Wireless communication method and related access point multi-link equipment

Abstract

The invention discloses a method and equipment for coordinating downlink transmission in a wireless roaming process. A first access point multilink device (AP MLD) performs frame exchange with a non-access point multilink device (non-AP MLD). In a roaming process in which a non-access point multi-link device is switched to a second access point multi-link device, a first access point multi-link device sends first information for indicating a sequence number (SN) restriction to the second access point multi-link device. In response, the second access point multilink device controls its downlink transmission based on the first information. This control is performed to ensure that MAC protocol data units (MPDUs) transmitted from the second access point multilink device to the non-access point multilink device are within a reordering buffer window of the non-access point multilink device. According to the invention, data loss can be prevented and communication reliability can be improved.

Description

[Technical Field]

[0001] This invention generally relates to a wireless communication technology, and more particularly, to a wireless communication method and related access point multilink devices for coordinating transmission window control during roaming in a multi-AP MLD. [Background Technology]

[0002] In modern wireless local area networks (WLANs), such as IEEE 802.11bn, a multi-access point (multi-AP) system is defined to facilitate seamless roaming of non-access point multi-link devices (non-AP MLDs). During roaming, a non-AP MLD simultaneously connects to both the current access point multi-link device (AP MLD) and the target access point multi-link device (AP MLD), resulting in parallel downlink transmissions between the two AP MLDs.

[0003] Typically, MPDUs (Multi-Processing Units) in a single traffic stream share a common sequence number (SN) space and are reordered using a reordering buffer at the non-AP MLD. The reordering window at the non-AP MLD dynamically moves forward as MPDUs are received. It's important to note that MPDUs with sequence numbers outside the current reordering buffer window (before) are discarded. However, in parallel transmission scenarios, the target AP MLD transmits new MPDUs with higher sequence numbers. Receiving these higher sequence number MPDUs causes the reordering buffer window at the non-AP MLD to move forward prematurely. The Block Acknowledgement (BA) window at the AP MLD is used to estimate / project the reordering buffer window at the non-AP MLD. The BA window at the AP MLD and the reordering buffer window at the non-AP MLD move forward synchronously, and the BA window and the reordering buffer window are of the same size. In other words, the BA window at the transmitter (TX) and the reordering buffer window at the receiver (RX) are synchronized.

[0004] However, uncoordinated window forwarding will cause the sequence numbers of older MPDUs still queued in the current AP MLD to fall outside the window's effective range (specifically, before). Therefore, even if these older MPDUs are successfully transmitted, they will be discarded by the non-AP MLD. This results in significant data loss and reduced reliability, contradicting the goals of an ultra-high reliability (UHR) system. [Summary of the Invention]

[0005] In a first aspect, the present invention provides a wireless communication method performed by a first access point multilink device (AP MLD), wherein the method includes: performing frame exchange with a non-access point multilink device (non-AP MLD); and, during roaming when the non-AP MLD switches from the first AP MLD to a second AP MLD, sending first information to the second AP MLD to indicate a sequence number (SN) restriction, thereby restricting the second AP MLD from exceeding the reordering buffer window of the non-AP MLD when performing downlink transmissions to the non-AP MLD.

[0006] In some embodiments, the first information is determined based at least on the start sequence number of at least one MAC Protocol Data Unit (MPDU) in the transmission queue of the first AP MLD.

[0007] In some embodiments, the first information is determined at least based on the starting sequence number of the block confirmation (BA) window of the first AP MLD.

[0008] In some embodiments, the first information is determined based on the window size and the starting sequence number of at least one MAC Protocol Data Unit (MPDU) in the transmission queue of the first AP MLD; or, the first information is determined based on the window size and the starting sequence number of the block acknowledgment (BA) window of the first AP MLD.

[0009] In some embodiments, the first information for indicating SN restrictions is configured to cause the second AP MLD to control downlink transmission to the non-AP MLD based on the first SN restriction value, so as to ensure that MAC Protocol Data Units (MPDUs) transmitted to the non-AP MLD are within the reordering buffer window, wherein the first SN restriction value is defined as the starting sequence number plus the window size minus one, and the first information is associated with the starting sequence number.

[0010] In some embodiments, the method further includes: after one or more MPDUs in the transmission queue of the first AP MLD are transmitted from the first AP MLD to the non-AP MLD or in response to a request from the second AP MLD, sending second information to the second AP MLD to indicate SN restrictions, so that the second AP MLD controls downlink transmission to the non-AP MLD based on a second SN restriction value; wherein the second SN restriction value is defined as an updated start sequence number plus the window size minus one, and the second information is related to the updated start sequence number, and the second SN restriction value is greater than the first SN restriction value.

[0011] In some embodiments, the method further includes sending a cancellation message to cancel the SN restriction.

[0012] In a second aspect, the present invention provides a wireless communication method performed by a second access point multilink device (AP MLD), wherein the method includes: during a roaming process in which a non-access point multilink device (non-AP MLD) switches from a first AP MLD to the second AP MLD, receiving first information from the first AP MLD for indicating sequence number (SN) restrictions; and controlling downlink transmission to the non-AP MLD based on the first information to ensure that MAC protocol data units (MPDUs) transmitted to the non-AP MLD are within the reordering buffer window of the non-AP MLD.

[0013] In some embodiments, the first information is determined based at least on the start sequence number of at least one MPDU in the transmission queue of the first AP MLD; or, the first information is determined based at least on the start sequence number of the block acknowledgment (BA) window of the first AP MLD.

[0014] In some embodiments, the first information is determined based on the window size and the starting sequence number of at least one MPDU in the transmission queue of the first AP MLD; or, the first information is determined based on the window size and the starting sequence number of the block acknowledgment (BA) window of the first AP MLD.

[0015] In some embodiments, controlling downlink transmission of the non-AP MLD based on the first information includes: controlling downlink transmission of the non-AP MLD based on a first SN limit value to ensure that the MPDU transmitted to the non-AP MLD does not exceed the first SN limit value; wherein the first SN limit value is defined as the starting sequence number plus the window size minus one, and the first information is related to the starting sequence number.

[0016] In some embodiments, the method further includes: receiving second information for indicating SN limitations, wherein the second information is different from the first information; and controlling downlink transmission of the non-AP MLD based on the second information.

[0017] In some embodiments, the method further includes: receiving a cancellation message indicating cancellation of the SN restriction; and, in response to receiving the cancellation message, independently transmitting a subsequent MPDU to the non-AP MLD without being restricted by the first information or the first AP MLD.

[0018] Thirdly, a first access point multilink device (AP MLD) is provided, including a transceiver and a processor coupled to the transceiver, wherein the processor is configured to: perform frame exchange with a non-access point multilink device (non-AP MLD); and, during roaming when the non-AP MLD switches from the first AP MLD to a second AP MLD, send first information via the transceiver to the second AP MLD for indicating sequence number (SN) restrictions to limit the second AP MLD from exceeding the reordering buffer window of the non-AP MLD when performing downlink transmissions to the non-AP MLD.

[0019] In some embodiments, the first information is determined based at least on the start sequence number of at least one MAC Protocol Data Unit (MPDU) in the transmission queue of the first AP MLD.

[0020] In some embodiments, the first information is determined at least based on the starting sequence number of the block confirmation (BA) window of the first AP MLD.

[0021] In some embodiments, the first information is determined based on the window size and the starting sequence number of at least one MAC Protocol Data Unit (MPDU) in the transmission queue of the first AP MLD; or, the first information is determined based on the window size and the starting sequence number of the block acknowledgment (BA) window of the first AP MLD.

[0022] In some embodiments, the first information for indicating SN restrictions is configured to cause the second AP MLD to control downlink transmission to the non-AP MLD based on the first SN restriction value, so as to ensure that MAC Protocol Data Units (MPDUs) transmitted to the non-AP MLD are within the reordering buffer window, wherein the first SN restriction value is defined as the starting sequence number plus the window size minus one, and the first information is associated with the starting sequence number.

[0023] In some embodiments, the processor is further configured to: after one or more MPDUs in the transmission queue of the first AP MLD are transmitted from the first AP MLD to the non-AP MLD or in response to a request from the second AP MLD, send second information via the transceiver to the second AP MLD for indicating SN limits, so that the second AP MLD controls downlink transmission to the non-AP MLD based on a second SN limit value; wherein the second SN limit value is defined as the updated start sequence number plus the window size minus one, and the second information is related to the updated start sequence number, and the second SN limit value is greater than the first SN limit value.

[0024] In some embodiments, the processor is further configured to send a cancellation message via the transceiver to cancel the SN restriction.

[0025] Fourthly, a second access point multilink device (AP MLD) is provided, including a transceiver and a processor coupled to the transceiver, wherein the processor is configured to: receive first information from the first AP MLD via the transceiver for indicating sequence number (SN) restrictions during roaming when a non-AP MLD switches from a first AP MLD to the second AP MLD; and control downlink transmissions to the non-AP MLD based on the first information to ensure that MAC protocol data units (MPDUs) transmitted to the non-AP MLD are within the reordering buffer window of the non-AP MLD.

[0026] In some embodiments, the first information is determined based at least on the start sequence number of at least one MPDU in the transmission queue of the first AP MLD; or, the first information is determined based at least on the start sequence number of the block acknowledgment (BA) window of the first AP MLD.

[0027] In some embodiments, the first information is determined based on the window size and the starting sequence number of at least one MPDU in the transmission queue of the first AP MLD; or, the first information is determined based on the window size and the starting sequence number of the block acknowledgment (BA) window of the first AP MLD.

[0028] In some embodiments, the processor is further configured to: control downlink transmission to the non-AP MLD based on a first SN limit value to ensure that the MPDU transmitted to the non-AP MLD does not exceed the first SN limit value; wherein the first SN limit value is defined as the starting sequence number plus the window size minus one, and the first information is related to the starting sequence number.

[0029] In some embodiments, the processor is further configured to: receive second information indicating SN limitations, wherein the second information is different from the first information; and control downlink transmission of the non-AP MLD based on the second information.

[0030] In some embodiments, the processor is further configured to: receive a cancellation message via the transceiver indicating the cancellation of the SN restriction; and, in response to receiving the cancellation message, independently transmit a subsequent MPDU to the non-AP MLD without being restricted by the first information or the first AP MLD.

[0031] This invention provides a method and apparatus for transmission window control during roaming in a multi-AP MLD system. By coordinating information indicating SN limits between the first and second AP MLDs, premature advance of the transmission window can be prevented (this limits the second AP MLD from exceeding the reordering buffer window of the non-AP MLD when making downlink transmissions to the non-AP MLD via the information indicating SN limits), thereby avoiding the erroneous discarding of old packets from the current AP MLD. Therefore, this invention significantly reduces data loss, improves communication reliability, and enhances spectral efficiency, thus supporting the ultra-high reliability (UHR) goals of modern WLAN systems.

[0032] These and other objectives of the invention will undoubtedly become apparent to those skilled in the art upon reading the following detailed description of preferred embodiments. [Attached Image Description]

[0033] A more complete understanding of the invention can be obtained by reading the following detailed description and referring to the examples given in the accompanying drawings.

[0034] Figure 1 This is a schematic diagram of a wireless communication system in an initial state during roaming, according to an embodiment of the present invention.

[0035] Figure 2 yes Figure 1 A schematic diagram of the restriction removal mechanism in a wireless communication system.

[0036] Figure 3A yes Figure 1 A schematic diagram of the first stage of the dynamic update mechanism of a Chinese wireless communication system.

[0037] Figure 3B yes Figure 1 A schematic diagram of the second stage (after the first stage) of the dynamic update mechanism of the wireless communication system.

[0038] Figure 4 yes Figure 1A schematic diagram of the pause restriction mechanism in a Chinese wireless communication system.

[0039] Figure 5 yes Figure 1 A schematic diagram of the permissive packet loss policy in a wireless communication system.

[0040] Figure 6 yes Figure 1 A schematic diagram of the first scenario of the request mechanism within a wireless communication system.

[0041] Figure 7 yes Figure 1 A schematic diagram of the second scenario of the request mechanism within a wireless communication system.

[0042] Figure 8 yes Figure 1 A flowchart illustrating the method for establishing transmission restrictions executed by the first access point multilink device (AP MLD) in a wireless communication system.

[0043] Figure 9 yes Figure 1 A flowchart illustrating the method for canceling transmission restrictions executed by the first AP MLD in a wireless communication system.

[0044] Figure 10 yes Figure 1 A flowchart illustrating the method for controlling transmission based on received restriction information executed by the second AP MLD in a wireless communication system.

[0045] In the following detailed description, numerous specific details are set forth for illustrative purposes so that those skilled in the art can more thoroughly understand the embodiments of the invention. However, it will be apparent that one or more embodiments may be practiced without these specific details, and different embodiments may be combined as needed, and should not be limited to the embodiments illustrated in the accompanying drawings. [Specific Implementation Examples]

[0046] The following description illustrates preferred embodiments of the present invention and is intended only to exemplify the technical features of the invention, not to limit the scope of the invention. Throughout this specification and claims, certain terms are used to refer to specific elements. Those skilled in the art should understand that manufacturers may use different names for the same element. Therefore, this specification and claims do not distinguish elements by differences in name, but rather by differences in function. The terms "element," "system," and "device" used in this invention can refer to computer-related entities, where the computer can be hardware, software, or a combination of hardware and software. The terms "comprising" and "including" as used in the following description and claims are open-ended terms and should be interpreted as "comprising, but not limited to...". Furthermore, the term "coupled" refers to an indirect or direct electrical connection. Therefore, if a device is described as coupled to another device, it means that the device can be directly electrically connected to the other device, or indirectly electrically connected to the other device through other devices or connection means.

[0047] Unless otherwise indicated, the corresponding numbers and symbols in the various figures generally refer to the corresponding parts. The figures are drawn to clearly illustrate the relevant parts of the embodiments and are not necessarily drawn to scale.

[0048] The terms "basically" or "roughly" as used in this document mean that, within an acceptable range, a person skilled in the art can solve the technical problem to be solved and basically achieve the desired technical effect. For example, "roughly equal to" means a method that a person skilled in the art can accept with a certain margin of error from "exactly equal to" without affecting the correctness of the result.

[0049] This invention provides a method and apparatus for coordinating parallel downlink transmissions during roaming of a non-access point multilink device (non-AP MLD) in a multi-access point (MAP) wireless local area network (WLAN) system. This coordination is initiated by a first (current) AP MLD, which determines information to indicate sequence number (SN) limits to manage / control / limit downlink transmissions from a second (target) AP MLD. Downlink transmissions from the second AP MLD to the non-AP MLD are limited to not exceeding the non-AP MLD's reordering buffer window; that is, ensuring that MAC Protocol Data Units (MPDUs) transmitted to the non-AP MLD are within the non-AP MLD's reordering buffer window. It is important to note that the block acknowledgment (BA) window at the AP MLD moves forward / slides with each MPDU transmission (successfully received), and correspondingly, the reordering buffer window at the non-AP MLD moves forward / slides with each MPDU reception (successfully received). Understandably, the Block Acknowledgment (BA) window used at the AP MLD is used to estimate / project the reordering buffer window of the non-AP MLD (ideally, the sequence number spaces of the two windows are the same). The BA window at the AP MLD and the reordering buffer window at the non-AP MLD move forward synchronously, and the BA window and the reordering buffer window are the same size (collectively referred to as the "window size"). That is, for downlink transmissions, the BA window of the sender (e.g., the first / second AP MLD) and the reordering buffer window of the receiver (e.g., the non-AP MLD) are synchronized and have the same window size. Therefore, "downlink transmissions from the second AP MLD to the non-AP MLD are restricted to not exceeding the reordering buffer window of the non-AP MLD" is equivalent to "downlink transmissions from the second AP MLD to the non-AP MLD are restricted to not exceeding the BA window of the AP MLD," that is, ensuring that MPDUs transmitted from the second AP MLD to the non-AP MLD are within the BA window maintained by the AP MLD, i.e., not exceeding the reordering buffer window maintained by the non-AP MLD. In embodiments of the invention, the information used to indicate SN limitations is determined at least based on the start sequence number (SN) of at least one MAC Protocol Data Unit (MPDU) in the transmission queue of the first AP MLD, or at least based on the start sequence number (SN) of the Block Acknowledgment (BA) window. Optionally, the information used to indicate SN limitations may further be determined based on the size of the transmission window (e.g., the BA window), i.e., the window size.It should be noted that the size of the transmission window is known to both the first AP MLD and the second AP MLD, and the size of the BA window is equal to the size of the reordering buffer window. In this embodiment of the invention, during roaming when the non-AP MLD switches from the first AP MLD to the second AP MLD, the first AP MLD sends information to the second AP MLD indicating SN limitations to restrict the second AP MLD from exceeding the reordering buffer window of the non-AP MLD when performing downlink transmissions to the non-AP MLD. It should be noted that the BA window and the reordering buffer window can be understood as the same window (or described as the transmission window), but different names and functions are used on the transmitter (TX) and receiver (RX) sides, respectively. The two windows move forward synchronously with the successful reception of data (MPDU). Therefore, restricting the second AP MLD from exceeding the reordering buffer window of the non-AP MLD is equivalent to restricting the second AP MLD from exceeding the BA window of the first AP MLD.

[0050] This coordination prevents the premature forwarding of transmission windows (e.g., BA windows), thus avoiding the erroneous discarding of older MPDUs from the first AP MLD. Therefore, this invention can reduce data loss, improve communication reliability during roaming transitions, and enhance spectrum efficiency. The disclosed mechanism is also adaptive, supporting dynamic updates and updates proactively requested from the second AP MLD to meet Quality of Service (QoS) requirements. These features provide a robust solution supporting the ultra-high reliability (UHR) goals of modern WLAN systems.

[0051] Figure 1 This is a schematic diagram of a wireless communication system 100 in the initial state of a roaming process according to an embodiment of the present invention. The wireless communication system 100 includes a first AP MLD 10, a second AP MLD 20, and a non-AP MLD 30. The first AP MLD 10 and the second AP MLD 20 operate in a coordinated network, such as a seamless mobility domain (SMD), which can facilitate seamless roaming of the non-AP MLD 30 in a mobility domain composed of multiple AP MLDs.

[0052] The first AP MLD 10 can be the current AP MLD that a non-AP MLD 30 connects to (or transmits data with) before roaming. The first AP MLD 10 may include a transceiver 10a, a memory 10b, and a processor 10c. The processor 10c may be coupled to the transceiver 10a and the memory 10b to control the operation of the first AP MLD 10 according to the present invention. The memory 10b is configured to store data, such as buffered transmission queue Q1. Figure 1 As shown, transmission queue Q1 includes a plurality of older data packets (i.e., MPDUs) having consecutive sequence numbers (SNs) starting from a starting sequence number (SN) of N, for example {N, N+1, N+2, N+3, ..., N+A}. In one embodiment, the value N+A represents the sequence number of the last data packet in transmission queue Q1. Therefore, in an exemplary embodiment of the invention, the total number of data packets or the size of transmission queue Q1 is A+1.

[0053] The second AP MLD 20 is the target AP MLD that the non-access point multilink device (non-AP MLD) 30 needs to switch to. The second AP MLD 20 may include a transceiver 20a, a memory 20b, and a processor 20c. The processor 20c may be coupled to the transceiver 20a and the memory 20b to control the operation of the second AP MLD 20 according to the present invention. After the data path switch at the network end (i.e., the data path from the distribution system (DS) to the first AP MLD 10 is switched to the data path from the DS to the second AP MLD 20), the memory 20b of the second AP MLD 20 may buffer transmission queue Q2, which includes multiple newer data packets with sequence numbers (SN) larger than those in transmission queue Q1, such as {N+A+1,...}. It should be noted that... Figure 1 This is merely an illustrative description and the invention should not be limited thereto. For example, the memory is used to store data required by the device during operation.

[0054] The non-AP MLD 30 is a site device configured to receive downlink transmissions from a first AP MLD 10 and a second AP MLD 20. The AP MLDs (e.g., the first / second AP MLD 10 / 20, which are the senders of the downlink transmissions) maintain a sender's transmission window Q3 (e.g., a BA window), while the non-AP MLD 30 (which is the receiver of the downlink transmissions) maintains a receiver's reordering buffer window to reorder received packets. In this embodiment, the transmission window Q3 is a block acknowledgment (BA) window with a predefined window size W. Figure 1In the example shown, the transmission window Q3 can cover the range of sequence numbers from N to N+W-1. It is important to note that the size of the reordering buffer window of the non-AP MLD 30 is equal to the size of the BA window of the first AP MLD 10, for example, both are W in size, and the starting sequence number of the BA window is the same as the starting sequence number of the reordering buffer window. In this embodiment of the invention, the starting sequence number of at least one MAC Protocol Data Unit (MPDU) in the transmission queue of the first AP MLD is the same as the starting sequence number of the reordering buffer BA window. It is understood that the BA window and the reordering buffer window specify / define the same range of sequence numbers.

[0055] In some embodiments, the coordination mechanism is initiated in response to a roaming request. The roaming request can be a standardized message, such as an ST (transition) execution request issued by non-AP MLD 30, indicating its intention to perform a Seamless Mobile Domain Basic Service Set (SMD BSS) handover. Upon receiving the roaming request, a comprehensive coordination process is triggered. First, the first AP MLD 10 can determine information indicating SN restrictions based on the state of its transmission queue (specifically, the start sequence number of at least one MPDU in the first AP MLD 10's transmission queue) or the start sequence number of the BA window. Then, the first AP MLD 10 sends the information indicating SN restrictions to the second AP MLD 20, thereby controlling AP MLD 20 to perform SN-restricted downlink transmissions according to that information. The information indicating SN restrictions can be dynamically updated; alternatively, the SN restriction can be lifted by explicitly sending a cancellation message when the first AP MLD 10 does not need to perform downlink transmissions to the non-AP MLD 30 (e.g., after completing all MPDUs in its transmission queue).

[0056] It is important to note that the roaming request triggering this method can be received by different physical devices (such as the first / second AP MLD) in different embodiments. For example, a Seamless Mobile Domain Basic Service Set (SMD BSS) handover (ST) can be initiated by a non-AP MLD 30 transmitting an ST execution request, which is functionally equivalent to a roaming request. In practice, the non-AP MLD 30 can transmit the roaming request (e.g., an ST execution request) to its current AP MLD (the first AP MLD 10). Alternatively, the non-AP MLD 30 can also directly transmit the roaming request (e.g., an ST execution request) to the target AP MLD (the second AP MLD 20). The coordination mechanism of this invention is applicable to both scenarios because, regardless of which AP MLD initially receives the request, the first AP MLD 10 is aware of the roaming intention of the non-AP MLD 30, thereby enabling the first AP MLD 10 to transmit information indicating SN restrictions to the second AP MLD 20. For ease of explanation and understanding, the following embodiments are illustrated by taking the first AP MLD 10 receiving a roaming request as an example, but the present invention is not limited thereto.

[0057] In this embodiment of the invention, the transmission window Q3 can be considered as a reordering buffer window, wherein the reordering buffer window and the BA window have the same SN range and the same window size (denoted as W). This allows the state of the BA window / transmission queue Q1 (e.g., starting sequence number / offset) to manage the boundaries of the reordering buffer window. Therefore, in this invention, the terms BA window and reordering buffer window can be used interchangeably to refer to the acknowledgment and reordering of the non-AP MLD 30.

[0058] Initially, before the non-AP MLD 30 sends a roaming request, data transmission can be performed between the first AP MLD 10 and the non-AP MLD 30; that is, the processor 10c of the first AP MLD 10 is configured to exchange frames with the non-AP MLD 30 via transceiver 10a. Next, as... Figure 1 As shown, during the roaming process of non-AP MLD 30 switching from the first AP MLD 10 to the second AP MLD 20, processor 10c is further configured to send first information to the second AP MLD 20 to indicate sequence number (SN) restrictions. The purpose of transmitting the first information is to restrict the second AP MLD 20 from exceeding the reordering buffer window of the non-AP MLD 30 when performing downlink transmissions to the non-AP MLD 30, that is, to cause the second AP MLD 20 to perform SN-restricted downlink transmissions according to the first information.

[0059] The first information used to indicate the SN limit can be implemented in various ways. In summary, the information used to indicate the SN limit is related to the starting sequence number and enables the second AP ML 20 to determine the boundaries (SN limit value) of a window (e.g., a BA window) based on this information, ensuring that all transmitted MPDUs are within the current window (i.e., not outside the window). For example, but not limited to, in a first exemplary embodiment, the first information may include the starting sequence number of the transmission queue Q1 / BA window (e.g., starting sequence number = N). In this case, the second AP MLD 20 may be configured to derive the first SN limit value based on the received starting sequence number and the known window size W. In a second exemplary embodiment, the first information may include a calculated SN limit value SN', determined by the first AP MLD 10 based on the starting sequence number and the window size W. In this case, SN' may be determined as the first SN limit value within the BA window, for example, SN' = N + W - 1.

[0060] It should be understood that, in embodiments of the present invention, the starting sequence number (e.g., N) serves as a core parameter of the coordination mechanism. In one embodiment, the first information sent by the first AP MLD 10 to indicate SN limits may include the starting sequence number (e.g., N) itself, thereby allowing the second AP MLD 20 to derive the final SN limit value based on the window size W. Various exemplary embodiments of the present invention are based on this core parameter. For example, in one embodiment, the SN limit value may be calculated as a function of the starting sequence number and the window size (e.g., N + W - 1) to prevent any packet loss (this scenario may be referred to as the security mode). In another embodiment, the SN limit value may be calculated as a function of the starting sequence number and the transmission queue size (e.g., N + A) to suspend downlink transmissions of the second AP MLD 20 (this scenario may be referred to as the priority mode). These different embodiments all rely on the starting sequence number as the fundamental basis for coordination, demonstrating the flexibility and broad applicability of the present invention. In embodiments of the present invention, the information indicating SN limits is determined based on the starting sequence number of at least one MPDU in the transmission queue of the first AP MLD 10; or, the first information is determined based on the starting sequence number of the BA window.

[0061] It should be noted that the following detailed description of embodiments, provided in conjunction with the accompanying drawings, is intended to illustrate rather than limit the scope of the invention. These exemplary embodiments are intended to enable those skilled in the art to more effectively understand and practice the core concepts of the invention. It should be understood that the invention is not limited to these specific examples.

[0062] Figure 2 This is a schematic diagram illustrating the restriction cancellation mechanism of the wireless communication system 100. Figure 2 Depicting in Figure 1This is an operational state following the initial state shown, for example, the first AP MLD 10 has completed downlink transmission to the non-AP MLD 30 or no longer needs to perform downlink transmission to the non-AP MLD 30. (As previously...) Figure 1 The wireless communication system 100 includes a first AP MLD 10, a second AP MLD 20, and a non-AP MLD 30.

[0063] like Figure 2 As shown, the transmission queue Q1 in the memory 10b of the first AP MLD 10 is now empty. The empty state of the transmission queue Q1 indicates that all older packets have been successfully transmitted to the non-AP MLD 30 or have been discarded; that is, the first AP MLD 10 does not need to perform downlink transmission to the non-AP MLD 30. This state can trigger the first AP MLD 10 to cancel the previously established SN restriction.

[0064] After determining that transmission queue Q1 is empty, the processor 10c of the first AP MLD 10 is configured to send a cancellation message indicating the cancellation of the SN restriction to the second AP MLD 20 via a direct / indirect path. The purpose of the cancellation message is to notify the second AP MLD 20 that the SN restriction is no longer valid, thereby allowing the second AP MLD 20 to independently perform downlink transmissions (i.e., independently transmit subsequent MPDUs) without being subject to the previously sent information indicating the SN restriction or the restriction imposed by the first AP MLD. Figure 2 As shown, cancellation messages can be transmitted via different paths. In one embodiment, the cancellation message is transmitted via a direct path, where the first AP MLD 10 sends the cancellation message directly to the second AP MLD 20, for example, via a backhaul network. In another embodiment, the cancellation message is transmitted via an indirect path, where the first AP MLD 10 sends the cancellation message to a non-AP MLD 30, which then relays the cancellation message to the second AP MLD 20. This invention does not impose any limitations on this approach.

[0065] Upon receiving the cancellation message, the processor 20c of the second AP MLD 20 processes the cancellation message and stops applying the SN restrictions indicated by the aforementioned information. Therefore, transmission window Q3 is no longer restricted by the transmission activity of the first AP MLD 10. In other words, the second AP MLD 20 is allowed to independently transmit data packets from its transmission queue Q2, thereby resuming normal, unrestricted downlink operation / transmission. This restriction cancellation mechanism ensures the timely and efficient completion of the roaming process.

[0066] Apart from Figure 2In addition to the explicit cancellation mechanism shown, the SN restriction can also be implicitly terminated. In some embodiments, the second AP MLD 20 may automatically relinquish the SN restriction when a defined event marking the end of a concurrent transmission period occurs. For example, the processor 20c of the second AP MLD 20 may be configured to automatically ignore the SN restriction when the roaming process is fully completed. Alternatively, the processor 20c may automatically relinquish the SN restriction when it is detected that there is no longer an active communication link between the non-AP MLD 30 and the first AP MLD 10. This automatic relinquishment implements a robust fail-safe function, ensuring that the second AP MLD 20 is not unnecessarily restricted even if an explicit cancellation message is not received.

[0067] Figure 3A This is a schematic diagram illustrating the first stage of the dynamic update mechanism of the wireless communication system 100, which is an exemplary embodiment of the present invention. The wireless communication system 100 is shown as being in... Figure 1 The operational state following the initial state shown, for example, after the first AP MLD 10 successfully transmits the initial portion of the old data packets in transmission queue Q1.

[0068] like Figure 3A As shown, the state of transmission queue Q1 in memory 10b has been updated. The first X data packets (e.g., MPDUs with sequence numbers ranging from N to N+X-1) have been transmitted, so the starting sequence number (SN) at the head of transmission queue Q1 is now updated to N+X. The remaining old data packets in transmission queue Q1 are those with sequence numbers in the range {N+X, N+X+1, ..., N+A}.

[0069] In this exemplary embodiment, the processor 10c of the first AP MLD 10 can be configured to send second (i.e., updated) information indicating SN limitations based on the progress of its own transmission. The second information is configured to cause the second AP MLD 20 to control its downlink transmission based on the second SN limitation value, ensuring that the sequence number of the MPDU sent by the second AP MLD 20 to the non-AP MLD 30 does not exceed the reordering window of the non-AP MLD 30. Figure 3AAs shown, the second SN limit value (e.g., the second SN limit value is N+X+W-1) can be defined as the updated starting sequence number (e.g., N+X) plus the window size (such as the size W of the BA window) minus one. Because X is a positive integer, the second SN limit value is greater than the previously established first SN limit value. Then, the processor 10c controls the transceiver 10a to send second information related to the updated starting sequence number to the second AP MLD 20. This updated SN limit / information allows the second AP MLD 20 to transmit additional data packets from its transmission queue Q2, thereby preventing the second AP MLD 20 from unnecessarily pausing.

[0070] Figure 3B This is a schematic diagram illustrating the second stage (following the first stage) of the dynamic update mechanism of the wireless communication system 100, which can be considered as another exemplary embodiment of the present invention. The wireless communication system 100 is shown as being in... Figure 3A The operation states shown are intended to demonstrate the iterative nature of the update process.

[0071] like Figure 3B As shown, downlink transmission of the first AP MLD 10 continues. Transmission queue Q1 advances further, indicating that since... Figure 3A Since the operation state shown, an additional Y data packets have been transmitted. The starting sequence number at the head of transmission queue Q1 is now updated to N+X+Y.

[0072] In this exemplary embodiment, the processor 10c of the first AP MLD 10 can be configured again to send third (updated) information indicating SN limitations. Figure 3B As shown, the updated SN limit will apply a third SN limit value, which (e.g., N+X+Y-1) can be defined as the new (updated) starting sequence number (e.g., N+X+Y) plus the window size (W) minus one. Then, processor 10c controls transceiver 10a to transmit the third information related to the new (updated) starting sequence number to the second AP MLD 20. This iterative update process, as... Figure 3A and Figure 3B As shown, by gradually updating SN restrictions, we can ensure the effective use of spectrum resources during the roaming transition.

[0073] Figure 4 A schematic diagram of a pause mechanism of a wireless communication system 100 is shown in another exemplary embodiment of the present invention. Figure 4 An embodiment is described in which a first AP MLD 10 establishes a specific SN restriction designed to temporarily suspend downlink transmissions of a second AP MLD 20, thereby granting the first AP MLD 10 exclusive transmission priority.

[0074] like Figure 4 As shown, to implement the pause mechanism, the processor 10c of the first AP MLD 10 can be configured to determine a specific SN limit value SN'. This specific SN limit value SN' corresponds to the sequence number of the last data packet in the transmission queue Q1 of the first AP MLD 10. For example, as Figure 4 As shown, the specific SN limit value SN' is equal to N+A, which is smaller than the baseline security value (i.e., the SN limit value for the transmission window described in the previous embodiment) N+W-1. Then, the processor 10c controls the transceiver 10a to transmit specific information indicating this specific SN limit value SN' to the second AP MLD 20. For example, this specific information may include the starting sequence number (e.g., N) and the size of the transmission queue Q1 (e.g., A), from which the second AP MLD 20 can derive the specific SN limit value SN'. Alternatively, the specific information may also directly include the specific SN limit value SN' (e.g., N+A) itself.

[0075] The technical effect of this mechanism is manifested in the "pause" state of the second AP MLD 20. After receiving the specific information and acquiring the specific SN limit value N+A, since all available SNs (e.g., starting from N+A+1) in the second AP MLD 20's transmission queue Q2 are greater than the specific SN limit value (e.g., N+A), the control logic of the processor 20c will prevent the transceiver 20a of the second AP MLD 20 from transmitting any data packets. This pause operation mode allows the first AP MLD 10 to preferentially clear old data packets in its transmission queue Q1 without contention from the second AP MLD 20.

[0076] Figure 5 A schematic diagram of a permissive packet drop mechanism of a wireless communication system 100 is shown in another exemplary embodiment of the present invention. Figure 5 An embodiment is described in which a first AP MLD 10 establishes a packet loss-allowing mechanism that prioritizes the timely transmission of new data packets from the second AP MLD 20 at the cost of allowing some older data packets from the first AP MLD 10 to be discarded.

[0077] like Figure 5As shown, to implement this packet loss-allowing mechanism, the processor 10c of the first AP MLD 10 is configured to determine a specific sequence number (SN) limit value SN' that is greater than the baseline security value N+W-1. Then, the processor 10c controls the transceiver 10a to send specific information to the second AP MLD 20 to indicate this higher SN limit value SN'.

[0078] The technical effect of this mechanism is to strike a trade-off between the latency of new data and the reliability of old data. By receiving a higher SN limit value, the second AP MLD 20 is immediately allowed to transmit newer data packets with SN greater than N+W-1. The reception of these packets on the non-AP MLD 30 side advances the transmission window Q3. For example... Figure 5 As indicated by the symbol α, this advance causes some older data packets in transmission queue Q1 to fall outside the range of the new advance in transmission window Q3. Therefore, the non-AP MLD 30 discards α older data packets. However, in exchange, the second AP MLD 20 is able to transmit α newer data packets that would otherwise have been paused. This operating mode is particularly useful for latency-sensitive applications, where, in some applications, timely delivery of the latest data is more important than ensuring the reception of every older data packet.

[0079] Figure 6 A schematic diagram of a first scenario of a request mechanism in a wireless communication system 100 is shown according to another embodiment of the present invention. Figure 6 An embodiment is described in which a second AP MLD 20 is configured to proactively request new SN restrictions / information from a first AP MLD 10 to obtain updated information for indicating SN restrictions.

[0080] like Figure 6 As shown, this scenario can occur after the second AP MLD 20 has transmitted all packets allowed under the currently valid SN limit. For example, if the initial SN limit is N+W-1, the second AP MLD 20 has transmitted all packets destined for this SN. The next available packet in the second AP MLD 20's transmission queue Q2 has an SN of N+W, exceeding the current SN limit, causing the second AP MLD 20 to suspend its transmission.

[0081] In this embodiment, the processor 20c of the second AP MLD 20 can be configured to generate a request message. The processor 20c then controls the transceiver 20a to send the request message to the first AP MLD 10. The request message requests the first AP MLD 10 to send new information indicating SN limitations (intended for restricted transmission based on a higher SN limitation value). In response to receiving the request message, the processor 10c of the first AP MLD 10 can determine and transmit second (updated) information indicating SN limitations to the second AP MLD 20, enabling the second AP MLD 20 to resume its downlink transmission based on the second information. This bidirectional communication mechanism achieves a more adaptive and efficient coordination process.

[0082] Figure 7 A schematic diagram of a second scenario of a request mechanism in a wireless communication system 100 is shown according to another exemplary embodiment of the present invention. Figure 7 Depicting something similar to Figure 6 The exemplary embodiment shown illustrates that the second AP MLD 20 actively requests a new SN restriction. However, Figure 7 This demonstrates a more predictable request trigger.

[0083] like Figure 7 As shown, even if the second AP MLD 20 still has some allowed packets available for transmission in its transmission queue Q2, the request mechanism can still be initiated. Transmission queue Q2 contains packets with SNs (such as N+W-2 and N+W-1), all of which are less than or equal to the initial SN limit value N+W-1. In this scenario, the processor 20c of the second AP MLD 20 can be configured to send a request message before all allowed packets in transmission queue Q2 are completely exhausted. Furthermore, the anticipated request can be triggered based on various conditions, which are not limited by this invention. For example, when the remaining number of allowed packets is below a certain threshold, it indicates that transmission queue Q2 is expected to be exhausted soon, and a request may be sent. Optionally, this triggering may be based on Quality of Service (QoS) standards. For example, if a packet with strict latency requirements is nearing its transmission deadline but is blocked by other packets ahead in the queue, the processor 20c can also send a request to obtain a higher SN limit value earlier. This mechanism ensures that time-sensitive data can be transmitted without delay once the preceding packets have been sent, thereby improving support for latency-sensitive applications.

[0084] Figure 8An exemplary embodiment of the present invention illustrates a method for establishing transmission restrictions performed by a first AP MLD 10 in a wireless communication system 100. This method provides a detailed, step-by-step procedure for initiating SN restrictions. It should be noted that... Figure 8 This is an exemplary description for ease of explanation and understanding, but the present invention is not limited to this exemplary embodiment.

[0085] The method begins at step S801, where a roaming request is received from non-AP MLD 30 via transceiver 10a. In step S802, processor 10c of the first AP MLD 10 processes the roaming request to obtain the identifier (ID) of the non-AP MLD 30 and the ID of the target AP MLD (e.g., the second AP MLD 20). Next, processor 10c collects necessary parameters. In step S803, processor 10c obtains the size W of the block acknowledgment (BA) window of the non-AP MLD 30. In step S804, processor 10c reads the starting sequence number (e.g., starting sequence number = N) from the head of the transmission (TX) queue Q1 stored in memory 10b.

[0086] Based on the acquired parameters N and W, the method proceeds to step S805, where the processor 10c calculates the SN limit value SN' according to a security mechanism, for example, SN' = N + W - 1. In particular, other embodiments of SN limiting may also be employed. This calculation is performed to define the upper limit SN value for downlink transmissions from the second AP MLD 20 to the non-AP MLD 30, thereby ensuring that the reordering buffer window of the non-AP MLD 30 can accommodate all SNs from the starting sequence number (e.g., N) to the upper limit SN value (e.g., N + W - 1) (i.e., the data packets for downlink transmissions sent by the second AP MLD 20 to the non-AP MLD 30 are within the reordering buffer window of the non-AP MLD 30). This mechanism guarantees that the oldest data packet with a starting sequence number of N remains within the valid range of the transmission window. It should be noted that in step S805, the starting sequence number may also be sent to the second AP MLD 20, which calculates the corresponding upper limit SN value (which can also be interchangeably described as the "SN limit value").

[0087] Finally, in step S806, processor 10c controls transceiver 10a to send information indicating SN restrictions to the target AP MLD (e.g., the second AP MLD 20). This information may include a calculated SN restriction value SN' or a start sequence number or other SN restriction value. This transmission of information may be part of a context transfer between the two AP MLDs. This example process ends in step S807.

[0088] Figure 9 This is a flowchart illustrating a method for canceling a transmission (TX) restriction (i.e., a serial number (SN) restriction) performed by a first access point (AP) MLD of a wireless communication system 100 according to an exemplary embodiment of the present invention. The method provides a detailed, step-by-step implementation process for the first AP MLD 10 to terminate a previously established SN restriction. The method begins at step S901, where a trigger condition is met, indicating that a transmission from the first AP MLD 10 to the non-AP MLD 30 has been completed. For example, this condition may be met when the processor 10c determines that the transmission queue Q1 in the memory 10b is empty, i.e., all data packets have been transmitted or discarded.

[0089] In response to the triggering condition, the method proceeds to step S902, which is the action of the cancellation process. In step S902, the processor 10c of the first AP MLD 10 controls the transceiver 10a to send a cancellation message to the target AP MLD (second AP MLD 20) via a direct or indirect path. The purpose of the cancellation message is to explicitly indicate the need to terminate the TX restriction (i.e., the SN restriction), thereby releasing the transmission constraint of the second AP MLD 20. Therefore, the second AP MLD 20 independently transmits subsequent MPDUs to the non-AP MLD 30 without being subject to the information indicating the SN restriction or the restriction of the first AP MLD 10. This process ends in step S903.

[0090] Figure 10 This is a flowchart illustrating a method for controlling downlink transmission based on received information indicating SN limitations, performed by a second AP MLD 20 according to an exemplary embodiment of the present invention. The method provides a detailed step-by-step process of the control logic of the second AP MLD 20 considering each transmitted data packet during roaming.

[0091] This method is executed by the processor 20c of the second AP MLD 20 and begins each time a data packet (e.g., a MAC Protocol Data Unit (MPDU)) is to be transmitted from the transmission queue Q2 (step S1001). In step S1002, the processor 20c first determines whether there is a transmission restriction (i.e., SN restriction) for the transmission queue Q2. If no restriction is active (also described as "valid"), the method proceeds directly to step S1006 for normal downlink transmission.

[0092] If it is determined in step S1002 that the transmission restriction (i.e., the SN restriction) is active, the processor 20c continues to execute the control logic. In step S1003, the processor 20c obtains the currently active SN restriction value SN'. In step S1004, the processor 20c obtains the specific SN of the MPDU that is being considered for transmission.

[0093] In step S1005, processor 20c performs a core comparison of the control logic by determining whether the SN of the MPDU is greater than the acquired SN limit value SN'. If the comparison result in step S1005 is negative (i.e., the SN of the MPDU is less than or equal to SN'), the MPDU is considered to meet the transmission conditions. The method then proceeds to step S1006, where processor 20c controls transceiver 20a to transmit the MPDU to non-AP MLD 30. However, if the comparison result in step S1005 is positive (i.e., the SN of the MPDU is greater than SN'), the MPDU is considered to not meet the conditions. In this case, the transmission of the MPDU is abandoned, and the MPDU process ends in step S1007. This logic ensures that the second AP MLD 20 strictly adheres to the SN limit established by the first AP MLD 10, thereby ensuring that the transmitted MPDU is within the BA window or within the reordering buffer window of the non-AP MLD 30.

[0094] In another exemplary embodiment, the processor 20c of the second AP MLD 20 can be configured to implement a transmission and discard mechanism to manage its transmission queue Q2. According to this mechanism, after the processor 20c controls the transceiver 20a to transmit all compliant data packets (i.e., those whose SN is less than or equal to the SN' value), the processor 20c can identify any remaining data packets in the transmission queue Q2 as ineligible. Instead of pausing and continuing to buffer these ineligible data packets, the processor 20c can choose to discard or clear the ineligible data packets from the memory 20b. This mode of operation may be useful for managing buffer resources or for applications that do not wish to transmit stale data after a potentially long pause.

[0095] In another embodiment, the processor 20c of the second AP MLD 20 can be configured to ignore sequence number (SN) limits under certain conditions to ensure system robustness. For example, a timer can be started when transmission of the second AP MLD 20 is paused due to SN limits. If the processor 20c does not receive updated SN limit information or a cancellation message from the first AP MLD 10 before the timer expires, the processor 20c can determine that the first AP MLD 10 is not responding or has encountered an error. In response to such a timeout event, the processor 20c can be configured to ignore the previously established SN limit value and continue to control the transceiver 20a to transmit data packets, even if their SNs exceed the previous SN limit value. This fault protection mechanism prevents the second AP MLD 20 from being blocked indefinitely, thereby avoiding system stagnation and ensuring the completion of data transmission.

[0096] It should be understood that the determination of SN limits and their corresponding SN limit values ​​can be performed by different entities in the wireless communication system 100. Although Figure 8 and Figure 9 The embodiments described illustrate a scenario where SN constraints are established by the first AP MLD 10, while Figure 6 and Figure 7 The embodiments described illustrate a scenario where a request from the second AP MLD 20 triggers an SN limit update, but other implementations are possible. In another embodiment, the SN limit value can be determined through negotiation between the first AP MLD 10 and the second AP MLD 20. For example, the two AP MLDs can exchange messages to reach a mutually acceptable SN limit value to balance the transmission demands of the two access points. In yet another embodiment, the SN limit can be determined based on a set of predefined coordination rules established by the SMD itself. In this case, both the first AP MLD 10 and the second AP MLD 20 will follow these centralized rules enforced by the network to ensure consistent and predictable behavior across the entire SMD.

[0097] In summary, these embodiments provide a method and apparatus for dynamically and adaptively controlling the transmission window during non-AP MLD roaming. Unlike conventional systems where uncoordinated transmissions lead to data loss, these embodiments enable the first AP MLD and the second AP MLD to coordinate downlink transmissions based on SN constraint information. Based on this coordination, the second AP MLD can control its downlink transmissions to limit them from exceeding the reordering buffer window of the non-AP MLD during downlink transmissions, preventing premature advancement of the transmission window on the AP side and thus avoiding the erroneous discarding of older data packets from the first AP MLD. Therefore, these embodiments provide a robust and mutually optimized communication mechanism with enhanced reliability and efficiency.

[0098] The present invention may be presented in other specific formats without departing from the spirit and scope thereof. The described embodiments are for illustrative purposes only and are not intended to limit the invention. The scope of protection of the present invention is defined by the appended claims. Those skilled in the art will be able to make various modifications and refinements without departing from the spirit and scope of the invention.

[0099] While the invention has been described by way of example and according to preferred embodiments, it should be understood that the invention is not limited to the disclosed embodiments. Rather, it is intended to cover various variations and similar structures (as will be apparent to those skilled in the art), such as combinations or substitutions of different features in different embodiments. Therefore, the scope of the appended claims should be given the broadest interpretation to cover all such variations and similar structures.

Claims

1. A method of wireless communication performed by a first access point multi- link device (AP MLD), wherein, The method comprises: exchanging frames with a non-access point multi-link device (non-AP MLD); and in a roaming process in which the non-AP MLD switches from the first AP MLD to a second AP MLD, sending, to the second AP MLD, first information indicating a sequence number (SN) limit to limit the second AP MLD from exceeding a reordering buffer window of the non-AP MLD when performing downlink transmission to the non-AP MLD.

2. The method of claim 1, wherein, The first information is determined based on at least a starting sequence number of at least one MAC protocol data unit (MPDU) in a transmission queue of the first AP MLD.

3. The method of claim 1, wherein, The first information is determined based on at least a starting sequence number of a block acknowledgement (BA) window of the first AP MLD.

4. The method of claim 1, wherein, The first information is determined based on a window size and a starting sequence number of at least one MAC protocol data unit (MPDU) in a transmission queue of the first AP MLD; or the first information is determined based on a window size and a starting sequence number of a block acknowledgement (BA) window of the first AP MLD.

5. The method of claim 1, wherein, The first information indicating the SN limit is configured to enable the second AP MLD to control downlink transmission to the non-AP MLD based on a first SN limit value to ensure that MAC protocol data units (MPDUs) transmitted to the non-AP MLD are within the reordering buffer window, wherein the first SN limit value is defined as the starting sequence number plus the window size minus one, and the first information is related to the starting sequence number.

6. The method of claim 5, wherein, The method further comprises: after one or more MPDUs in the transmission queue of the first AP MLD are transmitted from the first AP MLD to the non-AP MLD or in response to a request of the second AP MLD, sending, to the second AP MLD, second information indicating the SN limit to enable the second AP MLD to control downlink transmission to the non-AP MLD based on a second SN limit value; wherein the second SN limit value is defined as an updated starting sequence number plus the window size minus one, and the second information is related to the updated starting sequence number, and the second SN limit value is greater than the first SN limit value.

7. The method of claim 1, wherein, The method further comprises: sending a cancellation message to cancel the SN limit.

8. A method of wireless communication performed by a second access point multi- link device (AP MLD), wherein, The method comprises: in a roaming process in which a non-access point multi-link device (non-AP MLD) switches from a first AP MLD to the second AP MLD, receiving, from the first AP MLD, first information indicating a sequence number (SN) limit; and controlling downlink transmission to the non-AP MLD based on the first information to ensure that MAC protocol data units (MPDUs) transmitted to the non-AP MLD are within a reordering buffer window of the non-AP MLD.

9. The method of claim 8, wherein, The first information is determined based on at least a starting sequence number of at least one MPDU in a transmission queue of the first AP MLD; or the first information is determined based on at least a starting sequence number of a block acknowledgement (BA) window of the first AP MLD.

10. The method of claim 8, wherein, The first information is determined based on a window size and a starting sequence number of at least one MPDU in a transmission queue of the first AP MLD; or the first information is determined based on a window size and a starting sequence number of a block acknowledgement (BA) window of the first AP MLD.

11. The method of claim 8, wherein, Controlling the downlink transmission to the non-AP MLD based on the first information comprises: Controlling the downlink transmission to the non-AP MLD based on a first SN limit value to ensure that MPDUs transmitted to the non-AP MLD do not exceed the first SN limit value; wherein the first SN limit value is defined as the starting sequence number plus the window size minus one, and the first information is related to the starting sequence number.

12. The method of claim 8, wherein, The method further comprises: receiving second information indicating a SN limit, wherein the second information is different from the first information; and controlling the downlink transmission to the non-AP MLD based on the second information.

13. The method of claim 8, wherein, The method further comprises: receiving a cancellation message indicating cancellation of the SN limit; and in response to receiving the cancellation message, transmitting subsequent MPDUs to the non-AP MLD independently without being subject to the first information or the first AP MLD.

14. A first access point multi-link device (AP MLD) comprising a transceiver and a processor coupled with the transceiver, wherein, The processor is configured to: exchange frames with a non-access point multi-link device (non-AP MLD); and in a roaming process in which the non-AP MLD switches from a first AP MLD to a second AP MLD, send, via the transceiver, first information indicating a sequence number (SN) limit to the second AP MLD to limit the second AP MLD from exceeding a reordering buffer window of the non-AP MLD when performing downlink transmission to the non-AP MLD.

15. The first AP MLD of claim 14, wherein, The first information is determined based on at least a starting sequence number of at least one MAC protocol data unit (MPDU) in a transmission queue of the first AP MLD.

16. The first AP MLD of claim 14, wherein, The first information is determined based on at least a starting sequence number of a block acknowledgement (BA) window of the first AP MLD.

17. The first AP MLD of claim 14, wherein, The first information is determined based on a window size and a starting sequence number of at least one MAC protocol data unit (MPDU) in a transmission queue of the first AP MLD; or the first information is determined based on a window size and a starting sequence number of a block acknowledgement (BA) window of the first AP MLD.

18. The first AP MLD of claim 14, wherein, The first information indicating the SN limit is configured to cause the second AP MLD to control the downlink transmission to the non-AP MLD based on a first SN limit value to ensure that MAC protocol data units (MPDUs) transmitted to the non-AP MLD are within the reordering buffer window, wherein the first SN limit value is defined as the starting sequence number plus the window size minus one, and the first information is related to the starting sequence number.

19. The first AP MLD of claim 18, wherein, The processor is further configured to: transmit, by the transceiver, second information indicating a sequence number (SN) limit to the non-AP MLD based on a second SN limit value, after one or more MPDUs in a transmission queue of the first AP MLD are transmitted from the first AP MLD to the non-AP MLD or in response to a request of the second AP MLD, to cause the second AP MLD to control downlink transmissions to the non-AP MLD based on the second SN limit value. wherein the second SN limit value is defined as an updated starting sequence number plus the window size minus one, and the second information is related to the updated starting sequence number, and the second SN limit value is greater than the first SN limit value.

20. The first AP MLD of claim 14, wherein, The processor is further configured to: transmit, by the transceiver, a cancellation message for cancelling the SN limit.

21. A second access point multi-link device (AP MLD) comprising a transceiver and a processor coupled with the transceiver, wherein, The processor is configured to: receive, by the transceiver, first information indicating a sequence number (SN) limit from a first AP MLD during a roaming procedure in which a non-AP multi-link device (non-AP MLD) is switching from the first AP MLD to a second AP MLD; and control downlink transmissions to the non-AP MLD based on the first information to ensure that MAC protocol data units (MPDUs) transmitted to the non-AP MLD are within a reordering buffer window of the non-AP MLD.

22. The second AP MLD of claim 21, wherein, The first information is determined based on at least a starting sequence number of at least one MPDU in a transmission queue of the first AP MLD; or the first information is determined based on at least a starting sequence number of a block acknowledgement (BA) window of the first AP MLD.

23. The second AP MLD of claim 21, wherein, The first information is determined based on a window size and a starting sequence number of at least one MPDU in a transmission queue of the first AP MLD; or the first information is determined based on a window size and a starting sequence number of a block acknowledgement (BA) window of the first AP MLD.

24. The second AP MLD of claim 21, wherein, The processor is further configured to: control downlink transmissions to the non-AP MLD based on a first SN limit value to ensure that MPDUs transmitted to the non-AP MLD do not exceed the first SN limit value; wherein the first SN limit value is defined as a starting sequence number plus a window size minus one, and the first information is related to the starting sequence number.

25. The second AP MLD of claim 21, wherein, The processor is further configured to: receive second information indicating a SN limit, wherein the second information is different from the first information; and control downlink transmissions to the non-AP MLD based on the second information.

26. The second AP MLD of claim 21, wherein, The processor is further configured to: receive, by the transceiver, a cancellation message for cancelling the SN limit; and in response to receiving the cancellation message, independently transmit subsequent MPDUs to the non-AP MLD without being subject to the first information or the first AP MLD.

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