Patent Publication Number: US-2022225108-A1

Title: Channel indication sending method, channel scanning method, and apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of International Application No. PCT/CN2020/116533, filed on Sep. 21, 2020, which claims priority to Chinese Patent Application No. 201910937941.0, filed on Sep. 30, 2019. The disclosures of the aforementioned applications are hereby incorporated by reference in their entireties. 
    
    
     TECHNICAL FIELD 
     This application relates to the field of wireless communication technologies, and in particular, to a channel indication sending method, a channel scanning method, and an apparatus. 
     BACKGROUND 
     Operating frequency bands of a wireless local area network (wireless local area network, WLAN) include the 5 GHz frequency band. The 5 GHz frequency band is an unlicensed frequency band (unlicensed band). Countries and regions may deploy various radars such as weather radars and military radars on some channels (channel) of the frequency band. Therefore, these channels are referred to as radar channels. A radar channel is used by a WLAN device in a communication network. Therefore, to avoid a conflict caused when the radar channel is simultaneously used by the WLAN device and the various radars, the WLAN protocol specifies that: 
     1. Channels of radars deployed by countries and regions are dynamic frequency selection (dynamic frequency selection, DFS) channels. 
     2. An access point (access point, AP) in the WLAN device needs to be silent for one minute before operating on a DFS channel. If the AP does not find a radar within the one minute, that is, the DFS channel is not used by the radar, the AP is allowed to formally use the DFS channel. This process is referred to as channel availability check (channel available check, CAC). The AP can send a beacon frame (beacon frame) on the DFS channel only after the CAC succeeds. 
     3. The AP operating on the DFS channel needs to perform CAC in real time. Once a radar signal is detected, the AP needs to switch to another channel within 10 seconds to ensure that the radar preferentially uses the DFS channel. 
     After various behaviors of the AP are specified in the foregoing protocol, behaviors of a station (station, STA) associated with the AP are affected. Specifically, after accessing the WLAN, the STA is associated with an AP by using a basic service set (basic service set, BSS). The STA further scans another channel, so that the STA is associated with another AP when communication quality of the current BSS deteriorates. 
     After switching to the DFS channel, the STA needs to scan the DFS channel. The CAC performed by the AP on the DFS channel may fail, that is, the AP fails to complete the CAC on the DFS channel, or the AP detects a radar in the 1-minute silent period. As a result, the STA fails to scan the DFS channel, and the STA switching to the DFS channel wastes air interface resources. 
     SUMMARY 
     This application provides a channel indication sending method, to resolve a problem that a STA fails to scan a DFS channel on which CAC is not completed. Specifically, this application discloses the following technical solutions. 
     According to a first aspect, this application provides a channel indication sending method. The method may be applied to a first access point, and the first access point is associated with a station. The method includes: The first access point receives channel information from a second access point. The channel information indicates that an operating channel of the second access point is a first channel, and whether the second access point successfully performs CAC on the first channel. The second access point is a neighbor of the first access point, and the first channel is a DFS channel. 
     The first access point sends, on a second channel, a WLAN frame to a station associated with the first access point. The second channel is an operating channel of the first access point. The WLAN frame includes an indication about the first channel and an indication of whether the second access point successfully performs the CAC on the first channel. 
     If it indicates that the CAC on the first channel succeeds, the station switches to the first channel and scans the first channel. 
     According to the method provided in this aspect, the first access point sends a channel indication to the station associated with the first access point, to indicate whether the neighbor second access point successfully performs CAC on the DFS channel. In this case, the station can scan, only after the second access point successfully performs the CAC, the DFS channel on which the second access point is located. This avoids a waste of air interface resources caused by switching by the station to a DFS channel on which CAC fails. This method increases a success rate of scanning a DFS channel by the station, and further saves air interface resources. 
     Optionally, the first channel is in a channel report (Channel Report) information element. 
     Optionally, the channel report information element includes a vendor-specific (vendor specific) field. Further, the vendor-specific field is used to indicate whether the second access point successfully performs the CAC on the first channel. 
     Further, if content carried in the field is “1”, it indicates that the CAC succeeds. If the content carried in the field is “0”, it indicates that the CAC fails. Still further, the CAC failure includes the following two cases: 
     In one case, the second access point fails to complete the CAC on the DFS channel and is still in a 1-minute silent period. 
     In another case, the second access point completes the CAC on the DFS channel and finds a radar during the 1-minute silent period. 
     According to a second aspect, this application provides a channel scanning method. The method may be applied to a station, and the station is associated with a first access point. Specifically, the method includes: 
     The station receives a WLAN frame from a first access point. The WLAN frame includes an indication about a first channel on which a second access point operates and an indication of whether the second access point successfully performs CAC on the first channel, and the first channel is a DFS channel. 
     The station scans, based on the indication about the first channel and the indication of whether the CAC succeeds, the first channel only if the second access point successfully performs the CAC on the first channel. 
     Optionally, the first channel is in a channel report information element. 
     With reference to the second aspect, in a possible implementation of the second aspect, the scanning, by the station, the first channel includes: The station sends a probe request on the DFS channel immediately after the station switches to the DFS channel on which the CAC succeeds. 
     In addition, the method further includes: The station receives a probe response returned by the second access point. After receiving the probe response, the station completes scanning on the DFS channel and returns to the second channel. The second channel is an operating channel of the first access point. 
     In the method, when the WLAN frame sent by the first access point indicates that the CAC succeeds, the station switches to the DFS channel on which the second access point is located, immediately sends a probe request frame, and starts to scan the DFS channel. This reduces waiting time during which the station can start to send a probe request frame only after receiving a beacon frame on the DFS channel. The method significantly shortens dwell time of the station on the DFS channel, so that the station can return to the original operating channel after scanning the DFS channel. This avoids frame freezing of audio or video caused by long dwell time of the station on the DFS channel. 
     According to a third aspect, this application provides a channel indication sending method. The method may be applied to a first access point, and the first access point is associated with a station. The method includes: 
     The first access point receives channel information from at least one neighbor access point whose operating channel is a DFS channel. The channel information from each neighbor access point is used to indicate an operating channel of a corresponding access point, and whether the corresponding access point successfully performs CAC. 
     The first access point sends a channel list to the station associated with the first access point. The channel list includes only an indication about an operating channel of a neighbor access point that successfully performs CAC and an indication about an operating channel of a neighbor access point whose operating channel is a non-DFS channel. 
     In this embodiment, for a station associated with a neighbor access point that fails to perform CAC, the first access point sends, to the station only after the neighbor access point successfully performs the CAC, an indication about a DFS channel on which the corresponding access point is located, instead of delivering an indication about an operating channel to the station. This avoids a waste of air interface resources caused by switching by the station to a DFS channel on which CAC fails, and increases a success rate of scanning a DFS channel by the station. 
     In addition, in this method, compared with carrying, in a WLAN frame, an indication of whether the neighbor access point successfully performs CAC on a DFS channel, delivering, by the first access point, only an indication about a DFS channel on which CAC succeeds reduces content carried in a message. In this way, transmission overheads are reduced, a DFS channel scanning procedure is simplified, and channel scanning efficiency is further improved. 
     According to a fourth aspect, this application provides a channel indication sending apparatus. The apparatus may be used as a first access point, and the apparatus includes a receiving unit and a sending unit. 
     The receiving unit is configured to receive channel information from a second access point. The channel information indicates that an operating channel of the second access point is a first channel, and whether the second access point successfully performs CAC on the first channel, the second access point is a neighbor of the first access point, and the first channel is a DFS channel. 
     The sending unit is configured to send, on a second channel, a WLAN frame to a station associated with the first access point. The second channel is an operating channel of the first access point. The WLAN frame includes an indication about the first channel and an indication of whether the second access point successfully performs the CAC on the first channel. 
     Optionally, the first channel is in a channel report information element. 
     According to a fifth aspect, this application provides a channel scanning apparatus. The apparatus may be used as a station, and the apparatus includes a receiving unit and a processing unit. 
     The receiving unit is configured to receive a WLAN frame from a first access point. The WLAN frame includes an indication about a first channel on which a second access point operates and an indication of whether the second access point successfully performs CAC on the first channel, and the first channel is a DFS channel. 
     The processing unit is configured to scan, based on the indication about the first channel and the indication of whether the CAC succeeds, the first channel only if the second access point successfully performs the CAC on the first channel. 
     With reference to the fifth aspect, in a possible implementation of the fifth aspect, the processing unit is specifically configured to send a probe request on the DFS channel immediately after the station switches to the DFS channel on which the CAC succeeds. 
     According to a sixth aspect, this application provides a channel indication sending apparatus. The apparatus may be used as a first access point. The apparatus includes a receiving unit and a sending unit, and may further include a processing unit, a storage unit, and the like. 
     The receiving unit is configured to receive channel information from at least one neighbor access point whose operating channel is a DFS channel. The channel information from each neighbor access point is used to indicate an operating channel of a corresponding access point, and whether the corresponding access point successfully performs CAC. The sending unit is configured to send a channel list to a station associated with the first access point. The channel list includes only an indication about an operating channel of a neighbor access point that successfully performs CAC and an indication about an operating channel of a neighbor access point whose operating channel is a non-DFS channel. 
     According to a seventh aspect, this application further provides a communication apparatus. The communication apparatus may be an access point or a station. The communication apparatus includes a transceiver, a processor, and a memory. 
     The memory is configured to store program instructions. The processor is configured to invoke the program instructions in the memory, and control the transceiver to perform, according to the program instructions, the method in any one of the first aspect to the third aspect or the implementations of the second aspect. 
     Further, when the communication apparatus is a first access point, the channel indication sending method in the first aspect or the third aspect is performed by executing the program instructions. 
     When the communication apparatus is a station, the channel scanning method in either the second aspect or the implementations of the second aspect is performed by executing the program instructions. 
     According to an eighth aspect, this application further provides a computer-readable storage medium. The storage medium stores instructions. When the instructions are run on a computer or a processor, the method in any one of the first aspect to the third aspect or the implementations of the second aspect may be performed. 
     In addition, this embodiment further provides a computer program product. The computer program product includes computer instructions. When the instructions are executed by a computer or a processor, the method in any one of the first aspect to the third aspect or the implementations of the second aspect may be performed. 
     According to a ninth aspect, an embodiment further provides a WLAN system. The WLAN system includes at least two access points and at least one station. The at least two access points include a first access point and a second access point. The first access point is associated with the station, the second access point is a neighbor of the first access point, and a channel on which the second access point is located is a DFS channel. 
     Each access point or station may be the communication apparatus in the seventh aspect, and the communication apparatus is configured to implement the method in any one of the first aspect to the third aspect or the implementations of the second aspect. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a schematic diagram of a structure of a WLAN system according to an embodiment of this application; 
         FIG. 2  is a signaling flowchart of a channel indication sending method according to an embodiment of this application; 
         FIG. 3  is a schematic diagram of a format of an AP channel report information element according to an embodiment of this application; 
         FIG. 4  is a signaling flowchart of channel scanning by a STA according to an embodiment of this application; 
         FIG. 5  is a block diagram of a structure of an apparatus according to an embodiment of this application; 
         FIG. 6  is a schematic diagram of a structure of an access point according to an embodiment of this application; and 
         FIG. 7  is a schematic diagram of a structure of a station according to an embodiment of this application. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Before technical solutions in the embodiments of this application are described, an application scenario of the embodiments of this application is first described with reference to the accompanying drawings. 
     The technical solutions of this application may be applied to a WLAN system. The WLAN system includes at least two radio access network devices and at least one terminal device. In addition, the system may further include other network devices, for example, may further include a control device, a relay device, and a wireless backhaul device. 
     The terminal device is connected to a wireless access point (access point, AP) device in a wireless manner, and the radio access network devices are connected to the control device by using a network (for example, a local area network, a core network, or the Internet). The control device and the radio access network devices may be different independent physical devices, functions of the control device and logical functions of the radio access network devices may be integrated into one physical device, or some functions of the control device and some functions of the radio access network devices may be integrated into one physical device. The terminal device may be at a fixed location, or may be mobile. The control device may control the AP device. For example, the control device is an access controller (Access Controller, AC). The terminal device is a WLAN terminal. For example, the terminal includes a station (station, STA). 
     The radio access network devices and the terminal device may communicate with each other by using an unlicensed spectrum (unlicensed spectrum). The unlicensed spectrum may be the 5-gigahertz (gigahertz, GHz) frequency band, or may be another frequency band. A spectrum resource used between the network device and the terminal device is not limited in the embodiments. 
       FIG. 1  is a schematic diagram of a structure of a WLAN system according to an embodiment of this application. The system includes an AC, a switch, an AP 1 , an AP 2 , and a STA. The AC is connected to the AP 1  and the AP 2  through the switch, and is responsible for managing the AP 1  and the AP 2 , for example, radio frequency and authorization management. The switch is responsible for establishing a connection between the AP 1  and the network, a connection between the AP 2  and the network, and a connection between internal networks within a campus. In addition, the switch is also responsible for management of a routing table, forwarding of a data packet, and other functions. The AP 1  or the AP 2  is configured to exchange data with the STA, and is responsible for managing behaviors of the STA, such as dormancy and roaming. 
     The following describes a mechanism in which the STA scans another channel. 
     When the STA switches to the another channel, a service on a current operating channel is temporarily interrupted. Therefore, a tradeoff between the scanned channel and the operating channel needs to be maintained. Scanning occasions of the STA may be classified into three types in an existing scanning mechanism: periodic scanning, scanning after quality of a current BSS deteriorates, and scanning after disconnection of the current BSS. Further, the following table shows advantages and disadvantages of the three types of scanning. 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Advantages and disadvantages of channel scanning mechanisms 
               
            
           
           
               
               
               
            
               
                 Channel scanning 
                   
                   
               
               
                 mechanisms 
                 Advantages 
                 Disadvantages 
               
               
                   
               
               
                 Periodic scanning 
                 Because scanning is always 
                 Scanning overheads are huge. 
               
               
                   
                 performed, roaming can be 
               
               
                   
                 initiated immediately after a 
               
               
                   
                 current BSS deteriorates. 
               
               
                   
                 This significantly improves 
               
               
                   
                 roaming initiative. 
               
               
                 Scanning after quality 
                 Scanning overheads are 
                 This mechanism is inferior to periodic 
               
               
                 of the current BSS 
                 significantly reduced 
                 scanning because of low roaming 
               
               
                 deteriorates 
                 compared with those of 
                 initiative. Currently, most STAs use 
               
               
                   
                 periodic scanning. 
                 this mechanism. 
               
               
                 Scanning after 
                 A service is not affected 
                 Scanning is performed only after the 
               
               
                 disconnection of the 
                 because no scanning is 
                 disconnection. Service experience of a 
               
               
                 current BSS 
                 performed during the 
                 STA using this mechanism is poor 
               
               
                   
                 service. 
                 because the service is interrupted for a 
               
               
                   
                   
                 long time during scanning. Therefore, 
               
               
                   
                   
                 this mechanism is usually not 
               
               
                   
                   
                 considered. 
               
               
                   
               
            
           
         
       
     
     If the STA uses a passive scanning mode, the STA completes scanning on the channel only after receiving a beacon frame. Even if the STA uses an active scanning mode, a specific scanning behavior for a DFS channel is different from that for a non-DFS channel. On the non-DFS channel, there is no need to wait for an AP on the non-DFS channel to send a beacon (beacon) frame, that is, the STA may send a probe request (probe request) frame immediately after switching to the non-DFS channel, to scan the channel. After scanning is completed, the STA returns to an operating channel of an originally associated AP, to process a service on the operating channel. Dwell time of the STA on the non-DFS channel (or referred to as a common channel) is usually 50 ms to 60 ms. If the scanned channel is a DFS channel, it is specified that the STA can start to send a probe request (probe request) only after receiving the beacon frame broadcast by the AP on the DFS channel. Therefore, dwell time of the STA on the DFS channel is usually 1 to 2 times the dwell time of the common channel, for example, 100 ms to 200 ms. 
     The embodiments of this application provide the following technical solutions, to avoid the following phenomenon: Because of long dwell time of a STA on a DFS channel, a service between the STA and an originally associated AP is interrupted for a long period of time, leading to frame freezing and affecting user experience. 
       FIG. 2  is a flowchart of a message sending method according to an embodiment. The method is applied to the foregoing communication system, and is mainly used to indicate a station STA to perform channel scanning on an access point located on a DFS channel. Network elements in the method include a first access point (AP 1 ), a second access point (AP 2 ), and a station (STA). Further, the AP 1  is associated with the STA, the AP 2  is a neighbor of the AP 1 , and the AP 2  is located on the DFS channel. 
     Specifically, the method includes the following steps. 
     Step  201 : The second access point AP 2  sends channel information to the first access point AP 1 . Correspondingly, the first access point AP 1  receives the channel information from the second access point AP 2 . 
     After switching to the DFS channel, the AP 2  starts CAC, and sends a channel change message to the AP 1 , for example, indicates, by using the channel information, to report the DFS channel on which the AP 2  is located to the AP 1 . The channel cannot be used within one minute after the check. 
     The channel information indicates that an operating channel of the AP 2  is a first channel, and whether the AP 2  successfully performs CAC on the first channel. The first channel is the DFS channel. 
     The indicating whether the AP 2  successfully performs CAC on the DFS channel specifically includes: 
     If the indication is “1” or “YES”, it indicates that the CAC succeeds. 
     If the indication is “0” or “NO”, it indicates that the CAC fails. Further, the CAC failure includes the following two cases: 
     In one case, the AP 2  fails to complete the CAC on the DFS channel and is still in a 1-minute silent period. 
     In another case, the AP 2  completes the CAC on the DFS channel and finds a radar during the 1-minute silent period. 
     When either of the foregoing two cases occurs, it indicates that the AP 2  fails to perform the CAC on the DFS channel. 
     Step  202 : The first access point AP 1  sends, on a second channel, a WLAN frame to the station STA associated with the AP 1 . Correspondingly, the STA receives, on the second channel, the WLAN frame from the AP 1 . 
     The second channel is an operating channel of the AP 1 . 
     The WLAN frame includes an indication about the first channel (DFS channel) and an indication of whether the AP 2  successfully performs the CAC on the first channel. Specifically, the indication about the DFS channel is the same as an indication about the channel information received by the AP 1  in step  201 , and is used to determine the DFS channel on which the AP 2  is located. 
     The AP 2  may indicate, by using a specific field, whether the AP 2  successfully performs the CAC on the DFS channel. For example, a vendor-specific (vendor specific) field is used. 
     Content carried in the field includes “1” or “YES”, or includes “0” or “NO”. 
     Optionally, in a specific implementation, the STA sends a neighbor report request (neighbor report request) to the associated AP 1 . After receiving the request, the AP 1  sends a neighbor report (neighbor report) to the STA. The neighbor report includes an AP channel report (AP Channel Report) information element (information element) and a vendor-specific field. The AP channel report information element includes a channel list (or a channel set). The channel list includes all channels that can be scanned by the STA, including a DFS channel and a non-DFS channel. In this embodiment, the channel list of the AP channel report information element includes the first channel. The vendor-specific field is used to indicate a status of CAC performed by each AP on a corresponding operating channel. 
       FIG. 3  shows a format of the AP channel report information element. An element ID (Element ID), a length (Length), an operating class (Operating Class), and a channel list (Channel List) are included. The channel list includes the first channel on which the AP 2  is located. In addition, the vendor-specific (vendor specific) field may be further included. 
     Optionally, the AP 1  sends the AP channel report information element including the first channel to the STA by using the WLAN frame. 
     Optionally, the WLAN frame is a management frame, for example, an action (action) frame. The action frame includes the AP channel report information element, or includes the AP channel report information element and the vendor-specific field. The AP 1  may send the management frame in a broadcast or unicast manner. This is not limited in this embodiment of this application. 
     Step  203 : The station STA scans, based on the indication about the first channel and the indication of whether the CAC succeeds, the first channel only if the AP 2  successfully performs the CAC on the first channel. 
     Specifically, the STA determines, based on the channel indication in the WLAN frame, the first channel on which the AP 2  is located, and determines, based on the indication about the CAC, whether the STA scans the first channel. If the content of the vendor-specific field is “1” or “YES”, the STA switches to the first channel, and scans the first channel. If the content of the vendor-specific field is “0” or “NO”, the STA does not scan the first channel. 
     Further, a process of scanning the first channel in step  203  includes: The STA switches to the first channel, and sends a probe request (probe request) frame on the first channel. The AP 2  returns a probe response (probe response) frame to the STA after receiving the probe request frame. After receiving the probe response frame returned by the AP 2 , the STA completes scanning on the first channel and returns to the second channel. The second channel is the operating channel of the AP 1 . 
     In addition, if the STA does not scan the first channel when the content of the vendor-specific field is “0” or “NO”, the method further includes: The AP 1  receives the channel information from the AP 2  after a period of time, to indicate that the AP 2  successfully performs the CAC. In this case, after the AP 1  sends, to the STA, the WLAN frame indicating that the AP 2  completes the CAC on the first channel, the STA switches to the first channel based on the WLAN frame, and scans the first channel. It should be understood that, because the first channel on which the AP 2  is located does not change, a new WLAN frame sent by the AP 1  to the STA needs to carry only an indication indicating that the AP 2  successfully performs the CAC on the first channel, and does not carry the indication about the first channel. This reduces indication overheads. 
     Because the STA clearly knows that CAC on the DFS channel scanned by the STA has been performed successfully, the STA does not need to wait for the beacon frame on the channel, but may send the probe request frame immediately after switching to the DFS channel. 
     According to the method provided in this embodiment, the first access point sends the channel indication to the station associated with the first access point, to indicate whether the neighbor second access point successfully performs the CAC on the DFS channel. In this case, after determining that the second access point successfully performs the CAC, the station switches to the DFS channel on which the second access point is located, and starts to scan the DFS channel. This avoids a waste of air interface resources caused by switching by the station to a DFS channel on which CAC fails. This method increases a success rate of scanning a DFS channel by the station, and further saves air interface resources. 
     In addition, the station may send the probe request frame immediately after switching to the DFS channel, and start to scan the DFS channel, to reduce waiting time during which the station needs to start to send the probe request frame on the DFS channel only after the second access point sends the beacon frame. This significantly shortens dwell time of the station on the DFS channel, so that the station can return to the original operating channel after scanning the DFS channel. In this way, an interval between services such as voice or video is shortened, avoiding frame freezing of voice or video caused by long dwell time of the station on the DFS channel. 
     In addition, in this method, time overheads for scanning the DFS channel by the station are reduced without affecting a success rate of scanning the DFS channel by the station. 
     In addition, the channel list of the WLAN frame sent by the first access point to the station further includes channel information of another access point, for example, a non-DFS channel on which a third access point is located. In this case, after receiving channel information of the third access point, the station scans, according to a non-DFS channel check procedure, the non-DFS channel on which the third access point is located. A specific scanning process is the same as a process in which the STA scans the DFS channel in step  203 . Details are not described herein again. 
     According to the method provided in this embodiment, the associated AP sends a status of CAC performed by the neighbor AP on the DFS channel to the STA. In this case, during channel scanning, the STA does not switch to the DFS channel on which CAC fails, but switches to the DFS channel on which the CAC succeeds. In addition, after the STA switches to the DFS channel on which the CAC succeeds, the STA may send the probe request frame without receiving the beacon frame. In this way, waiting time is reduced, dwell time of the STA on the DFS channel is reduced, and time overheads for scanning the DFS channel by the STA are reduced. 
     After obtaining channel information sent by an AP associated with a non-DFS channel, the STA switches to the non-DFS channel, and completes scanning on the channel. 
     This application provides another embodiment. This embodiment is improvement on the foregoing embodiment. The improvement lies in that, for channel information of a DFS channel, the AP 1  sends, to the STA, only channel information of an AP that successfully performs CAC. If the neighbor AP 2  fails to perform CAC on a channel on which the neighbor AP 2  is located, the channel information of the DFS channel is not sent to the STA, to avoid a phenomenon that the WLAN frame further carries indication information of whether CAC succeeds. This further reduces signaling overheads and simplifies a scanning procedure of the STA. 
     Specifically, as shown in  FIG. 4 , the method includes the following steps. 
     Step  401 : A first access point receives channel information from at least one neighbor access point whose operating channel is a DFS channel. 
     The channel information from each neighbor access point is used to indicate an operating channel of a corresponding access point, and whether the corresponding access point successfully performs CAC. 
     For example, the AP 1  receives channel information from a neighbor AP 2  on a DFS channel and channel information from a neighbor AP 3  on a DFS channel. It is known that the DFS channel on which the AP 2  is located is a first channel, and the DFS channel on which the AP 3  is located is a second channel. In this case, a channel on which the AP 1  is located is a third channel, and the third channel is an operating channel of the AP 1  and a STA. 
     After switching to the second channel, the AP 2  performs CAC on the second channel, and sends, to the AP 1 , an indication about the second channel and an indication of whether CAC on the second channel succeeds. Similarly, after switching to the second channel, the AP 3  performs CAC on the second channel, and sends, to the AP 1 , an indication about the second channel and an indication of whether CAC on the second channel succeeds. 
     In addition, optionally, the AP 1  further receives a channel indication sent by an AP located on a non-DFS channel. For example, an AP 4  is located on a non-DFS channel, and it is known that the non-DFS channel is a fourth channel. In this case, the AP 4  sends an indication about the fourth channel to the AP 1  after switching to the fourth channel. 
     Step  402 : The first access point sends a channel list to the station associated with the first access point. Correspondingly, the station receives the channel list from the first access point. 
     The channel list includes only an indication about an operating channel of a neighbor access point that successfully performs CAC and an indication about an operating channel of a neighbor access point whose operating channel is a non-DFS channel. 
     Specifically, the AP 1  first receives channel information from all neighbor access points. The channel information includes the channel information from the AP 2  on the DFS channel, the channel information from the AP 3  on the DFS channel, and channel information from the AP 4  on the non-DFS channel. Then, channel information from all APs that successfully perform CAC is filtered. In this embodiment, the AP 2  successfully performs the CAC, but the AP 3  fails to perform the CAC. Finally, the AP 1  sends the channel list to the STA. The channel list includes the first channel (DFS channel) on which the AP 2  is located and the fourth channel (non-DFS channel) on which the AP 4  is located. 
     Optionally, the channel list may be carried in the foregoing AP channel report information element, or may be carried in a WLAN frame. 
     Step  403 : The station scans channels in the channel list according to the channel list. The channel list includes the first channel on which the AP 2  is located and the fourth channel on which the AP 4  is located. 
     For example, the STA scans the first channel based on the first channel indicated by the channel list. A specific scanning process is the same as step  203  in the foregoing embodiment, and details are not described herein again. Similarly, after scanning the first channel, the STA returns to the third channel on which the AP 1  is located, and then switches to the fourth channel to scan the fourth channel. Specifically, for a process in which the STA scans the fourth channel, refer to an existing process in which the STA scans a non-DFS channel. Details are not described in this embodiment. 
     In addition, if the AP 1  receives indication information from the AP 3  indicating that the CAC succeeds, the AP 1  adds the second channel on which the AP 3  is located to the channel list, and delivers the channel list to the STA, so that the STA scans the second channel. 
     In this embodiment, for a station associated with a neighbor AP that fails to perform CAC, the associated AP 1  delivers, to the station only after the neighbor AP successfully performs CAC, information about a DFS channel on which the neighbor AP is located, instead of delivering channel information to the station. Compared with Embodiment 1 in which the WLAN frame carries indication information of whether a neighbor AP performs CAC, this embodiment reduces content of a message sent to the STA. In this way, transmission overheads are reduced, and a procedure of the STA is simplified. 
     The following describes apparatus embodiments corresponding to the foregoing method embodiments. 
       FIG. 5  is a schematic diagram of a structure of an apparatus according to an embodiment of this application. The apparatus may be a channel indication sending apparatus or a channel scanning apparatus, and is configured to implement the methods in  FIG. 2  and  FIG. 4  in the foregoing embodiments. 
     Further, as shown in  FIG. 5 , the apparatus may include a receiving unit  501 , a processing unit  502 , and a sending unit  503 . In addition, the apparatus may further include a storage unit or another unit or module. This is not limited in this embodiment. 
     When the apparatus is used as a channel indication sending apparatus, for example, the sending apparatus is a first access point, the receiving unit  501  is configured to receive channel information from a second access point. The channel information indicates that an operating channel of the second access point is a first channel, and whether the second access point successfully performs CAC on the first channel, the second access point is a neighbor of the first access point, and the first channel is a DFS channel. 
     The sending unit  503  is configured to send, on a second channel, a WLAN frame to a station associated with the first access point. The second channel is an operating channel of the first access point. The WLAN frame includes an indication about the first channel and an indication of whether the second access point successfully performs the CAC on the first channel. 
     Optionally, the first channel is in a channel report information element. 
     When the apparatus is used as a channel scanning apparatus, for example, the channel scanning apparatus is a station, 
     the receiving unit  501  is configured to receive a WLAN frame from a first access point. The WLAN frame includes an indication about a first channel on which a second access point operates and an indication of whether the second access point successfully performs CAC on the first channel, and the first channel is a DFS channel. 
     The processing unit  502  is configured to scan, based on the indication about the first channel and the indication of whether the CAC succeeds, the first channel only if the second access point successfully performs the CAC on the first channel. 
     Optionally, in a specific implementation, the processing unit  502  is specifically configured to: switch to the first channel; send a probe request to the second access point on the first channel by using the sending unit  503 ; and after the receiving unit  501  receives a probe response returned by the second access point, completes scanning on the first channel, and returns to the operating channel of the first access point. 
     When the apparatus is used as a channel indication sending apparatus, for example, the sending apparatus is a first access point, the receiving unit  501  is configured to receive channel information from at least one neighbor access point whose operating channel is a DFS channel. The channel information from each neighbor access point is used to indicate an operating channel of a corresponding access point, and whether the corresponding access point successfully performs CAC. The sending unit  503  is configured to send a channel list to a station associated with the first access point. The channel list includes only an indication about an operating channel of a neighbor access point that successfully performs CAC and an indication about an operating channel of a neighbor access point whose operating channel is a non-DFS channel. 
     During specific implementation, an embodiment of this application further provides an access point.  FIG. 6  is a schematic diagram of a structure of an access point according to an embodiment of this application. The access point may be any one of an AP 1 , an AP 2 , and an AP 3 , to implement the functions or operations of the access point in the foregoing embodiments. 
     As shown in  FIG. 6 , the access point may include a first transceiver  601 , a second transceiver  602 , a processor  603 , a memory  604 , and a bus  605 . In addition, the access point may further include more or fewer components such as a power supply module and an antenna, combine some components, or have different component arrangements. This is not limited in this application. 
     The first transceiver  601  is configured to communicate with another neighbor access point. Specifically, the first transceiver  601  is configured to receive channel information from a second access point. The channel information indicates that an operating channel of the second access point is a first channel, and whether the second access point successfully performs CAC on the first channel. 
     The second transceiver  602  is configured to communicate with an associated station. Specifically, the second transceiver  602  is configured to send a WLAN frame to the station on a second channel. The WLAN frame includes an indication about the first channel and an indication of whether the second access point successfully performs the CAC on the first channel. 
     In addition, in the method procedure shown in  FIG. 4 , the first transceiver  601  is configured to receive channel information from at least one neighbor access point whose operating channel is a DFS channel. The channel information from each neighbor access point is used to indicate an operating channel of a corresponding access point, and whether the corresponding access point successfully performs CAC. 
     The second transceiver  602  is configured to send a channel list to a station associated with the first access point. The channel list includes only an indication about an operating channel of a neighbor access point that successfully performs CAC and an indication about an operating channel of a neighbor access point whose operating channel is a non-DFS channel. 
     The processor  603  is configured to control the first transceiver  601  and the second transceiver  602  to perform the method procedures performed by the first access points in  FIG. 2  and  FIG. 4 . 
     During specific implementation, the first transceiver  601  is a wired communication interface, and the access point may be connected (directly connected, or connected by using a switch or an AC) to another AP through the wired communication interface. 
     The second transceiver  602  is configured to establish a communication channel, so that an access point implements communication transmission with another terminal device (for example, a STA) through the communication channel. Further, the second transceiver  602  includes a transceiver module, for example, may include a WLAN module, and a corresponding radio frequency (radio frequency, RF) circuit. The RF circuit is configured to perform WLAN communication. 
     The processor  603  is a control center of the access point, and connects to all components of the entire device through various interfaces and the communication bus  605 . The processor  603  runs or executes a software program or module stored in the memory  604 , and invokes data stored in the memory  604 , to perform various functions of the access point or process data. 
     The processor  603  may be a central processing unit (central processing unit, CPU), a network processor (network processor, NP), or a combination of a CPU and an NP. The processor  603  may further include a hardware chip. The hardware chip may be an application-specific integrated circuit (application-specific integrated circuit, ASIC), a programmable logic device (programmable logic device, PLD), or a combination thereof. The PLD may be a complex programmable logic device (complex programmable logic device, CPLD), a field-programmable gate array (field-programmable gate array, FPGA), a generic array logic (generic array logic, GAL), or any combination thereof. In addition, the processor  603  further includes an RF circuit and a baseband chip. 
     The memory  604  may include a volatile memory (volatile memory), for example, a random access memory (random access memory, RAM); or may include a non-volatile memory (non-volatile memory), for example, a flash memory (flash memory), a hard disk drive (hard disk drive, HDD), or a solid-state drive (solid-state drive, SSD). The memory may alternatively include a combination of the foregoing types of memories. The memory may store a program, code, or data. The processor  603  may implement functions of the access point by executing the program or the code. 
     Further, functions to be implemented by the receiving unit  501  and the sending unit  503  in  FIG. 5  may be implemented by the first transceiver  601  or the second transceiver  602  of the access point, or implemented by the first transceiver  601  and the second transceiver  602  that are controlled by the processor  603 . Functions to be implemented by the processing unit  502  may be implemented by the processor  603 . 
     In addition, an embodiment of this application further provides a computer storage medium. The computer storage medium may store program instructions. When the program instructions are executed, some or all of the steps in the embodiments of the channel indication sending method and the channel scanning method that are provided in this application may be performed. The computer storage medium may be the memory  604 . In addition, the program instructions stored in the memory  604  may be transmitted from one network node, computer, server, or data center to another node, computer, or server in a wired or wireless manner. 
     All or some of the foregoing embodiments may be implemented by using software, hardware, firmware, or any combination thereof. When software is used to implement the embodiments, all or some of the embodiments may be implemented in a form of a computer program product. 
     An embodiment of this application further provides a terminal device. The terminal device is configured to implement functions or operations of the stations in the foregoing embodiments. Further, as shown in  FIG. 7 , the station includes a third transceiver  701 , a processor  702 , a memory  703 , a bus  704 , and the like. The processor  702  is coupled to the memory  703  through the bus  704 . 
     Further, the third transceiver  701  is configured to receive a WLAN frame from a first access point. The WLAN frame includes an indication about a first channel on which a second access point operates and an indication of whether the second access point successfully performs CAC on the first channel, and the first channel is a DFS channel. The processor  702  is configured to scan, based on the indication about the first channel and the indication of whether the CAC succeeds, the first channel only if the second access point successfully performs the CAC on the first channel. 
     The memory  703  stores computer program instructions. When the processor  702  invokes and executes the computer program instructions, the station is enabled to implement the channel scanning method in  FIG. 2  or  FIG. 4 . 
     Further, the third transceiver  701  is configured to communicate with an access point that is associated with or is adjacent to the third transceiver  701 . Further, the third transceiver  701  may include a transceiver module, an antenna, and the like. A structure of the memory  703  is the same as a structure of the memory  604  shown in  FIG. 6 , and a structure of the processor  702  is the same as a structure of the processor  603  of the access point. Refer to the foregoing description of the structures of the processor  603  and the memory  604 . Details are not described again in this embodiment. 
     This embodiment further provides a WLAN system. The WLAN system may be the system shown in  FIG. 1 , and is configured to implement the method described in  FIG. 2  or  FIG. 4 . Specifically, the system includes at least two access points and at least one station. In addition, the system may further include other network devices, such as a switch and an AC. 
     In the specification, claims, and accompanying drawings of this application, the terms “first”, “second”, “third”, and so on are intended to distinguish between similar objects but do not necessarily indicate a specific order or sequence. It should be understood that the data termed in such a way are interchangeable in proper circumstances so that the embodiments described herein can be implemented in other orders than the order illustrated or described herein. 
     For same or similar parts in the embodiments, refer to these embodiments. Especially, the embodiments related to the network device/node or apparatus/device are basically similar to the method embodiments, and therefore are described briefly. For related parts, refer to the description in the method embodiments.