PATENT ABSTRACT
A device and method for active scanning are disclosed. The active scanning method in a wireless LAN can comprise the steps of: allowing an AP to receive a probe request frame containing an the AP identifier; determining whether the AP is a target AP or a non-target AP on the basis of the AP identifier; and performing back-off for the transmission of a probe response frame from a second interval after a first interval is terminated in a minimum channel interval when the AP is a non-target AP. Therefore, the present invention can prevent probe response frames from flooding a STA in a short period of time by distributing intervals in which probe response frames are received to the STA. In addition, the time used by the STA to perform active scanning can be reduced.

PATENT DESCRIPTION
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is the National Stage filing under 35 U.S.C. 371 of International Application No. PCT/KR2013/001526, filed on Feb. 26, 2013, which claims the benefit of U.S. Provisional Application Ser. Nos. 61/605,767, filed on Mar. 2, 2012 and 61/609,897, filed on Mar. 12, 2012, the contents of which are all hereby incorporated by reference herein in their entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to the scanning method and apparatus of a station (STA) and, more particularly, to a method and an apparatus for performing active scanning by an STA. 
     2. Related Art 
     A recent Wireless LAN (WLAN) technology is basically evolving into three directions. There are Institute of Electrical and Electronic Engineers (IEEE) 802.11ac and IEEE 802.11ad as efforts to further increase the transfer rate on the extension line of the existing WLAN evolution direction. IEEE 802.11ad is a WLAN technology using a 60 GHz band. Furthermore, a wide area WLAN that utilizes a frequency band of less than 1 GHz in order to enable wider area transfer than that of the existing WLAN in distance is recently emerging. The wide-area WLAN includes IEEE 802.11af that uses a TV White Space (TVWS) band and IEEE 802.11ah that uses a 900 MHz band. A main object of the wide-area WLANs is to extend extended range Wi-Fi services as well as the smart grid and a wide-area sensor network. Furthermore, the existing WLAN Medium Access Control (MAC) technology is problematic in that an initial link setup time is very long according to circumstances. In order to solve such a problem and in order for an STA to rapidly access an AP, IEEE 802.11ai standardization is recently in progress actively. 
     IEEE 802.11ai is a MAC technology for handling a rapid authentication procedure in order to significantly reduce the initial setup and association time of a WLAN. Standardization activities for IEEE 802.11ai have been started as a formal task group on January, 2011. In IEEE 802.11ai, in order to enable a rapid access procedure, a discussion on the simplification of procedures in such fields AP discovery, network discovery, Time Synchronization Function (TSF) synchronization, authentication &amp; association, and a procedure convergence with a higher layer is in progress. From among them, ideas, such as procedure convergence using the piggyback of a Dynamic Host Configuration Protocol (DHCP), the optimization of a full Extensible Authentication Protocol (EAP) using a concurrent IP, and efficient and selective Access Point (AP) scanning, are being actively discussed. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide the active scanning method of a station (STA). 
     Another object of the present invention is to provide an apparatus for performing the active scanning method of a station (STA). 
     An active scanning method in a WLAN according to an aspect of the present invention for achieving the aforementioned object of the present invention may includes the steps of receiving, by an Access Point (AP), a probe request frame comprising an AP identifier, determining whether or not the AP is a target AP or a non-target AP based on the AP identifier, and performing back-off for the transmission of a probe response frame from a second interval after a first interval of a minimum channel interval expires if the AP is the non-target AP, wherein the AP may be the target AP if the AP identifier is indicative of the AP, the AP may be the non-target AP if the AP identifier is not indicative of the AP, the minimum channel interval may be a minimum time used to scan each channel, and the minimum channel interval may include the first interval and the second interval. A step of performing the back-off for the transmission of the probe response frame in the first interval if the AP is the target AP may be further include. The probe request frame may include a first interval use field indicative of whether the first interval is used or not. The probe request frame further may include at least one first interval time field comprising information about a period assigned as the first interval. The step of performing the back-off for the transmission of the probe response frame from the second interval after the first interval of the minimum channel interval expires if the AP is the non-target AP may include overhearing whether or not the probe response frame is transmitted by the target AP during the first interval if the AP is the non-target AP and sending the probe response frame during the second interval if whether or not the probe response frame is transmitted by the target AP is not overheard during the first interval. Information about the AP identifier may be at least one of at least one Basic Service Set IDentification (BSSID), at least one Service Set IDentification (SSID), a mesh ID, a Homogeneous Extended Service Set IDentifier (HESSID), and a network ID. 
     An AP for performing active scanning in a WLAN according to another aspect of the present invention for achieving the aforementioned object of the present invention may includes a processor. The processor may be configured to determine whether or not an AP is a target AP or a non-target AP based on an AP identifier included in a received probe request frame and to perform back-off for the transmission of a probe response frame from a second interval after a first interval of a minimum channel interval expires if the AP is the non-target AP. The AP may be the target AP if the AP identifier is indicative of the AP. The AP may be the non-target AP if the AP identifier is not indicative of the AP. The minimum channel interval may be a minimum time used to scan each channel, and the minimum channel interval may include the first interval and the second interval. The processor may be configured to perform the back-off for the transmission of the probe response frame in the first interval if the AP is the target AP. The probe request frame may include a first interval use field indicative of whether the first interval is used or not. The probe request frame further may include at least one first interval time field comprising information about a period assigned as the first interval. The processor may be configured to overhear whether or not the probe response frame is transmitted by the target AP during the first interval if the AP is the non-target AP and to send the probe response frame during the second interval if whether or not the probe response frame is transmitted by the target AP is not overheard during the first interval. Information about the AP identifier may be at least one of at least one Basic Service Set IDentification (BSSID), at least one Service Set IDentification (SSID), a mesh ID, a Homogeneous Extended Service Set IDentifier (HESSID), and a network ID. 
     A phenomenon in which probe response frames are crowded within a short time is prevented by distributing an interval in which the probe response frames received by a station (STA). Furthermore, the time that is taken for an STA to perform active scanning can be reduced. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a conceptual diagram illustrating the configuration of a Wireless Local Area Network (WLAN); 
         FIG. 2  is a conceptual diagram illustrating an active scanning procedure; 
         FIG. 3  is a conceptual diagram illustrating a method of sending a probe request frame; 
         FIG. 4  is a conceptual diagram illustrating an active scanning method in accordance with an embodiment of the present invention; 
         FIG. 5  is a conceptual diagram illustrating a probe request frame in accordance with an embodiment of the present invention; 
         FIG. 6  is a conceptual diagram illustrating an active scanning method in accordance with an embodiment of the present invention; 
         FIG. 7  is a conceptual diagram illustrating an active scanning method in accordance with an embodiment of the present invention; 
         FIG. 8  is a conceptual diagram illustrating an active scanning method in accordance with an embodiment of the present invention; 
         FIG. 9  is a conceptual diagram illustrating an active scanning method in accordance with an embodiment of the present invention; 
         FIG. 10  is a conceptual diagram illustrating an active scanning method in accordance with an embodiment of the present invention; 
         FIG. 11  is a flowchart illustrating a method of performing active scanning in accordance with an embodiment of the present invention; and 
         FIG. 12  is a block diagram illustrating a wireless apparatus to which an embodiment of the present invention may be applied. 
     
    
    
     DESCRIPTION OF EXEMPLARY EMBODIMENTS 
       FIG. 1  is a conceptual diagram illustrating the configuration of a Wireless Local Area Network (WLAN). 
       FIG. 1(A)  illustrates the configuration of an infrastructure network according to Institute of Electrical and Electronic Engineers (IEEE) 802.11. 
     Referring to  FIG. 1(A) , the WLAN system may include one or more Basic Service Sets (BSSs)  100  and  105 . Each of the BSSs  100  and  105  is a set of an AP and an STA, such as an Access Point (AP)  125  and a Station STA1  100 - 1  that are successfully synchronized with each other and are capable of communicating with each other. The BSS is not a concept indicative of a specific area. The BSS  105  may include one or more STAs  105 - 1  and  105 - 2  that may be associated with a single AP  130 . 
     An infrastructure BSS may include at least one STA, the APs  125  and  130  providing distribution service, and a Distribution Systems (DS)  110  coupling a plurality of APs. 
     The DS  110  may implement an Extended Service Set (ESS)  140  by coupling some BSSs  100  and  105  together. The ESS  140  may be used as a term indicative of a single network over which one or more APs  125  and  230  are connected through the DS  110 . APs included in a single ESS  140  may have the same Service Set IDentification (SSID). 
     A portal  120  may function as a bridge for performing connection between a WLAN network (i.e., IEEE 802.11) and another network (e.g., 802.X). 
     In an infrastructure network, such as that of  FIG. 1(A) , a network between the APs  125  and  130  and a network between the APs  125  and  130  and the STAs  100 - 1 ,  105 - 1 , and  105 - 2  may be implemented. However, a network may be configured between STAs so that the STAs may perform communication even without the APs  125  and  130 . A network configured between STAs so that the STAs may perform communication without the APs  125  and  130  is defined as an Ad-Hoc network or an independent Basic Service Set (BSS). 
       FIG. 1(B)  is a conceptual diagram illustrating an independent BSS. 
     Referring to  FIG. 1(B) , the Independent BSS (IBSS) is a BSS that operates in Ad-Hoc mode. The IBSS does not include a centralized management entity because it does not include an AP. That is, in the IBSS, STAs  150 - 1 ,  150 - 2 ,  150 - 3 ,  155 - 4 , and  155 - 5  are managed in a distributed manner. In the IBSS, all the STAs  150 - 1 ,  150 - 2 ,  150 - 3 ,  155 - 4 , and  155 - 5  may be mobile STAs, and they form a self-contained network because they cannot access a distribution system. 
     An STA is a specific function medium, including Medium Access Control (MAC) that complies with the rules of the Institute of Electrical and Electronics Engineers (IEEE) 802.11 standard and a physical layer interface for a radio medium, and may be used as a meaning including both an AP STA and a non-AP STA in a broad sense. 
     An STA may be called as various names, such as a mobile terminal, a wireless device, a Wireless Transmit/Receive Unit (WTRU), User Equipment (UE), a Mobile Station (MS), a mobile subscriber unit, or simply a user. 
       FIG. 2  is a conceptual diagram illustrating an active scanning procedure. 
     Referring to  FIG. 2 , the active scanning procedure may be performed in accordance with the following steps. 
     (1) An STA  200  determines whether it is ready to perform a scanning procedure. 
     The STA  200  may perform active scanning, for example, after a probe delay time expires or until specific signaling information (e.g., PHY-RXSTART.indication primitive) is received. 
     The probe delay time is delay generated before a probe request frame  210  is transmitted when the STA  200  performs active scanning. The PHY-RXSTART.indication primitive is a signal transmitted from a physical (PHY) layer to a local Medium Access Control (MAC) layer. The PHY-RXSTART.indication primitive may signal information indicative that a PLCP Protocol Data Unit (PPDU) including a valid PLCP header has been received in a Physical Layer Convergence Protocol (PLCP) to the MAC layer. 
     (2) The STA  200  performs basic access. 
     In the 802.11 MAC layer, some STAs may share a radio medium using, for example, a Distributed Coordination Function (DCF) that is a contention-based function. The DCF is an access protocol, and can prevent a collision between STAs through a back-off method using Carrier Sense Multiple Access/Collision Avoidance (CSMA/CA). The STA  200  may send the probe request frame  210  to APs  260  and  270  using a basic access method. 
     (3) The STA  200  may include information (e.g., information about a Service Set IDentification (SSID) and a Basic Service Set IDentification (BSSID)) for specifying the APs  260  and  270 , included in an MLME-SCAN.request primitive, in the probe request frame  210 , and may send the probe request frame  210 . 
     The BSSID is an indicator for specifying an AP, and may have a value corresponding to the Medium Access Control (MAC) address of the AP. A Service Set IDentification (SSID) is a network name for specifying an AP that may be read by a person who operates an STA. The BSSID and/or the SSID may be used to specify an AP. 
     The STA  200  may specify an AP based on information for specifying the APs  260  and  270  included in the MLME-SCAN.request primitive. The specified APs  260  and  270  may send probe response frames  250  and  240  to the STA  200 . The STA  200  may include the information about the SSID and the BSSID in the probe request frame  210  and sending the probe request frame  210  by unicasting, multicasting, or broadcasting the probe request frame  210 . A method of unicasting, multicasting, or broadcasting the probe request frame  210  using the information about the SSID and the BSSID is additionally described with reference to  FIG. 3 . 
     For example, if an SSID list is included in the MLME-SCAN.request primitive, the STA  200  may include the SSID list in the probe request frame  210  and send the probe request frame  210 . The APs  260  and  270  may receive the probe request frame  210 , may determine an SSID included in the SSID list included in the probe request frame  210 , and may determine whether or not to send the probe response frames  240  and  250  to the STA  200 . 
     (4) The STA  200  resets a probe timer to 0 and then drives the probe timer. 
     The probe timer may be used to check a minimum channel time ‘MinChanneltime’  220  and a maximum channel time ‘MaxChanneltime’  230 . The minimum channel time  220  and the maximum channel time  230  may be used to control the active scanning operation of the STA  200 . 
     The minimum channel time  220  may be used to perform an operation for changing a channel in which the STA  200  performs active scanning. For example, if the STA  200  has not received the probe response frames  240  and  250  until the minimum channel time  220 , the STA  200  may change a scanning channel and perform scanning in another channel. If the STA  200  has received the probe response frames  240  and  250  until the minimum channel time  220 , the STA  200  may wait until the maximum channel time  230  and process the received probe response frames  240  and  250 . 
     The STA  200  may detect a PHY-CCA.indication primitive until the probe timer reaches the minimum channel time  220 , and may determine whether or not the probe response frames  240  and  250  have been received by the STA  200  prior to the minimum channel time  220 . 
     The PHY-CCA.indication primitive includes information about the state of a medium, and may be transmitted from the physical layer to the MAC layer. The PHY-CCA.indication primitive may provide notification of the state of a current channel using a channel state parameter called ‘busy’ if the channel is not available, and may provide notification of the state of a current channel using a channel state parameter called ‘idle’ if the channel is not available. If the PHY-CCA.indication is detected as being busy, the STA  200  may determine that the probe response frames  240  and  250  received by the STA  200  are present. If the PHY-CCA.indication is detected as being idle, the STA  200  may determine that the probe response frames  240  and  250  received by the STA  200  are not present. 
     If the PHY-CCA.indication is detected as being idle, the STA  200  may set a Net Allocation Vector (NAV) to 0 and scan a next channel. If the PHY-CCA.indication is detected as being busy, the STA  200  may perform processing on the probe response frames  240  and  250  received after the probe timer has reached the maximum channel time  230 . After processing the received probe response frames  240  and  250 , the STA  200  may set a Net Allocation Vector (NAT) to 0 and scan a next channel. 
     Hereinafter, in an embodiment of the present invention, to determine whether the probe response frames  240  and  250  received by the STA  200  are present or not may include determining the state of a channel using the PHY-CCA.indication primitive. 
     (5) If all channels included in a channel list ‘ChannelList’ are scanned, the MLME may signal an MLME-SCAN.confirm primitive. The MLME-SCAN.confirm primitive may include BSSDescriptionSet including all pieces of information that have been obtained in the scanning process. 
     If the STA  200  uses an active scanning method, the STA  200  needs to perform monitoring for determining whether the parameter PHY-CCA.indication is busy or not until the probe timer reaches a minimum channel time. Accordingly, although a probe response frame has been received from an AP specified through the probe request frame  210  prior to the minimum channel time, there is a problem in that unnecessary channel monitoring continues to be performed until the minimum channel time is reached. 
     Furthermore, although a probe response frame has been received from a specified AP, unnecessary delay in performing active scanning may occur because processing on a probe request frame received after wait until the probe timer reaches a maximum channel time is performed. 
       FIG. 3  is a conceptual diagram illustrating a method of sending a probe request frame. 
       FIG. 3  discloses a method of broadcasting, multicasting, and unicasting a probe request frame. 
       FIG. 3(A)  is a method of broadcasting, by an STA  300 , a probe request frame  310 . 
     The STA  300  may include a wildcard SSID and a wildcard BSSID in the probe request frame  310 , and may broadcast the probe request frame  310 . 
     The wildcard SSID and the wildcard BSSID may be used as identifiers indicative of all APs  305 - 1 ,  305 - 2 ,  305 - 3 ,  305 - 4 , and  305 - 5  that are included in the coverage of the STA  300 . 
     If the STA  300  includes the wildcard SSID and the wildcard BSSID in the probe request frame  310  and sends the probe request frame  310 , the APs  305 - 1 ,  305 - 2 ,  305 - 3 ,  305 - 4 ,  305 - 5  that have received the probe request frame  310  transmitted by the STA  300  may send probe response frames to the STA  300  in response to the received probe request frame. 
     If the APs  305 - 1 ,  305 - 2 ,  305 - 3 ,  305 - 4 , and  305 - 5  that have received the broadcasted probe request frame  310  send the probe response frames to the STA  300  within a specific time in response to the received probe request frame  310 , there may be a problem in that the STA  300  has to receive and process too many probe response frames at once. 
       FIG. 3(B)  is a method of unicating, by an STA  320 , a probe request frame  330 . 
     Referring to  FIG. 3(B) , if the STA  320  unicasts the probe request frame  330 , the STA  320  may send the probe request frame  330  including information about a specific SSID/BSSID of an AP. The STA  320  may send the probe response frame to only an AP  325  that belongs to APs that have received the probe request frame  330  and that corresponds to a specific SSID/BSSID. 
       FIG. 3(C)  is a method of multicasting, by an STA  340 , a probe request frame  360 . 
     Referring to  FIG. 3(C) , the STA  340  may include an SSID list and a wildcard BSSID in the probe request frame  360  and send the probe request frame  360 . APs  350 - 1  and  350 - 2  that belong to APs that have received the probe request frame  360  and that correspond to SSIDs included in the SSID list included in the probe request frame may send probe response frames to the STA  340 . 
     When an STA unicasts/multicasts a probe request frame as in  FIGS. 3(B) and 3(C) , there may be a case where a probe response frame may not be received from an AP corresponding to an SSID that is specified in the probe request frame transmitted by the STA. In such a case, the STA that has not received the probe response frame waits until a minimum channel time, changes a scanning channel to another channel, and performs scanning in another channel. That is, unnecessary delay in performing active scanning may occur because the STA that has not received the probe response frame from the specified AP may change a scanning channel only after it waits until the minimum channel time. 
     Accordingly, an active scanning method according to an embodiment of the present invention discloses a method of reducing unnecessary delay generated when an STA performs active scanning and rapidly associating the STA with an AP. Furthermore, there is disclosed a method for solving a problem in that an STA receives too many probe response frames in a specific time interval in an existing active scanning method. 
       FIG. 4  is a conceptual diagram illustrating an active scanning method in accordance with an embodiment of the present invention. 
       FIG. 4  discloses a method of receiving, by an STA  400 , a probe response frame from a specified at least one target AP  410  only in a first minimum channel time  440 - 1 , that is, a set time interval, if the STA  400  specifies at least one AP that will send a probe response frame and sends a probe request frame  430  to the specified at least one AP. 
     Hereinafter, in an embodiment of the present invention, if an STA unicasts or multicasts a probe request frame using information for specifying an AP, such as an SSID, an SSID list, or a BSSID, the specified AP is called a target AP. The remaining APs not specified by the probe request frame are called non-target APs. 
     Referring to  FIG. 4 , the STA  400  may specify the target AP  410  that will respond to the transmitted probe request frame  430 , and may send the probe request frame  430  to the specified target AP  410 . For example, the STA  400  may include information for specifying an AP, such as an SSID or an SSID list, in the probe request frame  430 , and may send the probe request frame  430 . 
     The STA  400  may specify the first minimum channel time  440 - 1  that belongs to a minimum channel time  440  as an interval in which a probe response frame is received from the target AP  410  only, and may use the specified first minimum channel time  440 - 1 . That is, the STA  400  may define some specified interval that belongs to the minimum channel time  440  and in which the probe response frame is received from the target AP as the first minimum channel time  440 - 1 , and may define intervals, belonging to the minimum channel time  440  other than the first minimum channel time  440 - 1 , as a second minimum channel time  440 - 2 . 
     A first interval, that is, another term, may be used as the same meaning as the first minimum channel time  440 - 1 , and a second interval, that is, another term, may be used as the same meaning as the second minimum channel time  440 - 2 . That is, in an embodiment of the present invention, the terms called the first minimum channel time  440 - 1  and the second minimum channel time  440 - 2  are used and described, but the first interval and the second interval may be interpreted as being the same meanings. 
     The first minimum channel time  440 - 1  may mean a time interval that is preferentially used for the target AP  410  to send a probe response frame  415  and for the STA  400  to receive the probe response frame  415  from the target AP  410 . That is, in the first minimum channel time, the target AP may perform back-off for sending the probe response frame. 
     In the first minimum channel time  440 - 1 , the STA  400  may receive the probe response frame  415  from only the target AP  410  that has been specified through the probe request frame  430 . After the first minimum channel time  440 - 1 , the STA  400  may receive a probe response frame  425  even from a non-target AP  420  that is not specified through the probe request frame  430 . 
     The probe request frame  430  including information about the target AP  410  may be transmitted to the non-target AP  420  in addition to the target AP  410 . The probe request frame  430  may include information about the first minimum channel time, for example, information about whether the first minimum channel time is used or not and information about the time that is set as the first minimum channel time. 
       FIG. 5  is a conceptual diagram illustrating a probe request frame in accordance with an embodiment of the present invention. 
     Referring to  FIG. 5 , the probe request frame may include first minimum channel time information field ‘FirstMinChannel information field’  510  including information about whether or not a first minimum channel time is used. 
     For example, the first minimum channel time information field  510  may be assumed to operate on or off as flag information. If the first minimum channel time information field is on, an AP that has received a probe request frame may obtain information indicative that an STA will receive a probe response frame from a target AP using the first minimum channel time. If the first minimum channel time information field is off, an AP that has received a probe request frame may obtain information indicative that an STA does not use the first minimum channel time. If the first minimum channel time is used, the time assigned as the first minimum channel time is a predetermined value, and may have a value smaller than the minimum channel time. 
     A field including information about whether or not the first minimum channel time (or the first interval) is used may be called a first interval use field. 
     As another method, the first minimum channel time field  510  may include information about the time assigned as the first minimum channel time. A target AP that has received the probe request frame may send a probe response frame to an STA within the time assigned as a first minimum channel time based on information about the time assigned as the first minimum channel time included in the first minimum channel time field  510 . Information about a second minimum channel time may also be included in the probe request frame. That is, the probe request frame may include a field including information about a period (or an interval) that is assigned as a first minimum channel time (or a first interval). 
     Furthermore, the probe request frame may information for specifying an AP. For example, at least one BSSID, at least one SSID, a mesh ID, a Homogeneous Extended Service Set IDentifier (HESSID), or a network ID (e.g., a roaming consortium ID) may be used as the information for specifying an AP. An STA may specify an AP that will send a probe response frame by including information about at least one AP ID of the IDs in the probe request frame and then sending the probe request frame. Such information about an AP ID may be included in the identifier field of a target AP ‘Indication of target AP field’  500  and transmitted. 
     The names of the fields disclosed in  FIG. 5  are arbitrary, and other names may be used. Furthermore, the information included in the field disclosed in  FIG. 5  may be transmitted in various information formats in such a manner that the information is included in a field not an independent field and transmitted. 
     (2) After sending the probe request frame  430 , the STA  400  monitors the probe response frame  415  received from the target AP  410  during the first minimum channel time  440 - 1 . 
     (3) If the STA  400  receives the probe response frame  415  from the target AP  410  within the first minimum channel time  440 - 1 , the STA  400  may be associated with the target AP  410  by performing an authentication and association process along with the target AP  410 . 
     In the case of the existing active scanning method, if the STA  400  receives a probe response frame from an AP within the minimum channel time  440 , the STA  400  waits until a maximum channel time  450  is reached and then performs processing on the received probe response frame. Accordingly, unnecessary delay is generated in performing active scanning. If the active scanning method disclosed in the present invention is used, however, the STA  400  may preferentially receive the probe response frame  415  from the target AP  410  in the first minimum channel time  440 - 1 , and may directly perform an authentication and association procedure along with the target AP  410 . Accordingly, unnecessary delay generated in active scanning can be reduced because the STA  400  does not need to unnecessarily wait until the maximum channel time  450 . 
     Furthermore, in the case of the existing active scanning method, the availability of a channel was monitored using the PHY-CCA.indication primitive until the minimum channel time  440 . In the active scanning method disclosed in the present invention, however, an unnecessary monitoring section can be reduced because a section in which the availability of a channel is monitored using the PHY-CCA.indication primitive is limited to the first minimum channel time  440 - 1  and monitoring is performed on the channel. 
     In another embodiment, if a signal received from another AP (e.g., the non-target AP  420 ) has an SNR better than that received from the target AP  410 , in order to select the signal having the better SNR, the STA  400  may wait until the maximum channel time  450  without performing an authentication and association procedure along with the target AP  410 , and may process the received probe response frames  415  and  425  after the maximum channel time  450 . 
     If the probe response frame  415  is not received from the target AP  410  within the first minimum channel time  440 - 1 , the STA  400  may change a scanning channel to another channel and perform a scanning procedure. In another method, the STA  400  may additionally determine whether or not the probe response frame  415  transmitted by the non-target AP  420  is received until the minimum channel time  440 . An embodiment of the present invention to be described later discloses a method of additionally determining whether or not the probe response frame  425  transmitted by the non-target AP  420  is received until the minimum channel time  440 . 
     If the STA  400  uses a method of receiving the probe response frame  415  from the target AP  410  in the first minimum channel time  440 - 1 , a phenomenon in which probe response frames are simultaneously received within a specific time interval can be prevented and the STA  400  can be rapidly associated with the target AP  410  because the SAT  400  preferentially receives the probe response frame  415  of the target AP  410 . 
     (4) If the STA  400  does not receive the probe response frame  415  from the target AP  410  in the first minimum channel time  440 - 1 , the STA  400  may determine whether or not a probe response frame is received until the minimum channel time  440 . 
     Hereinafter, in an embodiment of the present invention, a time interval in which whether or not a probe response frame is received is determined by determining whether or not the probe response frame is received is determined is called the minimum channel time  440 . 
     The STA  400  may receive the probe response frame  425  from the non-target AP  420  after the first minimum channel time  440 - 1 . If the STA  400  does not receive a probe response frame from the target AP  410  and the non-target AP  420  until the first minimum channel time  440 - 1 , the STA  400  may change a scanning channel to another channel and perform a scanning procedure. 
     That is, if the second minimum channel time (i.e., the second interval) is started after the first minimum channel time, the non-target AP  420  may perform back-off for sending the probe response frame. 
     A case where the probe response frame  415  is not received from the target AP  410  in the first minimum channel time  440 - 1  may be generated, for example, in a case where the STA moves. If an STA is associated with an AP installed by the user of the STA indoors and then moved outdoors, the STA is unaware of information about the AP included in the coverage of the STA. In such a case, if the STA specifies an AP and sends a probe request frame to the specified AP, the STA is unable to receive a probe response frame from the specified AP. Accordingly, the STA may receive a probe response frame from a non-target AP other than a target AP after a first minimum channel time, and may perform association with the non-target AP. 
     If the STA  400  receives the probe response frame  425  from the non-target AP  420 , the STA  400  may perform an authentication and association procedure along with the non-target AP  420  that has sent the probe response frame  425  after the minimum channel time  440  expires. Alternatively, the STA  400  may wait until the maximum channel time  450 , may receive an additional probe response frame if the additional probe response frame is transmitted, may perform processing on the received probe response frame after the maximum channel time  450  expires, and may perform an authentication and association procedure. 
     (5) If the STA  400  does not receive a probe response frame during the minimum channel time  440 , the STA  400  may change a scanning channel to another channel and perform the aforementioned procedure of (1)-(4) again. 
     The various embodiments described with reference to  FIG. 4  are disclosed in detail below with reference to  FIGS. 6 to 8 . 
       FIG. 6  is a conceptual diagram illustrating an active scanning method in accordance with an embodiment of the present invention. 
       FIG. 6  discloses a method of preferentially receiving, by an STA  600 , a probe response frame  615  from a target AP  610  within a first minimum channel time  606  and performing an authentication and association procedure along with the target AP  610 . 
     Referring to  FIG. 6 , the STA  600  may send a probe request frame  625  including the SSID of the target AP  610 . The target AP  610  that belong to APs that have received the probe request frame  625  and that corresponds to the SSID may send the probe response frame  615  in the first minimum channel time  606 . The STA  600  may receive the probe response frame  615  from the target AP  610  and perform an authentication and association procedure along with the target AP  610 . 
     A non-target AP  620  may overhear whether or not the probe response frame  615  is transmitted by the target AP  610 . If the probe response frame  615  is transmitted by the target AP  610  based on a result of the overhearing, the non-target AP  620  may not send a probe response frame to the STA  600 . If such a method is used, an unnecessary probe response frame can be prevented from being transmitted to the STA  600 . Furthermore, the non-target AP  620  may send the probe response frame  615  to the STA  600  regardless of whether or not the probe response frame  615  is transmitted by the target AP  610 . 
     A non-target AP may be aware of whether or not a probe response frame is transmitted by a target AP based on an interface between APs instead of overhearing the probe response frame. 
     If the STA  600  does not receive the probe response frame  615  in the first minimum channel time  606 , the STA  600  may wait for a probe response frame, transmitted after the first minimum channel time  606 , until a minimum channel time  603 , or may change a scanning channel to another channel after the first minimum channel time  606  and perform scanning ( 650 ) in another channel. 
       FIG. 7  is a conceptual diagram illustrating an active scanning method in accordance with an embodiment of the present invention. 
       FIG. 7  discloses a method of performing, by an STA  700 , an authentication and association procedure along with a non-target AP  720  if the STA  700  receives a probe response frame  730  from the non-target AP  720  within a minimum channel time  703  without receiving a probe response frame from a target AP  710  in a first minimum channel time  706 . 
     Referring to  FIG. 7 , the STA  700  may receive the probe response frame  730  from the non-target AP  720  within the minimum channel time  703 . 
     If the STA  700  receives the probe response frame  730  from the non-target AP  720  within the minimum channel time  703 , the STA  700  may immediately perform an authentication and association procedure along with the non-target AP  720  that has sent the probe response frame  730 , or may wait until a maximum channel time  709  and perform an authentication and association procedure along with an AP that has sent the probe response frame  730 . 
     The target AP  710  may send a probe response frame within the minimum channel time  703  after the first minimum channel time  706  expires. In such a case, the STA  700  may preferentially process the probe response frame transmitted by the target AP  710  and perform an authentication and association procedure along with the target AP  710 . 
       FIG. 8  is a conceptual diagram illustrating an active scanning method in accordance with an embodiment of the present invention. 
       FIG. 8  discloses a case where an STA  800  has not received a probe response frame within a minimum channel time  803 . 
     If the STA  800  has not received a probe response frame within the minimum channel time  803 , the STA  800  may change a channel in which active scanning is performed to another channel, and may perform scanning. 
       FIG. 9  is a conceptual diagram illustrating an active scanning method in accordance with an embodiment of the present invention. 
       FIGS. 9(A) to 9(C)  are conceptual diagrams illustrating a method of sending, by a target AP  910  and non-targets AP  920  and  930 , a probe response frame if the target AP  910  and the non-target APs  920  and  930  are present in the coverage of the probe request frame of an STA  900 . 
     Referring to  FIG. 9(A) , the STA  900  may send a probe request frame  905  that includes information capable of specifying an AP, such as the SSID of the target AP  910 . 
     The probe request frame  905  may also be transmitted to the non-target APs  920  and  930  in addition to the target AP  910  specified through the SSID. The probe request frame  905  may include information related to a first minimum channel time. The target AP  910  and the non-target APs  920  and  930  may obtain information about timing at which a probe response frame has to be transmitted to the STA  900  based on the information related to the first minimum channel time that is included in the received probe request frame  905 . 
     Referring to  FIG. 9(B) , only a target AP  910  may send a probe response frame  915  to an STA  900  during a first minimum channel time. The target AP  910  may send the probe response frame  915  to the STA  900  within the first minimum channel time. 
     If the STA  900  receives the probe response frame  915  from the target AP  910  within the first minimum channel time and immediately performs authentication and association along with the target AP  910 , the STA  900  may not receive a probe response frame transmitted after the first minimum channel time. In another embodiment, in order to additionally overhear whether or not an AP that sends a probe response frame having a better SNR is present, the STA  900  may receive an additional probe response frame for a time that has been predetermined in order to perform authentication and association. 
     (3) After the first minimum channel time, non-target APs  920  and  930  send probe response frames  925  and  935 . 
     After the first minimum channel time, the non-target APs  920  and  930  may send the probe response frames  925  and  935  to the STA  900 . For example, if an STA is externally moved, an AP different from an AP previously registered with the STA needs to be used. In such a case, if the SSID of a specific AP is specified and a probe request frame is unicasted, there is a good possibility that the STA may not receive the probe response frame from the specified AP because the STA is unaware of the SSID of the external AP. In such a case, the STA  900  may receive the probe response frames  925  and  935  from the non-target APs  920  and  930  other than the target AP  910 , and may perform authentication and association. 
     If the target AP  910  and the non-target APs  920  and  930  do not send the probe response frames  925  and  935  to the STA  900  during a minimum channel time, the STA  900  may change a channel in which scanning is performed to another channel. 
     In accordance with an embodiment of the present invention, if the target AP  910  sends the probe response frame  915 , the non-target APs  920  and  930  may overhear the probe response frame  915  transmitted by the target AP  10  and not send the probe response frames  925  and  935 . 
       FIG. 10  is a conceptual diagram illustrating an active scanning method in accordance with an embodiment of the present invention. 
     Referring to  FIG. 10 , if an STA  1000  receives a probe response frame  1015  from a target AP  1010  with which the STA  1000  wants to be associated, an additional time may be assigned so that the STA  1000  does not need to receive a probe response frame from the non-target APs  1020  and  1030 . For example, in the case of an AP shared in a specific office, there is a possibility that the signal of the AP may be the greatest within the office. Accordingly, if the target AP  1010  sends the probe response frame  1015  and performs authentication and association, other non-target APs  1020  and  1030  may not need to send probe response frames. 
     The STA  1000  may send a specific probe request frame to the target AP  1010 , and the target AP  1010  may send the probe response frame  1015  to the STA  1000  within a first minimum channel time. In this case, the non-target APs  1020  and  1030  may overhear the probe response frame  1015  transmitted by the target AP  1010 . If the non-target APs  1020  and  1030  overhear the probe response frame  1015  transmitted by the target AP  1010 , the non-target APs  1020  and  1030  may not send probe response frames even after the first minimum channel time. If such a method is used, an unnecessary operation of generating and sending, by the non-target APs  1020  and  1030 , probe response frames can be reduced. A phenomenon in which the STA  1000  unnecessarily receives probe response frames can be prevented because the STA  1000  may not receive an unnecessary probe response frame. 
       FIG. 11  is a flowchart illustrating a method of performing active scanning in accordance with an embodiment of the present invention. 
     Referring to  FIG. 11 , an STA monitors whether or not a probe response frame transmitted by a target AP is received for the first minimum channel time (step S 1100 ). 
     The STA may send a probe request frame including information about the target AP and information related to the first minimum channel time. The target AP and a non-target AP may receive the probe request frame transmitted by the STA. The target AP may preferentially send the probe response frame to the STA within the first minimum channel time, and the non-target AP may send a probe response frame to the STA after the first minimum channel time. 
     The STA may monitor only the probe response frame transmitted by the target AP in the first minimum channel time. Since the target AP preferentially sends the probe response frame and the non-target AP additionally sends the probe response frame in the remaining time interval, a phenomenon in which the probe response frames are crowded and received in a specific time interval can be prevented because the probe response frames can be distributed and received. 
     The STA determines whether or not the probe response frame transmitted by the target AP has been received in the first minimum channel time (step S 1110 ). If, as a result of the determination, the probe response frame has been received from the target AP in the first minimum channel time, the STA processes the probe response frame transmitted by the target AP and performs an authentication and association process (step S 1120 ). 
     The STA may receive the probe response frame from the target AP in the first minimum channel time. In such a case, the STA may perform an authentication and association step along with the target AP right before a maximum channel time expires. If such a method is used, the delay of active scanning, that is, an existing problem generated because a received probe response frame is processed after the probe response frame is received until the maximum channel time, can be reduced. 
     If the probe response frame is transmitted by the target AP, the non-target AP may overhear the probe response frame transmitted by the target AP. If the non-target AP has overheard the probe response frame transmitted by the target AP, the non-target AP may not send the probe response frame even after the first minimum channel time. 
     In an embodiment different from step S 1120 , if the probe response frame transmitted by the target AP is received in the first minimum channel time, the STA may additionally receive a probe response frame transmitted after the first minimum channel time, and may perform an authentication and association procedure. 
     The STA monitors whether or not a probe response frame is transmitted during a minimum channel time (step S 1130 ). 
     The non-target AP may send the probe response frame to the STA even after the first minimum channel time. If the target AP does not send the probe response frame in the first minimum channel time, the STA may monitor the transmission of an additional probe response frame up to the minimum channel time. 
     The STA may monitor whether or not a probe response frame is transmitted during the minimum channel time (step S 1140 ). If the probe response frame is transmitted within the minimum channel time, the STA may perform an authentication and association procedure based on the received probe response frame (step S 1150 ). 
     If the probe response frame is transmitted within the minimum channel time, the STA may immediately perform the authentication and association procedure based on the received probe response frame. In another method, the STA may wait until the maximum channel time without performing an authentication and association procedure. If an additional probe response frame is transmitted within the maximum channel time, the STA may receive the additional probe response frame, may process a probe response frame received after the maximum channel time expires, and may perform an authentication and association procedure. 
     If a probe response frame is not received during the minimum channel time, the STA may change a scanning channel to another channel and perform active scanning (step S 1160 ). 
       FIG. 12  is a block diagram illustrating a wireless apparatus to which an embodiment of the present invention may be applied. 
     The wireless apparatus  1200  is an STA capable of implementing the aforementioned embodiments, and may be an AP or non-AP STA. 
     The wireless apparatus  1200  may include a processor  1220 , memory  1240 , and a Radio Frequency (RF) unit  1060 . 
     The RF unit  1260  is connected to the processor  1220 , and may send/receive radio signals. 
     The processor  1220  implements the functions, processes and/or methods proposed by the present invention. For example, the processor  1220  may be implemented to perform an active scanning method in accordance with an embodiment of the present invention. The processor  1220  may be implemented to determine whether an AP is a target AP or a non-target AP based on an AP identifier included in a received probe request frame. If the AP is a non-target AP, the processor  1220  may be implemented to perform back-off for the transmission of a probe response frame from a second interval after a first interval of a minimum channel interval expires. Furthermore, if the AP is a non-target AP, the processor  1220  may be implemented to overhear whether or not a probe response frame has been transmitted by the target AP during the first interval. If whether or not a probe response frame has been transmitted by the target AP is not overheard during the first interval, the processor  1220  may be implemented to send a probe response frame during the second interval. That is, the processor  1220  may be implemented to perform the aforementioned embodiments of the present invention. 
     The processor  1220  may include Application-Specific Integrated Circuits (ASICs), other chipsets, logic circuits, data processing devices and/or converters for mutually converting baseband signals and radio signals. The memory  1240  may include Read-Only Memory (ROM), Random Access Memory (RAM), flash memory, memory cards, storage media and/or other storage devices. The RF unit  1260  may include one or more antennas for sending and/or receiving radio signals. 
     When an embodiment is implemented in software, the aforementioned scheme may be implemented using a module (process or function) which performs the aforementioned functions. The module may be stored in the memory  1240  and executed by the processor  1220 . The memory  1240  may be present inside or outside the processor  1220 , and may be connected to the processor  1220  using a variety of well-known means.