         [0070]    According to the first embodiment, the wireless communication interfaces  15 A and  15 B are wireless communication circuits on the wireless LAN. The wireless communication interface  15 A is an interface for communication with the terminal  2 . The wireless communication interface  15 B is an electromagnetic wave measurement interface for measuring the strength of an electromagnetic wave that is received on a utilization channel for a different AP  1 . The wireless communication interface  15 A is an example of a “first wireless communication unit”. The wireless communication interface  15 B is an example of a “second wireless communication unit”. 
         [0071]    The wired network interface  16  is, for example, a circuit that performs coupling to a wired network link cable such as a local area network (LAN) cable. The AP  1 , for example, performs coupling to a different AP  1  through the wired network interface  16 . The wired network interface  16  is an example of a “communication unit”. 
         [0072]    Moreover, a hardware configuration of the AP  1  that is illustrated in  FIG. 5  is an example, and is not limited to what is described above. According to the embodiment, it is possible to suitably omit or replace the constituent element, or to suitably add a new constituent element. For example, the AP  1  may include a portable recording medium drive device, and may use a portable recording medium, such as an SD card, as the secondary storage device. 
         [0073]      FIG. 6  is a diagram illustrating an example of functional constituents of the AP  1 . As the functional constituents, the AP  1  includes a terminal wireless communication interface (IF)  101 , a measurement wireless communication IF  102 , and an AP communication IF  103 , a coupling terminal management unit  104 , a congestion level determination unit  105 , a measurement terminal management unit  106 , an AP control unit  107 , a coupling destination determination unit  108 , a neighboring AP management unit  109 , a probe response control unit  110 , and a beacon control unit  111 . The coupling terminal management unit  104 , the congestion level determination unit  105 , the measurement terminal management unit  106 , the AP control unit  107 , the coupling destination determination unit  108 , the neighboring AP management unit  109 , the probe response control unit  110 , and the beacon control unit  111  are functional constituents that are accomplished by the execution by the CPU  11  of the terminal coupling control program  13 P within the secondary storage device  13 . Moreover, these functional constituents may be accomplished by a piece of software that uses electric circuits such as a field-programmable gate array (FPGA). 
         [0074]    The terminal wireless communication IF  101  is an interface for the communication with the terminal  2 , and is equivalent to a wireless communication IF  15 A. According to instructions from the probe response control unit  110  and the beacon control unit  111  that will be described below, the terminal wireless communication IF  101  changes the starting and the stopping of the probe response and of beacon transmission. Furthermore, the terminal wireless communication IF  101  measures an identifier and received electromagnetic wave strength of the terminal  2  that is performing coupling to the terminal wireless communication IF  101 , and outputs a result of the measurement to the coupling terminal management unit  104 . Furthermore, the terminal wireless communication IF  101  notifies the congestion level determination unit  105  of pieces of transfer quality information, such as a wireless occupancy rate, the number of coupling terminals, and the number of times that retransmission occurs, with a given period. The wireless occupancy rate is, for example, a rate of a communication time to a given unit time. 
         [0075]    The coupling terminal management unit  104  manages the terminal  2  that is being connected to the terminal wireless communication IF  101 . The details are as follows. For example, with a notification from the terminal wireless communication IF  101 , the coupling terminal management unit  104  acquires the identifier of the terminal  2  that is being connected to the terminal wireless communication IF  101 , and the received electromagnetic wave strength of the electromagnetic wave transmitted by each terminal  2 . The coupling terminal management unit  104  stores information that is input from the terminal wireless communication IF  101 , in the coupling terminal management table  132  that will be described below. Furthermore, the coupling terminal management unit  104 , when requested, outputs the information that is retained in the coupling terminal management table  132 , to the coupling destination determination unit  108  and the congestion level determination unit  105 . The information (the identifier of the terminal  2  that is performing coupling and the received electromagnetic wave strength of the electromagnetic wave transmitted by each terminal  2 ) that is stored in the coupling terminal management table  132  is referred to as coupling terminal information. 
         [0076]    The congestion level determination unit  105 , for example, acquires the congestion level, based on the wireless occupancy rate that is input from the terminal wireless communication IF  101 , the coupling terminal information that is input from the coupling terminal management unit  104 , and the like. For the congestion level, for example, any one of the wireless occupancy rate, the number of coupling terminals, and the like, or a combination of these may be used. The congestion level is output to the AP control unit  107 . The congestion level is output from the AP control unit  107  to the AP communication IF  103 , and is transmitted by the AP communication IF  103  to a different AP  1  over the wired LAN  50 . The congestion level is transmitted by any one of broadcasting, multicasting, or unicasting. The period with which the congestion level is calculated and transmitted is, for example, set by a manager on the minute basis. 
         [0077]    Furthermore, in a case where the congestion level is the congestion threshold or above, the congestion level determination unit  105  determines that the utilization channel for the terminal wireless communication IF  101  is in congestion. The congestion threshold is a threshold for measuring the congestion in the utilization channel for the terminal wireless communication IF  101 . For example, in a case where the congestion level is expressed in terms of the wireless occupancy rate, the congestion threshold is set to 80% or more, and in a case where the congestion level is expressed in terms of the number of coupling terminals, the congestion value is set to 30 or more. When determining that the utilization channel for the terminal wireless communication IF  101  is congested, the congestion level determination unit  105  notifies the AP control unit  107  of the congestion in the utilization channel for the terminal wireless communication IF  101 . 
         [0078]    The AP communication IF  103  is a wired interface for communicating with a different AP  1 , and is equivalent to a wired network IF  16 . The AP communication IF  103  performs coupling to the wired LAN  50 . The AP communication IF  103  receives a notification of control information, such as the congestion level, from the AP control unit  107 , and transmits the control information to a different AP  1 . Furthermore, the AP control unit  107  outputs the control information from the different AP  1  to the AP control unit  107 . 
         [0079]    The neighboring AP management unit  109  manages the congestion level of the different AP  1  and the received electromagnetic wave strength of the electromagnetic wave transmitted by the different AP  1 . The congestion level of a different AP  1  is input, with a given period, from the different AP  1  through the AP control unit  107  to the neighboring AP management unit  109 . The received electromagnetic wave strength of the electromagnetic wave transmitted by the different AP  1  is acquired by measuring the beacon that is transmitted from the different AP  1 . For the received electromagnetic wave strength of the electromagnetic wave transmitted by the different AP  1 , any one of the terminal wireless communication IF  101  and the measurement wireless communication IF  102  may be used. A period with which the beacon transmitted by the different AP  1  is measured, for example, by the manager on the one-hour basis or on the one-day basis. 
         [0080]    The neighboring AP management unit  109  stores the congestion level of the different AP  1  and the received electromagnetic wave strength of the electromagnetic wave transmitted by the different AP  1  in the neighboring AP management table  133  that will be described below. Furthermore, the neighboring AP management unit  109 , when requested, outputs information that is stored in the neighboring AP management table  133 , to the coupling destination determination unit  108 . The information (the congestion level of the different AP  1  and the received electromagnetic wave strength of the electromagnetic wave transmitted by the different AP  1 ) that is stored in the neighboring AP management table  133  is hereinafter referred to as neighboring AP information. 
         [0081]    The measurement wireless communication IF  102  is a measurement wireless interface for measuring the received electromagnetic wave strength on the utilization channel for the different AP  1 , and is equivalent to a wireless communication IF  15 B. In a case where an instruction that causes the received electromagnetic wave strength of the utilization channel to start to be measured is input from a different AP  1 , according to an instruction from the AP control unit  107 , the measurement wireless communication IF  102  performs adjustment to a frequency of the utilization channel for the different AP  1  and measures the received electromagnetic wave strength of the electromagnetic wave that is received on that channel. The measurement wireless communication IF  102  notifies the measurement terminal management unit  106  of the detected terminal  2  and the received electromagnetic wave strength of each terminal  2 , as a result of the measurement. The result of the measurement by the measurement wireless communication IF  102  is hereinafter referred to as measurement terminal information. Included in the measurement terminal information are an identifier of the detected terminal  2  and received electromagnetic wave strength of each detected terminal  2 . 
         [0082]    The measurement terminal management unit  106  manages the measurement terminal information that is input from the measurement wireless communication IF  102 . The information that is managed by the measurement terminal management unit  106  includes the identifier of the terminal  2  that is detected, with the measurement of the electromagnetic wave strength, by the measurement wireless communication IF  102 , and the received electromagnetic wave strength of each terminal  2 . The measurement terminal management unit  106  outputs the measurement terminal information to the AP control unit  107 . The measurement terminal information is output from the AP control unit  107  to the AP communication IF  103 , and is output to the AP  1  that is a transmission source of the instruction that causes the received electromagnetic wave strength on a designated channel to start to be measured. 
         [0083]    In a case where the congestion is detected by the congestion level determination unit  105 , the coupling destination determination unit  108  receives the instruction from the AP control unit  107 , and determines the coupling destination switching target terminal  2 , among the terminals  2  that are performing coupling to the host AP  1 , and the AP  1  that is the next coupling destination of that terminal  2 . This determination is performed based on the neighboring AP information that is input from the neighboring AP management unit  109 , and the measurement terminal information from the different AP  1 , which is input from the AP control unit  107 . 
         [0084]    For example, the coupling destination determination unit  108  selects the AP  1  that has the lowest congestion level in the neighboring AP information, as the next coupling destination. Furthermore, the terminal  2  that indicates the greatest received electromagnetic wave strength in the measurement terminal information of the AP  1  that is selected as the next coupling destination is selected as the coupling destination switching target terminal. Moreover, the coupling destination switching target terminal and the method of selecting the next coupling destination of that terminal is not limited to the method described above. The coupling destination determination unit  108  outputs the switch terminal information including an identifier of the coupling destination switching target terminal and an identifier of the AP  1  that is the next coupling destination of that terminal, to the AP control unit  107 . The switch terminal information is output from the AP control unit  107  to the AP communication IF  103 , and is transmitted from the AP communication IF  103  to a different AP  1 . The switch terminal information may be transmitted by any one of broadcasting, multicasting, and unicasting. 
         [0085]    The probe response control unit  110  receives the instruction from the AP control unit  107  and controls the starting and the stopping of the probe response by the terminal wireless communication IF  101 . For example, in a case where the congestion level determination unit  105  determines the congestion in the utilization channel for the terminal wireless communication IF  101 , the probe response control unit  110  receives the instruction from the AP control unit  107 , and stops the probe response by the terminal wireless communication IF  101 . Furthermore, in a case where the identifier of the host AP  1  is included in the switch terminal information from a different AP  1 , the probe response control unit  110  temporarily sets the terminal wireless communication IF  101  in such a manner that the probe response is temporarily stopped and that the reply to the probe request from the terminal  2  that has the identifier which is included in that switch terminal information is made. 
         [0086]    The beacon control unit  111  receives the instruction from the AP control unit  107  and controls the starting and the stopping of the beacon transmission by the terminal wireless communication IF  101 . For example, in a case where the congestion level determination unit  105  determines the congestion in the utilization channel for the terminal wireless communication IF  101 , the beacon control unit  111  receives the instruction from the AP control unit  107  and stops the beacon transmission by the terminal wireless communication IF  101 . 
         [0087]    The AP control unit  107  controls each functional constituent based on the control information that is input from each functional constituent. For example, the AP control unit  107  controls the starting or the stopping of the probe response and the beacon transmission by each of the probe response control unit  110  and the beacon control unit  111 . For example, the AP control unit  107  instructs the measurement terminal management unit  106  to adjust the measurement wireless communication IF  102  to the utilization channel for a different AP  1 , and to measure the received electromagnetic wave strength. For example, the AP control unit  107  instructs the coupling destination determination unit  108  to select the coupling destination switching target terminal and the next coupling destination of that terminal based on the detection of the congestion in the utilization channel for the terminal wireless communication IF  101 . 
         [0088]    Furthermore, for example, the AP control unit  107  outputs the pieces of control information, such as the congestion level that is input from the congestion level determination unit  105 , the measurement terminal information that is input from the measurement terminal management unit  106 , and the switch terminal information that is input from the coupling destination determination unit  108 , to the AP communication IF  103 , and transmits the pieces of control information to a different AP  1 . Furthermore, for example, the AP control unit  107  receives input of the pieces of control information from a different AP  1 , such as the measurement terminal information and the congestion level, from the AP communication IF  103 , and outputs the pieces of control information to the coupling destination determination unit  108  and the neighboring AP management unit  109 . 
         [0089]    Furthermore, the AP control unit  107  gives an instruction to a different AP  1 . For example, in a case where the congestion in the utilization channel for the terminal wireless communication IF  101  is detected, a measurement starting instruction that causes the received electromagnetic wave strength on that utilization channel to start to be measured is transmitted to a different AP  1  through the AP communication IF  103 . Processing by the AP control unit  107  will be described in more detail below. 
         [0090]      FIG. 7  is a diagram illustrating an example of the coupling terminal management table  132 . Information on the terminal that is performing coupling to the terminal wireless communication IF  101  is stored in the coupling terminal management table  132 . The coupling terminal management table  132  is managed by the coupling terminal management unit  104 . 
         [0091]    For entry, the heading of terminal identifier and the heading of received electromagnetic wave strength are included in the coupling terminal management table  132 . A MAC address that is allocated to a wireless LAN interface of the terminal  2  which is performing coupling to the terminal wireless communication IF  101  is stored under the heading of terminal identifier in the coupling terminal management table  132  that is illustrated in  FIG. 7 . The received electromagnetic wave strength of the terminal  2 , which is measured by the terminal wireless communication IF  101 , is stored under the heading of received electromagnetic wave strength. Because the terminal wireless communication IF  101  measures the received electromagnetic wave strength with a given period, the heading of the received electromagnetic wave strength in the coupling terminal management table  132  is updated with that given period. 
         [0092]    Because the received electromagnetic wave strength of the terminal  2  that is performing coupling to the terminal wireless communication IF  101  is stored in the coupling terminal management table  132 , the number of terminals that are being connected the terminal wireless communication IF  101  is acquired based on the number of entries in the coupling terminal management table  132 . 
         [0093]    Furthermore, the measurement terminal information of the measurement wireless communication IF  102  has the same configuration as the coupling terminal management table  132  as well. However, the measurement terminal information is associated with the identifier of the AP  1  that is a notification source. That is, when the AP  1  transmits an instruction that causes the utilization channel to start to be measured with the measurement wireless communication IF  102 , as many pieces of measurement terminal information as the number of APs  1  that are the notification sources are notified. 
         [0094]      FIG. 8  is a diagram illustrating an example of the neighboring AP management table  133 . Information on the AP  1  that is positioned in the vicinity is stored in the neighboring AP management table  133 . The neighboring AP management table  133  is managed by the neighboring AP management unit  109 . 
         [0095]    For entry, the heading of AP identifier, the heading of congestion level, and the heading of received electromagnetic wave strength are included in the neighboring AP management table  133  that is illustrated in  FIG. 8 . A MAC address that is allocated to the wired network IF  16  of each AP  1  which is present within the wireless communication network system  100  is stored in the heading of AP identifier. The MAC address that is allocated to the wired network IF  16  of each AP  1  that is present within the wireless communication network system  100  is managed in advance by the manager. 
         [0096]    A congestion level of each AP  1  that is notified from each AP  1  is stored under the heading of congestion level. Because the congestion level is notified with a given period, a value under the heading of congestion level is updated whenever the congestion level is notified. 
         [0097]    A measurement value of the received electromagnetic wave strength of the beacon from each AP  1  is stored under the received electromagnetic wave strength. Although the AP  1  is positioned within the wireless communication network system  100 , the received electromagnetic wave strength of the AP  1  that is positioned so far away that the electromagnetic wave does not reach that AP  1  is blank because the beacon does not reach that AP  1 . Moreover, a MAC Address that is allocated to the terminal wireless communication IF  101  of the AP  1  is stored in the beacon. The neighboring AP management unit  109 , in advance, retains a table (not illustrated) for correspondence between the MAC Address of the terminal wireless communication IF  101  a MAC address of the AP communication IF  103 , and the MAC address of the AP communication IF  103  of the AP  1  that is a transmission source of the beacon is acquired from the table for correspondence. 
         [0098]    A relationship in distance between the host AP  1  and each AP  1  is known by the received electromagnetic wave strength of each AP  1  that is stored in the neighboring AP management table  133 . Moreover, for entry, the heading for the host AP  1  is also included in the neighboring AP management table  133 . 
         [0099]    Based on as many pieces of terminal information as the number of the APs  1  that are the notification sources, the coupling terminal management table  132 , and the neighboring AP management table  133 , the coupling destination determination unit  108  selects the coupling destination switching target terminal and the coupling destination of that terminal. For example, in a case where the AP  1  that has the lowest congestion level is selected as the next coupling destination, the coupling destination determination unit  108  selects the AP  1  that is the next coupling destination, referring to the neighboring AP management table  133 . 
         [0100]    Furthermore, in a case where a terminal that indicates the greatest received electromagnetic wave strength in the AP  1  which is selected as the next coupling destination is selected as the coupling destination switching target terminal, the coupling destination determination unit  108  refers to the measurement terminal information of the AP  1  that is selected as the next coupling destination, and the coupling terminal management table  132 . The coupling destination determination unit  108  selects a terminal that has the greatest received electromagnetic wave strength among the terminals  2  of which the received electromagnetic wave strengths are registered in both the measurement terminal information of the AP  1  that is selected as the next coupling destination and the coupling terminal management table  132 , as a terminal that releases the coupling. 
         [0101]    &lt;Flow of Processing&gt; 
         [0102]      FIG. 9  is a flowchart illustrating an example of processing by the AP control unit  107  in the first-phase processing of the terminal coupling control processing. Processing that is illustrated in  FIG. 9  starts when the congestion level determination unit  105  notifies the AP control unit  107  of the detection of the congestion in the utilization channel for the terminal wireless communication IF  101 . Moreover, for convenience, the description is provided with the AP control unit  107  as a main subject, but a main subject for the processing that is illustrated in  FIG. 9  is in practice the CPU  11  that executes the terminal coupling control program  13 P. The same is true for the following flowchart. 
         [0103]    In OP 1 , the AP control unit  107  instructs each of the beacon control unit  111  and the probe response control unit  110  to stop the beacon transmission and the transmission of the probe response. Accordingly, a coupling from a new terminal is not made. Moreover, in a case where the beacon transmission and the transmission of the probe response are already in a stopped state, OP 1  in the processing is omitted. Next, the processing proceeds to OP 2 . 
         [0104]    In OP 2 , the AP control unit  107  transmits an instruction that causes the received electromagnetic wave strength on the utilization channel to start to be measured with the measurement wireless communication IF  102 , to the AP  1  that is positioned in the vicinity, through the AP communication IF  103 . Next, the processing proceeds to OP 3 . 
         [0105]    In OP 3 , the AP control unit  107  receives the measurement terminal information that is a response from the neighboring AP  1  to the instruction that causes the received electromagnetic wave strength on the utilization channel to start to be measured with the measurement wireless communication IF  102 . For example, when the pieces of measurement terminal information from all the APs  1 , of which the received electromagnetic wave strengths are recorded in the neighboring AP management table  133  are received, next, the processing proceeds to OP 4 . For example, after a given time elapses after the instruction that causes the received electromagnetic wave strength to start to be measured is transmitted, the processing may proceed to OP 4 . 
         [0106]    In OP 4 , the AP control unit  107  determines whether or not the neighboring AP  1  that has the measurement terminal information indicating that at least one terminal  2  has received an electromagnetic wave strength higher than the threshold is present. The threshold that is used for this determination is one for determining whether or not the coupling destination switching target terminal is able to perform communication through the neighboring AP  1  after coupling switching, and is set to a value of the received electromagnetic wave strength that sufficiently guarantees communication quality on the wireless LAN. 
         [0107]    In a case where the AP  1  that has the measurement terminal information indicating that the received electromagnetic wave strength of at least one terminal  2  is higher than the threshold is present (Yes in OP 4 ), the processing proceeds to OP 5 . In a case where the AP  1  that has the measurement terminal information indicating that the received electromagnetic wave strength of at least one terminal  2  is higher than the threshold is not present (No in OP 4 ), because the AP  1  that is suitable for the next coupling destination is not present in the vicinity, the processing that is illustrated in  FIG. 9  ends without performing coupling destination switching of the terminal  2 . 
         [0108]    In OP 5 , the AP control unit  107  instructs the coupling destination determination unit  108  to determine the coupling destination switching target terminal  2  and the AP  1  that is the next coupling destination, and acquires pieces of information on the coupling destination switching target terminal  2  and on the AP  1  that is the next coupling destination, from the coupling destination determination unit  108 . A method for the coupling destination determination unit  108  to determine the coupling destination switching target terminal  2  and the AP  1  that is the next coupling destination is as described above. Specifically, the AP  1  that has the lowest congestion level is selected as the next coupling destination, and in the selected AP  1 , the terminal  2  that has the greatest received electromagnetic wave strength is selected as the terminal  2 . Moreover, multiple terminals  2  may be selected as the coupling destination switching targets. How many of the terminal coupling destinations are switched is determined depending on the congestion level. For example, in a case where the congestion level is the number of coupling terminals and the congestion is detected with the number of coupling terminals being 30 or more, the number of the coupling destination switching target terminals is determined in such a manner that the number of coupling terminals is less than 30. Next, the processing proceeds to OP 6 . 
         [0109]    In OP 6 , the AP control unit  107  activates the second-phase processing of the terminal coupling control processing. Thereafter, the processing that is illustrated in  FIG. 9  ends. 
         [0110]      FIG. 10  is a flowchart illustrating an example of the processing by the AP control unit  107  in the second-phase processing of the terminal coupling control processing. Processing that is illustrated in  FIG. 10  starts after the coupling destination switching target terminal  2  and the AP  1  that is the next coupling destination are selected in the first phase of the terminal coupling control processing. 
         [0111]    In OP 11 , the AP control unit  107  notifies the neighboring AP  1  of switch terminal information including an identifier of the coupling destination switching target terminal  2  and an identifier of the AP  1  that is the next coupling destination, through the AP communication IF  103 . Next, the processing proceeds to OP 12 . 
         [0112]    In OP 12 , the AP control unit  107  receives a notification of setting completion from the neighboring AP  1  through the AP communication IF  103 . For example, when the notification of the setting completion is received from the APs  1  of which the received electromagnetic wave strengths are recorded in the neighboring AP management table  133 , next, the processing proceeds to OP 13 . For example, a given time has elapsed after the switch terminal information is transmitted, the processing may proceed to OP 13 . 
         [0113]    In OP 13 , the AP control unit  107  releases a coupling to the coupling destination switching target terminal  2 . Next, the processing proceeds to OP 14 . 
         [0114]    In OP 14 , the AP control unit  107  determines whether or not the utilization channel for the terminal wireless communication IF  101  of the host AP  1  is in congestion. The AP control unit  107  requests the congestion level determination unit  105  to determine whether or not the utilization channel for the terminal wireless communication IF  101  is in congestion. By a current congestion level being a given threshold or above, the congestion level determination unit  105  detects that the utilization channel for the terminal wireless communication IF  101  of the device is in congestion. 
         [0115]    In a case where the utilization channel for the terminal wireless communication IF  101  of the host AP  1  (Yes in OP 14 ) is in congestion, the processing that is illustrated in  FIG. 10  ends. Thereafter, when the next timing for calculation of the congestion level of the congestion level determination unit  105  comes, the congestion in the utilization channel for the terminal wireless communication IF  101  is again detected, and the first phase of the terminal coupling control processing in  FIG. 9  starts. A new coupling destination switching target terminal  2  is selected. 
         [0116]    In a case where the utilization channel for the terminal wireless communication IF  101  of the host AP  1  is not congested (No in OP 14 ), the processing proceeds to OP 15 . 
         [0117]    In OP 15 , because the congestion in the utilization channel for the terminal wireless communication IF  101  is alleviated, the AP control unit  107  instructs the beacon control unit  111  and the probe response control unit  110  to start the beacon transmission and the transmission of the probe response, respectively, with the terminal wireless communication IF  101 . Accordingly, a coupling by a new terminal to the host AP  1  is resumed. Thereafter, the processing that is illustrated in  FIG. 10  ends. 
         [0118]      FIGS. 11A and 11B  are flowcharts, each of which illustrates an example of the processing that is performed by the AP control unit  107  when the terminal wireless communication IF  101  of the host AP  1  does not detect the congestion. Processing that is illustrated in each of  FIGS. 11A and 11B  starts together with the activation of the AP  1 , and is repeatedly performed when the AP  1  is in operation. The processing of each of  FIGS. 11A and 11B  is also processing that is performed by the CPU  11  executing a terminal coupling control program  13 P. That is, the terminal coupling control program  13 P is a program that, after the AP  1  is activated, automatically starts to be executed. 
         [0119]    In OP 21 , in a case where the AP control unit  107  receives an instruction that causes the received electromagnetic wave strength on the utilization channel, of the in-congestion AP  1  to start to be measured, from that in-congestion AP  1  through the AP communication IF  103  (Yes in OP 21 ), the processing proceeds to OP 22 . In a case where the measurement starting instruction is not received (No in OP 21 ), the processing proceeds to OP 31 . 
         [0120]    OP 22  to OP 24  are processing operations that are performed in a case where the instruction that causes the received electromagnetic wave strength on the utilization, of the in-congestion AP  1  is received from that in-congestion AP  1 . In OP 22 , the AP control unit  107  instructs the measurement wireless communication IF  102  to measure the received electromagnetic wave strength on the utilization channel, of the in-congestion AP  1 . The measurement wireless communication IF  102  starts to measure the received electromagnetic wave strength, in a state of being adjusted to the utilization channel for the in-congestion AP  1 . Next, the processing proceeds to OP 23 . 
         [0121]    In OP 23 , the AP control unit  107  is in a waiting state for a given time for which the measurement wireless communication IF  102  performs measurement. When the given time elapses (Yes in OP 23 ), a result of (measurement terminal information on) the measurement by the measurement wireless communication IF  102  of the received electromagnetic wave strength on the utilization channel, of the in-congestion AP  1  is input into the AP control unit  107  through the measurement terminal management unit  106 . Next, the processing proceeds to OP 24 . 
         [0122]    IN OP 24 , the AP control unit  107  transmits the measurement terminal information including the identifier of the terminal  2  that is detected by the measurement of the received electromagnetic wave strength on the utilization channel, of the in-congestion AP  1 , and the measured received electromagnetic wave strength, and the congestion level of the host AP  1 , to the in-congestion AP  1  through the AP communication IF  103 . Thereafter, the processing that is illustrated in  FIG. 11A  ends and the processing again starts from OP 21 . 
         [0123]    In OP 31  in  FIG. 11B , in a case where the AP control unit  107  receives the switch terminal information from the in-congestion AP  1  through the AP communication IF  103  (Yes in OP 31 ), the processing proceeds to OP 32 . In a case where neither the switch terminal information nor the instruction that causes the received electromagnetic wave strength on the utilization channel, of the in-congestion AP  1  to start is received (No in OP 31 ), the processing that is illustrated in  FIG. 11B  ends, and the processing again starts from OP 21  in  FIG. 11A . 
         [0124]    The processing operations in OP 32  to OP 42  are ones that are performed in a case where the switch terminal information is received from the in-congestion AP  1 . Included in the switch terminal information are the identifier of the coupling destination switching target terminal  2  and the identifier of the AP  1  that is the next coupling destination. 
         [0125]    In OP 32 , the AP control unit  107  determines whether or not the identifier of the host AP  1  is included, as the next coupling destination, in the switch terminal information. In a case where the identifier of the host AP  1  is included, as the next coupling destination (Yes in OP 32 ), in the switch terminal information, the processing proceeds to OP 35 . In a case where the identifier of the host AP  1  is not included in the switch terminal information (No in OP 32 ), the processing proceeds to OP 33 . 
         [0126]    OP 33  to OP 34  are processing operations that are performed in the case where the identifier of the host AP  1  is not included in the switch terminal information, that is, in a case where no terminal performs coupling to the host AP  1 . In OP 33 , the AP control unit  107  instructs the beacon control unit  111  and the probe response control unit  110  to stop the beacon transmission and the transmission of the probe response, respectively, with the terminal wireless communication IF  101 . Accordingly, with the switch terminal information, new terminals  2  including a terminal  2  that is designated as the coupling destination switching target terminal are suppressed from newly performing coupling. Next, the processing proceeds to OP 34 . 
         [0127]    In OP 34 , the AP control unit  107  transmits a setting completion notification that setting is completed according to the switch terminal information, to the in-congestion AP  1  through the AP communication IF  103 . Next, the processing proceeds to OP 41 . 
         [0128]    OP 35  to OP 40  are processing operations that are performed in a case where the identifier of the host AP  1  is included, as the next coupling destination, in the switch terminal information. In OP 35 , the AP control unit  107  instructs the beacon control unit  111  and the probe response control unit  110  to stop the beacon transmission and the transmission of the probe response, respectively, with the terminal wireless communication IF  101 . Accordingly, a coupling by a new terminal  2  is suppressed. Next, the processing proceeds to OP 36 . 
         [0129]    In OP 36 , the AP control unit  107  instructs the probe response control unit  110  to reply to the probe request from the terminal in which, with the switch terminal information, the host AP  1  is designated as the next coupling destination. The terminal in which, with the switch terminal information, the host AP  1  is the next coupling destination is hereinafter referred to as an acceptance-designated terminal. The probe response control unit  110  performs the setting in such a manner that the reply to the probe request from the acceptance-designated terminal is made in the terminal wireless communication IF  101 . Next, the processing proceeds to OP 37 . 
         [0130]    In OP 37 , the AP control unit  107  transmits the setting completion notification to the in-congestion AP  1  through the AP communication IF  103 . Next, the processing proceeds to OP 38 . 
         [0131]    In OP 38 , the AP control unit  107  is in a state of waiting to receive the probe request from the acceptance-designated terminal. In a case where the probe request is received in the terminal wireless communication IF  101  from the acceptance-designated terminal (Yes in OP 38 ), the processing proceeds to OP 39 . 
         [0132]    In OP 39 , in the terminal wireless communication IF  101 , the probe response control unit  110  transmits the probe response to the probe request from the acceptance-designated terminal and notifies the AP control unit  107  of the completion of the probe response to the acceptance-designated terminal. Thereafter, the coupling processing is performed between the terminal wireless communication IF  101  and the acceptance-designated terminal. Next, the processing proceeds to OP 40 . 
         [0133]    In OP 40 , the AP control unit  107  transmits a coupling completion response for notifying that the coupling to the acceptance-designated terminal is completed, to the in-congestion AP  1 . Next, the processing proceeds to OP 41 . 
         [0134]    In OP 41 , the AP control unit  107  determines whether or not the utilization channel for the terminal wireless communication IF  101  of the host AP  1  is in congestion. In the case where the utilization channel for the terminal wireless communication IF  101  of the host AP  1  is in congestion (Yes in OP 41 ), the processing that is illustrated in  FIG. 11B  ends and instead the terminal coupling control processing that is illustrated in  FIG. 9  starts. In the case where the utilization channel for the terminal wireless communication IF  101  of the host AP  1  is not congested (No in OP 41 ), the processing proceeds to OP 42 . 
         [0135]    In OP 42 , the AP control unit  107  instructs the beacon control unit  111  and the probe response control unit  110  to start the beacon transmission and the transmission of the probe response, respectively, with the terminal wireless communication IF  101 . This is because the utilization channel for the terminal wireless communication IF  101  is not congested and it is possible to accept a coupling by a new terminal  2 . Accordingly, the coupling by the new terminal  2  to the host AP  1  is resumed. Thereafter the processing that is illustrated in  FIG. 11B  ends and the processing starts again from OP 21  in  FIG. 11A . 
       Operational Example 
       [0136]      FIG. 12  is a diagram illustrating an example of a sequence of terminal coupling control processing by the wireless communication network system  100  that is illustrated in  FIG. 1 . In an example that is illustrated in  FIG. 12 , output strengths of electromagnetic waves of the AP # 1 , the AP # 2 , and the AP # 3  are assumed to be same. Furthermore, it is assumed that the terminal # 1  is positioned the closest to the AP # 1 , the AP # 2  is positioned the second closest to the terminal # 1 , and the AP # 3  is positioned the third closest to the terminal # 1 . In the example that is illustrated in  FIG. 12 , the terminal # 1  performs coupling to the AP # 1  and performs communication. 
         [0137]    In S 1 , the AP # 1  detects the congestion in the utilization channel for the terminal wireless communication IF  101 . In S 2 , the AP # 1  stops the beacon transmission and the transmission of the probe response from the terminal wireless communication IF  101  (OP 1  in  FIG. 9 ). 
         [0138]    In S 3 , the AP # 1  transmits the instruction that causes the received electromagnetic wave strength on the utilization channel, of the AP # 1  to start to be measured (OP 2  in  FIG. 9 ). The instruction that causes the received electromagnetic wave strength on the utilization channel, of the AP # 1  to start to be measured may be transmitted by broadcasting, multicasting, and unicasting. 
         [0139]    In S 4 , each of the AP # 2  and the AP # 3  receives the instruction that causes the received electromagnetic wave strength on the utilization channel, of the AP # 1  to start to be measured, from the AP # 1  (Yes in OP 21  in  FIG. 11A ) and performs the measurement of the received electromagnetic wave strength with the measurement wireless communication IF  102  being adjusted to the utilization channel for the AP # 1  (OP 22  and OP 23  in  FIG. 11A ). Because any of the AP # 2  and the AP # 3  is positioned within a range where the electromagnetic wave transmitted by the terminal # 1  propagates, the AP # 2  and the AP # 3  are able to measure the strength of the electromagnetic wave received from the terminal # 1 . 
         [0140]    In S 5  and S 6 , each of the AP # 2  and the AP # 3  transmits the measurement terminal information and the congestion level to the AP # 1  (OP 24  in  FIG. 11A ). In the example that is illustrated in  FIG. 12 , the identifier of the AP # 1  and the received electromagnetic wave strength of the electromagnetic wave from the terminal # 1  are included in any one of the measurement terminal information of the AP # 2  and the measurement terminal information of the AP # 3 . Moreover, the measurement terminal information is transmitted by unicasting with the AP # 1  as the destination. 
         [0141]    In S 7 , the AP # 1  receives the measurement terminal information from each of the AP # 2  and the AP # 3  (OP 3  in  FIG. 9 ), and determines the coupling destination switching target terminal and the next coupling destination (OP 5  in  FIG. 9 ). Moreover, any one of the received electromagnetic wave strengths of the terminal # 1  that are included in the measurement terminal information of the AP # 2  and the measurement terminal information of the AP # 3  is assumed to be a value that is greater than the threshold and at which communication is possible (Yes in OP 4  in  FIG. 9 ). The AP # 1  is assumed to select the terminal # 1  as the coupling destination switching target terminal and the AP # 2  as the next coupling destination. 
         [0142]    In S 8 , the AP # 1  transmits the switch terminal information (OP 11  in  FIG. 10 ). The identifier of the terminal # 1  and the identifier of the AP # 2  are included, in a state of being associated with each other, in the switch terminal information that is transmitted from the AP # 1  in S 8 . The switch terminal information may be transmitted by any one of broadcasting, multicasting, and unicasting. 
         [0143]    In S 9 , each of the AP # 2  and the AP # 3  receives the switch terminal information (Yes in OP 31  in  FIG. 11B ) and stops the beacon transmission and the transmission of the probe response with the terminal wireless communication IF  101  (OP 33  and OP 35  in  FIG. 11B ). 
         [0144]    In S 10 , because with the switch terminal information, the AP # 2  is designated as the next coupling destination, the setting is performed in such a manner that the probe response to the probe request from the terminal # 1  is transmitted (OP 36  in  FIG. 11B ) and the setting completion notification is transmitted to the AP # 1  (OP 37  in  FIG. 11B ). 
         [0145]    In S 11 , because an identifier of the AP # 3  itself is not included in the switch terminal information, when the processing ends in S 9 , without any change, the AP # 3  transmits the setting completion notification to the AP # 1  (OP 34  in  FIG. 11B ). Moreover, the setting completion notification is transmitted by unicasting with the AP # 1  as the destination. 
         [0146]    In S 12 , the AP # 1  receives the setting completion notification from each of the AP # 2  and the AP # 3  (OP 12  in  FIG. 10 ) and releases a coupling to the terminal # 1  (OP 13  in  FIG. 10 ). 
         [0147]    In S 13 , because the coupling is released by the AP # 1 , the terminal # 1  starts recoupling processing in order to perform handover. In S 14 , as one of the retransmission processing operations, the terminal # 1  transmits the probe request on the utilization channel for the terminal wireless communication IF  101  of each of the AP # 1 , the AP # 2 , and the AP # 3 . 
         [0148]    In S 15 , the AP # 2  receives the probe request from the terminal # 1  (OP 38  in  FIG. 11B ), and transmits the probe response to the terminal # 1  (OP 39  in  FIG. 11B ). Although the AP # 1  and the AP # 3  receives the probe request from the terminal # 1 , because the transmission of the probe response is stopped, the probe response is not transmitted to the terminal # 1 . 
         [0149]    In S 16 , the terminal # 1  receives the probe response from the AP # 2 . Because the probe response is not transmitted from the AP # 1  and the AP # 3  to the terminal # 1 , the terminal # 1  recognizes the presence of the AP # 2 , but does not recognize the presence of the AP # 1  and the AP # 3 . In S 17 , the terminal # 1  performs coupling to the AP # 2  of which the presence is recognized. 
         [0150]    In S 18 , the AP # 2  transmits the coupling completion notification to the AP # 1  (OP 40  in  FIG. 11B ). It is assumed that the terminal # 1  performs a handover from the AP # 1  to the AP # 2  and thus that the congestion in the AP # 1  is alleviated. Furthermore, it is assumed that the AP # 2  and the AP # 3  are consecutively in a non-congested state. 
         [0151]    In S 19 , because none of the AP # 1 , the AP # 2 , and the AP # 3  is congested, the beacon transmission and the transmission of the probe response with the terminal wireless communication IF  101  is resumed (OP 15  in  FIG. 10  and OP 42  in  FIG. 11B ). 
         [0152]    &lt;Operational Effect of the First Embodiment&gt; 
         [0153]    According to the first embodiment, the AP  1  that is not congested is selected as the next coupling destination of the coupling destination switching target terminal  2 , and the selected AP  1  is notified to the other APs  1 . The APs other than the AP  1  that is selected as the next coupling destination do not reply to the probe request from the coupling destination switching target terminal  2 . Accordingly, because the presence of the APs other than the AP  1  that is selected as the next coupling destination is not viewed from the coupling destination switching target terminal  2 , the coupling destination switching target terminal  2  performs the handover from the in-congestion AP  1  to the AP  1  that is selected as the next coupling destination. Accordingly, the congestion in the in-congestion AP  1  is reduced, and a situation where the congestion also occurs in the AP  1  that the coupling destination switching target terminal  2  performs coupling to is suppressed. 
         [0154]    Therefore, according to the first embodiment, concentration of the coupling by the terminals on the in-congestion AP  1  is alleviated and the coupling is distributed. Furthermore, a communication interruption and the like due to the switching of the coupling destination are also suppressed, and a comfortable communication environment is provided to a user of the terminal. 
         [0155]    Furthermore, according to the first embodiment, in addition to the terminal wireless communication IF  101 , each AP  1  includes the measurement wireless communication IF  102 . According to an instruction from the in-congestion AP  1 , each AP  1  performs the measurement of the received electromagnetic wave strength on the utilization frequency for the terminal wireless communication IF  101  of the in-congestion AP  1 , with the measurement wireless communication IF  102 , and notifies the in-congestion AP  1  of the measurement terminal information. The in-congestion AP  1  determines the coupling destination switching target terminal  2  and the next coupling destination of that terminal  2 , from the measurement terminal information that is notified by a different AP  1 . Accordingly, the AP  1  that is present outside of a reach range of the electromagnetic wave of the terminal  2  that is disconnected by the in-congestion AP  1 , or the AP  1  that is in congestion is suppressed from being selected as the next coupling destination by that terminal  2 . 
         [0156]    The AP  1  that has the lowest congestion level is selected as the next coupling destination of the coupling destination switching target terminal  2 , and thus the AP  1  that is the next coupling destination accepts the coupling by the coupling destination switching target terminal  2 . This suppresses the congestion. Furthermore, accordingly, the number of times that the coupling destination of the terminal  2  is switched is decreased. 
         [0157]    Furthermore, the terminal  2  that has the greatest received electromagnetic wave strength in the measurement terminal information of the AP  1  which is selected as the next coupling destination is selected as the coupling destination switching target terminal  2 , and thus the communication is suppressed from being unstable after the switching of the coupling destination by that terminal  2 . This provides a more comfortable communication service. 
         [0158]    According to the first embodiment, in a case where the measurement starting instruction from the in-congestion AP  1  is present, because the AP  1  performs the measurement of the received electromagnetic wave strength with the measurement wireless communication IF  102 , power consumption and a processing load due to the measurement wireless communication IF  102  are suppressed. 
         [0159]    Furthermore, according to the first embodiment, the terminal  2  performs an operation according to regulation of the wireless LAN, which states that when the coupling to the AP  1  is released, the probe request is transmitted, and does not perform an operation outside of a range of the regulation. For this reason, the terminal  2  may not be equipped with a function according to the first embodiment, and the wireless communication network system  100  that is described according to the first embodiment has great versatility. 
       Modification Example of the First Embodiment 
       [0160]    According to the first embodiment, the in-congestion AP  1  determines the coupling destination switching target terminal  2  based on the measurement terminal information that is notified by a different AP  1 . Included in the measurement terminal information are the identifier of the terminal  2  and the received electromagnetic wave strength that are detected in the utilization frequency for the in-congestion AP  1 , which are notified by the different AP  1 . 
         [0161]    According to a modification example of the first embodiment, the coupling destination switching target terminal  2  is determined based in the information that is acquired from the terminal  2  that performs coupling to the in-congestion AP  1 . According to the modification example of the first embodiment, a remaining capacity of a battery and information on an application in use are used as pieces of information that are acquired from the terminal  2 . However, the pieces of information that are acquired from the terminal  2  are not limited to these. The remaining capacity of the battery is hereinafter referred to as an amount of remaining battery power. 
         [0162]    According to the modification example of the first embodiment, because the information is acquired from the terminal  2 , a terminal situation measurement program and a terminal situation transmission program are added to a secondary storage device of the terminal  2 . A terminal situation reception program is added to the secondary storage device  13  of the AP  1 . 
         [0163]    The terminal situation measurement program is a program with which the terminal  2  acquires pieces of information on the amount of remaining battery power and the application in use with a given period. The terminal situation transmission program is a program with which the terminal  2  transmits the pieces of information on the amount of remaining battery power and the application in use, which are acquired by performing the terminal situation measurement program, to the AP  1  to which a coupling is made. 
         [0164]    The terminal situation reception program is a program with which the AP  1  receives the pieces of information on the amount of remaining battery power and the application in use from the terminal  2 . The CPU  11  of the AP  1  executes the terminal situation reception program, and thus the coupling terminal management unit  104  receives the pieces of information on the amount of remaining battery power and the application in use in the terminal  2  from the terminal wireless communication IF  101  and stores the received information in a terminal coupling management table  131 . 
         [0165]      FIG. 13  is a diagram illustrating an example of the terminal coupling management table  131  according to the modification example of the first embodiment. For entry, the terminal coupling management table  131  according to the modification example of the first embodiment includes the heading of type of application in use and the heading of amount of remaining battery power, in addition to the heading of identifier and the heading of received electromagnetic wave strength. 
         [0166]    For example, a type of application for performing real-time communication, such as a video website and a voice over Internet protocol (IP) (VoIP), are stored under the heading of application in use. For example, a parameter for the amount of remaining battery power, which is set against a maximum value for the capacity of the battery of the terminal  2 , is stored under the heading of amount of remaining battery power. 
         [0167]    According to the first embodiment, the in-congestion AP  1  selects the AP  1  that has the lowest congestion level, as the next coupling destination, and selects the terminal  2  that is performing coupling to the in-congestion AP  1  itself and that has the greatest received electromagnetic wave strength in the AP  1  which is selected as the next coupling destination. 
         [0168]    On the other hand, according to the modification example of the first embodiment, the coupling destination determination unit  108  of the in-congestion AP  1 , for example, determines the coupling destination switching target terminal  2  in terms of the following headings in the terminal coupling management table  131 : (1) amount of remaining battery power, (2) type of application, (3) received electromagnetic wave strength in this order. 
         [0169]    First, the coupling destination determination unit  108  extracts the terminals  2  that have an amount of remaining battery power which is a threshold or above, as candidate terminals. Next, the coupling destination determination unit  108  further extracts candidate terminals  2  from the extracted terminals  2  in terms of type of application. For example, the video website has a higher priority than the VoIP, and thus the coupling destination determination unit  108  further extracts terminals  2  that are using the video website, from the terminals  2  that are extracted in terms of amount of remaining battery power. 
         [0170]    Next, the coupling destination determination unit  108  selects a terminal  2  that has the lowest received electromagnetic wave strength in the coupling terminal management table  132 , as a coupling destination switching target, from the terminals  2  that are selected as the candidates. In this case, an AP  1  that has the greatest received electromagnetic wave strength of the coupling destination switching target terminal  2  in the measurement terminal information, is selected as the next selection destination. 
         [0171]    In a terminal coupling management table  131  that is illustrated in  FIG. 13 , for example, in a case where a threshold of the amount of remaining battery power is 40%, a terminal B, which is using the type of application, that is, the video website, with the amount of remaining battery power being 80%, is selected as the coupling destination switching target. However, a method of determining the coupling destination switching target terminal  2  is not limited to this. 
         [0172]    According to the modification example of the first embodiment, the coupling destination switching target terminal  2  is selected, additionally using the amount of remaining battery power that is obtained from the terminal  2  and a type of application in communication. In a case where the terminal  2  that has a small amount of remaining battery power is selected as the coupling destination switching target, there is a concern that battery power will be further consumed due to the handover. According to the first embodiment, because the terminal  2  that has an amount of remaining battery power which is the threshold or above is selected as the coupling destination switching target, power consumption is suppressed in the terminal  2  that has a small amount of remaining battery power. 
         [0173]    Furthermore, when terminal  2  that is using an application for performing real-time communication, such as the VoIP is caused to perform the handover, there is a concern that the user will remarkably feel that the communication is temporarily disconnected due to the switching of the AP  1  that is the coupling destination. According to the first embodiment, because the terminal  2  that is using the application for performing the real-time communication has a lower priority, a comfortable communication environment is provided to the terminal  2  that is using the application for performing the real-time communication. 
       Second Embodiment 
       [0174]    According to the first embodiment, when the congestion level of the terminal wireless communication IF  101  is the congestion threshold or above, the congestion is detected, and the detection of that congestion causes the terminal coupling control processing to start. According to the second embodiment, the detection of movement of the terminal  2  that is performing coupling without the detection of the congestion causes the terminal coupling control processing to start. That is, the first-phase processing of the terminal coupling control processing is different from that according to the first embodiment. Description of what the second embodiment has in common with the first embodiment is hereinafter omitted. Moreover, a hardware configuration and functional constituents of an AP  1  according to the second embodiment are the same as those according to the first embodiment. 
         [0175]      FIG. 14  is a diagram illustrating an example of a flow of a first-phase processing of terminal coupling control processing according to the second embodiment. In  FIG. 14 , the wireless communication network system  100  in  FIG. 3  is illustrated, and it is assumed that the terminal # 1  is present at a location where the terminal # 1  is able to receive the beacon from any one of the AP # 1 , the AP # 2 , and the AP # 3 . The terminal # 1  is assumed to move in the direction of AP # 2 . The AP # 3  is assumed to be positioned at a destination in the direction of the AP # 2 . 
         [0176]    (1) Each AP  1  measures the received electromagnetic wave strength on a channel for a different AP  1  in the vicinity with a given period with the measurement wireless communication IF  102 , and retains measurement histories including a measurement point in time, the detected terminal  2 , and the received electromagnetic wave strength, for a given time. In a case where multiple other APs  1  are present in the vicinity, setting of the frequency for the measurement wireless communication IF  102  is switched while tuning in to the utilization channel for each AP  1  in the vicinity, but the utilization channel for each AP  1  in the vicinity is monitored by including multiple measurement wireless communications IF. 
         [0177]    (2) The AP # 1  detects that the received electromagnetic wave strength of the terminal # 1  that performs coupling to the terminal wireless communication IF  101  is below a movement detection threshold. The AP # 1  detects the movement of the terminal # 1 , and notifies other APs # 2  and AP # 3  in the vicinity of the identifier of the terminal # 1 . The movement detection threshold is a threshold that is used to detect the movement of the terminal  2 . The movement detection threshold, for example, is set to approximately the same value as the received electromagnetic wave strength that is measured in a case where the terminal  2  is positioned around a boundary of an area in the reach range of the electromagnetic wave of the AP  1 . 
         [0178]    (3) When notified of the identifier of the terminal # 1  by the AP # 1 , each of the AP # 2  and the AP # 3  notifies the AP # 1  of a history of the terminal # 1  among the measurement histories of the received electromagnetic wave strength that is measured with the measurement wireless communication IF  102 . For example, the time for which the history is set by the manager to be in a range of several seconds to several minutes. 
         [0179]    (4) The AP # 1  estimates a movement direction and movement speed of the terminal # 1  from the histories of the received electromagnetic wave strength of the terminal # 1 , which are received from the AP # 2  and the AP # 3 . The AP # 1  recognizes the movement direction of the terminal # 1 , using a position of the AP  1 , for example, like the expression a direction in which the AP # 2  is positioned. Furthermore, the movement direction of the terminal # 1  is estimated by specifying the AP  1  that retains the history of the received electromagnetic wave strength, in which the received electromagnetic wave strength of the terminal # 1  increases with the passage of time. 
         [0180]    Movement speed of the terminal # 1  is estimated from a change in the received electromagnetic wave strength of the terminal # 1  in the history in at least two APs  1 . The AP  1  estimates the movement speed of the terminal # 1  from the history of the received electromagnetic wave strength of the terminal # 1 , which is received from a different AP  1 . For example, the AP  1  retains, in advance, a relationship between a distance between the AP  1  and the terminal  2  and the received electromagnetic wave strength, acquires a change in the distance between the AP  1  and the terminal  2  from a change in the received electromagnetic wave strength, and acquires a movement distance. Furthermore, the AP  1  acquires the time demanded for the movement of the terminal # 1 , from the measurement point in time of the received electromagnetic wave strength in the history. The AP  1  acquires an estimation value of the movement speed of the terminal # 1  by dividing the acquired movement history by the time demanded for the acquired movement. 
         [0181]    (5) The AP # 1  selects the next coupling destination in such a manner that the number of times that the terminal # 1  performs the handover decreases. For example, in  FIG. 14 , in a case where the terminal # 1  moves in the direction of the AP # 2 , the AP # 3  that is positioned farther from the AP # 1  than the AP # 2  is selected as the next coupling destination. Accordingly, in a case where the terminal # 1  continues to move farther from the AP # 2  toward the direction of the AP # 3 , the terminal # 1  can omit the switching of the coupling destination to the AP # 2 . However, a condition that the AP has to be selected as the next coupling destination is that the congestion level is below the congestion threshold. 
         [0182]    The second-phase processing after it is determined that the AP  1  is the next coupling destination of the terminal # 1  is the same as that according to the first embodiment. That is, the AP # 1  notifies the AP # 2  and the AP # 3  of the switch terminal information including the identifier of the terminal # 1  and the identifier of the AP # 3  as the next coupling destination. The AP # 2  and the AP # 3  reply to a probe request from the terminal  2 , which is notified along with the identifier of the host AP  1 , but does not reply to probe requests from terminals  2  other than that probe request. 
         [0183]    Accordingly, in a case where the terminal # 1  moves in the direction from the AP # 1  to the AP # 2  to the AP # 3 , the terminal # 1  performs the handover from the AP # 1  to the AP # 3 , not the handover from the AP # 1  to the AP # 2  and from the AP # 2  to the AP # 3 . Therefore, the number of times that the terminal  2  in motion performs the handover is decreased, and the number of times that the communication is interrupted momentarily is reduced. 
         [0184]      FIG. 15  is a diagram illustrating an example of a RSSI change history management table. The RSSI change history management table is table for storing the history of the measurement of the received electromagnetic wave strength on the utilization channel for a different AP  1  in the vicinity with the measurement wireless communication IF  102  of the AP  1 . The RSSI change history management table is retained in the secondary storage device  13  of the AP  1 . 
         [0185]    In  FIG. 15 , an RSSI change history table for one terminal  2  is illustrated. For entry, the heading of time and the heading of received electromagnetic wave strength are included in the RSSI change history management table that is illustrated in  FIG. 15 . A point in time at which the measurement is performed is stored under the heading of time. The received electromagnetic wave strength that is the measurement result is stored under the heading of received electromagnetic wave strength. 
         [0186]    As many RSSI change history management tables as the number of terminals that are detected by the measurement of the received electromagnetic wave strength are present. Furthermore, as many pieces of information as the length of a given time are stored in the RSSI change history management table, and when, for entry, new information is stored, the oldest information is deleted. Furthermore, although, for entry, new information is not stored, when the given time elapsed, information is deleted. The length of the given time for which the information is retained in the RSSI change history management table, for example, is on the minute basis, such as on the one-minute basis, and is able to be arbitrarily set by the manager. Moreover, the received electromagnetic wave strength on the utilization channel for the host AP  1  is also stored in the RSSI change history management table. 
         [0187]    According to the second embodiment, the measurement wireless communication IF  102  and the measurement terminal management unit  106  are activated along with the AP  1 . The measurement wireless communication IF  102  measures the received electromagnetic wave strength with a given period, while tuning in to the utilization channel for each AP  1  in the vicinity. When the result of the measurement is input from the measurement wireless communication IF  102 , the measurement terminal management unit  106  stores the result of the measurement in the RSSI change history management table of the terminal  2 . 
         [0188]    In a case where the notification of the identifier of the terminal  2  from a different AP  1  is received, the measurement terminal management unit  106  receives an instruction from the AP control unit  107 , and outputs the RSSI change history management table of the terminal  2  that is designated by the notification to the AP control unit  107 . The RSSI change history management table transmits the AP  1  that is the notification source, through the AP communication IF  103 . In the AP  1  that is the notification source, the coupling destination determination unit  108  determines the next coupling destination of the terminal  2  of which the movement is detected, based on the RSSI change history management table that is received from a different AP  1  in the vicinity. A method of measuring the next coupling destination of the terminal  2  of which the movement is detected will be described in detail below. 
         [0189]      FIG. 16  is a flowchart illustrating an example of a flowchart of the processing by the AP control unit  107  in a first phase of terminal coupling control processing according to the second embodiment. When the movement of the terminal  2  that is performing coupling to the host AP  1  is detected, the processing that is illustrated in  FIG. 16  starts. The movement of the terminal  2  is detected by the coupling terminal management unit  104 . In a case where the received electromagnetic wave strength of any terminal  2 , which is input from the terminal wireless communication IF  101 , is below the movement detection threshold, the coupling terminal management unit  104  detects the movement of the terminal  2 . The terminal  2  of which the movement is detected is hereinafter referred to as a target terminal  2 . 
         [0190]    In OP 51 , the AP control unit  107  instructs the beacon control unit  111  and the probe response control unit  110  to stop the beacon transmission and the transmission of the probe response, respectively, from the terminal wireless communication IF  101 . Next, the processing proceeds to OP 52 . 
         [0191]    In OP 52 , the AP control unit  107  transmits the identifier of the target terminal  2  to a different AP  1  through the AP communication IF  103 . Next, the processing proceeds to OP 53 . 
         [0192]    In OP 53 , the AP control unit  107  receives the history (the RSSI change history table) of the received electromagnetic wave strength of the target terminal  2  from a different AP  1  through the AP communication IF  103 . The completion of the procession in OP 53 , for example, is determined by receiving the history from all the APs  1  that are registered in the neighboring AP management table  133  or by a given time having elapsed after the transmission of the identifier of the target terminal  2 . Next, the processing proceeds to OP 54 . 
         [0193]    In OP 54 , the AP control unit  107  instructs the coupling destination determination unit  108  to determines the next coupling destination of the target terminal  2 , and acquires the AP  1  of the next coupling destination of the target terminal  2  from the coupling destination determination unit  108 . Thereafter, the processing that is illustrated in  FIG. 16  ends, and next, the second-phase processing in  FIG. 10  starts in the same manner as in the first embodiment. 
         [0194]      FIG. 17  is a flowchart illustrating an example of processing by the coupling destination determination unit  108 , which determines the next coupling destination of the target terminal  2 . When an instruction to determine the next coupling destination of the target terminal  2  is input from the AP control unit  107 , the processing that is illustrated in  FIG. 17  starts. 
         [0195]    In OP 61 , the coupling destination determination unit  108  determine whether or not movement speed of the target terminal  2  is a speed threshold or above. The speed threshold is arbitrarily set by the manager to be within walking speed of the user. In a case where the movement speed of the target terminal  2  is the speed threshold or above (Yes in OP 61 ), the processing proceeds to OP 62 . In a case where the movement speed of the target terminal  2  is below the speed threshold (No in OP 61 ), the processing proceeds to OP 66 . 
         [0196]    In OP 62 , the coupling destination determination unit  108  determines whether or not a different AP  1  is present farther in the movement direction of the target terminal  2  than the AP  1  that is the closest to the host AP  1 . The position of the different AP  1  is retained in advance. In a case where the different AP  1  is present farther in the movement direction of the target terminal  2  than the AP  1  that is the closest to the host AP  1  (Yes in OP 62 ), the processing proceeds to OP 63 . In a case where the different AP  1  is not present farther in the movement direction of the target terminal  2  than the AP  1  that is the closest to the host AP  1  (No in OP 62 ), the processing proceeds to OP 66 . The AP  1  that is present farther in the movement direction of the target terminal  2  than the AP  1  that is the closest to the host AP  1  is referred to as a candidate AP. 
         [0197]    In OP 63 , the coupling destination determination unit  108  determine whether or not an electromagnetic wave of the target terminal  2  reaches the candidate AP  1 . It is illustrated that if the RSSI change history management table of the target terminal  2  is received from the candidate AP  1 , the electromagnetic wave of the target terminal  2  reaches the candidate AP  1 . In a case where the electromagnetic wave of the target terminal  2  reaches the candidate AP  1  (Yes in OP 63 ), the processing proceeds to OP 64 . In a case where the electromagnetic wave of the target terminal  2  does not reach the candidate AP  1  (No in OP 63 ), the processing proceeds to OP 66 . 
         [0198]    In OP 64 , the coupling destination determination unit  108  extracts the AP  1  of which the congestion level is below the congestion threshold, from candidates APs  1 , and the extracted AP  1  is set to be a new candidate AP  1 . Next, the processing proceeds to OP 65 . Moreover, in a case where the AP  1  of which the congestion level is below the congestion threshold is not present, the processing may proceed to OP 66 . 
         [0199]    In OP 65 , the coupling destination determination unit  108  determines the AP  1  that is the closest to the AP  1 , among the APs  1  that are extracted from the candidates APs  1 , as the next coupling destination, and notifies the AP control unit  107  of the determined AP  1 . Thereafter, the processing that is illustrated in  FIG. 17  ends. 
         [0200]    In OP 66 , it is determined whether or not the congestion level of the AP  1  that is the closest to the host AP  1  is below the congestion threshold. In a case where the congestion level of the AP  1  that is the closest to the host AP  1  is below the congestion threshold (Yes in OP 66 ), the processing proceeds to OP 67 . In a case where the congestion level of the AP  1  that is the closest to the host AP  1  is the congestion threshold or above (No in OP 66 ), the coupling destination determination unit  108  notifies the AP control unit  107  that there is no next coupling destination, and ends the processing that is illustrated in  FIG. 17 . In this case, the AP  1  does not release a coupling to the target terminal  2 . 
         [0201]    In OP 67 , the coupling destination determination unit  108  determines the AP  1  that is the closest to the host AP  1 , as the next coupling destination, and notifies the AP control unit  107  of the determined AP  1 . Thereafter, the processing that is illustrated in  FIG. 17  ends. 
         [0202]    Moreover, in an example that is illustrated in  FIG. 17 , the AP  1  that is the closest to the host AP  1  is selected as the next coupling destination of the target terminal  2 , from among the candidate APs  1 , but a method of selecting the next coupling destination of the target terminal  2  is not limited to this. Although the AP  1  that is the closest to the host AP  1  is selected from among the candidate APs  1 , the AP  1  that is positioned between the AP  1  that is the closest to the host AP  1  and the AP  1  that is the farthest from the host AP  1  may be selected as the next coupling destination of the target terminal  2 . 
         [0203]      FIG. 18  is a diagram illustrating an example of a sequence of the terminal coupling control processing according to the second embodiment. In an example that is illustrated in  FIG. 18 , output strengths of electromagnetic waves of the AP # 1 , the AP # 2 , the AP # 3  are assumed to be same. Furthermore, it is assumed that the AP # 1  is positioned the closest to the terminal # 1 , the AP # 2  is positioned the second closest to the terminal # 1 , and the AP # 3  is positioned the third closest to the terminal # 1 . In the example that is illustrated in  FIG. 18 , the terminal # 1  performs coupling to the AP # 1  and performs communication. Furthermore, it is assumed that the congestion is not detected from any of the AP # 1 , the AP # 2 , and the AP # 3 . 
         [0204]    In S 21 , the AP # 1  detects the movement of the terminal # 1  of which the received electromagnetic wave strength is below a given threshold. In S 22 , the AP # 1  stops the beacon transmission and the transmission of the beacon transmission from the terminal wireless communication IF  101  (OP 51  in  FIG. 16 ). 
         [0205]    In S 23 , the AP # 1  notifies a different AP  1  in the vicinity of the identifier of the terminal # 1  (OP 52  in  FIG. 16 ). This notification may be transmitted by any one of broadcasting, multicasting, and unicasting. 
         [0206]    In S 24  and S 25 , the AP # 2  and the AP # 3 , respectively, transmit the RSSI change history management table of the terminal # 1  to the AP # 1 . Moreover, the RSSI change history management table of the terminal # 1  is transmitted by unicasting with the AP # 1  as a destination. 
         [0207]    In S 26 , the AP # 1  receives the RSSI change history management table of the terminal # 1  from the AP # 2  and the AP # 3  (OP 53  in  FIG. 16 ), and determines the next coupling destination of the terminal # 1  (OP 54  in  FIG. 16 ). It is assumed that the AP # 1  detects that the terminal # 1  moves in the direction of the AP # 2  and that the AP # 3  that is positioned farther in the movement direction of the terminal # 1  than the AP # 2  is selected as the next coupling destination of the terminal # 1 . 
         [0208]    In S 27 , the AP # 1  transmits the switch terminal information (OP 11  in  FIG. 10 ). The identifier of the terminal # 1  and the identifier of the AP # 3  are included, in a state of being associated with each other, in the switch terminal information that is transmitted from the AP # 1  in S 27 . The switch terminal information may be transmitted by any one of broadcasting, multicasting, and unicasting. 
         [0209]    In S 28 , each of the AP # 2  and the AP # 3  receives the switch terminal information (Yes in OP 31  in  FIG. 11B ), and stops the beacon transmission and the transmission of the probe response with the terminal wireless communication IF  101  (OP 33  and OP 35  in  FIG. 11B ). 
         [0210]    In S 29 , because the identifier of the host AP is not included in the switch terminal information, when the processing in S 28  ends, without any change, the AP # 2  transmits the setting completion notification to the AP # 1  (OP 34  in  FIG. 11B ). 
         [0211]    In S 30 , because with the switch terminal information, the AP # 3  is designated as the next coupling destination, the setting is performed in such a manner that the probe response to the probe request from the terminal # 1  is transmitted (OP 36  in  FIG. 11B ) and the setting completion notification is transmitted to the AP # 1  (OP 37  in  FIG. 11B ). 
         [0212]    In S 31 , the AP # 1  receives the setting completion notification from the AP # 2  and the AP # 3  (OP 12  in  FIG. 10 ), and releases a coupling to the terminal # 1  (OP 13  in  FIG. 10 ). 
         [0213]    In S 32 , because the coupling is released by the AP # 1 , the terminal # 1  starts re-coupling processing in order to perform the handover. In S 33 , as one of the re-coupling processing operations, the terminal # 1  transmits the probe request on the utilization channel for the terminal wireless communication IF  101  of each of the AP # 1 , and the AP # 2 , and the AP # 3 . 
         [0214]    In S 34 , the AP # 3  receives the probe request from the terminal # 1  (OP 38  in  FIG. 11B ), and transmits the probe response to the terminal # 1  (OP 39  in  FIG. 11B ). Because the AP # 1  and the AP # 2  also receives the probe request from the terminal # 1 , but because the transmission of the probe response is stopped, the probe response is not transmitted to the terminal # 1 . 
         [0215]    In S 35 , the terminal # 1  receives the probe response from the AP # 3 . Because the probe response is not transmitted from the AP # 1  and the AP # 2  to the terminal # 1 , the terminal # 1  recognizes the presence of the AP # 3 , but does not recognize the presence of the AP # 1  and the AP # 2 . In S 36 , the terminal # 1  performs coupling to the AP # 3  of which the presence the terminal # 1  recognizes. 
         [0216]    In S 37 , the AP # 3  transmits the coupling completion notification to the AP # 1  (OP 40  in  FIG. 11B ). In S 38 , because none of the AP # 1 , the AP # 2 , and the AP # 3  is congested, the beacon transmission and the transmission of the probe response with the terminal wireless communication IF  101  is resumed (OP 15  in  FIG. 10  and OP 42  in  FIG. 11B ). 
         [0217]    &lt;Operational Effect of the Second Embodiment&gt; 
         [0218]    In the example that is illustrated in  FIG. 18 , when the terminal # 1  moves in the direction of the AP # 2  and the AP # 1  detects the movement of the terminal # 1 , the AP # 3  is selected as the next coupling destination of the terminal # 1 . Accordingly, in a case where the terminal # 1  moves in the direction from the AP # 1  to the AP # 2  to the AP # 3 , the handover from the AP # 1  to the AP # 2  is omitted. Accordingly, the number of times that the communication with the terminal # 1  is momentarily interrupted is reduced. Furthermore, because the AP # 1  excludes the AP  1  that is in congestion, from the candidate for the next coupling destination of the target terminal, the coupling by the terminal # 1  to the AP  1  that is in congestion is suppressed and local concentration of the coupling by the terminals is suppressed. 
         [0219]    Furthermore, after the terminal # 1  switches the coupling destination to the AP # 3 , in a case where the user of the terminal # 1  stops in the vicinity of the AP # 2  or changes the direction, the AP # 3  detects that the terminal # 1  is positioned at a great distance and performs the coupling destination switching described above. As a result, the coupling destination of the terminal # 1  is switched from the AP # 3  to a different AP  1 . 
         [0220]    Furthermore, in a case where the terminal # 1  moves in a different direction from that estimated, and the coupling destination that is the AP # 3  is not switched, the terminal # 1  performs coupling to the AP  1  that has the greatest received electromagnetic wave strength, at a timing when the AP # 1 , the AP # 2 , and the AP # 3  resumes the beacon transmission and the transmission of the probe response. 
       Third Embodiment 
       [0221]    According to the first and second embodiments, the next coupling destination of the terminal that have made a coupling to the AP  1  is determined. According to a third embodiment, even in a case where a new terminal that does not perform coupling to any AP  1  performs the coupling processing, the AP  1  performs control in such a manner that a coupling to the AP  1  that is not congested can be made. A description of what the third embodiment has in common with the first and second embodiments is omitted. Moreover, a hardware configuration of an AP  1  according to the third embodiment is the same as that according to the first embodiment. 
         [0222]    According to the third embodiment, when the AP  1  detects the congestion in the host AP  1 , the AP  1  stops the beacon transmission and the transmission of the probe response in such a manner that the coupling by the terminal  2  is no longer accepted. Along with this, based on the congestion level of a different AP  1 , the AP  1  selects the AP  1  that is not congested, as the coupling destination, notifies the different AP  1  of the selected AP  1 , and performs setting in such a manner that the selected AP  1  replies to the probe request of a new terminal  2 . 
         [0223]    Additionally, according to the third embodiment, the AP  1  controls the congestion level that is mutually exchanged between the AP  1  and a different AP  1 , based on the history of the past congestion level. For example, there is a high likelihood that the congestion will also occur at a current point in time during a period of time, the history for which indicates that the congestion occurred in the past. For this reason, in order to suppress the concentration of coupling by new terminals  2  in advance, even in a case where the congestion does not occur actually, the AP  1  changes the congestion level to a value that is greater than is assigned to the actual congestion level, and transmits the changed congestion level to a different AP  1 . Accordingly, because the congestion appears to occur when viewed from a different AP  1 , this decreases the likelihood that the host AP  1  will be selected as the coupling destination of a new terminal  2 . 
         [0224]      FIG. 19  is a diagram illustrating an example of functional constituents of the AP  1  according to the third embodiment. As the functional constituents, the AP  1  includes the terminal wireless communication IF  101 , the AP communication IF  103 , the coupling terminal management unit  104 , the congestion level determination unit  105 , the AP control unit  107 , the coupling destination determination unit  108 , the neighboring AP management unit  109 , the probe response control unit  110 , the beacon control unit  111 , a congestion level management unit  112 , and a notification congestion level determination unit  113 . These functional constituents are functions that are accomplished by the execution by the CPU  11  of the AP  1  of the terminal coupling control program  13 P that is stored in the secondary storage device  13 . 
         [0225]    The terminal wireless communication IF  101 , the AP communication IF  103 , the coupling terminal management unit  104 , the congestion level determination unit  105 , the AP control unit  107 , the coupling destination determination unit  108 , the neighboring AP management unit  109 , the probe response control unit  110 , and the beacon control unit  111  are as described according to the first embodiment. 
         [0226]    According to the third embodiment, the congestion level determination unit  105  outputs the congestion level to the neighboring AP management unit  109  and the congestion level management unit  112 . A period with which the congestion level is calculated, for example, is arbitrarily set by the manager on the minute basis, such as on the one-minute basis. 
         [0227]    The congestion level management unit  112  receives the congestion level from the congestion level determination unit  105 , and manages the congestion level of the host AP  1 . Specifically, the congestion level management unit  112  calculates an average congestion level for every given unit time, and stores a history of the average congestion level in an average congestion level history management table that will be described below. For example, the given unit time for which the average congestion level is calculated is set by the manager at intervals of several minutes to several hours. 
         [0228]    The notification congestion level determination unit  113  measures the congestion level that is notified to a different AP  1 , from a past congestion level and a current congestion level. The congestion level that is notified to the different AP  1  is hereinafter referred to as a notification congestion level. For example, in a case where a congestion level for the same period of time in the past as a current period of time indicates that the congestion occurred, the notification congestion level determination unit  113  sets the notification congestion level to be higher than the current congestion level, in such a manner that the coupling by the terminals  2  is not concentrated on the host AP 1 . Processing by the notification congestion level determination unit  113  will be described in detail below. 
         [0229]      FIG. 20  is a diagram illustrating an example of the average congestion level history management table. The average congestion level history management table is a table that contains the history of the average congestion level which is an average value per unit time of the congestion levels of the host AP  1 . The average congestion level history management table is retained in the secondary storage device  13  of the AP  1 . 
         [0230]    In the average congestion level history management table that is illustrated in  FIG. 20 , the unit time is 30 minutes. Furthermore, in the average congestion level history management table that is illustrated in  FIG. 20 , the average congestion level is stored for every period of time that results from dividing one day by 30 minutes. In an example that is illustrated in  FIG. 20 , the congestion level is a wireless occupancy rate of the terminal wireless communication IF  101 , and is illustrated as a percentage. 
         [0231]      FIG. 21  is a diagram illustrating an example of the congestion level that is exchanged between the AP # 1 , the AP # 2 , and the AP # 3  for a period of time from 13:00 to 13:30 on Tuesday. The average congestion level of the AP # 1  for the same period of time on the last Tuesday is 20 and the current congestion level is 30. The average congestion level of the AP # 2  for the same period of time on the last Tuesday is 70 and the current congestion level is 30. The average congestion level of the AP # 3  for the same period of time on the last Tuesday is 30 and the current congestion level is 50. The average congestion level for the same period of time one week ago as the current period of time is hereinafter referred to as a “preceding average congestion level”. 
         [0232]    The AP  1  acquires the preceding average congestion level of the host AP  1  from the average congestion level history management table. With the notification from the different AP  1 , the AP  1  acquires the preceding average congestion level of a different AP  1 , along with the congestion level of the different AP  1 . 
         [0233]    According to the third embodiment, the AP  1  determines the notification congestion level, with an average value of the preceding average congestion level of the host AP  1  and the preceding average congestion level of a different AP  1  as a congestion estimation threshold. The congestion estimation threshold is a threshold for estimating the occurrence of the congestion. 
         [0234]    (1) In a case where a current congestion level of any AP  1  is a congestion level estimation threshold or above, the notification congestion level is assumed to be the current congestion level. 
         [0235]    (2) In a case where the current congestion level is below the congestion level estimation threshold, the congestion level estimation threshold of the AP  1  that is expected to be congested is assumed to be the notification congestion level. The AP  1  that is expected to be congested is the AP  1  of which the preceding average congestion level is the congestion level estimation threshold or above. 
         [0236]    (3) In a case where the current congestion level is below the congestion level estimation threshold, the notification congestion level of an AP  1  other than the AP  1  that is expected to be congested is assumed to be a value that is smaller than is assigned to the notification congestion level of the AP  1  that is expected to be congested. 
         [0237]    In an example that is illustrated in  FIG. 21 , the congestion level estimation threshold is 40 that is an average value of the preceding average congestion level. Furthermore, the AP # 2  of which the preceding average congestion level is the congestion level estimation threshold (40) or above is the AP  1  that is expected to be congested. 
         [0238]    Because the AP # 1  is not the AP  1  that is expected to be congested and the current congestion level of the AP # 1  is 30 that is below the congestion level estimation threshold (40), the notification congestion level of the AP # 1  is determined as 10 that is a value smaller than the notification congestion level 40 of the AP # 2  that is expected to be congested. Because the AP # 2  is the AP  1  that is expected to be congested and the current congestion level of the AP # 2  is 30 that is below the congestion level estimation threshold (40), the notification congestion level of the AP # 2  is determined as the congestion level estimation threshold (40). Because the current congestion level of the AP # 3  is 50 that is the congestion level estimation threshold (40) or above, the notification congestion level of the AP # 3  is determined as the current congestion level (50). 
         [0239]    On the assumption of a state of the congestion level of each AP  1  that is illustrated in  FIG. 21 , when the AP # 3  detects the congestion, the AP # 3  selects the AP  1  that is the coupling destination of a new terminal  2 . For example, in a case where the AP  1  that has the lowest notification congestion level is selected as the coupling destination of a new terminal  2 , the AP # 3  selects the AP # 1  as the coupling destination of a new terminal  2 . According to the policy described above, although the notification congestion level of each AP  1  is determined and thus the AP # 1  and thus the AP # 2  has the same current congestion level, because the AP # 2  that is expected to be congested has a greater notification congestion level, the AP # 1  is selected as the coupling destination of the new terminal  2 . Accordingly, control is performed in such a manner that the coupling by the new terminals  2  is not concentrated on the AP # 2  that is expected to be congested. 
         [0240]      FIG. 22  is a flowchart illustrating an example of processing by the notification congestion level determination unit  113 , which determines the notification congestion level. When the congestion level is input from the congestion level determination unit  105  to the notification congestion level determination unit  113 , the processing that is illustrated in  FIG. 22  starts. That is, the processing that is illustrated in  FIG. 22  is performed with a period of the calculation of the congestion level. Moreover, the period of the calculation of the congestion level, for example, is arbitrarily set by the manager on the minute basis, such as on the one-minute basis, or on the second basis. 
         [0241]    In OP 71 , the notification congestion level determination unit  113  determines whether or not the current congestion level is the congestion level estimation threshold or above. The congestion level estimation threshold, for example, is an average value of the average congestion level of each AP  1  at the same point time one week ago as the current point in time and for the same period of time one week ago as the current period of time. In a case where the current congestion level is the congestion level estimation threshold or above (Yes in OP 71 ), the processing proceeds to OP 72 . In a case where the current congestion level is below the congestion level estimation threshold (No in OP 71 ), the processing proceeds to OP 73 . 
         [0242]    In OP 72 , the notification congestion level determination unit  113  determines the current congestion level as the notification congestion level. Thereafter, the processing that is illustrated in  FIG. 22  ends. 
         [0243]    In OP 73 , the notification congestion level determination unit  113  determines whether or not the preceding average congestion level is the congestion level estimation threshold or above. In a case where the preceding average congestion level is the congestion level estimation threshold or above, (Yes in OP 73 ), the processing proceeds to OP 74 . In a case where the preceding average congestion level is below the congestion level estimation threshold (No in OP 73 ), the processing proceeds to OP 75 . 
         [0244]    In OP 74 , the notification congestion level determination unit  113  determines the average value of the preceding average congestion level. Thereafter, the processing that is illustrated in  FIG. 22  ends. 
         [0245]    In OP 75 , because the host AP  1  is not the AP  1  that is expected to be congested, the notification congestion level determination unit  113  determines a value that is smaller than the notification congestion level of a different AP  1  that is expected to be congested, as the notification congestion level. From the AP  1  that is expected to be congested, it is possible for the manager to arbitrarily set to what extent the notification congestion level is set to be small. Thereafter, the processing that is illustrated in  FIG. 22  ends. 
         [0246]    The determined notification congestion level is output to the AP control unit  107 , and is transmitted to a different AP  1  through the AP communication IF  103 . Moreover, the congestion level estimation threshold is not limited to the average value of the average congestion level of each AP  1  at the same point time one week ago as the current point in time and for the same period of time one week ago as the current period of time, and for example, may be set to a fixed value. Furthermore, the congestion level estimation threshold may be the same value as a congestion level threshold. 
         [0247]      FIG. 23  is a flowchart illustrating an example of processing by an AP control unit  107  according to the third embodiment, which is perform when the congestion is detected. The processing that is illustrated in  FIG. 23  starts when the detection of the congestion is input from the congestion level determination unit  105 . 
         [0248]    In OP 81 , the AP control unit  107  instructs the beacon control unit  111  and the probe response control unit  110  to stop the beacon transmission and the transmission of the probe response, respectively. Next, the processing proceeds to OP 82 . 
         [0249]    In OP 82 , the AP control unit  107  instructs the coupling destination determination unit  108  to determine the AP  1  that is the coupling destination of a new terminal  2 , and acquires the AP  1  that is the coupling destination of the new terminal  2  from the coupling destination determination unit  108 . The coupling destination determination unit  108 , for example, selects the AP  1  that has the lowest notification congestion level, as the AP that is the coupling destination of the new terminal  2 . Alternatively, the coupling destination determination unit  108  may select all the APs  1  that have the notification congestion level that is smaller than the congestion threshold, as the APs that are the coupling destinations of the new terminal  2 . Next, the processing proceeds to OP 83 . 
         [0250]    In OP 83 , the AP control unit  107  transmits new terminal coupling destination information including the identifier of the AP  1  that is the coupling destination of the new terminal, to a different AP  1  through the AP communication IF  103 . Next, the processing proceeds to OP 84 . 
         [0251]    In OP 84 , the AP control unit  107  receives a setting completion notification from a different AP  1  through the AP communication IF  103 . For example, after a given time has elapsed from when the new terminal coupling destination information was received, the processing may proceed to OP 85 . 
         [0252]    In OP 85 , the AP control unit  107  determines whether or not the utilization channel for the terminal wireless communication IF  101  of the host AP  1  is in congestion. The AP control unit  107  requests the congestion level determination unit  105  to determine whether or not the utilization channel for the terminal wireless communication IF  101  is in congestion. If the current congestion level is the congestion threshold or above, the congestion level determination unit  105  detects that the utilization channel for the terminal wireless communication IF  101  of the device is congested. 
         [0253]    The AP control unit  107  is a waiting state until the congestion in the utilization channel for the terminal wireless communication IF  101  of the host AP  1  is alleviated. When the congestion in the utilization channel for the terminal wireless communication IF  101  of the host AP  1  is alleviated (No in OP 85 ), the processing proceeds to OP 86 . 
         [0254]    In OP 86 , because the congestion in the utilization channel for the terminal wireless communication IF  101  is alleviated, the AP control unit  107  instructs the beacon control unit  111  and the probe response control unit  110  to start the beacon transmission and the transmission of the probe response, respectively, with the terminal wireless communication IF  101 . Accordingly, the coupling by the new terminal  2  to the host AP  1  is resumed. Thereafter, the processing that is illustrated in  FIG. 23  ends. 
         [0255]      FIG. 24  is an example of a flowchart of the processing that, according to the third embodiment, is performed by the AP control unit  107  when the terminal wireless communication IF  101  does not detect the congestion. The processing that is illustrated in  FIG. 24  starts together with the activation of the AP  1 , and is repeatedly performed while the AP  1  is in operation. 
         [0256]    In OP 91 , in a case where the AP control unit  107  receives new terminal coupling destination information from the in-congestion AP  1  through the AP communication IF  103  (Yes IN OP 91 ), the processing proceeds to OP 92 . In a case where the new terminal coupling destination information is not received, (No in OP 91 ), the processing that is illustrated in  FIG. 24  ends, and again starts. 
         [0257]    In OP 92 , with the new terminal coupling destination information, the AP control unit  107  determines whether or not the host AP  1  is designated as the coupling destination, that is, whether or not the identifier of the host AP  1  is included in the new terminal coupling destination information. In a case where with the new terminal coupling destination information, the host AP  1  is designated as the coupling destination (Yes in OP 92 ), the processing that is illustrated in  FIG. 24  ends. In a case where with the new terminal coupling destination information, the host AP  1  is not designated as the coupling destination (No in OP 92 ), the processing proceeds to OP 93 . 
         [0258]    In OP 93 , the AP control unit  107  instructions the beacon control unit  111  and the probe response control unit  110  to stop the beacon transmission and the transmission of the probe response, respectively, with the terminal wireless communication IF  101 . Accordingly, the new terminal  2  is suppressed from newly performing coupling to the host AP  1 . Next, the processing proceeds to OP 94 . 
         [0259]    In OP 94 , the AP control unit  107  transmits the setting completion notification that the setting in accordance with the new terminal coupling destination information is completed, to the in-congestion AP  1  through the AP communication IF  103 . Next, the processing proceeds to OP 95 . 
         [0260]    In OP 95 , the AP control unit  107  determines whether or not the utilization channel for the terminal wireless communication IF  101  of the host AP  1  is in congestion. In the case where the utilization channel for the terminal wireless communication IF  101  of the host AP  1  is in congestion (Yes in OP 95 ), the processing that is illustrated in  FIG. 24  ends and instead the processing that is illustrated in  FIG. 23  starts. In the case where the utilization channel for the terminal wireless communication IF  101  of the host AP  1  is not congested (No in OP 95 ), the processing proceeds to OP 96 . 
         [0261]    In OP 96 , the AP control unit  107  instructs the beacon control unit  111  and the probe response control unit  110  to start the beacon transmission and the probe response, respectively, with the terminal wireless communication IF  101 . This is because the utilization channel for the terminal wireless communication IF  101  is not congested and it is possible to accept a coupling by a new terminal  2 . Accordingly, the coupling by the new terminal  2  to the host AP  1  is resumed. Thereafter, the processing that is illustrated in  FIG. 24  ends and the processing again starts. 
       Operational Example 
       [0262]      FIG. 25  is a diagram illustrating an example of a sequence of terminal coupling control processing according to the third embodiment. In  FIG. 25 , the wireless communication network system  100  that is illustrated in  FIG. 1  is assumed. 
         [0263]    In S 41 , the AP # 1  detects the congestion in the utilization channel for the terminal wireless communication IF  101 . In S 42 , the AP # 1  stops the beacon transmission and the transmission of the probe response from the terminal wireless communication IF  101  (OP 81  in  FIG. 23 ). 
         [0264]    In S 43 , the AP # 1  determines the coupling destination of a new terminal  2  (OP 82  in  FIG. 23 ). The AP # 1  is assumed to select the AP # 2  as the coupling destination of the new terminal  2 . 
         [0265]    In S 44 , the AP # 1  transmits a new terminal coupling destination information (OP 83  in  FIG. 23 ). The identifier of the AP # 2  is included in the new terminal coupling destination information that is transmitted from the AP # 1  in S 44 . The new terminal coupling destination information may be transmitted by an one of broadcasting, multicasting, and unicasting. 
         [0266]    In S 45 , each of the AP # 2  and the AP # 3  receive the new terminal coupling destination information (Yes in OP 91  in  FIG. 24 ). Because with the new terminal coupling destination information, the AP # 3  is not designated as the coupling destination of a new terminal  2  (No in OP 92  in  FIG. 24 ), the AP # 3  stops the beacon transmission and the transmission of the probe response with the terminal wireless communication IF  101  in order for the new terminal  2  not to perform coupling (OP 93  in  FIG. 24 ). Because with the new terminal coupling destination information, the AP # 2  is designated as the coupling destination of the new terminal  2  (Yes in OP 92  in  FIG. 24 ), the AP # 2  does not stop the beacon transmission and the transmission of the probe response in such a manner that the new terminal  2  can perform coupling. 
         [0267]    In S 46 , the AP # 3  transmits the setting completion notification to the AP # 1  (OP 94  in  FIG. 24 ). Moreover, the setting completion notification is transmitted by unicasting with the AP # 1  as the destination. 
         [0268]    In S 47 , a new terminal # 1  that does not perform coupling to any AP  1  starts the coupling processing. In S 48 , as one of the coupling processing operations, the terminal # 1  transmits the probe request on the utilization channel for the terminal wireless communication IF  101  of any one of the AP # 1 , and the AP # 2 , and the AP # 3 . 
         [0269]    In S 49 , the AP # 2  receives the probe request from the terminal # 1 , and transmits the probe response to the terminal # 1 . Although the AP # 1  and the AP # 3  receives the probe request from the terminal # 1 , because the transmission of the probe response is stopped, the probe response is not transmitted to the terminal # 1 . 
         [0270]    In S 50 , the terminal # 1  receives the probe response from the AP # 2 . Because the probe response is not transmitted from the AP # 1  and the AP # 3  to the terminal # 1 , the terminal # 1  recognizes the presence of the AP # 2 , but does not recognize the presence of the AP # 1  and the AP # 3 . In S 51 , the terminal # 1  performs coupling to the AP # 2  of which the presence the terminal # 1  recognizes. 
         [0271]    &lt;Operational Effect of the Third Embodiment&gt; 
         [0272]    In an example that is illustrated in  FIG. 25 , when the congestion is detected, the in-congestion AP # 1  stops the probe response of the host AP  1  and designates the AP # 2  that is not congested, as the coupling destination of a new terminal  2 . The AP # 3  that is not designated stops the probe response, and the designated AP # 2  replies to the probe request form the terminal  2 . Accordingly, the new terminal  2  performs coupling to the AP # 2  that is not congested, and the concentration of the coupling by the terminals on the AP # 1  is suppressed. 
         [0273]    According to the third embodiment, the AP  1  determines the notification congestion level based on the current congestion level and the past congestion level. For example, in a case where the host AP  1  neither has the history indicating that the host AP  1  was in congestion for the same period of time in the past as the current period of time, nor is in congestion, the AP  1  determines the notification congestion level, with a value smaller than the congestion level that is notified by a different AP  1  that has the history indicating that the different AP  1  was in congestion for the same period of time in the past. The AP  1  that has the history indicating that the AP  1  was in congestion for the same period of time in the past is expected to be also in congestion for the current period of time. The likelihood that the designation as the coupling destination of a new terminal  2  will be performed is increased by notifying that the congestion level is lower than that of the AP  1  that is expected to be congested. Accordingly, the coupling by new terminals  2  is distributed to the APs  1  that are not expected to be congested, and the coupling to the AP  1  that is expected to be congested is suppressed. 
         [0274]    Moreover, according to the third embodiment, the AP  1  that is not designated with new terminal coupling destination information stops the transmission of the probe response and of the beacon without any change. After a given time has elapsed from when the transmission of the probe response and the beacon transmission was stopped, the AP  1  may resume the transmission of the probe response and the beacon transmission. The given time from the stopping of the transmission of the probe response and the beacon transmission to the resumption thereof is arbitrarily set by the manager. The given time is, for example, one minute or approximately several minutes. 
       Modification Examples of the First to Third Embodiments 
       [0275]    According to the first to third embodiments, each AP  1  individually performs processing that determines the coupling destination of the terminal  2 . In contrast, a master-slave type system configuration may be employed in which a server that generally manages all APs  1  performs the processing by the terminal  2 , which determines the coupling destination. In this case, the server performs processing that is performed by each of the AP control unit  107  and the coupling destination determination unit  108 . 
         [0276]      FIG. 26  is a diagram illustrating an example of a system configuration of a wireless communication network system  200  according to a modification example of each of the first to third embodiments. The wireless communication network system  200  includes the multiple APs  1 , the terminal  2  that performs coupling to any one of the multiple APs  1  and performs communication, and a server  3  that generally manages the multiple APs  1 . In terms of a hardware configuration, the server  3  is, for example, an information processing apparatus that includes a CPU, a memory, an ancillary storage device, and a network interface. Furthermore, the server  3  includes a terminal coupling control program in the auxiliary storage device. By executing the program, the server  3  performs the processing that is performed by each of the AP control unit  107  and the coupling destination determination unit  108 . The server is an example of a “communication apparatus”. 
         [0277]    For example, according to the first embodiment, each AP  1  transmits the congestion level to the server  3  with a given period. When the congestion level that is the congestion threshold or above is detected, each AP  1  notifies the server  3  of the congestion detection. When receiving the notification of the congestion detection from the in-congestion AP  1 , the server  3  notifies a different AP  1  of the instruction that causes the received electromagnetic wave strength on the utilization channel for the in-congestion AP  1  to start to be measured. The server  3  receives the measurement terminal information from the different AP  1 , determines the coupling destination switching target terminal and the AP  1  that is the next coupling destination, based on the measurement terminal information, and transmits the switch terminal information to each AP  1 . 
         [0278]    According to the switch terminal information from the server  3 , the AP  1  is set in such a manner that the AP  1  stops the probe response if the AP  1  is not designated as the next coupling destination and the AP  1  replies to the probe request if the AP  1  is designated as the next coupling destination. When receiving the switch terminal information from the server  3 , the in-congestion AP  1  releases a coupling to the designated terminal and stops the probe response. 
         [0279]    For example, according to the second embodiment, when receiving from the AP  1  the notification that the terminal  2  moves, the server  3  makes a request to a different AP  1  for the RSSI change history management table of that terminal  2 , receives the RSSI change history management table, determines the next coupling destination based on that table, and transmits the switch terminal information to each AP  1 . The operation of each AP  1  is the same as in the first embodiment. 
         [0280]    For example, according to the third embodiment, with a given period, each AP  1  determines the notification congestion level and notifies the server  3  of the notification congestion level. When receiving the notification of the congestion detection from the in-congestion AP  1 , the server  3  determines the AP  1  that is the next coupling destination of a new terminal  2  and notifies each AP  1  of new terminal coupling destination information. The AP  1  that is not designated with the new terminal coupling destination information stops a probe response. 
         [0281]    According to any one of the embodiments, realization is possible by notifying the server  3  of information that is exchanged between the APs  1  and by the server  3  retaining information that is used according to each of the embodiments. According to the first embodiment, for example, the server  3  may retain the congestion level of each AP  1 , identification information of the terminal that performs coupling to the in-congestion AP  1 , the measurement terminal information of the received electromagnetic wave strength on a designated channel, and the like. According to the second embodiment, the server  3  may retain the congestion level of each AP  1  and a result of the measurement of the received electromagnetic wave strength on each utilization channel for the AP  1  in the vicinity, the identification information of the terminal of which the movement is detected, and pieces of information such as positional information of each AP  1 . According to the third embodiment, the server  3  may retain the congestion level of each AP  1  and pieces of information such as the history of the congestion level of each AP  1 . 
         [0282]    The server  3  performs the processing that is performed by each of the AP control unit  107  and the coupling destination determination unit  108 , and thus a load of terminal coupling control processing does not have to be placed on the AP  1 . For this reason, the AP  1  can reassign as a resource for the unnecessary terminal coupling control processing to the processing for the coupling by the terminal  2 . Furthermore, according to the first embodiment and the third embodiment, because the in-congestion AP  1  performs the processing that determines the AP  1  which is the coupling destination, the server is instead made to perform that determination processing. Thus, an effect of alleviating a load on the operation of the in-congestion AP  1  is remarkable. 
         [0283]    Moreover, one AP  1  of the APs  1  within the wireless communication network system  100 , as a master, may operate as the server described above without introducing the server. The AP  1  that operates as the master is an example of a “communication apparatus”. 
       Other Modification Examples 
       [0284]    According to the first to third embodiments, for wireless communication network system  100 , a network is assumed as the wireless LAN, but the assumed network is not limited to the wireless LAN. The wireless communication network system  100  may be a network such as 3G or Long Term Evolution (LTE). In a case where the wireless communication network system  100  is the network such as the 3G or the LTE, a “wireless base station” is a base station. Furthermore, the wireless communication interfaces  15 A and  15 B are wireless communication circuits for the 3G and the LTE. Furthermore, instead of the probe request and the probe response on the wireless LAN, an acknowledgment request and an acknowledgment in the 3G and the LTE that are equivalent to the probe request and the probe response are used. 
         [0285]    Furthermore, for the first to third embodiments, combinations of the embodiments are also possible. For example, the notification congestion level according to the third embodiment may be introduced into the first embodiment or the second embodiment. Accordingly, the switching to the coupling by the terminal  2  to the AP  1  that holds the record indicating that the congestion was detected in the past is suppressed, and the congestion level of that AP  1  is suppressed from being raised. 
         [0286]    The first to third embodiments, for example, is able to be also applied to a wireless communication network system including an AP  1  that is installed a moving object such as a train. 
         [0287]    &lt;Recording Medium&gt; 
         [0288]    A program that realizes any one of the functions described above on a computer, and on a machine and an apparatus other than the computer (which are hereinafter referred to as the computer or the like) can be recorded on a computer-readable recording medium. The computers are caused to read and execute the program on the recording medium, and thus the functions described above can be provided. 
         [0289]    The recording medium that is readable by the computers refers to a non-temporary recording medium in which pieces of information, such as data and a programs, are accumulated by performing an electrical, magnetic, optical, mechanical, or chemical operation and which can be read from the computers. Among these recording media, media that are removable from the computers include, for example, a flexible disk, a magneto-optical disk, a CD-ROM, a CD-R/W, a DVD, a Blu-ray disk, a DAT, an 8 mm tape, and a memory card such as a flash memory. Furthermore, the recording media that are fixed to the computers includes a hard disk and a read only memory (ROM). Additionally, a solid state drive (SSD) is used not only as a recording medium that is removable from the computers, but also as a recording medium that is fixed to the computers. 
         [0290]    All examples and conditional language recited herein are intended for pedagogical purposes to aid the reader in understanding the invention and the concepts contributed by the inventor to furthering the art, and are to be construed as being without limitation to such specifically recited examples and conditions, nor does the organization of such examples in the specification relate to a showing of the superiority and inferiority of the invention. Although the embodiments of the present invention have been described in detail, it should be understood that the various changes, substitutions, and alterations could be made hereto without departing from the spirit and scope of the invention.