Patent Publication Number: US-2023156755-A1

Title: Communication apparatus, control method, and storage medium

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a Continuation of International Patent Application No. PCT/JP2021/025744, filed Jul. 8, 2021, which claims the benefit of Japanese Patent Application No. 2020-125693, filed Jul. 22, 2020, both of which are hereby incorporated by reference herein in their entireties. 
    
    
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to a communication apparatus for wireless communication, and a wireless communication method. 
     Background Art 
     Institute of Electrical and Electronics Engineers (IEEE) 802.11 series is known as a Wireless Local Area Network (WLAN) communication specification standard developed by the IEEE. WLAN stands for Wireless Local Area Network. The IEEE802.11 series standard includes the IEEE802.11a/b/g/n/ac/ax standards. 
     Patent Document 1 discloses execution of wireless communication using, in a case of communication compliant with the IEEE802.11ax standard, Orthogonal Frequency Division Multiple Access (OFDMA). The IEEE802.11ax standard achieves a high peak throughput by execution of wireless communication using OFDMA. 
     The IEEE has been studying the development of the IEEE802.11be standard as a new standard of the IEEE802.11 series to further improve the throughput and the frequency use efficiency. In the IEEE802.11be standard, a technique for establishment of a plurality of connections by an Access Point (AP) with a Station (STA) via a plurality of different frequency channels has being studied to achieve wireless communication at higher speeds. 
     CITATION LIST 
     Patent Literature 
     
         
         PTL 1: Japanese Patent Laid-Open No. 2018-50133 
       
    
     When a plurality of links are established between an AP and a STA and communication is performed, the AP and the STA communicate capability information by using a probe request frame and a probe response frame. However, in the communication from the AP to provide capability information to the STA, if the AP provides a notification about capability information on all frequency bands or frequency channels via which the AP can establish links, a frame size is increased and a communication overhead is increased. 
     SUMMARY OF THE INVENTION 
     In view of the above-described issues, the present invention is directed to reducing communication overhead in transmission of capability information about communication to be performed via a plurality of links established with another communication apparatus. 
     To achieve the above-described object, a communication apparatus includes a reception unit configured to receive a probe request frame from another communication apparatus, a decision unit configured to decide, as capability information indicating a capability related to multi-link communication that is performed via a plurality of links established with a partner apparatus, the capability information to be included in a probe response frame corresponding to a frequency band or a frequency channel of which selection is based on information about a frequency band or a frequency channel included in the probe request frame received by the reception unit from the another communication apparatus, and a transmission unit configured to transmit the probe response frame including the capability information corresponding to the frequency band or the frequency channel decided by the decision unit. 
     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 
         FIG.  1    is a diagram illustrating a configuration of a network to which an AP  102  belongs. 
         FIG.  2    is a diagram illustrating a hardware configuration of the AP  102 . 
         FIG.  3    is a diagram illustrating a functional configuration of the AP  102 . 
         FIG.  4    is a sequence diagram illustrating an example of processing that is executed when the AP  102  provides a notification about capability information to an STA  103 . 
         FIG.  5    is a diagram illustrating a frame configuration example of a probe request frame. 
         FIG.  6    is a diagram illustrating a frame configuration example of a probe response frame. 
         FIG.  7    is a flowchart illustrating processing that is executed when the AP  102  provides a notification about capability information. 
         FIG.  8    is a sequence diagram illustrating another example of processing to that is executed when the AP  102  provides a notification about capability information to the STA  103 . 
         FIG.  9    is a flowchart illustrating another processing that is executed when the AP  102  provides a notification about capability information. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     Exemplary embodiments of the present invention will be described in detail below with reference to the accompanying drawings. Configurations described in the following exemplary embodiments are merely examples, and the present invention is not limited to the illustrated configurations. 
     First Exemplary Embodiment 
       FIG.  1    illustrates a configuration of a network in which an access point (AP)  102  according to an exemplary embodiment of the present invention participates. The AP  102  is a communication apparatus having a role of establishing a network  101 . A station (STA)  103  is a communication apparatus having a role of participating in the network  101  established by the access point. Each of the communication apparatuses is compliant with the IEEE 802.11 be (EHT) standard and can execute wireless communication compliant with the IEEE 802.11 be standard via the network  101 . IEEE stands for Institute of Electrical and Electronics Engineers. EHT stands for Extremely High Throughput. EHT can be interpreted as an abbreviation for Extreme High Throughput. Each of the communication apparatuses can perform communication in frequency bands of a 2.4 GHz band, a 5 GHz band, and a 6 GHz band. The frequency bands used by each of the communication apparatuses are not limited to the above-described frequency bands. Any other different frequency bands, such as a 60 GHz band, can also be used. Each of the communication apparatuses can perform communication using bandwidths of 20 MHz, 40 MHz, 80 MHz, 160 MHz, and 320 MHz. 
     The AP  102  and the STA  103  establish a link via a plurality of frequency channels and perform multi-link communication. Each frequency channel used herein refers to the frequency channel which is defined in the IEEE 802.11 series standards and via which wireless communication compliant with the IEEE 802.11 series standards can be executed. In the IEEE 802.11 series standards, a plurality of frequency channels are defined in each of the frequency bands of the 2.4 GHz band, the 5 GHz band, and the 6 GHz band. The IEEE 802.11 series standards also define the bandwidth for each frequency channel as 20 MHz. A bandwidth of 40 MHz or more created by channel bonding of the adjacent frequency channels may be used in a frequency channel. For example, the AP  102  and the STA  103  can establish a first link  104  via a first frequency channel in the 2.4 GHz band and a second link  105  with the STA  103  via a second frequency channel in the 5 GHz band, to communicate with each other via both of the links. In this case, the AP  102  maintains the second link  105  via the second frequency channel in parallel with the first link  104  via the first frequency channel. The AP  102  establishes the links with the STA  103  via a plurality of frequency channels in this way, whereby the throughput in communication with the STA  103  can be improved. 
     The AP  102  and the STA  103  may establish a plurality of links in different frequency bands in multi-link communication. For example, the AP  102  and the STA  103  may establish a third link in the 6 GHz band in addition to the first link  104  in the 2.4 GHz band and the second link  105  in the 5 GHz band. Alternatively, the AP  102  and the STA  103  may establish links via a plurality of different channels included in the same frequency band. For example, the AP  102  and the STA  103  may establish the first link  104  via channel 1 in the 2.4 GHz band and the second link  105  via channel 5 in the 2.4 GHz band. Links in the same frequency band and links in different frequency bands can also be established. For example, the AP  102  and the STA  103  may establish the third link via channel 36 in the 5 GHz band in addition to the first link  104  via channel 1 in the 2.4 GHz band and the second link  105  via channel 5 in the 2.4 GHz band. Establishment of a plurality of connections in different frequency bands with the STA  103  in this way allows the AP  102  to communicate, when a certain band is in congestion, with the STA  103  in other bands, which prevents degradation in the throughput in communication with the STA  103 . 
     The AP  102  and the STA  103  can execute communication in three modes as multi-link communication. The modes include an asynchronous mode (async mode). In this mode, communication via each link in multi-link communication is asynchronously performed. Specifically, the communication via the first link and the communication via the second link are executed at separate timings, respectively. Accordingly, the communication via the first link and the communication via the second link can be performed at any timing regardless of the timing for communication via other links. 
     The modes include a synchronous mode. In this mode, communication via a plurality of links is synchronously executed. Specifically, the communication via the first link and the communication via the second link are started at the same timing. In this case, the communication via each link is started at the same time, which eliminates the need for one link to perform carrier sensing on the communication via the other link. 
     The modes include a semi-asynchronous mode (semi-async mode). This mode is a mode in which when data is communicated via a certain link and a frequency channel via another link is vacant, communications via the both links are simultaneously executed. For example, when a first link backoff counter indicates “0” and a frequency channel via the second link is vacant, the communication via the first link and the communication via the second link are started at the same timing. In this case, when a second link backoff counter may indicate a value other than “0”. When the first link backoff counter indicates “0” and a frequency channel via the second link is not vacant, only the communication via the first link is started and the communication via the second link is not started. 
     The AP  102  and the STA  103  support at least any one of the above-described three modes as a multi-link communication mode. In a case where the AP  102  and the STA  103  support a plurality of modes, at least one of the AP  102  and the STA  103  selects any one of the modes for multi-link communication according to a user instruction, an instruction from an application, a communication status, or the like. 
     While the AP  102  and the STA  103  are compliant with the IEEE 802.11 be standard, these apparatuses may be compliant with at least any one of the legacy standards that are standards prior to the IEEE 802.11 be standard. The legacy standards include the IEEE 802.11 a/b/g/n/ac/ax standards. In the present exemplary embodiment, at least any one of the IEEE 802.11 a/b/g/n/ac/ax/be standards is referred to as the IEEE 802.11 series standards. In addition to the IEEE 802.11 series standards, the AP  102  and the STA  103  may be compliant with other communication standards, such as Bluetooth®, Near Field Communication (NFC), Ultra Wide Band (UWB), Zigbee, and Multi Band OFDM Alliance (MBOA). UWB stands for Ultra Wide Band, and MBOA stands for Multi Band OFDM Alliance. OFDM stands for Orthogonal Frequency Division Multiplexing. NFC stands for Near Field Communication. The UWB includes wireless universal serial bus (USB), wireless 1394, and Winet. The AP  102  and the STA  103  may also be compliant with communication standards for wired communication, such as wired LAN. 
     While, specific examples of the AP  102  include a wireless LAN router and a personal computer (PC), the AP  102  is not limited to these examples. The AP  102  may be any communication apparatus that can execute multi-link communication with another communication apparatus. The AP  102  may also be an information processing device, such as a wireless chip, capable of wireless communication compliant with the IEEE 802.11 be standard. Specific examples of the STA  103  include a camera, a tablet, a smartphone, a PC, a mobile phone, and a video camera. However, the STA is not limited to these examples. The STA  103  may be any communication apparatus capable of multi-link communication with another communication apparatus. Alternatively, the STA  103  may be any information processing device, such as a wireless chip, capable of wireless communication compliant with the IEEE 802.11 be standard. While the network illustrated in  FIG.  1    includes one AP and one STA, the number of APs and the number of STAs are not limited to this example. The information processing apparatus, such as a wireless chip, may include an antenna to transmit generated signals. 
       FIG.  2    illustrates a hardware configuration of the AP  102  in the present exemplary embodiment. The AP  102  includes a storage unit  201 , a control unit  202 , a function unit  203 , an input unit  204 , an output unit  205 , a communication unit  206 , and an antenna  207 . 
     The storage unit  201  includes one or more memories, such as a ROM and a RAM, and stores computer programs for various operations to be described below, and various information, such as communication parameters, for wireless communication. ROM stands for Read-Only Memory, and RAM stands for Random Access Memory. As the storage unit  201 , not only memories, such as a ROM and a RAM, but also storage media, such as a flexible disk, a hard disk, an optical disk, a magneto-optical disk, a CD-ROM, a CD-R, a magnetic tape, a non-volatile memory card, and a DVD may be used. The storage unit  201  may include a plurality of memories. 
     The control unit  202  includes, for example, one or more processors, such as a CPU and an MPU, and executes computer programs stored in the storage unit  201  to control the entire AP  102 . The control unit  202  may control the entire AP  102  by cooperating computer programs and an operating system (OS) stored in the storage unit  201 . Further, the control unit  202  generates data and signals (wireless frames) to be transmitted in communication with another communication apparatus. CPU stands for Central Processing Unit, and MPU stands for Micro Processing Unit. The control unit  202  may include a plurality of processors, such as a multicore, and the plurality of processors may control the entire AP  102 . 
     Further, the control unit  202  controls the function unit  203  to execute wireless communication and predetermined processing, such as image capturing, printing, projection, and the like. The function unit  203  is hardware that enables the AP  102  to execute predetermined processing. 
     The input unit  204  receives various operations from a user. The output unit  205  performs various output operations for the user via a monitor screen or a speaker. In this case, the output operations by the output unit  205  may include a display on a monitor screen, an audio output from a speaker, and a vibration output. Both the input unit  204  and the output unit  205  may be implemented in one module like a touch panel. The input unit  204  and the output unit  205  may be integrally formed into the AP  102  or may be separated from the AP  102 . 
     The communication unit  206  controls wireless communication compliant with the IEEE 802.11 be standard. Further, the communication unit  206  may control wireless communication compliant not only with the IEEE 802.11 be standard, but also with other IEEE 802.11 series standards, and may control wired communication via a wired LAN or the like. The communication unit  206  controls the antenna  207  to transmit and receive signals for wireless communication generated by the control unit  202 . In a case where the AP  102  is compliant not only with the IEEE 802.11 be standard, but also with the NFC standard, the Bluetooth® standard, and the like, the AP  102  may control wireless communication compliant with these communication standards. In a case where the AP  102  can execute wireless communication compliant with a plurality of communication standards, the AP  102  may include communication units and antennas that are compliant with the respective communication standards. The AP  102  communicates data, such as image data, document data, or video data, via the communication unit  206 . The antenna  207  may be separated from the communication unit  206  or may be integrally formed into the communication unit  206  as one module. 
     The antenna  207  is an antenna that supports communication in the 2.4 GHz band, the 5 GHz band, and the 6 GHz band. While, in the present exemplary embodiment, the AP  102  incudes one antenna, different antennas may be provided for each frequency band. In a case where the AP  102  includes a plurality of antennas, the AP  102  may include the communication units  206  each corresponding to a different one of the plurality of antennas. 
     The STA  103  includes a hardware configuration similar to that of the AP  102 . 
       FIG.  3    illustrates a functional configuration of the AP  102  in the present exemplary embodiment. The AP  102  includes a probe request frame analysis unit  301 , a probe response frame generation unit  302 , a channel information management unit  303 , and a data transmission/reception unit  304 . 
     The probe request frame analysis unit  301  is a block that analyzes a probe request frame received from the STA  103 . The probe request frame is one of management frames compliant with the IEEE 802.11 series standards and is a frame to request network information. In the present exemplary embodiment, the probe request frame includes information indicating whether the STA  103  supports multi-link communication, and information about a frequency channel supported by the STA  103  in the communication. 
     The probe response frame generation unit  302  is a block that generates a probe response frame to be transmitted from the AP  102 . The probe response frame is one of management frames compliant with the IEEE 802.11 series standards and is a frame that is transmitted as a response to the probe request frame and provides network information. In the present exemplary embodiment, the probe response frame includes capability information about multi-link communication with the AP  102 . In the present exemplary embodiment, the probe response frame generation unit  302  generates a frame including information about multi-link communication corresponding to the frequency channel that is indicated by the received probe request frame as a channel supported by the STA  103  in the communication. In this processing, excluding information about multi-link communication corresponding to a frequency channel that is supported by the AP  102  but is not supported by the STA  103  can reduce a communication overhead. 
     The channel information management unit  303  is a block that manages information about a frequency channel supported by the AP  102  in multi-link communication. Specifically, the channel information management unit  303  manages information about at least one of pieces of information on a frequency channel via which the AP  102  currently executes multi-link communication and information on a frequency channel via which the AP  102  can execute multi-link communication. 
     The data transmission/reception unit  304  is a block that transmits and receives management frames and data frames compliant with the IEEE 802.11 series standards. 
       FIG.  4    is a sequence diagram illustrating an example of processing that is executed when the AP  102  provides the STA  103  with a notification about capability information. The processing in this sequence is started in response to power-on of the AP  102  and the STA  103 . Alternatively, the processing may be started in response to issuance of a multi-link communication start instruction from the user or application in at least one of the AP  102  and the STA  103 . Yet alternatively, the processing may be started when the amount of data to be communicated with a partner device is more than or equal to a predetermined threshold in at least one of the AP  102  and the STA  103 . 
     In the present exemplary embodiment, the AP  102  supports multi-link communication using at most three links and includes connection processing units each corresponding to a different link of the plurality of links. Specifically, the AP  102  includes connection processing units AP 1 to AP 3 that use frequency bands different from each other. On the other hand, the STA  103  supports multi-link communication using at most two links and includes connection processing units STA 1 and STA 2 that use frequency bands different from each other. In the present exemplary embodiment, the STA  103  does not support wireless communication in the 6 GHz band. 
     The STA 1 that is included in the STA  103  and operates in the 2.4 GHz band transmits the probe request frame to request capability information about multi-link communication for the AP  102  (S 401 ). In this processing, the STA  103  may transmit a probe request frame in this step as a response to a beacon frame (not illustrated) received from the AP  102  prior to S 401 . 
     The probe request frame that is transmitted from the STA  103  in this processing includes a multi-link (ML) element and channel information indicating at least one of frequency channels and a frequency band supported by the STA  103 . The ML element is information indicating that the STA  103  supports multi-link communication. In the present exemplary embodiment, the STA  103  supports wireless communication in the 2.4 GHz band and the 5 GHz band, and thus the probe request frame includes information indicating that the 2.4 GHz band and the 5 GHz band are supported, as channel information. 
     While, in the present exemplary embodiment, the probe request frame includes channel information separately from the ML element, the configuration of the probe request frame is not limited to this example, and channel information may be included in the ML element. 
     The AP  102  transmits the probe response frame including the capability information decided based on the channel information about the STA  103  included in the received probe request frame (S 402 ). The AP  102  compares the channel information indicating frequency channels or a frequency band for the own apparatus supporting multi-link communication with the channel information about the STA  103 . Then, the AP  102  includes only the capability information about the frequency band or channel supported by the STA  103  in the probe response frame. In  FIG.  4   , the STA  103  supports only the 2.4 GHz band and the 5 GHz band, and thus the AP  102  transmits the probe response frame including capability information about the 2.4 GHz band and the 5 GHz band. In this case, the probe response frame to be transmitted does not include capability information about the 6 GHz band. Consequently, the amount of information included in the probe response frame is reduced, which leads to a reduction in communication overhead. 
     While, in the present exemplary embodiment, the STA  103  does not support wireless communication in the 6 GHz band, the configuration is not limited to this example, and the STA  103  may have a configuration in which the STA  103  can execute wireless communication in the 6 GHz band but does not support multi-link communication. In this case, the AP  102  decides capability information to be included in the probe response frame to be transmitted, based on the channel information included in the received ML element. 
     While, in the present exemplary embodiment, information indicating the frequency band supported by the STA  103  is included as channel information, information indicating the frequency channel supported by the STA  103  may also be included in addition to or in place of the information indicating the frequency band. In this case, the AP  102  may transmit the probe response frame including only the capability information about the frequency channel supported by the STA  103 . For example, in a case where the STA  103  transmits the probe request frame including information indicating channel 1 to channel 5 in the 2.4 GHz band as channel information, the AP  102  may transmit the probe response frame that includes capability information about channel 1 to channel 5 and does not include capability information about channel 6 to channel 13. 
     The AP 1 to AP 3 may be configured as independent pieces of hardware or may be configured as software. The same can be applied to STA 1 and STA 2. 
       FIG.  5    illustrates a frame configuration example of the probe request frame. In the present exemplary embodiment, the probe request frame includes fields of a Service Set IDentifier (SSID) element  501 , a Supported Rates element  502 , and an HT Capabilities element  504 . HT stands for High Throughput. Further, the probe request frame includes fields of a Very High Throughput (VHT) Capabilities element  505  and an ML element  506 . Further, the probe request frame includes fields of a Multi-band element  507  and a Supported Operating element  508 . 
     The STA  103  sequentially transmits the fields illustrated in  FIG.  5    from the SSID element  501  on the left side. The STA  103  may start to transmit the probe request frame after completion of generation of all the fields included in the probe request frame. Alternatively, the STA  103  may start the transmission after completion of generation of some fields and may perform generation of the other fields and transmission of the generated fields in parallel. 
     There is no need for the probe request frame to include all the fields illustrated in  FIG.  5   , and at least one of the fields may be omitted. 
     The SSID element  501  is a field to designate the SSID of the AP  102  for the STA  103  to request capability information. 
     The Supported Rates element  502  is a field indicating a list of data rates supported by the STA  103 . 
     A field  503  to be transmitted after the Supported Rates element  502  includes a field indicating the capability information about the STA  103  in the channel via which the STA  103  transmits the probe request frame. In the present exemplary embodiment, specifically, the field  503  includes the HT Capabilities element  504  and the VHT Capabilities element  505 . However, the configuration of the field  503  is not limited to this example. 
     The HT Capabilities element  504  is a field that is for communication compliant with the IEEE 802.11 n standard and indicates the capability information about the STA  103 . 
     The VHT Capabilities element  505  is a field that is for communication compliant with the IEEE 802.11 ac standard and indicates the capability information about the STA  103 . 
     The field  503  may include an EHT Capabilities element in place of or in addition to the HT Capabilities element  504  and the VHT Capabilities element  505 . The EHT Capabilities element is a field that is for communication compliant with the IEEE 802.11 be standard is performed and indicates the capability information about the STA  103 . 
     The multi-link (ML) element  506  is a field indicating capability information in multi-link communication for the STA  103 . The probe request frame including the ML element  506  indicates that the STA  103  supports multi-link communication. 
     The probe request frame transmitted from a STA not supporting multi-link communication includes no ML element. 
     The ML element  506  includes an Element ID field  511  and a Length field  512 , like the other information elements defined in the IEEE 802.11 series standards. The ML element  506  further includes fields indicating information unique to each element. The fields indicating information unique to each element include a Common Info field  513 , which includes information common to all links, and a field  514  including at least one Per Link Info  515  including information unique to each link. 
     In the present exemplary embodiment, the ML element  506  includes information about a frequency channel, via which the STA  103  has already established a link with another communication apparatus, and/or information about the frequency channel, via which the STA  103  can execute multi-link communication. 
     The Element ID field  511  is a field including information for identifying each element. 
     The Length field  512  is a field including information indicating the data length of each element. 
     The Per Link Info  515  further includes sub-fields  521  to  524 . 
     The Band ID  521  is a sub-field including information for identifying each frequency band. Specifically, as illustrated in Table 1, a value indicating a frequency band is included in the sub-field. 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Value 
                 Content 
               
               
                   
                   
               
             
            
               
                   
                 0 
                 2.4 GHz band 
               
               
                   
                 1 
                   5 GHz band 
               
               
                   
                 2 
                   6 GHz band 
               
               
                   
                   
               
            
           
         
       
     
     A Supported Bandwidth  522  is a sub-field including information indicating the bandwidth supported by the STA  103  in multi-link communication in the frequency band indicated by the Band ID  521 . Specifically, as illustrated in Table 2, a value indicating a bandwidth is included in the sub-field. 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 2 
               
               
                   
                   
               
               
                   
                 Value 
                 Content 
               
               
                   
                   
               
             
            
               
                   
                 0 
                  20 MHz bandwidth 
               
               
                   
                 1 
                  40 MHz bandwidth 
               
               
                   
                 2 
                  80 MHz bandwidth 
               
               
                   
                 3 
                 160 MHz bandwidth 
               
               
                   
                 4 
                 240 MHz bandwidth 
               
               
                   
                 5 
                 320 MHz bandwidth 
               
               
                   
                   
               
            
           
         
       
     
     A Supported Channel (CH)  523  is a sub-field including information indicating a channel via which the STA  103  can execute multi-link communication in the frequency band indicated by the Band ID  521 . This sub-field may include information indicating a channel via which the STA  103  has already established a link, in place of or in addition to this information. For example, in a case where the Band ID  521  includes information indicating the 2.4 GHz band, the value indicating the channel as illustrated in Table 3 is included in the sub-field. This sub-field may include only information indicating one frequency channel or may include information indicating a plurality of frequency channels. 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 3 
               
               
                   
                   
               
               
                   
                 Bit 
                 Content 
               
               
                   
                   
               
             
            
               
                   
                  0 
                 CHANNEL 1 
               
               
                   
                  1 
                 CHANNEL 2 
               
               
                   
                  2 
                 CHANNEL 3 
               
               
                   
                 12 
                 CHANNEL 13 
               
               
                   
                   
               
            
           
         
       
     
     The Supported Nss  524  is a sub-field including information indicating a maximum number of multiple input/multiple output (MIMO) streams supported by the STA  103  in multi-link communication in the frequency band indicated in the Band ID  521 . 
     The Per Link Info  515  may include all or some of the sub-fields  521  to  524 . For example, the Per Link Info  515  may be configured to include the Band ID  521  and configured not to include the Supported CH  523 . 
     The ML element  506  may be configured to include one Per Link Info  515  for each frequency band or may be configured to include one Per Link Info  515  for each frequency channel. 
     The Multi-band element  507  is a field including information indicating the frequency band or the frequency channel supported by the STA  103 . The probe request frame including the Multi-band element  507  indicates that the STA  103  supports communication in a plurality of frequency bands. The ML element  506  is a field including the frequency band and the frequency channel via which the STA  103  can execute multi-link communication. However, this field includes information indicating the frequency band and the frequency channel via which the STA  103  can execute not only multi-link communication, but also wireless communication. 
     The Multi-band element  507  includes the Band ID including information indicating the frequency band in which the STA  103  can execute wireless communication. This element further includes operating classes including information indicating the frequency channel via which the STA  103  can execute wireless communication in the frequency band indicated by the Band ID. The operating classes include information indicating a set of a plurality of frequency channels via which the STA  103  can execute wireless communication. The Multi-band element  507  may include only one of the Band ID and the operating classes. 
     The Supported Operating element  508  is a field including information about the frequency channel supported by the STA  103  in each country. Specifically, this field includes information indicating the frequency channel supported by the STA  103  in the country in which the STA  103  is present. This field includes a value for identifying a set of one or more frequency channels and a frequency band as information. 
     The Multi-band element  507  and the Supported Operating element  508  are information defined in the standards prior to the IEEE 802.11 ax standard. 
     In the present exemplary embodiment, the STA  103  includes at least one of the Multi-band element  507  and the Supported Operating element  508  as channel information. The STA  103  may use the Per Link Info  515  included in the ML element  506  as channel information, in addition to or in place of the Multi-band element  507  and the Supported Operating element  508 . In a case where the Per Link Info  515  is used as channel information, the Per Link Info  515  may be configured to include one or both of the Band ID  521  and the Supported CH  523 . 
     The STA  103  transmits the probe request frame including channel information to the AP  102 , whereby the AP  102  is provided with a notification about the frequency band and/or the frequency channel via which the own apparatus can execute communication. Further, the STA  103  transmits the probe request frame including the ML element  506 , whereby the STA  103  provides the AP  102  with a notification indicating that the own apparatus is compliant with multi-link communication. 
     The name of each of the fields illustrated in  FIG.  5    and the order of the transmission and the generation of the fields are not limited to those in this example. Similar information in fields having different names may be included, and the transmission and the generation may be performed in different orders. Some of the fields or sub-fields illustrated in  FIG.  5    may be omitted. 
       FIG.  6    illustrates a frame configuration example of the probe response frame. In the present exemplary embodiment, the probe response frame includes fields of an SSID element  601 , a Supported Rates element  602 , and an HT Capabilities element  604 . Further, the probe response frame includes fields of a VHT Capabilities element  605  and an ML element  606 . Further, the probe response frame includes fields of a Multi-band element  607  and a Supported Rates element  608 . 
     The AP  102  sequentially transmits the fields illustrated in  FIG.  6    from the SSID element  601  on the left side. The AP  102  may start to transmit the probe response frame after completion of generation of all the fields included in the probe response frame. Alternatively, the AP  102  may start the transmission after completion of generation of some fields and may perform generation of the other fields and transmission of the generated fields in parallel. 
     The fields  601  to  624  illustrated in  FIG.  6    include information similar to that in the fields  501  to  524  illustrated in  FIG.  5   . While the fields  501  to  524  illustrated in  FIG.  5    include information indicating the capability information about the STA  103 , the fields  601  to  624  illustrated in  FIG.  6    include information indicating the capability information about the AP  102 . 
     In the probe response frame, Per Link Info  615  may include a field  625  indicating capability information in the frequency band indicated in a Band ID  621  of the AP  102 . Alternatively, the field  625  may include information indicating the capability information about the AP  102  in the frequency channel indicated in Supported CH  623 . 
     Specifically, the field  625  is configured to include an HT Capabilities element  626  and a VHT Capabilities element  627 . Each of the fields includes information similar to that in the fields  504  and  505  illustrated in  FIG.  5   . An EHT Capabilities element field may be included in addition to or in place of these fields. 
     The AP  102  decides channel information to be included in the probe response frame, based on the channel information about the STA  103  included in the request frame received from the STA  103 . The AP  102  decides to include, in the probe response frame, only the capability information about the frequency band and/or the frequency channel indicated by the channel information received from the STA  103 . Specifically, the AP  102  includes, in the ML element  606 , only the Per Link Info corresponding to the frequency band and/or the frequency channel indicated by the channel information included in the probe request frame received from the STA  103 . The AP  102  does not include, in the probe response frame, capability information about the frequency band and frequency channel which are not supported by the STA  103 , whereby the communication overhead can be reduced. 
     The name of each of the fields illustrated in  FIG.  6    and the order of transmission and generation of the fields are not limited to those in this example. Similar information in fields having different names may be included, and the transmission and the generation may be performed in different orders. Some of the fields or sub-fields illustrated in  FIG.  6    may be omitted. 
       FIG.  7    is a flowchart illustrating processing that is executed by the control unit  202  reading out computer programs stored in the storage unit  201  and executing the computer programs when the AP  102  provides a notification about capability information. The processing in this flowchart is started when at least one of the connection processing units AP 1 to AP 3 in the AP  102  receives the probe request frame from the STA. The AP 1 to AP 3 may execute the processing in this flowchart independently or in parallel. 
     In step S 700 , the AP  102  determines whether the probe request frame received from the STA  103  includes the ML element. In a case where the received probe request frame includes the ML element, the AP  102  determines that a result in this step is “Yes” and performs processing of step S 702 . On the other hand, in a case where the probe request frame does not include the ML element, the AP  102  determines that a result in this step is “No” and performs processing of step S 701 . 
     In a case where the received probe request frame does not include the ML element, then in step S 701 , the AP  102  generates the probe response frame in which a ML element is not included. Specifically, the probe response frame that does not include the ML element  606  illustrated in  FIG.  6    is generated. Since this probe response frame does not include a ML element, the probe response frame has backward compatibility with a system compliant with the IEEE 802.11 series standards prior to the IEEE 802.11 ax standard. Since the received probe request frame in which a ML element is not included, the AP  102  determines that the STA  103  does not support multi-link communication. After completion of processing of this step, the AP  102  performs processing of step S 709 . 
     In a case where the determination result in step S 700  is “Yes”, then in step S 702 , the AP  102  determines whether the received probe request frame includes information indicating a frequency channel. In this case, the received probe request frame includes the ML element. Accordingly, the AP  102  determines that the STA  103  supports multi-link communication. Specifically, the information indicating the frequency channel is information indicating the frequency channel included in the Supported CH  523  in the Per Link Info illustrated in  FIG.  5   . Alternatively, the information may be information indicating the frequency channel included in the operating classes in the Multi-band element  507 . Yet alternatively, the information may be information indicating the frequency channel included in the Supported Operating element  508 . In a case where the received probe request frame does not include the information indicating the frequency channel, the AP  102  determines that a result in this step is “No” and performs processing of step S 704 . On the other hand, in a case where the probe request frame includes the information indicating the frequency channel, the AP  102  determines that a result in this step is “Yes” and performs processing of step S 703 . 
     In step S 703 , the AP  102  generates the probe response frame including capability information corresponding to the frequency channel designated by the information indicating the frequency channel included in the received probe request frame. Specifically, the AP  102  generates the probe response frame including the ML element  606  including only the Per Link Info  615  including the Supported CH  523  corresponding to the designated frequency channel. After completion of processing of this step, the AP  102  performs processing of step S 709 . 
     In the present exemplary embodiment, the AP  102  generates the probe response frame including only the capability information corresponding to the designated frequency channel in step S 703 . However, the configuration of the probe response frame is not limited to this example. The AP  102  may generate the probe response frame including only the capability information corresponding to the frequency band to which the designated frequency channel belongs. Also, in this case, the probe response frame includes no capability information corresponding to the frequency bands to which the designated frequency channel does not belong, which leads to a reduction in communication overhead. 
     In a case where the determination result in step S 702  is “No”, then in step S 704 , the AP  102  determines whether the received probe request frame includes the information indicating the frequency band. Specifically, the information indicating the frequency band is information that is included in the Band ID  521  in the Per Link Info  515  illustrated in  FIG.  5    and indicates the frequency band. Alternatively, the information may be information that is included in the band ID in the Multi-band element  507  and indicates the frequency band. Yet alternatively, the information may be information that is included in the Supported Operating element  508  and indicates the frequency band. In a case where the received probe request frame does not include the information indicating the frequency band, the AP  102  determines that a result in this step is “No” and performs processing of step S 707 . On the other hand, in a case where the probe request frame includes the information indicating the frequency band, the AP  102  determines that a result in this step is “Yes” and performs processing of step S 705 . 
     In step S 705 , the AP  102  generates the probe response frame including capability information corresponding to the frequency band designated by the information indicating the frequency band included in the received probe request frame. Specifically, the AP  102  generates the probe response frame including the ML element  606  including only the Per Link Info  615  including the Band ID  521  corresponding to the designated frequency band. After completion of processing of this step, the AP  102  performs processing of step S 709 . 
     In a case where the determination result in step S 704  is “No”, then in step S 707 , the AP  102  generates the probe response frame including capability information corresponding to all frequency bands and/or frequency channels supported by the own apparatus. Specifically, the AP  102  generates the probe response frame including the ML element  606  including the Per Link Info  615  corresponding to all frequency bands and/or frequency channels via which the own apparatus can execute multi-link communication. After completion of processing of this step, the AP  102  performs processing of step S 709 . 
     In step S 709 , the AP  102  transmits the generated probe response frame to the STA  103  that has transmitted the received probe request frame. 
     As described above with reference to  FIG.  7   , the AP  102  decides the capability information to be included in the probe response frame, based on the information included in the received probe request frame, whereby the communication overhead can be reduced. In a case where the STA  103  does not support multi-link communication, the STA  103  does not include the ML element in the probe response frame, whereby the communication overhead can be further reduced. 
     In a case where, among links already established by the AP  102 , there is a link of which communication load status is high and which is desired not to be used for communication with the STA  103 , the probe response frame excluding the capability information corresponding to the link may be generated. The high communication load status indicates, for example, a case where the amount of data to be communicated within a predetermined period in the link exceeds a predetermined threshold, a case where the amount of data that cannot be successfully communicated within the predetermined period in the link exceeds the predetermined threshold, or a case where the reception strength of a signal received from another communication apparatus via the link is less than the predetermined threshold. Thus, the capability information about the link to be excluded from the communication with the STA  103  is not included in the probe response frame. This configuration enables the AP  102  to further reduce the communication overhead. 
     Second Exemplary Embodiment 
     The first exemplary embodiment described above illustrates processing that is executed by the AP  102  when the AP  102  receives the probe request frame from the STA  103  via one frequency channel. A second exemplary embodiment illustrates processing that is executed by the AP  102  when the AP  102  receives the probe request frame from the STA  103  via two or more frequency channels. 
     The network configuration of the network in which the AP  102  participates is similar to that illustrated in  FIG.  1    according to the first exemplary embodiment. The hardware configuration of the AP  102  is similar to that illustrated in  FIG.  2    according to the first exemplary embodiment. The functional configuration of the AP  102  is similar to that illustrated in  FIG.  3    according to the first exemplary embodiment. The frame configuration of the probe request frame is similar to that illustrated in  FIG.  5   . The frame configuration of the probe response frame is similar to that illustrated in  FIG.  6   . Only differences from the first exemplary embodiment will now be described. 
       FIG.  8    is a sequence diagram illustrating another example of processing that is executed when the AP  102  provides the STA  103  with a notification about capability information. The processing in this sequence is started in response to power-on of the AP  102  and the STA  103 . Alternatively, the processing may be started in response to issuance of a multi-link communication start instruction from the user or application in at least one of the AP  102  and the STA  103 . Yet alternatively, the processing may be started when the amount of data to be communicated with the partner device is more than or equal to a predetermined threshold in at least one of the AP  102  and the STA  103 . 
     The AP  102  and the STA  103  support multi-link communication, like in  FIG.  4    according to the first exemplary embodiment, and include connection processing units each corresponding to a different link of the plurality of links. The AP 1 to AP 3 may be configured as independent pieces of hardware or may be configured as software. The same can be applied to the STA 1 and STA 2. 
     In step S 801 , the STA 1 that is included in the STA  103  and operates in the 2.4 GHz band transmits the probe request frame to request capability information about multi-link communication for the AP  102 . In this processing, the STA  103  may transmit the probe request frame in this step as a response to a beacon frame (not illustrated) received from the AP  102  prior to step S 801 . 
     In this processing, the probe request frame transmitted from the STA  103  includes an ML element and channel information indicating at least one of the frequency channels and the frequency band supported by the STA  103 . In the present exemplary embodiment, the STA  103  supports wireless communication in the 2.4 GHz band and the 5 GHz band, and thus the probe request frame includes information indicating that the 2.4 GHz band and the 5 GHz band are supported as channel information. 
     In the present exemplary embodiment, the probe request frame includes channel information separately from the ML element. However, the configuration of the probe request frame is not limited to this example, and channel information may be included in the ML element. 
     Next, in step S 802 , the STA 2 that is included in the STA  103  and operates in the 5 GHz band transmits the probe request frame to request capability information about multi-link communication for the AP  102 . This step may occur, for example, when communication in the frequency channel currently used by the STA 1 is temporarily congested and the AP  102  cannot transmit the probe response frame for a certain period. In a case where the STA  103  fails to receive a response after the lapse of a certain period of time from the transmission of the probe request frame via the STA 1, the STA  103  can transmit the probe request frame again via the STA 2 that uses a different frequency band. The probe request frame transmitted in this step includes information indicating that the 2.4 GHz band and the 5 GHz band are supported as channel information, like the probe request frame transmitted in step S 801 . 
     In a case where the AP  102  receives the probe request frame from the STA  103  via a plurality of frequency channels, the AP  102  may transmit the probe response frame to the STA  103  via any one of the frequency channels. In this sequence, in step S 803 , the AP 2 that is included in the AP  102  and operates in the 5 GHz band transmits the probe response frame to the STA  103 . The probe response frame transmitted in this step includes the capability information decided based on the channel information about the STA  103  included in the probe request frame received by the AP  102 , like in step S 402  illustrated in  FIG.  4   . Instead of the AP 2 that has subsequently received the probe request frame and operates in the 5 GHz band, the AP 1 that has received the probe request frame first and operates in the 2.4 GHz band may transmit the probe response frame. The AP  102  may receive three or more probe request frames from the STA  103 . 
     As described above with reference to  FIG.  8   , even when the AP  102  receives the probe request frame from the same STA via two or more frequency channels, the AP  102  transmits the probe response frame via only one frequency channel. This configuration prevents repeating transmission of the probe response frame of the same content from the AP  102 , which can prevent an increase in communication overhead. 
       FIG.  9    is a flowchart illustrating another processing that is executed by the control unit  202  reading out computer programs stored in the storage unit  201  and executing the programs when the AP  102  provides a notification about capability information. The processing in this flowchart is started when at least one of the connection units AP 1 to AP 3 in the AP  102  receives the probe request frame from the STA. The AP 1 to AP 3 may independently execute the processing in this flowchart in parallel. 
     In step S 900 , in response to receipt of the probe request frame from the STA  103 , the AP  102  sets a flag indicating that the probe request frame for the received link is being processed to “1”. 
     Steps S 901  to S 909  are processing similar to steps S 700  to S 707 , respectively, in  FIG.  7    according to the first exemplary embodiment. 
     After generating the probe response frame to be transmitted, in step S 910 , the AP  102  determines whether the flag indicating that the probe request frame for another link is being processed is set to “1”. Specifically, determination of whether the flag is set for another link different from the link for which the flag is set in step S 900  is performed. In a case where the flag is not set for another link, the AP  102  determines that a result in this step is “No” and performs processing of step S 911 . On the other hand, in a case where the flag is set for another link, the AP  102  determines that a result in this step is “Yes” and performs processing of step S 913 . 
     In step S 913 , The AP  102  determines whether the transmission source of the probe request frame received via the link for which the flag is set in step S 900  matches the transmission source of the probe request frame received via another link for which the flag is set in step S 910 . Specifically, the AP  102  compares pieces of identification information each indicating the transmission source included in the respective received probe request frames. In a case where the pieces of identification information match each other, the AP  102  determines that a result in this step is “Yes” and performs processing of step S 912 . On the other hand, in a case where the pieces of identification information each indicating the transmission source do not match each other, the AP  102  determines that a result in this step is “No” and performs processing of step S 911 . 
     In step S 911 , the AP  102  transmits the generated probe response frame to the STA that has transmitted the probe request frame received in step S 900 . After completion of processing of step S 911 , the AP  102  performs processing of step S 912 . 
     In step S 912 , the AP  102  sets the flag indicating that the probe request frame for the received link is being processed to “0”. After completion of processing of step S 912 , the AP  102  terminates this processing flow. 
     As described above with reference to  FIG.  9   , the AP  102  transmits the probe response frame via only one frequency channel even when the AP  102  receives the probe request frames from the same STA via a plurality of frequency channels. This configuration prevents repeating transmission of the probe response frame including the same capability information from the AP  102 , which can prevent an increase in communication overhead. 
     The processing according to the first exemplary embodiment and the processing according to the second exemplary embodiment are executed by different APs, but instead may be executed by one AP. In this case, the AP includes a mode for the processing according to the first exemplary embodiment and a mode for the processing according to the second exemplary embodiment. This AP may select which of the modes to be used for operation to execute the processing according to the exemplary embodiments, based on a user instruction or based on an instruction from an application. 
     At least a part or all of the flowcharts for the AP  102  illustrated in  FIGS.  7  and  9    may be implemented using hardware. In a case where the flowcharts are implemented using hardware, for example, a predetermined compiler may be used to generate a dedicated circuit on an FPGA based on computer programs for implementing each step, and the generated circuit may be used. FPGA stands for Field Programmable Gate Array. A gate array circuit may be formed like the FPGA, and may be implemented as hardware, or may be implemented as an application specific integrated circuit (ASIC). 
     The present invention can also be implemented by processing in which a program for implementing one or more functions according to the above-described exemplary embodiments is supplied to a system or an apparatus via a network or a storage medium, and one or more processors in a computer in the system or the apparatus read out and executes the program. The present invention can also be implemented by a circuit (e.g., an ASIC) for implementing one or more functions. 
     The present invention is not limited to the above-described exemplary embodiments, and various modifications and changes can be made without departing from the spirit and scope of the present invention. Accordingly, the following claims are appended to disclose the scope of the present invention. 
     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. 
     According to the present invention, it is possible to reduce a communication overhead in transmission of capability information about communication to be performed via a plurality of links established with another communication apparatus. 
     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.