CHANNEL ACCESS METHOD FOR MULTI-LINK DEVICE AND RELATED APPARATUS.

Abstract

This application relates to the field of wireless communication technologies, specifically to a wireless local area network compliant with the 802.11be standard, and in particular, to a channel access method for a multilink device and related equipment. The method includes: when the length of a first PPDU transmitted on a first link by a first multilink device is less than or equal to a first value, skipping the start, via the first multilink device, of a media synchronization delay timer on a second link, wherein the first multilink device is not permitted to perform simultaneous transmission and reception on the first and second links. In the modalities of this application, channel access efficiency can be improved when the non-STR MLD is in a blind / self-interfering state.

Description

CHANNEL ACCESS METHOD FOR MULTI-LINK DEVICE AND RELATED APPARATUS This application claims priority from Chinese patent application No. 202010924423.8, filed with the National Intellectual Property Administration of China on September 4, 2020, and entitled CHANNEL ACCESS METHOD FOR MULTI-LINK DEVICE AND RELATED APPARATUS, which is incorporated herein by reference in its entirety. TECHNICAL FIELD This application relates to the field of wireless communication technologies and, in particular, to a channel access method for a multi-link device and related apparatus. BACKGROUND As wireless communication technologies develop, an increasing number of wireless communication devices support multi-link communication, for example, by communicating simultaneously on the 2.4 GHz, 5 GHz, and 6 GHz frequency bands, or by communicating simultaneously on different channels within the same frequency band. This type of wireless communication device is generally called a multi-link device (MLD). Obviously, a multi-link device can perform parallel communication using multiple links, thus greatly increasing the transmission rate. Although a multilink device can perform parallel communication by using multiple links to increase transmission speed, signal transmission in one frequency band affects signal reception in another when the frequency gap between multiple bands supported by an extremely high-throughput (EHT) multilink device is small. For example, if the EHT multilink device transmits on link 1, and the frequency separation between link 1 and link 2 is small, signal transmission on link 1 causes channel interference on link 2, affecting channel access and data reception on link 2.Therefore, the device is not permitted to independently perform simultaneous transmit and receive operations across multiple frequency bands to avoid mutual interference. In accordance with the current progress of the 802.11 TGbe standard group, the EHT multilink device is defined as having either simultaneous transmitting and receiving (STR) capability or non-simultaneous transmitting and receiving (non-STR) capability. When a multi-link device (MLD) with no STR capability (known as a non-STR MLD for short) transmits on a link, the non-STR MLD is in a blind state (a period of blindness or deafness) because interference affects the clear channel assessment (CCA) performed on another link. The blind state means that no information can be received on the channel. Therefore, when the non-STR MLD is in a blind state on some links, how the non-STR MLD accesses the channel on these links becomes a critical issue that needs to be addressed. BRIEF DESCRIPTION The modalities of this application provide a channel access method for a multilink device and related apparatus, to improve channel access efficiency when a non-STR MLD is in a blind / self-interference state. The following describes this request from different perspectives. It should be understood that the following implementations and beneficial effects of the different aspects can be mutually referenced. According to the first aspect, this application provides a channel access method for a multilink device. The method includes: When the length of a first PPDU transmitted on a first link by a first multilink device is less than or equal to a first value, the first multilink device does not initiate a half synchronization delay timer on a second link. The first multilink device cannot perform simultaneous transmissions and receptions on the first and second links. The first multilink device not initiating a half synchronization delay timer on a second link includes: When channel contention is performed on the second link, the first multilink device sets an energy sensing threshold that is used by the clear channel CCA evaluation performed on the second link to a first threshold, where the first threshold is -62 dBm; or after a backoff counter on the second link goes back to 0, the first multilink device is allowed to transmit a frame other than an RTS frame and a MU-RTS frame. In this solution, when the length of a PPDU transmitted on one link is less than or equal to a specific value, the half-synchronization delay timer is not started on the other link; or when channel contention occurs on the other link, the energy detection threshold used by CCA is set to -62 dBm; or there is no need to use an RTS frame on the other link to attempt to detect channel protection / availability. Therefore, the channel access efficiency or success rate of the first multilink device on the other link is improved, or the opportunities for channel access by the first multilink device on the other link are increased. With regard to the first aspect, in a possible implementation, the method also includes: The first multilink device receives the first value. The first value can be carried in a beacon frame, an association response frame, or a reassociation response frame. Optionally, the first value can be carried in a multi-link element, an extremely high-performance operation element, or a newly defined element. With reference to the first aspect, in a possible implementation, the method also includes: When the length of the first PPDU is greater than the first value, the first multilink device determines an initial value that is the middle synchronization delay timer and that corresponds to the length of the first PPDU, and starts the middle synchronization delay timer with the initial value on the second link. Optionally, the method also includes: The first multilink device receives the first indication information. The first indication information is used to indicate a mapping relationship between the length of a physical layer protocol data unit (PPDU) and an initial value of the medium's synchronization delay timer. In this solution, the initial value of the average synchronization delay timer is determined based on the length of the first PPDU, allowing for more flexible configuration of the average synchronization delay timer. With reference to the first aspect, in a possible implementation, the method also includes: When the length of the first PPDU is greater than the first value, the first multilink device starts the half synchronization delay timer on the second link; and during a period of time during which the half synchronization delay timer runs, if the first multilink device performs channel contention on the second link, the first multilink device sets the energy sensing threshold using CCA performed on the second link to a threshold corresponding to the length of the first PPDU. Optionally, before the first multilink device transmits the first PPDU on the first link, the method also includes: The first multilink device receives the second indication information. The second indication information is used to indicate a mapping relationship between a PPDU length and an energy detection threshold. In this solution, the energy detection threshold is determined based on the length of the first PPDU. In this case, a more flexible channel access mechanism is used in the second link, improving channel access efficiency. According to a second aspect, this application provides a first multilink device or a chip in the first multilink device, for example, a Wi-Fi chip. The first multilink device may be a non-STR MLD. The first multilink device includes a processing unit. The processing unit is configured to: when the length of a first PPDU transmitted on a first link by a first multilink device is less than or equal to a first value, skip the start of a half-time synchronization delay on a second link. The first multilink device cannot perform simultaneous transmissions and receptions on the first and second links. The processing unit is specifically configured to establish an energy detection threshold used by the Clear Channel Assessment (CCA) performed on the second link, up to a first threshold of -62 dBm. Alternatively, the first multilink device also includes a transceiver unit. This transceiver unit is configured to transmit, after a backoff counter on the second link resets to zero, a non-RTS frame and a MURTS frame. With regard to the second aspect, in a possible implementation, the first multilink device also includes the transceiver unit. The transceiver unit is further configured to receive the first value. The first value can be carried in a beacon frame, an association response frame, or a reassociation response frame. Optionally, the first value can be carried in a multi-link element, an extremely high-performance operation element, or a newly defined element. With reference to the second aspect, in a possible implementation, the processing unit is further configured to: when the length of the first PPDU is greater than the first value, determine an initial value of the middle synchronization delay timer that corresponds to the length of the first PPDU and start the middle synchronization delay timer with the initial value on the second link. Optionally, the first multilink device also includes the transceiver unit. The transceiver unit is further configured to receive the first indication information. This first indication information is used to establish a correspondence between a PPDU length and an initial value of the medium's synchronization delay timer. With reference to the second aspect, in a possible implementation, the processing unit is further configured to: when the length of the first PPDU is greater than the first value, start the half synchronization delay timer on the second link; and during a period of time during which the half synchronization delay timer runs, if the first multilink device performs channel contention on the second link, set the energy detection threshold using CCA performed on the second link to a threshold corresponding to the length of the first PPDU. Optionally, the first multilink device also includes the transceiver unit. The transceiver unit is further configured to receive the second indication information. This second indication information is used to define a mapping relationship between a PPDU length and an energy detection threshold. According to a third aspect, this application provides a channel access method for a multilink device. The method includes: When a first frame transmitted on a first link by a first multilink device is of a first type, the first multilink device does not initiate a half synchronization delay timer on a second link. The first multilink device cannot perform simultaneous transmissions and receptions on the first and second links. That the first multilink device does not initiate a half synchronization delay timer on a second link includes: When channel contention is performed on the second link, the first multilink device sets an energy sensing threshold that is used by clear channel assessment (CCA) to a first threshold, where the first threshold is -62 dBm; or after a backoff counter on the second link goes back to 0, the first multilink device is allowed to transmit a frame other than an RTS frame and a MU-RTS frame. Optionally, when the first frame is any of the following frames, the type of the first frame is the first type: a request to send (RTS) frame, a multiple user RTS (MU-RTS) frame, a power save-poll (PS-poll) frame, a CTS frame, a buffer status report (BSR) frame, a bandwidth query report (BQR) frame, a null data packet (NDP) frame, an acknowledgment (ACK) frame, and a block acknowledgment (BA) frame. Optionally, the first frame is a request to send (RTS) frame or a multiple user (MU-RTS) frame. If the first multilink device does not receive a clear to send (CTS) frame on the first link within a preset time period, the first multilink device does not start the half-sync delay timer on the second link. Optionally, the first frame is a Power Save Poll (PS Poll). If the first multilink device does not receive a data frame or an acknowledgment frame on the first link within a preset time period, the first multilink device does not start the half-synchronization delay timer on the second link. Optionally, the first frame is a CTS frame. Before the first multilink device transmits a first PPDU on the first link, the method also includes: The first multilink device receives an RTS frame or a MU-RTS frame on the first link. Optionally, the first multilink device receives a status report BSR frame. Before the first multilink device transmits its first PPDU on the first link, the method also includes: The first multilink device receives a status report polling BSRP activation frame on the first link. Optionally, the first frame is a BQR bandwidth query report frame. Before the first multilink device transmits a first PPDU on the first link, the method also includes: The first multilink device receives a BQRP bandwidth query report probe trigger frame. Optionally, the first frame is a null data packet NDP frame. Before the first multilink device transmits a first PPDU on the first link, the method also includes: The first multilink device receives a beamforming report probe BFRP activation frame on the first link. Optionally, the first frame is an ACK frame or a BA frame. Before the first multilink device transmits a first PPDU on the first link, the method also includes: The first multilink device receives a data frame or a management frame on the first link. According to a fourth aspect, this application provides a first multilink device or a chip in the first multilink device, for example, a Wi-Fi chip. The first multilink device may be a non-STR MLD. The first multilink device includes a processing unit. The processing unit is configured to: when the type of a first frame transmitted on a first link by a first multilink device is a first type, skip the start of a half synchronization delay timer on a second link. The first multilink device cannot perform simultaneous transmissions and receptions on the first and second links. The processing unit is specifically configured to establish an energy detection threshold used by the Clear Channel Assessment (CCA) performed on the second link, up to a first threshold of -62 dBm. Alternatively, the first multilink device also includes a transceiver unit. This transceiver unit is configured to transmit, after a backoff counter on the second link resets to zero, a non-RTS frame and a MURTS frame. Optionally, when the first frame is any of the following frames, the type of the first frame is the first type: a request to send (RTS) frame, a multiple user RTS (MU-RTS) frame, a power save-poll (PS-poll) frame, a CTS frame, a buffer status report (BSR) frame, a bandwidth query report (BQR) frame, a null data packet (NDP) frame, an acknowledgment (ACK) frame, and a block acknowledgment (BA) frame. Optionally, the first frame is an RTS frame or a MU-RTS frame. The processing unit is specifically configured to: when the first multilink device does not receive a clear-to-send (CTS) frame on the first link within a preset time period, skip the start of the half-synchronization delay timer on the second link. Optionally, the first frame is a PS-Poll frame. The processing unit is specifically configured so that when the first multilink device does not receive a PS-Poll frame ready to send on the first link within a preset time period, it skips the start of the middle synchronization delay timer on the second link. Optionally, the first frame is a CTS frame. The first multilink device also includes the transceiver unit. The transceiver unit is configured to receive either an RTS frame or an MU-RTS frame on the first link. Optionally, the first PPDU is a BSR status report frame. The first multilink device also includes the transceiver unit. The transceiver unit is configured to receive a BSRP status report probe trigger frame on the first link. Optionally, the first PPDU is a BQR bandwidth query report frame. The first multilink device also includes the transceiver unit. The transceiver unit is configured to receive a BQRP bandwidth query report probe trigger frame on the first link. Optionally, the first PPDU is a null data packet NDP frame. The first multilink device also includes the transceiver unit. The transceiver unit is configured to receive a beamforming report probe BFRP activation frame on the first link. Optionally, the first PPDU is an ACK frame or a BA frame. The first multilink device also includes the transceiver unit. The transceiver unit is configured to receive either a data frame or a management frame on the first link. According to a fifth aspect, this application provides a method for determining the initial duration of a half-time synchronization delay timer. The method includes: A first multilink device receives the first indication information, where the first indication information is used to establish a mapping relationship between a PPDU length and an initial value (or initial duration) of a medium synchronization delay timer. Based on the length of a first PPDU transmitted on a first link, the first multilink device determines an initial value of the medium synchronization delay timer that corresponds to the length of the first PPDU. This initial value is then used to determine whether to start the medium synchronization delay timer on a second link. The first multilink device cannot perform simultaneous transmissions and receptions on the first and second links. Optionally, the first multilink device determines, based on the initial value of the mean synchronization delay timer and corresponding to the length of the first PPDU, whether to start the mean synchronization delay timer on the second link. Optionally, if the initial value of the medium synchronization delay timer is 0, the first multilink device does not start the medium synchronization delay timer on the second link. If the initial value of the medium synchronization delay timer is 0, the first multilink device starts the medium synchronization delay timer with that initial value on the second link. The first multilink device initiating the mediumSyncDelay timer on the second link can be understood as (or described as): During the time the mediumSyncDelay timer is running, the first multilink device may use a more conservative channel access mechanism on the second link. This more conservative channel access mechanism includes, but is not limited to: (1) using a lower energy detection threshold (defined as an ED threshold below -62 dBm in this document) to determine if a channel is busy; and (2) requiring the transmission of an RTS frame to attempt to detect channel availability. In this solution, different PPDU lengths / byte lengths correspond to different initial values ​​of the mediumSyncDelay timer, allowing for more flexible configuration of the mediumSyncDelay timer and improving channel access efficiency. According to a sixth aspect, this application provides a method for determining the initial duration of a medium synchronization delay timer. The method includes: A second multilink device generates and transmits the first indication information. The first indication information is used to establish a correspondence between a PPDU length and an initial value (or initial duration) of the medium synchronization delay timer. According to a seventh aspect, this application provides a first multilink device or a chip in the first multilink device, for example, a Wi-Fi chip. The first multilink device may be a non-STR MLD. The communication apparatus includes: a transceiver unit, configured to receive first indication information, where the first indication information is used to indicate a mapping relationship between a PPDU length and an initial value of a medium synchronization delay timer; and a processing unit, configured to determine, based on the length of a first PPDU transmission sent on the first link, an initial value of the medium synchronization delay timer that corresponds to the length of the first PPDU. The communication apparatus cannot perform simultaneous transmissions and receptions on the first and second links. Optionally, the processing unit is also configured to determine, based on the initial value of the mean synchronization delay timer, which corresponds to the length of the first PPDU, whether to start the mean synchronization delay timer on the second link. Optionally, the processing unit is specifically configured to: if the initial value determined by the middle synchronization delay timer is equal to 0, skip the start of the middle synchronization delay timer on the second link; or if the initial value determined by the middle synchronization delay timer is equal to 0, start the middle synchronization delay timer on the second link. According to an eighth aspect, this application provides a second multilink device or a chip in the second multilink device, for example, a Wi-Fi chip. The second multilink device may or may not be an STR MLD. The communication apparatus includes: a processing unit, configured to generate the first indication information, where the first indication information is used to indicate a mapping relationship between a PPDU length and an initial value (or initial duration) of a medium synchronization delay timer; and a transceiver unit, configured to transmit the first indication information. According to a ninth aspect, this application provides a method for determining an energy sensing threshold in a CCA process. The method includes: A first multilink device receives second indication information, where the second indication information is used to indicate a mapping relationship between a PPDU length and an energy direction threshold. The first multilink device transmits a first PPDU on a first link. The first multilink device determines, based on the length of the first PPDU transmitted on the first link, an energy sensing threshold corresponding to the length of the first PPDU, where the energy sensing threshold is used to determine whether to start a media synchronization delay timer on a second link. The first multilink device cannot perform simultaneous transmissions and receptions on the first and second links. Optionally, the first multilink device determines, based on the energy detection threshold corresponding to the length of the first PPDU, whether to start the half synchronization delay timer on the second link. Optionally, if the determined energy detection threshold is equal to -62 dBm, the first multilink device does not start the half synchronization delay timer on the second link; or if the determined energy detection threshold is less than -62 dBm, the first multilink device starts the half synchronization delay timer on the second link. In this solution, different PPDU lengths / byte lengths correspond to different energy detection thresholds. In this case, a more flexible channel access mechanism is used on the second link, improving channel access efficiency. According to a tenth aspect, this application provides a method for determining an energy detection threshold in a CCA process. The method includes: A second multilink device generates and transmits second indication information. This second indication information is used to indicate a mapping relationship between a PPDU length and an energy detection threshold. According to an eleventh aspect, this application provides a first multilink device or a chip in the first multilink device, for example, a Wi-Fi chip. The first multilink device may be a non-STR MLD. The communication apparatus includes: a transceiver unit, configured to receive second indication information, wherein the second indication information is used to indicate a mapping relationship between a PPDU length and an energy sensing threshold; and a processing unit, configured to determine, based on the length of a first PPDU transmitted on a first link, an energy sensing threshold corresponding to the length of the first PPDU, wherein the energy sensing threshold is used to determine whether to initiate a synchronization delay timer medium on a second link.The first multilink device cannot perform simultaneous transmissions and receptions on the first link and the second link. Optionally, the processing unit is also configured to determine, based on the energy detection threshold corresponding to the length of the first PPDU, whether to start the half-time synchronization delay timer on the second link. The communication device cannot perform simultaneous transmissions and receptions on the first and second links. Optionally, the processing unit is specifically configured to: if the determined energy detection threshold is equal to -62 dBm, skip starting the half synchronization delay timer on the second link; or if the determined energy detection threshold is less than -62 dBm, start the half synchronization delay timer on the second link. According to a twelfth aspect, this application provides a second multilink device or a chip in the second multilink device, for example, a Wi-Fi chip. The second multilink device may be an STR MLD. The communication apparatus includes: a processing unit, configured to generate second indication information, where the second indication information is used to indicate a mapping relationship between a PPDU length and an energy detection threshold; and a transceiver unit, configured to transmit the second indication information. According to a thirteenth aspect, this application provides a first multilink device, which includes a processor. Optionally, a transceiver is also included. The processor is configured to: when the length of a first PPDU transmitted on a first link by a first multilink device is less than or equal to a first value, skip the start of a half-synchronization delay timer on a second link. The first multilink device cannot perform simultaneous transmissions and receptions on the first and second links. In one possible design, the processor is configured to: when the type of the first frame transmitted on the first link by the first multilink device is a first type, skip the start of a half-synchronization delay timer on the second link. The first multilink device cannot perform simultaneous transmissions and receptions on the first and second links. In one possible design, the transceiver is configured to receive first indication information, where the first indication information is used to indicate a mapping relationship between a PPDU length and an initial value (or initial duration) of the medium synchronization delay timer; and the processor is configured to determine, based on the length of the first PPDU transmission sent on the first link, an initial value of the medium synchronization delay timer that corresponds to the length of the first PPDU. The communication apparatus cannot perform simultaneous transmissions and receptions on the first and second links. In one possible design, the transceiver is configured to receive a second indication piece of information, where the second indication piece is used to indicate a mapping relationship between a PPDU length / byte length and an energy sensing threshold; and the processor is configured to determine, based on the length of a first PPDU transmitted on a first link, an energy sensing threshold corresponding to the length of the first PPDU, where the energy sensing threshold is used to determine whether to initiate a synchronization delay timer half on a second link. The first multilink device cannot perform simultaneous transmissions and receptions on the first and second links. According to a fourteenth aspect, this request provides a second multilink device, which includes a processor and a transceiver. The processor is configured to generate the first indication information, where the first indication information is used to indicate a mapping relationship between a PPDU length and an initial value (or initial duration) of a medium synchronization delay timer. The transceiver is configured to transmit the first indication information. In a possible design, the processor is configured to generate a second indication information, where the second indication information is used to indicate a mapping relationship between a PPDU length and an energy detection threshold; and the transceiver is configured to transmit the second indication information. According to a fifteenth aspect, this application provides for a first multilink device. The first multilink device may exist as a single-chip product. The structure of the first multilink device includes an input / output interface and a processing circuit. The input / output interface is configured to receive code instructions and transmit them to the processing circuit. The processing circuit is configured to: when the length of a first PPDU transmitted on the first link is less than or equal to a first value, omit the start of a media synchronization delay timer on the second link. The first multilink device cannot perform simultaneous transmissions and receptions on the first and second links. In one possible design, the input / output interface is configured to receive code instructions and transmit them to the processing circuit. The processing circuit is configured to: when a frame type in the first link transmitted by the first multilink device is of type 1, skip the start of the synchronization delay timer on the second link. The first multilink device cannot perform simultaneous transmissions and receptions on the first and second links. In a possible design, a transceiver is configured to receive the first indication information, and the input / output interface is configured to receive the first indication information from the transceiver and transmit it to the processing circuit for processing. This process establishes a mapping relationship between a PPDU length and an initial value (or initial duration) of the medium synchronization delay timer, as indicated by the first indication information. The processing circuit is then configured to determine, based on the length of the first PPDU transmitted on the first link, an initial value for the medium synchronization delay timer that corresponds to the length of the first PPDU. This initial value is used to determine whether to start the medium synchronization delay timer on the second link.The first multilink device cannot perform simultaneous transmissions and receptions on the first link and the second link. In a possible design, the transceiver is configured to receive the second indication information, and the input / output interface is configured to receive this second indication information from the transceiver and transmit it to the processing circuit. This circuit establishes a mapping relationship between a PPDU length and an energy sensing threshold, as indicated by the second indication information. The processing circuit is then configured to determine, based on the length of the first PPDU transmitted on the first link, a corresponding energy sensing threshold. This energy sensing threshold is used to determine whether to initiate the mid-synchronization delay timer on the second link. The first multilink device cannot perform simultaneous transmissions and receptions on both the first and second links. According to a sixteenth aspect, this application provides a first multilink device. The second multilink device may exist as a one-chip product. The structure of the second multilink device includes an input / output interface and a processing circuit. The input / output interface is configured to receive code instructions and transmit them to the processing circuit. The processing circuit is configured to generate the first indication information, where the first indication information is used to indicate a mapping relationship between a PPDU length and an initial value (or initial duration) of a half-synchronization delay timer. The input / output interface is configured to transmit the first indication information to a transceiver. The transceiver is configured to transmit the first indication information. In a possible design, the input / output interface is configured to receive code instructions and transmit the code instructions to the processing circuit; the processing circuit is configured to generate a second indication information, where the second indication information is used to indicate a mapping relationship between a PPDU length and an energy detection threshold; the input / output interface is configured to transmit the second indication information to the transceiver; and the transceiver is configured to transmit the second indication information. According to a seventeenth aspect, this application provides a computer-readable storage medium. The computer-readable storage medium stores instructions, and when the instructions are executed on a computer, the computer is enabled to perform the method according to the first aspect, the third aspect, the fifth aspect, the seventh aspect, the ninth aspect, or the tenth aspect. According to an eighteenth aspect, this application provides a computer program product that includes program instructions. When the program instructions are executed on a computer, the computer is enabled to perform the method according to the first, third, fifth, seventh, ninth, or tenth aspect. When implementing modalities of this request, channel access efficiency can be improved when the non-STR MLD is in a blind / self-interference state. BRIEF DESCRIPTION OF THE DRAWINGS To describe the technical solutions in the modalities of this application more clearly, the following briefly describes the attached drawings to illustrate the modalities. FIG. 1 is a schematic diagram of communication between a non-AP MLD and an AP MLD according to one modality of this request. FIG. 2 is a schematic diagram of a wireless communication system architecture according to one modality of this application. FIG. 3a is a schematic diagram of a multilink device structure according to one modality of this application. FIG. 3b is a schematic diagram of another structure of a multilink device according to one modality of this application. FIG. 4 is a schematic flowchart of a channel access method for a multilink device according to one modality of this application. FIG. 5 is another schematic flowchart of a channel access method for a multilink device according to one modality of this application. FIG. 6a is a schematic diagram of a frame structure of a multilinked element according to one modality of this application. FIG. 6b is a schematic diagram of a frame structure of an EHT operating element according to a modality of this application. FIG. 6c is a schematic diagram of a frame structure of a non-STR MLD parameter set element according to a modality of this application. FIG. 7 is a flowchart of a method for determining the initial duration of a media synchronization delay timer according to a modality of this application. FIG. 8 is a schematic diagram of a mapping relationship between a PPDU length and an initial value of a medium synchronization delay timer according to a modality of this application. FIG. 9 is a schematic flowchart of a method for determining an energy direction threshold in a COA process according to a modality of this application. FIG. 10 is a schematic diagram of a mapping relationship between a PPDU length and an energy detection threshold according to a modality of this application. FIG. 11 is a schematic diagram of a structure of a first multilink device according to one modality of this application. FIG. 12 is a schematic diagram of a structure of a second multilink device according to one modality of this application. DESCRIPTION OF THE MODALITIES The following clearly and completely describes the technical solutions in the forms of this application with reference to the drawings attached in the forms of this application. To facilitate understanding of a channel access method for a multilink device provided in the modalities of this request, a system architecture and / or application scenario for the channel access method for a multilink device provided in the modalities of this request is described below. It is understood that the system architecture and / or scenario described in the modalities of this request are intended to describe the technical solutions in the modalities of this request more clearly and do not constitute a limitation on the technical solutions provided in the modalities of this request. The modalities in this application provide a channel access method applicable to a non-STR MLD. Using this method, channel access efficiency can be improved when the non-STR MLD is in a blind / self-interfering state. The channel access method for a multi-link device can be implemented by a communication device within the wireless communication system or by a chip or processor within the communication device. The communication device can be a wireless communication device that supports parallel transmission over multiple links. For example, the communication device might be referred to as a multi-link device or a multi-band device. Compared to a communication device that only supports single-link transmission, the multi-link device offers higher transmission efficiency and throughput. The multilink device includes one or more affiliated stations (STAs). An affiliated station is a logical station and can operate on a link, a frequency band, or a channel. An affiliated station can be an access point (AP) or a non-access point station (non-AP STA). For ease of description, a multilink device whose affiliated station is an AP is referred to as a multilink AP, multilink AP device, or AP multi-link device (AP MLD), and a multilink device whose affiliated station is a non-AP STA is referred to as a non-AP multi-link device, non-AP multi-link device, or non-AP multi-link device (Non-AP MLD) in this application. Optionally, a multi-link device may include a plurality of logical stations; each logical station operates on one link, but a plurality of logical stations are allowed to operate on the same link. Optionally, one or more STAs in a non-AP MLD can establish an association relationship with one or more APs in an AP MLD and then perform communication. Figure 1 is a schematic diagram of communication between a non-AP MLD and an AP MLD according to one modality of this application. As shown in Figure 1, the AP MLD includes AP1, AP2, ..., and APn, and the non-AP MLD includes STA1, STA2, ..., and STAn. The AP MLD and the non-AP MLD can communicate in parallel using link 1, link 2, ..., and link n. STA1 in the non-AP MLD establishes an association relationship with AP1 in the AP MLD; STA2 in the non-AP MLD establishes an association relationship with AP2 in the AP MLD; STAn in the non-AP MLD establishes an association relationship with APn in the AP MLD; and so on. Optionally, the multi-link device can comply with the IEEE 802.11 series protocols to implement wireless communication. For example, a station that meets Extremely High Throughput (EHT) standards or a station that complies with or supports IEEE 802.11be implements communication with another device. The channel access method for a multi-link device provided in the modalities of this application can be applied to a scenario where one node communicates with one or more nodes, a single-user uplink / downlink communication scenario, a multi-user uplink / downlink communication scenario, or a device-to-device (D2D) communication scenario. Any of the aforementioned nodes can be an MLD AP or a non-MLD AP; for example, a scenario where an MLD AP communicates with a non-MLD AP, or a scenario where an MLD AP communicates with another MLD AP, or a scenario where a non-MLD AP communicates with another non-MLD AP. This is not limited to the modalities of this application. Optionally, in any of the above scenarios, there is at least one node that is not allowed to perform simultaneous transmission and reception, i.e., it is not STR capable. Optionally, for ease of description, the system architecture in this application is described below using the scenario where an MLD AP communicates with a non-MLD AP as an example. The channel access method for a multilink device provided in the modalities of this application can be applied to a wireless local area network (WLAN). Figure 2 is a schematic diagram of a wireless communication system architecture according to one modality of this application. As shown in Figure 2, the wireless communication system includes at least one MLD AP and at least one non-MLD AP. The MLD AP is a multilink device that provides service to the non-MLD AP. The non-MLD AP can communicate with the MLD AP using multiple links. An AP on the MLD AP can communicate with a STA on the non-MLD AP using one link.It can be understood that the number of AP MLDs and the number of non-AP MLDs in FIG. 2 are merely examples. Optionally, the wireless communication system includes at least one MLD that is capable of non-STR. For example, a multilink device (which may be a non-AP MLD or an AP MLD in this document) is an appliance with a wireless communication function. The appliance may be an integrated device, or it may be a chip, processing system, or similar component installed in the integrated device. A device in which the chip or processing system is installed may implement, under the control of the chip or processing system, the method and functions provided in the modalities of this application. For example, the non-AP multilink device in the modalities of this application has a wireless transceiver function, may support the 802.11 series protocols, and may communicate with the AP multilink device or another non-AP multilink device. For example, the non-AP multilink device is any user communication device that allows a user to communicate with an AP and then communicate with a WLAN.For example, a non-AP multilink device can be a user device that connects to a network, such as a tablet, desktop computer, laptop, small notebook computer, ultra-mobile personal computer (UMPC), handheld computer, mini PC, personal digital assistant (PDA), or mobile phone; it can be an Internet of Things (IoT) node in the Internet of Things; or it can be a vehicle-mounted communication device in the Internet of Vehicles (IoV). Alternatively, the non-AP multilink device can be a chip and processing system in the aforementioned terminals. The AP multilink device can be a device that provides service to the non-AP multilink device and can be compatible with 802.11 series protocols.For example, a multi-link access point (AP) device can be a communication entity such as a communications server, router, switch, or network bridge. Alternatively, it can include various types of macro base stations, micro base stations, repeater stations, and the like. A multi-link AP device can also be a chip or processing system within these various device forms. The 802.11 protocol can be either 802.11be or 802.11be compatible. It can be understood that the multi-link device can support high-speed, low-latency transmission. With the continuous evolution of wireless local area network application scenarios, the multi-link device can be applied to more scenarios, for example, a sensor node (e.g., a smart meter, a smart electricity meter, and a smart air detector node) in a smart city, a smart device (e.g., a smart camera, a projector, a display, a television, a stereo, a refrigerator, and a washing machine) in a smart home, a node in the Internet of Things, an entertainment terminal (e.g., a wearable device such as AR and VR), a smart device (such as a printer and a projector) in a smart office, a IoT device in the Internet of Vehicles, and some infrastructure (e.g., a vending machine, a supermarket self-service navigation station,a self-service cash register device and a self-service ordering machine) in everyday life scenarios. A specific form of the multi-link device is not limited in this modality of this application, and only examples are provided herein. Optionally, FIG. 2 is a schematic diagram of a multilink device structure according to one modality of this application. The 802.11 IEEE standard focuses on an 802.11 physical layer (PHY) and an 802.11 media access control (MAC) layer in a multilink device. As shown in FIG. 3a, a plurality of STAs included in the multilink device are independent of each other at a low MAC layer and a PHY layer, and are also independent of each other at a high MAC layer. FIG. 3b is a schematic diagram of another multilink device structure according to one modality of this application. As shown in FIG. 3b, a plurality of STAs included in the multilink device are independent of each other at a low MAC layer and a PHY layer, and share a high MAC layer.Certainly, a non-AP multilink device can use a structure in which the high MAC layers are independent of each other, or it can use a structure in which the high MAC layers are shared. Similarly, an AP multilink device can use a structure in which the high MAC layers are shared, or it can use a structure in which the high MAC layers are independent of each other. A schematic diagram of the internal structure of a multilink device is not limited in the modalities of this application. Figures 3a and 3b are simply examples for illustrative purposes. For example, the high MAC layer or the low MAC layer can be implemented by a single processor on a multilink device's system-on-a-chip, or they can be implemented by different processing modules on a system-on-a-chip. For example, the multilink device in the modalities of this application may be a single-antenna device or a multi-antenna device, for example, a device with more than two antennas. The number of antennas included in the multilink device is not limited in the modalities of this application. In the modalities of this application, the multilink device may allow services of the same access category (AC) to be transmitted over different links, and may even allow the same data packet to be transmitted over different links; and alternatively, it may not allow services of the same access category to be transmitted over different links, but may allow services of different access categories to be transmitted over different links. A frequency band in which the multi-link device operates may include one or more frequency bands of less than 1 GHz, 2.4 GHz, 5 GHz, 6 GHz and a high frequency of 60 GHz. For a non-STR MLD, when the non-STR MLD transmits on one link (e.g., link 1), channel interference causes the non-STR MLD to incorrectly determine the states of channels on another link or links (link 2 is used as an example). This affects the reception of an Overlapped Basic Service Set (OBSS) frame on link 2 by the non-STR MLD. The OBSS frame is used by a station to update a network allocation vector (NAV). Therefore, before the transmission on one link is complete, the non-STR MLD may miss an OBSS frame on another link and thus miss the NAV update. In this case, when the non-STR MLD performs channel contention on link 2 and accesses a channel after the transmission on link 1 has finished, the data transmitted on link 2 collides with a received OBSS frame.This is known as a blind problem or a self-interference problem. The NAV can be understood as a countdown timer that gradually decreases over time. When the NAV is 0, a medium is considered to be in an idle state. Specifically, after a station receives a frame, if the frame's receive address is not the station, the station can update its NAV based on the duration field in the received frame. If the frame's receive address is the station, it indicates that the station is a receiving station. In this case, the station cannot update its NAV. Before updating the NAV, the station can further determine if the duration field value in the current frame is greater than the station's current NAV value. If the duration field value in the current frame is greater than the station's current NAV value, the station updates its NAV.If the duration field value in the current frame is less than or equal to the station's current NAV value, the station does not update the NAV. The comparison uses a NAV value existing at a time when frame reception ends. To address the blind spot problem of the non-STR MLD, one modality of this application proposes a medium sync delay mechanism. Specifically, the mechanism works as follows: After transmitting on one link (e.g., link 1), the non-STR MLD needs to start a timer on another link, i.e., a medium sync delay timer. During a period of time specified by the medium sync delay timer, the non-STR MLD needs to use a more conservative channel access mechanism on link 2. The more conservative channel access mechanism includes, but is not limited to: (1) Whether a channel is busy is determined using a lower energy detection (ED) threshold. In a channel access mechanism, -62 dBm is typically used as the energy detection threshold. If the energy on a channel is detected to exceed this threshold, i.e., exceeds -62 dBm, the channel is considered busy.When using an ED threshold lower than -62 dBm, a more distant signal causes a channel to be occupied by CCA detection. Therefore, channel access is more conservative. The lower energy detection threshold can be -82 dBm, -72 dBm, or similar. (2) A request to send (RTS) frame must be transmitted to attempt to detect channel availability. Optionally, there can be only one attempt (or only one transmission of the RTS frame) or a limited number of attempts. In the medium synchronization delay mechanism, whenever the non-STR MLD transmits on link 1, it uses the most conservative channel access mechanism on link 2, regardless of the frame type transmitted by the non-STR MLD on link 1. However, frames transmitted by the non-STR MLD on link 1 are of various types and can include a control frame, a data frame, or a management frame, and the data frame can be either long or short. Therefore, when the non-STR MLD transmits a short frame on link 1, it is in a blind state on link 2 for a brief period, and consequently, there is a low chance (or probability) that the non-STR MLD will lose important information (e.g., a NAV) on link 2.In conclusion, in the medium synchronization delay mechanism, whenever the non-STR MLD transmits on link 1, channel access for the non-STR MLD on link 2 must be restricted. This results in low channel access efficiency, a low channel access success rate, or decreased channel access opportunities on link 2. A non-STR MLD being in a blind state on a link in this request can be alternatively understood as an STA that is in the non-STR MLD and operating on the link being in a blind state. It may be understood that the blind state mentioned in this application may also be called a state of self-interference, a state of inability to receive, a state of deafness, or similar. It can be understood that the non-STR MLD in this request may be an EHT MLD that is not allowed to perform simultaneous transmission and reception. It can be understood that the long frame and short frame mentioned in this application are distinguished from each other by the length of time a frame occupies an air interface. For example, a long frame might be a frame that occupies an air interface for a time greater than or equal to a preset value A, and a short frame might be a frame that occupies an air interface for a time less than or equal to a preset value B. The preset value A and the preset value B might be the same or different. For example, preset value A might be 1 ms (milliseconds) and preset value B might be 100 ps (microseconds). One modality of this request provides a channel access method for a multilink device, to improve the efficiency of channel access or the channel access success rate of a non-STR MLD on some links or to increase the channel access opportunities of the non-STR MLD on these links when the non-STR MLD is in a blind / self-interfering state on these links. The technical solutions provided in this application are described in detail below, with reference to further attached drawings. It can be understood that, in this application, a first multilink device may be a non-STR MLD and a second multilink device may be an STR MLD. For ease of further explanation, this application uses as an example a scenario in which two MLDs communicate over two or more links. In the following modalities, two links are used as an example to describe the technical solutions in this application. However, the technical solutions in this application are also applicable to two MLDs that support multiple links. The technical solutions provided in this request are developed using Modes 1 through 4. Mode 1 details how to perform channel access on one link when a frame of a specific type is transmitted on another link. Mode 2 explains in detail how to determine, based on the length of a frame transmitted on one link, whether a more conservative channel access mechanism should be used on another link. Mode 3 details how to determine the initial duration of a mediumSyncDelay timer. Mode 4 details how to determine an ED threshold used in a CCA process. The following describes Modalities 1 through 4 in detail separately. It may be understood that the technical solutions described in Modalities 1 through 4 of this application may be combined arbitrarily to form a new modality. Mode 1 In Modality 1 of this application, we describe how to determine, based on the type of frame transmitted on one link, whether it is necessary to use a more conservative channel access mechanism on another link. Figure 4 is a schematic flowchart of a channel access method for a multilink device according to one modality of this application. As shown in Figure 4, the channel access method for a multilink device includes, but is not limited to, the following step. S101: When a type of a first frame transmitted on a first link by a first multilink device is a first type, the first multilink device does not start a half synchronization delay timer on a second link, where the first multilink device is not allowed to perform simultaneous transmissions and receptions on the first link and the second link. When the first frame is any of the following frames, the type of the first frame is the first type: a request to send frame, a multiple user RTS frame, a power save-poll (PS-poll) frame, a clear to send (CTS) frame, a buffer status report (BSR) frame, a bandwidth query report (BQR) frame, a null data packet (NDP) frame, an acknowledgment (ACK) frame, and a block acknowledge (BA) frame. In a first implementation, the first frame is either an RTS frame or a MURTS frame. Specifically, if the first multilink device transmits an RTS frame or a MU-RTS frame on the first link but does not receive a forwarding-ready frame within a predetermined time period, the first multilink device does not initiate the medium sync delay timer on the second link. The first multilink device cannot perform simultaneous transmissions and receptions on the first and second links.In other words, if the first multilink device does not receive a CTS frame that is on the first link within the preset time period (for example, the sum of the length of a short inter-frame space (SIFS), a length of a slot, and a physical layer receive delay, i.e., a SIFS time + a slot time + an RxPHYStartDelay) after transmitting the RTS / MU-RTS frame on the first link, the first multilink device does not start the mediumSyncDelay timer on the second link. The fact that the first multilink device does not initiate the mediumSyncDelay timer on the second link can be understood (or described as): When the first multilink device performs channel contention on the second link, a power detection threshold used by a CCA operation is a first threshold; or after a backoff counter on the second link resets to 0, the first multilink device is allowed to directly transmit a frame other than an RTS frame and a MU-RTS frame. In other words, after the backoff counter on the second link resets to 0, the first multilink device does not transmit an RTS / MU-RTS frame to attempt to detect channel protection / availability. The first threshold can be -62 dBm. It can be understood that the reasons why the first multilink device does not receive a CTS frame within the preset time period (e.g., a SIFS time + an interval time + an RxPHYStartDelay) can be the following: (a) The RTS frame transmitted by the first multilink device collides with a frame transmitted by another device; (b) a receiver corresponding to the RTS frame transmitted by the first multilink device cannot successfully receive the RTS frame; and (c) a receiver corresponding to the RTS frame transmitted by the first multilink device is in a busy state. Optionally, if the first multilink device starts the mediumSyncDelay timer on the second link after transmitting the RTS / MU-RTS frame on the first link, and the first multilink device does not receive a CTS frame within the preset time period, the first multilink device closes (or stops or cancels) the mediumSyncDelay timer. Optionally, if the first multilink device receives a CTS frame within the preset time period, it can initiate the mediumSyncDelay timer. The first multilink device initiating the mediumSyncDelay timer on the second link can be understood as (or described as): The first multilink device uses a more conservative channel access mechanism on the second link. Specifically, a lower power detection threshold (a threshold below -62 dBm, for example, -82 dBm) is used to determine if a channel is busy, and an RTS / MU-RTS frame must be transmitted once to detect channel availability. Optionally, there can be only one attempt (or only one transmission of the RTS / MU-RTS frame) or a limited number of attempts. Optionally, the RTS or MU-RTS frame in the first implementation can be replaced with a Power Save Poll (PS-Poll), and the CTS frame in the first implementation can be replaced with a Data Frame or an Acknowledge (ACK) frame. Therefore, the first implementation can be alternatively described as follows: If the first multilink device transmits a PS-Poll frame on the first link but does not receive a Data Frame or an Acknowledge frame within a preset time period, the first multilink device does not start the medium Sync Delay timer on the second link. Optionally, if the first multilink device transmits a PS-Poll frame on the first link and receives a Data Frame or an Acknowledge frame within the preset time period, the first multilink device can start the mediumSyncDelay timer. It can be learned that when a non-STR MLD (i.e., the first multilink device) in this application mode transmits an RTS (or MU-RTS) frame on the first link but does not receive a CTS frame, the non-STR MLD does not initiate the mediumSyncDelay timer on the second link. Therefore, the non-STR MLD performs common channel contention on the second link; that is, the energy detection threshold used by the CCA operation is -62 dBm, or RTS / CTS frames cannot be used to attempt to detect channel protection. Thus, the channel access efficiency or success rate of the non-STR MLD on the second link is improved, or the channel access opportunities for the non-STR MLD on the second link are increased. In a second implementation, the first frame is a CTS frame. Specifically, if the first multilink device receives an RTS frame or a MU-RT frame on the first link and responds with / transmits an RTS frame on the first link, the first multilink device does not start the medium SyncDelay timer on the second link. The first multilink device cannot perform simultaneous transmissions and receptions on the first and second links. In other words, a second multilink device transmits an RTS frame or a MU-RTS frame on the first link. Consequently, the first multilink device receives the RTS frame or the MU-RTS frame on the first link and responds with / transmits a CTS frame on the first link. After transmitting the CTS frame on the first link, the first multilink device does not start the medium SyncDelay timer on the second link. The fact that the first multilink device does not initiate the mediumSyncDelay timer on the second link can be understood (or described as): When the first multilink device performs channel contention on the second link, a power detection threshold used by a CCA operation is a first threshold; or after a backoff counter on the second link resets to 0, the first multilink device is allowed to directly transmit a frame other than an RTS frame and a MU-RTS frame. In other words, after the backoff counter on the second link resets to 0, the first multilink device does not transmit an RTS / MU-RTS frame to attempt to detect channel protection / availability. The first threshold can be -62 dBm. Optionally, if the first multilink device starts the mediumSyncDelay timer on the second link after transmitting the CTS frame on the first link, the first multilink device closes (or stops or cancels) the mediumSyncDelay timer. Optionally, the RTS / CTS frame in the second implementation can be replaced with a buffer status report poly trigger (BSRP Trigger), buffer status report (BSR) frame, or bandwidth query report poly trigger (BQRP Trigger), bandwidth query report (BQR) frame, beamforming report poly trigger (BFRP Trigger), null data packet (NDP) frame, or an acknowledgment (ACK) frame, or a management / ACK frame, or a block acknowledge (BA) frame.Therefore, step S201 can be alternatively described as: The first multilink device receives a BSRP wake-up frame on the first link and responds with / transmits a BSR frame on the first link; or the first multilink device receives a BQRP wake-up frame on the first link and responds with / transmits a BQR frame on the first link; or the first multilink device receives a BFRP wake-up frame on the first link and responds with / transmits an NDP frame on the first link; or the first multilink device receives a data frame or a management frame on the first link and responds with / transmits an ACK frame on the first link; or the first multilink device receives a data frame on the first link and responds with / transmits a BA frame on the first link.Therefore, the second implementation can be alternatively described as: After transmitting a BQR frame, a BQR frame, or an NDP frame on the first link, the first multilink device does not start the middle synchronization delay timer on the second link. It can be understood that after responding with / transmitting a CTS frame, an NDP frame, a BSR frame, a BQR frame, an ACK frame, or a BA frame on the first link, the first multilink device is in a receive state on the first link. Therefore, receiving on the first link does not affect channel contention performed on the second link. In this case, the first multilink device can perform common channel contention on the second link; that is, the energy detection threshold used by CCA operation is -62 dBm, or RTS / CTS frames may not be used to attempt to detect channel protection. It can be learned that after a non-STR MLD (i.e., the first multilink device) in this request mode receives an RTS (or MU-RTS) frame on the first link and responds with a CTS frame, the non-STR MLD does not initiate the mediumSyncDelay timer on the second link. This can improve the channel access efficiency or success rate of the non-STR MLD on the second link, or increase the chances of the non-STR MLD accessing the channel on the second link. In this mode of this request, when a frame of a specific type is transmitted on the first link, the medium synchronization delay timer is not started on the second link. Therefore, when the non-STR MLD is in a blind / self-interfering state on some links, the channel access efficiency or success rate of the non-STR MLD on these links can be improved, or the opportunities for channel access by the non-STR MLD on these links can be increased. Mode 2 In Mode 2 of this application, it is described how a non-STR MLD accesses the channel on a second link when the length of a PPDU transmitted on a first link by the non-STR MLD is less than a preset value. Figure 5 is another schematic flowchart of a channel access method for a multilink device according to one modality of this application. As shown in Figure 5, the channel access method for a multilink device includes, but is not limited to, the following steps. S201: When a length of a first PPDU transmitted on a first link by a first multilink device is less than or equal to a first value, the first multilink device does not start a media synchronization delay timer on a second link, where the first multilink device is not allowed to perform simultaneous transmission and reception on the first link and the second link. The fact that the first multilink device does not initiate the mediumSyncDelay timer on the second link can be understood (or described as): When the first multilink device performs channel contention on the second link, a power detection threshold used by a CCA operation is reached; or after a backoff counter on the second link resets to 0, the first multilink device is allowed to directly transmit a frame other than an RTS frame and a MU-RTS frame. In other words, after the backoff counter on the second link resets to 0, the first multilink device does not transmit an RTS / MU-RTS frame to attempt to detect channel protection / availability. The first threshold can be -62 dBm. Optionally, if the first multilink device starts the mediumSyncDelay timer on the second link after transmitting the first PPDU on the first link, the first multilink device closes (or stops or cancels) the mediumSyncDelay timer when it determines that the length of the first PPDU is less than or equal to the first value. Optionally, when the media synchronization delay timer is started on the second link, if the length of the first PPDU transmitted on the first link by the first multilink device is less than or equal to a first value, the first multilink device does not update the media synchronization delay timer located on the second link. Optionally, when the media synchronization delay timer starts on the second link, if the length of the PPDU transmitted on the first link by the first multilink device is greater than or equal to the first value, the first multilink device needs to update the media synchronization delay timer on the second link. The first multilink device updating the media synchronization delay timer on the second link can be understood as: updating the media synchronization delay timer on the second link to the initial value of the media synchronization delay timer that exists when the media synchronization delay timer starts, which is equivalent to resetting the media synchronization delay timer on the second link.Conversely, the fact that the first multilink device does not update the media synchronization delay timer that is on the second link can be understood as: not updating the media synchronization delay timer that is on the second link to an initial value of the existing media synchronization delay timer when the media synchronization delay timer is started. Optionally, the first value can be a fixed value stipulated in a protocol, for example, 50 ps, ​​100 ps or 200 ps. Optionally, the first value can be 28 ps. 28 ps is an existing PPDU length when the CTS and ACK frames are transmitted in a non-HT 24 Mbps PPDU format or a duplicate non-HT 24 Mbps PPDU. Optionally, the first value can be 32 ps. 32 ps is a PPDU length that exists when transmitting a BA (block acknowledge, block ACK) frame with a bitmap length of 64 in a 24 Mbps non-HT PPDU format or a 24 Mbps non-HT duplicate PPDU. Optionally, the first value can be 44 ps. 44 ps is an existing PPDU length when the CTS and ACK frames are transmitted in a 6 Mbps non-HT PPDU format or a duplicated 6 Mbps non-HT PPDU. Optionally, the first value can be 40 ps. 40 ps is a PPDU length that exists when transmitting a BA frame with a bitmap length of 256 in a 24 Mbps non-HT PPDU format or a 24 Mbps non-HT duplicate PPDU. Optionally, the first value can be 36 ps. 36 ps is a PPDU length that exists when transmitting a QoS-Null frame in a 24 Mbps non-HT PPDU format or a 24 Mbps non-HT duplicate PPDU. Optionally, the first value can be 68 ps. 68 ps is a PPDU length that exists when transmitting a BA frame with a bitmap length of 64 in a 6 Mbps non-HT PPDU format or a 6 Mbps non-HT duplicate PPDU. Optionally, the first value may be determined by an access point (or an AP MLD) and transmitted to a station (i.e., a non-AP MLD). Specifically, prior to step S201, the channel access method for a multilink device in this application mode may also include: Step S202: A second multilink device transmits indication information, where the indication information is used to indicate the first value. The first multilink device then receives the indication information. The indication information may be carried in a beacon frame, an association response frame, or a reassociation response frame. The first multilink device may be a non-STR MLD, and specifically, a non-AP MLD of a non-STR. The second multilink device may be an STR MLD, and specifically, an AP MLD of an STR. In one implementation, the indication information may be located in a multi-link element. Figure 6a is a schematic diagram of a multi-link element frame structure according to one modality of this application. As shown in Figure 6a, the multi-link element may include an element ID field, a length field, an element ID extension field, a multi-link control field, a medium SyncDelay timer threshold field, an optional subelement field, and the like. The medium SyncDelay timer threshold field is used to indicate the first value. In another implementation, the indication information can be located in an EHT operation element. Figure 6b is a schematic diagram of the frame structure of an EHT operation element according to one modality of this application. As shown in Figure 6b, the EHT operation element can include an element ID field, a length field, an element ID extension field, a medium sync delay timer threshold field, and the like. The medium sync delay timer threshold field is used to indicate the first value. In yet another implementation, a new information element can be defined to carry the indication information. This new information element is used to carry a configuration parameter of a non-STR MLD. Optionally, the new information element can be called a non-STR MLD parameter set element. It can be understood that the new information element may have another name. This is not limited in this modality of this application. Figure 6c is a schematic diagram of a frame structure of a non-STR MLD parameter set element according to one modality of this application. As shown in Figure 6c,6c, the No STR MLD parameter set element may include an element ID field, a length field, an element ID extension field, a medium sync delay timer threshold field, and the like. The medium sync delay timer threshold field is used to indicate the first value. Optionally, when the length of the first PPDU exceeds the first value, the first multilink device may initiate the media synchronization delay timer on the second link. During the period while the media synchronization delay timer is running, the first multilink device may use a more conservative channel access mechanism on the second link. This more conservative channel access mechanism includes, but is not limited to: (1) using a lower energy detection threshold (defined as an ED threshold below -62 dBm in this document) to determine if a channel is busy; and (2) requiring the transmission of an RTS frame to attempt to detect channel availability. Optionally, there may be only one attempt (or only one transmission of the RTS frame) or a limited number of attempts.It can be understood that when the length of the first PPDU is equal to the first value, an operation performed by the first multilink device could be either not to start the media synchronization delay timer on the second link or to start the media synchronization delay timer on the second link. In this modality of this application, the operation performed by the first multilink device when the length of the first PPDU is equal to the first value can be established based on a real-world situation. Optionally, before starting the media synchronization delay timer on the second link, the first multilink device can determine an initial value for the media synchronization delay timer that corresponds to the length of the first PPDU. It can be understood that the initial value of the media synchronization delay timer started on the second link by the first multilink device is the predetermined initial value corresponding to the length of the first PPDU. A mapping relationship between a PPDU length and an initial value (or initial duration) of a medium synchronization delay timer can be stipulated in a standard protocol. Alternatively, before the first multilink device transmits the first PPDU on the first link, the multilink device transmits the first indication information. Correspondingly, the first multilink device receives the first indication information. The first indication information is used to indicate the mapping relationship between a PPDU length and an initial value (or initial duration) of the medium synchronization delay timer. Optionally, after or at the time of starting the media synchronization delay timer on the second link, the first multilink device determines an energy sensing threshold corresponding to the length of the first PPDU, and when channel contention is performed on the second link, the first multilink device sets an energy sensing threshold used by a CCA operation at a threshold corresponding to the length of the first PPDU. A mapping relationship between a PPDU length and an energy detection threshold can be stipulated in a standard protocol. Optionally, before the first multilink device transmits the first PPDU on the first link, the second multilink device transmits the second indication information. Consequently, the first multilink device receives the second indication information. This second indication information is used to define the mapping relationship between a PPDU length and an energy detection threshold. The first and second indication pieces can be understood as a single piece of indication information. That is, a single piece of indication information indicates both the mapping relationship between a PPDU length and the initial value (or initial duration) of a media synchronizer delay timer, and the mapping relationship between a PPDU length and a power sensing threshold. In other words, the first and second indication pieces are carried in a single frame. It can be understood that in this mode of this request, after transmitting a short frame on one link, a non-STR MLD is restricted from initiating the mediumSyncDelay timer on another link; or when channel contention occurs on another link, the energy detection threshold used by CCA is set to -62 dBm; or there is no need to use an RTS frame on another link to attempt to detect channel protection / availability. Therefore, the channel access efficiency or success rate of the non-STR MLD on the other link is improved, or the opportunities for channel access by the non-STR MLD on the other link are increased. In an optional mode, the length of the first PPDU can be replaced by the length (in bytes or bits) of a medium access control (MAC) frame in the first PPDU. Consequently, step S301 can be replaced by: When the length of a MAC frame in a first PPDU transmitted on a first link by a first multilink device is less than or equal to a second value, the first multilink device does not initiate a media synchronization delay timer on a second link, where the first multilink device is not permitted to perform simultaneous transmission and reception on the first and second links. In another optional mode, the channel access methods provided in Mode 1 and Mode 2 can also be applied to a single-link, multi-access channel scenario. Two channels are used as an example. An access point (AP) can use two channels for channel access, but it can only access one channel at a time and cannot access two channels simultaneously. Specifically, the AP performs channel contention on a primary channel (e.g., the first channel). When the primary channel is busy, the AP can switch to the other channel (e.g., the second channel) to perform channel contention. After a backoff counter on the second channel resets to zero, the AP transmits on the second channel. For the single-link, multi-channel access scenario, this modality of this application proposes the following: After the AP transmits a short frame (e.g., an RTS frame, a CTS frame, a block acknowledge (BA) frame, a BSR frame, a BQR frame, a PS-Poll frame, or an NDP frame) on the second channel, the AP does not start a timer on the first channel. The timer can be a media synchronization delay timer. Optionally, this modality of this application further proposes the following: The AP transmits a first PPDU on the second channel; and when the PPDU length of the first PPDU is less than or equal to a first value, the AP does not start the media synchronization delay timer on the first channel. Optionally, the AP not starting a timer on the first channel can be understood (or described as): When the AP performs channel contention on the first channel, an energy detection threshold used by an OCA operation is used as a first threshold; or after a backoff counter on the first channel resets to 0, the AP is allowed to directly transmit a frame other than an RTS frame and a MU-RTS frame. In other words, after the backoff counter on the first channel resets to 0, the AP does not transmit an RTS / MU-RTS frame to attempt to detect channel protection / availability. The first threshold can be -62 dBm. It can be understood that the second channel in this modality of this request is equivalent to the first links in Modality 1 and Modality 2, and the first channel in this modality of this request is equivalent to the second links in Modality 1 and Modality 2. It can be learned that the channel access method provided in this application mode can also be applied to a single-link and multi-access channel scenario, thus expanding a method scenario and improving channel access efficiency or the AP's channel access success rate on the first channel. Mode 3 Mode 3 of this application provides a method for determining the initial duration of a medium synchronization delay timer. According to the method for determining the initial duration of a medium synchronization delay timer, the initial duration of a medium synchronization delay timer is determined based on the length of a frame transmitted on a first link (or a second channel). Figure 7 is a flowchart of a method for determining the initial duration of a medium synchronization delay timer according to one modality of this application. As shown in Figure 7, the method for determining the initial duration of a medium synchronization delay timer includes, but is not limited to, the following steps. S301: A second multilink device transmits the first indication information, where the first indication information is used to indicate a mapping relationship between a PPDU length / byte length and an initial value (or initial duration) of a medium synchronization delay timer. Specifically, the second multilink device can be an MLD AP, and the MLD AP is capable of STR. The MLD AP can transmit the first indication information on a first link or another link. This is not limited in this application. The first indication information can be used to indicate a correspondence between a PPDU length and an initial value (or initial duration) of the medium's synchronization delay timer. In one example, FIG. 8 is a schematic diagram of a mapping relationship between a PPDU length and an initial value of a media synchronization delay timer according to one modality of this application. As shown in FIG. 8, when the PPDU length is in the range of 0 to 100 ps (microseconds) (i.e., interval [0, 100 ps], interval (0, 100 ps), interval (0, 100 ps], or interval [0, 100 ps)), the initial value of the media synchronization delay timer is 0 ms; When the PPDU length is in the range of 100 ps to 1 ms (i.e., an interval [100 ps, ​​1000 ps], an interval (100 ps, ​​1000 ps), an interval (100 ps, ​​1000 ps), or an interval [100 ps, ​​1000 ps]), the initial value of the medium synchronization delay timer is 3 ms; and when the PPDU length is greater than or equal to 1 ms, the initial value of the medium synchronization delay timer is 6 ms. The mapping relationship shown in FIG. 8 can be summarized as shown in Table 2 below. Table 1 PPDU Length Initial value (or initial duration) of the media synchronization delay timer s 100 ps 0 >100 ps and <1 ms 3 ms >1 ms 6 ms It should be understood that the mapping relationship shown in Figure 8 and Table 1 is merely an example, and in the actual application, the mapping relationship may be determined based on a specific application scenario. For example, the mapping relationship could alternatively be as follows: When the PPDU length is less than or equal to 50 ps, ​​the initial value of the medium synchronization delay timer is 0 ms; when the PPDU length is greater than or equal to 50 ps and less than or equal to 200 ps, ​​the initial value of the medium synchronization delay timer is 1 ms; when the PPDU length is greater than or equal to 200 ps and less than or equal to 500 ps, ​​the initial value of the medium synchronization delay timer is 3 ms; and when the PPDU length is greater than or equal to 500 ps, ​​the initial value of the medium synchronization delay timer is 5 ms. This is not limited in this application. Optionally, the initial indication information can include an array. For example, an array (0,100,0) indicates that when the PPDU length is in the range of 0 to 100 ps, ​​the initial value of the media synchronization delay timer is 0 ms; an array (100,1000,3) indicates that when the PPDU length is in the range of 100 ps to 1 ms, the initial value of the media synchronization delay timer is 3 ms; and an array (1000,maximum PPDU length,6) indicates that when the PPDU length is in the range of 1 ms to the maximum PPDU length, the initial value of the media synchronization delay timer is 6 ms. The maximum PPDU length is stipulated in a standard protocol. Optionally, the initial indication information can include two fields. The first field is used to define N intervals. The second field is used to indicate an initial value for the media synchronization delay timer, corresponding to each of the N intervals. The first field can include N+1 subfields. The values ​​of the N+1 subfields increase monotonically, and the values ​​of two adjacent subfields can define an interval. Therefore, the N+1 subfields can define N intervals. For example, the value of a first subfield might be 0, and the value of an (N+1)th subfield might be the maximum PPDU length, or a value greater than the maximum PPDU length, for example, 6 ms. Optionally, the first subfield (or the (N+1)th subfield) cannot be included in the first field. The second field includes N subfields. A value in a subfield represents an initial value from the media synchronization delay timer and corresponds to an interval. S302: A first multi-link device receives the first indication information. S303: The first multilink device determines, based on the length of the first PPDU transmitted on the first link, an initial value for the mean synchronization delay timer that corresponds to the length of the first PPDU. This initial value is used to determine whether to start the mean synchronization delay timer on a second link. The first multilink device cannot perform simultaneous transmissions and receptions on the first and second links. Specifically, the first multilink device can be a non-AP MLD, and the non-AP MLD is capable of non-STR. The first multilink device can determine, based on the mapping relationship between a PPDU length and the initial value (or initial duration) of a medium synchronization delay timer (indicated by the first indication information), and based on the PPDU length of the first PPDU, the initial value (or initial duration) of the medium synchronization delay timer that corresponds to the PPDU length of the first PPDU. For example, the mapping relationship is shown in Table 1 above, and assuming the length of the first PPDU is 200 ps, ​​the initial value (or initial duration) of the medium synchronization delay timer is 3 ms. Optionally, the first multilink device determines, based on the initial value (or initial duration) of the mean synchronization delay timer and which corresponds to the length of the first PPDU, whether to start the mean synchronization delay timer on the second link. Specifically, if the initial value (or initial duration) of the medium synchronization delay timer is equal to 0, the first multilink device does not start the medium synchronization delay timer on the second link; or if the initial value (or initial duration) of the medium synchronization delay timer is greater than 0, the first multilink device starts the medium synchronization delay timer on the second link. The initial value / initial duration of the medium synchronization delay timer is the value determined in step S404. The first multilink device initiating the mediumSyncDelay timer on the second link can be understood (or described) as follows: During the time the mediumSyncDelay timer is running, the first multilink can use a more conservative channel access mechanism on the second link. This more conservative channel access mechanism includes, but is not limited to: (1) using a lower energy detection threshold (defined as an ED threshold below -62 dBm in this document) to determine if a channel is busy; and (2) requiring the transmission of an RTS frame to attempt to detect channel availability. Optionally, there can be only one attempt (or only one transmission of the RTS frame) or a limited number of attempts. The fact that the first multilink device does not initiate the mediumSyncDelay timer on the second link can be understood as (or described as): When the first multilink device performs channel contention on the second link, a power sensing threshold used by a CCA operation is a first threshold; or after a backoff counter on the second link resets to 0, the first multilink device is allowed to directly transmit a non-RTS frame and a MU-RTS frame. The first threshold can be -62 dBm. It can be understood that the method for determining the initial duration of a media synchronization delay timer provided in this modality of this application can be applied alternatively to a single-link, multi-access channel scenario. A first channel in the single-link, multi-access channel scenario is equivalent to the second link, and a second channel in the single-link, multi-access channel scenario is equivalent to the first link. The details are not described again herein. It can be learned that in this mode of this application, the mapping relationship between a PPDU length and an initial value (or initial duration) of a medium synchronization delay timer is indicated by the first indication information, so that the first multilink device determines, based on the mapping relationship and the length of the first PPDU transmitted on the first link, the initial value of the medium synchronization delay timer that corresponds to the length of the first PPDU. It does not start the mediumSyncDelay timer on the second link when the initial value is equal to 0, and starts the mediumSyncDelay timer on the second link when the initial value is greater than 0. Different PPDU lengths correspond to different initial values ​​of the mediumSyncDelay timer, allowing for more flexible configuration of the mediumSyncDelay timer and improving channel access efficiency. In an optional mode, the mapping relationship between a PPDU length and an initial value (or initial duration) of a medium synchronization delay timer may be stipulated in a standard protocol. When the mapping relationship is stipulated in a standard protocol, the method for determining the initial duration of a medium synchronization delay timer shown in FIG. 7 may not include steps S301 and S302; that is, the method for determining the initial duration of a medium synchronization delay timer may include step S303. Mode 4 Mode 4 of this application provides a method for determining an energy detection threshold in a CCA process. According to the method for determining an energy detection threshold in a CCA process, an ED threshold used in a CCA process when backtracking occurs on a second link during a media synchronization delay is determined based on the length of a frame transmitted on a first link (or a second channel). Figure 9 is a schematic flowchart of a method for determining an energy direction threshold in a CCA process according to one modality of this application. As shown in Figure 9, the method for determining an energy detection threshold in a CCA process includes, but is not limited to, the following steps. S401: A second multilink device transmits a second indication information, wherein the second indication information is used to indicate a mapping relationship between a PPDU length and a power direction threshold. Specifically, the second multilink device can be an MLD AP, and the MLD AP is capable of STR. The MLD AP can transmit the second indication information on a first link or another link. This is not limited in this application. The second indication information can be used to indicate the mapping relationship between a PPDU length and an energy detection threshold. In one example, FIG. 10 is a schematic diagram of a mapping relationship between a PPDU length and an energy detection threshold according to one modality of this application. As shown in FIG. 10, when the PPDU length is in the range of 0 to 100 ps (microseconds) (i.e., interval [0,100 ps], interval (0,100 ps), interval (0,100 ps), or interval [0,100 ps)), the energy detection threshold is -62 dBm; When the PPDU length is in the range of 100 ps to 1 ms (i.e., an interval [100 ps, ​​1000 ps], an interval (100 ps, ​​1000 ps), an interval (100 ps, ​​1000 ps] or an interval [100 ps, ​​1000 ps)), the energy detection threshold is -72dBm; and when the PPDU length is greater than or equal to 1 ms, the energy detection threshold is -82 dBm. The mapping relationship shown in FIG. 10 can be summarized as shown in Table 2 below. Table 2 PPDU Length Energy Detection Threshold <100 μδ -62 dBm >100 μδ and <1 πίδ -72 dBm >1 ηΐδ -82 dBm It should be understood that the mapping ratio shown in Figure 10 and Table 2 is merely an example, and in the actual application, the mapping ratio may be determined based on a specific application scenario. For example, the mapping ratio could alternatively be as follows: When the PPDU length is less than or equal to 50 ps, ​​the energy detection threshold is -62 dBm; when the PPDU length is greater than or equal to 50 ps and less than or equal to 200 ps, ​​the energy detection threshold is -67 dBm; when the PPDU length is greater than or equal to 200 ps and less than or equal to 500 ps, ​​the energy detection threshold is 72 dBm; and when the PPDU length is greater than or equal to 500 ps, ​​the energy detection threshold is -82 dBm. This is not limited in this application. Optionally, the second indication information can include an array. For example, an array (0,100,-62) indicates that when the PPDU length is in the range of 0 to 100 ps, ​​the energy detection threshold is -62 dBm; an array (100,1000,-72) indicates that when the PPDU length is in the range of 100 ps to 1 ms, the energy detection threshold is -72 dBm; and an array (1000,maximum PPDU length,-82) indicates that when the PPDU length is in the range of 1 ms to the maximum PPDU length, the energy detection threshold is -62 dBm. The maximum PPDU length is stipulated in a standard protocol. Optionally, the second indication information can include two fields. A first field is used to define N intervals. A second field is used to indicate an energy detection threshold corresponding to each of the N intervals. The first field can include N+1 subfields. The values ​​of the N+1 subfields increase monotonically, and the values ​​of two adjacent subfields can define an interval. Therefore, the N+1 subfields can define N intervals. For example, the value of a first subfield might be 0, and the value of an (N+1)th subfield might be the maximum PPDU length, or a value greater than the maximum PPDU length, for example, 6 ms. Optionally, the first subfield (or the (N+1)th subfield) cannot be included in the first field. The second field includes N subfields. A value in a subfield represents an energy detection threshold corresponding to an interval. S402: A first multi-link device receives the second indication information. S403: The first multilink device determines, based on the length of the first PPDU transmitted on the first link, an energy sensing threshold corresponding to the length of the first PPDU. The energy sensing threshold is used to determine whether to start the half-synchronization delay timer on a second link. Specifically, the first multilink device can be a non-AP MLD, and the non-AP MLD is capable of non-STR. The first multilink device can determine, based on the mapping relationship between a PPDU length and an energy detection threshold (indicated by the second indication information), and based on the length of the first PPDU, the energy detection threshold corresponding to that length. For example, the mapping relationship is shown in Table 2 above, and assuming the length of the first PPDU is 200 ps, ​​the energy detection threshold is -72 dBm. Optionally, the first multilink device determines, based on the energy detection threshold corresponding to the length of the first PPDU, whether to start the media synchronization delay timer on the second link. Specifically, if the energy detection threshold determined in step S403 is equal to -62 dBm, the first multilink device does not start the media synchronization delay timer on the second link; or if the energy detection threshold determined in step S403 is less than -62 dBm, the first multilink device starts the media synchronization delay timer on the second link.If the first multilink device starts the medium sync delay timer on the second link, it indicates that, during a mediumSyncDelay, the first multilink device sets an energy sensing threshold used by CCA to the energy sensing threshold corresponding to the duration of the first PPDU (i.e., the energy sensing threshold determined in step S504) when channel contention is performed on the second link. The first multilink device initiating the mediumSyncDelay timer on the second link can be understood (or described) as follows: During the mediumSyncDelay, the first multilink device may use a more conservative channel access mechanism on the second link. This more conservative channel access mechanism includes, but is not limited to: (1) using a lower energy detection threshold (defined as an ED threshold below -62 dBm in this document) to determine if a channel is busy; and (2) requiring the transmission of an RTS frame to attempt to detect channel availability. Optionally, there may be only one attempt (or only one transmission of the RTS frame) or a limited number of attempts. The fact that the first multilink device does not initiate the mediumSyncDelay timer on the second link can be understood as (or described as): When the first multilink device performs channel contention on the second link, a power sensing threshold used by a CCA operation is a first threshold; or after a backoff counter on the second link resets to 0, the first multilink device is allowed to directly transmit a non-RTS frame and a MU-RTS frame. The first threshold can be -62 dBm. It can be understood that the method for determining an energy detection threshold in a CCA process provided in this modality of this application can be applied alternatively to a single-link, multi-access channel scenario. A first channel in the single-link, multi-access channel scenario is equivalent to the second link, and a second channel in the single-link, multi-access channel scenario is equivalent to the first link. The details are not described again herein. It can be learned that in this application mode, the mapping relationship between a PPDU length and an energy detection threshold is indicated by the second indication information. The first multilink device determines, based on the mapping relationship and the length of the first PPDU transmitted on the first link, the energy detection threshold corresponding to that PPDU length. It does not start the mediumSyncDelay timer on the second link when the energy detection threshold is -62 dBm, and starts the mediumSyncDelay timer on the second link when the energy detection threshold is less than -62 dBm. Different PPDU lengths correspond to different energy detection thresholds. In this case, the channel access mechanism used on the second link is more flexible, improving channel access efficiency. In an optional mode, the mapping relationship between a PPDU length and an energy detection threshold can be stipulated in a standard protocol. When the mapping relationship is stipulated in a standard protocol, the method for determining an energy detection threshold in a CCA process shown in FIG. 9 may not include steps S401 and S402; that is, the method for determining an energy detection threshold in a CCA process may include step S403. In another optional mode, the first indication information in Mode 3 and the second indication information in Mode 4 can be a single piece of indication information, or the first and second indication information can be carried in the same frame. Therefore, Mode 3 and Mode 4 can be combined into one mode.Specifically, a second multilink device transmits indication information, where the indication information is used to indicate a mapping relationship between a PPDU length and an initial value (or initial duration) of a medium synchronization delay timer and a mapping relationship between a PPDU length and an energy sensing threshold; a first multilink device receives the indication information; the first multilink device transmits a first PPDU on a first link; and the first multilink device determines, based on a length of the first PPDU, an initial value of the medium synchronization delay timer that corresponds to the length of the first PPDU, and an energy sensing threshold that corresponds to the length of the first PPDU.Optionally, the first multilink device can also determine, based on the energy detection threshold corresponding to the length of the first PPDU or based on the initial value of the mean synchronization delay timer corresponding to the length of the first PPDU, whether to start the mean synchronization delay timer on the second link. The preceding content describes in detail the methods provided in this application. To better implement the above solutions in the modalities of this application, the modalities of this application also provide the corresponding apparatus or devices. In the modalities of this application, the communication device can be divided into functional modules based on the examples of the previous methods. For example, each functional module can be divided according to each function, or two or more functions can be integrated into a single processing module. The integrated module can be implemented in hardware form or as a software function module. It should be noted that the division of modules in the modalities of this application is an example and only a logical functional division; other forms of division may exist in actual implementation. When an integrated unit is used, FIG. 11 is a schematic diagram of a first multilink device structure according to one modality of this application. As shown in FIG. 11, the first multilink device includes a transceiver unit 11 and a processing unit 12. In one design, processing unit 12 is configured to: when the length of a first PPDU transmitted on a first link by a first multilink device is less than or equal to a first value, skip the start of a half synchronization delay timer on a second link. The first multilink device cannot perform simultaneous transmissions and receptions on the first and second links. Processing unit 12 is specifically configured to: when channel contention occurs on the second link, establish an energy detection threshold used by a COA operation for a first threshold. Alternatively, transceiver unit 11 is configured to: after a backoff counter on the second link resets to 0, transmit a non-RTS frame and a MU-RTS frame. The first threshold can be -62 dBm. It should be understood that the first multilink device in this design can execute Mode 2 accordingly, or the operations or functions of the units in the first multilink device are used separately to implement the corresponding operations performed by the first multilink device in Mode 2. For brevity, the details are not described again herein. In one design, processing unit 12 is configured to: when the type of a first frame transmitted on a first link by a first multilink device is a first type, skip the start of a half synchronization delay timer on a second link. The first multilink device cannot perform simultaneous transmissions and receptions on the first and second links. Processing unit 12 is specifically configured to: when channel contention occurs on the second link, set the energy detection threshold used by a CCA operation to the first threshold. Alternatively, transceiver unit 11 is configured to: after the backoff counter on the second link resets to 0, transmit a non-RTS frame and an MU-RTS frame. The first threshold can be -62 dBm. It should be understood that the first multilink device in this design can execute Mode 1 accordingly, or the operations or functions of the units in the first multilink device are used separately to implement the corresponding operations performed by the first multilink device in Mode 1. For brevity, the details are not described again herein. In one design, transceiver unit 11 is configured to receive first indication information, where the first indication information is used to indicate a mapping relationship between a PPDU length and an initial value of a medium synchronization delay timer; and processing unit 12 is configured to determine, based on the length of a first PPDU transmission sent on the first link, an initial value of the medium synchronization delay timer that corresponds to the length of the first PPDU. The initial value is used to determine whether to start the medium synchronization delay timer on the second link. The first multilink device cannot perform simultaneous transmissions and receptions on the first and second links. Optionally, processing unit 12 is also configured to determine, based on the initial value of the mean synchronization delay timer and corresponding to the length of the first PPDU, whether to start the mean synchronization delay timer on the second link. Optionally, processing unit 12 is specifically configured for: If the initial value determined for the middle synchronization delay timer is equal to 0, skip the start of the middle synchronization delay timer on the second link; or if the initial value determined for the middle synchronization delay timer is equal to 0, start the middle synchronization delay timer on the second link. It should be understood that the first multilink device in this design can execute Mode 3 accordingly, or the operations or functions of the units in the first multilink device are used separately to implement the corresponding operations performed by the first multilink device in Mode 3. For brevity, the details are not described again herein. In one design, transceiver unit 11 is configured to receive second indication information, where the second indication information is used to indicate a mapping relationship between a PPDU length and the energy detection threshold; and processing unit 12 is configured to determine, based on the length of the first PPDU transmission sent on the first link, an initial value for the medium synchronization delay timer that corresponds to the length of the first PPDU. The communication apparatus cannot perform simultaneous transmissions and receptions on the first and second links. Optionally, processing unit 12 is also configured to determine, based on the energy detection threshold corresponding to the length of the first PPDU, whether to start the half synchronization delay timer on the second link. Optionally, processing unit 12 is specifically configured to: if the determined energy detection threshold is equal to -62 dBm, skip starting the middle synchronization delay timer on the second link; or if the determined energy detection threshold is less than -62 dBm, start the middle synchronization delay timer on the second link. It should be understood that the first multilink device in this design can execute Mode 4 accordingly, or the operations or functions of the units in the first multilink device are used separately to implement the corresponding operations performed by the first multilink device in Mode 4. For brevity, the details are not described again herein. FIG. 12 is a schematic diagram of a second multilink device structure according to one modality of this application. As shown in FIG. 12, the second multilink device includes a processing unit 21 and a transceiver unit 22. In one design, the processing unit 21 is configured to generate the first indication information 22 where the first indication information is used to indicate a mapping relationship between a PPDU length and an initial value (or initial duration) of a medium synchronization delay timer; and the transceiver unit 22 is configured to transmit the first indication information. It should be understood that the second multilink device in this design can execute Mode 3 accordingly, and the operations or functions of the units in the second multilink device are used separately to implement the corresponding operations performed by the second multilink device in Mode 3. For brevity, the details are not described again herein. In another design, the processing unit 21 is configured to generate a second indication information, where the second indication information is used to indicate a mapping relationship between a PPDU length and an energy detection threshold; and the transceiver unit 22 is configured to transmit the second indication information. It should be understood that the second multilink device in this design can execute Mode 4 accordingly, and the operations or functions of the units in the second multilink device are used separately to implement the corresponding operations performed by the second multilink device in Mode 4. For brevity, the details are not described again herein. The foregoing describes the first multilink device and the second multilink device in the modalities of this application. Possible product forms of the first multilink device and the second multilink device are described below. It is understood that any product in any form with functions of the first multilink device in FIG. 11, and any product in any form with functions of the second multilink device in FIG. 12, falls within the scope of protection of the modalities of this application. It is further understood that the following descriptions are merely examples and do not limit which product forms of the first multilink device and the second multilink device in the modalities of this application are limited to them. In a possible product form, the first multilink device and the second multilink device described in the modalities of this application can be implemented using a generic bus architecture. The first multi-link device includes a processor and a transceiver that communicates with the processor through an internal connection. In one design, the processor is configured to: when the length of the first PPDU transmitted on the first link by the first multilink device is less than or equal to a first value, skip the start of a half-sync delay timer on the second link. The first multilink device cannot perform simultaneous transmits and receives on the first and second links. Optionally, the transceiver is configured to transmit the first PPDU on the first link. In one design, the processor is configured to: when the type of the first frame transmitted on the first link by the first multilink device is a first type, skip the start of a half-synchronization delay timer on the second link. The first multilink device cannot perform simultaneous transmissions and receptions on the first and second links. In one design, the transceiver is configured to receive first indication information, where the first indication information is used to establish a mapping relationship between a PPDU length and an initial value of a medium synchronization delay timer. The processor is configured to determine, based on the length of the first PPDU transmission on the first link, an initial value for the medium synchronization delay timer that corresponds to the length of the first PPDU. This initial value is used to determine whether to start the medium synchronization delay timer on the second link. The first multilink device cannot perform simultaneous transmissions and receptions on both the first and second links. In one design, the transceiver is configured to receive second indication information, where the second indication information is used to indicate a mapping relationship between a PPDU length / byte length and an energy detection threshold; and the processor is configured to determine, based on the length of the first PPDU transmission sent on the first link, an initial value for the medium synchronization delay timer that corresponds to the length of the first PPDU. The communication apparatus cannot perform simultaneous transmissions and receptions on the first and second links. The second multi-link device includes a processor and a transceiver that communicates with the processor through an internal connection. In one design, the processor is configured to generate the first indication information, where the first indication information is used to indicate a mapping relationship between a PPDU length and an initial value (or initial duration) of a medium synchronization delay timer; and the transceiver is configured to transmit the first indication information. In another design, the processor is configured to generate a second indication information, where the second indication information is used to indicate a mapping relationship between a PPDU length and an energy detection threshold; and the transceiver is configured to transmit the second indication information. In a possible product form, the first multilink device and the second multilink device described in the modalities of this application can be implemented using a chip. A chip implementing the first multi-link device includes a processing circuit and an input / output interface that communicates with the processing circuit via an internal connection. In one design, the input / output interface is configured to receive code instructions and transmit them to the processing circuit. The processing circuit is configured to skip the start of the synchronization delay timer on the second link when the length of the first PPDU transmitted on the first link is less than or equal to a certain value. The first multi-link device cannot perform simultaneous transmissions and receptions on the first and second links. In this design, the input / output interface is configured to receive code instructions and transmit them to the processing circuit. The processing circuit is configured to: when a frame type in the first link transmitted by the first multilink device is of type 1, skip the start of the synchronization delay timer on the second link. The first multilink device cannot perform simultaneous transmissions and receptions on the first and second links. In one design, a transceiver is configured to receive the first indication information, and the input / output interface is configured to receive the first indication information from the transceiver and transmit it to the processing circuit for processing. This process establishes a mapping relationship between a PPDU length and an initial value (or initial duration) of the medium synchronization delay timer, as indicated by the first indication information. The processing circuit is then configured to determine, based on the length of the first PPDU transmitted on the first link, an initial value for the medium synchronization delay timer that corresponds to the length of the first PPDU. This initial value is used to determine whether to start the medium synchronization delay timer on the second link.The first multilink device cannot perform simultaneous transmissions and receptions on the first link and the second link. In one design, the transceiver is configured to receive the second indication information, and the input / output interface is configured to receive the second indication information from the transceiver and transmit it to the processing circuit to obtain a mapping relationship between a PPDU length and an energy sensing threshold, as indicated by the second indication information. The processing circuit is configured to determine, based on the length of the first PPDU transmitted on the first link, a corresponding energy sensing threshold. This energy sensing threshold is used to determine whether to start the half-synchronization delay timer on the second link. The first multilink device cannot perform simultaneous transmissions and receptions on both the first and second links. A chip implementing the second multi-link device includes a processing circuit and an input / output interface that communicates with the processing circuit via an internal connection. In one design, the input / output interface is configured to receive code instructions and transmit the code instructions to the processing circuit; the processing circuit is configured to generate a first indication information, where the first indication information is used to indicate a mapping relationship between a PPDU length and an initial value (or initial duration) of a medium synchronization delay timer; the input / output interface is configured to transmit the first indication information to a transceiver; and the transceiver is configured to transmit the first indication information. In another design, the input / output interface is configured to receive code instructions and transmit the code instructions to the processing circuit; the processing circuit is configured to generate a second indication information, where the second indication information is used to indicate a mapping relationship between a PPDU length and an energy detection threshold; the input / output interface is configured to transmit the second indication information to the transceiver; and the transceiver is configured to transmit the second indication information. In one possible product form, the first multilink device and the second multilink device described in the modality of this application may be alternatively implemented using the following: one or more FPGAs (field-programmable gate arrays), a PLD (programmable logic device), a controller, a state machine, gate logic, a discrete hardware component, any other suitable circuit, or any combination of circuits that is capable of performing various functions described throughout this application. It should be understood that communication devices in various product forms have some function of the first multilink device or the second multilink device in the modalities of the previous method. The details are not described again herein. One form of this application further provides a computer-readable storage medium. The computer-readable storage medium stores the instructions; when the instructions are executed on a computer, the computer is enabled to perform the method in accordance with any of the preceding forms. One form of this application also provides a computer program product. When the computer program product is run on a computer, the computer is enabled to perform the method in any of the above forms. One modality of this application further provides a communication device. The device may exist in the form of a chip product. One device structure includes a processor and an interface circuit. The processor is configured to communicate with another device through the interface circuit, so that the device implements the method in any of the above modalities. The steps of the method or algorithm described in combination with the content disclosed in this application may be implemented by hardware, or they may be implemented by a processor through the execution of software instructions. The software instructions may include a corresponding software module. The software module may be stored in random access memory (RAM), flash memory, erasable programmable read-only memory (EPROM), electrically programmable read-only memory (EEPROM), a cash register, a hard disk drive, a removable hard disk, a compact disc (CD-ROM), or any other form of storage medium known in the art.For example, a storage medium is coupled to a processor so that the processor can read information from the storage medium or write information to the storage medium. The storage medium can also be a component of the processor. The processor and the storage medium can be integrated into an ASIC. Furthermore, the ASIC can be integrated into a core network interface device. The processor and the storage medium can also exist as discrete components within the core network interface device. A person skilled in the art will know that, as in one or more of the preceding examples, the functions described in this application can be implemented using hardware, software, firmware, or any combination thereof. When this application is implemented using software, the above functions can be stored on a computer-readable medium or transmitted as one or more instructions or code on a computer-readable medium. A computer-readable medium includes a computer-readable storage medium and a communication medium. A communication medium includes any means that enables the transmission of a computer program from one location to another. A storage medium can be any available medium accessible to a general-purpose or dedicated computer. In the specific modalities above, the objectives, technical solutions, and benefits of this application are described in more detail. It should be understood that the above descriptions are merely specific implementations of this application and are not intended to limit the scope of protection provided by this application. Any modification, equivalent substitution, improvement, or similar action based on the technical solutions of this application will fall within the scope of protection provided by this application.

Claims

1. A channel access method for a multilink device, characterized in that it comprises: when a length of a first physical layer protocol data unit (PPDU) transmitted on a first link by a first multilink device is less than or equal to a first value, skipping the start, by means of the first multilink device, of a media synchronization delay timer on a second link, wherein the first multilink device is not permitted to perform simultaneous transmission and reception on the first and second links.

2. A channel access method for a multilink device, characterized in that it comprises: when a media synchronization delay timer on a second link is started, if the length of a first PPDU transmitted on a first link by a first multilink device is less than or equal to a first value, skip update, by means of the first multilink device, the media synchronization delay timer that is on the second link, wherein the first multilink device is not permitted to perform simultaneous transmission and reception on the first link and the second link.

3. The method according to claim 1, characterized in that the initial hop, by the first multilink device, to a medium synchronization delay timer on a second link comprises: setting an energy detection threshold used by the clear channel OCA evaluation performed on the second link to a first threshold, wherein the first threshold is -62 dBm; or after a backoff counter on the second link resets to 0, a frame other than an RTS frame and a MU-RTS frame are transmitted by the first multilink device.

4. The method according to any of claims 1 to 3, characterized in that the method further comprises: receiving, by means of the first multilink device, the first value, wherein the first value is carried in a beacon frame, an association response frame, or a reassociation response frame.

5. The method according to any of claims 1 to 4, characterized in that the method further comprises: when the length of the first PPDU is greater than the first value, determining, by means of the first multilink device, an initial value of the mean synchronization delay timer that corresponds to the length of the first PPDU, and starting the mean synchronization delay timer with the initial value on the second link.

6. The method according to claim 5, characterized in that the method further comprises: receiving, by means of the first multilink device, the first indication information, wherein the first indication information is used to indicate a mapping relationship between a PPDU length and an initial value of the mean synchronization delay timer.

7. A first multilink device, characterized in that it comprises: a processing unit, configured to: when a length of a first PPDU transmitted on a first link by a first multilink device is less than or equal to a first value, skip start, a media synchronization delay timer on a second link, wherein the first multilink device is not permitted to perform simultaneous transmission and reception on the first and second links.

8. A first multilink device, characterized in that it comprises: a processing unit, configured so that when a media synchronization delay timer on a second link starts, and a length of a first PPDU transmitted on a first link by a first multilink device is less than or equal to a first value, it skips the update, the media synchronization delay timer that is on the second link, wherein the first multilink device is not allowed to perform simultaneous transmission and reception on the first link and the second link.

9. The first multilink device according to claim 7, characterized in that the processing unit is specifically configured to: establish an energy detection threshold that is used by the clear channel assessment (CCA) performed on the second link to a first threshold, wherein the first threshold is -62 dBm; the first multilink device further comprises a transceiver unit, wherein the transceiver unit is configured to: after a backoff counter on the second link resets to 0, transmit a frame other than an RTS frame and an MU-RTS frame.

10. The first multilink device according to any of claims 7 to 9, characterized in that the first multilink device further comprises the transceiver unit, the transceiver unit is configured to receive the first value, and the first value is carried in a beacon frame, an association response frame, or a reassociation response frame.

11. The first multilink device according to any of claims 7 to 10, characterized in that the processing unit is further configured to: when the length of the first PPDU is greater than the first value, determine an initial value of the mean synchronization delay timer that corresponds to the length of the first PPDU and start the mean synchronization delay timer with the initial value on the second link.

12. The first multilink device according to claim 11, characterized in that the first multilink device further comprises the transceiver unit, the transceiver unit is configured to receive the first indication information, and the first indication information is used to indicate a mapping relationship between a PPDU length and an initial value of the medium synchronization delay timer.

13. A first multilink device, comprising a processor, characterized in that the processor is configured to: when the length of a first PPDU transmitted on a first link by a first multilink device is less than or equal to a first value, skip the start of a half synchronization delay timer on a second link, wherein the first multilink device is not permitted to perform simultaneous transmission and reception on the first and second links.

14. A first multilink device, comprising a processor, characterized in that the processor is configured so that: when a media synchronization delay timer on a second link is started, and when a length of a first PPDU transmitted on a first link by a first multilink device is less than or equal to a first value, the update is skipped, the media synchronization delay timer that is on the second link, wherein the first multilink device is not allowed to perform simultaneous transmission and reception on the first link and the second link.

15. A first multilink device, comprising an input / output interface and a processing circuit, characterized in that the input / output interface is configured to receive code instructions and transmit the code instructions to the processing circuit; and the processing circuit is configured to: when a length of a first PPDU transmitted on a first link is less than or equal to a first value, skip the start of a half synchronization delay timer on a second link, wherein the first multilink device is not permitted to perform simultaneous transmission and reception on the first and second links.

16. A first multilink device, comprising an input / output interface and a processing circuit, characterized in that the input / output interface is configured to receive code instructions and transmit the code instructions to the processing circuit; and the processing circuit is configured so that when a media synchronization timer on a second link starts, and the length of a first PPDU transmitted on a first link by a first multilink device is less than or equal to a first value, the update is skipped by the media synchronization delay timer on the second link, wherein the first multilink device is not permitted to perform simultaneous transmission and reception on the first and second links.

17. A computer-readable storage medium, characterized in that the computer-readable storage medium stores program instructions, and when the program instructions are executed on a computer, the computer is enabled to perform the method in accordance with any one of claims 1 to 11.

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