Wireless communication method and related access point multi-link device

Abstract

A method and apparatus for coordinating downlink transmissions during wireless roaming are disclosed. A first access point multilink device (AP MLD) exchanges frames with a non-access point multilink device (non-AP MLD). During roaming, when the non-AP MLD switches to a second AP MLD, the first AP MLD sends first information to the second AP MLD indicating sequence number (SN) restrictions. In response, the second AP MLD controls its downlink transmissions based on the first information. This control is executed to ensure that MAC Protocol Data Units (MPDUs) transmitted from the second AP MLD to the non-AP MLD are within the reordering buffer window of the non-AP MLD, thereby preventing data loss and improving communication reliability.

Description

Technical Field

[0001] This disclosure generally relates to wireless communication technology, and more particularly, to a wireless communication method and related access point multilink device for coordinating transmission window control during roaming of multiple access point MLDs. Prior 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 transmission between the two AP MLDs.

[0003] Typically, MPDUs 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 valid 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] The following summary is illustrative only and is not intended to be limiting in any way. That is, it provides an overview to introduce the concepts, key points, benefits, and advantages of the novel and non-obvious techniques described herein. Selected embodiments are further described in the detailed description below. Therefore, the following summary is neither intended to identify the essential features of the claimed subject matter nor to define the scope of the claimed subject matter.

[0006] In a first aspect, the present invention provides a wireless communication method performed by a first access point multi-link device (AP MLD), wherein the method includes: performing frame exchange with a non-access point multi-link 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 transmission to the non-AP MLD.

[0007] 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.

[0008] 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.

[0009] 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.

[0010] In some embodiments, the first information indicating SN limitation is configured to cause the second AP MLD to control downlink transmission to the non-AP MLD based on the first SN limitation 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 limitation 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.

[0011] 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 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 restriction value is greater than the first SN restriction value.

[0012] In some embodiments, the method further includes sending a cancellation message to cancel the SN restriction.

[0013] 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.

[0014] 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.

[0015] 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.

[0016] 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.

[0017] In some embodiments, the method further includes: receiving second information for indicating SN restrictions, wherein the second information is different from the first information; and controlling downlink transmission of the non-AP MLD based on the second information.

[0018] 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.

[0019] Thirdly, the present invention provides a first access point multilink device (AP MLD), 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, 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.

[0020] 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.

[0021] 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.

[0022] 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.

[0023] In some embodiments, the first information indicating SN limitation is configured to cause the second AP MLD to control downlink transmission to the non-AP MLD based on the first SN limitation 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 limitation 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.

[0024] 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.

[0025] In some embodiments, the processor is further configured to send a cancellation message via the transceiver to cancel the SN restriction.

[0026] Fourthly, the present invention provides a second access point multilink device (AP MLD), including a transceiver and a processor coupled to the transceiver, wherein the processor is configured to: during roaming when a non-AP MLD switches from a first AP MLD to the second AP MLD, receive first information from the first AP MLD via the transceiver for indicating sequence number (SN) restrictions; 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.

[0027] 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.

[0028] 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.

[0029] 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.

[0030] 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.

[0031] 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.

[0032] This invention provides a method and apparatus for transmission window control during roaming in a multi-AP MLD system. By coordinating information used to indicate SN limits between the first and second AP MLDs, premature advance of the transmission window can be prevented (by limiting the second AP MLD to not exceed the reordering buffer window of the non-AP MLD when making downlink transmissions to the non-AP MLD using 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.

[0033] The present invention has been provided by way of example and is not intended to be limiting. Other embodiments and advantages are described in the following detailed description. The invention is defined by the claims. Simple Explanation of the Diagram

[0034] The accompanying drawings (in which the same numerals denote the same components) illustrate embodiments of the present invention. The included drawings are used to provide a further understanding of embodiments of the present disclosure, and are incorporated in and constitute a part of the present disclosure. The drawings illustrate implementations of embodiments of the present disclosure and, together with the description, serve to explain the principles of the embodiments of the present disclosure. It is understood that the drawings are not necessarily drawn to scale, as some components may be shown out of proportion to actual dimensions in order to clearly illustrate the concepts of the embodiments of the present disclosure. Figure 1 is a schematic diagram of a wireless communication system in an initial state during roaming, according to an embodiment of the present invention. Figure 2 is a schematic diagram of the restriction removal mechanism of the wireless communication system in Figure 1. Figure 3A is a schematic diagram of the first stage of the dynamic update mechanism of the wireless communication system in Figure 1. Figure 3B is a schematic diagram of the second stage (after the first stage) of the dynamic update mechanism of the wireless communication system in Figure 1. Figure 4 is a schematic diagram of the pause restriction mechanism of the wireless communication system in Figure 1. Figure 5 is a schematic diagram of the permissive packet loss policy of the wireless communication system in Figure 1. Figure 6 is a schematic diagram of the first scenario of the request mechanism within the wireless communication system in Figure 1. Figure 7 is a schematic diagram of the second scenario of the request mechanism within the wireless communication system in Figure 1. Figure 8 is a flowchart illustrating the method for establishing transmission restrictions executed by the first access point multi-link device (AP MLD) of the wireless communication system in Figure 1. Figure 9 is a flowchart illustrating the method for canceling transmission restrictions executed by the first AP MLD of the wireless communication system in Figure 1. Figure 10 is a flowchart illustrating the method for controlling transmission based on received restriction information executed by the second AP MLD of the wireless communication system in Figure 1. In the following detailed description, numerous specific details are set forth for illustrative purposes so that those skilled in the art may 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 or different features disclosed in different embodiments may be combined as needed, and should not be limited to the embodiments illustrated in the accompanying drawings. Implementation

[0035] The following description illustrates preferred embodiments of the present invention. These embodiments are merely illustrative of the technical features of the invention and are not intended to limit the scope of the invention. Certain terms are used throughout this specification and the claims to refer to specific components. Those skilled in the art will understand that manufacturers may use different terms to refer to the same components. This specification and the claims do not distinguish components by differences in name, but by differences in function. The scope of the invention should be determined with reference to the appended claims. The terms "comprising" and "including" 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. The terms "basically" or "roughly" used in this text refer to a technical problem that can be solved and the desired technical effect achieved by someone with ordinary knowledge in the relevant technical field, within an acceptable range. For example, "roughly equal to" refers to a method that, without affecting the correctness of the result, is acceptable to someone with ordinary knowledge in the relevant technical field, with a certain margin of error compared to "exactly equal to".

[0036] 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 indicating 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 advances / slides with the transmission (successful reception) of MPDUs, and correspondingly, the reordering buffer window at the non-AP MLD advances / slides with the reception (successful reception) of MPDUs. 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.

[0037] 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.

[0038] Figure 1 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, for example, 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.

[0039] The first AP MLD 10 may be the current AP MLD that the non-AP MLD 30 connects to (or data transfers 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 buffering a transmission queue Q1. As shown in Figure 1, the transmission queue Q1 includes a plurality of older data packets (i.e., MPDUs) having consecutive sequence numbers (SNs) starting from a starting sequence number (starting 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 the transmission queue Q1. Therefore, in an exemplary embodiment of the present invention, the total number of data packets or the size of the transmission queue Q1 is A+1.

[0040] 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, memory 20b, and a processor 20c. The processor 20c may be coupled to the transceiver 20a and memory 20b to control the operation of the second AP MLD 20 according to the present invention. After a data path switch at the network end (i.e., a switch from the data path from the distribution system (DS) to the first AP MLD 10 to a data path from the DS to the second AP MLD 20), the memory 20b of the second AP MLD 20 may buffer a transmission queue Q2, which includes multiple newer data packets having a sequence number (SN) larger than that in transmission queue Q1, for example, {N+A+1, ...}. It should be noted that Figure 1 is only an illustrative description, and the invention should not be limited thereto. For example, the memory may be used to store data required by the device during operation.

[0041] The non-AP MLD 30 is a site device configured to receive downlink transmissions from the first AP MLD 10 and the 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. In the example shown in Figure 1, the transmission window Q3 can cover a sequence number range from N to N+W-1. It should be noted that the size of the non-AP MLD 30's reordering buffer window is equal to the size of the first AP MLD 10's BA window, 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 sequence number range.

[0042] 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 canceled 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).

[0043] 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, and thus the first AP MLD 10 transmits 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.

[0044] 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.

[0045] Initially, before the non-AP MLD 30 sends a roaming request, data transfer 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 shown in Figure 1, during the roaming process of the non-AP MLD 30 switching from the first AP MLD 10 to the second AP MLD 20, the 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.

[0046] 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 boundary (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 exceeding the window / 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', which is 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.

[0047] 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 by 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.

[0048] 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.

[0049] Figure 2 is a schematic diagram illustrating the restriction cancellation mechanism of the wireless communication system 100. Figure 2 depicts an operational state following the initial state shown in Figure 1, for example, where 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 described in Figure 1 above. The wireless communication system 100 includes a first AP MLD 10, a second AP MLD 20, and a non-AP MLD 30.

[0050] As shown in Figure 2, the transmission queue Q1 in memory 10b of the first AP MLD 10 is now empty. The empty state of 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 transmissions to the non-AP MLD 30. This state can trigger the first AP MLD 10 to cancel the previously established SN restriction.

[0051] 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 transmission (i.e., independently transmit subsequent MPDUs) without being restricted by the previously sent information indicating the SN restriction or by the first AP MLD. As shown in Figure 2, the cancellation message 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.

[0052] Upon receiving the cancellation message, the processor 20c of the second AP MLD 20 processes the cancellation message and stops applying the SN restriction 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 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.

[0053] In addition to the explicit cancellation mechanism shown in Figure 2, 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 the 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.

[0054] Figure 3A 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 in an operational state following the initial state shown in Figure 1, for example, after the first AP MLD 10 successfully transmits the initial portion of the old data packets in the transmission queue Q1.

[0055] As shown in Figure 3A, the state of transmission queue Q1 in memory 10b has been updated. The first X 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 packets in transmission queue Q1 are those with sequence numbers in the range {N+X, N+X+1, ..., N+A}.

[0056] 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 limits 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 limit 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. As shown in Figure 3A, 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 (e.g., 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. The processor 10c then controls the transceiver 10a to send the 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 packets from its transmission queue Q2, thus preventing the second AP MLD 20 from unnecessarily pausing.

[0057] Figure 3B is a schematic diagram illustrating the second stage (after 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 in an operating state following the operating state shown in Figure 3A, intended to demonstrate the iterative nature of the update process.

[0058] As shown in Figure 3B, downlink transmission of the first AP MLD 10 continues. Transmission queue Q1 advances further, indicating that an additional Y packets have been transmitted since the operating state shown in Figure 3A. The starting sequence number at the head of transmission queue Q1 is now updated to N+X+Y.

[0059] In this exemplary embodiment, the processor 10c of the first AP MLD 10 can be reconfigured to send third (updated) information indicating SN restrictions. As shown in Figure 3B, the updated SN restrictions will apply a third SN restriction value, which (e.g., N+X+Y-1) can be defined as the new (updated) start sequence number (e.g., N+X+Y) plus the window size (W) minus one. The processor 10c then controls the transceiver 10a to transmit the third information related to the new (updated) start sequence number to the second AP MLD 20. This iterative update process, as shown in Figures 3A and 3B, ensures efficient use of spectrum resources during roaming transitions by progressively updating the SN restrictions.

[0060] Figure 4 illustrates a schematic diagram of a pause mechanism in a wireless communication system 100 according to another exemplary embodiment of the present invention. Figure 4 depicts an embodiment 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.

[0061] As shown in Figure 4, 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 packet in the transmission queue Q1 of the first AP MLD 10. For example, as shown in Figure 4, this 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.

[0062] 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.

[0063] Figure 5 illustrates a schematic diagram of a permissive packet drop mechanism in a wireless communication system 100 according to another exemplary embodiment of the present invention. Figure 5 depicts an embodiment in which a first AP MLD 10 establishes a permissive packet drop mechanism that prioritizes the timely transmission of new data packets from a second AP MLD 20 at the cost of allowing some older data packets from the first AP MLD 10 to be discarded.

[0064] As shown in Figure 5, 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'.

[0065] The technical effect of this mechanism is to strike a trade-off between the delay 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 data packets on the non-AP MLD 30 side causes the transmission window Q3 to advance. As indicated by symbol α in Figure 5, this advance causes some older data packets in the transmission queue Q1 to fall outside the range of the new advance in the transmission window Q3. Therefore, the non-AP MLD 30 will discard 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 suspended. This operating mode is particularly useful for latency-sensitive applications, because in some applications, timely delivery of the latest data is more important than ensuring the reception of every older data packet.

[0066] Figure 6 illustrates a schematic diagram of a first scenario of a request mechanism in a wireless communication system 100 according to another embodiment of the present invention. Figure 6 depicts an embodiment in which a second AP MLD 20 is configured to actively request new SN restrictions / information from a first AP MLD 10 to obtain updated information for indicating SN restrictions.

[0067] As shown in Figure 6, 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 already transmitted all packets destined for this SN. The next available packet in the second AP MLD 20's transmission queue Q2 has a SN of N+W, exceeding the current SN limit, causing the second AP MLD 20 to suspend its transmission.

[0068] 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.

[0069] Figure 7 illustrates a second scenario of the request mechanism in the wireless communication system 100 according to another exemplary embodiment of the present invention. Figure 7 depicts an exemplary embodiment similar to that shown in Figure 6, wherein the second AP MLD 20 actively requests a new SN restriction. However, Figure 7 shows a more anticipated request triggering.

[0070] As shown in Figure 7, the request mechanism can be initiated even if the second AP MLD 20 still has some allowed packets available for transmission in its transmission queue Q2. Transmission queue Q2 shows packets containing 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, indicating that transmission queue Q2 is expected to be exhausted soon, a request may be sent. Optionally, this trigger may be based on Quality of Service (QoS) standards. For example, if a packet with strict delay requirements is nearing its transmission deadline but is blocked by other packets ahead in the queue, the processor 20c may 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 data packets have been sent, thereby improving support for latency-sensitive applications.

[0071] Figure 8 illustrates a schematic diagram of a method for establishing transmission restrictions performed by a first AP MLD 10 in a wireless communication system 100 according to an exemplary embodiment of the present invention. This method provides a detailed, step-by-step process for initiating SN restrictions. It should be noted that Figure 8 is an exemplary description for ease of illustration and understanding, but the present invention is not limited to this exemplary embodiment.

[0072] 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 the 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.

[0073] 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., packets from 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 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 can also be sent to the second AP MLD 20, and the second AP MLD 20 can calculate the corresponding SN upper limit value (which can also be interchangeably described as "SN limit value").

[0074] 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. The transmission of this information may be part of a context transfer between the two AP MLDs. This example process ends in step S807.

[0075] Figure 9 is a flowchart illustrating a method for canceling transmission (TX) restrictions (i.e., SN restrictions) performed by a first 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 previously established SN restrictions. The method begins at step S901, where a trigger condition is met, indicating that 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 memory 10b is empty, i.e., all packets have been transmitted or discarded.

[0076] In response to the triggering condition, the method proceeds to step S902, which is 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.

[0077] Figure 10 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.

[0078] 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 there is no restriction in an active state (also described as "valid"), the method proceeds directly to step S1006 for normal downlink transmission.

[0079] 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 (to be transmitted).

[0080] 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.

[0081] 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 packets (i.e., those with SN less than or equal to SN' values), the processor 20c can identify any remaining packets in the transmission queue Q2 as ineligible. Instead of pausing and continuing to buffer these ineligible packets, the processor 20c can choose to discard or clear the ineligible packets from 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.

[0082] In another embodiment, the processor 20c of the second AP MLD 20 can be configured to ignore sequence number (SN) restrictions under certain conditions to ensure system robustness. For example, when transmission of the second AP MLD 20 is suspended due to SN restrictions, a timer can be started. If the processor 20c does not receive updated SN restriction 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 has not responded or has encountered an error. In response to such a timeout event, the processor 20c can be configured to ignore the previously established SN restriction value and continue to control the transceiver 20a to transmit data packets, even if their SNs exceed the previous SN restriction 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.

[0083] 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. While the embodiments described in Figures 8 and 9 illustrate a scenario where SN limits are established by the first AP MLD 10, and the embodiments in Figures 6 and 7 demonstrate a scenario where SN limit updates are triggered by a request from the second AP MLD 20, other implementations are also 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.

[0084] 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 transmission leads 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 packets from the first AP MLD. Therefore, these embodiments provide a robust and mutually optimized communication mechanism with enhanced reliability and efficiency.

[0085] The use of ordinal terms such as “first,” “second,” and “third” in the scope of a patent application to modify patent elements does not in itself indicate any priority, order, or sequence of one patent element relative to another, or the temporal order of the execution of method actions, but is merely used as a marker to distinguish one patent element with the same name from another element with the same name.

[0086] While the embodiments and advantages of the present invention have been described in detail, it should be understood that various changes, substitutions, and modifications can be made to the present invention without departing from the spirit and scope defined by the appended claims. For example, new embodiments can be derived by combining several parts of different embodiments. 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 can make various modifications and refinements without departing from the spirit and scope of the present invention. The above description is only a preferred embodiment of the present invention. All equivalent changes and modifications made in accordance with the claims of the present invention shall be covered by the present invention.

[0087] 100: Wireless Communication System Q3: Transmission Window Q1, Q2: Transmission queues 10: First AP MLD 20: Second AP MLD 30:non-AP MLD 10b, 20b: Memory 10c, 20c: Processor 10a, 20a: Transceiver S801, S802, S803, S804, S805, S806, S807: Steps S901, S902: Steps S1001, S1002, S1003, S1004, S1005, S1006, S1007: Steps

Claims

1. A wireless communication method performed by a first access point multi-link 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 the 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; wherein the first information is determined at least based on the start 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 at least based on the start sequence number of the block acknowledgment (BA) window of the first AP MLD.

2. The method as described in request item 1, wherein, 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.

3. The method as described in request item 1, wherein, The first information used to indicate the SN limit is configured to cause the second AP MLD to control downlink transmission to the non-AP MLD based on the first SN limit 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 limit 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.

4. The method as described in request item 3, wherein, 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 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 restriction value is greater than the first SN restriction value.

5. The method as described in request item 1, wherein, The method also includes sending a cancellation message to cancel the SN restriction.

6. A wireless communication method performed by a second access point multi-link 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 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; wherein the first information is determined at least based 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 at least based on the start sequence number of a block acknowledgment (BA) window of the first AP MLD.

7. The method as described in claim 6, wherein, 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.

8. The method as described in request item 6, wherein, 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.

9. The method as described in claim 6, wherein, 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.

10. The method as described in claim 6, wherein, The method further includes: receiving a cancellation message indicating the 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.

11. A first access point multilink device (AP MLD) comprising 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 the second AP MLD, send first information via the transceiver to the second AP MLD indicating a sequence number (SN) restriction 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; wherein the first information is determined at least based on the start 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 at least based on the start sequence number of the block acknowledgment (BA) window of the first AP MLD.

12. The first AP MLD as described in claim 11, wherein, 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.

13. The first AP MLD as described in claim 11, wherein, The first information used to indicate the SN limit is configured to cause the second AP MLD to control downlink transmission to the non-AP MLD based on the first SN limit 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 limit 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.

14. The first AP MLD as described in claim 13, wherein, 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 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.

15. The first AP MLD as described in claim 11, wherein, The processor is also configured to send a cancellation message via the transceiver to cancel the SN restriction.

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