Patent Publication Number: US-9906937-B2

Title: Method and apparatus for changing state of nan terminal in wireless communication system

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is the National Stage filing under 35 U.S.C. 371 of International Application No. PCT/KR2014/011023, filed on Nov. 17, 2014, which claims the benefit of U.S. Provisional Application No. 61/904,469, filed on Nov. 15, 2013, 61/906,904, filed on Nov. 21, 2013 and 61/912,551, filed on Dec. 6, 2013, the contents of which are all hereby incorporated by reference herein in their entirety. 
    
    
     TECHNICAL FIELD 
     The present invention relates to a wireless communication system, and more particularly, to a method and apparatus for changing a state of a NAN (neighbor awareness networking) terminal. 
     BACKGROUND ART 
     Recently, various wireless communication technologies have been developed with the advancement of information communication technology. Among the wireless communication technologies, a wireless local area network (WLAN) is the technology capable of accessing the Internet by wireless in a home, a company or a specific service provided area through portable terminal such as a personal digital assistant (PDA), a laptop computer, a portable multimedia player (PMP), etc. based on a radio frequency technology. 
     DISCLOSURE OF THE INVENTION 
     Technical Task 
     The technical task of the present invention is to define a state change/transition of a NAN (neighbor awareness networking) terminal. 
     Technical tasks obtainable from the present invention are non-limited by the above-mentioned technical task. And, other unmentioned technical tasks can be clearly understood from the following description by those having ordinary skill in the technical field to which the present invention pertains. 
     Technical Solutions 
     In a 1 st  technical aspect of the present invention, provided herein is a method of changing a state of a NAN (neighbor awareness networking) terminal in a wireless communication system, including the steps of receiving a synchronization beacon frame from less than three terminals within a discovery window and changing the state based on anchor master information of the synchronization beacon frame, wherein when an RSSI (received signal strength indication) of the synchronization beacon frame is between a first value and a second value and when an anchor master rank value included in the synchronization beacon frame is higher than that stored in the terminal, the terminal changes from a non-synchronization state to a synchronization state. 
     In a 2 nd  technical aspect of the present invention, provided herein is a NAN (neighbor awareness networking) terminal in a wireless communication system, including a receiving module and a processor, wherein the processor is configured to receive a synchronization beacon frame from less than three terminals within a discovery window and to change a state based on anchor master information of the synchronization beacon frame and wherein when an RSSI (received signal strength indication) of the synchronization beacon frame is between a first value and a second value and when an anchor master rank value included in the synchronization beacon frame is higher than that stored in the terminal, the terminal changes from a non-synchronization state to a synchronization state. 
     The following matters may be included in the 1 st  and 2 nd  technical aspects of the present invention. 
     When the RSSI of the synchronization beacon frame is between the first value and the second value and when the anchor master rank value included in the synchronization beacon frame is equal to that stored in the terminal, the terminal may change to the synchronization state only if a hop count value of each of the less than three terminals is lower than a hop count value of the terminal. 
     When the RSSI of the synchronization beacon frame is between the first value and the second value and when the anchor master rank value and a hop count value included in the synchronization beacon frame are equal to those stored in the terminal respectively, the terminal may change to the synchronization state only if a master rank value of each of the less than three terminals is higher than a master rank value of the terminal. 
     The first value and the second value may correspond to an RSSI_middle and an RSSI_close, respectively. 
     The first value may be greater than −60 dBm and the second value may be greater than −75 dBm and less than the first value. 
     The anchor master information may include an anchor master rank, a hop count, and an anchor master beacon transmission time. 
     The state change may be performed at an end of the discovery window. 
     When the terminal enters a cluster in a Sync state, the terminal may omit synchronization beacon frame transmission in a first discovery window. 
     When the terminal enters a cluster in a Sync state, the terminal may transmit a synchronization beacon frame after setting a back-off count value based on a synchronization beacon frame received in a first discovery window. 
     When the terminal enters a cluster from intervals except the discovery window in a Sync state, the terminal may transmit a synchronization beacon frame after setting a back-off count value based on a hop count value included in a discovery beacon frame. 
     When the terminal enters a cluster in a Sync state, the terminal may perform anchor master selection in a first discovery window and transmit a synchronization beacon frame from a second discovery window. 
     The terminal can enter a cluster only in a Non-Master Non-Sync state. 
     Advantageous Effects 
     According to the present invention, a NAN terminal can perform a state change/transition correctly. 
     Effects obtainable from the present invention are non-limited by the above mentioned effect. And, other unmentioned effects can be clearly understood from the following description by those having ordinary skill in the technical field to which the present invention pertains. 
    
    
     
       DESCRIPTION OF DRAWINGS 
       The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and together with the description serve to explain the principles of the invention. 
         FIG. 1  is a diagram illustrating an exemplary structure of IEEE 802.11 system. 
         FIGS. 2 and 3  are diagrams illustrating examples of a NAN cluster. 
         FIG. 4  illustrates an example of a structure of a NAN device (terminal). 
         FIGS. 5 and 6  illustrate relations between NAN components. 
         FIG. 7  is a diagram illustrating a state transition of a NAN device (terminal). 
         FIG. 8  is a diagram illustrating a discovery window and the like. 
         FIG. 9  is a diagram for describing anchor master selection. 
         FIG. 10  is a block diagram illustrating a configuration of a wireless device according to one embodiment of the present invention. 
     
    
    
     BEST MODE FOR INVENTION 
     Reference will now be made in detail to the preferred embodiments of the present invention, examples of which are illustrated in the accompanying drawings. The detailed description, which will be given below with reference to the accompanying drawings, is intended to explain exemplary embodiments of the present invention, rather than to show the only embodiments that can be implemented according to the present invention. The following detailed description includes specific details in order to provide the full understanding of the present invention. However, it will be apparent to those skilled in the art that the present invention may be implemented without such specific details. 
     The following embodiments can be achieved by combinations of structural elements and features of the present invention in prescribed forms. Each of the structural elements or features should be considered selectively unless specified separately. Each of the structural elements or features may be carried out without being combined with other structural elements or features. Also, some structural elements and/or features may be combined with one another to constitute the embodiments of the present invention. The order of operations described in the embodiments of the present invention may be changed. Some structural elements or features of one embodiment may be included in another embodiment, or may be replaced with corresponding structural elements or features of another embodiment. 
     Specific terminologies in the following description are provided to help the understanding of the present invention. And, these specific terminologies may be changed to other formats within the technical scope or spirit of the present invention. 
     Occasionally, to avoid obscuring the concept of the present invention, structures and/or devices known to the public may be skipped or represented as block diagrams centering on the core functions of the structures and/or devices. In addition, the same reference numbers will be used throughout the drawings to refer to the same or like parts in this specification. 
     The embodiments of the present invention can be supported by the disclosed standard documents disclosed for at least one of wireless access systems including IEEE 802 system, 3GPP system, 3GPP LTE system, LTE-A (LTE-Advanced) system and 3GPP2 system. In particular, the steps or parts, which are not explained to clearly reveal the technical idea of the present invention, in the embodiments of the present invention may be supported by the above documents. Moreover, all terminologies disclosed in this document can be supported by the above standard documents. 
     The following embodiments of the present invention can be applied to a variety of wireless access technologies, for example, CDMA (code division multiple access), FDMA (frequency division multiple access), TDMA (time division multiple access), OFDMA (orthogonal frequency division multiple access), SC-FDMA (single carrier frequency division multiple access) and the like. CDMA can be implemented with such a radio technology as UTRA (universal terrestrial radio access), CDMA 2000 and the like. TDMA can be implemented with such a radio technology as GSM/GPRS/EDGE (Global System for Mobile communications)/General Packet Radio Service/Enhanced Data Rates for GSM Evolution). OFDMA can be implemented with such a radio technology as IEEE 802.11 (Wi-Fi), IEEE 802.16 (WiMAX), IEEE 802.20, E-UTRA (Evolved UTRA), etc. For clarity, the following description focuses on IEEE 802.11 systems. However, technical features of the present invention are not limited thereto. 
     Structure of WLAN System 
       FIG. 1  is a diagram illustrating an exemplary structure of IEEE 802.11 system to which the present invention is applicable. 
     IEEE 802.11 structure may include a plurality of components and WLAN supportive of transparent STA mobility for an upper layer can be provided by interactions between the components. A basic service set (BSS) may correspond to a basic component block in IEEE 802.11 WLAN.  FIG. 1  shows one example that two basic service sets BSS  1  and BSS  2  exist and that 2 STAs are included as members of each BSS. In particular, STA  1  and STA  2  are included in the BSS  1  and STA  3  and STA  4  are included in the BSS  2 . In  FIG. 1 , an oval indicating the BSS can be understood as indicating a coverage area in which the STAs included in the corresponding BSS maintain communication. This area may be called a basic service area (BSA). Once the STA moves out of the BSA, it is unable to directly communicate with other STAs within the corresponding BSA. 
     A most basic type of BSS in IEEE 802.11 WLAN is an independent BSS (IBSS). For instance, IBSS can have a minimum configuration including 2 STAs only. Moreover, the BSS (e.g., BSS  1  or BSS  2 ) shown in  FIG. 1 , which has the simplest configuration and in which other components are omitted, may correspond to a representative example of the IBSS. Such a configuration is possible if STAs can directly communicate with each other. Moreover, the above-mentioned WLAN is not configured according to a devised plan but can be configured under the necessity of WLAN. And, this may be called an ad-hoc network. 
     If an STA is turned on/off or enters/escapes from a BSS area, membership of the STA in a BSS can be dynamically changed. In order to obtain the membership of the BSS, the STA can join the BSS using a synchronization procedure. In order to access all services of the BSS based structure, the STA should be associated with the BSS. This association may be dynamically configured or may include a use of a DSS (distribution system service). 
     Additionally,  FIG. 1  shows components such as a DS (distribution system), a DSM (distribution system medium), an AP (access point) and the like. 
     In WLAN, a direct station-to-station distance can be restricted by PHY capability. In some cases, the restriction of the distance may be sufficient enough. However, in some cases, communication between stations located far away from each other may be necessary. In order to support extended coverage, the DS (distribution system) may be configured. 
     The DS means a structure in which BSSs are interconnected with each other. Specifically, the BSS may exist as an extended type of component of a network consisting of a plurality of BSSs instead of an independently existing entity as shown in  FIG. 1 . 
     The DS corresponds to a logical concept and can be specified by a characteristic of the DSM. Regarding this, IEEE 802.11 standard logically distinguishes a wireless medium (WM) from the DSM. Each of the logical media is used for a different purpose and is used as a different component. According to the definition of the IEEE 802.11 standard, the media are not limited to be identical to each other or to be different from each other. Since a plurality of the media are logically different from each other, flexibility of IEEE 802.11 WLAN structure (a DS structure or a different network structure) can be explained. In particular, the IEEE 802.11 WLAN structure can be implemented in various ways and the WLAN structure can be independently specified by a physical characteristic of each implementation case. 
     The DS can support a mobile device in a manner of providing seamless integration of a plurality of BSSs and logical services necessary for handling an address to a destination. 
     The AP enables associated STAs to access the DS through the WM and corresponds to an entity having STA functionality. Data can be transferred between the BSS and the DS through the AP. For instance, as shown in  FIG. 1 , while each of the STA  2  and STA  3  have STA functionality, the STA  2  and STA  3  provide functions of enabling associated STAs (STA  1  and STA  4 ) to access the DS. And, since all APs basically correspond to an STA, all APs correspond to an addressable entity. An address used by the AP for communication in the WM should not be identical to an address used by the AP for communication in the DSM. 
     Data transmitted from one of STAs associated with an AP to an STA address of the AP is always received in an uncontrolled port and the data can be processed by an IEEE 802.1X port access entity. Moreover, if a controlled port is authenticated, transmission data (or frame) can be delivered to a DS. 
     Layer Structure 
     Operations of the STA which operates in a wireless LAN system can be explained in terms of the layer structure. In terms of a device configuration, the layer structure can be implemented by a processor. The STA may have a structure of a plurality of layers. For example, a main layer structure handled in the 802.11 standard document includes a MAC sublayer and a physical (PHY) layer on a data link layer (DLL). The PHY layer may include a physical layer convergence procedure (PLCP) entity, a physical medium dependent (PMD) entity, etc. The MAC sublayer and the PHY layer conceptually include management entities called MAC sublayer management entity (MLME) and physical layer management entity (PLME), respectively. These entities provide a layer management service interface for performing a layer management function. 
     A station management entity (SME) is present within each STA in order to provide an accurate MAC operation. The SME is a layer-independent entity that may be considered as existing in a separate management plane or as being off to the side. Detailed functions of the SME are not specified in this document but it may be generally considered as being responsible for functions of gathering layer-dependent status from the various layer management entities (LMEs), setting values of layer-specific parameters similar to each other. The SME may perform such functions on behalf of general system management entities and may implement a standard management protocol. 
     The aforementioned entities interact with each other in various ways. For example, the entities may interact with each other by exchanging GET/SET primitives. The primitive means a set of elements or parameters related to a specific purpose. XX-GET.request primitive is used for requesting a value of a given MIB attribute (management information based attribute). XX-GET.confirm primitive is used for returning an appropriate MIB attribute value if a status is ‘success’, otherwise it is used for returning an error indication in a status field. XX-SET.request primitive is used to request that an indicated MIB attribute be set to a given value. If this MIB attribute implies a specific action, this requests that the action be performed. And, XX-SET.confirm primitive is used such that, if the status is ‘success’, this confirms that the indicated MIB attribute has been set to the requested value, otherwise it is used to return an error condition in the status field. If this MIB attribute implies a specific action, this confirms that the action has been performed. 
     Moreover, the MLME and the SME may exchange various MLME_GET/SET primitives through an MLME SAP (service access point). Furthermore, various PLME_GET/SET primitives may be exchanged between the PLME and the SME through PLME_SAP and may be exchanged between the MLME and the PLME through an MLME-PLME_SAP. 
     NAN (Neighbor Awareness Network) Topology 
     A NAN network can be constructed with NAN devices (terminals) that use a set of identical NAN parameters (e.g., a time interval between consecutive discovery windows, an interval of a discovery window, a beacon interval, a NAN channel, etc.). A NAN cluster can be formed by NAN terminals and the NAN cluster means a set of NAN terminals that are synchronized on the same discovery window schedule. And, a set of the same NAN parameters is used in the NAN cluster.  FIG. 2  illustrates an example of the NAN cluster. A NAN terminal included in the NAN cluster may directly transmit a multicast/unicast service discovery frame to a different NAN terminal within a range of the discovery window. As shown in  FIG. 3 , at least one NAN master may exist in a NAN cluster and the NAN master may be changed. Moreover, the NAN master may transmit all of a synchronization beacon frame, discovery beacon frame and service discovery frame. 
     NAN Device Architecture 
       FIG. 4  illustrates an example of a structure of a NAN device (terminal). Referring to  FIG. 4 , the NAN terminal is based on a physical layer in 802.11 and its main components correspond to a NAN discovery engine, a NAN MAC (medium access control), and NAN APIs connected to respective applications (e.g., Application 1, Application 2, . . . , Application N). 
       FIGS. 5 and 6  illustrate relations between NAN components. Service requests and responses are processed through the NAN discovery engine, and the NAN beacon frames and the service discovery frames are processed by the NAN MAC. The NAN discovery engine may provide functions of subscribing, publishing, and following-up. The publish/subscribe functions are operated by services/applications through a service interface. If the publish/subscribe commands are executed, instances for the publish/subscribe functions are generated. Each of the instances is driven independently and a plurality of instances can be driven simultaneously in accordance with the implementation. The follow-up function corresponds to means for the services/applications that transceive specific service information. 
     Role and State of NAN Device 
     As mentioned in the foregoing description, a NAN device (terminal) can serve as a NAN master and the NAN master can be changed. In other words, roles and states of the NAN terminal can be shifted in various ways and related examples are illustrated in  FIG. 7 . The roles and states, which the NAN terminal can have, may include a master (hereinafter, the master means a state of master role and sync), a Non-master sync, and a Non-master Non-sync. Transmission availability of the discovery beacon frame and/or the synchronization beacon frame can be determined according to each of the roles and states and it may be set as illustrated in Table 1. 
     
       
         
           
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                 Role and State 
                 Discovery Beacon 
                 Synchronization Beacon 
               
               
                   
               
             
            
               
                 Master 
                 Transmission Possible 
                 Transmission Possible 
               
               
                 Non-Master Sync 
                 Transmission Impossible 
                 Transmission Possible 
               
               
                 Non-Master 
                 Transmission Impossible 
                 Transmission Impossible 
               
               
                 Non-Sync 
               
               
                   
               
            
           
         
       
     
     The state of the NAN terminal can be determined according to a master rank (MR). The master rank indicates the preference of the NAN terminal to serve as the NAN master. In particular, a high master rank means strong preference for the NAN master. The NAN MR can be determined by Master Preference, Random Factor, Device MAC address, and the like according to Formula 1.
 
MasterRank=MasterPreference*2 56 +RandomFactor*2 48 −MAC[5]*2 40 + . . . +MAC[0]  [Formula 1]
 
     In Formula 1, the Master Preference, Random Factor, Device MAC address may be indicated through a master indication attribute. The master indication attributes may be set as illustrated in Table 2. 
     
       
         
           
               
               
               
               
             
               
                 TABLE 2 
               
               
                   
               
               
                 Field Name 
                 Size (Octets) 
                 Value 
                 Description 
               
               
                   
               
             
            
               
                 Attribute ID 
                 1 
                 0x00 
                 Identifies the type of 
               
               
                   
                   
                   
                 NAN attribute. 
               
               
                 Length 
                 2 
                 2 
                 Length of the 
               
               
                   
                   
                   
                 following field in the 
               
               
                   
                   
                   
                 attribute 
               
               
                 Master Preference 
                 1 
                 0-255 
                 Information that is 
               
               
                   
                   
                   
                 used to indicate a 
               
               
                   
                   
                   
                 NAN Device&#39;s 
               
               
                   
                   
                   
                 preference to serve 
               
               
                   
                   
                   
                 as the role of Master, 
               
               
                   
                   
                   
                 with a larger value 
               
               
                   
                   
                   
                 indicating a higher 
               
               
                   
                   
                   
                 preference. 
               
               
                 Random Factor 
                 1 
                 0-255 
                 A random number 
               
               
                   
                   
                   
                 selected by the 
               
               
                   
                   
                   
                 sending NAN 
               
               
                   
                   
                   
                 Device. 
               
               
                   
               
            
           
         
       
     
     Regarding the above MR, in case of a NAN terminal that activates a NAN service and initiates a NAN cluster, each of the Master Preference and the Random Factor is set to 0 and NANWarmUp is reset. The NAN terminal should set a Master Preference field value in the master indication attribute to a value greater than 0 and a Random Factor value in the master indication attribute to a new value until when the NANWarmUp expires. When a NAN terminal joins a NAN cluster in which the Master Preference of an anchor master is set to a value greater than 0, the corresponding NAN terminal may set the Master Preference to a value greater than 0 and the Random Factor to a new value irrespective of expiration of the NANWarmUp. 
     Moreover, a NAN terminal can become an anchor master of a NAN cluster depending on an MR value. That is, all NAN terminals have capabilities of operating as the anchor master. The anchor master means the device that has a highest MR and a smallest AMBTT (anchor master beacon transmit time) value and has a hop count (HC) (to the anchor master) set to 0 in the NAN cluster. In the NAN cluster, two anchor masters may exist temporarily but a single anchor master is a principle of the NAN cluster. If a NAN terminal becomes an anchor master of a currently existing NAN cluster, the NAN terminal adopts TSF used in the currently existing NAN cluster without any change. 
     The NAN terminal can become the anchor master in the following cases: if a new NAN cluster is initiated; if the master rank is changed (e.g., if an MR value of a different NAN terminal is changed or if an MR value of the anchor master is changed); or if a beacon frame of the current anchor master is not received any more. In addition, if the MR value of the different NAN terminal is changed or if the MR value of the anchor master is changed, the NAN terminal may lose the status of the anchor master. The anchor master can be determined according to an anchor master selection algorithm in the following description. In particular, the anchor master selection algorithm is the algorithm for determining which NAN terminal becomes the anchor master of the NAN cluster. And, when each NAN terminal joins the NAN cluster, the anchor master selection algorithm is driven. 
     If a NAN terminal initiates a new NAN cluster, the NAN terminal becomes the anchor master of the new NAN cluster. If a NAN synchronization beacon frame has a hop count in excess of a threshold, the NAN synchronization beacon frame is not used by NAN terminals. And, other NAN synchronization beacon frames except the above-mentioned NAN synchronization beacon frame are used to determine the anchor master of the new NAN cluster. 
     If receiving the NAN synchronization beacon frame having the hop count equal to or less than the threshold, the NAN terminal compares an anchor master rank value in the beacon frame with a stored anchor master rank value. If the stored anchor master rank value is greater than the anchor master value in the beacon frame, the NAN terminal discards the anchor master value in the beacon frame. If the stored anchor master value is less than the anchor master value in the beacon frame, the NAN terminal newly stores values greater by 1 than the anchor master rank and the hop count included in the beacon frame and an AMBTT value in the beacon frame. If the stored anchor master rank value is equal to the anchor master value in the beacon frame, the NAN terminal compares hop counters. Then, if a hop count value in the beacon frame is greater than a stored value, the NAN terminal discards the received beacon frame. If the hop count value in the beacon frame is equal to (the stored value—1) and if an AMBTT value is greater than the stored value, the NAN terminal newly stores the AMBTT value in the beacon frame. If the hop count value in the beacon frame is less than (the stored value—1), the NAN terminal increases the hop count value in the beacon frame by 1. The stored AMBTT value is updated according to the following rules. If the received beacon frame is transmitted by the anchor master, the AMBTT value is set to the lowest four octets of time stamp included in the received beacon frame. If the received beacon frame is transmitted from a NAN master or non-master sync device, the AMBTT value is set to a value included in a NAN cluster attribute in the received beacon frame. 
     Meanwhile, a TSF timer of a NAN terminal exceeds the stored AMBTT value by more than 16*512 TUs (e.g., 16 DW periods), the NAN terminal may assume itself as an anchor master and then update an anchor master record. In addition, if any of MR related components (e.g., Master Preference, Random Factor, MAC Address, etc.) is changed, a NAN terminal not corresponding to the anchor master compares the changed MR with a stored value. If the changed MR of the NAN terminal is greater than the stored value, the corresponding NAN terminal may assume itself as the anchor master and then update the anchor master record. 
     Moreover, a NAN terminal may set anchor master fields of the cluster attributes in the NAN synchronization and discovery beacon frames to values in the anchor master record, except that the anchor master sets the AMBTT value to a TSF value of corresponding beacon transmission. The NAN terminal, which transmits the NAN synchronization beacon frame or the discovery beacon frame, may be confirmed that the TSF in the beacon frame is derived from the same anchor master included in the cluster attribute. 
     Moreover, a NAN terminal may adopt a TSF timer value in a NAN beacon received with the same cluster ID in the following case: i) if the NAN beacon indicates an anchor master rank higher than a value in an anchor master record of the NAN terminal; or ii) if the NAN beacon indicates an anchor master rank equal to the value in the anchor master record of the NAN terminal and if a hop count value and an AMBTT value in the NAN beacon frame are larger values in the anchor master record. 
     NAN Synchronization 
     NAN terminals (devices) participating in the same NAN Cluster may be synchronized with respect to a common clock. A TSF in the NAN cluster can be implemented through a distributed algorithm that should be performed by all the NAN terminals. Each of the NAN terminals participating in the NAN cluster may transmit NAN synchronization beacon frame (NAN sync beacon frame) according to the above-described algorithm. The NAN device may synchronize its clock during a discovery window (DW). A length of the DW corresponds to 16 TUs. During the DW, one or more NAN terminals may transmit synchronization beacon frames in order to help all NAN terminals in the NAN cluster synchronize their own clocks. 
     NAN beacon transmission is distributed. A NAN beacon frame is transmitted during a DW period existing at every 512 TU. All NAN terminals can participate in generation and transmission of the NAN beacon according to their roles and states. Each of the NAN terminals should maintain its own TSF timer used for NAN beacon period timing. A NAN synchronization beacon interval can be established by the NAN terminal that generates the NAN cluster. A series of TBTTs are defined so that the DW periods in which synchronization beacon frames can be transmitted are assigned exactly 512 TUs apart. Time zero is defined as a first TBTT and the discovery window starts at each TBTT. 
     Each NAN terminal serving as a NAN master transmits a NAN discovery beacon frame from out of a NAN discovery window. On average, the NAN terminal serving as the NAN master transmits the NAN discovery beacon frame every 100 TUs. A time interval between consecutive NAN discovery beacon frames is smaller than 200 TUs. If a scheduled transmission time overlaps with a NAN discovery window of the NAN cluster in which the corresponding NAN terminal participates, the NAN terminal serving as the NAN master is able to omit transmission of the NAN discovery beacon frame. In order to minimize power required to transmit the NAN discovery beacon frame, the NAN terminal serving as the NAN master may use AC_VO (WMM Access Category—Voice) contention setting.  FIG. 8  illustrates relations between a discovery window and a NAN discovery beacon frame and transmission of NAN synchronization/discovery beacon frames. Particularly,  FIG. 8( a )  shows transmission of NAN discovery and synchronization beacon frames of a NAN terminal operating in 2.4 GHz band.  FIG. 8( b )  shows transmission of NAN discovery and synchronization beacon frames of a NAN terminal operating in 2.4 GHz and 5 GHz bands. 
     In the following description, explained are state changes/transitions of a NAN terminal and methods for a NAN terminal to create/enter a cluster according to the embodiments of the present invention. 
     State Change/Transition of NAN Terminal 
     As briefly described above with reference to  FIG. 7 , a NAN terminal may have states of Master, Non-Master Sync, Non-Master Non-Sync and the like. The transition from the Non-Master Sync state to the Non-Master Non-Sync state can be performed according to the following description only. 
     When a NAN terminal changes its state based on anchor master information in synchronization beacon frame(s) after receiving the synchronization beacon frame from less than three terminals within a discovery window, if RSSI (received signal strength indication) of the synchronization beacon frame is between a first value and a second value and if an anchor master rank value contained in the synchronization beacon frame is higher than that stored in the NAN terminal, the NAN terminal can changes its state from Non-Sync to Sync. In this case, if the RSSI of the synchronization beacon frame is between the first value and the second value and if the anchor master rank value contained in the synchronization beacon frame is equal to that stored in the NAN terminal, the NAN terminal can change to the Sync state only when a hop count value of each of the less than three terminals is lower than that of the NAN terminal. Here, the first and second values correspond to RSSI_middle and RSSI_close, respectively. The first value may be greater than −60 dBm and the second value may be greater than −75 dBm and less than the first value. 
     The aforementioned transition from the Non-Master Sync state to the Non-Master Non-Sync state can be performed in the following cases. 
     At the end of a DW (discovery window), a NAN terminal in a Non-Master role should change its state from Non-Sync to Sync if all of the following conditions are met. 
     First of all, the NAN terminal does not receive a synchronization beacon frame with the RSSI higher than RSSI_close (&gt;−60 dBm, the second value) from a NAN terminal in the same NAN cluster, an anchor master rank field value of the synchronization beacon frame is equal to a value stored in the NAN terminal, and a hop count field value of the terminal that transmits the synchronization beacon frame is lower than a hop count value of the NAN terminal. Alternatively, the NAN terminal does not receive a synchronization beacon frame with the RSSI higher than RSSI_close (&gt;−60 dBm, the second value) from a NAN terminal in the same NAN cluster, an anchor master rank field value of the synchronization beacon frame is equal to the value stored in the NAN terminal, hop count field values are equal to each other, and a master rank value of the terminal that transmits the synchronization beacon frame is higher than a master rank of the NAN terminal. 
     Secondly, the NAN terminal does not receive a synchronization beacon frame with the RSSI higher than RSSI_close (&gt;−60 dBm) (the second value) from a NAN terminal in the same NAN cluster and an anchor master rank field value of the synchronization beacon frame is higher than the value stored in the NAN terminal. Alternatively, the NAN terminal does not receive a synchronization beacon frame with the RSSI higher than RSSI_close (&gt;−60 dBm) (the second value) from a NAN terminal in the same NAN cluster and a master rank of the terminal that transmits the synchronization beacon is higher than that of the NAN terminal. 
     Thirdly, the NAN terminal receives synchronization beacon frame(s) with the RSSI higher than RSSI_middle (&gt;−75 dBm, the first value) from less than three NAN terminals within the NAN cluster, an anchor master rank of the synchronization beacon frame is equal to that stored in the NAN terminal, and a hop count field value of the terminal that transmits the synchronization beacon frame is lower than the hop count value of the NAN terminal. Alternatively, the NAN terminal receives synchronization beacon frame(s) with the RSSI higher than RSSI_middle (&gt;−75 dBm, the first value) from less than three NAN terminals within the NAN cluster, an anchor master rank of the synchronization beacon frame is equal to that stored in the NAN terminal, hop count field value are equal to each other, and a master rank value of the terminal that transmits the synchronization beacon is higher than the master rank of the NAN terminal 
     Finally, the NAN terminal receives synchronization beacon frame(s) with the RSSI higher than RSSI_middle (&gt;−75 dBm, the first value) from less than three NAN terminals within the NAN cluster and an anchor master rank field value of the synchronization beacon frame is higher than the value stored in the NAN terminal. Alternatively, the NAN terminal receives synchronization beacon frame(s) with the RSSI higher than RSSI_middle (&gt;−75 dBm, the first value) from less than three NAN terminals within the NAN cluster and a master rank of the terminal that transmits the synchronization beacon frame is higher than that of the NAN terminal. 
     Next, the transition from the Non-Master Sync state to the Non-Master Non-Sync state may occur if one of the following four conditions is met. 
     First of all, a NAN device (terminal) receives a synchronization beacon frame with the RSSI higher than RSSI_close from a NAN device (terminal) within the same NAN cluster, an anchor master rank value of the synchronization beacon frame is equal to that stored in the NAN device, and a hop count value of the device that transmits the synchronization beacon is lower than that of the NAN device. Alternatively, the NAN device (terminal) receives a synchronization beacon frame with the RSSI higher than RSSI_close from a NAN device (terminal) within the same NAN cluster, an anchor master rank value of the synchronization beacon frame is equal to that stored in the NAN device, hop counts are equal to each other, and a master rank value of the device that transmits the synchronization beacon is higher than a master rank of the NAN device. 
     Secondly, the NAN device receives synchronization beacon frames each of having the RSSI higher than RSSI_middle from three or more NAN devices within the same NAN cluster, an anchor master rank value of the synchronization beacon frame is equal to that stored in the NAN device, and a hop count value of each device that transmits the synchronization beacon frame is lower than that of the NAN device. Alternatively, the NAN device receives synchronization beacon frames each of having the RSSI higher than RSSI_middle from three or more NAN devices within the same NAN cluster, an anchor master rank value of the synchronization beacon frame is equal to that stored in the NAN device, hop counts are equal to each other, and a master rank value of each device that transmits the synchronization beacon is higher than that of the NAN device. 
     Thirdly, the NAN device receives a synchronization beacon frame with the RSSI higher than RSSI_close from a NAN terminal within the same NAN cluster and an anchor master rank value of the synchronization beacon frame is higher than that stored in the NAN device. Alternatively, the NAN device receives a synchronization beacon frame with the RSSI higher than RSSI_close from a NAN terminal within the same NAN cluster and a master rank value of the synchronization beacon frame is higher than that of the NAN device. 
     Finally, the NAN device receives synchronization beacon frames each of having the RSSI higher than RSSI_middle from three or more NAN devices within the same NAN cluster and an anchor master rank value of the synchronization beacon frame is higher than that stored in the NAN device. Alternatively, the NAN device receives a synchronization beacon frame with the RSSI higher than RSSI_close from a NAN terminal within the same NAN cluster and a master rank value of the synchronization beacon frame is higher than that of the NAN device. 
     Methods for NAN Terminal to Create/Enter Cluster 
     When a NAN device intends to start a cluster or to join a previously created cluster, the NAN device initially set its role and state to Master and Sync. If a NAN device is in the Sync state (e.g., Master or Non-Master Sync state), the NAN device transmits a synchronization beacon frame through transmission and reception of synchronization beacon frames. To this end, a back-off count is required and the value of the back-off count is determined based on a HC (Hop Count) value to the anchor master in a random manner. However, when a NAN device joins the cluster as a master, the NAN device is unable to transmit the synchronization beacon frame since there is no hop count value in a first discovery window (where transmission of the synchronization beacon frame is expected (or should be performed). Thus, one of the following methods may be adopted in order for the NAN device to operate correctly. 
     During the first discovery window interval, the NAN Device does not transmit the synchronization beacon frame but can perform anchor master selection and a master/anchor master selection procedure. 
     Alternatively, during the first discovery window interval, the NAN device receives one or more synchronization beacon frames and sets a back-off count value based on the received synchronization beacon frames. Thereafter, the NAN device can transmit its own synchronization beacon frame. 
     Alternatively, if starting a related procedure before the first discovery window interval, the NAN receives one or more discovery beacon frames, obtains a hop count value from information of a cluster attribute in the received discovery beacon frames, sets a back-off count value based on a value of (hop count+1) (or the hop count value), and then transmits the synchronization beacon frame in the discovery window interval. When joining the existing NAN cluster, the NAN device can transmit the synchronization beacon frame within the discovery window after updating current anchor master information (e.g., anchor master rank, hop count to anchor master, anchor master beacon transmission time (AMBTT)) based on the information of the cluster attribute obtained from the discovery beacon frames. 
     Alternatively, during the first discovery window interval, the NAN device performs anchor master selection and then updates a current anchor master record. The NAN device can transmit the synchronization beacon frame in a next discovery window. 
     Alternatively, when joining the existing NAN cluster, the NAN device starts in the Non-Master Non-Sync state instead of the Sync state. During the discovery window, the NAN device does not transmit the synchronization beacon frame but the NAN device can perform master/anchor master selection and transmit a service discovery frame. 
     Alternatively, when receiving the synchronization beacon frame only and joining the existing cluster, the NAN device updates its current anchor master record during one or more discovery window intervals. In doing so, the NAN device can transmit the synchronization beacon frame in a next discovery window. 
     Alternatively, after setting anchor master information to default values, the NAN device can enter the discovery window in the Master state. 
     Alternatively, before joining the existing cluster, the NAN device has WarmUpTime (i.e., prescribed time interval). In particular, the NAN device can update anchor master information during this time interval. 
     Meanwhile, if a terminal receives a synchronization beacon during a DW interval, the terminal performs anchor master selection and then performs master selection and state transition. In a series of the processes, if anchor master information stored in the terminal needs to be updated during the anchor master selection, the update is set to be launched before the master selection and state transition. However, if the terminal operates as described above, it may cause an undesired operation in the course of the transition from the Non-Master Sync state to the Non-Master Non-Sync state. Thus, during the above state transition, the terminal is configured to operate by comparing stored anchor master information with anchor master information obtained from the synchronization beacon. However, if the stored anchor master information is updated during the anchor master selection, it may cause a problem to the above-mentioned operation. 
     Therefore, in case of a NAN terminal, if receiving one synchronization beacon, the NAN terminal performs update after completing the master selection and state transition. This may be interpreted as that if a NAN terminal needs to perform update during the anchor master selection, the NAN terminal first stores corresponding elements in temporary space and then perform the update after completing the master selection and state transition. 
     On the other hand, in case that a stored anchor master rank and hop count are equal to an anchor master rank and hop count included in a synchronization beacon frame, related processing is not clearly defined in the conventional anchor master selection. Regarding this issue, referring to  FIG. 9 , when a discovery window is changed from discovery window 1 (i.e., DW1 in  FIG. 9 ( a ) ) to discovery window 2 (i.e., DW2 in  FIG. 9  (b 1 ) or  FIG. 9  (b 2 )), a master rank of NAN terminal 2 next to an anchor master is changed, whereby the NAN terminal 2 updates stored anchor master information (e.g., anchor master rank, hop count, AMBTT, etc.) with its own values and then assumes itself as the anchor master. In this case, a NAN terminal that becomes the anchor master may set an AMBTT value not only to 0x00000000 as shown in  FIG. 9  (b 2 ) but also to a value of a current TSF Timer as shown in  FIG. 9  (b 1 ). 
     Moreover, if the stored anchor master rank value is equal to an anchor master value of a beacon frame, a NAN terminal compares hop counts. In this case, if a hop count value of the beacon frame is higher than the stored value, the NAN terminal may discard the hop count value of the beacon frame. 
     If the stored AMR (anchor master rank) value is equal to that of the received synchronization beacon frame and if the stored hop count value is equal to that of the received synchronization beacon frame, 
     As shown in  FIG. 9  (b 1 ), when the hop count value is 0, if a stored AMBTT value is lower than an AMBTT value of the received synchronization beacon frame, the AMBTT value is updated with the received AMBTT value. Alternatively, as shown in  FIG. 9  (b 2 ), when the hop count value is 0, if the stored AMBTT value is lower than the AMBTT value of the received synchronization beacon frame, it is discarded. On the contrary, if the stored AMBTT value is greater than the AMBTT value of the received synchronization beacon frame, the AMBTT value is updated with the received AMBTT value. If a NAN terminal becomes an anchor master, the NAN terminal sets the stored AMBTT value to 0x00000000 as shown in  FIG. 9  (b 2 ). Thus, when the AMBTT value in the received synchronization beacon frame is 0x0000000, the NAN terminal updates its stored AMBTT value with 0x0000000. 
     If two adjacent NAN terminals have the same AMR, hop count value, and AMBTT in the same DW, the two NAN terminals compare time stamp values of the synchronization beacon frame and the perform update with a larger value. In addition, it may be considered that the AMBTT value is set to the value of the current TSF Timer. Alternatively, if a NAN terminal becomes the anchor master, the NAN terminal resets the TSF Timer to 0x00000000. Thereafter, the NAN terminal sets the AMBTT value to an actual value of the TSF Timer at which transmission of an actual synchronization beacon frame is expected (or performed) after the reset. 
       FIG. 10  is a block diagram illustrating a configuration of a wireless device according to one embodiment of the present invention. 
     Referring to  FIG. 10 , a wireless device  10  may include a processor  11 , a memory  12 , and a transceiver  13 . The transceiver  13  can transmit/receive radio signals and implement a physical layer according to, for example, IEEE 802 system. The processor  11  is connected to the transceiver  13  electrically and can then implement the physical layer and/or a MAC layer according to the IEEE 802 system. Moreover, the processor  11  may be configured to perform at least one operation of the application, the service and the ASP layer according to the various embodiments of the present invention mentioned in the foregoing description. Alternatively, the processor  11  may be configured to perform operations related to a device operating as an AP/STA. Moreover, a module for implementing the operations of the wireless device according to the various embodiments of the present invention mentioned in the foregoing description may be saved in the memory  12  and then driven by the processor  11 . The memory  12  may be included inside the processor  11  or be provided outside the processor  11 . And, the memory  12  can be connected to the processor  11  through known means. 
     The detailed configuration of the wireless device  10  in  FIG. 10  may be implemented such that each of the various embodiments of the present invention described above is applied independently or at least two thereof are simultaneously applied. And, redundant description shall be omitted for clarity. 
     The embodiments of the present invention mentioned in the foregoing description can be implemented using various means. For instance, the embodiments of the present invention can be implemented using hardware, firmware, software and/or any combinations thereof. 
     In case of the implementation by hardware, a method according to the embodiments of the present invention can be implemented by at least one selected from the group consisting of ASICs (application specific integrated circuits), DSPs (digital signal processors), DSPDs (digital signal processing devices), PLDs (programmable logic devices), FPGAs (field programmable gate arrays), processor, controller, microcontroller, microprocessor and the like. 
     In case of the implementation by firmware or software, a method according to the embodiments of the present invention can be implemented by modules, procedures, and/or functions for performing the above-explained functions or operations. Software code is stored in the memory unit and can be driven by the processor. The memory unit is provided within or outside the processor to exchange data with the processor through the various means known to the public 
     As mentioned in the foregoing description, the detailed descriptions for the preferred embodiments of the present invention are provided to enable those skilled in the art to implement and practice the invention. While the present invention has been described herein with reference to the preferred embodiments thereof, it will be apparent to those skilled in the art that various modifications and variations can be made therein without departing from the spirit and scope of the invention. Therefore, the present invention is not limited to the embodiments disclosed herein but intends to give a broadest scope that matches the principles and new features disclosed herein. 
     INDUSTRIAL APPLICABILITY 
     Although the various embodiments of the present invention have been described above mainly with reference to an IEEE 802.11 system, the present invention can be applied to various mobile communication systems in the same manner.