Wireless communication method using roaming and wireless communication terminal using same

Abstract

Disclosed is a non-access point (AP) multi-link device (MLD) operating on a plurality of links in a wireless LAN. The non-AP MLD comprises a transmission / reception unit and a processor. The processor: while the non-AP MLD is connected to a first AP MLD, transmits, to the first AP MLD, a roaming preparation request requesting roaming to a second AP MLD belonging to an SMD to which the first AP MLD belongs; receives, from the first AP MLD, a roaming preparation request response accepting the roaming preparation request; transmits, to the first AP MLD, a roaming execution instruction instructing execution of the roaming; receives, from the first AP MLD, information about a drain time indicating a time required to complete transmission of traffic to the non-AP MLD; and when the drain time ends, transmits, to the second AP MLD, information indicating the end of the drain time.

Description

Wireless communication method using roaming and wireless communication terminal using the same

[0001] The present invention relates to a wireless communication method that supports roaming and a wireless communication terminal that uses the same.

[0002] With the recent expansion of mobile device adoption, Wireless LAN technology, capable of providing fast wireless internet services to these devices, is gaining significant attention. Based on short-range wireless communication technology, Wireless LAN enables mobile devices—such as smartphones, smart pads, laptop computers, portable multimedia players, and embedded devices—to connect to the internet wirelessly in homes, businesses, or specific service areas.

[0003] Since supporting early wireless LAN technology using the 2.4GHz frequency, IEEE (Institute of Electrical and Electronics Engineers) 802.11 has been commercializing or developing standards for various technologies. First, IEEE 802.11b uses the 2.4GHz band and supports a maximum communication speed of 11Mbps. IEEE 802.11a, which was commercialized after IEEE 802.11b, improved communication speeds to up to 54Mbps by using the 5GHz band instead of the 2.4GHz band, employing technology that reduces the impact of interference compared to the significantly congested 2.4GHz band. However, IEEE 802.11a has the disadvantage of a shorter communication range compared to IEEE 802.11b. IEEE 802.11g, like IEEE 802.11b, uses the 2.4GHz band frequency to achieve a maximum communication speed of 54Mbps and has received considerable attention for satisfying backward compatibility, and it also has an advantage over IEEE 802.11a in terms of communication distance.

[0004] In addition, IEEE 802.11n is a technical standard established to overcome the limitations on communication speed that have been pointed out as a vulnerability in wireless LANs. IEEE 802.11n aims to increase network speed and reliability, and to extend the operating range of wireless networks. More specifically, IEEE 802.11n supports High Throughput (HT) with a data processing speed of up to 540 Mbps or higher, and is based on MIMO (Multiple Inputs and Multiple Outputs) technology, which uses multiple antennas at both the transmitter and receiver ends to minimize transmission errors and optimize data speeds. Furthermore, this standard allows for the use of coding schemes that transmit multiple redundant copies to enhance data reliability.

[0005] As the adoption of wireless LANs has increased and applications utilizing them have diversified, a need has arisen for new wireless LAN systems capable of supporting Very High Throughput (VHT) higher than the data processing speeds supported by IEEE 802.11n. Among these, IEEE 802.11ac supports a wide bandwidth (80MHz to 160MHz) in the 5GHz frequency band. Although the IEEE 802.11ac standard is defined only in the 5GHz band, early 11ac chipsets will also support operation in the 2.4GHz band to ensure backward compatibility with existing 2.4GHz band products. Theoretically, according to this standard, multi-station wireless LAN speeds can reach a minimum of 1Gbps, and maximum single-link speeds can reach a minimum of 500Mbps. This is achieved by extending the wireless interface concepts adopted in 802.11n, such as a wider wireless frequency bandwidth (up to 160 MHz), more MIMO spatial streams (up to 8), multi-user MIMO, and high-density modulation (up to 256 QAM). Additionally, there is IEEE 802.11ad, which transmits data using the 60 GHz band instead of the existing 2.4 GHz / 5 GHz bands. IEEE 802.11ad is a transmission standard that provides speeds of up to 7 Gbps using beamforming technology, making it suitable for high-bitrate video streaming, such as large amounts of data or uncompressed HD video. However, the 60 GHz frequency band has the disadvantage of being difficult to penetrate through obstacles, limiting its use to only between devices in close proximity.

[0006] Meanwhile, as a wireless LAN standard following 802.11ac and 802.11ad, the IEEE 802.11ax (High Efficiency WLAN, HEW) standard is in the final stages of development to provide high-efficiency and high-performance wireless LAN communication technology in high-density environments where APs and terminals are densely packed. In an 802.11ax-based wireless LAN environment, high frequency-efficiency communication must be provided indoors and outdoors in the presence of high-density stations and APs (Access Points), and various technologies have been developed to implement this.

[0007] In addition, development of a new wireless LAN standard has begun to increase maximum transmission speeds to support new multimedia applications such as high-definition video and real-time games. The 7th generation wireless LAN standard, IEEE 802.11be (Extremely High Throughput, EHT), is currently under development with the goal of supporting transmission rates of up to 30Gbps in the 2.4 / 5 / 6 GHz bands through wider bandwidth, increased spatial streams, and multiple AP cooperation.

[0008] Recently, discussions have begun on Ultra High Reliability (UHR) wireless LAN communication technology as a wireless LAN standard following 802.11be to overcome the reliability issues that have been pointed out as a limitation of wireless LANs. The Ultra High Reliability wireless LAN standard is currently under development with the goal of supporting low latency and low jitter for wireless LAN traffic with a high probability (e.g., over 99.9999%).

[0009] One embodiment of the present invention aims to provide a wireless communication method that supports roaming and a wireless communication terminal that uses the same.

[0010] According to one embodiment of the present invention, a non-AP (access point) multi-link device (MLD) operating on a plurality of links in a wireless LAN includes a transceiver; and a processor. When the non-AP MLD is connected to a first AP MLD, the processor transmits a roaming preparation request to the first AP MLD requesting roaming to a second AP MLD belonging to the SMD to which the first AP MLD belongs, receives a roaming preparation request response from the first AP MLD accepting the roaming preparation request, transmits a roaming execution instruction to the first AP MLD instructing the execution of the roaming, receives information regarding a drain time instructing the time required to complete the transmission of traffic to the non-AP MLD from the first AP MLD, and when the drain time ends, transmits information instructing the end of the drain time to the second AP MLD.

[0011] When the second AP MLD receives information indicating the end of the drain time and instructs the first AP MLD that roaming is completed, the first AP MLD may delete the link between the first AP MLD and the non-AP MLD.

[0012] The above roaming execution instruction may include an identifier of the second AP MLD that is the target of the roaming.

[0013] For a predetermined period of time from when the non-AP MLD transmits information indicating the end of the drain time, the non-AP MLD may not be allowed to transmit a roaming preparation request to any AP MLD included in the SMD.

[0014] Until the non-AP MLD sets up a link with the second AP MLD and completes or cancels roaming, the second AP MLD may not be allowed to change information regarding the operation of the basic service set (BSS) on the link set up with the non-AP MLD or the operational status of the set-up link.

[0015] Information regarding the operation of the above-mentioned BSS (basic service set) may include information regarding the operation channel of the above-mentioned BSS.

[0016] According to an embodiment of the present invention, a first access point (AP) multi-link device (MLD) operating on a plurality of links in a wireless LAN includes a transceiver; and a processor. When the first AP MLD is connected to a non-AP MLD, the processor receives a roaming preparation request from the non-AP MLD requesting roaming to a second AP MLD belonging to the SMD to which the first AP MLD belongs, transmits a roaming preparation request response to the non-AP MLD accepting the roaming preparation request, receives a roaming execution instruction from the non-AP MLD instructing the execution of the roaming, and transmits information regarding a drain time to the non-AP MLD instructing the time required to complete the transmission of traffic to the non-AP MLD.

[0017] When information indicating that roaming is completed is received from the second AP MLD, the link between the first AP MLD and the non-AP MLD can be deleted.

[0018] The above roaming execution instruction may include an identifier of the second AP MLD that is the target of the roaming.

[0019] For a predetermined period of time from when the non-AP MLD transmits information indicating the end of the drain time, the non-AP MLD may not be allowed to transmit a roaming preparation request to any AP MLD included in the SMD.

[0020] Until the non-AP MLD sets up a link with the second AP MLD and completes or cancels roaming, the second AP MLD may not be allowed to change information regarding the operation of the basic service set (BSS) on the link set up with the non-AP MLD or the operational status of the set-up link.

[0021] Information regarding the operation of the above-mentioned BSS (basic service set) may include information regarding the operation channel of the above-mentioned BSS.

[0022] According to an embodiment of the present invention, a method of operation of a non-AP (access point) multi-link device (MLD) operating on a plurality of links in a wireless LAN may include: a step of transmitting a roaming preparation request to the first AP MLD requesting roaming to a second AP MLD belonging to the SMD to which the first AP MLD belongs, while the non-AP MLD is connected to the first AP MLD; a step of receiving a roaming preparation request response from the first AP MLD accepting the roaming preparation request; a step of transmitting a roaming execution instruction to the first AP MLD instructing the execution of the roaming; a step of receiving information regarding a drain time from the first AP MLD instructing the time required to complete the transmission of traffic to the non-AP MLD; and a step of transmitting information instructing the end of the drain time to the second AP MLD when the drain time has ended.

[0023] When the second AP MLD receives information indicating the end of the drain time and instructs the first AP MLD that roaming is completed, the first AP MLD may delete the link between the first AP MLD and the non-AP MLD.

[0024] The above roaming execution instruction may include an identifier of the second AP MLD that is the target of the roaming.

[0025] For a predetermined period of time from when the non-AP MLD transmits information indicating the end of the drain time, the non-AP MLD may not be allowed to transmit a roaming preparation request to any AP MLD included in the SMD.

[0026] Until the non-AP MLD sets up a link with the second AP MLD and completes or cancels roaming, the second AP MLD may not be allowed to change information regarding the operation of the basic service set (BSS) on the link set up with the non-AP MLD or the operational status of the set-up link.

[0027] Information regarding the operation of the above-mentioned BSS (basic service set) may include information regarding the operation channel of the above-mentioned BSS.

[0028] One embodiment of the present invention efficiently

[0029] A wireless communication method and a wireless communication terminal using the same are provided.

[0030] FIG. 1 shows a wireless LAN system according to one embodiment of the present invention.

[0031] FIG. 2 shows a wireless LAN system according to another embodiment of the present invention.

[0032] FIG. 3 shows the configuration of a station according to one embodiment of the present invention.

[0033] FIG. 4 shows the configuration of an access point according to one embodiment of the present invention.

[0034] Figure 5 schematically illustrates the process of a station establishing a link with an access point.

[0035] Figure 6 shows an example of a CSMA (Carrier Sense Multiple Access) / CA (Collision Avoidance) method used in wireless LAN communication.

[0036] Figure 7 shows various standard generational PPDU (physical layer protocol data unit) formats according to an embodiment of the present invention.

[0037] FIG. 8 shows an EHT / UHR PPDU format according to an embodiment of the present invention.

[0038] FIG. 9 shows a transmission / TXOP protection method using an RTS frame and a CTS frame according to an embodiment of the present invention.

[0039] FIG. 10 shows a transmission / TXOP protection method using MU-RTS frames and CTS frames according to an embodiment of the present invention.

[0040] FIG. 11 shows an AP MLD and a non-AP MLD that have undergone a multi-link setup according to an embodiment of the present invention.

[0041] FIG. 12 shows that an AP MLD and a non-AP MLD according to an embodiment of the present invention perform a multi-link setup in a plurality of links.

[0042] FIG. 13 shows that an AP MLD and a non-AP MLD perform multi-link resetting according to an embodiment of the present invention.

[0043] FIG. 14 shows a roaming process performed by a non-AP MLD according to an embodiment of the present invention.

[0044] FIG. 15 shows that an AID collision occurs while a non-AP MLD according to an embodiment of the present invention is performing roaming.

[0045] FIG. 16 shows a change in the AID applied to each link when a non-AP MLD according to an embodiment of the present invention performs roaming.

[0046] FIG. 17 shows that a non-AP MLD according to an embodiment of the present invention transmits PS-Poll frames on different links depending on the AP MLD that transmitted the TIM element.

[0047] FIG. 18 shows a detailed procedure for roaming according to an embodiment of the present invention and frames exchanged in the detailed procedure.

[0048] FIG. 19 shows an operation in which a non-AP MLD according to an embodiment of the present invention stops a roaming procedure that was in progress.

[0049] FIG. 20 shows an operation in which a non-AP MLD changes a target AP MLD according to one embodiment of the present invention.

[0050] FIG. 21 shows a minimum time interval limit applied to the re-execution of a roaming-related operation of a non-AP MLD according to an embodiment of the present invention.

[0051] The terms used in this specification have been selected to be as widely used as possible, taking into account their functions in the present invention; however, these may vary depending on the intent, convention, or emergence of new technologies of those skilled in the art. In addition, in certain cases, terms have been arbitrarily selected by the applicant, and in such cases, their meanings will be described in the relevant description of the invention. Therefore, it should be noted that the terms used in this specification should be interpreted based on their actual meanings and the overall content of this specification, rather than merely their names.

[0052] Throughout the specification, when a configuration is described as being "connected" to another configuration, this includes not only cases where they are "directly connected" but also cases where they are "electrically connected" with other components interposed between them. Furthermore, when a configuration is described as "including" a specific component, this means that, unless specifically stated otherwise, it does not exclude other components but may include additional components. In addition, limitations such as "greater than or equal to" or "less than or equal to" based on a specific threshold value may be appropriately replaced with "greater than" or "less than," respectively, depending on the embodiment.

[0053] Hereinafter, in the present invention, fields and sub-fields may be used interchangeably.

[0054] FIG. 1 shows a wireless LAN system according to one embodiment of the present invention.

[0055] A wireless LAN system includes one or more Basic Service Sets (BSS), where a BSS represents a set of devices that can communicate with each other by successfully synchronizing. Generally, BSSs can be classified into infrastructure BSSs and independent BSSs (IBSS), and Figure 1 shows an infrastructure BSS.

[0056] As illustrated in FIG. 1, the infrastructure BSS (BSS1, BSS2) includes one or more stations (STA1, STA2, STA3, STA4, STA5), access points (AP-1, AP-2) that are stations providing distribution services, and a distribution system (DS) that connects multiple access points (AP-1, AP-2).

[0057] A Station (STA) is any device comprising a Medium Access Control (MAC) and a Physical Layer interface for a wireless medium that conforms to the specifications of the IEEE 802.11 standard, and in a broad sense includes both non-Access Point (non-AP) stations and Access Points (APs). Additionally, in this specification, the term "terminal" may be used to refer to a non-AP STA or an AP, or to refer to both. A station for wireless communication includes a processor and a communication unit, and may further include a user interface unit and a display unit, etc., depending on the embodiment. The processor generates frames to be transmitted over a wireless network or processes frames received over said wireless network, and may perform various other processing to control the station. Furthermore, the communication unit is functionally connected to said processor and transmits and receives frames over the wireless network for the station. In the present invention, the term "terminal" may be used to include user equipment (UE).

[0058] An Access Point (AP) is an entity that provides access to a Distribution System (DS) via a wireless medium for stations associated with it. In principle, communication between non-AP stations in an Infrastructure BSS is conducted via the AP, but direct communication between non-AP stations is possible if a direct link is established. Meanwhile, in the present invention, the term AP is used to include the Personal BSS Coordination Point (PCP), and in a broader sense, it may include concepts such as a centralized controller, a Base Station (BS), a Node-B, a Base Transceiver System (BTS), or a site controller. In the present invention, the AP may also be referred to as a base wireless communication terminal, and in a broad sense, the term base wireless communication terminal may be used to include the AP, base station, eNB (eNodeB), and transmission point (TP). In addition, the base wireless communication terminal may include various types of wireless communication terminals that allocate medium resources and perform scheduling in communication with multiple wireless communication terminals.

[0059] Multiple infrastructure BSSs can be interconnected through a distribution system (DS). At this time, multiple BSSs connected through the distribution system are called an Extended Service Set (ESS).

[0060] FIG. 2 illustrates an independent BSS, which is a wireless LAN system according to another embodiment of the present invention. In the embodiment of FIG. 2, parts that are identical or corresponding to the embodiment of FIG. 1 are omitted from redundant description.

[0061] BSS3 shown in Fig. 2 is an independent BSS and does not include an AP, so all stations (STA6, STA7) are not connected to an AP. An independent BSS is not allowed to connect to a distribution system and forms a self-contained network. In an independent BSS, each station (STA6, STA7) can be directly connected to one another.

[0062] FIG. 3 is a block diagram showing the configuration of a station (100) according to an embodiment of the present invention. As shown, the station (100) according to an embodiment of the present invention may include a processor (110), a communication unit (120), a user interface unit (140), a display unit (150), and a memory (160).

[0063] First, the communication unit (120) transmits and receives wireless signals such as wireless LAN packets and may be built into or externally provided in the station (100). According to an embodiment, the communication unit (120) may include at least one communication module using different frequency bands. For example, the communication unit (120) may include communication modules of different frequency bands such as 2.4 GHz, 5 GHz, 6 GHz, and 60 GHz. According to one embodiment, the station (100) may be equipped with a communication module using a frequency band of 7.125 GHz or higher and a communication module using a frequency band of 7.125 GHz or lower. Each communication module may perform wireless communication with an AP or an external station according to the wireless LAN standard of the frequency band supported by the communication module. Depending on the performance and requirements of the station (100), the communication unit (120) may operate only one communication module at a time or operate multiple communication modules together simultaneously. When the station (100) includes a plurality of communication modules, each communication module may be provided in an independent form, or the plurality of modules may be integrated into a single chip. In an embodiment of the present invention, the communication unit (120) may represent an RF communication module that processes RF (Radio Frequency) signals.

[0064] Next, the user interface unit (140) includes various types of input / output means provided in the station (100). That is, the user interface unit (140) can receive user input using various input means, and the processor (110) can control the station (100) based on the received user input. In addition, the user interface unit (140) can perform output based on the commands of the processor (110) using various output means.

[0065] Next, the display unit (150) outputs an image to the display screen. The display unit (150) can output various display objects, such as content executed by the processor (110) or a user interface based on control commands of the processor (110). Additionally, the memory (160) stores a control program used in the station (100) and various data associated therewith. This control program may include a connection program necessary for the station (100) to establish a connection with an AP or an external station.

[0066] The processor (110) of the present invention can execute various commands or programs and process data within the station (100). In addition, the processor (110) can control each unit of the station (100) described above and control the transmission and reception of data between the units. According to an embodiment of the present invention, the processor (110) can execute a program for connection with an AP stored in memory (160) and receive a communication setting message transmitted by the AP. In addition, the processor (110) can read information regarding the priority conditions of the station (100) included in the communication setting message and request a connection to the AP based on the information regarding the priority conditions of the station (100). The processor (110) of the present invention may refer to the main control unit of the station (100), and, depending on the embodiment, may refer to a control unit for individually controlling a part of the station (100), such as a communication unit (120). That is, the processor (110) may be a modem or a modulator and / or demodulator that modulates and / or demodulates wireless signals transmitted and received from the communication unit (120). The processor (110) controls various operations of wireless signal transmission and reception of the station (100) according to an embodiment of the present invention. Specific embodiments thereof will be described later.

[0067] The station (100) illustrated in FIG. 3 is a block diagram according to an embodiment of the present invention, and the separated blocks represent the elements of the device, logically distinguished. Accordingly, the elements of the device described above may be mounted as a single chip or as multiple chips depending on the design of the device. For example, the processor (110) and the communication unit (120) may be implemented as a single integrated chip or as separate chips. In addition, in an embodiment of the present invention, some components of the station (100), such as the user interface unit (140) and the display unit (150), may be optionally provided in the station (100).

[0068] FIG. 4 is a block diagram showing the configuration of an AP (200) according to an embodiment of the present invention. As shown, the AP (200) according to an embodiment of the present invention may include a processor (210), a communication unit (220), and a memory (260). In FIG. 4, redundant descriptions of parts of the configuration of the AP (200) that are identical to or corresponding to the configuration of the station (100) of FIG. 3 are omitted.

[0069] Referring to FIG. 4, the AP (200) according to the present invention is equipped with a communication unit (220) for operating a BSS in at least one frequency band. As described above in the embodiment of FIG. 3, the communication unit (220) of the AP (200) may also include a plurality of communication modules using different frequency bands. That is, the AP (200) according to the embodiment of the present invention may be equipped with two or more communication modules among different frequency bands, such as 2.4 GHz, 5 GHz, 6 GHz, and 60 GHz. Preferably, the AP (200) may be equipped with a communication module using a frequency band of 7.125 GHz or higher and a communication module using a frequency band of 7.125 GHz or lower. Each communication module may perform wireless communication with a station according to the wireless LAN standard of the frequency band supported by the communication module. Depending on the performance and requirements of the AP (200), the communication unit (220) may operate only one communication module at a time or operate multiple communication modules together simultaneously. In an embodiment of the present invention, the communication unit (220) may represent an RF communication module that processes RF (Radio Frequency) signals.

[0070] Next, the memory (260) stores a control program used in the AP (200) and various data associated therewith. This control program may include a connection program that manages the connection of the station. Additionally, the processor (210) controls each unit of the AP (200) and can control the transmission and reception of data between the units. According to an embodiment of the present invention, the processor (210) executes a program for connection with a station stored in the memory (260) and can transmit a communication setting message for one or more stations. At this time, the communication setting message may include information regarding the connection priority conditions of each station. Additionally, the processor (210) performs connection settings according to the connection request of the station. According to one embodiment, the processor (210) may be a modem or a modulator and / or demodulator that modulates and demodulates wireless signals transmitted and received from the communication unit (220). The processor (210) controls various operations of wireless signal transmission and reception of the AP (200) according to an embodiment of the present invention. Specific embodiments thereof will be described later.

[0071] Figure 5 schematically illustrates the process of a station establishing a link with an access point.

[0072] Referring to FIG. 5, the link between STA (100) and AP (200) is established through three stages: scanning, authentication, and association. First, the scanning stage is a stage in which STA (100) obtains connection information of the BSS operated by AP (200). Methods for performing scanning include a passive scanning method, which obtains information by utilizing only beacon messages (S101) periodically transmitted by AP (200), and an active scanning method, in which STA (100) transmits a probe request to AP (S103) and receives a probe response from AP (S105) to obtain connection information.

[0073] STA (100), having successfully received wireless access information during the scanning phase, transmits an authentication request (S107a) and receives an authentication response from AP (200) (S107b) to perform an authentication step. After the authentication step is performed, STA (100) transmits an association request (S109a) and receives an association response from AP (200) (S109b) to perform an association step. In this specification, association basically refers to wireless association, but the present invention is not limited thereto, and association in a broad sense may include both wireless association and wired association.

[0074] Meanwhile, additionally, an 802.1X-based authentication step (S111) and an IP address acquisition step via DHCP (S113) may be performed. In FIG. 5, the authentication server (300) is a server that processes 802.1X-based authentication with the STA (100), and may exist by being physically coupled to the AP (200) or as a separate server.

[0075] Figure 6 shows an example of a CSMA (Carrier Sense Multiple Access) / CA (Collision Avoidance) method used in wireless LAN communication.

[0076] A terminal performing wireless LAN communication performs Carrier Sensing before transmitting data to check whether the channel is busy. If a wireless signal of a certain strength or higher is detected, the channel is determined to be busy, and the terminal delays access to the channel. This process is called Clear Channel Assessment (CCA), and the level determining whether such a signal is detected is called the CCA threshold. If a wireless signal of a strength greater than the CCA threshold is received by the terminal and targets the terminal, the terminal processes the received wireless signal. Meanwhile, if no wireless signal is detected on the channel, or if a wireless signal of a strength lower than the CCA threshold is detected, the channel is determined to be idle.

[0077] When it is determined that the channel is idle, each terminal with data to transmit performs a backoff procedure after an Inter Frame Space (IFS) time, such as Arbitration IFS (AIFS) or PCF IFS (PIFS), depending on the situation of each terminal. According to an embodiment, the AIFS may be used as a configuration to replace the existing DCF IFS (DIFS). Each terminal waits by decreasing the slot time by a random number determined for that terminal during the interval of the channel's idle state, and the terminal that has exhausted all slot times attempts to access the channel. The period during which each terminal performs the backoff procedure in this manner is called the contention window period. At this time, the random number may be referred to as the backoff counter. That is, the initial value of the backoff counter is set by an integer, which is a random number obtained by the terminal. If the terminal detects that the channel is idle during the slot time, the terminal may decrease the backoff counter by 1. Additionally, when the backoff counter reaches 0, the terminal may be allowed to perform channel access on the corresponding channel. Thus, transmission by the terminal may be allowed when the channel is idle during the AIFS time and the slot time of the backoff counter.

[0078] If a specific terminal successfully accesses the channel, the terminal can transmit data through the channel. However, if the terminal attempting access collides with another terminal, the colliding terminals are each assigned a new random number and perform a backoff procedure again. According to one embodiment, the random number newly assigned to each terminal may be determined within a range (2*CW) that is twice the range of the random number (contention window, CW) previously assigned to the terminal. Meanwhile, each terminal attempts access by performing a backoff procedure again in the next contention window period, at which time each terminal performs the backoff procedure starting from the slot time remaining from the previous contention window period. In this way, each terminal performing wireless LAN communication can avoid collisions with each other regarding a specific channel.

[0079] <Examples of Various PPDU Formats>

[0080] Figure 7 shows various standard generational PPDU (physical layer protocol data unit) formats according to an embodiment of the present invention.

[0081] More specifically, FIG. 7(a) illustrates an example of a legacy PPDU format based on 802.11a / g, FIG. 7(b) illustrates an example of an HE PPDU format based on 802.11ax, and FIG. 7(c) illustrates an example of a non-legacy PPDU (i.e., EHT PPDU) format based on 802.11be. Additionally, FIG. 7(d) shows the detailed field configuration of L-SIG and RL-SIG commonly used in the above PPDU formats.

[0082] Referring to FIG. 7(a), the preamble of the legacy PPDU includes L-STF (Legacy Short Training field), L-LTF (Legacy Long Training field), and L-SIG (Legacy Signal field). In an embodiment of the present invention, the L-STF, L-LTF, and L-SIG may be referred to as the legacy preamble.

[0083] Referring to FIG. 7(b), the preamble of the HE PPDU additionally includes RL-SIG (Repeated Legacy Short Training field), HE-SIG-A (High Efficiency Signal A field), HE-SIG-B (High Efficiency Signal B field), HE-STF (High Efficiency Short Training field), and HE-LTF (High Efficiency Long Training field) in addition to the legacy preamble. In an embodiment of the present invention, the RL-SIG, HE-SIG-A, HE-SIG-B, HE-STF, and HE-LTF may be referred to as the HE preamble. The specific configuration of the HE preamble may be modified according to the HE PPDU format. For example, HE-SIG-B may be used only in the HE MU PPDU format.

[0084] Referring to FIG. 7(c), the preamble of the EHT PPDU additionally includes RL-SIG (Repeated Legacy Short Training field), U-SIG (Universal Signal field), EHT / UHR-SIG-A (Extremely High Throughput / Ultra High Reliability Signal A field), EHT / UHR-SIG-A (Extremely High Throughput / Ultra High Reliability Signal B field), EHT-STF (Extremely High Throughput Short Training field), and EHT-LTF (Extremely High Throughput Long Training field) in addition to the legacy preamble. In an embodiment of the present invention, the RL-SIG, EHT-SIG-A, EHT-SIG-B, EHT-STF, and EHT-LTF may be referred to as the EHT preamble. The specific configuration of the non-legacy preamble may be modified according to the EHT PPDU format. For example, EHT-SIG-A and EHT-SIG-B can be used in only some of the EHT PPDU formats.

[0085] As such, PPDUs used in the UHR standard can have a format similar to that of PPDUs used in the EHT standard. This is because the EHT PPDU format defined in 802.11be includes the U-SIG field, which is agreed upon for common use by multiple wireless LAN generations. In this case, the value of the PHY Version Identifier field within the U-SIG field included in the EHT PPDU is 0, while the value of the PHY Version Identifier field within the U-SIG field included in the UHR PPDU can be a non-zero value, such as 1. The EHT PPDU includes the EHT-STF (Extremely High Throughput Short Training field) in the STF field and the EHT-LTF (Extremely High Throughput Long Training field) in the LTF field. The UHR PPDU includes the UHR-STF (Ultra High Reliability Short Training field) in the STF field and the UHR-LTF (Ultra High Reliability Long Training field) in the LTF field.

[0086] The L-SIG field included in the preamble of the PPDU applies 64FFT OFDM and consists of a total of 64 subcarriers. Of these, 48 subcarriers, excluding the guard subcarrier, DC subcarrier, and pilot subcarrier, are used for L-SIG data transmission. Since the L-SIG applies a Modulation and Coding Scheme (MCS) of BPSK and Rate=1 / 2, it can contain a total of 24 bits of information. Figure 7(d) shows the 24-bit information configuration of the L-SIG.

[0087] Referring to FIG. 7(d), the L-SIG includes the L_RATE field and the L_LENGTH field. The L_RATE field consists of 4 bits and indicates the MCS used for data transmission. Specifically, the L_RATE field represents a value among transmission speeds of 6 / 9 / 12 / 18 / 24 / 36 / 48 / 54 Mbps, which are a combination of modulation schemes such as BPSK / QPSK / 16-QAM / 64-QAM and buoyancy rates such as 1 / 2, 2 / 3, and 3 / 4. By combining the information from the L_RATE field and the L_LENGTH field, the total length of the corresponding PPDU can be indicated. In non-legacy PPDU formats, the L_RATE field is set to the minimum speed of 6 Mbps.

[0088] The L_LENGTH field is allocated a total of 12 bits in bytes, allowing it to signal up to 4095, and can represent the length of the corresponding PPDU in combination with the L_RATE field. In this case, legacy terminals and non-legacy terminals may interpret the L_LENGTH field in different ways.

[0089] First, the method by which legacy or non-legacy terminals interpret the length of the corresponding PPDU using the L_LENGTH field is as follows. If the value of the L_RATE field is set to indicate 6 Mbps, 3 bytes (i.e., 24 bits) can be transmitted during 4 µs, which is the duration of one symbol in a 64 FFT. Therefore, by adding the 3 bytes corresponding to the SVC and Tail fields to the L_LENGTH field value and dividing this by 3 bytes, which is the transmission amount of one symbol, the number of symbols based on the 64 FFT after L-SIG is obtained. By multiplying the obtained number of symbols by 4 µs, which is the duration of one symbol, and adding 20 µs, which is required for the transmission of L-STF, L-LTF, and L-SIG, the length of the corresponding PPDU, i.e., the receive time (RXTIME), is obtained. This can be expressed mathematically as Equation 1 below.

[0090]

[0091] At this time, represents the smallest natural number greater than or equal to x. Since the maximum value of the L_LENGTH field is 4095, the length of the PPDU can be set up to a maximum of 5.484ms. Non-legacy terminals transmitting the PPDU must set the L_LENGTH field as shown in Equation 2 below.

[0092]

[0093] Here, TXTIME is the total transmission time constituting the corresponding PPDU, as shown in Equation 3 below. In this case, TX represents the transmission time of X.

[0094]

[0095] Referring to the above formulas, the length of the PPDU is calculated based on the rounded-up value of L_LENGTH / 3. Therefore, for any value of k, three different values ​​of L_LENGTH={3k+1, 3k+2, 3(k+1)} indicate the same PPDU length.

[0096] Referring to Fig. 7(e), the U-SIG (Universal SIG) field continues to exist in EHT / UHR PPDUs and subsequent generations of wireless LAN PPDUs, and serves to distinguish which generation the PPDU belongs to, including EHT / UHR. Additionally, the U-SIG field can facilitate spatial reuse of EHT / UHR and subsequent generations of wireless LANs. U-SIG is a 64FFT-based OFDM 2 symbol that can transmit a total of 52 bits of information. Of these, 43 bits, excluding the 9 bits for CRC / Tail, are broadly divided into the VI (Version Independent) field and the VD (Version Dependent) field.

[0097] The VI bits maintain the current bit configuration in the future, allowing current EHT / UHR terminals to obtain information about a PPDU through its VI fields even when a subsequent generation of PPDUs is defined. To this end, the VI fields consist of PHY version, UL / DL, BSS Color, TXOP, and Reserved fields. The PHY version ID field is 3 bits long and serves to sequentially distinguish versions of EHT / UHR and subsequent generation wireless LAN standards. The PHY version ID field of an EHT (11be) PPDU has a value of 000b, while the PHY version ID field of a UHR PPDU has a non-000b value. The UL / DL field distinguishes whether the PPDU is an uplink or downlink PPDU. The BSS Color refers to the BSS-specific identifier defined in 11ax and has a value of 6 bits or more. TXOP refers to the Transmit Opportunity Duration transmitted in the MAC header; by adding it to the PHY header, the length of the TXOP containing the corresponding PPDU can be inferred without the need to decode the MPDU, and it has a value of 7 bits or more.

[0098] The VD field of EHT contains signaling information useful only for PPDUs of version 11be, and can be composed of fields that are common to any PPDU format, such as PPDU format and BW, and fields that are defined differently for each PPDU format. PPDU format is a identifier that distinguishes EHT SU (Single User), EHT MU (Multiple User), EHT TB (Trigger-based), EHT ER (Extended Range) PPDUs, etc.

[0099] The BW field signals five basic PPDU BW options of 20, 40, 80, 160 (80+80), and 320 (160+160) MHz (BWs expressible in the form of 20*2 powers can be referred to as basic BWs), as well as various remaining PPDU BWs configured through Preamble Puncturing. Additionally, a signal can be transmitted in a punctured form of 80 MHz after being signaled at 320 MHz. Furthermore, the punctured and modified channel form can be signaled directly in the BW field, or by utilizing the BW field together with fields appearing after it (e.g., fields within the EHT-SIG field). If the BW field is set to 3 bits, a total of 8 BW signals are possible, so a maximum of only 3 puncturing modes can be signaled. If the BW field is 4 bits, a total of 16 BW signals are possible, so the puncturing mode can signal up to 11.

[0100] The VD field of the UHR is a field that indicates signaling information useful only to the UHR PPDU. However, the information indicated by each field included in the VD field of the UHR PPDU may be the same as or more extended than the information indicated by the field that performs the same role as the VD field of the EHT (11be). For example, the field indicating a puncturing pattern included in the VD field of the UHR PPDU may indicate a more diverse form of pattern than the field indicating a puncturing pattern included in the VD field of the EHT PPDU. Alternatively, the field indicating a puncturing pattern included in the VD field of the UHR PPDU may be interpreted in combination with the BW field. Through this, a more diverse form of puncturing pattern may be indicated.

[0101]

[0102] FIG. 8 shows an EHT / UHR PPDU format according to an embodiment of the present invention.

[0103] The EHT / UHR PPDU format can be indicated by the PPDU Format field of the U-SIG field of the PPDU. FIG. 8(a) shows an EHT / UHR SU PPDU according to an embodiment of the present invention. The EHT / UHR SU PPDU is a PPDU used for single-user transmission between an AP and a single station and may include an EHT-SIG-A field for additional signaling after U-SIG.

[0104] FIG. 8(b) shows an EHT / UHR Trigger-based PPDU according to an embodiment of the present invention. The EHT / UHR Trigger-based PPDU is an uplink PPDU used for transmission in response to a trigger frame and may not have a separate EHT / UHR-SIG-A field after U-SIG.

[0105] FIG. 8(c) shows an EHT / UHR MU PPDU according to an embodiment of the present invention. An EHT / UHR MU PPDU is a PPDU used for transmission to one or more terminals. The EHT / UHR MU PPDU format may include HE-SIG-B after the U-SIG field.

[0106] FIG. 8(d) shows an EHT / UHR ER SU PPDU according to an embodiment of the present invention. The EHT / UHR ER SU PPDU is used for single-user transmission to stations in an extended range. In the EHT / UHR ER SU PPDU format, U-SIG can be repeated in the time axis.

[0107] The EHT / UHR MU PPDU described through FIG. 8(c) can be used by an AP to perform downstream transmission to multiple stations. In this case, the EHT / UHR MU PPDU may include scheduling information for multiple stations to receive the PPDU simultaneously. In this case, the EHT / UHR MU PPDU may convey the AID information of the receiver or sender of the PPDU through the user-specific field of the EHT / UHR-SIG-B. A station that receives the EHT / UHR MU PPDU may perform a spatial reuse operation based on the AID information obtained from the preamble of the PPDU. More specifically, the resource unit allocation (RA) field of the EHT / UHR-SIG-B may include information regarding the resource unit (RU) partitioning form in a specific bandwidth (e.g., 20 MHz) in the frequency domain. Additionally, information on the station assigned to each partitioned resource unit may be transmitted through user-specific fields of EHT / UHR-SIG-B. User-specific fields may include one or more user fields corresponding to each partitioned resource unit.

[0108] Among the multiple divided resource units, the AID of the receiver or sender may be inserted into the user field corresponding to the resource unit where data transmission is performed. A pre-specified Null STA ID may be inserted into the user field corresponding to the remaining resource units where data transmission is not performed.

[0109] Two or more PPDUs described through FIG. 8 may be indicated by the same PPDU format. For example, the value of the U-SIG PPDU format subfield indicating an EHT / UHR SU PPDU and the value of the U-SIG PPDU format subfield indicating an EHT / UHR MU PPDU may be the same.

[0110] Some fields or parts of the information within the fields included in the PPDU format described above may be omitted. This may be referred to as compression mode or compressed mode.

[0111]

[0112] <Wi-Fi 단말의 채널 액세스 방법>

[0113] Since Wi-Fi terminals (APs, non-AP STAs, etc.) communicate using unlicensed bands, they check whether the channel they intend to transmit on is being used by another device before transmitting a frame. Carrier Sense Multiple Access (CSMA) is a channel access method in which a terminal intending to transmit a packet performs Carrier Sense to check whether the channel is being used by another device, and transmits only when it is determined that the channel is not being used by another device (i.e., is idle). Because a terminal using CSMA can perform an action of not attempting transmission when it is confirmed that another device is using the medium (channel) (when it is determined to be busy), transmissions that have already started can be protected from other devices.

[0114] However, multiple terminals that recognize that the medium has been occupied by another device experience a transmission collision by attempting to transmit packets simultaneously when it is confirmed that the media occupancy by the other device has ended (the medium changes to Idle). In other words, as multiple other terminals attempt to transmit packets at the same time as a specific terminal attempts to transmit a packet, the terminal required to receive the packet transmitted by the specific terminal is unable to properly receive and decode the packet due to interference caused by the transmissions performed by the other multiple terminals.

[0115] As described above, CSMA / CA (CSMA with collision avoidance) is a channel access mechanism that prevents multiple terminals from attempting packet transmission simultaneously upon detecting that the medium has changed to Idle. Terminals accessing the medium (channel) using CSMA / CA attempt transmission after waiting for a random amount of time when the observed state of the medium changes to Idle. This random amount of time may be an aslottime (typically 9 us) equal to a random number (random backoff counter) generated by each terminal attempting transmission. In other words, since terminals accessing the medium using CSMA / CA attempt transmission after waiting for different random amounts of time, they attempt transmission at different times, unlike when CSMA alone is used. At this time, if a specific terminal that has waited for the shortest random amount of time after the medium changes to Idle attempts transmission first, other terminals may stop the channel access procedure after realizing that the medium has been occupied (changed to busy) due to said specific terminal. At this time, the specific terminal may perform the operation of decreasing the backoff counter it maintains by 1 at every aslottime while the medium is kept in Idle, and attempt transmission when the backoff counter becomes 0, or when aslottime has passed after the backoff counter has become 0. At this time, the specific terminal that performed the transmission may generate a new random number (new backoff counter) after the transmission is finished, and attempt transmission when the new random number becomes 0 again, or after it becomes 0.

[0116] The CSMA / CA and random backoff procedures briefly explained above apply to both DCF (Distributed Coordination Function) and EDCAF (Enhanced Distributed Channel Access), which are the basic functions used by Wi-Fi terminals when attempting channel access. Since these are well-known and widely utilized methods for accessing unlicensed band channels, further detailed explanations will be omitted.

[0117]

[0118] The DCF and EDCAF utilized by the MAC of a Wi-Fi terminal evaluate the channel condition by considering not only the channel status (idle / busy status) confirmed by each terminal directly performing Physical Carrier Sense (CS), but also the results of Virtual CS. More specifically, even if the result of the Physical CS performed on the channel is idle, if the Virtual CS result is busy, the Wi-Fi terminal considers the channel status to be busy. In this case, the Virtual CS is a channel evaluation method that determines the channel as busy if the Network Allocation Vector (NAV) is not zero. The NAV may be a value maintained for future traffic predicted to occupy the medium. To explain in more detail, when a Wi-Fi MAC receives an RTS / CTS frame, it sets the NAV (NAV count) based on the duration information of the received frame, such as the value of the duration field, so that the NAV can be maintained as a non-zero value for the expected time during which the medium will be occupied after the RTS / CTS frame exchange. In other words, the value maintained as NAV decreases over time. If the NAV value of a specific MAC is 0, it can be interpreted as a state where future traffic perceived by the specific MAC no longer occupies the medium. If the NAV is 0, the MAC can determine the virtual CS result as Idle. In this case, the Wi-Fi MAC may set the NAV based on duration values ​​obtained from other received MAC frames, not just RTS / CTS frames.

[0119] The channel evaluation method (determine the state of the medium) that considers both the physical and virtual CS results, briefly explained above, is also a well-known Wi-Fi MAC function, so a detailed explanation is omitted.

[0120]

[0121] <EDCA와 TXOP>

[0122] EDCA provides a mechanism for managing traffic by differentiating it into four types of access categories (ACs) based on traffic characteristics. These four types of ACs are AC_VO (AC Voice), AC_VI (AC Video), AC_BE (AC Best Effort), and AC_BK (AC Background), and each AC can have different contention window (CW), transmit opportunity (TXOP), and AIFSN parameters. Simply put, EDCA is a mechanism that controls the transmission priority of traffic transmitted by utilizing each AC by differentiating the CW, TXOP, and AIFSN parameters for the four types of ACs. To this end, EDCA can map traffic (MSDU) that a MAC must service to one of the four ACs based on the traffic category (TC) or traffic stream (TS). At this time, the traffic mapped to one of the four ACs by EDCA is divided and managed into four queues for each AC. In this case, the four queues may be logically separated queues rather than physically separated queues.

[0123] AC_VO is an AC that can be used for traffic that is vulnerable to transmission delays, such as voice traffic, where the absolute amount of traffic is not large. It has relatively small CW and AIFSN parameter values ​​to increase the probability of being serviced preferentially over traffic from other ACs. The TXOP parameter of AC_VO is limited to a value relatively smaller than the TXOP parameter of other ACs, so only a shorter transmission time than other ACs is guaranteed.

[0124] AC_VI is an AC that is more robust to transmission delay than voice traffic, but can be used for traffic such as video that still requires low-latency transmission and needs to handle a large amount of traffic. AC_VI has CW and AIFSN parameter values ​​that are larger than AC_VO but smaller than other ACs, and instead, TXOP is about twice as long as AC_VI.

[0125] AC_BE is an AC that can be used for traffic robust to transmission delay, and most general traffic, excluding voice data and streaming video data, can be classified as AC_BE. AC_BE uses larger values ​​for the CW and AIFSN parameters than AC_VO and AC_VI. Additionally, AC_BE does not have a separate TXOP. Therefore, traffic corresponding to AC_BE cannot be used in a TXOP transmission sequence in which a PPDU is transmitted, an ACK is received, and a PPDU is transmitted again after SIFS.

[0126] AC_BK is an AC that is robust to transmission delay, similar to AC_BE, but can be used for traffic with a lower priority than BE traffic. AC_BK uses the same CW parameter values ​​as AC_BE, and uses larger AIFSN parameter values ​​than AC_BE. Additionally, traffic corresponding to AC_BK does not have a separate TXOP, just like AC_BE, so it cannot be used in a TXOP transmission sequence.

[0127] The four types of EDCA ACs described above are mapped to the UP (user-priority) of 802.1D, and the EDCA AC is determined based on the UP value of the traffic received via wire or the TID of the MSDU indicated by the upper layer. In this case, if the TID of the MSDU indicates a value from 0 to 7, the value indicated by the TID can correspond one-to-one with the UP.

[0128] In addition, the default CW (CWmin, CWmax), AIFSN, and TXOP parameters for each of the four types of EDCA ACs described above are defined in the standard, and the parameter values ​​of each AC can be changed by the AP and different values ​​can be used for each BSS.

[0129]

[0130] When utilizing the EDCA mechanism, Wi-Fi traffic is stored in one of four queues corresponding to four ACs, and can be transmitted to a destination device only if the AC containing the traffic wins the channel access competition against another AC. In this case, during the channel access competition between ACs, each AC competes using the access parameters (CW[AC], AIFSN[AC]) assigned to it, and the channel access competition operation performed by each AC is the same as DCF. In this case, if a specific AC does not have any traffic to transmit in the queue, said specific AC may not participate in the competition.

[0131] However, as mentioned above, since the CW and AIFSN parameter values ​​utilized by each AC differ, the AC_VO with the smallest CW and AIFSN parameters has a high probability of winning the channel access competition against other ACs, and therefore, it is highly likely that the traffic of AC_VO will be served preferentially over the traffic of other ACs.

[0132] In addition, the EDCA mechanism stipulates internal competition rules, such as when an internal collision occurs between ACs, the AC with higher priority wins and increases the CW of the other AC that caused the collision, and rules for configuring PPDUs by including traffic from ACs other than the AC that won the competition (primary AC), but since these details are not significantly relevant to the proposal of the present invention, a detailed explanation is omitted.

[0133] As described above, EDCA provides the EDCA TXOP (EDCA Transmission Opportunity) function, along with the ability to operate differentiated ACs based on the type of traffic (frames, packets, etc.) to enhance QoS. EDCA TXOP refers to the time during which a specific AC's EDCAF (EDCA Function) can control the medium without interference from other devices during the TXOP duration when it acquires a channel access opportunity, that is, when it becomes a TXOP holder. At this time, the EDCA TXOP may be limited by a TXOP limit advertised by the AP. The TXOP holder must ensure that the transmission of their own transmission and the transmission of any response frames resulting from their transmission are completed within the TXOP limit.

[0134] A TXOP holder can transmit multiple frames (multiple PPDUs) during an EDCA TXOP interval. If the transmission of each frame is performed within the acquired TXOP interval, the TXOP holder can transmit multiple frames continuously without performing a separate channel access procedure, such as a backoff procedure, between the transmissions of each frame. In this case, if the multiple frames are MPDUs or A-MPDUs (Aggregated MAC protocol data units) that do not request an immediate ack, the transmission of multiple frames may be performed at intervals of SIFS (short interframe space) or RIFS (reduced interframe space). In this case, if there is an MPDU or A-MPDU among the multiple frames that requests an immediate ack, the TXOP holder can transmit the frame requesting the immediate ack, receive the ack, and then transmit the next frame after SIFS.

[0135] At this time, traffic (packets, frames, etc.) of an AC other than the specific AC that is the TXOP holder may also be transmitted together within the TXOP acquired by the TXOP holder (specific AC) when certain conditions are satisfied. The transmission of traffic of an AC other than the TXOP holder within the TXOP may be an operation due to TXOP sharing between ACs, and details regarding the above specific conditions are omitted as they are not relevant to the present invention.

[0136]

[0137] As described above, the TXOP holder can perform continuous frame transmission within the TXOP without performing a separate channel access procedure. This operation can be achieved when other terminals understand and protect the TXOP segment acquired by the TXOP holder. That is, in order for the TXOP holder to acquire medium control authority over the EDCA TXOP segment, a procedure to notify other terminals so that they can recognize the acquired TXOP segment may be necessary.

[0138] To this end, a terminal (AC) that has become a TXOP holder or has started transmission after completing the channel access procedure may attempt to enable other terminals to recognize the TXOP period by transmitting an RTS frame. At this time, the RTS frame refers to a frame in which the Type subfield of the Frame Control field of the MAC frame header (the fourth bit (B3) and third bit (B2) of the Frame Control field) is set to 01b (Type = Control frame), and the Subtype subfield of the Frame Control field (the eighth bit (B7), seventh bit (B6), sixth bit (B5), and fifth bit (B4) of the Frame Control field) is set to 1011b. Another terminal that receives the RTS frame from the TXOP holder may set a NAV based on information related to the duration included in the RTS frame, for example, the value of the Duration field. The set NAV may be maintained as a non-zero value for the duration corresponding to the TXOP of the TXOP holder. However, the terminal designated as the destination device of the RTS frame must respond with a CTS frame instead of setting the NAV based on the information in the RTS frame. In this case, the destination device of the RTS frame transmitted to initiate the TXOP is the TXOP responder, and must transmit a CTS frame as a response to the RTS (after the RTS frame is received and SIFS). In this case, the Duration field of the responding CTS frame is set to a value calculated as: the value indicated in the Duration field of the received RTS frame - the CTS frame transmission time - SIFS. Terminals that receive the CTS frame may set the NAV based on information related to the duration included in the CTS frame (e.g., the value of the Duration field).

[0139] Accordingly, the NAV of the terminal that received the RTS frame from the TXOP holder and the terminal that received the CTS frame from the TXOP responder are set to 0 after the TXOP acquired by the TXOP holder is terminated. Through this, the Wi-Fi MAC mechanism can protect the TXOP holder and the TXOP responder from exchanging multiple frames during the TXOP without interference.

[0140] However, if the TXOP holder transmits an RTS frame as a non-HT duplicate PPDU across the primary 80 MHz band, but the CTS frame (non-HT duplicate PPDU) responded to by the TXOP responder is responded only in the primary 40 MHz band, the TXOP holder may use only the primary 40 MHz or a bandwidth less than the primary 40 MHz, e.g., primary 20 MHz, for frame exchange during the acquired TXOP. The CH_BANDWIDTH (a type of TXVECTOR parameter) of the PPDU transmitted by the TXOP holder must be set to a value equal to or smaller than the CH_BANDWIDTH_IN-NON_HT (a type of RXVECTOR parameter) of the received CTS frame. In this case, the RTS frame may be an RTS frame that allows the CTS frame to be responded to with a bandwidth smaller than the bandwidth in which the RTS frame was transmitted. The RTS frame may be an RTS frame transmitted with DYN_BANDWIDTH_IN_NON_HT (a type of TXVECTOR parameter) set to Dynamic. If the RTS frame is transmitted from the TXOP holder with DYN_BANDWIDTH_IN_NON_HT set to Static, the TXOP responder may respond with a CTS frame with the same BW as the BW at which the RTS frame was received.

[0141]

[0142] FIG. 9 shows a transmission / TXOP protection method using an RTS frame and a CTS frame according to an embodiment of the present invention.

[0143] Before transmitting the PPDU, the first station (STA1) transmits an RTS frame to the second station (STA2), which is the destination device of the PPDU, and the second station (STA2) responds with a CTS frame after SIFS, after acknowledging that the received RTS frame is an RTS frame with itself as the destination device.

[0144] STA1_Neighbor, a neighbor station (Neighbor STA) of the first station (STA1), receives an RTS frame transmitted by the first station (STA1) and sets the NAV based on the value indicated by the Duration field of the RTS frame. STA2_Neighbor, a neighbor station of the second station (STA2), receives a CTS frame transmitted by the second station (STA2) and sets the NAV based on the information indicated by the Duration field of the CTS frame. After receiving the RTS / CTS frames, STA1_Neighbor and STA2_Neighbor determine that the virtual CS is busy while the set NAV (counter) remains a non-zero value and perform actions such as not decreasing the backoff counter. Consequently, the neighbor terminal that receives the RTS / CTS frame does not attempt to transmit during the period in which the NAV remains a non-zero value. Therefore, the first station (STA1) and the second station (STA2) can be free from interference by surrounding terminals while exchanging PPDU and Ack frames.

[0145] Even if the first station (STA1) and STA2_Neighbor are in a hidden relationship where no signal is detected from each other's transmission, STA2_Neighbor can perform an operation that takes into account that the channel (channel, WM, Wireless medium) is in use while the first station (STA1) is transmitting a PPDU.

[0146] Meanwhile, a Wi-Fi terminal (non-AP STA) can transmit a UL PPDU to the AP without directly acquiring a TXOP or performing channel access via DCF and EDCAF. More specifically, the non-AP STA can transmit a UL PPDU using the RU assigned to it after receiving a trigger frame transmitted by the AP. In this case, the UL PPDU is a TB (trigger-based) PPDU.

[0147] A STA that responds with a UL PPDU after receiving a trigger frame can perform transmissions without acquiring direct channel access opportunities through DCF and EDCAF, and thus obtains more transmission opportunities compared to a STA that does not transmit a trigger frame-based UL PPDU; consequently, a STA that transmits a UL PPDU via a trigger frame may cause a fairness issue in terms of channel access. To resolve this fairness issue, 11ax defines a constraint that requires an HE non-AP STA to perform EDCAF using the MU (Multi-user)-EDCA parameter when it successfully transmits at least one MPDU via the UL PPDU transmitted after receiving a trigger frame. Accordingly, a STA that transmits a UL PPDU via a trigger frame must perform channel access using the MU-EDCA parameter instead of the EDCA parameter. The MU-EDCA parameters include parameters related to the size of the contention window for each of AC_VO, AC_VI, AC_BE, and AC_BK, as well as the MU-EDCA timer, and the contention window included in MU-EDCA can be set larger than the parameters of EDCA. An STA that has transmitted a TB PPDU via a trigger frame and has successfully transmitted at least one MPDU will succeed in channel access with a lower probability than an STA using EDCA parameters when performing channel access within the time interval corresponding to the MU-EDCA timer by utilizing the MU-EDCA parameters instead of the EDCA parameters.In this way, by reducing the channel accessibility of the STA that transmits the UL PPDU (TB PPDU) based on the trigger frame, the equity issue regarding channel accessibility between the STA that transmits the UL PPDU without performing direct channel access and the STA that does not transmit the UL PPDU based on the trigger frame can be resolved or mitigated.

[0148] <MU-RTS 트리거 프레임을 이용한 TXOP 보호>

[0149] In 11ax (6th generation Wi-Fi, Wi-Fi 6, HEW, High Efficiency WLAN), a MU-RTS Trigger / CTS frame exchange procedure is defined to add a feature that allows an AP to initiate a TXOP using a MU-RTS trigger frame (hereinafter referred to as MU-RTS, MU-RTS frame) and protect the TXOP frame exchange procedure. A MU-RTS frame is a type of trigger frame; upon receiving a MU-RTS frame, a station whose AID12 (the LSB 12 bits of the Association ID) is indicated in the User field included in the MU-RTS frame simultaneously responds with a CTS frame. When an AP protects a TXOP using a MU-RTS frame, multiple stations respond with CTS frames, thereby protecting the TXOP from the peripheral devices of each of the multiple stations that are the destination devices of a DL MU PPDU (Down-link multi-user PPDU). Additionally, MU-RTS frames can be used to protect an UL MU PPDU. More specifically, before requesting a Trigger-based PPDU (TB) from multiple stations via a trigger frame, the AP can transmit a MU-RTS frame to cause multiple stations responding to the TB PPDU to respond to a CTS frame. At this time, the CTS frames responded to by multiple stations induce the neighboring stations of each station to set up NAVs that protect the TB PPDU and the Ack frames (Ack, Block Ack, etc.) to be transmitted after the TB PPDU, thereby allowing legacy stations (STAs) that cannot recognize (interpret, decode) the trigger frame and the TB PPDU to not perform channel access during the packet switching sequence period (or TXOP) initiated by the trigger frame.

[0150]

[0151] FIG. 10 shows a transmission / TXOP protection method using MU-RTS frames and CTS frames according to an embodiment of the present invention.

[0152] In the embodiment of FIG. 10, prior to transmitting the MU PPDU, the AP transmits a MU-RTS frame to the first station (STA1) and the second station (STA2), which are the destination devices of the MU PPDU, and the first station (STA1) and the second station (STA2) receive the MU-RTS frame and, after SIFS, each respond to the MU-RTS frame with a CTS frame.

[0153]

[0154] After receiving the CTS frame transmitted by the first station (STA1), the neighboring station STA1_Neighbor sets the NAV based on the information indicated by the Duration field of the CTS frame. After receiving the CTS frame transmitted by the second station (STA2), the neighboring station STA2_Neighbor sets the NAV based on the information indicated by the Duration field of the CTS frame. While the NAV (counter) set by STA1_Neighbor and STA2_Neighbor remains a non-zero value after receiving the CTS frame, the Virtual CS (Virtual Carrier Sense) determines it to be busy and performs actions such as not decreasing the back-off counter. Therefore, the neighboring terminal that received the CTS frame does not attempt to transmit during the period in which the NAV remains a non-zero value. This allows the AP to transmit MU PPDU and the first station (STA1) and the second station (STA2) to transmit Ack frames without being interfered with by surrounding terminals.

[0155] The trigger frame described above is a frame type defined in 11ax, in which the Type (fourth bit (B3) and third bit (B2)) and Subtype (eighth bit (B7), seventh bit (B6), sixth bit (B5), and fifth bit (B4)) subfields of the Frame Control field are set to 01b and 0010b, respectively. The trigger frame is a frame of the Control Type with the Type subfield of the Frame Control field being 01b, and the Subtype value 0010 indicates that it is a Trigger frame type. In 11ax, trigger frames are defined to allow an AP to request response frames from multiple stations at once, and MU-RTS frames are used for an AP to request CTS frames from multiple stations (non-AP STAs). Other Trigger Types, excluding the MU-RTS frame, include the Basic Trigger frame requesting UL MU PPDU, the BRP Trigger frame requesting a Beamforming Report (Beamforming Report Poll Trigger frame), the MU-BAR Trigger frame (BlockAck request), the BSRP Trigger frame requesting a Buffer Status Report (Buffer Status Report Poll Trigger frame), the GCR MU-BAR Trigger frame, the BQRP (Bandwidth Query Report Poll) Trigger frame, and the NDP Feedback Report Poll Trigger frame. Since other Trigger Types, excluding the MU-RTS frame, are not related to the content of the present invention, a detailed description is omitted.

[0156] Multi-link Device (MLD)

[0157] In Wi-Fi 7's EHT (Extremely High Throughput), MLDs are defined. An MLD refers to a logical entity containing one or more STAs, and an AP MLD may be affiliated with one or more APs (AP STAs), and a non-AP (STA) MLD may be affiliated with one or more non-AP STAs.

[0158] Each AP belonging to an AP MLD can operate an independent Basic Service Set (BSS), and the operating bandwidth (Operating BW) and operating channel of the BSSs operated by the APs may differ. When an AP MLD and a non-AP MLD are associated, setup can be performed between multiple APs belonging to a single AP MLD and multiple non-AP STAs belonging to a single non-AP MLD. In this case, since each AP belonging to the AP MLD operates a BSS on its own Link (Operating channel), the non-AP MLD associated with each of the multiple APs belonging to the single AP MLD is considered to have performed a Multi-Link setup. In other words, the AP MLD and non-AP MLD defined in Wi-Fi 7 can perform a Multi-Link setup connected across multiple Links.

[0159] Each MLD can have up to 15 STAs (AP STAs, non-AP STAs) attached. That is, 15 APs can be attached to an AP MLD, and each of the 15 APs operates an independent BSS. At this time, each AP attached to the AP MLD provides a level of service equivalent to that of a conventional Wi-Fi AP. In other words, each AP attached to the AP MLD functions as an independent AP and can perform services for non-AP STAs not attached to the MLD (e.g., legacy non-AP STAs). At this time, each AP attached to the AP MLD operates on a mutually independent Link, and the meaning of the Link refers only to the operating channel in which each AP operates, not a Link that distinguishes between 2.4 / 5 / 6 GHz. That is, the first AP attached to the AP MLD operates on the first Link, and the second AP can operate on the second Link. At this time, the first Link where the first AP is operated and the second Link where the second AP is operated can both be located in the 6 GHz band.

[0160] In addition, AP MLDs and non-AP MLDs can complete setup on multiple links through a Multi-Link setup procedure performed on a specific link. In this case, the Multi-Link setup procedure refers to the exchange of Multi-Link Probe Request / Response and Multi-Link Association Request / Response frames performed to establish a connection for one or more links. In the present invention, since the procedure for performing Multi-Link setup between AP MLDs and non-AP MLDs is not critical, a detailed description is omitted.

[0161] When two MLDs are connected via multiple links, it is possible for the two MLDs to operate the traffic transmitted and received through each link separately. This may be achieved through TID-to-Link mapping negotiations performed between the two MLDs or by applying the TID-to-Link mapping status instructed by the AP MLD. In this case, the TID-to-Link mapping status instructed by the AP MLD to non-AP MLDs is indicated by management frames (e.g., Beacon or Probe Response frames) transmitted by the AP MLD, and non-AP MLDs associated with the AP MLD through at least one link must operate each link according to the TID-to-Link mapping instructed by the AP MLD. However, if a new TID-to-Link mapping negotiation is performed between the AP MLD and the non-AP MLD, the traffic (MPDU) of each TID may be transmitted and received through different links according to the method determined by the new TID-to-Link mapping negotiation. For example, if an AP MLD and a non-AP MLD are connected through two links, and TIDs 0 to 3 are mapped to Link 1 and TIDs 4 to 7 are mapped to Link 2, then the AP MLD and the non-AP MLD must transmit and receive only MPDUs with TIDs 0 to 3 through Link 1, and transmit and receive MPDUs with TIDs 4 to 7 through Link 2.

[0162] If the AP MLD has not indicated a separate TID-to-Link mapping state and there is no TID-to-Link mapping performed between the AP MLD and the non-AP MLD, the AP MLD and the non-AP MLD have a Default TID-to-Link mapping state. The Default TID-to-Link mapping state means that all TIDs are mapped to each Link, and in this case, the AP MLD and the non-AP MLD send and receive MPDUs of all TIDs (TID = 0 to 7) on each Link.

[0163] Since Wi-Fi 8 (UHR, Ultra High Reliability) is planned to be developed based on Wi-Fi 7, the MLD concept, connection procedures between MLDs, and methods of operating links through TID-to-Link mapping will still be inherited in Wi-Fi 8. In other words, it is possible for an AP belonging to an AP MLD to be a UHR STA, and it is also possible for a non-AP STA belonging to a non-AP MLD to be a UHR STA.

[0164]

[0165] Wireless Terminal Roaming

[0166] Wireless LAN technology can be used not only for devices operating in fixed locations but also for portable electronic devices such as mobile phones and laptops. When a user terminal (non-AP Station, non-AP STA (STA)) moves, the base station (Access Point Station, AP STA (AP)) most suitable for the user terminal to connect to may change. The user terminal performs roaming from the previously connected AP to another AP.

[0167] The roaming procedures provided by conventional wireless LAN technology offer a poor experience, to the extent that users are required to recognize that a connection has been dropped. This roaming performance stands in stark contrast to the smooth roaming experience provided to mobile communication service users, where they do not even feel that roaming is taking place. There are various reasons why the roaming experience of wireless LAN technology is inferior to that of mobile communication.

[0168] Roaming in mobile communication technology is roaming performed between base stations operated by a single operator. All plans regarding which base station to use to service a user terminal are managed by a centralized processor. For example, if a mobile communication terminal receiving service through a first base station moves away from the first base station, roaming to a second base station located in the direction of the mobile communication terminal's movement can be planned in advance by the operator. In this case, at the time the connection of the mobile communication terminal receiving service through the first base station changes to the second base station, service preparation for the mobile communication terminal is already completed at the second base station, thereby allowing roaming between the first base station and the second base station to be completed without any disconnection.

[0169] Roaming in wireless LAN technology refers to movement between Access Points (APs) that operate independently. There is a problem in that prior coordination between different APs is difficult. Furthermore, wireless LAN terminals performing roaming must decide for themselves when to initiate roaming. Generally, when the connection with the existing AP is lost, the non-AP station searches for another adjacent AP. At this point, the non-AP station must find a new AP and complete the connection and authentication procedures with it to regain service. This can lead to service interruptions for the non-AP station. A method is needed to minimize such wireless LAN service interruptions. Although technologies have been introduced to search for other adjacent APs when the existing AP's Receive Signal Strength Indicator (RSSI) weakens, or to have the existing AP continuously provide information about other adjacent APs, service interruptions still occur.

[0170]

[0171] <MLD(Multi-Link Device)>

[0172] In the 7th generation wireless LAN standard (802.11be), a Multi-Link Device (MLD) was defined as a device operating on multiple links. An MLD includes one or more stations, and each station affiliated with the MLD performs the function of a wireless LAN station. An MLD containing one or more APs is an AP MLD, and an MLD containing one or more non-AP stations is a non-AP MLD.

[0173] AP MLDs and non-AP MLDs may have a multi-link setup in which one or more links are established. When two MLDs perform a multi-link setup, the two MLDs may communicate using one or more links simultaneously or independently. Each link subject to the multi-link setup may be located in different bands, for example, the 2.4 GHz, 5 GHz, and 6 GHz bands, or may be included in different channels. The maximum number of links that an MLD can operate may be 15. Additionally, an MLD may operate at least one link in each of the bands supported by the MLD.

[0174] FIG. 11 shows an AP MLD and a non-AP MLD that have undergone a multi-link setup according to an embodiment of the present invention.

[0175] In the embodiment of FIG. 11, the AP MLD includes a first AP (AP1), a second AP (AP2), and a third AP (AP3). In this case, each of the first AP (AP1), the second AP (AP2), and the third AP (AP3) operates on a first link (Link1), a second link (Link2), and a third link (Link3), respectively. In this case, the first AP (AP1), the second AP (AP2), and the third AP (AP3) may be referred to as the affiliated APs of the AP MLD. Additionally, the non-AP MLD includes a first non-AP station (STA1), a second non-AP station (STA2), and a third non-AP station (STA3). In this case, each of the first non-AP station (STA1), the second non-AP station (STA2), and the third non-AP station (STA3) operates on a first link (Link1), a second link (Link2), and a third link (Link3), respectively. At this time, the first non-AP station (STA1), the second non-AP station (STA2), and the third non-AP station (STA3) may be referred to as affiliated non-AP stations of the non-AP MLD. The AP MLD and the non-AP MLD set up the first link (Link1), the second link (Link2), and the third link (Link3). The AP MLD and the non-AP MLD can operate independently or simultaneously on each of the first link (Link1), the second link (Link2), and the third link (Link3). Through this, the MLD can have higher channel accessibility than the existing station.

[0176] AP MLDs and non-AP MLDs can set up multiple links at once. In particular, AP MLDs and non-AP MLDs can set up multiple links on a single link. This can be referred to as the aforementioned multi-link setup. Additionally, when performing a multi-link setup on a first link, AP MLDs and non-AP MLDs can exchange information regarding stations operating on a second or third link. Through this, one affiliated AP and one affiliated non-AP station can set up multi-links for another AP and another non-AP station. The procedure for multi-link setup may include at least one of a probing procedure, an association procedure, or an authentication procedure. In this case, the probing procedure may include the exchange of a probe request frame and a probe response frame. Additionally, the association procedure may include the exchange of an association request frame and an association response frame.

[0177] An AP affiliated with an AP MLD can indicate that it belongs to the AP MLD. Additionally, an AP affiliated with an AP MLD can indicate information about other APs in the AP MLD to which it is affiliated. Specifically, an AP can indicate information about other APs in the AP MLD to which it is affiliated by transmitting a Reduced Neighbor Report (RNR) element. In this case, the RNR element may include one or more Neighbor AP Information fields. If the value of the AP MLD ID subfield of the Neighbor AP Information field is 0, the Neighbor AP Information field corresponds to the AP MLD to which the AP that transmitted the RNR element is affiliated. If the value of the AP MLD ID subfield of the Neighbor AP Information field is 0, a non-AP station receiving the RNR element can determine that the corresponding Neighbor AP Information field corresponds to another AP affiliated with the AP MLD to which the AP that transmitted the RNR element is affiliated.

[0178] A non-AP MLD that receives an RNR element can perform a multi-link setup with an AP MLD. A station performing the multi-link setup may include a Multi-Link element in a management frame on a specific link, such as a probe request frame, a probe response frame, a connection request frame, or a connection response frame. In this case, the Multi-Link element may indicate information about other stations of the MLD to which the station transmitting the Multi-Link element is affiliated. The Multi-Link element may indicate information about one or more stations. The Multi-Link element may include one or more Per-STA Profile sub-elements. Each of the one or more Per-STA Profile sub-elements may indicate information about one or more stations.

[0179] FIG. 12 shows that an AP MLD and a non-AP MLD according to an embodiment of the present invention perform a multi-link setup for a plurality of links.

[0180] In the embodiment of FIG. 12, the AP MLD includes a first AP (AP1), a second AP (AP2), and a third AP (AP3). In this case, each of the first AP (AP1), the second AP (AP2), and the third AP (AP3) operates on a first link (Link1), a second link (Link2), and a third link (Link3), respectively. Additionally, the non-AP MLD includes a first non-AP station (STA1), a second non-AP station (STA2), and a third non-AP station (STA3). In this case, each of the first non-AP station (STA1), the second non-AP station (STA2), and the third non-AP station (STA3) operates on a first link (Link1), a second link (Link2), and a third link (Link3), respectively. The non-AP MLD receives a beacon frame from the first AP (AP1) on the first link (Link1). A beacon frame includes an RNR element, and the RNR element may include a Neighbor AP Information field for each of the second AP (AP2) and the third AP (AP3). In this case, the non-AP MLD can obtain information about the second AP (AP2) and the third AP (AP3) from the RNR element. Specifically, the non-AP MLD can determine that the second AP (AP2) is operating on the second link (Link2) and the third AP (AP3) is operating on the third link (Link3).

[0181] The non-AP MLD and the AP MLD exchange multi-link probe request frames and multi-link probe response frames on the first link (Link 1). The multi-link probe request frame transmitted by the non-AP MLD includes a Per-STA Profile sub-element corresponding to the second AP (AP2) and a Per-STA Profile sub-element corresponding to the third AP (AP3). The value of the Complete Profile Requested sub-field for each of the Per-STA Profile sub-element corresponding to the second AP (AP2) and the Per-STA Profile sub-element corresponding to the third AP (AP3) is set to 1. A value of 1 for the Complete Profile Requested sub-field indicates a request for complete information regarding the second AP (AP2) and the third AP (AP3). In this case, the complete information may represent information regarding the sub-fields that define complete information in the 802.11 standard. Upon receiving a multi-link probe request frame, the AP MLD transmits a multi-link probe response frame on the first link (Link1) that includes information regarding the first AP (AP1), the second AP (AP2), and the third AP (AP3). Specifically, the multi-link probe response frame includes a Per-STA Profile subelement that indicates information regarding each of the first AP (AP1), the second AP (AP2), and the third AP (AP3). The multi-link probe response frame includes all possible information regarding each of the second AP (AP2) and the third AP (AP3). Specifically, the multi-link probe response frame transmitted by the first AP may include all information contained in the probe response frame transmitted by the second AP (AP2) and the probe response frame transmitted by the third AP (AP3).Through this, a non-AP MLD that has received a multi-link probe response frame from the first AP can obtain the same information as when it has received probe response frames from both the second AP and the third AP.

[0182] The non-AP MLD and the AP MLD exchange a connection request frame and a connection response frame on the first link (Link1) for multi-link setup. At this time, the connection request frame includes a Multi-Link element. Specifically, the Multi-Link element includes a Per-STA Profile sub-element corresponding to each of the second non-AP station (non-AP STA2) and the third non-AP station (non-AP STA3). The Link ID sub-field of the Per-STA Profile sub-element corresponding to the second non-AP station (non-AP STA2) indicates the second link (Link2), and the Link ID sub-field of the Per-STA Profile sub-element corresponding to the third non-AP station (non-AP STA3) indicates the third link (Link3). The AP MLD can determine which link setup is requested based on the link indicated by the Per-STA Profile sub-element. Additionally, the AP MLD can perform a multi-link setup by transmitting a connection response frame indicating the links to which the multi-link setup is to be performed. The AP MLD performs a multi-link setup by transmitting a connection response frame indicating the first link (Link1), the second link (Link2), and the third link (Link3).

[0183]

[0184] <Multi-Link Setup Change>

[0185] According to the embodiments described above, the AP MLD and non-AP MLD can perform a multi-link setup. After the multi-link setup is configured, the AP MLD and non-AP MLD may request the addition or deletion of some links. To do this, it may be inefficient to disable the existing multi-link setup and perform the multi-link setup again. Therefore, the AP MLD and non-AP MLD can perform a multi-link reconfiguration. The AP MLD and non-AP MLD that have performed the multi-link setup may perform a multi-link reconfiguration to change the link settings. Through this, new links may be added or existing links may be deleted. At this time, links that are not subject to reconfiguration may not be affected.

[0186] FIG. 13 shows that an AP MLD and a non-AP MLD perform multi-link resetting according to an embodiment of the present invention.

[0187] In FIG. 13(a), the AP MLD and the non-AP MLD perform a multi-link reconfiguration to add a new link. In the embodiment of FIG. 13(a), the AP MLD and the non-AP MLD establish a first link (Link1) and a second link (Link2) through a multi-link setup. The non-AP MLD transmits a link reconfiguration request frame to add a third link (Link3). At this time, the AP MLD transmits a link reconfiguration response frame accepting the addition of the third link (Link3) as a response to the link reconfiguration request frame. Through this, the third link (Link3) is added between the AP MLD and the non-AP MLD.

[0188] In FIG. 13(b), the AP MLD and the non-AP MLD perform a multi-link reconfiguration to delete a new link. In the embodiment of FIG. 13(b), the AP MLD and the non-AP MLD establish a first link (Link1), a second link (Link2), and a third link (Link3) through a multi-link setup. The non-AP MLD transmits a link reconfiguration request frame to delete the third link (Link3). At this time, the AP MLD transmits a link reconfiguration response frame accepting the deletion of the third link (Link3) as a response to the link reconfiguration request frame. Through this, the third link (Link3) is deleted between the AP MLD and the non-AP MLD.

[0189]

[0190] <Multi-Link setup 상태를 활용한 로밍 기법>

[0191] As previously explained, a non-AP MLD can be connected to an AP MLD through multiple links (Multi-Link setup state), and it is possible to perform data frame communication with the AP MLD through any one of the multiple links. If a non-AP MLD is connected to a first AP MLD through some links and connected to a second AP MLD through the remaining links, the non-AP MLD can be connected to both the first AP MLD and the second AP MLD simultaneously, that is, it can perform frame exchange through either one of the two AP MLDs or simultaneously with both AP MLDs. This connection state can be utilized by a non-AP MLD that performs roaming from an existing AP MLD to a new Target AP MLD, and a non-AP MLD that is connected to two different AP MLDs simultaneously can complete roaming without experiencing data service interruption during the roaming process. In other words, the non-AP MLD can perform uninterrupted roaming by utilizing the connection status connected to both AP MLDs. For the convenience of explanation, the AP MLD connected first is referred to as the serving AP MLD or current AP MLD.

[0192]

[0193] FIG. 14 shows a roaming process performed by a non-AP MLD according to an embodiment of the present invention.

[0194] In the embodiment of FIG. 14 (a), the non-AP MLD is connected to the serving AP MLD. The links established between the non-AP MLD and the serving AP MLD are the first link (Link 1) and the second link (Link 2). The non-AP MLD moves from the serving AP MLD to the target AP MLD. At this time, the non-AP MLD moves to the target AP MLD. When the non-AP MLD determines that the signal sensitivity of the serving AP MLD is decreasing, it can perform roaming to the target AP MLD. The non-AP MLD can terminate the connection of some links established between the non-AP MLD and the serving AP MLD and establish a new link with the target AP MLD. Specifically, the non-AP MLD can terminate a single link established between the non-AP MLD and the serving AP MLD. In the embodiment (b) of FIG. 14, the non-AP MLD terminates the connection at the first link (Link1) established between the serving AP MLD and the non-AP MLD, and connects with the target AP MLD at the second link (Link2).

[0195] Subsequently, the non-AP MLD can terminate all connections on the links established between the serving AP MLDs and maintain only the connection with the target AP MLD. In the embodiment (c) of FIG. 14, the non-AP MLD terminates all connections on the first link (Link1) and the second link (Link2) established between the serving AP MLDs and maintains the state in which the target AP MLD is connected to the first link (Link1) and the second link (Link2).

[0196]

[0197] The serving AP MLD can transmit information about an AP MLD adjacent to the serving AP MLD to the non-AP MLD. The non-AP MLD can determine the target AP MLD for roaming based on the information about the adjacent AP MLD received from the serving AP MLD. The serving AP MLD can transmit a management frame, such as a beacon frame, to the non-AP MLD instructing it on the information about the AP MLD adjacent to the serving AP MLD. In this case, the management frame may include an RNR element instructing it on the information about the AP MLD adjacent to the serving AP MLD. Specifically, the RNR element may instruct it on information about an adjacent AP MLD that allows the non-AP MLD to perform seamless roaming. For example, the RNR element may indicate whether the adjacent AP MLD is an AP MLD belonging to the same SMD (seamless mobility domain) as the serving AP MLD. The non-AP MLD can determine that an AP MLD belonging to the same SMD as the serving AP MLD is an AP MLD that supports seamless roaming.

[0198] Additionally, the Serving AP MLD can support the non-AP MLD establishing a link with the Target AP MLD. Specifically, the non-AP MLD can establish a link between the non-AP MLD and the Target AP MLD through the Serving AP MLD. The non-AP MLD can send a link reset request frame to the Serving AP MLD, indicating the link the non-AP MLD intends to establish with the Target AP MLD. The Serving AP MLD can send the link reset request frame received from the non-AP MLD to the Target AP MLD. At this time, the Serving AP MLD can transmit to the Target AP MLD according to the link reset request frame received from the non-AP MLD. The Target AP MLD can send a link reset response frame, which is a response to the link reset request frame, to the Serving AP MLD. The Serving AP MLD can send the received link reset response frame to the non-AP MLD. The Serving AP MLD and the Target AP MLD can exchange frames or information through a distribution system or OTA (Over the Air).

[0199] In another specific embodiment, the non-AP MLD may request a direct link setup from the target AP MLD. In this case, the non-AP MLD may transmit information regarding the serving AP MLD to the target AP MLD. The information transmitted by the non-AP MLD to the target AP MLD may include an identifier for the serving AP MLD.

[0200] Additionally, the target AP MLD can obtain information regarding the non-AP MLD from the serving AP MLD. Specifically, the target AP MLD can obtain information regarding the non-AP MLD from the serving AP MLD based on the received identifier. The information regarding the non-AP MLD may include information about the service status of the non-AP MLD. The information regarding the non-AP MLD may be referred to as the context information of the non-AP MLD. The information about the service status may include the sequence number of the last packet successfully transmitted by the serving AP MLD to the non-AP MLD, such as an MSDU, A-MSDU, or MMPDU. The target AP MLD can determine the sequence number of the packet to be transmitted to the non-AP MLD based on the sequence number obtained from the serving AP MLD. Through this, the non-AP MLD can receive a packet having a number consecutive to the sequence number of the previously received packet, even if roaming occurs. The information about the service status may include information about the BlockAck scoreboard. The target AP MLD can generate a BlockAck frame based on information regarding the received BlockAck scoreboard and transmit it to the non-AP MLD. Through this, the reception status of a packet successfully received by the serving AP MLD can be conveyed from the target AP MLD to the non-AP MLD.

[0201] Additionally, information regarding the service status may include information regarding the negotiation established between the non-AP MLD and the serving AP MLD. In this case, the information regarding the negotiation may include at least one of the agreed BlockAck bitmap size or information regarding the TWT agreement. In this case, the non-AP MLD may operate as if the agreement is complete without further negotiation with the target AP MLD. This reduces the time required for the non-AP MLD to perform negotiation again with the target AP MLD.

[0202]

[0203] <Operation of AP MLDs and non-AP MLDs performing roaming>

[0204] In the roaming described in Fig. 14, the non-AP MLD temporarily maintains a state of being simultaneously connected to the serving AP MLD and the target AP MLD. Conventionally, the non-AP MLD was allowed to be connected to only one AP MLD. This is because, even though the MLD is a logical entity, in the link layer and distribution system, the MLD was treated as a wireless LAN terminal using a single MAC address.

[0205] Additionally, all non-AP stations affiliated with a non-AP MLD may use the same Association ID (AID). In this case, the AP MLD assigns a single AID to the non-AP MLD. Furthermore, the non-AP MLD uses a single AID across all links established to the non-AP MLD. In these embodiments, when the non-AP MLD receives a PPDU from the AP MLD, it may receive a PSDU at the RU indicated by the User field of the PPDU, which has a STA-ID subfield indicating the non-AP MLD's AID, regardless of the link through which the PPDU is transmitted. Additionally, the non-AP MLD that receives a trigger frame may transmit a TB PPDU at the RU indicated by the User Info field, which has a value in the AID12 subfield of the trigger frame indicating the non-AP MLD's AID, regardless of the link. In these embodiments, the non-AP MLD may not be assigned a new AID from the AP MLD upon multi-link reconfiguration, because the non-AP MLD uses a single AID. However, if a non-AP MLD uses a single AID, AID conflicts may become an issue if the non-AP MLD is simultaneously associated with both the serving AP MLD and the target AP MLD.

[0206] FIG. 15 shows that an AID collision occurs while a non-AP MLD according to an embodiment of the present invention is performing roaming.

[0207] In the embodiment of FIG. 15, the non-AP MLD performs a roaming operation as described in FIG. 14. At this time, the value of the AID assigned to the non-AP MLD by the serving AP MLD is x. However, the target AP MLD has already assigned an AID value of x to the third non-AP station (non-AP STA3). The second non-AP station (non-AP STA2) of the non-AP MLD is connected to the fourth AP (AP4) of the target AP MLD. When the target AP MLD transmits a trigger frame containing a User Info field having an AID12 subfield indicating x, the second non-AP station (non-AP STA2) and the third non-AP station (non-AP STA3) may attempt to transmit a TB PPDU from the RU indicated by the User Info field having the AID12 subfield indicating x. A method is required to prevent such AID collisions.

[0208] In a specific embodiment, the serving AP MLD and the target AP MLD may share an AID space representing the range of AIDs assigned by the AP MLD. Through this, the values ​​of the AIDs assigned by the serving AP MLD and the target AP MLD can be managed so that they do not overlap. For example, if the serving AP MLD assigns an AID named x to any one MLD or station, the target AP MLD cannot assign x as an AID. In this embodiment, the serving AP MLD and the target AP MLD can exchange information regarding the assigned AIDs. Additionally, the serving AP MLD and the target AP MLD may assign AIDs within a range assigned to each. In this case, the ranges assigned to the serving AP MLD and the target AP MLD may not overlap with each other. For example, if the range of AIDs that the serving AP MLD can assign is from 1 to x, the range of AIDs that the target AP MLD can assign may be from x+1 to y. In these embodiments, when a non-AP MLD performs roaming, no AID conflict occurs, but the number of AIDs that an AP MLD can allocate may be reduced.

[0209] In another specific embodiment, when a non-AP station of a non-AP MLD connects to an AP of a target AP MLD, the AP of the target AP MLD may assign a new AID to the non-AP station of the non-AP MLD. In this case, the non-AP MLD may use different AIDs on the link connected to the serving AP MLD and the link connected to the target AP MLD. Additionally, the non-AP MLD may use the AID assigned by the serving AP MLD on the link connected to the serving AP MLD, and the AID assigned by the target AP MLD on the link connected to the target AP MLD. In this embodiment, the target AP MLD may instruct the non-AP MLD to the AID assigned to the non-AP MLD. Specifically, the target AP MLD may directly transmit information instructing the non-AP MLD to the AID assigned to the non-AP MLD. Specifically, the target AP MLD may include information indicating the AID assigned to the non-AP station of the non-AP MLD in a response frame to a frame transmitted by the non-AP MLD to establish a link. The frame transmitted to establish the link may be a link reset request frame, and the response frame may be a link reset response frame. Additionally, the frame transmitted to establish the link may be a connection request frame, and the response frame may be a connection response frame. In another specific embodiment, the target AP MLD may transmit information indicating the AID assigned to the non-AP MLD to the non-AP MLD via the serving AP MLD. Specifically, the target AP MLD may include information indicating the AID assigned to the non-AP station of the non-AP MLD in a response frame to a frame transmitted by the serving AP MLD to establish a link between the non-AP MLD and the target AP MLD.The frame transmitted by the serving AP MLD to establish a link between the non-AP MLD and the target AP MLD may be a roaming request frame, and the response frame may be a roaming response frame. In these embodiments, the non-AP MLD may transmit a link reset request frame to the serving AP MLD. Additionally, the serving AP MLD may include information obtained from the roaming response frame, such as the AID assigned to the non-AP station of the non-AP MLD, in the link reset response frame.

[0210] FIG. 16 shows a change in the AID applied to each link when a non-AP MLD according to an embodiment of the present invention performs roaming.

[0211] In the embodiment of FIG. 16(a), the non-AP MLD is connected to the serving AP MLD via two links. At this time, the non-AP MLD uses an AID value of x. When the non-AP MLD moves and the signal sensitivity received from the serving AP MLD decreases, the non-AP MLD performs roaming to the target AP MLD. Before roaming is completed, the non-AP MLD maintains the link with the serving AP MLD and establishes a link with the target AP MLD as shown in FIG. 16(b). At this time, the non-AP MLD uses x as the AID in the link connected to the serving AP MLD and y as the AID in the link connected to the target AP MLD. Accordingly, the first non-AP station (non-AP STA1) operating on the link connected to the serving AP MLD uses x as the AID, and the second non-AP station (non-AP STA2) operating on the link connected to the target AP MLD uses y as the AID. After roaming is complete, the non-AP MLD terminates the connection with the serving AP MLD as shown in Fig. 16 (c) and connects to the target AP MLD via two links. At this time, the non-AP MLD uses y as the AID.

[0212] A non-AP MLD performing roaming in this manner can maintain connections with two AP MLDs. In this case, the non-AP MLD can determine the link to which to send a response to the frame based on the link through which the frame was transmitted or the AP MLD that transmitted the frame. When the non-AP MLD receives a TID element, the non-AP MLD can determine the information of the TIM element based on the AP MLD that transmitted the TIM element. Specifically, the non-AP MLD can determine whether the AP MLD that transmitted the TIM element is the serving AP MLD or the target AP MLD. In this case, the non-AP MLD can determine whether the AP MLD that transmitted the TIM element is the serving AP MLD or the target AP MLD based on the value of the TA field of the frame containing the TIM element. The non-AP MLD can determine the information of the TIM element based on the AID assigned by the AP MLD that transmitted the TIM element. Specifically, if the bit corresponding to the AID assigned to the non-AP MLD from the AP MLD that transmitted the TIM element is 1, the non-AP MLD can determine that the AP MLD that transmitted the TIM element is buffering a BU (bufferable unit) for the non-AP MLD. If the AP MLD is buffering a BU for the non-AP MLD, the non-AP MLD can receive the BU and the AP MLD transmit a PS-Poll frame. The TIM element may be included in a beacon frame or a TIM frame.

[0213] FIG. 17 shows that a non-AP MLD according to an embodiment of the present invention transmits PS-Poll frames on different links depending on the AP MLD that transmitted the TIM element.

[0214] In the embodiment of FIG. 17, the non-AP MLD is connected to the serving AP MLD via a first link (Link 1) and a second link (Link 2), and to the target AP MLD via a fifth link (Link 5). FIG. 17 (a) shows the connection between the non-AP MLD, the serving AP MLD, and the target AP MLD.

[0215] In FIG. 17(b), the non-AP MLD receives a TIM element from the first AP (AP1) of the serving AP MLD via the first link (Link 1). At this time, the value of the bit corresponding to x allocated from the serving AP MLD is set to 1 in the TIM element. Since the non-AP MLD has received the TIM element from the first AP (AP1), it determines whether the bit corresponding to x is 1, and since it is 1, it determines that a BU for the non-AP MLD is buffered in the serving AP MLD. Therefore, the non-AP MLD transmits a PS-Poll frame to the serving AP MLD via the first link (Link 1) or the second link (Link 2). The non-AP MLD receives a TIM element from the second AP (AP2) of the serving AP MLD via the second link (Link 2). At this time, the value of the bit corresponding to x allocated from the serving AP MLD is set to 1 in the TIM element. Since the non-AP MLD receives a TIM element from the second AP (AP2), it determines whether the bit corresponding to x is 1, and since it is 1, it determines that a BU for the non-AP MLD is buffered in the serving AP MLD. Accordingly, the non-AP MLD transmits a PS-Poll frame to the serving AP MLD from the first link (Link 1) or the second link (Link 2). The non-AP MLD receives a TIM element from the fifth AP (AP5) of the target AP MLD on the fifth link (Link 5). At this time, the value of the bit corresponding to y allocated from the target AP MLD is set to 1 in the TIM element. Since the non-AP MLD receives a TIM element from the fifth AP (AP5), it determines whether the bit corresponding to x is 1, and since it is 1, it determines that a BU for the non-AP MLD is buffered in the serving AP MLD. Therefore, the non-AP MLD transmits a PS-Poll frame to the target AP MLD on Link 5.

[0216]

[0217] <SMD(seamless mobility domain)>

[0218] An SMD is an AP MLD group for supporting seamless roaming. An SMD includes one or more AP MLDs. When a non-AP MLD performs roaming between AP MLDs included in a single SMD, the non-AP MLD can perform seamless roaming according to the embodiments described above. The AP MLDs included in the SMD can perform operations for seamless roaming. Specifically, according to the embodiments described above, the AP MLDs can exchange information regarding the MLD context. Additionally, the AP MLDs can support link establishment according to the embodiments described above.

[0219] In addition, AP MLDs included in a single SMD can use a common security key, such as PMK (Pairwise master key), PTK (Pairwise transient key), or GTK (Group temporal key). Specifically, non-AP MLDs can generate and manage security keys in relation to the SMD. Through this, non-AP MLDs performing roaming can use the same security key they used with the serving AP MLD without generating a new security key when adding a new link with the target AP MLD.

[0220] FIG. 18 shows a detailed procedure for roaming according to an embodiment of the present invention and frames exchanged in the detailed procedure.

[0221] As explained earlier, a non-AP MLD connected to a serving AP MLD belonging to an SMD can perform roaming to a target AP MLD belonging to an SMD. This roaming can be referred to as an SMD BSS transition.

[0222] A non-AP MLD may instruct a serving AP MLD to perform roaming to a target AP MLD by transmitting a link reset request frame. In this case, the Type field of the link reset request frame may be a value indicating preparation for roaming, known as Preparation Type. Additionally, the link reset request frame may include information identifying the target AP MLD, such as the target AP MLD's MAC address or the target AP MLD's AP MLD ID. Based on the information identifying the target AP MLD, the serving AP MLD may determine the target AP MLD to which the non-AP MLD intends to perform roaming. Additionally, the link reset request frame may include information regarding the ID of the link that the non-AP MLD intends to establish with the target AP MLD. The number of links that the non-AP MLD intends to establish with the target AP MLD may be one or more. The serving AP MLD may transmit information regarding the links that the non-AP MLD intends to establish with the target AP MLD, such as the link ID and the number of links, to the target AP MLD. The target AP MLD may respond to the serving AP MLD regarding whether to accept the link setup for each link that the non-AP MLD intends to establish with the target AP MLD. If the target AP MLD accepts any of the links that the non-AP MLD intends to establish with the target AP MLD, the roaming of the non-AP MLD is accepted. In these embodiments, the serving AP MLD and the target AP MLD may exchange information via wireless LAN packets or through an interface between AP MLDs belonging to the SMD.

[0223] The target AP MLD can establish a link based on context information obtained from the serving AP MLD for the link that has received link setup. Through this, after roaming, the non-AP MLD can perform frame exchanges with the target AP MLD continuously with the frame exchanges it was performing with the serving AP MLD.

[0224] The serving AP MLD may include information in the link reset response frame indicating whether the target AP MLD has accepted the link setup for the link indicated by the non-AP MLD. The non-AP MLD may determine whether the target AP MLD has accepted roaming based on the link reset response frame received from the serving AP MLD. If the target AP MLD has accepted the link setup for at least one link, the non-AP MLD may decide whether to perform roaming to the target AP MLD.

[0225] Within a pre-specified execution timeout, the non-AP MLD may send a frame to the serving AP MLD instructing whether to perform roaming. In this case, the pre-specified timeout is referred to as the execution timeout. If the non-AP MLD does not send a frame instructing whether to perform roaming within the execution timeout, the target AP MLD may cancel the roaming accepted by the non-AP MLD. Specifically, the target AP MLD may cancel the procedures performed to support roaming and delete related information. The procedures performed to support roaming may include applying context information or setting up links. The frame instructing whether to perform roaming may be a link reset request frame. In this case, the value of the type of the link reset request frame may be 'Execution', a value instructing execution. Upon receiving a frame instructing that roaming be executed within the execution timeout, the serving AP MLD may send a response frame. In this case, the response frame may be a link reset response frame. Additionally, the response frame may include information indicating the time (DLDrainTime) required for the serving AP MLD to transmit traffic buffered in the serving AP MLD's queue to the non-AP MLD. For the sake of convenience, the time required to transmit to the non-AP MLD (DLDrainTime) is referred to as the drain time. The serving AP MLD can perform downlink transmission to the non-AP MLD until the drain time expires. The non-AP MLD can maintain a state capable of receiving downlink transmission from the non-AP MLD until the drain time expires. At this time, the non-AP MLD performs a power save operation and can perform operations according to the power save state (awake / doze).

[0226] The non-AP MLD can transmit a frame instructing the target AP MLD to execute roaming instead of the serving AP MLD. In this case, the embodiments described above may be applied to the frame instructing whether to execute roaming. Additionally, the non-AP MLD can transmit a frame instructing the target AP MLD to execute roaming through a link established between the target AP MLD and the non-AP MLD. The link established between the target AP MLD and the non-AP MLD is a link accepted by the target AP MLD.

[0227] A serving AP MLD or a target AP MLD that receives a frame instructing a roaming execution from a non-AP MLD may update the distribution system (DS) mapping for the non-AP MLD. After the serving AP MLD or the target AP MLD receives the frame instructing a roaming execution, traffic from the non-AP MLD transmitted from the DS may be forwarded to the port of the target AP MLD. Even if it is stated in this specification that the serving AP MLD or the target AP MLD updates the DS mapping, the entity of the SMD to which the serving AP MLD and the target AP MLD belong may update the DS mapping. If the serving AP MLD transmits all traffic for the non-AP MLD buffered in the serving AP MLD’s queue and the drain time has not yet expired, the serving AP MLD may transmit information instructing the non-AP MLD to complete downlink data transmission. Even if the serving AP MLD fails to transmit all traffic for non-AP MLDs buffered in its queue before the drain time expires, the serving AP MLD may not perform downlink transmission to the non-AP MLDs. When the drain time expires, the non-AP MLD may determine that the serving AP MLD's downlink transmission to the non-AP MLDs is complete.

[0228] If the non-AP MLD determines that the downlink transmission from the serving AP MLD to the non-AP MLD is complete, the non-AP MLD may transmit information indicating that the drain time is complete (DLDraintTime termination indication) to the target AP MLD. The target AP MLD may receive the information indicating the drain time termination (DLDraintTime termination indication) and respond with an ack. Through this, the non-AP MLD and the target AP MLD can complete the roaming procedure. At this time, the target AP MLD may indicate to the serving AP MLD that roaming is complete. Specifically, if the target AP MLD receives information indicating that the drain time is complete from the non-AP MLD, the target AP MLD may indicate to the serving AP MLD that roaming is complete. If the serving AP MLD receives from the target AP MLD that roaming is complete, the serving AP MLD may delete the link with the non-AP MLD. When the roaming process is completed, the non-AP MLD may delete information regarding the link with the serving AP MLD. At this time, the non-AP MLD may consider the link with the serving AP MLD to be deleted without exchanging link reset frames or disassociation frames.

[0229]

[0230] <Cancellation of Roaming>

[0231] In the embodiments described above, roaming can be canceled by the non-AP MLD not transmitting a frame instructing the execution of roaming. At this time, the target AP MLD can delete the link set up for the non-AP MLD. Additionally, the target AP MLD can delete the context information for the non-AP MLD.

[0232] When a frame instructing the execution of roaming is received, a DS mapping update for the non-AP MLD may be performed. Therefore, since the DS mapping update must be performed again, additional actions by the target AP MLD are required following the cancellation of roaming. First, the DS mapping for the non-AP MLD must be changed to the serving AP MLD. Additionally, the target AP MLD may forward traffic intended for the non-AP MLD to the serving AP MLD. Consequently, this may increase the additional communication burden. Therefore, the execution of roaming needs to be performed with caution.

[0233] During roaming, for example, after receiving information instructing roaming execution, the non-AP MLD may transmit information instructing roaming cancellation to the target AP MLD or the serving AP MLD. Specifically, the non-AP MLD may cancel roaming by transmitting a link reset request frame having a type of a value indicating roaming cancellation, such as Cancel, to the serving AP MLD or the target AP MLD. At this time, the serving AP MLD or the target AP MLD may respond to the link reset frame with a link reset response frame or an ack. At this time, the link reset response frame must always accept the link reset request frame. The serving AP MLD that receives information instructing roaming cancellation may transmit information instructing roaming cancellation to the target AP MLD. At this time, the target AP MLD may delete the link set up for the non-AP MLD. Additionally, the target AP MLD may forward traffic for the non-AP MLD delivered to the target AP MLD to the serving AP MLD.

[0234] Additionally, the non-AP MLD can cancel roaming by not transmitting information indicating the completion of the drain time to the target AP MLD. Furthermore, roaming may be automatically canceled when the non-AP MLD is unable to transmit information indicating the completion of the drain time or fails to transmit it. Specifically, if the non-AP MLD does not receive information indicating the completion of the drain time by a certain point in time, the non-AP MLD may determine that roaming has been canceled.

[0235] At this time, the specific point in time may be when a predetermined period has elapsed since the time when information indicating the drain time is received from the serving AP MLD. For example, the specific point in time may be the time indicated as the Completion Timeout in FIG. 19. The predetermined period may be a value predetermined by the SMD. At this time, the target AP MLD may transmit information indicating that roaming is canceled to the serving AP MLD. At this time, the target AP MLD may delete the link set up for the non-AP MLD. Additionally, the target AP MLD may forward traffic for the non-AP MLD transmitted to the target AP MLD to the serving AP MLD. Furthermore, as previously explained, the DS mapping for the non-AP MLD may be changed to the serving AP MLD.

[0236] FIG. 19 shows an operation in which a non-AP MLD according to an embodiment of the present invention stops a roaming procedure that was in progress.

[0237] In the embodiment of FIG. 19, the non-AP MLD requests roaming in accordance with the embodiments described above and transmits information regarding the execution of roaming to the serving AP. The non-AP MLD receives all traffic for the non-AP MLD buffered by the serving AP MLD from the serving AP MLD. At this time, the non-AP MLD decides to cancel the roaming. Accordingly, the non-AP MLD transmits a link reset request frame with type Cancele, which is a value indicating the cancellation of roaming, to the serving AP MLD. The serving AP MLD receives the link reset request frame and notifies the target AP MLD to cancel the roaming procedure. The serving AP MLD or the target AP MLD updates the DS mapping for the non-AP MLD from the target AP MLD to the serving AP MLD.

[0238] A target AP MLD that receives a notification regarding roaming cancellation can forward traffic to a non-AP MLD to a serving AP MLD. Additionally, the target AP MLD can delete the link set up for the non-AP MLD. Additionally, the target AP MLD can delete the context information for the non-AP MLD.

[0239] The serving AP MLD sends a link reset response frame to the non-AP MLD in response to the link reset request frame. In this case, the link reset response frame may have 'Cancel' as the result, a value indicating that the cancellation has been completed.

[0240] As previously explained, if the target AP MLD does not receive information from the non-AP MLD indicating the completion of the drain time by a certain point in time (Completion Timeout), it may determine that the roaming of the non-AP MLD has been canceled. In this case, the certain point in time may be when a pre-specified period has elapsed since the serving AP MLD transmitted information indicating the drain time. At this time, the target AP MLD may transmit information indicating that roaming has been canceled to the serving AP MLD. At this time, the target AP MLD may delete the link set up for the non-AP MLD. Additionally, the target AP MLD may forward the traffic for the non-AP MLD delivered to the target AP MLD to the serving AP MLD. The serving AP MLD transmits an unsolicited link reset response frame to the non-AP MLD. An unsolicited link reset response frame may be a link reset response frame transmitted without receiving a link reset request frame. At this time, the link reset response frame may have Cancel as TYPE, which is a value indicating that the running roaming procedure has been canceled.

[0241]

[0242] <Target AP MLD의 변경>

[0243] When the communication environment changes due to the movement of a non-AP MLD, roaming is required. Therefore, the target AP MLD that the non-AP MLD initially requested to roam may not be the optimal roaming target. Consequently, not only is it necessary to cancel the roaming, but also to change the target AP MLD that is the target of the roaming. However, it may not be permitted for the non-AP MLD to change the target AP MLD during the roaming execution phase, for example, after the non-AP MLD has transmitted information instructing the execution of roaming. In this case, instead of changing the target AP MLD, the non-AP MLD may cancel the ongoing roaming or complete the roaming.

[0244] A non-AP MLD preparing for roaming to a first target AP MLD may additionally prepare for roaming to a second AP MLD. Specifically, the non-AP MLD may transmit a frame requesting roaming to the first target AP MLD, such as a link reset request frame having a type of 'Preparation', which is a value indicating preparation for roaming, and may also transmit the said frame requesting roaming to the second target AP MLD. At this time, the non-AP MLD may selectively execute roaming with either the first target AP MLD or the second target AP MLD that performed the roaming preparation step. Information instructing the execution of roaming to the first target AP MLD or the second target AP MLD may be a link reset request frame having a type of 'Execution', which is a value indicating execution of roaming. At this time, the link reset request frame may include an identifier identifying the roaming for which execution is requested. Additionally, a link reset response frame accepting the roaming may include an identifier identifying the roaming. An identifier identifying roaming may include the value of a dialogue token of a link reset request frame having type Preparation requesting roaming, or an indicator of a target AP MLD, such as the MAC address or AP MLD ID of the target AP MLD. In this case, the value of the dialogue token may be specified by a non-AP MLD, and the non-AP MLD may select one of the integers that is not currently being used as the value of the dialogue token. Through these embodiments, a roaming preparation request for a first target AP MLD and a roaming preparation request for a second target AP MLD each have different identifiers, and based on the identifiers, it can be identified which AP MLD is the target AP MLD of the roaming preparation request.

[0245] The roaming preparation procedure performed by a non-AP MLD with multiple target AP MLDs may be valid only until the non-AP MLD executes roaming. Specifically, the roaming preparation procedure performed by a non-AP MLD with multiple target AP MLDs may be terminated when the non-AP MLD executes roaming with a specific target AP MLD.

[0246] In another specific embodiment, when a non-AP MLD performs a roaming preparation procedure with multiple target AP MLDs, only the last performed roaming preparation procedure may be defined as valid. Specifically, the non-AP MLD may cancel roaming to the first target AP MLD by transmitting a frame requesting roaming to the first target AP MLD, such as a link reset request frame, and transmitting a frame requesting roaming to the second target AP MLD.

[0247] In another specific embodiment, a non-AP MLD may not be allowed to request a new roaming until the execution timeout of the previously requested roaming has expired. Specifically, the non-AP MLD may transmit a frame requesting roaming to a first target AP MLD, such as a link reset request frame, and only after the execution timeout to the first target AP MLD has expired may it transmit a frame requesting roaming to a second target AP MLD.

[0248] FIG. 20 shows an operation in which a non-AP MLD changes a target AP MLD according to one embodiment of the present invention.

[0249] In the embodiment of FIG. 20, the non-AP MLD transmits a link reset request frame requesting roaming to the first target AP MLD (Target AP MLD1) in accordance with the embodiments described above. At this time, the link reset request frame indicates the link requesting setup. Also, the type of the link reset request frame is Preparation. Also, the value of the dialog token of the link reset request frame is x. Additionally, the link reset request frame may include information indicating the first target AP MLD (Target AP MLD1). The serving AP MLD transmits a link reset response frame to the non-AP MLD, having Success as the result, which is a value indicating acceptance. As in the embodiments described above, the link reset response frame may indicate the link being set up for roaming. Also, the link reset response frame indicates x as the value of the dialog token identifying roaming.

[0250] The non-AP MLD decides to perform roaming to the second target AP MLD (Target AP MLD2). At this time, the non-AP MLD transmits a roaming reset request frame requesting roaming with the second target AP MLD (Target AP MLD2). At this time, the link reset request frame indicates the link requesting setup. Also, the type of the link reset request frame is Preparation. Also, the value of the dialog token in the link reset request frame is y. Additionally, the link reset request frame may include information indicating the second target AP MLD (Target AP MLD2). The serving AP MLD transmits a link reset response frame to the non-AP MLD, having Success as the result, which is a value indicating acceptance. As with the embodiments described above, the link reset response frame may indicate the link being set up for roaming. Also, the link reset response frame indicates y as the value of the dialog token identifying the roaming.

[0251] A non-AP MLD can transmit information instructing the execution of roaming to a second target AP MLD (Target AP MLD2). The information instructing the execution of roaming to the second target AP MLD (Target AP MLD2) may be a link reset request frame having a type of Execution, which is a value instructing the execution of roaming. Additionally, the value of the dialog token of the link reset request frame is y. A serving AP MLD receives a link reset request frame having a type of Execution, which is a value instructing the execution of roaming, and identifies the roaming instructed to be executed by the dialog token value y of the link reset request frame.

[0252]

[0253] <Roaming Usage Restrictions>

[0254] If roaming is requested too frequently, it puts a strain on the AP MLD's operation and may delay the AP MLD's communication. Therefore, roaming requests may be allowed under certain limits.

[0255] In specific embodiments, a non-AP MLD may not be allowed to perform roaming again within a predetermined time interval. Specifically, a non-AP MLD may not be allowed to perform roaming again within a predetermined time interval within a single SMD. For example, a non-AP MLD may not be allowed to perform roaming within a first SMD and then perform roaming again within a predetermined time within the first SMD. However, a non-AP MLD may be allowed to perform roaming within a first SMD and then perform roaming again within a second SMD within a predetermined time. In this case, the predetermined time interval may be a value that applies commonly within the SMD. Additionally, the predetermined interval may be specified by the SMD. Furthermore, the predetermined interval may be indicated by the SMD when the non-AP MLD connects to the SMD. In these embodiments, a non-AP MLD may not be allowed to transmit a second link reset request frame instructing to perform roaming within a predetermined time from the time it transmitted a first link reset request frame instructing to perform roaming. Additionally, a non-AP MLD may not be allowed to transmit a second link reset request frame having a type of Preparation having a roaming request within a predetermined time from the time it transmits a first link reset request frame having a type of Preparation having a roaming request. Additionally, a non-AP may not be allowed to transmit information indicating the termination of the drain time again within a predetermined time from the time it transmits information indicating the termination of the drain time (DLDraintTime termination indication).

[0256] In a specific embodiment, the minimum time interval limit may be applied to at least one of the transmissions of the following frames or information.

[0257] - Transmission of a link reset request frame with a type value of Execution, directing sequential roaming execution

[0258] - Transmission of a link reset request frame of type value 'Execution' instructing the execution of roaming, and transmission of a link reset request frame of type value 'Prepration' instructing the preparation of roaming

[0259] - Transmission of a link reset request frame with a type value of Preparation, requesting roaming from the point of transmission of information indicating the end of the drain time

[0260] - Transmission of a link reset request frame with a type value of Execution, instructing roaming execution from the point of transmission of information indicating the end of the drain time

[0261] - Transmission of a link reset request frame with a type value of Preparation requesting roaming upon receiving a response frame containing information indicating the end of the drain time

[0262] - Transmission of a link reset request frame with a type value of Execution, instructing roaming execution upon receipt of a response frame containing information indicating the end of the drain time

[0263] - The response frame for the information indicating the end of the drain time can be a link reset response frame, as previously explained.

[0264] FIG. 21 shows a minimum time interval limit applied to the re-execution of a roaming-related operation of a non-AP MLD according to an embodiment of the present invention.

[0265] In the embodiment of FIG. 21, the non-AP MLD is connected to the first AP MLD (AP MLD1) according to the embodiments described above, and receives information indicating a value (Minimum ST Interval) that is limited to the minimum time interval applied to the SMD to which the first AP MLD (AP MLD1) belongs. While connected to the first AP MLD (AP MLD1), the non-AP MLD requests roaming to the second AP MLD (AP MLD2). The non-AP MLD performs roaming to the second AP MLD (AP MLD2) and performs frame exchange with the second AP MLD (AP MLD2). At this time, the non-AP MLD intends to perform roaming to the third AP MLD (AP MLD3). At this time, the minimum time interval limit is applied to the non-AP MLD. In the embodiment of FIG. 21, from the time when the non-AP MLD transmits a link reset request frame with type Execution instructing roaming execution (T_1) until a value limited to the minimum time interval (Minimum ST Interval) has elapsed ((T_2)), the non-AP MLD cannot transmit a link reset request frame with type Preparation instructing a roaming request. After the time limiting to the minimum time interval (T_2), the non-AP MLD performs roaming to the third AP MLD (AP MLD3) and performs frame exchange with the third AP MLD (AP MLD3).

[0266]

[0267] <Restrictions on AP MLD Operation to Prevent Roaming Failure>

[0268] An AP MLD operates on multiple links and can dynamically enable and disable links. Additionally, an AP MLD can change the operating channel of each link. An AP MLD can announce that such changes are scheduled in advance via beacon frames. However, a non-AP MLD performing roaming may not receive beacon frames on the link set up with the target AP MLD. This is because the number of links that a non-AP MLD can use simultaneously may be limited. For example, if a non-AP MLD supports only one link, it may not receive beacon frames from the target AP MLD when it performs roaming. Consequently, even if the non-AP MLD performs roaming, it may not be able to apply changes to the link or operating channel, and thus may not be able to perform frame exchanges with the target AP MLD.

[0269] When a non-AP MLD performs a roaming procedure, the AP operating the BSS of the setup link on which the non-AP MLD operates may not be allowed to change information regarding the operation of the BSS or the operational status of the link.

[0270] When a non-AP MLD within an SMD performs a roaming procedure, an AP MLD within the SMD may not be allowed to change information regarding the operation of the BSS or the operating status of the link set up with the non-AP MLD. In this case, the information regarding the operation of the BSS may include at least one of the BSS operating channel, BSS color, BSS operating bandwidth, or whether the link is active.

[0271] In these embodiments, when the non-AP MLD performs a roaming procedure, it may indicate a state where the non-AP MLD sets up a link with the target AP MLD and the roaming is not completed or canceled. Additionally, when the non-AP MLD performs a roaming procedure, it may indicate a case where the non-AP MLD transmits a roaming request and receives an acceptance of the roaming request, and the roaming is not canceled or completed.

[0272]

[0273] Through the embodiments described above, a non-AP MLD can switch a connection from the first AP MLD to the second AP MLD with minimal interruption of frame switching through the exchange of information between the first AP MLD and the second AP MLD. The exchange of information between the first AP MLD and the second AP MLD may include the context information described above. The first AP MLD and the second AP MLD may be AP MLDs belonging to a single SMD described above.

[0274] Specifically, the non-AP MLD transmits a roaming request to the second AP MLD to the first AP MLD that is currently connected, and the first AP MLD can forward the roaming request to the second AP MLD. Additionally, the second AP MLD transmits whether it accepts the roaming request to the first AP MLD, and the first AP MLD can transmit whether it accepts the received roaming request to the non-AP MLD. If the second AP MLD accepts the roaming, the non-AP MLD can transmit a roaming execution instruction to the first AP MLD. The first AP MLD can forward the roaming execution instruction to the second AP MLD. Additionally, the first AP MLD can transmit information regarding the drain time to the non-AP MLD, which indicates the time required to complete the transmission of traffic to the non-AP MLD. When the drain time ends, the non-AP MLD can transmit information indicating the end of the drain time to the second AP MLD. When the drain time expires or the transmission of the first AP MLD is determined to be complete, the non-AP MLD may determine that the drain time has ended (completed).

[0275] Additionally, the non-AP MLD may transmit a roaming request via a link reset request frame and receive whether the roaming request is accepted via a link reset response frame. However, the embodiments described above may not be limited to these frame formats. In this case, the specific operations of the non-AP MLD, the first AP MLD, and the second AP MLD are<Multi-link setup 상태를 활용한 로밍 기법> , <behavior of AP MLDs and non-AP MLDs performing roaming>, and <smd>You can follow the description of and the description of Figures 14 to 18.

[0276] In addition, the non-AP MLD may cancel roaming or change the target AP MLD in accordance with the restrictions described above. Specifically, the specific actions of the non-AP MLD, the first AP MLD, and the second AP MLD are <cancellation of roaming> and<Target AP MLD의 변경> You can follow the description of and the description of FIGS. 19 to 20.

[0277] In these embodiments, certain restrictions may be applied to the roaming operation of the non-AP MLD. Specifically, the non-AP MLD may not be allowed to perform roaming again within a predetermined time interval. The specific operation of the non-AP MLD may follow the description in <Restrictions on the Use of Roaming> and 21.

[0278] In addition, some restrictions may be applied to the operation of the second AP MLD while the roaming procedure is being performed. The specific operation of the second AP MLD may follow the description regarding <Restrictions on the operation of AP MLD to prevent roaming failure>.

[0279]

[0280] Although the present invention has been described above using wireless LAN communication as an example, the invention is not limited thereto and can be applied in the same way to other communication systems, such as cellular communication. Furthermore, while the method, apparatus, and system of the present invention have been described in relation to specific embodiments, some or all of the components and operations of the present invention may be implemented using a computer system having a general-purpose hardware architecture.

[0281] The features, structures, effects, etc. described in the embodiments above are included in at least one embodiment of the present invention and are not necessarily limited to only one embodiment. Furthermore, the features, structures, effects, etc. exemplified in each embodiment may be combined or modified and implemented in other embodiments by a person skilled in the art to which the embodiments belong. Therefore, details regarding such combinations and modifications should be interpreted as being included within the scope of the present invention.

[0282] Although the above description has focused on exemplary embodiments, this is merely illustrative and does not limit the invention. Those skilled in the art will understand that various modifications and applications not exemplified above are possible within the scope of the essential characteristics of the embodiments. For example, each component specifically shown in the embodiments may be modified. Furthermore, differences related to such modifications and applications should be interpreted as being included within the scope of the invention as defined in the appended claims.< / smd>

Claims

1. In a non-AP (access point) multi-link device (MLD) operating on multiple links in a wireless LAN, Transmitter / receiver; and Includes a processor, The above processor While the above non-AP MLD is connected to the first AP MLD, a roaming preparation request requesting roaming to the second AP MLD belonging to the SMD to which the first AP MLD belongs is transmitted to the first AP MLD, and Receive a roaming readiness request response from the first AP MLD that accepts the roaming readiness request, and Transmitting a roaming execution instruction to the first AP MLD to instruct the execution of the roaming, and Receive information regarding a drain time indicating the time required to complete the transmission of traffic to the non-AP MLD from the first AP MLD, and When the above drain time ends, information indicating the end of the above drain time is transmitted to the second AP MLD. non-AP MLD.

2. In Paragraph 1, If the second AP MLD receives information indicating the end of the drain time and instructs the first AP MLD that roaming is completed, the first AP MLD deletes the link between the first AP MLD and the non-AP MLD. non-AP MLD.

3. In Paragraph 2, The above roaming execution instruction includes the identifier of the second AP MLD that is the target of the roaming. non-AP MLD.

4. In Paragraph 1, For a predetermined time from the time the non-AP MLD transmits information indicating the end of the drain time, the non-AP MLD is not allowed to transmit a roaming readiness request to any AP MLD included in the SMD. non-AP MLD.

5. In Paragraph 1, Until the non-AP MLD sets up a link with the second AP MLD and completes or cancels roaming, the second AP MLD is not permitted to change information regarding the operation of the basic service set (BSS) on the link set up with the non-AP MLD or the operational status of the set-up link. non-AP MLD.

6. In Paragraph 5, Information regarding the operation of the above BSS (basic service set) includes information regarding the operation channel of the above BSS. non-AP MLD.

7. In a first access point (AP) multi-link device (MLD) operating on multiple links in a wireless LAN, Transmitter / receiver; and Includes a processor, The above processor While the first AP MLD is connected to a non-AP MLD, a roaming preparation request is received from the non-AP MLD requesting roaming to a second AP MLD belonging to the SMD to which the first AP MLD belongs, and Sending a roaming readiness request response to the above non-AP MLD accepting the above roaming readiness request, and Receives a roaming execution instruction from the above non-AP MLD instructing the execution of the above roaming, and Transmitting information regarding drain time to the above non-AP MLD, which indicates the time required to complete the transmission of traffic to the above non-AP MLD. 1st AP MLD.

8. In Paragraph 7, When information indicating that roaming is completed is received from the second AP MLD, the link between the first AP MLD and the non-AP MLD is deleted. 1st AP MLD.

9. In Paragraph 8, The above roaming execution instruction includes the identifier of the second AP MLD that is the target of the roaming. 1st AP MLD.

10. In Paragraph 7, For a predetermined time from the time the non-AP MLD transmits information indicating the end of the drain time, the non-AP MLD is not allowed to transmit a roaming readiness request to any AP MLD included in the SMD. 1st AP MLD.

11. In Paragraph 7, Until the non-AP MLD sets up a link with the second AP MLD and completes or cancels roaming, the second AP MLD is not permitted to change information regarding the operation of the basic service set (BSS) on the link set up with the non-AP MLD or the operational status of the set-up link. 1st AP MLD.

12. In Paragraph 11, Information regarding the operation of the above BSS (basic service set) includes information regarding the operation channel of the above BSS. 1st AP MLD.

13. A method of operation for a non-AP (access point) multi-link device (MLD) operating on multiple links in a wireless LAN, A step of transmitting a roaming preparation request to the first AP MLD, requesting roaming to a second AP MLD belonging to the SMD to which the first AP MLD belongs, while the non-AP MLD is connected to the first AP MLD; A step of receiving a roaming readiness request response from the first AP MLD that accepts the roaming readiness request; A step of transmitting a roaming execution instruction to the first AP MLD to instruct the execution of the roaming; A step of receiving information regarding a drain time indicating the time required to complete the transmission of traffic from the first AP MLD to the non-AP MLD; and When the drain time ends, the method includes the step of transmitting information indicating the end of the drain time to the second AP MLD. Method of operation.

14. In Paragraph 13, If the second AP MLD receives information indicating the end of the drain time and instructs the first AP MLD that roaming is completed, the first AP MLD deletes the link between the first AP MLD and the non-AP MLD. Method of operation.

15. In Paragraph 14, The above roaming execution instruction includes the identifier of the second AP MLD that is the target of the roaming. Method of operation.

16. In Paragraph 13, For a predetermined time from the time the non-AP MLD transmits information indicating the end of the drain time, the non-AP MLD is not allowed to transmit a roaming readiness request to any AP MLD included in the SMD. Method of operation.

17. In Paragraph 13, Until the non-AP MLD sets up a link with the second AP MLD and completes or cancels roaming, the second AP MLD is not permitted to change information regarding the operation of the basic service set (BSS) on the link set up with the non-AP MLD or the operational status of the set-up link. Method of operation.

18. In Paragraph 17, Information regarding the operation of the above BSS (basic service set) includes information regarding the operation channel of the above BSS. Method of operation.

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