Patent Publication Number: US-2023155754-A1

Title: Communication device, method for controlling communication device, and medium

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a Continuation of International Patent Application No. PCT/JP2021/017612, filed May 10, 2021, which claims the benefit of Japanese Patent Application No. 2020-132538, filed Aug. 4, 2020, both of which are hereby incorporated by reference herein in their entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to wireless communication techniques. 
     Background Art 
     In recent years, the amount of Internet usage has been growing as information communication technology develops, and a variety of communication technologies are being developed to meet the increase in demand. Among these, wireless local area network (WLAN) technology has improved the throughput of Internet communications such as packet data, voice, and video by terminal devices, and various technological developments are currently underway. 
     Numerous standardization efforts by the Institute of Electrical and Electronics Engineers (IEEE) 802, the standardization organization for WLAN technology, have played an important role in the development of WLAN technology. The IEEE 802.11 series of standards are known as one of the standard WLAN communication standards, and include the IEEE 802.11n/a/b/g/ac, IEEE 802.11ax, and other standards (PTL 1). For example, 802.11ax uses Orthogonal Frequency Division Multiple Access (OFDMA) to achieve high peak throughput of up to 9.6 gigabits per second (Gbps), as well as higher communication speeds under congested conditions. 
     Recently, a task group called IEEE 802.11be was launched as a successor standard to further improve throughput, frequency utilization efficiency, and communication latency. In IEEE 802.11be, multi-link techniques are being considered to transmit to a single STA (station/terminal device) using a plurality of frequency bands (radio channels) in the 2.4 GHz, 5 GHz, and 6 GHz bands simultaneously. Thus far, STAs compliant with the IEEE 802.11 standard have been connected to an access point (AP) and communicated data with the access point over a single frequency band. Multi-link techniques make it possible to improve throughput by having APs and STAs communicate data over at least two radio channels simultaneously. 
     CITATION LIST 
     Patent Literature 
     PTL 1: Japanese Patent Laid-Open No. 2018-50133 
     Communication using multi-link techniques (multi-link communication) includes a plurality of communication modes. For example, there is an asynchronous mode in which APs and STAs transmit and receive independently over a plurality of frequency bands, and a synchronous mode in which APs and STAs transmit and receive synchronously over a plurality of frequency bands. There is also a semi-asynchronous mode which uses the synchronous mode or the asynchronous modes depending on the situation. 
     The asynchronous mode is a mode in which the AP and STA transmit and receive independently in each frequency band used, and each device (AP and STA) may transmit and receive simultaneously. When the same device transmits and receives simultaneously, if there is only a small interval between the respective frequency bands used for transmission and reception, the transmitted signal may enter into the reception circuit as radio interference, causing intra-device interference that adversely affects the reception characteristics. Therefore, frequency bands with small intervals such as those that cause intra-device interference cannot be used in the asynchronous mode. On the other hand, synchronous mode is a mode in which the AP and STA transmit or receive simultaneously in each frequency band used. It is therefore possible to use frequency bands with small intervals such as those where intra-device interference can occur. With respect to STAs, an STA that supports multi-link techniques will be called an “ML-STA”, and an STA that does not support multi-link techniques will be called a “Non-ML-STA″. 
     As described above, multi-link communication can improve throughput, but if the AP and an ML-STA are not communicating in the appropriate communication mode, throughput may decrease. For example, assume a state where the AP and the ML-STA are performing multi-link communication in the synchronous mode using a plurality of frequency bands at small intervals that cause intra-device interference. In this state, when a Non-ML-STA communicates with the AP using one frequency band, it is necessary for the ML-STA and the AP to consider intra-device interference in other frequency bands. In other words, in communication between the ML-STA and the AP using the stated other frequency bands, only transmission is possible when the AP is transmitting to the Non-ML-STA, whereas only reception is possible when the AP is receiving from the Non-ML-STA. As a result, communication downtime occurs in the ML-STA and the throughput drops. 
     In addition, when the AP and the ML-STA are performing multi-link communication in synchronous mode, it is not possible to transmit and receive simultaneously when used in an application where data is transmitted and received bidirectionally in real time. This has sometimes affected the use of the application. 
     SUMMARY OF THE INVENTION 
     According to one aspect of the present invention, a communication device compliant with an IEEE 802.11 series standard, the communication device comprises: a communication unit configured to communicate in a communication mode among a first mode, in which transmission and reception are performed independently over each of frequency bands, and a second mode, in which transmission or reception is performed simultaneously over each of the frequency bands; a judgment unit configured to judge whether a predetermined condition pertaining to changing the communication mode is satisfied; and a determination unit configured to determine, in a case where the judgment unit judges that the predetermined condition is satisfied, at least one frequency band to be used for communication in the communication mode after the changing, wherein in a case where the judgment unit judges that the predetermined condition is satisfied while the communication unit is communicating with a first other communication device using each frequency band of a plurality of frequency bands in the first mode or the second mode, the determination unit determines, based on the plurality of frequency bands, a plurality of frequency bands for performing communication in the communication mode after the changing, and the communication unit communicates with the first other communication device in the communication mode after the changing using the plurality of frequency bands determined. 
     Further features of the present invention will become apparent from the following description of exemplary embodiments with reference to the attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate embodiments of the invention and, together with the description, serve to explain principles of the invention. 
         FIG.  1    is a diagram illustrating an example of the configuration of a network. 
         FIG.  2 A  is a block diagram illustrating an example of the functional configuration of a communication device (an AP, an ML-STA). 
         FIG.  2 B  is a block diagram illustrating an example of the functional configuration of a communication device (a Non-ML-STA). 
         FIG.  3 A  is a block diagram illustrating an example of the hardware configuration of a communication device (the AP, the ML-STA). 
         FIG.  3 B  is a block diagram illustrating an example of the hardware configuration of a communication device (the Non-ML-STA). 
         FIG.  4    is a conceptual diagram illustrating a frequency interval value required to ensure intra-device interference does not occur. 
         FIG.  5    is a flowchart illustrating processing for connecting with an ML-STA executed by an AP in a first embodiment. 
         FIG.  6    is a sequence chart illustrating connection processing executed between an AP and an ML-STA in the first embodiment. 
         FIG.  7    is a diagram illustrating frequency intervals of links in the first embodiment. 
         FIG.  8    is a flowchart illustrating processing for connecting with a Non-ML-STA executed by an AP in the first embodiment. 
         FIG.  9    is a sequence chart illustrating connection processing executed between an AP and a Non-ML-STA in the first embodiment. 
         FIG.  10    is a conceptual diagram illustrating a frequency band change for a link in the first embodiment. 
         FIG.  11    is a flowchart illustrating frequency band change processing for a link after disconnection from a Non-ML-STA, executed by an AP, in the first embodiment. 
         FIG.  12    is a sequence chart illustrating frequency band change processing for a link after disconnection from a Non-ML-STA, in the first embodiment. 
         FIG.  13    is a sequence chart illustrating processing for connecting with an ML-STA executed by an AP in a second embodiment. 
         FIG.  14    is a flowchart illustrating processing for connecting with a Non-ML-STA executed by an AP in the second embodiment. 
         FIG.  15    is a sequence chart illustrating connection processing between an AP and a Non-ML-STA in the second embodiment. 
         FIG.  16    is a sequence chart illustrating processing for re-changing a frequency band used based on a type of transmitted data in a third embodiment. 
         FIG.  17    is a sequence chart illustrating processing for re-changing a frequency band used based on a type of transmitted data in the third embodiment. 
         FIG.  18    is a sequence chart illustrating processing based on a type of transmitted data in a fourth embodiment. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     Hereinafter, embodiments will be described in detail with reference to the attached drawings. Note, the following embodiments are not intended to limit the scope of the claimed invention. Multiple features are described in the embodiments, but limitation is not made to an invention that requires all such features, and multiple such features may be combined as appropriate. Furthermore, in the attached drawings, the same reference numerals are given to the same or similar configurations, and redundant description thereof is omitted. 
     In the following descriptions, frequency channels are defined according to the IEEE  802  series of standards, and channel numbers in the 2.4 GHz band, the 5 GHz band, and the 6 GHz band are not limited to those disclosed in the present specification, and may be any number (channel). 
     First Embodiment 
     The present embodiment is an embodiment concerning processing for changing a link being used to a frequency band over which transmission and reception can be performed simultaneously when a Non-ML-STA connects to an AP in a state where the AP and an ML-STA are performing multi-link communication in communication modes including a synchronous mode. As described above, the ML-STA is an STA (station/terminal device) that supports a multi-link technique, whereas the Non-ML-STA is an STA that does not support a multi-link technique. “Link” refers to a frequency channel (frequency band) over which data can be transmitted and received. 
     Configuration of Wireless Communication System 
       FIG.  1    illustrates an example of the configuration of a network according to the present embodiment.  FIG.  1    illustrates a configuration including an access point (AP)  102 , an ML-STA  103 , and a Non-ML-STA  104  as communication devices supporting IEEE  802 .11be. The AP  102  is a communication device that supports a multi-link technique. A network formed by the AP  102  is indicated by the circle  101 . Signals transmitted and received by the AP  102  can be transmitted and received by the ML-STA  103  and the Non-ML-STA  104 . 
     The present embodiment assumes that the AP  102  and the ML-STA  103  include a plurality of wireless LAN control units, and are therefore capable of transmitting and receiving frames at the same time using a plurality of frequency bands, as will be described later with reference to  FIGS.  2 A and  2 B . However, the Non-ML-STA  104  is assumed to include only one wireless LAN control unit, and transmit and receive frames using only one frequency band. 
     Note that these illustrations are merely examples, and the following descriptions can be applied, for example, to networks containing many ML-STAs and Non-ML-STAs in an even broader area, as well as to a variety of positional relationships among the communication devices. 
     Configurations of Communication Devices 
     The configurations of the communication devices in the present embodiment (the AP  102 , the ML-STA  103 , and the Non-ML-STA  104 ) will be described next. The functional configurations of the communication devices will be described first with reference to  FIGS.  2 A and  2 B .  FIG.  2 A  is a block diagram illustrating an example of the functional configuration of the AP  102  and the ML-STA  103 . The AP  102  and the ML-STA  103  each includes the following as an example of the functional configuration: wireless LAN control units  201   a ,  201   b , and  201   c ; a frame generation unit  202 ; a frame analysis unit  203 ; a user interface (UI) control unit  204 ; a frequency band determination unit  205 ; a multi-link communication control unit  206 ; and a communication mode control unit  207 . 
     The wireless LAN control units  201   a ,  201   b , and  201   c  are configured including programs that perform control for transmitting and receiving wireless signals to and from other wireless LAN devices. The wireless LAN control units  201   a ,  201   b , and  201   c  execute wireless LAN communication control based on frames generated by the frame generation unit  202  in accordance with the IEEE  802 .11 standard series. The present embodiment assumes that the wireless LAN control units  201   a ,  201   b , and  201   c  communicate (perform communication control) in one of the 2.4 GHz band, the 5 GHz band, or the 6 GHz band through antennas  306   a ,  306   b , and  306   c  (see  FIG.  3 A ), respectively. Note that the number of wireless LAN control units is not limited to three, and may be two, or four or more. 
     The frame generation unit  202  generates wireless control frames to be transmitted by the wireless LAN control units  201   a ,  201   b , and  201   c . The content of the wireless control frames generated by the frame generation unit  202  may be constrained by settings stored in a storage unit  301  (see  FIG.  3 A ). The content of the wireless control frames may be changed according to settings made by a user through an input unit  304  (see  FIG.  3 A ) and the UI control unit  204 . 
     The frame analysis unit  203  interprets frames received by the wireless LAN control units  201   a ,  201   b , and  201   c  and reflects the content of those frames in the wireless LAN control units  201   a ,  201   b , and  201   c . Regardless of which wireless LAN control unit received the frame, passing the frame through the frame analysis unit  203  makes it possible to control even the wireless LAN control units that did not receive the frame. 
     The UI control unit  204  is configured including a program that controls operations made by a user (not shown) on the input unit  304  (see  FIG.  3 A ) of the AP  102  and the ML-STA  103 . The UI control unit  204  also has a function for presenting information to the user through an output unit  305  (see  FIG.  3 A ), in the form of a display of images or the like, audio output, and so on. The frequency band determination unit  205  has a function for determining which frequency band can be used for data communication. The multi-link communication control unit  206  has a function for performing control pertaining to multi-link communication. For example, the multi-link communication control unit  206  performs control for determining (including changing or maintaining) a plurality of frequency bands to be used for communication. The communication mode control unit  207  controls the communication mode (synchronous mode, asynchronous mode, and the like). For example, the communication mode control unit  207  determines whether a predetermined condition pertaining to changing the communication mode is satisfied. The synchronous mode is a communication mode in which an AP and an STA transmit and receive over a plurality of frequency bands in synchronization, whereas the asynchronous mode is a communication mode in which an AP and an STA transmit and receive independently over a plurality of frequency bands. 
       FIG.  2 B  is a block diagram illustrating an example of the functional configuration of the Non-ML-STA  104 . A wireless LAN control unit  211  is configured including a program that performs control for transmitting and receiving wireless signals to and from other wireless LAN devices. The wireless LAN control unit  211  executes wireless LAN communication control based on frames generated by a frame generation unit  212  in accordance with the IEEE  802 .11 standard series. The present embodiment assumes that the wireless LAN control unit  211  communicates (performs communication control) in one of the 2.4 GHz band, the 5 GHz band, or the 6 GHz band through an antenna  316  (see  FIG.  3 B ). The frame generation unit  212  generates wireless control frames to be transmitted by the wireless LAN control unit  211 . The content of the wireless control frames generated by the frame generation unit  212  may be constrained by settings stored in a storage unit  311  (see  FIG.  3 B ). The content of the wireless control frames may be changed according to settings made by a user through a UI control unit  214 . A frame analysis unit  213  interprets frames received by the wireless LAN control unit  211 . The UI control unit  214  is configured including a program that controls operations made by a user (not shown) on an input unit  314  (see  FIG.  3 B ) of the Non-ML-STA  104 . The UI control unit  214  also has a function for presenting information to the user through an output unit  315  (see  FIG.  3 B ), in the form of a display of images or the like, audio output, and so on. A frequency band determination unit  215  has a function for determining which frequency band can be used for data communication. 
     The hardware configurations of the communication devices will be described next with reference to  FIGS.  3 A and  3 B .  FIG.  3 A  is a block diagram illustrating an example of the hardware configuration of the AP  102  and the ML-STA  103 . The AP  102  will be described here as an example, but the same descriptions also apply to the ML-STA  103 . The AP  102  includes the following as an example of the hardware configuration: the storage unit  301 ; a control unit  302 ; a function unit  303 ; the input unit  304 ; the output unit  305 ; a communication unit  307 ; and the antennas  306   a ,  306   b , and  306   c . 
     The storage unit  301  is constituted by one or both of a ROM and a RAM, and stores various information such as programs for performing various operations (described later), communication parameters for wireless communication, and the like. Note that in addition to memory such as a ROM, a RAM, and the like, storage media such as flexible disks, hard disks, optical disks, magneto-optical disks, CD-ROMs, CD-Rs, magnetic tape, non-volatile memory cards, DVDs, and the like may be used as the storage unit  301 . 
     The control unit  302  is constituted by, for example, a processor such as a CPU or an MPU, an Application Specific Integrated Circuit (ASIC), a Digital Signal Processor (DSP), a Field Programmable Gate Array (FPGA), or the like. Here, “CPU” is an acronym for “Central Processing Unit”, and “MPU” is an acronym for “Micro Processing Unit”. The control unit  302  controls the AP  102  as a whole by executing programs stored in the storage unit  301 . Note that the control unit  302  may control the AP  102  as a whole by operating in cooperation with programs and an Operating System (OS) stored in the storage unit  301 . 
     The control unit  302  also executes predetermined processing such as capturing images, printing, projection, and the like by controlling the function unit  303 . The function unit  303  is hardware for the AP  102  to execute the predetermined processing. For example, if the AP  102  is a camera, the function unit  303  is an image capture unit, and performs image capture processing. If, for example, the AP  102  is a printer, the function unit  303  is a printing unit, and performs printing processing. Furthermore, if, for example, the AP  102  is a projector, the function unit  303  is a projection unit, and performs projection processing. The data processed by the function unit  303  may be data stored in the storage unit  301 , or may be data communicated with another communication device through the communication unit  307  (described later). 
     The input unit  304  accepts various operations from the user. The output unit  305  provides various outputs to the user. Here, the “output” by the output unit  305  includes at least one of making a display on a screen, outputting audio through a speaker, outputting vibrations, and the like. Note that both the input unit  304  and the output unit  305  may be implemented as a single module, as in the case of a touch panel. The input unit  304  and the output unit  305  may be integrated parts of the AP  102 , or may be separate from the AP  102 . 
     The communication unit  307  controls wireless communication compliant with the IEEE  802 .11 standard series, controls IP communication, and the like. In the present embodiment, the communication unit  307  can execute processing compliant with at least the IEEE  802 .11be standard. The communication unit  307  also transmits and receives wireless signals for wireless communication by controlling the antennas  306   a ,  306   b , and  306   c . The AP  102  communicates content such as image data, document data, video data, and the like with other communication devices through the communication unit  307 . The communication unit  307  also includes a transmission queue that holds data to be transmitted. 
     The antennas  306   a ,  306   b , and  306   c  are antennas capable of communicating in at least one of the 2.4 GHz band, the 5 GHz band, and the 6 GHz band, respectively. The present embodiment assumes that the antennas  306   a ,  306   b , and  306   c  are used for communication by the wireless LAN control units  201   a ,  201   b , and  201   c  (see  FIG.  2 A ), respectively. The antennas  306   a ,  306   b , and  306   c  may each be physically constituted by at least one antenna for implementing Multi-Input and Multi-Output (MIMO) transmission and reception. 
       FIG.  3 B  is a block diagram illustrating an example of the hardware configuration of the Non-ML-STA  104 . In  FIG.  3 B , the storage unit  311 , a control unit  312 , a function unit  313 , the input unit  314 , the output unit  315 , and a communication unit  307  are similar to the storage unit  301 , the control unit  302 , the function unit  303 , the input unit  304 , the output unit  305 , and the communication unit  307  illustrated in  FIG.  3 A , and will therefore not be described. However, the communication unit  307  need not be compliant with the IEEE  802 .11be standard. The Non-ML-STA  104  includes the one antenna  316 . The antenna  316  is an antenna capable of communicating in at least one of the 2.4 GHz band, the 5 GHz band, and the 6 GHz band. The present embodiment assumes that the antenna  316  is used for communication by the wireless LAN control unit  211  (see  FIG.  2 B ). The antenna  316  may be physically constituted by at least one antenna for implementing MIMO transmission and reception. 
     Flow of Processing 
     The flow of processing by the communication devices in the present embodiment will be described next. Frequency interval values required to ensure that intra-device interference does not occur, which are pre-set in the AP  102  and the ML-STA  103  that support multi-link techniques, will be described first. With multi-link communication in the asynchronous mode (when the same device transmits and receives simultaneously), if there is only a small frequency interval between the respective links used for transmission and reception, the transmitted signal may enter into the reception circuit as radio interference, causing intra-device interference that adversely affects the reception characteristics. To prevent the occurrence of intra-device interference, frequency interval values required to ensure that intra-device interference does not occur are set in the AP  102  and the ML-STA  103 . 
       FIG.  4    is a conceptual diagram illustrating a frequency interval value F required to ensure intra-device interference does not occur. As illustrated in  FIG.  4   , the frequency interval value F is a frequency interval value required to ensure intra-device interference when two links (Link 1 and Link 2) are used. In the present embodiment, the frequency interval value F for the AP  102  (“F AP ” hereinafter) is set to 200 MHz, and the frequency interval value F for the ML-STA  103  (“F ML-STA ” hereinafter) is set to 300 MHz. 
     Connection Processing Between AP  102  and ML-STA  103   
     Connection processing between the AP  102  and the ML-STA  103  (processing up to data transmission and reception) will be described with reference to  FIGS.  5  and  6   .  FIG.  5    is a flowchart illustrating processing for connecting with the ML-STA  103 , performed by the AP  102  in the present embodiment, and  FIG.  6    is a sequence chart illustrating the connection processing executed by the AP  102  and the ML-STA  103  in the present embodiment. As described above, the AP  102  and the ML-STA  103  each includes the wireless LAN control units  201   a ,  201   b , and  201   c , each of which are capable of communicating in one of the 2.4 GHz band, the 5 GHz band, and the 6 GHz band. 
     In the present embodiment, the frequency at which the AP  102  and the ML-STA  103  communicate management frames for the connection processing is assumed to be 2.4 GHz, and connections in other frequency bands are also controlled through such communication. Note that using 2.4 GHz is merely an example, and the frequency at which the management frames are transmitted and received is not limited thereto. Management frames are defined in the IEEE  802 .11 series standard and include Beacon frames, Probe Request/Response, Authentication Request/Response, Association Request/Response, Reassociation Request/Response frames, and the like. Processing for generating such frames is performed by the frame generation unit  202 . 
     In  FIG.  5   , first, the frequency band determination unit  205  of the AP  102  determines which frequency bands can be used (S 501 ). The AP  102  may make this determination based on the degree of congestion in the surrounding wireless environment, but is not limited thereto. The present embodiment assumes that the 2.4 GHz band, the 5 GHz band, and the 6 GHz band can be used. 
     After the frequency band determination unit  205  of the AP  102  determines the frequency bands that can be used, the AP  102  transmits frequency band information using one of the frequency bands among the frequency bands which can be used (S 502 ). It is assumed here that the wireless LAN control unit  201   a  transmits using the 2.4 GHz band (in both the AP  102  and the ML-STA  103 ). The frequency band information includes information on the frequency bands which the AP  102  can use, information indicating that the AP  102  supports multi-link communication, and information on F AP  (the same applies in the following descriptions as well). The wireless LAN control unit  201   a  of the AP  102  adds the frequency band information to a Beacon frame, for example, and transmits that frame in the 2.4 GHz band (F 601 ). Note that the frequency band information may be added to a Probe Response, Authentication Response, Association Response, or Reassociation Response frame transmitted by the AP  102 , rather than the Beacon frame. 
     After the AP  102  transmits the frequency band information to the ML-STA  103 , the wireless LAN control unit  201   a  of the ML-STA  103 , which has received the Beacon frame, starts scanning operations by transmitting a Probe Request frame at 2.4 GHz (F 602 ). The wireless LAN control unit  201   a  of the ML-STA  103  may add frequency band information of the ML-STA  103  to the Probe Request frame. The frequency band information can include information on the frequency bands which the ML-STA  103  can use, information indicating that the ML-STA  103  supports multi-link communication, and information on F ML-STA  (the same applies in the following descriptions as well). Note that the ML-STA  103  may notify the AP  102  of its own usable frequency information by including that information in Authentication Request, Association Request, and Reassociation Request frames. 
     Having received the Probe Request frame from the ML-STA  103 , the wireless LAN control unit  201   a  of the AP  102  transmits a Probe Response frame to the ML-STA  103  (F 603 ). The Probe Response frame can include information on the frequencies which the AP  102  can use. Using the information on the frequencies that can be used, included in the Beacon frame, the Probe Response frame, and the like from the AP  102 , the frame analysis unit  203  of the ML-STA  103  detects the frequencies supported by the AP  102  and the channels operating on those frequencies. 
     Thereafter, the AP  102  and the ML-STA  103  establish a connection by communicating through the respective wireless LAN control units  201   a  (S 503 , F 604 , F 605 ). When a secure connection is to be established using encryption, the AP  102  and the ML-STA  103  may then perform communication processing such as Wi-Fi Protected Access (WPA), WPA2, WPA3, or the like. Although the present embodiment describes an unencrypted connection, the connection is not limited thereto. The AP  102  and the ML-STA  103  may establish connections in at least two frequency bands that can be used. For example, when there are three frequency bands which can be used, connections may be established using two or all of the frequency bands. Thereafter, the channel (frequency band) over which data can be transmitted and received in this manner becomes a “link”. The present embodiment assumes that channel 5 in the 2.4 GHz band and channels 36 and  100  in the 5 GHz band are links. It is also assumed that the AP  102  and the ML-STA  103  have functions enabling communication at a maximum bandwidth of 40 MHz in the 2.4 GHz band and 160 MHz in the 5 GHz band and the 6 GHz band. 
     Once a connection is established between the AP  102  and the ML-STA  103 , the multi-link communication control unit  206  of the AP  102  optionally determines transmission and reception parameters (S 504 , F 606 ). The transmission and reception parameters are information (parameters) for determining how transmitted/received data is to be distributed for each connection when a plurality of connections are established. Data distribution amounts can be determined, for example, according to the maximum throughput that can be used in each frequency band, or according to the current throughput calculated by actually sending test packets. These values may be changed at any time. For example, after transmitting and receiving data for a set period, the AP  102  may determine the data distribution amount for the next set period from the amount of data that was actually able to be transmitted and received. Alternatively, the bandwidth for transmission and reception may be divided between control packets and data packets. 
     Additionally, when performing multi-link communication, the communication mode control unit  207  of the AP  102  determines the communication mode for each link, i.e., whether to transmit and receive simultaneously among the links. If the frequency interval between two of the links used is less than either F AP  (= 200 MHz) or F ML-STA  (= 300 MHz), the communication mode control unit  207  determines not to transmit and receive simultaneously (that is, not use the asynchronous mode) using those two links. In other words, when the frequency interval between two links is less than the frequency interval required for both the AP  102  and the ML-STA  103  to transmit and receive simultaneously over those two links, the communication mode control unit  207  can determine to communicate in the synchronous mode. When the frequency interval between two links is greater than or equal to the stated frequency interval, the communication mode control unit  207  can determine to communicate in the synchronous mode and/or the asynchronous mode. The wireless LAN control unit  201   a  of the AP  102  notifies the ML-STA  103  of that determination as the (included in the) transmission and reception parameters. 
     Taking into account the maximum bandwidth that can be used for communication, the frequency intervals between the links used in multi-link communication are as follows in the present embodiment.  FIG.  7    illustrates the frequency intervals of links used in multi-link communication in the present embodiment.
     · Frequency interval between channels 5 and 36: 2708 MHz   · Frequency interval between channels 36 and 100: 160 MHz   · Frequency interval between channels 5 and 100: 3028 MHz   
Comparing these interval values with F AP  (= 200 MHz) and F ML-STA  (= 300 MHz), the frequency interval between channels 36 and  100  is smaller than both F AP  and F ML-STA . Accordingly, the communication mode control unit  207  of the AP  102  determines to communicate in the synchronous mode between the links for channels 36 and  100  rather than transmitting and receiving simultaneously (using the asynchronous mode). On the other hand, the frequency intervals between the links of channels 5 and 36 and channels 5 and  100  are greater than both F AP  and F ML-STA . Accordingly, the communication mode control unit  207  of the AP  102  determines that transmission and reception may be performed at the same time between these links (i.e., either the synchronous mode or the asynchronous mode may be used). The wireless LAN control unit  201   a  of the AP  102  notifies the ML-STA  103  of that determination as the (included in the) transmission and reception parameters.
     Using those transmission and reception parameters, the AP  102  and the ML-STA  103  start transmitting and receiving data using multi-link communication (S 505 , F 607  to F 609 , F 617  to F 618 , and F 627  to F 628 ). In the example in  FIG.  6   , the wireless LAN control units  201   a  of the AP  102  and the ML-STA  103  perform multi-link communication on channel 5 (the 2.4 GHz band) using the asynchronous mode. Additionally, the wireless LAN control units  201   b  of the AP  102  and the ML-STA  103  perform multi-link communication on channel 36 (the 5 GHz band) using the synchronous mode. Additionally, the wireless LAN control unit  201   c  of the AP  102  and the ML-STA  103  perform multi-link communication on channel  100  (the 5 GHz band) using the synchronous mode. Note that multi-link communication using the synchronous mode can be implemented as synchronous communication by, for example, the AP  102  transmitting a trigger frame regarding the communication timing to the ML-STA  103 . 
     Connection Processing Between AP  102  and Non-ML-STA  104 , Processing for Changing Frequency Band Used 
     Connection processing performed between the AP  102  and the Non-ML-STA  104  when the AP  102  establishes multi-link communication with the ML-STA  103 , and processing performed by the AP  102  for changing the frequency band used in consideration of intra-device interference, will be described next with reference to  FIGS.  8  and  9   .  FIG.  8    is a flowchart illustrating processing for connecting with the Non-ML-STA  104 , performed by the AP  102  in the present embodiment, and  FIG.  9    is a sequence chart illustrating the connection processing executed by the AP  102  and the Non-ML-STA  104  in the present embodiment. As described above, the Non-ML-STA  104  includes the wireless LAN control unit  211 , which is capable of communicating in one of the 2.4 GHz band, the 5 GHz band, and the 6 GHz band. 
     In  FIG.  9   , the AP  102  and the ML-STA  103  are assumed to be transmitting and receiving data (F 901 , F 902 , F 911 , F 912 , F 931 , and F 932 ) following the data transmission and reception illustrated in  FIGS.  5  and  6    (S 505 , F 607  to F 609 , F 617  to F 618 , and F 627  to F 628 ). At this time, the frequency band determination unit  205  of the AP  102  periodically determines which frequency bands can be used (S 801 ). The AP  102  may make this determination based on the degree of congestion in the surrounding wireless environment, but is not limited thereto. The present embodiment assumes that the 2.4 GHz band, the 5 GHz band, and the 6 GHz band can be used. 
     In S 802 , the AP  102  periodically transmits a Beacon frame using one of the frequency bands among the usable frequency bands (F921). It is assumed here that the wireless LAN control unit  201   b  transmits a management frame containing a Beacon frame to the Non-ML-STA  104  using the 5 GHz band. The wireless LAN control unit  201   b  adds the frequency band information of the AP  102  to the Beacon frame. The frequency band information is as described above, and can include information indicating that the AP  102  supports multi-link communication. 
     Upon receiving the Beacon frame, the wireless LAN control unit  211  of the Non-ML-STA  104  starts scanning operations by transmitting a Probe Request frame over the one frequency band which can be used (5 GHz, here) (F 922 ). The wireless LAN control unit  211  of the Non-ML-STA  104  may add frequency band information of the Non-ML-STA  104  to the Probe Request frame. The frequency band information can include information on the frequency bands which the Non-ML-STA  104  can use and information indicating that the Non-ML-STA  104  does not support multi-link communication. Alternatively, the frequency band information may be configured not to include information indicating that the Non-ML-STA  104  supports multi-link communication. Note that the Non-ML-STA  104  may notify the AP  102  of its own frequency information by including that information in Authentication Request, Association Request, and Reassociation Request frames. 
     The frame analysis unit  203  of the AP  102  analyzes the usable frequency band information of the Non-ML-STA  104  added to the Probe Request frame from the Non-ML-STA  104 . The frame analysis unit  203  detects that the Non-ML-STA  104  does not support multi-link communication from the fact that information indicating multi-link communication is not supported is included (or from the fact that information indicating multi-link communication is supported is not included). 
     When the AP  102  is performing multi-link communication with an STA aside from the Non-ML-STA  104  (i.e., the ML-STA  103 ) in the synchronous mode, as in this example, if the Non-ML-STA  104  connects to a link being used in that synchronous mode (channel 36), the communication throughput of the ML-STA  103  will drop. Accordingly, when the Non-ML-STA  104  is performing processing for connecting with a link, such as channel 36, for which the frequency interval with at least one adjacent link is smaller than F AP  (= 300 MHz) and F ML-   STA  (= 200 MHz), the communication mode control unit  207  of the AP  102  determines that a condition pertaining to changing the communication mode is satisfied. The multi-link communication control unit  206  then determines to change a plurality of links used such that the frequency intervals between the links become greater than both F AP  and F ML-STA , and performs change processing. In this example, the 160 MHz interval between the two links, namely channels 36 and  100 , is smaller than F AP  (= 300 MHz) and F ML-STA  (= 200 MHz), as described above. The AP  102  therefore changes channel  100 . In this example, AP  102  changes to channel 1 of the 6 GHz band (center frequency of 5945 MHz), where bandwidth of 160 MHz can be secured. 
       FIG.  10    is a conceptual diagram illustrating a frequency band change for a link in the present embodiment. As illustrated in  FIG.  10   , changing the frequency band of the link produces a frequency interval of 605 MHz, which is greater than F AP  and F ML-STA . Accordingly, the AP  102  and the ML-STA  103  can transmit and receive simultaneously between the links of channel 36 in the 5 GHz band and channel 1 in the 6 GHz band, without intra-device interference. 
     The wireless LAN control unit  201   a  of the AP  102  transmits a channel switch announcement to notify the ML-STA  103  that the frequency band of the link is being changed and the post-change frequency band (F 903 ). The multi-link communication control units  206  of the AP  102  and the ML-STA  103  then perform control for changing the link (F 933 ). Furthermore, the communication mode control unit  207  of the AP  102  determines that transmission and reception may be executed simultaneously over any link in the multi-link communication with the ML-STA  103 . In other words, the communication mode control unit  207  of the AP  102  determines that the communication may use either the synchronous mode or the asynchronous mode. The ML-STA  103  is notified of that determination by the wireless LAN control unit  201   a  of the AP  102 . 
     The wireless LAN control unit  201   b  of the AP  102  transmits a Probe Response frame (F 923 ) as a response to the Probe Request frame from the Non-ML-STA  104  (F 922 ). The Probe Response frame includes information on the frequency bands which can be used after the link is changed. Based on the information on the frequency bands that can be used after the link is changed, included in the Probe Response frame from the AP  102 , the frame analysis unit  213  of the Non-ML-STA  104  detects the frequencies supported by the AP  102  and the channels operating on those frequencies. 
     Then, the AP  102  and the Non-ML-STA  104  establish a connection by communicating through the wireless LAN control unit  201   b  and the wireless LAN control unit  211  (S 803 , F 924 , F 925 ). When a secure connection is to be established using encryption, the AP  102  and the Non-ML-STA  104  may then perform communication processing such as WPA, WPA2, WPA3, or the like. Although the present embodiment describes an unencrypted connection, the connection is not limited thereto. 
     Then, the AP  102  and the ML-STA  103  transmit and receive data using multi-link communication (F 904 , F 905 , F 913 , F 914 , F 934  to F 937 ). The AP  102  and the Non-ML-STA  104  transmit and receive data using single-link communication in a single frequency band (F 926 ). 
     Processing for Changing Frequency Band Used After Disconnecting STA Not Supporting Multi-Link 
     Processing for returning the frequency band used for the multi-link communication between the AP  102  and the ML-STA  103  to its original band (from before the change) after the Non-ML-STA  104  disconnects from the AP  102  will be described next with reference to  FIGS.  11  and  12   . The AP  102  can continue to communicate with the ML-STA  103  without changing the frequency band of the link, even after the Non-ML-STA  104  disconnects from the AP  102 . On the other hand, the AP  102  can also change back to the original frequency band and change to synchronous mode in response to the communication conditions in the frequency band being congested or the like. 
       FIG.  11    is a flowchart illustrating frequency band change processing for a link after disconnection from the Non-ML-STA  104 , executed by the AP  102 , in the present embodiment.  FIG.  12    is a sequence chart illustrating frequency band change processing for a link after disconnection from a Non-ML-STA, in the present embodiment. 
     In  FIG.  12   , the AP  102 , the ML-STA  103 , and the Non-ML-STA  104  are assumed to be transmitting and receiving data following the data transmission and reception illustrated in  FIGS.  9  and  10    (F 904 , F 905 , F 913 , F 914 , F 934  to F 937 , and F 926 ). It is further assumed here that the Non-ML-STA  104  has stopped transmitting and receiving data or the like and disconnects from the AP  102 . At this time, the wireless LAN control unit  211  of the Non-ML-STA  104  transmits a De-authentication frame to the AP  102  (F 1221 ). Through this, the AP  102  disconnects from the Non-ML-STA  104  (S 1101 ). 
     Thereafter, the transmission and reception of data between the AP  102  and the ML-STA  103  continues (F 1201 , F 1211 , F 1212 , F 1231 , and F 1232 ), while if necessary (e.g., due to the degree of congestion (communication conditions) of the surrounding wireless environment, an input operation made by the user, or the like), the AP  102  can return the frequency band to the original band and change to the synchronous mode. This example assumes that if the Non-ML-STA  104  has disconnected from the AP  102 , the communication mode control unit  207  of the AP  102  determines that a condition pertaining to changing the communication mode is satisfied according to the communication conditions. Thereafter, the multi-link communication control unit  206  of the AP  102  determines to return to the frequency band changed in F 933 , and the communication mode control unit  207  determines to change to the synchronous mode. The wireless LAN control unit  201   a  then transmits a channel switch announcement to the ML-STA  103  (F 1202 , S 1102 ). As a result, the AP  102  notifies the ML-STA of the change in the frequency band of the link and the post-change frequency band. The multi-link communication control units  206  of the AP  102  and the ML-STA  103  then change the link (F 1233 ). In this example, the AP  102  returns the link of channel 1 in the 6 GHz band to channel  100  in the 5 GHz band. The AP  102  then transmits and receives data to and from the ML-STA  103  through multi-link communication (S 1103 , F 1203 , F 1204 , F 1213 , F 1214 , F 1234 , and F 1235 ). 
     Note that the configuration may be such that after the AP  102  disconnects from the Non-ML-STA  104 , the multi-link communication control unit  206  changes the frequency band used regardless of the determination by the communication mode control unit  207 , and the communication mode control unit  207  then determines the communication mode. 
     In this manner, in the present embodiment, if, when an AP and an STA that support multi-link communication are performing multi-link communication in the synchronous mode, an STA that does not support multi-link connects to the AP, the link is changed to a frequency band over which transmission and reception can be executed simultaneously. This makes it possible to suppress a drop in communication throughput in the STA that supports multi-link communication. Additionally, a procedure was also described in which when the STA that does not support multi-link disconnects from an AP after having been connected to that AP, the AP returns to the original frequency band and communicates in the synchronous mode. 
     Second Embodiment 
     The present embodiment is an embodiment concerning processing performed when, in a state where an AP and an ML-STA are performing multi-link communication, a Non-ML-STA communicates with the AP. The configuration of the wireless communication system, and the configurations of the AP  102 , the ML-STA  103 , and the Non-ML-STA  104 , are similar to those of the first embodiment, and will therefore not be described here. The frequency interval values F required to ensure intra-device interference does not occur is also similar to that in the first embodiment. In other words, the frequency interval values F required to ensure that intra-device interference does not occur in the AP  102  and the ML-STA  103  are F AP  = 200 MHz and F ML-STA  = 300 MHz, respectively. 
     Connection Processing Between AP  102  and ML-STA  103   
     Connection processing between the AP  102  and the ML-STA  103  (processing up to data transmission and reception) will be described with reference to  FIGS.  5  and  13   .  FIG.  5    is a flowchart illustrating processing for connecting with the ML-STA  103  executed by the AP  102  in the present embodiment, and is the same as in the first embodiment.  FIG.  13    is a sequence chart illustrating connection processing executed between the AP  102  and the ML-STA  103  in the present embodiment. As described above, the AP  102  and the ML-STA  103  each includes the wireless LAN control units  201   a ,  201   b , and  201   c , each of which are capable of communicating in one of the 2.4 GHz band, the 5 GHz band, and the 6 GHz band. 
     In the present embodiment, the frequency at which the AP  102  and the ML-STA  103  communicate management frames for the connection processing is assumed to be 2.4 GHz, and connections in other frequency bands are also controlled through such communication. Note that using 2.4 GHz is merely an example, and the frequency at which the management frames are transmitted and received is not limited thereto. The management frame is as described in the first embodiment. 
     The processing from F 1301  to F 1305  in  FIG.  13    is similar to that in the first embodiment (corresponding to S 501  to S 503  in  FIG.  5    and F 601  to F 605  in  FIG.  6   ), and will therefore not be described. Note that the present embodiment assumes that channel 5 in the 2.4 GHz band, channel 36 in 5 GHz band, and channel 1 in the 6 GHz band are links. It is also assumed that the AP  102  and the ML-STA  103  have functions enabling communication at a maximum bandwidth of 40 MHz in the 2.4 GHz band and 160 MHz in the 5 GHz band and the 6 GHz band. 
     Once a connection is established between the AP  102  and the ML-STA  103 , the multi-link communication control unit  206  of the AP  102  optionally determines transmission and reception parameters (S 504 , F 1306 ). The transmission and reception parameters are information (parameters) for determining how transmitted/received data is to be distributed for each connection when a plurality of connections are established, and are as described in the first embodiment. 
     Additionally, when performing multi-link communication, the communication mode control unit  207  of the AP  102  determines the communication mode for each link, i.e., whether to transmit and receive simultaneously among the links. Similar to the first embodiment, if the frequency interval between two of the links used is less than either F AP  (= 200 MHz) or F ML-   STA  (= 300 MHz), the communication mode control unit  207  determines not to transmit and receive simultaneously (that is, not use the asynchronous mode) using those two links. The wireless LAN control unit  201   a  of the AP  102  notifies the ML-STA  103  of that determination as the (included in the) transmission and reception parameters. 
     Taking into account the maximum bandwidth that can be used for communication, the frequency intervals between the links used in multi-link communication are as follows in the present embodiment.
     Frequency interval between channels 5 and 36: 2708 MHz   Frequency interval between channel 36 and channel 1 in the 6 GHz band: 605 MHz   Frequency interval between channel 5 and channel 1 in the 6 GHz band: 3473 MHz   
 Comparing these interval values with F AP  (= 200 MHz) and F ML-STA  (= 300 MHz), both frequency intervals are smaller than both F AP  and F ML-STA . Accordingly, the communication mode control unit  207  of the AP  102  determines that transmission and reception may be performed at the same time between the links (i.e., either the synchronous mode or the asynchronous mode may be used). The wireless LAN control unit  201   a  of the AP  102  notifies the ML-STA  103  of that determination as the (included in the) transmission and reception parameters.
     Using those transmission and reception parameters, the AP  102  and the ML-STA  103  start transmitting and receiving data using multi-link communication (S 505 , F 1307  to  1309 , F 1311  to F 1313 , and F 1321  to F 1322 ). In the example in  FIG.  13   , the wireless LAN control units  201   a  of the AP  102  and the ML-STA  103  perform multi-link communication on channel 5 (the 2.4 GHz band) using the asynchronous mode. Additionally, the wireless LAN control units  201   b  of the AP  102  and the ML-STA  103  perform multi-link communication on channel 36 (the 5 GHz band) using the asynchronous mode. Additionally, the wireless LAN control units  201   c  of the AP  102  and the ML-STA  103  perform multi-link communication on channel 1 (the 6 GHz band) using the asynchronous mode. 
     Connection Processing Between AP  102  and Non-ML-STA  104   
     Connection processing performed between the AP  102  and the Non-ML-STA  104  after the AP  102  establishes multi-link communication with the ML-STA  103  will be described next with reference to  FIGS.  14  and  15   .  FIG.  14    is a flowchart illustrating processing for connecting with the Non-ML-STA  104 , performed by the AP  102  in the present embodiment, and  FIG.  15    is a sequence chart illustrating the connection processing executed by the AP  102  and the Non-ML-STA  104  in the present embodiment. As described above, the Non-ML-STA  104  includes the wireless LAN control unit  211 , which is capable of communicating in one of the 2.4 GHz band, the 5 GHz band, and the 6 GHz band. 
     In  FIG.  15   , the AP  102  and the ML-STA  103  are assumed to be transmitting and receiving data (F 1501 , F 1502 , F 1511 , F 1512 , F 1531 , and F 1532 ) following the data transmission and reception illustrated in  FIGS.  5  and  13    (S 505 , F 1307  to  1309 , F 1311  to F 1313 , and F 1321  to F 1322 ). At this time, the frequency band determination unit  205  of the AP  102  periodically determines which frequency bands can be used (S 1401 ). The AP  102  may make this determination based on the degree of congestion in the surrounding wireless environment, but is not limited thereto. The present embodiment assumes that the 2.4 GHz band, the 5 GHz band, and the 6 GHz band can be used. 
     In S 1402 , the AP  102  periodically transmits a Beacon using one of the frequency bands among the usable frequency bands (F 1521 ). It is assumed here that the wireless LAN control unit  201   b  transmits a management frame containing a Beacon frame to the Non-ML-STA  104  using the 5 GHz band. The wireless LAN control unit  201   b  adds the frequency band information of the AP  102  to the Beacon frame. The frequency band information is as described above, and the frequency band information can include information indicating that the AP  102  supports multi-link communication. 
     Upon receiving the Beacon frame, the wireless LAN control unit  211  of the Non-ML-STA  104  starts scanning operations by transmitting a Probe Request frame over the one frequency band which can be used (5 GHz, here) (F 1522 ). The wireless LAN control unit  211  of the Non-ML-STA  104  may add frequency band information of the Non-ML-STA  104  to the Probe Request frame. The frequency band information can include information on the frequency bands which the Non-ML-STA  104  can use and information indicating that the Non-ML-STA  104  does not support multi-link communication. Alternatively, the frequency band information may be configured not to include information indicating that the Non-ML-STA  104  supports multi-link communication. Note that the Non-ML-STA  104  may notify the AP  102  of its own frequency information by including that information in Authentication Request, Association Request, and Reassociation Request frames. 
     The frame analysis unit  203  of the AP  102  analyzes the usable frequency band information of the Non-ML-STA  104  added to the Probe Request frame from the Non-ML-STA  104 . The frame analysis unit  203  detects that the Non-ML-STA  104  does not support multi-link communication from the fact that information indicating multi-link communication is not supported is included (or from the fact that information indicating multi-link communication is supported is not included). 
     In this example, the frequency intervals between the links are greater than both F AP  and F ML-STA , and thus the communication mode control unit  207  of the AP  102  determines that the condition pertaining to changing the communication mode is not satisfied. Accordingly, the multi-link communication control unit  206  does not change the frequency band of the link as in the first embodiment. In response to the Probe Request frame from the Non-ML-STA  104  (F 1522 ), the wireless LAN control unit  201   b  of the AP  102  transmits a Probe Response frame including information on the usable frequency bands (F 1523 ). Based on the information on the frequency bands that can be used, included in the Probe Response frame from the AP  102 , the frame analysis unit  213  of the Non-ML-STA  104  detects the frequencies supported by the AP  102  and the channels operating on those frequencies. 
     Then, the AP  102  and the Non-ML-STA  104  establish a connection by communicating through the wireless LAN control unit  201   b  and the wireless LAN control unit  211  (S 1403 , F 1524 , F 1525 ). When a secure connection is to be established using encryption, the AP  102  and the Non-ML-STA  104  may then perform communication processing such as WPA, WPA2, WPA3, or the like. Although the present embodiment describes an unencrypted connection, the connection is not limited thereto. 
     Thereafter, the AP  102  and the ML-STA  103  continue to perform multi-link communication (in the asynchronous mode), in which transmission and reception can be performed simultaneously (F 1503  to F 1505 , F 1513  to F 1514 , and F 1533  to F 1537 ). The AP  102  and the Non-ML-STA  104  transmit and receive data using single-link communication in a single frequency band (F 1526 ). 
     Changing Frequency Band Used After Disconnecting STA Not Supporting Multi-Link 
     Processing for returning the frequency band used for the multi-link communication between the AP  102  and the ML-STA  103  to its original band after the Non-ML-STA  104  disconnects from the AP  102  is similar to that in the first embodiment (described with reference to  FIGS.  11  and  12   ). In other words, if the Non-ML-STA  104  has disconnected from the AP  102 , the communication mode control unit  207  of the AP  102  determines that a condition pertaining to changing the communication mode is satisfied according to the communication conditions. The AP  102  then transmits a channel switch announcement to the ML-STA  103  as described with reference to  FIG.  12    (F 1202 ). As a result, the AP  102  notifies the ML-STA of the change in the frequency band of the link and the post-change frequency band. The multi-link communication control units  206  of the AP  102  and the ML-STA  103  then change the link (F 1233 ). 
     In this manner, in the present embodiment, if, when an AP and an STA that support multi-link are performing multi-link communication in a mode in which transmission and reception can be executed simultaneously, an STA that does not support multi-link connects to the AP, the connection can be made appropriately. Through this, the AP and the STA that supports multi-link can continue communication without causing a drop in communication throughput. 
     Third Embodiment 
     The present embodiment is an embodiment concerning processing for changing to a mode in which transmission and reception can be executed simultaneously (the asynchronous mode), if, when an AP and an ML-STA are performing multi-link communication in synchronous mode, an application where data is transmitted and received bidirectionally in real time is used. The configuration of the wireless communication system, and the configurations of the AP  102 , the ML-STA  103 , and the Non-ML-STA  104 , are similar to those of the first embodiment, and will therefore not be described here. In the present embodiment, the communication mode of multi-link communication is changed to the asynchronous mode based on an access category as defined in IEEE 802.11e. The access category (AC) indicates a priority of frame (packet) transmission. 
     Changing Frequency Band Used Based on Type of Transmitted Data 
     Processing for changing to the asynchronous mode according to the type of transmitted data after the AP  102  and the ML-STA  103  establish multi-link communication will be described with reference to  FIG.  16   .  FIG.  16    is a sequence chart illustrating processing for re-changing a frequency band used based on a type of transmitted data in the present embodiment. The present embodiment assumes that the type of the transmitted data is distinguished by the access category defined in IEEE  802 .11e. It is also assumed that “AC_VO” (a voice data access category) is set in the multi-link communication control unit  206  of the AP  102  as an access category serving as a condition for changing to the asynchronous mode. AC_VO is the access category having the highest transmission priority. 
     In  FIG.  16   , the AP  102  and the ML-STA  103  are communicating in the synchronous mode through, for example, the procedure described in the first embodiment, using channel 5 in the 2.4 GHz band, channel 36 in the 5 GHz band, and channel  100  in the 5 GHz band as links (F 1601 , F 1602 , F 1611 , F 1622 , F 1621 , and F 1622 ). It is assumed here that the AP  102  and the ML-STA  103  are executing applications that transmit and receive data bidirectionally in real time. 
     Assume that in F 1603 , data addressed to the ML-STA  103 , generated by the application that transmits and receives data bidirectionally in real time, has entered the transmission queue within the communication unit  307  of the AP  102  as a frame in the AC_VO category. Based on the access category of the data addressed to the ML-STA  103  (the transmitted data), the multi-link communication control unit  206  of the AP  102  detects that the transmitted data is data of a specific type that satisfies a condition pertaining to changing the communication mode. In this case, the communication mode control unit  207  determines that the condition pertaining to changing the communication mode is satisfied. The multi-link communication control unit  206  then determines to change the link from the synchronous mode to a frequency band that transmits and receives simultaneously (that is, in which the asynchronous mode is possible), and then performs the change processing. Specifically, the multi-link communication control unit  206  changes the link such that the frequency intervals between the links become greater than both F AP  and F ML-STA . In this example, the 160 MHz interval between the two links, namely channels 36 and  100 , is smaller than F AP  (= 300 MHz) and F ML-STA  (= 200 MHz). The AP  102  therefore changes channel  100 . Specifically, as described in the first embodiment with reference to  FIG.  10   , the AP  102  changes to channel 1 in the 6 GHz band (center frequency of 5945 MHz), where a bandwidth of 160 MHz can be secured. As a result, the frequency interval becomes 605 MHz, which is greater than both F AP  and F ML-STA , and thus the AP  102  and the ML-STA  103  can transmit and receive simultaneously between the links of channel 36 in the 5 GHz band and channel 1 in the 6 GHz band, without intra-device interference. 
     The wireless LAN control unit  201   a  of the AP  102  transmits a channel switch announcement to notify the ML-STA  103  that the frequency band of the link is being changed and the post-change frequency band (F1604). The multi-link communication control units  206  of the AP  102  and the ML-STA  103  then perform control for changing the link (F1623). Furthermore, the multi-link communication control unit  206  of the AP  102  determines that transmission and reception may be executed simultaneously over any link in the multi-link communication with the ML-STA  103 . In other words, the multi-link communication control unit  206  of the AP  102  determines that the communication may use either the synchronous mode or the asynchronous mode. The ML-STA  103  is notified of that determination by the wireless LAN control unit  201   a  of the AP  102 . Thereafter, the AP  102  and the ML-STA  103  transmit and receive data through multi-link communication (F 1605  to F 1607 , F 1613  to F 1615 , and F 1624  to F 1627 ). 
     Although the present embodiment describes changing from the synchronous mode to the asynchronous mode under the condition that a frame in the AC_VO category enters the transmission queue, another access category may be used instead. Furthermore, a different parameter indicating the transmission priority of data may be used rather than an access category. 
     Processing for Re-Changing Frequency Band Used Based on Type of Transmitted Data 
     Processing for returning the frequency band used in the multi-link communication between the AP  102  and the ML-STA  103  to its original frequency band after a frame in the AC_VO category is no longer in the transmission queue of the AP  102  will be described next with reference to  FIG.  17   . Even after a frame in the AC_VO category is no longer in the transmission queue due to the application that transmits and receives data bidirectionally in real time no longer being used or the like, communication with the ML-STA  103  can be continued without changing the frequency band of the link. On the other hand, it is also possible to change back to the original frequency band and change to synchronous mode in response to the communication conditions in the frequency band being congested or the like. 
       FIG.  17    is a sequence chart illustrating processing for re-changing a frequency band used based on a type of transmitted data (access category) in the present embodiment. First, the multi-link communication control unit  206  of the AP  102  determines whether a frame in the AC_VO category is no longer in the transmission queue (F 1701 ). The multi-link communication control unit  206  makes this determination using, as a condition, no frame in the AC_VO category being in the transmission queue for a pre-set period of time (e.g., 300 seconds), for example. Accordingly, the communication mode control unit  207  of the AP  102  determines that the condition for changing the communication mode is satisfied according to the communication conditions. The subsequent processing (F 1702  to F 1725 ) is similar to that described in the first embodiment with reference to  FIG.  12    and will therefore not be described. 
     In this manner, in the present embodiment, if, when an AP and an STA that support multi-link communication are performing multi-link communication in the synchronous mode, the real-time transmission and reception of data bidirectionally is started, the frequency band used is changed to a frequency band in which multi-link communication can be performed in the asynchronous mode, which enables simultaneous transmission and reception. This makes it possible to prevent affecting the use of the application that transmits and receives data bidirectionally in real time, even when performing multi-link communication in the synchronous mode. 
     Fourth Embodiment 
     The present embodiment is an embodiment concerning processing performed when using an application that transmits and receives data bidirectionally in real time, in a state where an AP and a ML-STA are performing multi-link communication using a frequency band in which transmission and reception can be executed simultaneously. The configuration of the wireless communication system, and the configurations of the AP  102 , the ML-STA  103 , and the Non-ML-STA  104 , are similar to those of the first embodiment, and will therefore not be described here. Similar to the third embodiment, the present embodiment will describe an example of using an access category, as defined in IEEE  802 .11e, as the type of the transmitted data. 
     Processing Based on Type of Transmitted Data 
     Processing according to the type of transmitted data after the AP  102  and the ML-STA  103  establish multi-link communication will be described with reference to  FIG.  18   .  FIG.  18    is a sequence chart illustrating processing based on a type of transmitted data in the present embodiment. The AP  102  and the ML-STA  103  are assumed to be communicating in the synchronous mode, using channel 5 in the 2.4 GHz band, channel 36 in the 5 GHz band, and channel 1 in the 6 GHz band as links ((F 1801 , F 1802 , F 1811 , F 1812 , F 1821 , and F 1822 ). It is assumed here that the AP  102  and the ML-STA  103  are executing applications that transmit and receive data bidirectionally in real time. It is also assumed that “AC VO” (a voice data access category) is set in the multi-link communication control unit  206  of the AP  102  as an access category serving as a condition for changing to the asynchronous mode. AC_VO is the access category having the highest transmission priority. 
     Assume that in F 1803 , data addressed to the ML-STA  103 , generated by the application that transmits and receives data bidirectionally in real time, has entered the transmission queue within the communication unit  307  of the AP  102  as a frame in the AC_VO category. Based on the access category of the data addressed to the ML-STA  103  (the transmitted data), the multi-link communication control unit  206  of the AP  102  detects that the transmitted data is data of a specific type that satisfies a condition pertaining to changing the communication mode. In this case, the communication mode control unit  207  determines that the condition pertaining to changing the communication mode is satisfied. 
     The multi-link communication control unit  206  of the AP  102  determines the frequency band used for the link such that the frequency intervals between the links become greater than both F AP  and F ML-STA . In this example, multi-link communication is already being performed using a frequency band in which transmission and reception can be executed simultaneously, and thus the multi-link communication control unit  206  does not change the frequency band of the link. The AP  102  continues transmitting and receiving data through multi-link communication using a mode in which data can be transmitted and received to and from the ML-STA  103  simultaneously (F 1804  to F 1806 , F 1813  to F 1815 , and F 1823  to F 1826 ). 
     Changing Frequency Band Used 
     Even after a frame in the AC_VO category is no longer in the transmission queue, the AP  102  can continue communicating with the ML-STA  103  without changing the frequency band of the link. On the other hand, the AP  102  can also change the frequency band and change to synchronous mode in response to the communication conditions in the frequency band being congested or the like. The processing for returning to the synchronous mode is similar to that described in the third embodiment with reference to  FIG.  17    and will therefore not be described. 
     In this manner, in the present embodiment, processing can be performed appropriately when starting to use an application that transmits and receives data bidirectionally in real time, when an AP and an STA that supports multi-link are performing multi-link communication using a frequency band in which transmission and reception can be executed simultaneously. 
     The present invention makes it possible to determine an appropriate frequency band for use in multi-link communication. 
     Other Embodiments 
     Embodiment(s) of the present invention can also be realized by a computer of a system or apparatus that reads out and executes computer executable instructions (e.g., one or more programs) recorded on a storage medium (which may also be referred to more fully as a ‘non-transitory computer-readable storage medium’) to perform the functions of one or more of the above-described embodiment(s) and/or that includes one or more circuits (e.g., application specific integrated circuit (ASIC)) for performing the functions of one or more of the above-described embodiment(s), and by a method performed by the computer of the system or apparatus by, for example, reading out and executing the computer executable instructions from the storage medium to perform the functions of one or more of the above-described embodiment(s) and/or controlling the one or more circuits to perform the functions of one or more of the above-described embodiment(s). The computer may comprise one or more processors (e.g., central processing unit (CPU), micro processing unit (MPU)) and may include a network of separate computers or separate processors to read out and execute the computer executable instructions. The computer executable instructions may be provided to the computer, for example, from a network or the storage medium. The storage medium may include, for example, one or more of a hard disk, a random-access memory (RAM), a read only memory (ROM), a storage of distributed computing systems, an optical disk (such as a compact disc (CD), digital versatile disc (DVD), or Blu-ray Disc (BD)™), a flash memory device, a memory card, and the like. 
     While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions.