Abstract:
A method and an apparatus for scanning in a wireless LAN are disclosed. The method for scanning in a wireless LAN comprises the steps of: enabling a station (STA) to receive, from a first access point (AP) operated in a first channel, a detection frame comprising identifier information of a second AP operated in a second channel and beacon frame transmission time information of the second AP, and enabling the STA to scan a beacon frame of the second AP on the basis of the beacon frame transmission time information of the second AP. Thus, the present invention can rapidly perform a scanning operation.

Description:
This application is a 35 USC §371 National Stage entry of International Application No. PCT/KR2013/006179 filed on Jul. 11, 2013, and claims priority to U.S. Provisional Application No. 61/670,583 filed on Jul. 12, 2012, all of which are hereby incorporated by reference in their entireties as if fully set forth herein. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a scanning method and apparatus, and more specifically, to a scanning method and apparatus by an STA. 
     2. Related Art 
     Recent wireless LAN technologies are evolving largely in three ways. Efforts to further increase transmission speed include IEEE (Institute of Electrical and Electronic Engineers) 802.11ac and IEEE 802.11ad as extensions to the existing WLAN evolution. IEEE802.11ad is a wireless LAN technique that employs a 60GH band. Further, broad band wireless LAN utilizing a frequency band of less than 1 GHz is nowadays on the rise to enable transmission in a broader area than by the existing WLAN and such WLAN technologies include IEEE 802.11af utilizing a TVWS (TV White Space) band and IEEE 802.11ah utilizing a 900 MHz band. These standards primarily target expansion of extended range Wi-Fi services as well as smart grid and wide-area sensor networks. Further, the conventional WLAN MAC (Medium Access Control) techniques suffer from the problem that the initial link setup time is significantly increased in some cases. Standardization of IEEE 802.11ai is actively going on to address such issue to thus enable quick access from an STA to an AP. 
     IEEE 802.11ai is oriented towards the MAC technology that copes with quick authentication procedures for significantly saving initial setup and association time of wireless LAN, and its standardization activities were started with a regular task group on January, 2011. In order to enable a quick access procedure, the IEEE 802.11ai task group is discussing the simplification of the procedures in AC discovery, network discovery, TSF (Time Synchronization Function) synchronization, authentication &amp; association, and process merging with higher layers. Among others, the ideas such as procedure merging utilizing piggyback of DHCP (Dynamic Host Configuration Protocol), optimization of the full EAP (Extensible Authentication Protocol) using concurrent IP, and efficient selective AP (Access Point) scanning are actively under discussion. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a scanning method. 
     Another object of the present invention is to provide an apparatus of performing scanning. 
     To achieve the above objects of the present invention, according to an aspect of the present invention, a scanning method in a wireless LAN may comprise receiving by an STA (station), from a first AP (Access Point) operating on a first channel, a discovery frame including identifier information of a second AP operating on a second channel and beacon frame transmission time information of the second AP and scanning by the STA a beacon frame of the second AP based on the beacon frame transmission time information of the second AP, wherein the beacon frame transmission time information of the second AP may include information on a difference between a transmission time of the discovery frame and a transmission time of a beacon frame of the second AP transmitted after the discovery frame is transmitted, and wherein a transmission period of the discovery frame may be shorter than a transmission period of a beacon frame of the first AP. 
     To achieve the above objects of the present invention, according to another aspect of the present invention, an STA (station) of a wireless LAN system may include a processor, wherein the processor may be implemented to receive, from a first AP (Access Point) operating on a first channel, a discovery frame including identifier information of a second AP operating on a second channel and beacon frame transmission time information of the second AP and to scan a beacon frame of the second AP based on the beacon frame transmission time information of the second AP, wherein the beacon frame transmission time information of the second AP may include information on a difference between a transmission time of the discovery frame and a transmission time of a beacon frame of the second AP transmitted after the discovery frame is transmitted, and wherein a transmission period of the discovery frame may be shorter than a transmission period of a beacon frame of the first AP. 
     A scanning procedure may be rapidly performed. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIGS. 1(A) -(B) are concept views illustrating the structure of a wireless local area network (WLAN). 
         FIG. 2  is a view illustrating a layer architecture of a WLAN system supported by IEEE 802.11. 
         FIGS. 3(A) -(B) are concept views illustrating a scanning method in a WLAN. 
         FIG. 4  is a concept view illustrating an authentication and association process after scanning between an AP and an STA. 
         FIG. 5  is a concept view illustrating an active scanning procedure. 
         FIGS. 6(A) -(C) are concept views illustrating a probe request frame transmission method. 
         FIG. 7  is a concept view illustrating a method of performing scanning on a plurality of channels by an STA. 
         FIG. 8  is a concept view illustrating a discovery frame according to an embodiment of the present invention. 
         FIG. 9  is a concept view illustrating a procedure of association to a target AP by an STA according to an embodiment of the present invention. 
         FIG. 10  is a concept view illustrating an operation of an AP according to an embodiment of the present invention. 
         FIG. 11  is a flowchart illustrating an operation of an STA according to an embodiment of the present invention. 
         FIG. 12  is a concept view illustrating an operation mode switching method by an STA according to an embodiment of the present invention. 
         FIG. 13  is a concept view illustrating a scanning method by an STA according to an embodiment of the present invention. 
         FIG. 14  is a concept view illustrating an operation mode switching method by an STA according to an embodiment of the present invention. 
         FIG. 15  is a flowchart illustrating an operation of an STA according to an embodiment of the present invention. 
         FIG. 16  is a block diagram illustrating a wireless device to which an embodiment of the present invention may apply. 
     
    
    
     DESCRIPTION OF EXEMPLARY EMBODIMENTS 
       FIG. 1  is a concept view illustrating the structure of a wireless local area network (WLAN). 
       FIG. 1(A)  shows the structure of the IEEE (institute of electrical and electronic engineers) 802.11 infrastructure network. 
     Referring to  FIG. 1(A) , the WLAN system may include one or more basic service sets (BSSs,  100  and  105 ). The BSSs  100  and  105  are sets of APs (Access Points) and STAs (stations), such as AP  125  and STA 1   100 - 1 , which may successfully sync with each other to perform communication, and the BSSs  100  and  105  do not mean specific areas. The BSS  105  may include one AP  130  and one or more STAs  105 - 1  and  105 - 2  connectible to the AP  130 . 
     The infrastructure BSS may include at least one STA, APs  125  and  130  providing a distribution service, and a distribution system (DS)  110  connecting multiple APs. 
     The distribution system  110  may implement an extended service set (ESS)  140  by connecting a number of BSSs  100  and  105 . The ESS  140  may be used as a term to denote one network configured of one or more APs  125  and  230  connected via the distribution system  110 . The APs included in one ESS  140  may have the same SSID (service set identification). 
     The portal  120  may function as a bridge that performs connection of the WLAN network (IEEE 802.11) with other network (for example, 802.X). 
     In the infrastructure network as shown in  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, without the APs  125  and  130 , a network may be established between the STAs to perform communication. The network that is established between the STAs without the APs  125  and  130  to perform communication is defined as an ad-hoc network or an independent BSS (basic service set). 
       FIG. 1(B)  is a concept view illustrating an independent BSS. 
     Referring to  FIG. 1(B) , an independent BSS (IBSS) is a BSS operating in ad-hoc mode. The IBSS does not include an AP, so that it lacks a centralized management entity. In other words, in the IBSS, the STAs  150 - 1 ,  150 - 2 ,  150 - 3 ,  155 - 1 , and  155 - 2  are managed in a distributed manner. In the IBSS, all of the STAs  150 - 1 ,  150 - 2 ,  150 - 3 ,  155 - 1 , and  155 - 2  may be mobile STAs, and access to the distribution system is not allowed so that the IBSS forms a self-contained network. 
     The STA is some functional medium that includes a medium access control (MAC) following the IEEE (Institute of Electrical and Electronics Engineers) 802.11 standards and that includes a physical layer interface for radio media, and the term “STA” may, in its definition, include both an AP and a non-AP STA (station). 
     The STA may be referred to by various terms such as mobile terminal, wireless device, wireless transmit/receive unit (WTRU), user equipment (UE), mobile station (MS), mobile subscriber unit, or simply referred to as a user. 
       FIG. 2  is a view illustrating a layer architecture of a WLAN system supported by IEEE 802.11. 
       FIG. 2  conceptually illustrates a layer architecture (PHY architecture) of a WLAN system. 
     The WLAN system layer architecture may include an MAC (medium access control) sub-layer  220 , a PLCP (Physical Layer Convergence Procedure) sub-layer  210 , and a PMD (Physical Medium Dependent) sub-layer  200 . The PLCP sub-layer  210  is implemented so that the MAC sub-layer  220  is operated with the minimum dependency upon the PMD sub-layer  200 . The PMD sub-layer  200  may serve as a transmission interface to communicate data between a plurality of STAs. 
     The MAC sub-layer  220 , the PLCP sub-layer  210 , and the PMD sub-layer  200  may conceptually include a management entity. 
     The management entity of the MAC sub-layer  220  is denoted an MLME (MAC layer management entity,  225 ), and the management entity of the physical layer is denoted a PLME (PHY layer management entity,  215 ). Such management entities may offer an interface where a layer management operation is conducted. The PLME  215  is connected with the MLME  225  to be able to perform a management operation on the PLCP sub-layer  210  and the PMD sub-layer  200 , and the MLME  225  is also connected with the PLME  215  to be able to perform a management operation on the MAC sub-layer  220 . 
     There may be an SME (STA management entity,  250 ) to perform a proper MAC layer operation. The SME  250  may be operated as a layer independent component. The MLME, PLME, and SME may communicate information between the mutual components based on primitive. 
     The operation of each sub-layer is briefly described below. The PLCP sub-layer  110  delivers an MPDU (MAC protocol data unit) received from the MAC sub-layer  220  according to an instruction from the MAC layer between the MAC sub-layer  220  and the PMD sub-layer  200  to the PMD sub-layer  200  or delivers a frame from the PMD sub-layer  200  to the MAC sub-layer  220 . The PMD sub-layer  200  is a PLCP sub-layer and the PMD sub-layer  200  may communicate data between a plurality of STAs by way of a radio medium. The MPDU (MAC protocol data unit) delivered from the MAC sub-layer  220  is denoted a PSDU (Physical Service Data Unit) on the side of the PLCP sub-layer  210 . The MPDU is similar to the PSDU, but in case an A-MPDU (aggregated MPDU), which is obtained by aggregating a plurality of MPDUs, has been delivered, each MPDUs may differ from the PSDU. 
     The PLCP sub-layer  210  adds an additional field including information required by the physical layer transceiver while receiving the PSDU from the MAC sub-layer  220  and delivering the same to the PMD sub-layer  200 . At this time, the added field may include a PLCP preamble to the PSDU, a PLCP header, and tail bits necessary to return the convolution encoder to zero state. The PLCP preamble may play a role to allow the receiver to prepare for syncing and antenna diversity before the PSDU is transmitted. The data field may include padding bits to the PSDU, a service field including a bit sequence to initialize the scrambler, and a coded sequence in which a bit sequence added with tail bits has been encoded. In this case, as the encoding scheme, one of BCC (Binary Convolutional Coding) encoding or LDPC (Low Density Parity Check) encoding may be selected depending on the encoding scheme supported by the STA receiving the PPDU. The PLCP header may include a field containing information on the PPDU (PLCP Protocol Data Unit) to be transmitted. 
     The PLCP sub-layer  210  adds the above-described fields to the PSDU to generate the PPDU (PLCP Protocol Data Unit) and transmits the same to a receiving station via the PMD sub-layer  200 , and the receiving station receives the PPDU and obtains information necessary for data restoration from the PLCP preamble and PLCP header to thus restore the same. 
       FIG. 3  is a concept view illustrating a scanning method in a WLAN. 
     Referring to  FIG. 3 , scanning methods may be divided into passive scanning  300  and active scanning  350 . 
     Referring to  FIG. 3(A) , the passive scanning  300  may be performed by a beacon frame  330  that is periodically broadcast from the AP  300 . The AP  300  over the wireless LAN broadcasts a beacon frame  330  to a non-AP STA  340  at every particular period (e.g., every 100 msec). The beacon frame  330  may contain information on a current network. The non-AP STA  340  may perform scanning on the channel with the AP  310  to perform the authentication/association process by obtaining the network information from the beacon frame  330  periodically broadcast. 
     The passive scanning method  300  only receives the beacon frame  330  transmitted from the AP  310  without requiring the non-AP STA  340  to transmit a frame. Accordingly, the passive scanning  300  is advantageous of a reduction in the overall overhead that is created upon data transmission/reception over the network. However, since the scanning is obliged to be passively performed in proportion to the period of the beacon frame  330 , the time taken to perform scanning may be increased. The details of the beacon frame are set forth in IEEE Draft P802.11-REVmb™/D12, November 2011 ‘IEEE Standard for Information Technology Telecommunications and information exchange between systems—Local and metropolitan area networks—Specific requirements Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications (hereinafter, IEEE 802.11)’ Ch. 8.3.3.2 Beacon Frame, as disclosed in November 2011. IEEE 802.11ai may additionally use other format of a beacon frame, and such beacon frame may be referred to as a FILS (fast initial link setup) beacon frame. Further, the measurement pilot frame is a frame containing only some information of the beacon frame, and the measurement pilot frame may be used in the scanning procedure. The measurement pilot frame is set forth in IEEE 802.11 8.5.8.3 measurement pilot format. 
     Referring to  FIG. 3(B) , the active scanning  350  refers to a method in which the non-AP STA  390  leads scanning by transmitting a probe request frame  370  to the AP  360 . 
     The AP  360  may wait a random time after receiving the probe request frame  370  from the non-AP STA  390  to prevent frame collision and may then transmit a probe response frame  380 , which includes network information, to the non-AP STA  390 . The non-AP STA  390  may obtain the network information based on the received probe response frame  380  to stop the scanning process. 
     The active scanning  350  allows the non-AP STA  390  to lead the scanning process, and the active scanning  350  has the advantage of a short scanning time. However, the non-AP STA  390  should transmit the probe request frame  37 , resulting in an increase in the network overhead for frame transmission and reception. The probe request frame  370  is set forth in IEEE 802.11 Ch. 8.3.3.9, and the probe response frame  380  is set forth in IEEE 802.11 Ch. 8.3.3.1. 
     After the scanning is done, the AP and the STA may perform an authentication and association process. 
       FIG. 4  is a concept view illustrating an authentication and association process after scanning between an AP and an STA. 
     Referring to  FIG. 4 , after passive/active scanning, the authentication and association may be conducted with one of the scanned APs. 
     The authentication and association process may be carried out by way of, e.g., 2-way handshaking.  FIG. 4(A)  is a concept view illustrating an authentication and association process after passive scanning, and  FIG. 4(B)  is a concept view illustrating an authentication and association after active scanning. 
     The authentication and association process may be likewise performed by exchanging an authentication request frame  410 /authentication response frame  420  and an association request frame  330 /association response frame  440  between the APs  400  and  450  and the non-AP STAs  405  and  455  regardless of whether the scanning scheme used is active scanning or passive scanning. 
     The authentication process may be conducted by transmitting the authentication request frame  410  from the non-AP STA  405  or  455  to the AP  400  or  450 . In response to the authentication request frame  410 , the authentication response frame  420  may be transmitted from the AP  400  or  450  to the non-AP STA  405  or  455 . The authentication frame format is set forth in IEEE 802.11 Ch. 8.3.3.11. 
     The association process may be conducted by transmitting the association request frame  410  from the non-AP STA  405  or  455  to the AP  400  or  405 . In response to the association request frame  430 , the association response frame  440  may be transmitted from the AP  405  or  455  to the non-AP STA  400  or  450 . The transmitted association request frame  430  contains information regarding the capability of the non-AP STAs  405  and  455 . Based on the information on the capability of the non-AP STA  405  or  455 , the AP  400  or  350  may determine whether the non-AP STA  405  or  355  may be supported. In case such support is possible, the AP  300  or  450  may include in the association response frame  440  whether to accept the association request frame  440  and a reason therefore, and its supportable capability information, and the AP  300  or  450  may send the same to the non-AP STA  405  or  455 . The association frame format is set forth in IEEE 802.11 Chs. 8.3.3.5/8.3.3.6. 
     After the association step is done, normal data transmission and reception is carried out. The association, unless done, is re-conducted based on the reason for which the association is not performed, or association with other AP may be performed. 
       FIG. 5  is a concept view illustrating an active scanning procedure. 
     Referring to  FIG. 5 , the active scanning procedure may be performed in the following steps. 
     (1) It is determined whether the STA  500  is ready to perform the scanning procedure. 
     The STA  500  may wait, e.g., until the probe delay time expires or particular signaling information (for example, PHY-RXSTART.indication primitive) is received, to perform active scanning. 
     The probe delay time is a delay that occurs before the STA  500  sends a probe request frame  510  when performing active scanning. PHY-RXSTART.indication primitive is a signal that is transmitted from the physical (PHY) layer to the local MAC (medium access control) layer. PHY-RXSTART.indication primitive may signal information indicating that the PLCP (physical layer convergence protocol) has received a PPDU (PLCP protocol data unit) including a valid PLCP header to the MAC layer. 
     (2) Basic access is performed. 
     In the 802.11 MAC layer, a number of STAs may share a radio medium using a distributed coordination function (DCF) that is a contention-based function. The DCF may use carrier sense multiple access/collision avoidance (CSMA/CA) as an access protocol to prevent collision between STAs in a backoff scheme. The STA  500  may transmit the probe request frame  510  to the APs  560  and  570  using a basic access method. 
     (3) Information for specifying the APs  560  and  570  included in MLME-SCAN.request primitive (for example, SSID (service set identification) and BSSID (basic service set identification) information) may be included in the probe request frame  510  and may be transmitted. 
     BSSID may have a value corresponding to the MAC address of an AP as an indicator to specify the AP. SSID (Service Set Identification) is a network term to specify an AP, which may be read by a person operating an STA. BSSID and/or SSID may be used to specify an AP. 
     The STA  500  may specify an AP based on information for specifying the APs  560  and  570  included by MLME-SCAN.request primitive. The specified APs  560  and  570  may transmit the probe response frames  550  and  550  to the STA  500 . The STA  500  may include the SSID and BSSID information in the probe request frame  510  and send the same, thereby unicasting, multicasting, or broadcasting the probe request frame  510 . A method of unicasting, multicasting, or broadcasting the probe request frame  510  using the SSID and BSSID information is further described with reference to  FIG. 5 . 
     For example, in case an SSID list is included in MLME-SCAN.request primitive, the STA  500  may include the SSID list in the probe request frame  510  and transmit the same. APs  560  and  570  may receive the probe request frame  510 , determine the SSIDs included in the SSID list contained in the received probe request frame  510 , and determine whether to send the probe response frames  550  and  550  to the STA  200 . 
     (4) A probe timer is initialized as 0 and is then operated. 
     The probe timer may be used to check a minimum channel time (MinChanneltime,  520 ) and a maximum channel time (MaxChanneltime,  530 ). The minimum channel time  520  and the maximum channel time  530  may be used to control the active scanning operation of the STA  500 . 
     The minimum channel time  520  may be used to perform the operation for varying the channel for conducting active scanning. For example, in case the STA  500  fails to receive the probe response frames  550  and  550  until the minimum channel time  520 , the STA  500  shifts scanning channels to perform scanning on other channel. In case the STA  500  receives the probe response frame  550  until the minimum channel time  520 , it may process the received probe response frames  550  and  550  after waiting until the maximum channel time  530 . 
     The STA  500  may detect PHY-CCA.indication primitive until the probe timer reaches the minimum channel time  520  and may determine whether other frame (for example, probe response frames  550  and  550 ) has been received by the STA  500  until before the minimum channel time  520 . 
     PHY-CCA.indication primitive may transmit information on the state of the medium from the physical layer to the MAC layer. PHY-CCA.indication primitive may indicate the current state of the channel using channel state parameters such as “busy” when the channel is unavailable and “idle” when the channel is available. The STA  500  may determine that there are probe response frames  550  and  550  received by the STA  500  when PHY-CCA.indication is detected to be busy and may determine that there are no probe response frames  550  and  550  received by the STA  500  when PHY-CCA.indication is detected to be idle. 
     In case PHY-CCA.indication is detected to be idle, the STA  500  may set an NAV (net allocation vector) to 0, and the STA  500  may scan a next channel. In case PHY-CCA.indication is detected to be busy, the STA  500  may perform a process on the received probe response frames  550  and  550  after the probe timer reaches the maximum channel time  530 . After the process on the received probe response frames  550  and  550  is done, the STA  500  may set the NAV (net allocation vector) to 0 and may then scan a next channel. 
     Hereinafter, in embodiments of the present invention, determining whether there are probe response frames  550  and  550  received by the STA  500  may also mean that the channel state is determined using PHY-CCA.indication primitive. 
     (5) In case all the channels included in the channel list (ChannelList) are scanned, the MLME may signal MLME-SCAN.confirm primitive. MLME-SCAN.confirm primitive may contain BSSDescriptionSet including all the information obtained in the scanning process. 
     In case the STA  500  uses the active scanning method, the STA  500  should perform monitoring to determine whether the parameter of PHY-CCA.indication is busy until the probe timer reaches the minimum channel time. 
     The specific information included in the above-described MLME-SCAN.request primitive is as follows. In order for the STA to perform scanning, the MLME may receive MLME-SCAN.request primitive. MLME-SCAN.request primitive is a primitive created by the SME. MLME-SCAN.request primitive may be used to determine whether there is other BSS to which the STA is to be connected. 
     MLME-SCAN.request primitive may contain information specifically such as BSSType, BSSID, SSID, ScanType, ProbeDelay, ChannelList, MinChannelTime, MaxChannelTime, RequestInformation, SSID List, ChannelUsage, AccessNetworkType, HESSID, MeshID, VendorSpecificlnfo. The details of MLME-SCAN.request primitive are set forth in IEEE Draft P802.11-REVmb™/D12, November 2011 ‘IEEE Standard for Information Technology Telecommunications and information exchange between systems—Local and metropolitan area networks—Specific requirements Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications’ 6.3.3.2 MLME-SCAN.request disclosed on November, 2011. 
     The following Table 1 briefly represents example information included in MLME-SCAN.request primitive. 
     
       
         
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 name 
                 description 
               
               
                   
               
             
             
               
                 BSSType 
                 Determine whether infrastructure BSS, IBSS MBSS(mesh basic 
               
               
                   
                 service set), or all are included in the scan 
               
               
                 BSSID 
                 Identifies a specific or wildcard BSSID 
               
               
                 SSID 
                 Specifies the desired SSID or the wildcard SSID 
               
               
                 ScanType 
                 Indicates either active or passive scanning 
               
               
                 ProbeDelay 
                 Delay(in microseconds) to be used prior to transmitting a probe 
               
               
                   
                 request frame during active scanning 
               
               
                 ChannelList 
                 Specifies a list of channels that are examined when scanning for 
               
               
                   
                 a BSS 
               
               
                 MinChannelTime 
                 The minimum time(in TU) to spend on each channel when 
               
               
                   
                 scanning 
               
               
                 MaxChannelTime 
                 The maximum time(in TU) to spend on each channel when 
               
               
                   
                 scanning 
               
               
                 RequirementInforamtion 
                 This element is optionally present if 
               
               
                   
                 dot11RadioMeasurementActivated is true and is placed in a 
               
               
                   
                 probe request frame to request that the responding STA include 
               
               
                   
                 the requested information in the probe response frame 
               
               
                 SSID List 
                 One or more SSID element that are optionally present when 
               
               
                   
                 dot11MgmtOptionSSIDListActivated is true 
               
               
                 ChannelUsage 
                 Specifies request types for the ChannelUsage request 
               
               
                 AccessNetworkType 
                 Specifies a desired specific access networktype or the wildcard 
               
               
                   
                 access network type. 
               
               
                   
                 This field is present when dot11InterworkingSeviceActivated is 
               
               
                   
                 true 
               
               
                 HESSID 
                 Specifies the desired specific HESSID network identifier or the 
               
               
                   
                 wildcard network identifier. This field is present when 
               
               
                   
                 dot11InterworkingServiceActivated is true 
               
               
                 Mesh ID 
                 Only present if BSS Type = MESH or BSSType = ANY_BSS. 
               
               
                   
                 Specifies the desired Mesh ID or wildcard Mesh ID 
               
               
                 RequestParameters 
                 The parameter define the responding STAs 
               
               
                 ReportingOption 
                 Indicates the result reporting mode 
               
               
                 APConfigurationChangeCount 
                 When a specific BSSID is indicated in the MLME- 
               
               
                   
                 SCAN.request, the APConfigurationChangeCount associated 
               
               
                   
                 with the stored configuration of the AP is optionally provided 
               
               
                 VendorSpecificInfo 
                 Information added depend on the vendor 
               
               
                   
               
             
          
         
       
     
     The request parameter included in MLME-SCAN.request primitive may be used to determine whether the responding STA is to transmit a probe response frame. The request parameter may contain information for requesting that other BSS&#39;s information be included in the probe response frame. Also, the request parameter may include a report request field, a delay reference field, and a maximum delay limit field. 
     The report request field contains information to request that other BSS&#39;s information be included in the probe response frame, the delay reference field contains information on the delay type applied as a response to the probe request frame, and the maximum delay limit field may contain the maximum access delay information on the delay type indicated by the delay reference field. 
     Besides, the request parameter may include a minimum data rate field and/or a received signal strength limit field. The minimum data rate field contains information on the lowest overall data rate in transmitting an MSDU or A-MSDU. The received signal strength limit field may further contain information on the limit value of the signal necessary for a recipient of the probe request frame to respond. 
       FIG. 6  is a concept view illustrating a probe request frame transmission method. 
       FIG. 6  discloses methods of broadcasting, multicasting, and unicasting probe request frames from an STA. 
       FIG. 6(A)  shows a method in which the STA  600  broadcasts the probe request frame  610 . 
     The STA  600  may include a wildcard SSID and a wildcard BSSID in the probe request frame  610  and broadcast the probe request frame  610 . 
     The wildcard SSID and the wildcard BSSID may be used as identifiers to indicate all of the APs  606 - 1 ,  606 - 2 ,  606 - 3 ,  606 - 4 , and  606 - 6  included in the transmission range of the STA  600 . 
     In case the STA  600  transmits the probe request frame  610  with the wildcard SSID and the wildcard BSSID included in the probe request frame  610 , the APs  606 - 1 ,  606 - 2 ,  606 - 3 ,  606 - 4 , and  606 - 6  that have received the probe request frame  610  from the STA  600  may send probe response frames to the STA  600  in response to the received probe request frame. 
     In case the APs  606 - 1 ,  606 - 2 ,  606 - 3 ,  606 - 4 , and  606 - 6  that have received the broadcast probe request frame  610  send the probe response frames to the STA  600  in response to the received probe request frame  610  within a predetermined time, the problem may occur that the STA  600  should simultaneously receive and process too many probe response frames. 
       FIG. 6(B)  shows a method in which the STA  620  unicasts the probe request frame  630 . 
     Referring to  FIG. 6(B) , in case the STA  620  unicasts the probe request frame  630 , the STA  620  may transmit the probe request frame  630  containing particular SSID/BSSID information of the AP. Among the APs that receive the probe request frame  630 , only the AP  626  corresponding to the SSID/BSSID specified by the STA  620  may transmit a probe response frame to the STA  620 . 
       FIG. 6(C)  shows a method in which the STA  640  multicasts the probe request frame  660 . 
     Referring to  FIG. 6(C) , the STA  640  may include an SSID list and a wildcard BSSID in the probe request frame  660  and transmit the same. Among the APs receiving the probe request frame  660 , the APs  660 - 1  and  660 - 2  corresponding to the SSIDs included in the SSID list contained in the probe request frame may transmit a probe response frame to the STA  640 . 
       FIG. 7  is a concept view illustrating a method of performing scanning on a plurality of channels by an STA. 
     It is assumed in  FIG. 7  that an STA performs scanning through a plurality of frequency channels (first channel  710 , second channel  720 , and third channel  730 ). Further, a target AP on which the STA intends to perform initial access is assumed to be on the third channel  730  (for example, seventh AP  770 ). Further, the STA is assumed to be in operation on the first channel  710 . The AP where the STA intends to perform scanning or intends to be associated is defined as a target AP. For example, the target AP may be an AP specified by information (e.g., SSID and/or BSSID information) to specify an AP included in the MLME-SCAN.request primitive. 
     The STA may monitor a frame that is transmitted from the AP operating on the first channel  710 . The first AP, the second AP, and the third AP are APs operating on the first channel  710 . The first AP, second AP, and third AP may transmit a discovery frame through the first channel  710 . The discovery frame is a frame that is transmitted within a transmission period of a beacon frame from each AP, and the discovery frame may be a frame transmitted with a shorter period than the beacon frame. In other words, the discovery frame is a frame that is transmitted with a smaller period than a transmission period of the beacon frame. The discovery frame may contain identifier information (SSID, BSSID) of an AP that transmits the discovery frame. The discovery frame is transmitted before the beacon frame is transmitted to the STA, so that the STA may previously detect that the AP is present on the corresponding channel. An interval at which the discovery frame is transmitted from one AP is referred to as a frame transmission interval  750 . 
     That is, the STA may detect whether an AP that the STA desires to be associated with is present on the current operation channel, based on the discovery frame. 
     The STA may receive the discovery frame  705  transmitted from the first AP on the first channel  710 , and the STA may sequentially receive a discovery frame transmitted from the second AP and a discovery frame transmitted from the third AP. The STA, in case the discovery frame transmission interval  750  of the first AP elapses, may re-receive a discovery frame  707  transmitted from the first AP. The STA may be aware that there is no target AP (seventh AP,  770 ) on the first channel through the discovery frame received during the discovery frame transmission interval. The STA may shift the scanning channel to other channel (second channel,  720 ) in order to discover the seventh AP  770 . 
     The STA may shift the scanning channel from the first channel  710  to the second channel  720 . The STA may receive a discovery frame  723  transmitted from the fourth AP on the second channel  720 . Next, the STA may receive a discovery frame transmitted from the fifth AP and a discovery frame transmitted from the sixth AP. The STA failed to receive a discovery frame from the target AP  770  not only on the first channel but also on the second channel until the passage of the discovery frame transmission interval of the fourth AP that has first transmitted the discovery frame. 
     In such case, the STA may shift the scanning channel to a next channel, the third channel  730 , and the STA may perform scanning. 
     The STA receives the discovery frame  735  transmitted from the eighth AP on the third channel  730 . Subsequently, the STA may receive a discovery frame transmitted from the ninth AP and a discovery frame  775  transmitted from the seventh AP  770 . When receiving the discovery frame  775  from the seventh AP  770  that is the target AP, the STA may recognize that the seventh AP  770 , the target AP, is operating on the third channel  730 . In such case, the STA may monitor a beacon frame transmitted from the seventh AP  770  on the corresponding channel, and the STA may receive the beacon frame to proceed with initial access. 
     In case the existing initial access method is used, the STA may consume, as its scanning time, the time calculated by adding the time of the reception of the beacon frame to the value obtained by multiplying the discovery frame transmission interval by the number of frequency channels on which scanning has been performed substantially until before the STA is associated with the target AP. 
     The scanning method according to an embodiment of the present invention includes an initial access method by the STA, which allows the STA to save the scanning time when the STA performs scanning. 
     In the scanning method according to an embodiment of the present invention, various additional pieces of information may be defined in a discovery frame and the same may be transmitted in order to reduce the scanning time of a terminal. 
       FIG. 8  is a concept view illustrating a discovery frame according to an embodiment of the present invention. 
     Referring to  FIG. 8 , a discovery frame may include discovery frame interval information  800 , beacon frame transmission time information  810 , BSS load information  820 , neighbor AP information  830 , and information  840  on a neighbor AP on other channel. 
     The discovery frame interval information  800 , beacon frame transmission time information  810 , BSS load information  820 , neighbor AP information  830 , and information  840  on a neighbor AP on other channel, each, may contain what is set forth in the following table. 
     
       
         
               
               
             
           
               
                 TABLE 2 
               
               
                   
               
               
                 Information 
                 Details 
               
               
                   
               
             
             
               
                 Discovery frame interval 
                 Transmission period of discovery frame of 
               
               
                   
                 current AP (AP having transmitted discovery 
               
               
                   
                 frame) 
               
               
                 Beacon frame 
                 May indicate transmission time of beacon 
               
               
                 transmission time 
                 frame in offset intervals, US (micro second), 
               
               
                   
                 or TS (symbol time) from discovery frame 
               
               
                   
                 transmission time of AP as transmission time 
               
               
                   
                 of beacon frame first transmitted after 
               
               
                   
                 discovery frame received from current AP 
               
               
                   
                 (AP having transmitted discovery frame) 
               
               
                 BSS load information 
                 Refer to following Table 3 
               
               
                 Neighbor AP information 
                 Information on other AP operating on current 
               
               
                   
                 channel, transmitting discovery frame (refer 
               
               
                   
                 to Table 4) 
               
               
                 Neighbor AP information 
                 Information on other AP operating on other 
               
               
                 of other frequency 
                 channel than current channel where discovery 
               
               
                 channel 
                 frame is transmitted (refer to Table 5) 
               
               
                   
               
             
          
         
       
     
     The BSS load information contained in the discovery frame may include additional information as follows. 
     
       
         
               
             
               
               
             
           
               
                 TABLE 3 
               
             
             
               
                   
               
               
                 BSS load information 
               
             
          
           
               
                 information 
                 description 
               
               
                   
               
               
                 STA count 
                 The STA Count filed is interpreted as an unsigned integer that 
               
               
                   
                 indicates the total number of STAs currently associated with 
               
               
                   
                 this BSS 
               
               
                 BSS average access delay 
                 The BSS Average Access Delay is a scalar indication of the 
               
               
                   
                 relative level of loading at an AP. A low value indicates more 
               
               
                   
                 available more capacity than a higher value 
               
               
                 BSS AC access delay 
                 The BSS AC access dealy is a scalar indication of the average 
               
               
                   
                 access dealy at a QoS AP for services for each of the indicated 
               
               
                   
                 Access Categories 
               
               
                 Available admission capacity 
                 The Available Admission Capacity field is 2 octets long and 
               
               
                   
                 contains an unsigned integer that specifies the remaining 
               
               
                   
                 amount of medium time available via explicit admission control, 
               
               
                   
                 in units of 32us/s/ The field is helpful for roaming STAs to 
               
               
                   
                 select an AP that is likely to accept future admission control 
               
               
                   
                 request. 
               
               
                   
               
             
          
         
       
     
     The neighbor AP information contained in the discovery frame may include additional information as shown in the following Table 4. 
     
       
         
               
             
               
               
             
           
               
                 TABLE 4 
               
             
             
               
                   
               
               
                 Neighbor AP information 
               
             
          
           
               
                 Information 
                 details 
               
               
                   
               
               
                 For (i=0; i&gt;=number 
                   
               
               
                 of APs; i++){ 
               
               
                 Discovery interval 
                 Discovery frame transmission period of 
               
               
                   
                 neighbor AP 
               
               
                 Beacon transmission time 
                 Transmission time of beacon frame first 
               
               
                   
                 transmitted by AP after discovery frame 
               
               
                   
                 transmitted from neighbor AP. 
               
               
                   
                 May indicate beacon frame transmission 
               
               
                   
                 points in offset intervals, US (micro second), 
               
               
                   
                 or TS (symbol time) from discovery frame 
               
               
                   
                 transmission time of neighbor AP 
               
               
                 BSS load information 
                 Refer to Table 3 
               
               
                   
               
             
          
         
       
     
     Referring to Table 4, as the neighbor AP information, the discovery interval for each of the APs operating on the current channel, beacon transmission time, and BSS load information may be included in the discovery frame, and the same may be transmitted. The beacon transmission time interval included in the neighbor AP information as set forth in Table 4 may be information regarding a beacon transmission time that is transmitted from a neighbor AP with respect to the discovery frame time received by the current STA. In other words, the STA may obtain the information on the beacon transmission time transmitted from the neighbor AP with respect to the time of the received discovery frame. 
     The information on the neighbor AP on other frequency channel contained in the discovery frame may include additional information as shown in the following Table 5. 
     
       
         
               
             
               
               
             
           
               
                 TABLE 5 
               
             
             
               
                   
               
               
                 Information on neighbor AP on other channel 
               
             
          
           
               
                 Information 
                 details 
               
               
                   
               
               
                 For (i=0; i&gt;=number of 
                   
               
               
                 frequency channels; i++){ 
               
               
                 For (i=0; i&gt;=number 
                 Discovery frame transmission period of 
               
               
                 of APs; i++){ 
                 neighbor AP 
               
               
                 Discovery interval 
                 Discovery frame transmission period of 
               
               
                   
                 neighbor AP on other frequency channel 
               
               
                 Beacon transmission time 
                 Transmission time of beacon frame first 
               
               
                   
                 transmitted by neighbor AP on other 
               
               
                   
                 frequency channel after discovery frame 
               
               
                   
                 transmitted from neighbor AP on other 
               
               
                   
                 frequency channel. 
               
               
                   
                 May indicate beacon frame transmission 
               
               
                   
                 points in offset intervals, US (micro second), 
               
               
                   
                 or TS (symbol time) from discovery frame 
               
               
                   
                 transmission time of neighbor AP on other 
               
               
                   
                 frequency channel 
               
               
                 BSS load information 
                 Refer to Table 3 
               
               
                   
               
             
          
         
       
     
     Referring to Table 5, as the neighbor AP information, the discovery interval for each of the APs operating on a channel other than the current channel, beacon transmission time, and BSS load information may be included in the discovery frame, and the same may be transmitted. The beacon transmission time interval included in the information on the neighbor AP on other channel as set forth in Table 5 may be information regarding a beacon transmission time that is transmitted from a neighbor AP on other channel with respect to the discovery frame time received by the current STA. In other words, the STA may obtain the information on the beacon transmission time transmitted from the neighbor AP on other channel with respect to the time of the received discovery frame. 
     A scanning method by an STA when the STA receives a discovery frame including information as set forth in Table 2 is now disclosed. 
       FIG. 9  is a concept view illustrating a procedure of association to a target AP by an STA according to an embodiment of the present invention. 
     It is assumed in  FIG. 9  that the STA performs scanning through a plurality of frequency channels (first channel  910 , second channel  920 , and third channel  930 ). Further, a target AP to which the STA intends to gain initial access is assumed to be an AP (e.g., fifth AP) on the third channel. 
     The STA may receive a discovery frame  905  transmitted from the first AP on the first channel. The discovery frame received by the STA may contain the information set forth above in connection with Table 2. The STA may transmit information on other AP operating on the first channel  910  on which the first AP is currently in operation and information on APs operating on the second channel  920  and the third channel  930  based on the neighbor AP information and information on a neighbor AP on other frequency channel which are included in the discovery frame  905  transmitted from the first AP. 
     The AP may obtain information on the channel on which the fifth AP, the target AP to which the STA intends to gain access, is in operation, based on the information on the neighbor AP on other frequency channel included in the received discovery frame. 
     The STA may recognize that the fifth AP is in operation on the third channel  930  based on the discovery frame, and the STA may shift the scanning channel to the third channel  930 . The scanning channel may be previously shifted to the channel on which the target AP operates, or the shift of scanning channel may be also performed based on the information on the time during which the target AP transmits a beacon frame. The STA may obtain the information on the beacon transmission time of the fifth AP through the discovery frame received on the first channel. Since the STA may be aware of the transmission time of the beacon frame  950  transmitted from the target AP, the STA may operate in sleep mode until before the beacon frame  950  is transmitted, thereby saving battery. For example, the STA may switch to sleep mode to perform the operation in case the time during which the beacon frame  950  is transmitted is left by a predetermined time or more. Such operation of the STA is described below in further detail. 
       FIG. 10  is a concept view illustrating an operation of an AP according to an embodiment of the present invention. 
       FIG. 10  discloses a method of sharing information between an AP operating on a current channel and an AP operating on other channel. 
     Referring to  FIG. 10 , it is assumed that the first AP  1000  is an AP operating on a current first channel, and the second AP  1050  is an AP operating on a current second channel. 
     The first AP  1000  and the second AP  1050  may be APs connected to a single extended service set based on a distribution system. In other words, a plurality of APs connected to one another in an extended service set (ESS) may be extended through a distribution system  1030 , and information on each AP may be transmitted and shared with other APs between the plurality of APs based on the distribution system  1030 . For example, the first AP  1000  may transmit its discovery interval, beacon transmission time, and BSS load information to the second AP  1050 . Further, the second AP  1050  may transmit its discovery interval, beacon transmission time, and BSS load information to the first AP  1000 . 
     As another method, an AP may obtain information on other APs based on measurement information transmitted from the STA. For example, the first STA  1020  associated with the first AP  1000  to operate may transmit, to the first AP  1000 , information on a result of scanning the current channel and other channel in response to a request from the first AP  1000 . The information on the result of scanning the other channel, which is transmitted from the first STA  1020  to the AP, may contain information on an AP operating on the current channel and information on an AP operating on other channel. That is, the information on the scanning result may contain BSS load information, beacon frame transmission time, and discovery interval information, etc. of the other APs. The AP may be reported a result of scanning by an STA present in the BSS to obtain the information on the operation of the other APs. 
       FIG. 10  illustrates an exemplary method by which the AP obtains information on APs operating on the current channel and other channel, and the AP may also obtain such information on the APs by other methods. 
       FIG. 11  is a flowchart illustrating an operation of an STA according to an embodiment of the present invention. 
     Referring to  FIG. 11 , the STA receives a first discovery frame (step S 1100 ). 
     When receiving the first discovery frame, the STA determines whether the AP having transmitted the first discovery frame is a target AP (step S 1110 ). 
     The STA may determine whether the AP that has transmitted the current discovery frame is the target AP based on identifier information of the AP included in the first discovery frame. In case the AP that has transmitted the first discovery frame is the target AP, the STA need not shift channels to perform scanning. The STA may obtain information on the beacon frame transmission time of the target AP based on the beacon transmission time information included in the received first discovery frame. 
     The beacon frame is monitored (step S 1130 ). 
     In case the AP that has transmitted the first discovery frame is the target AP, the STA may obtain information on the time during which the target AP transmits the beacon frame through the beacon transmission time included in the discovery frame. The STA may receive a beacon frame transmitted from the target AP based on the information on the time during which the beacon frame is transmitted, thereby performing a passive scanning procedure. 
     In an additional embodiment, the STA may determine whether to switch to sleep mode based on the beacon frame transmission time information as described above. In other words, in case it is determined there is a plenty of time remaining until the target AP transmits a beacon frame based on the beacon frame transmit information, the STA may switch to sleep mode to reduce power consumption of the STA. 
     In case the STA determines that the AP that has transmitted the discovery frame is not the target AP, the STA determines a channel on which the target AP operates (step S 1120 ). 
     In case the STA determines that the AP having transmitted the discovery frame is not the target AP, the STA may obtain the information regarding the channel on which the target AP operates based on the neighbor AP information and information on the neighbor AP on other channel, which are included in the first discovery frame. 
     In case the neighbor AP information contains the information on the target AP, the STA may be aware that the target AP is present on the current channel. In case the target AP is present on the current channel, the STA need not shift the scanning channel. The STA may monitor the beacon frame transmitted from the target AP n the current channel based on the beacon frame transmission time information of the target AP which is included in the discovery frame (step S 1130 ). 
     In case the information on the neighbor AP on other channel contains the target AP information, this indicates that the target AP is not present on the current channel, and thus, the STA shifts the scanning channel to other channel on which the target AP is present. The STA that has shifted the scanning channel to other channel may monitor a beacon frame transmitted from the target AP based on the beacon frame transmission time information contained in the discovery frame (step S 1130 ). 
     The scanning channel may be previously shifted to the channel on which the target AP operates, or the shift of scanning channel may be also performed based on the information on the time during which the target AP transmits a beacon frame. In an additional embodiment, the STA that has shifted the scanning channel to other channel may also determine whether the STA switches to sleep mode based on the time taken until the target AP transmits a beacon frame. By using such method, in case there is a plenty of time left until a beacon frame is transmitted, the STA may switch to sleep mode without unnecessarily operating in active mode, thereby reducing power consumption. 
       FIG. 12  is a concept view illustrating an operation mode switching method by an STA according to an embodiment of the present invention. 
     Referring to  FIG. 12 , the STA may determine whether to switch the operation modes of an AP by setting a predetermined threshold. In case the time taken until the target AP transmits a beacon frame is smaller than the operation mode switching threshold  1220 , the STA might not switch the operation mode from active mode to sleep mode. In contrast, in case the time taken until the target AP transmits a beacon frame is larger than or equal to the operation mode switching threshold, the STA may switch the operation mode from active mode to sleep mode. The STA that has switched to sleep mode may switch back to active mode to operate, at the time when a beacon frame is transmitted afterwards. 
     The STA may switch from active mode to sleep mode after switching the scanning channel, or the STA may switch from active mode to sleep mode before switching the scanning channel and the STA may then switch back to active mode to switch the scanning channel. 
     The beacon frame transmitted from the target AP may be assumed to be a first beacon frame  1200 . The STA may calculate the time until the first beacon frame  1200  is transmitted through the beacon transmission time information of the first beacon frame  1200  included in the received discovery frame. Since the first beacon frame transmission time calculated by the STA is smaller than the operation mode switching threshold  1220 , the STA may remain in active mode to monitor the beacon frame. 
     In contrast, the beacon frame transmitted from the target AP may be assumed to be a second beacon frame  1250 . The STA may calculate the time until the second beacon frame  1250  is transmitted through the beacon transmission time information of the second beacon frame  1250  included in the received discovery frame. Since the second beacon frame transmission time calculated by the STA is larger than the operation mode switching threshold  1220 , the STA may switch from active mode to sleep mode. The STA that has switched to sleep mode may switch back to active mode to operate at the timing when the second beacon frame  1250  is transmitted. 
       FIG. 13  is a concept view illustrating a scanning method by an STA according to an embodiment of the present invention. 
       FIG. 13  is a concept view illustrating a method of receiving a discovery frame by an STA and switching the scanning mode from passive mode to active mode based on information of a target AP included in the discovery frame. Like  FIG. 8 , it is assumed in  FIG. 12  that the STA performs scanning through a plurality of frequency channels (first channel, second channel, and third channel). Further, a target AP to which the STA intends to gain initial access is assumed to be an AP (e.g., fifth AP) on the third channel. 
     Referring to  FIG. 13 , the STA may receive a discovery frame transmitted from the first AP on the first frequency channel. 
     The discovery frame may contain the information set forth above in connection with Table 2. The STA may receive information on an AP operating on the first channel and information on APs operating on the second channel and third channel based on the discovery frame transmitted from the first AP. 
     The AP may obtain information on the channel on which the fifth AP, the target AP to which the STA intends to gain access, is in operation, and the discovery interval, beacon transmission time, and BSS load information of the fifth AP, based on the information on the neighbor AP on other channel included in the received discovery frame. 
     For example, the STA may recognize that the fifth AP is in operation on the third channel through the information on the neighbor AP on other channel included in the discovery frame. The STA may shift the scanning channel to the third channel and the STA may transmit a probe request frame  1300  including the identifier information of the fifth AP. The STA may receive a probe response frame transmitted from the target AP in response to the probe request frame  1300 . 
     The STA may obtain the information on the target AP, shift to the channel on which the target AP operates, and immediately perform active scanning. Or, the STA may also determine which one of active scanning and passive scanning the STA is to perform through an additional determination. 
     For example, the STA may determine whether there is much load of the target AP through the BSS load information of the target AP. In case the load of the target AP is high, it may be difficult to obtain a medium to transmit a probe request frame  1300  that is transmitted to separately perform an active scanning procedure. Accordingly, in case the load of the target AP is high, the STA may perform a scanning procedure by monitoring a beacon frame transmitted from the target AP as described above in connection with  FIG. 8 , without transmitting a probe request frame  1300  for transmitting active scanning. 
     As another example, the STA may also determine whether to switch scanning modes through the information on the time left until a beacon frame is transmitted. For example, in case the time taken until the target AP transmits a beacon frame is larger than a scanning mode switching threshold, the STA may switch the scanning mode to active mode, performing scanning. In contrast, in case the time taken until the target AP transmits a beacon frame is smaller than or equal to the scanning mode switching threshold, the STA may maintain the scanning mode at passive mode, performing scanning. The scanning mode switching threshold may be a threshold for the STA to determine whether to switch the scanning mode from passive mode to active mode based on the beacon frame transmission time information. 
       FIG. 14  is a concept view illustrating an operation mode switching method by an STA according to an embodiment of the present invention. 
     Referring to  FIG. 14 , the STA may set a predetermined threshold and switch from passive mode to active mode to determine whether to perform scanning. 
     In case the time taken until the target AP transmits a beacon frame is smaller than or equal to the scanning mode switching threshold  1420 , the STA may maintain the scanning mode at passive mode. In contrast, in case the time taken until the target AP transmits a beacon frame is currently larger than the scanning mode switching threshold, the STA may switch the scanning mode from passive mode to active mode. 
     The beacon frame transmitted from the target AP may be assumed to be a first beacon frame  1400 . The STA may calculate the time until the first beacon frame  1400  is transmitted through the beacon transmission time information of the first beacon frame  1400  included in the received discovery frame. Since the first beacon frame transmission time calculated by the STA is smaller than the scanning mode switching threshold  1420 , the STA may remain in passive mode to monitor the beacon frame. 
     In contrast, the beacon frame transmitted from the target AP may be assumed to be a second beacon frame  1450 . The STA may calculate the time until the second beacon frame  1450  is transmitted through the beacon transmission time information of the second beacon frame  1450  included in the received discovery frame. Since the second beacon frame transmission time calculated by the STA is larger than the scanning mode switching threshold  1420 , the STA may switch the scanning mode from passive mode to active mode. The STA may transmit a probe request frame  1470  including the identifier information of the target AP to the AP. 
       FIG. 15  is a flowchart illustrating an operation of an STA according to an embodiment of the present invention. 
     Referring to  FIG. 15 , the STA receives a first discovery frame (step S 1500 ). 
     When receiving the first discovery frame, the STA determines whether the AP having transmitted the first discovery frame is a target AP (step S 1510 ). 
     In case the STA determines that the AP that has transmitted the first discovery frame is the target AP, the STA may obtain information on the time during which the target AP transmits the beacon frame based on the beacon transmission time information included in the received first discovery frame. The STA monitors a beacon frame transmitted from the target AP (step S 1520 ). 
     In case the STA determines that the AP that has transmitted the first discovery frame is the target AP, the STA may obtain information on the time during which the target AP transmits the beacon frame through the first discovery frame. The STA may receive the beacon frame transmitted from the target AP to perform a scanning procedure. 
     In an additional embodiment, the STA may determine whether to switch to sleep mode based on the beacon frame transmission time information as described above. In other words, in case it is determined there is a plenty of time remaining until the target AP transmits a beacon frame based on the beacon frame transmit information, the STA may switch to sleep mode to reduce power consumption of the STA. 
     In case the STA determines that the AP that has transmitted the first discovery frame is not the target AP, the STA determines a channel on which the target AP operates (step S 1530 ). 
     In case the STA determines that the AP having transmitted the first discovery frame is not the target AP, the STA may obtain the information regarding the channel on which the target AP operates based on the neighbor AP information and information on the neighbor AP on other channel, which are included in the first discovery frame. 
     The STA transmits a probe request frame to the target AP (step S 1540 ). 
     In case the target AP information is included in the neighbor AP information, this indicates that the target AP is present on the current channel, and the STA thus transmits a probe request frame including an identifier of the target AP on the current channel without the need of shifting the scanning channel. 
     In case the information on the neighbor AP on other channel contains the target AP information, this indicates that the target AP is not present on the current channel, and thus, the STA shifts the scanning channel to other channel on which the target AP is present. The STA that has shifted the scanning channel to other channel transmits a probe request frame including the identifier information of the target AP. 
     That is, unlike  FIG. 11 , the STA may switch the scanning mode from passive mode to active mode at step S 1540  and transmits a probe request frame, thereby terminating the scanning procedure more quickly. 
     In an additional embodiment, as described above, the BSS load information of a discovery frame may be further considered in order to determine whether to switch the scanning mode from passive mode to active mode as in step S 1540 . For example, in case the BSS load is high, the STA may perform operation in passive mode without switching to active mode, and in case the BSS load is low, the STA may switch to active mode to perform operation. 
     Further, in a further additional embodiment, the STA may determine the target AP based on the BSS load information included in the discovery frame. For example, the STA may obtain the BSS load information of an AP present on each channel from the received discovery frame. The STA may obtain, through the received discovery frame, not only the BSS load information of the AP operating on the channel on which the STA is currently in operation but also the BSS load information of the AP operating on a channel other than the channel on which the STA currently operates. 
     The STA may determine the target AP based on the received discovery frame or may re-determine the determined target AP. For example, in case there are a plurality of APs to which the STA may be associated with, the STA may determine an AP to which the STA is to be associated with, based on the BSS load information. In another embodiment, in case the STA determines that the BSS load information of the target AP to which the STA attempted to be associated is high, the STA may change the target AP to other AP and may perform association with the changed AP. 
       FIG. 16  is a block diagram illustrating a wireless device to which an embodiment of the present invention may apply. 
     Referring to  FIG. 16 , the wireless device  1600  may be an STA that may implement the above-described embodiments, and the wireless device  1600  may be an AP or a non-AP STA (station). 
     The wireless device  1600  includes a processor  1620 , a memory  1640 , and an RF (Radio Frequency) unit  1660 . 
     The RF unit  1660  may be connected with the processor  1620  to transmit/receive radio signals. 
     The processor  1620  implements functions, processes, and/or methods as proposed herein. For example, the processor  1620  may be implemented to perform the operation of the above-described wireless device according to an embodiment of the present invention. 
     For example, the processor  1620 , in case the wireless device is an AP, may transmit a discovery frame including information on other AP operating on the channel on which the AP operates and information on an AP operating on other channel. The transmission period of the discovery frame may be shorter than the transmission period of a beacon frame of the AP. The information on the other AP on the current channel and other AP on the other channel may be BSS load information and identifier information of the AP and beacon frame transmission time information calculated with respect to the currently transmitted discovery frame. 
     Further, the processor  1620 , in case the wireless device is an STA, may be implemented to receive, from a first AP operating on a first channel, identifier information and beacon frame transmission time information of a second AP operating on a second channel and to scan a beacon frame of the second AP based on the beacon frame transmission time information of the second AP. 
     The processor  1620  may include an ASIC (Application-Specific Integrated Circuit), other chipset, a logic circuit, a data processing device, and/or a converter that performs conversion between a baseband signal and a radio signal. The memory  1640  may include a ROM (Read-Only Memory), a RAM (Random Access Memory), a flash memory, a memory card, a storage medium, and/or other storage device. The RF unit  1660  may include one or more antennas that transmit and/or receive radio signals. 
     When an embodiment is implemented in software, the above-described schemes may be embodied in modules (processes, or functions, etc.) performing the above-described functions. The modules may be stored in the memory  1540  and may be executed by the processor  1520 . The memory  1540  may be positioned in or outside the processor  1520  and may be connected with the processor  1520  via various well-known means.