WO2018012765A1

WO2018012765A1 - Method for automatically changing channel in wireless lan system and apparatus therefor

Info

Publication number: WO2018012765A1
Application number: PCT/KR2017/006878
Authority: WO
Inventors: 신종화; 송영금; 정양석; 허시영
Original assignee: 주식회사 케이티
Priority date: 2016-07-15
Filing date: 2017-06-29
Publication date: 2018-01-18
Also published as: KR20180008193A

Abstract

The present invention relates to a method and an apparatus for automatically changing a channel in a wireless LAN system. Disclosed is an automatic channel change method comprising: a channel change notification step for determining a channel change from a first channel to a second channel and transmitting a channel change notification message to a station (STA) associated with an access point (AP) by the AP; a channel change completion determination step for, after the channel change, determining whether the STA has also completed the channel change to the second channel in a predetermined first time; and a channel change re-notification step for, when the STA has not completed the channel change to the second channel, changing the channel to the first channel to retransmit the channel change notification message to the relevant STA and re-changing the channel to the second channel. Therefore, the present invention can minimize a service interruption for an STA, which may occur when a message notifying of a channel change does not normally reach the relevant STA due to interference although an AP of a wireless LAN system has automatically performed the channel change.

Description

Automatic channel change method and device therefor in WLAN system

The present invention relates to a method and apparatus for automatically changing a channel in a WLAN system. More specifically, the present invention relates to a station due to interference or the like when an access point (AP) automatically changes a channel in a WLAN system. The present invention relates to an automatic channel changing method and apparatus for minimizing service disconnection that may occur in a station (hereinafter, STA).

A wireless LAN is a method of communicating over a network using radio waves without using a cable. The emergence of wireless LAN has emerged as an alternative to solve the difficulties of installation, maintenance, and movement due to cabling, and the necessity is increasing due to the increase of mobile users.

When interference occurs in a channel currently used by an AP in a WLAN environment, a channel change is performed through a process as shown in FIG. 1 to maintain service quality. In detail, in the WLAN system, the AP may change the channel to a new channel after determining the channel change, but a case in which the CSA frame transmitted by the AP may not reach the STA and may be lost due to interference or the like may occur. have. In this case, the AP moves to a new channel, while an STA that does not receive a CSA frame cannot change a channel to a new channel, and the STA finds an AP for a predetermined time, and if the AP does not exist in the corresponding channel, The channel must be scanned again to find and reassociate the AP. At this time, the STA has a problem in that the service disconnection occurs in the STA during the time for determining that the AP disappears, the time for rescanning the AP through the full channel scan, and the time for reconnection to the AP.

The present invention is to solve the above problems, even if the AP fails to change the channel even though the AP transmits a channel change notification message to the STA in the WLAN system, the service disconnection for the STA occurs An object of the present invention is to provide an automatic channel changing method and apparatus for minimizing the number of channels.

Automatic channel change method in a wireless LAN system according to the present invention,

A channel change notification step of determining, by an access point (AP), a channel change from a first channel to a second channel, and transmitting a channel change notification message to an STA associated with the AP; A channel change determining step of determining whether the STA also completes the channel change to the second channel for a preset first time after changing the channel to the second channel; And when the STA fails to complete the channel change to the second channel, change the channel to the first channel, retransmit the channel change notification message to the corresponding STA, and change the channel back to the second channel. It may include a notification step.

Furthermore, the method may further include generating a list of QoS_STA lists for generating a list of STAs requiring QoS guarantee among the STAs connected to the AP, wherein the determining of channel change comprises: the STA included in the QoS_STA list being selected by the STA. It may be determined whether a channel change is completed with two channels.

The generating of the QoS_STA list may include: determining whether the corresponding STA is an STA requiring QoS guarantee based on the type of data packet received from the STA; If the STA is an STA requiring QoS guarantee, the STA may include adding to the QoS_STA list.

The generating of the QoS_STA list may further include allocating priorities based on the types of data packets received from the STA requiring the QoS guarantee. The re-notifying of the channel change may include: The QoS_STA may retransmit the channel change notification message more preferentially.

In addition, in the method, if there is an STA that has not completed channel change to the second channel even after performing each step, the corresponding STA completes the channel change to the second channel in a preset second time range. The channel change determination step and the channel change re-notification step may be repeatedly performed.

Furthermore, the method may further include: when the STA satisfies at least one or more of a condition in which the signal quality in the first channel is less than or equal to a first reference value and a signal quality in the second channel is greater than or equal to a second reference value, the STA is assigned to the STA. The method may further include transmitting a control message for transmitting the control message so as not to switch to the low power mode.

The channel change determining step may determine whether a data packet is received from an STA on the second channel.

The channel change re-notification step may change the channel to the first channel for a time shorter than a beacon frame period, retransmit the channel change notification message to the corresponding STA, and change the channel to the second channel again. have.

Automatic channel change AP in the wireless LAN system according to the present invention,

A channel changer that determines and performs a channel change from the first channel to the second channel; A channel change notification unit for transmitting a channel change notification message to an associated STA (Station) based on the channel change determination; And a channel change determining unit determining whether the STA completes the channel change to the second channel for a preset first time after changing the channel to the second channel, and changing the channel change to the second channel. If the STA fails to complete the channel change to the second channel for a preset first time, the AP changes the channel to the first channel, retransmits the channel change notification message to the corresponding STA, and then the second channel. You can change the channel to 2 channels.

Furthermore, the apparatus further includes a QoS_STA list generation unit for generating a list of STAs requiring QoS guarantee among the STAs connected to the AP, wherein the channel change determining unit changes the channel of the STA included in the QoS_STA list into the second channel. It can be determined whether or not to complete.

Furthermore, when the signal quality in the first channel is equal to or less than a first reference value and when at least one or more conditions of the signal quality in the second channel are equal to or greater than a second reference value, the STA switches to the low power mode. The STA mode control unit may further include a control message for transmitting the control message.

According to an embodiment of the present invention, even if the AP of the WLAN system performs automatic channel change, service disconnection for the corresponding STA, which may occur when a message indicating a channel change does not reach the STA properly due to interference, may be minimized. have.

BRIEF DESCRIPTION OF THE DRAWINGS The accompanying drawings, included as part of the detailed description in order to provide a thorough understanding of the present invention, provide examples of the present invention and together with the description, describe the technical idea of the present invention.

1 illustrates a procedure of a method performed when a channel change notification message transmitted from a conventional AP does not reach an STA.

2 shows each configuration of a wireless LAN system according to the present invention.

3 is a flowchart illustrating an automatic channel change method in a WLAN system according to a first embodiment of the present invention.

4 illustrates an example of a channel change notification message according to the present invention.

5 is a flowchart illustrating an automatic channel change method in a WLAN system according to a second embodiment of the present invention.

6 illustrates a part of an order of an automatic channel changing method in a WLAN system according to a third embodiment of the present invention.

7 illustrates a part of an order of an automatic channel changing method in a WLAN system according to a fourth embodiment of the present invention.

8 illustrates a configuration of an automatic channel change AP in a WLAN system according to the present invention.

As the inventive concept allows for various changes and numerous embodiments, particular embodiments will be described in detail with reference to the accompanying drawings.

The following examples are provided to assist in a comprehensive understanding of the methods, devices, and / or systems described herein. However, this is only an example and the present invention is not limited thereto.

In describing the embodiments of the present invention, when it is determined that the detailed description of the known technology related to the present invention may unnecessarily obscure the gist of the present invention, the detailed description thereof will be omitted. In addition, terms to be described below are terms defined in consideration of functions in the present invention, which may vary according to the intention or custom of a user or an operator. Therefore, the definition should be made based on the contents throughout the specification. The terminology used in the description is for the purpose of describing particular embodiments only and should not be limiting. Unless expressly used otherwise, the singular forms “a,” “an,” and “the” include plural forms of meaning. In this description, expressions such as "comprises" or "equipment" are intended to indicate certain features, numbers, steps, actions, elements, portions or combinations thereof, and one or more than those described. It should not be construed to exclude the presence or possibility of other features, numbers, steps, actions, elements, portions or combinations thereof.

In addition, terms such as first and second may be used to describe various components, but the components are not limited by the terms, and the terms are used to distinguish one component from another component. Used only as

Hereinafter, a method and apparatus for automatic channel change in a WLAN system according to the present invention will be described in detail with reference to the accompanying embodiments.

Referring to FIG. 2, the WLAN system according to the present invention may include an AP 100, a STA 200, and a communication network 10 wirelessly connecting the AP 100 and the STA 200. have.

First, the AP 100 is a functional entity that provides data through a wireless medium for an associated station (STA) associated with it. Since the WLAN environment is constantly changing, the AP 100 periodically measures the signal quality in each channel for stable data transmission to the STA 200, and based on the corresponding measurement value, the channel to a channel having a better condition. The change can be determined. In addition, since the AP 100 may provide data only to the STA 200 existing on the same channel, the STA 200 may transmit a channel change notification message to the combined STA 200 after determining a channel change. Also, channel change related information may be provided to enable channel change.

In addition, the AP 100 may separately store and manage a list of STAs requiring QoS guarantee from among the STAs 200.

In addition, the AP 100 may transmit a control message for partially controlling the operation of the STA 200.

The STA 200 is a functional medium including a medium access control (MAC) conforming to the IEEE 802.11 standard and a physical layer interface to a wireless medium. The concept includes all of the STA. However, in the present invention, it is assumed that the STA 200 is a non-AP STA for convenience of description. The STA 200 receives data from the AP 100 and operates according to the function of each STA 200. As shown in FIG. 2, the STA 200 may include an IPTV, a PC, a laptop, a smart phone, and the like.

The communication network 10 mediates data transmission and reception between the AP 100 and the STA 200, and may include a wired network and a wireless network. However, the communication network 10 is assumed to be a wireless network. Specifically, the communication network 10 may include a local area network (LAN), a metropolitan area network (MAN), a wide area network (WAN), and the like. However, the communication network 10 according to the present invention is not limited to the networks listed above, but may include a known wireless data network.

Referring to FIG. 3, in the automatic channel changing method in the WLAN system according to the first embodiment of the present invention, the AP 100 determines a channel change and transmits a channel change notification message (S320). Thereafter, the method may include determining whether to change a channel for each STA 210 and 220 (S330), and resending the channel change notification message to the STA 210 that has not completed channel change (S330). However, prior to the above steps, it is assumed that the AP 100 and the

STAs

210 and 220 in the WLAN system are transmitting and receiving data (WLAN Data) on the first channel.

First, in step S320, the AP 100 may determine the channel change when a predetermined channel change condition is satisfied while periodically measuring the signal quality of each channel of the WLAN system. Here, the signal quality may include delay, packet loss, jitter, bandwidth, throughput, reliability, availability, and the like. The channel change condition may mean whether data load, interference, signal strength, etc. satisfy a preset condition.

Upon determining the channel change, the AP 100 transmits a channel change notification message to the STA 200 that is associated with the current channel (first channel) before performing the actual channel change. The channel change notification message may include identification information about a channel (second channel) to be changed, a channel change time, and the like.

4 illustrates an example of a channel switch announcement (CSA) frame as an example of the channel change notification message. It may include a Channel Switch Announcement Element used in IEEE 802.11. The channel switch notification element may include a channel switch mode, a new channel number, a channel switch count, and the like. Upon receiving the channel change notification message, the STA 200 stops data transmission until the channel is changed and changes the channel to the second channel when the Channel Switch Count becomes 0.

However, as can be seen in step S320 of Figure 3, despite the transmission of the channel change notification message of the AP 100, the channel change notification message is not properly received by some STA 210 due to interference in the WLAN environment. Cases may occur.

Next, in step S320, after the AP 100 completes the channel change to the second channel, the information included in the channel change notification message by the STA 200 that has been associated with the AP 100. It may be determined whether or not to complete the channel change to the second channel for a predetermined first time by. That is, it is determined whether a data packet is received from the STA 200 during the first time. Here, the first time may be set in consideration of a delay variation or jitter of a packet, a buffer size, and the like, and a time taken for several data packets to reach the AP 100 from the STA 200. It may be set in consideration. In addition, among the plurality of STAs 200 connected to the AP 100, the data packet transmission period may be set based on the data packet transmission period of the longest STA.

As shown in FIG. 3, the STA 220 receiving the channel change notification message in the previous step S320 completes the channel change to the second channel based on the channel change notification message received from the AP 100. In addition, the AP 100 may perform data transmission / reception (WLAN Data) smoothly. On the other hand, since the STA 210 that has not received the channel change notification message due to interference or the like is present on a different channel from the AP 100, smooth data transmission and reception are impossible. At this time, the AP 100 determines that the STA 210 has failed to change the channel based on the absence of a data packet received from the STA 210 during the first time.

In step S340, the AP 100 changes the channel back to the first channel for the STA 210 that has not completed the channel change to the second channel, retransmits the channel change notification message to the STA 210, and again. The channel may be changed to the second channel.

Although not shown, if the STA 210 fails to change the channel because the STA 210 does not receive the channel change notification message, the steps S330 and S340 are performed until the corresponding STA 210 completes the channel change. May be repeated.

However, when the AP 100 performs the step S340 for the STA 210 that fails to change the channel, the AP 100 should not interfere with data transmission and reception with the STA 220 that has completed the channel change, so the step S340 is a beacon. ) Is preferably performed for a time shorter than the period of the frame.

In addition, in a wireless LAN environment with the STA 210 that fails to change the channel, it may occur that the channel change problem cannot be solved simply by repeatedly transmitting the channel notification change message. In this case, since repeating steps S330 and S340 is meaningless, a second time is set in advance, and although the steps S330 and S340 are repeatedly performed within the second time range, the STA 210 channels. If the change is not completed, the step S330 and S340 may not be performed any more. At this time, if the time taken to repeatedly perform steps S330 and S340 is longer than the service disconnection time in FIG. 1, the object of the present invention may not be achieved, and thus, the second time is set within the service disconnection time. It is preferable.

5 illustrates an example of a procedure of an automatic channel changing method in a WLAN system according to a second embodiment of the present invention.

Referring to FIG. 5, in the automatic channel change method in the WLAN system according to the second embodiment of the present invention, the AP 100 generates a QoS_STA list (S510), determines a channel change, and transmits a notification message. And a step S520 of determining whether to change the channel for the QoS_STA 210 after the channel change, and resending the channel change notification message to the QoS_STA 210 that has not completed the channel change (S540). can do. However, prior to the above steps, it is assumed that the AP 100 and the

STAs

In operation S510, the AP 100 may generate a list of STAs (hereinafter, referred to as QoS_STAs) for which quality of service (QoS) guarantee is required. More specifically, when the AP 100 and the

STAs

210 and 220 are initially associated with each other, the STA is based on the characteristics and types of data packets received from the

STAs

210 and 220 to the AP 100. It is determined whether it is QoS_STA, and if the corresponding STA is QoS_STA, the identification information is stored in the QoS_STA list. For example, an STA generating traffic such as a Real Time Protocol (RTP) packet, AC_VI, AC_VO, and the like may be determined as QoS_STA. In FIG. 5, the QoS_STA 210 is stored in the QoS_STA list, and the STA 220 is not stored.

The STA (QoS_STA) requiring the QoS guarantee may include an STA that should allocate radio communication resources preferentially to another STA, an STA that plays a high capacity multimedia, and the like. For example, among the STA 200 of FIG. 2, the IPTV 210 including the set top box may be classified as QoS_STA. In general, the IPTV 210 requires transmission and reception of high-capacity data such as video and audio, and STA STAs 220a and 220b have different user inconveniences when data transmission and reception are not smooth in relation to the AP 100 due to interference or the like. This is because it is comparable to 220c).

Since step S520 is the same as step S320, detailed description thereof will be omitted. However, it is assumed that the channel change notification message has not reached both the QoS_STA 210 and the STA 220.

In operation S530, the AP 100 may not determine whether the channel is changed for all

connected STAs

210 and 220, but may determine whether the second channel is changed only for the QoS_STA included in the QoS_STA list. In FIG. 5, both the QoS_STA 210 and the STA 220 have failed to change the channel, but the AP 100 may determine that only the QoS_STA 210 has changed the channel.

In step S540, the AP 100 changes the channel back to the first channel for the QoS_STA 210 that has not completed the channel change to the second channel and retransmits the channel change notification message to the QoS_STA 210. The channel may be changed back to the second channel. At this time, even if the STA 220 also fails to change the channel in the previous step (S530), since the STA 220 is not included in the QoS_STA list, the STA 220 may not retransmit the channel change notification message.

Like the first embodiment, steps S530 and S540 may be repeatedly performed for a preset second time, and in particular, step S540 may be performed for a shorter time than a period of a beacon frame.

In the third embodiment of the present invention, based on the second embodiment described above with reference to FIG. 5, the STA 200 is further subdivided to allocate priorities and retransmit the channel change notification message according to the priority. It is about.

First, in step S510, when the AP and the STA are connected, the AP receives a data packet from the STA (S511). The STA determines whether the corresponding STA is a QoS_STA based on the characteristics and types of the received data packet, allocates a priority according to preset DB information, and stores the identification information in the QoS_STA list. The priority may be set such that higher priority is assigned to data traffic such as video and audio sensitive to latency or data traffic having a high loss rate.

Steps S520 and S530 are the same as those described with reference to FIG. 5, and thus will be omitted.

In step S540, the AP 100 changes the channel back to the first channel for the QoS_STA 210 that has not completed the channel change to the second channel (S541), and prioritizes each STA stored in the QoS_STA list. Based on the higher priority, the channel change notification message may be retransmitted (S542), and the channel may be changed back to the second channel (S543).

As described above, the AP continuously measures the channel environment in the WLAN system, determines a channel change when a specific channel change condition is satisfied, and transmits a channel change notification message to an STA connected to the AP. However, at this time, if the STA receiving the channel change notification message is operating in a low power mode, the probability of not receiving the channel change notification message increases.

In order to solve this problem, when the signal quality of the current channel is deteriorating enough to cause a channel change in the WLAN system or the signal quality of another channel other than the current channel is improving (that is, the AP has a high probability of determining the channel change). At this point, the AP may transmit a control message to the STA to control the STA from operating in a low power mode. Here, it is assumed that the channel change is performed when the signal quality in the current channel is lower than the third reference value or when the signal quality in a channel other than the current channel is better than the fourth reference value.

Referring to FIG. 7, in the automatic channel changing method in the WLAN system according to the fourth embodiment of the present invention, the AP 100 measures signal quality of all channels on the first channel (S710). Determining whether the signal quality of the current channel (first channel) is deteriorating or the signal quality of another channel (second channel) other than the current channel is improving to such an extent (S720). It may be configured to include a step (S730) for transmitting.

In step S710, the AP measures the signal quality of the current channel that is transmitting and receiving data with the STA and the signal quality of a channel other than the current channel. Here, the signal quality may include delay, packet loss, jitter, bandwidth, throughput, reliability, availability, and the like.

Step S720 is a step of determining when the AP has a high probability of determining a channel change, more specifically, determining whether a signal quality of a first channel is equal to or less than a first reference value (S721) and at least one other than the first channel. The method may include determining whether the signal quality of the second channel is greater than or equal to the second reference value (S722). Here, the first reference value may be greater than the third reference value, and the second reference value may be smaller than the fourth reference value. However, the method of determining the point of time when the AP has a high probability of determining a channel change is not limited to steps S721 and S722, and may further consider various variables such as a change amount of signal quality in each channel. In operation S720, operation S722 may be performed before operation S721.

In operation S730, when one or more of the conditions of the operations S721 and S722 are satisfied, a control message may be transmitted to the STA connected to the AP to prevent operation in the low power mode. Furthermore, it may be determined whether to transmit the control message by further considering the remaining battery capacity or the power supply state of the STA.

8 illustrates a configuration of an automatic channel changing apparatus in a wireless LAN system according to the present invention.

Referring to FIG. 8, in the WLAN system according to the present invention, the automatic channel change AP 100 generates a channel change unit 110, a channel change notification unit 120, a channel change determination unit 130, and a QoS_STA list. It may be configured to include the unit 140 and the STA mode control unit. Additionally, the DB 300 may further include the QoS_STA list, a channel change condition, priority information according to a data packet, and the like.

The channel changer 110 may determine and perform a channel change from a current channel to which the AP 100 is transmitting / receiving data to a channel other than the current channel. The channel change may be performed when the signal quality of the current channel and other channels other than the current channel are measured, and the measured signal quality satisfies a preset condition.

The channel change notification unit 120 may transmit a channel change notification message to the STA connected to the AP based on the channel change determination. The channel change notification message may include information such as identification information about a channel to be changed, channel change time, and the like.

The channel change determination unit 130 may determine whether the STA completes the channel change to the channel to be changed for a preset first time. More specifically, it may be determined whether a channel of the corresponding STA is changed based on a data packet received from the STA during the preset first time. Here, the first time may be set in consideration of a delay variation or jitter of a packet, a buffer size, and the like, and a time taken for several data packets to reach the AP 100 from the STA 200. It may be set in consideration. In addition, among the STAs 200 connected to the AP 100, the data packet transmission period may be set to the data packet transmission period of the longest STA 200.

The QoS_STA list generation unit 140 determines whether the corresponding STA is a STA (QoS_STA) requiring QoS guarantee based on the characteristics and types of data packets received from the STA when the AP and the STA are associated. If the corresponding STA is QoS_STA, the identification information is stored in the QoS_STA list. Furthermore, priority may be allocated and stored according to QoS_STA.

The STA mode controller 150 may control the STA not to operate in the low power mode by transmitting a control message to the STA connected to the AP. When the STA operates in the low power mode, it is difficult to properly receive the channel change notification message. Therefore, the STA does not operate in the low power mode before transmitting the channel change notification message by determining a time when the AP has a high probability of determining the channel change. It is not allowed.

The above description is merely illustrative of the technical idea of the present invention, and those skilled in the art to which the present invention pertains may make various modifications and changes without departing from the essential characteristics of the present invention. Therefore, the embodiments described in the present invention are not intended to limit the technical idea of the present invention but to describe the present invention, but are not limited to these embodiments. The scope of protection of the present invention should be interpreted by the following claims, and all technical ideas falling within the scope of the present invention should be construed as being included in the scope of the present invention.

Claims

A channel change notification step of determining, by an access point (AP), a channel change from a first channel to a second channel, and transmitting a channel change notification message to an STA associated with the AP;

A channel change determining step of determining whether the STA also completes the channel change to the second channel for a preset first time after changing the channel to the second channel; And

When the STA fails to complete the channel change to the second channel, the channel change re-notification of changing the channel to the first channel to resend the channel change notification message to the corresponding STA and changing the channel to the second channel again. Automatic channel change method comprising the step of.
The method of claim 1,

The method,

A QoS_STA list generation step of generating a list of STAs requiring QoS guarantee among STAs connected to the AP;

The channel change determination step,

And determining whether the STA included in the QoS_STA list completes the channel change to the second channel.
The method of claim 2,

The QoS_STA list generation step,

Determining whether the corresponding STA is an STA requiring QoS guarantee based on the type of data packet received from the STA;

If the STA is an STA requiring QoS guarantee, adding to the QoS_STA list.
The method of claim 2,

The QoS_STA list generation step,

The method may further include assigning a priority based on a type of a data packet received from an STA requiring the QoS guarantee.

The channel change re-notification step,

The higher priority QoS_STA retransmits the channel change notification message more preferentially.
The method of claim 1,

The method,

If there is an STA that has not completed channel change to the second channel even after performing each step,

And repeating the channel change determination step and the channel change re-notification step until the corresponding STA completes the channel change in the second channel in a preset second time range.
The method of claim 1,

The method,

When the signal quality in the first channel is equal to or less than a first reference value and when at least one or more of the conditions in which the signal quality in the second channel is greater than or equal to a second reference value are satisfied, the STA is not switched to the low power mode by the STA. And a control message transmitting step of transmitting a control message.
The method of claim 1,

The channel change determination step,

And determining whether a data packet is received from the STA on the second channel.
The method of claim 1,

The channel change re-notification step,

And changing the channel to the first channel for a time shorter than a beacon frame period, retransmitting the channel change notification message to the corresponding STA, and changing the channel to the second channel again.
A channel changer that determines and performs a channel change from the first channel to the second channel;

A channel change notification unit for transmitting a channel change notification message to an associated STA (Station) based on the channel change determination;

And a channel change determination unit to determine whether the STA completes the channel change to the second channel for a preset first time after changing the channel to the second channel.

If the STA fails to complete the channel change to the second channel for a preset first time after changing the channel change to the second channel, the AP changes the channel to the first channel to the corresponding STA. And resending a change notification message and changing the channel back to the second channel.
The method of claim 9,

Further comprising a QoS_STA list generation unit for generating a list for the STA that requires QoS guarantee among the STAs connected to the AP,

The channel change determination unit,

And determining whether the STA included in the QoS_STA list completes the channel change to the second channel.
The method of claim 9,

When the signal quality in the first channel is less than or equal to a first reference value and when at least one or more conditions of the signal quality in the second channel are greater than or equal to a second reference value, the STA is not switched to the low power mode by the STA. Automatic channel change AP device further comprises a STA mode control unit for transmitting a control message.