Patent Publication Number: US-9888439-B2

Title: Method for communication based on identifying information assignment and apparatus for the same

Description:
This application is a Continuation of U.S. application Ser. No. 13/980,012 filed Aug. 13, 2013, which is a National Stage under 35 U.S.C. 371 of International Application No. PCT/KR2012/000353 filed Jan. 16, 2012, which claims the benefit of U.S. Provisional Application Nos. 61/433,250 filed Jan. 16, 2011; 61/447,706 filed Mar. 1, 2011; 61/448,207 filed Mar. 2, 2011 and 61/556,186 filed Nov. 5, 2011, all of which are hereby incorporated by reference in their entirety. 
    
    
     TECHNICAL FIELD 
     The present invention relates to a wireless local area network (WLAN) system, and more particularly, to a communication method of a station (STA) based on identifying information assignment in the WLAN system and an apparatus supporting the method. 
     BACKGROUND ART 
     With the advancement of information communication technologies, various wireless communication technologies have recently been developed. Among the wireless communication technologies, a wireless local area network (WLAN) is a technology whereby Internet access is possible in a wireless fashion in homes or businesses or in a region providing a specific service by using a portable terminal such as a personal digital assistant (PDA), a laptop computer, a portable multimedia player (PMP), etc. 
     The IEEE 802.11n is a technical standard relatively recently introduced to overcome a limited data rate which has been considered as a drawback in the WLAN. The IEEE 802.11n is devised to increase network speed and reliability and to extend an operational distance of a wireless network. More specifically, the IEEE 802.11n supports a high throughput (HT), i.e., a data processing rate of up to above 540 Mbps, and is based on a multiple input and multiple output (MIMO) technique which uses multiple antennas in both a transmitter and a receiver to minimize a transmission error and to optimize a data rate. 
     The WLAN system supports an active mode and a power save mode as an operation mode of a station (STA). The active mode implies an operation mode in which the STA operates in an awake state capable of transmitting and receiving a frame. On the other hand, the power save mode is supported for power saving of an STA which does not require the active state to receive the frame. An STA supporting the power save mode (PSM) can avoid unnecessary power consumption by operating in a doze mode when it is not a time duration in which the STA can access to its radio medium. That is, the STA operates in the awake state only for a time duration in which a frame can be transmitted to the STA or a time duration in which the STA can transmit the frame. 
     In the WLAN system, an access point (AP) manages traffic to be transmitted to STAs that operate in the power save mode. A method is required in which, if buffered traffic to be transmitted to a specific STA exists, the AP reports the existence of the buffered traffic to the STA and transmits a frame. Further, a method is required in which the STA determines whether there is buffered traffic for the STA when the STA operates in the doze state, and if there is buffered traffic for the STA, the STA transitions to the awake state to be able to normally receive the frame. 
     As such, transmitting of a frame for buffered traffic to an STA operating in the power save mode can be performed based on information capable of identifying the STA. Meanwhile, in a WLAN environment in which the same STA identifying information can be assigned to a plurality of STAs, a method can be required in which communication is performed by assigning new STA identifying information or by re-defining or changing the old STA identifying information according to a specific condition. 
     SUMMARY OF INVENTION 
     Technical Problem 
     The present invention provides a communication method based on a protocol that assigns identifying information to a station (STA) operating in a power save mode in a wireless local area network (WLAN) system, and an apparatus supporting the method. 
     Technical Solution 
     In an aspect, a communication method based on identifying information assignment in a wireless local area network (WLAN) system, performed by a station (STA), is provided. The method includes receiving an identifier assignment message from an access point (AP), wherein the identifier assignment message comprises identifying information for the STA, and TIM offset information for a time point at which at least one traffic indication map (TIM) element for the STA starts to be transmitted; receiving a first TIM element from the AP at a time point indicated by the TIM offset information; determining whether the first TIM element comprises the identifying information; and receiving a first data frame from the AP if the first TIM element contains the identifying information. 
     The identifier assignment message may further include TIM interval information indicating an interval in which the at least one TIM element is transmitted 
     The method may further include receiving a second TIM element at a time point at which the first TIM element is received after the elapse of a time point indicated by the TIM interval information; determining whether the second TIM element comprises the identifying information; and receiving a second data frame from the AP if the second TIM element comprises the identifying information. 
     The method may further include receiving a new identifier assignment message comprising new identifying information; receiving a third TIM element; determining whether the third TIM element comprises the new identifying information; and receiving a third data frame from the AP if the third TIM element comprises the new identifying information. 
     The new identifier assignment message may further include new TIM offset information, and the receiving of the third TIM element may be performed at a time point indicated by the new TIM offset information. 
     The first TIM element may transmitted by being included in a beacon frame which is periodically transmitted, and the TIM offset information may indicate the number of beacon frames transmitted while receiving the first TIM element after the STA receives the identifier assignment message. 
     The second TIM element may be transmitted by being included in a beacon frame which is transmitted periodically, and the interval in which the TIM element may be transmitted is set to a multiple of an interval of the beacon frame. 
     The first TIM element may further include traffic class information as information indicating traffic related to the first data frame. 
     If the first TIM element does not comprises the identifying information, the STA may operate by transitioning to a doze state. 
     The identifier assignment message may be transmitted by being included in an association response frame transmitted by the AP to the STA in response to an association request frame transmitted to associate the STA with the AP. 
     The identifying information may be an association ID (AID) assigned when the STA is associated with the AP. 
     In an another aspect, a wireless apparatus is provided. The apparatus includes a transceiver for transmitting and receiving a radio signal; and a processor operably coupled to the transceiver. The processor is configured for: receiving an identifier assignment message from an access point (AP), wherein the identifier assignment message comprises identifying information for the wireless apparatus, and TIM offset information for a time point at which at least one traffic indication map (TIM) element for the wireless apparatus starts to be transmitted; receiving a first TIM element from the AP at a time point indicated by the TIM offset information; determining whether the first TIM element comprises the identifying information; and receiving a first data frame from the AP if the first TIM element comprises the identifying information. 
     The processor is further configured for: receiving a new identifier assignment message comprising new identifying information from the AP; receiving a second TIM element from the AP; determining whether the second TIM element comprises the new identifying information; and receiving a second data frame from the AP if the second TIM element comprises the new identifying information. 
     In still another aspect, a communication method in a wireless local area network (WLAN) system, performed by a STA, is provided. The method includes transmitting a traffic indication request message requesting the AP to indicate whether a buffered traffic for the STA exists, receiving a traffic indication response message from the AP, the traffic indication response message comprising an identifier field including identifier for at least one buffered STA having a buffered traffic, and, a timer field for time synchronization between the STA and the AP, determining a time point at which the STA enters a awake state on a basis of the timer field, entering the awake state at the time point, and, receiving a data frame for the buffered for the STA traffic from the AP. 
     The timer field may include a timestamp field indicating a time point at which the traffic indication response message is transmitted; a timer accuracy field indicating a margin of error for a timer synchronization function; and, a timer accuracy error limit field a limitation of the margin of error. 
     Advantageous Effects 
     Since two or more stations (STAs) to which the same identifying information is assigned can selectively receive a traffic indication map (TIM), it is possible to perform communication based on a TIM protocol in a wireless local area network (WLAN) environment in which the identifying information can be assigned in an overlapping manner. Therefore, an STA having no buffered traffic can be prevented from unnecessary power consumption while maintaining an awake state. 
     If buffered traffic having a high importance level exists for a specific STA, identifying information for the STA can be modified and a TIM element reception interval can be changed to be short. On the other hand, if there is buffered traffic having a low importance level, the TIM element reception interval can be changed to be long. That is, a power save mode operation can be dynamically performed according to an importance level of traffic. 
     Unnecessary power consumption of STAs can be avoided by providing a timing synchronization request/response procedure for the power save mode operation. 
    
    
     
       DESCRIPTION OF DRAWINGS 
         FIG. 1  is a diagram showing the configuration of a WLAN system to which embodiments of the present invention may be applied. 
         FIG. 2  is a diagram showing the PHY architecture of a WLAN system which is supported by IEEE 802.11. 
         FIG. 3  is a diagram showing an example of a PPDU format used in a WLAN system. 
         FIG. 4  shows an example of a power management operation. 
         FIG. 5  shows an example of a response procedure of an AP in a TIM protocol. 
         FIG. 6  shows another example of a response procedure of an AP in a TIM protocol. 
         FIG. 7  shows a procedure of a TIM protocol based on a DTIM. 
         FIG. 8  shows a format of an AID assignment management frame according to an embodiment of the present invention. 
         FIG. 9  is a flow diagram showing an example of a communication method of an STA on the basis of an AID assignment method according to an embodiment of the present invention. 
         FIG. 10  shows an AID assignment management frame format and an AID assignment information element format according to an embodiment of the present invention. 
         FIG. 11  is a block diagram representing a PPDU format for SU transmission in WLAN system supporting M2M according to an embodiment of the present invention. 
         FIG. 12  is a block diagram representing a PPDU format for MU transmission in WLAN system supporting M2M according to an embodiment of the present invention. 
         FIG. 13  shows a traffic indication request frame according to an embodiment of the present invention. 
         FIG. 14  shows a traffic indication response frame according to an embodiment of the present invention. 
         FIG. 15  shows a TSF timer accuracy information element. 
         FIG. 16  shows a revised time stamp field format according to an embodiment of the present invention. 
         FIG. 17  is a block diagram showing a format of a short beacon frame according to an embodiment of the present invention. 
         FIG. 18  is a block diagram of a wireless apparatus according to an embodiment of the present invention. 
     
    
    
     MODE FOR INVENTION 
       FIG. 1  is a diagram showing the configuration of a WLAN system to which embodiments of the present invention may be applied. 
     A WLAN system includes one or more Basic Service Set (BSSs). The BSS is a set of stations (STAs) which can communicate with each other through successful synchronization. The BSS is not a concept indicating a specific area 
     An infrastructure BSS includes one or more non-AP STAs STA 1 , STA 2 , STA 3 , STA 4 , and STA 5 , an AP (Access Point) providing distribution service, and a Distribution System (DS) connecting a plurality of APs. In the infrastructure BSS, an AP manages the non-AP STAs of the BSS. 
     On the other hand, an Independent BSS (IBSS) is operated in an Ad-Hoc mode. The IBSS does not have a centralized management entity for performing a management function because it does not include an AP. That is, in the IBSS, non-AP STAs are managed in a distributed manner. In the IBSS, all STAs may be composed of mobile STAs. All the STAs form a self-contained network because they are not allowed to access the DS. 
     An STA is a certain functional medium, including Medium Access Control (MAC) and wireless-medium physical layer interface satisfying the Institute of Electrical and Electronics Engineers (IEEE) 802.11 standard. Hereinafter, the STA refers to both an AP and a non-AP STA. 
     A non-AP STA is an STA which is not an AP. The non-AP STA may also be referred to as a mobile terminal, a wireless device, a wireless transmit/receive unit (WTRU), a user equipment (UE), a mobile station (MS), a mobile subscriber unit, or simply a user. For convenience of explanation, the non-AP STA will be hereinafter referred to the STA. 
     The AP is a functional entity for providing connection to the DS through a wireless medium for an STA associated with the AP. Although communication between STAs in an infrastructure BSS including the AP is performed via the AP in principle, the STAs can perform direct communication when a direct link is set up. The AP may also be referred to as a central controller, a base station (BS), a node-B, a base transceiver system (BTS), a site controller, etc. 
     A plurality of infrastructure BSSs including the BSS can be interconnected by the use of the DS. An extended service set (ESS) is a plurality of BSSs connected by the use of the DS. APs and/or STAs included in the ESS can communicate with each another. In the same ESS, an STA can move from one BSS to another BSS while performing seamless communication. 
     In  FIG. 1 , an association ID (AID) can be assigned to each of STAs  21 ,  22 ,  23 ,  24 , and  25  while the STAS are associated with an AP  10 . The AID is used uniquely in one BSS. For example, in a current WLAN system, the AID can be given to any one of values 1 to 2007. In this case, for the AID, 14 bits can be assigned to a frame transmitted by the AP and/or the STA, and the AID value can be given to up to 16383. In this case, 2008 to 16383 may be reserved. 
     In a WLAN system based on IEEE 802.11, a basic access mechanism of a medium access control (MAC) is a carrier sense multiple access with collision avoidance (CSMA/CA) mechanism. The CSMA/CA mechanism is also referred to as a distributed coordinate function (DCF) of the IEEE 802.11 MAC, and basically employs a “listen before talk” access mechanism. In this type of access mechanism, an AP and/or an STA senses a wireless channel or medium before starting transmission. As a result of sensing, if it is determined that the medium is in an idle status, frame transmission starts by using the medium. Otherwise, if it is sensed that the medium is in an occupied status, the AP and/or the STA does not start its transmission but sets and waits for a delay duration for medium access. 
     The CSMA/CA mechanism also includes virtual carrier sensing in addition to physical carrier sensing in which the AP and/or the STA directly senses the medium. The virtual carrier sensing is designed to compensate for a problem that can occur in medium access such as a hidden node problem. For the virtual carrier sending, the MAC of the WLAN system uses a network allocation vector (NAV). The NAV is a value transmitted by an AP and/or an STA, currently using the medium or having a right to use the medium, to anther AP or another STA to indicate a remaining time before the medium returns to an available state. Therefore, a value set to the NAV corresponds to a period reserved for the use of the medium by an AP and/or an STA transmitting a corresponding frame. 
     An IEEE 802.11 MAC protocol, together with a DCF, provides a Hybrid Coordination Function (HCF) based on a Point Coordination Function (PCF) in which a reception AP or a reception STA or both periodically poll a data frame using the DCF and a polling-based synchronous access scheme. The HCF includes Enhanced Distributed Channel Access (EDCA) in which a provider uses an access scheme for providing a data frame to a number of users as a contention-based scheme and HCF Controlled Channel Access (HCCA) employing a non-contention-based channel access scheme employing a polling mechanism. The HCF includes a medium access mechanism for improving the Quality of Service (QoS) of a WLAN and can transmit QoS data both in a Contention Period (CP) and a Contention-Free Period (CFP). 
       FIG. 2  is a diagram showing the PHY architecture of a WLAN system which is supported by IEEE 802.11. 
     The PHY architecture of IEEE 802.11 includes a PHY Layer Management Entity (PLME), a Physical Layer Convergence Procedure (PLCP) sublayer  210 , and a Physical Medium Dependent (PMD) sublayer  200 . The PLME provides the management function of a physical layer in association with a MAC Layer Management Entity (MLME). The PLCP sublayer  210  transfers a MAC Protocol Data Unit (MPDU), received from a MAC sublayer  220 , to the PMD sublayer  200  or transfers a frame, received from the PMD sublayer  200 , to the MAC sublayer  220  according to an instruction of a MAC layer between the MAC sublayer  220  and the PMD sublayer  200 . The PMD sublayer  200 , as a PLCP sublayer, enables the transmission and reception of a physical entity between two STAs through a radio medium. The MPDU transmitted by the MAC sublayer  220  is referred to as a Physical Service Data Unit (PSDU) in the PLCP sublayer  210 . The MPDU is similar to the PSDU, but if an Aggregated MPDU (A-MPDU) in which a plurality of MPDUs is aggregated is transferred, each MPDU and each PSDU may be different from each other. 
     In a process of transferring the PSDU, received from the MAC sublayer  220 , to the PMD sublayer  200 , the PLCP sublayer  210  adds a supplementary subfield, including information necessary for a physical transceiver, to the PSDU. The field added to the PSDU may include tail bits necessary to restore a PLCP preamble, a PLCP header, and a convolution encoder to a zero state. The PLCP sublayer  210  receives a TXVECTOR parameter, including control information necessary to generate and transmit a Physical Layer Convergence Procedure (PLCP) Protocol Data Unit (PPDU) and control information necessary for a receiving STA to receive and interpret the PPDU, from the MAC sublayer  220 . The PLCP sublayer  210  uses the information included in the TXVECTOR parameter in order to generate the PPDU including the PSDU. 
     The PLCP preamble functions to enable a receiver to be prepared for a synchronization function and an antenna diversity before the PSDU is transmitted. A data field may include padding bits, a service field including a bit sequence for resetting a scrambler, and a coded sequence in which the bit sequence having tail bits added thereto has been encoded in the PSDU. Here, an encoding scheme may be either a Binary Convolutional Coding (BCC) encoding scheme or a Low Density Parity Check (LDPC) encoding scheme according to an encoding scheme supported by an STA that receives a PPDU. The PLCP header includes a field including information about a PLCP Protocol Data Unit (PPDU) to be transmitted. The PLCP header will be described in more detail later with reference to  FIG. 3 . 
     The PLCP sublayer  210  generates the PPDU by adding the field to the PSDU and transmits the generated PPDU to a receiving STA via the PMD sublayer  200 . The receiving STA receives the PPDU, obtains information necessary to restore data from a PLCP preamble and a PLCP header, and restores the data. The PLCP sublayer of the receiving STA transfers an RXVECTOR parameter, including control information included in a PLCP preamble and a PLCP header, to an MAC sublayer so that the MAC sublayer can interpret the PPDU and obtain data in a reception state. 
     A WLAN system supports transmit channels of a more contiguous 160 MHz band and a discontiguous 80+80 MHz band in order to support a higher throughput. Furthermore, the WLAN system supports a Multiple User-Multiple Input Multiple Output (MU-MIMO) transmission scheme. In a WLAN system supporting the MU-MIMO transmission scheme, an AP or an STA or both that try to transmit data may transmit data packets to one or more MU-MIMO-paired receiving STAs at the same time. 
     Referring back to  FIG. 1 , in a WLAN system, such as that shown in  FIG. 1 , the AP  10  may transmit data to an STA group including at least one STA, from among the plurality of STAs  21 ,  22 ,  23 ,  24 , and  30  associated therewith, at the same time. An example where the AP performs MU-MIMO transmission to the STAs is shown in  FIG. 1 . In a WLAN system supporting Tunneled Direct Link Setup (TDLS), Direct Link Setup (DLS), or a mesh network, however, an STA trying to send data may send a PPDU to a plurality of STAs by using the MU-MIMO transmission scheme. An example where an AP sends a PPDU to a plurality of STAs according to the MU-MIMO transmission scheme is described below. 
     The data respectively transmitted to each of the STAs may be transmitted through different spatial streams. The data packet transmitted by the AP  10  may be a PPDU, generated and transmitted by the physical layer of a WLAN system, or a data field included in the PPDU, and the data packet may be referred to as a frame. That is, a data field included in a PPDU for SU-MIMO or MU-MIMO or both may be called an MIMO packet. In an example of the present invention, it is assumed that a target transmission STA group MU-MIMO-paired with the AP  10  includes the STA  1   21 , the STA  2   22 , the STA  3   23 , and the STA  4   24 . Here, data may not be transmitted to a specific STA of the target transmission STA group because spatial streams are not allocated to the specific STA. Meanwhile, it is assumed that the STA 5   25  is associated with the AP  10 , but not included in the target transmission STA group. 
     In order to support MU-MIMO transmission in a WLAN system, an identifier may be allocated to a target transmission STA group, and the identifier may be called a group ID. An AP transmits a group ID management frame, including group definition information, to STAs supporting MU-MIMO transmission in order to allocate a group ID to the STAs. The group ID is allocated to the STAs based on the group ID management frame prior to PPDU transmission. A plurality of group IDs may be allocated to one STA. 
     Table 1 below shows information elements included in the group ID management frame. 
     
       
         
           
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                 Order 
                 Information 
               
               
                   
               
             
            
               
                 1 
                 Category 
               
               
                 2 
                 VHT action 
               
               
                 3 
                 Membership status 
               
               
                 4 
                 Spatial stream position 
               
               
                   
               
            
           
         
       
     
     The frames of the category field and the VHT action field correspond to management frames. The category field and the VHT action field are set to identify that the relevant frames are group ID management frames used in the next-generation WLAN system supporting MU-MIMO. 
     As in Table 1, group definition information includes the membership status information, indicating whether an STA belongs to a specific group ID, and spatial stream position information indicating that what place is the spatial stream set of a relevant STA located from all the spatial streams according to MU-MIMO transmission if the STA belongs to the relevant group ID. 
     Since a plurality of group IDs is managed by one AP, membership status information provided to one STA needs to indicate whether the STA belongs to each of the group IDs managed by the AR Accordingly, the membership status information may exist in an array form of subfields, indicating whether the STA belongs to each group ID. The spatial stream position information may exist in an array form of subfields, indicating a position of a spatial stream set occupied by an STA regarding each group ID, because the spatial stream position information indicates a position for each group ID. Furthermore, the membership status information and the spatial stream position information for one group ID may be implemented within one subfield. 
     If an AP transmits a PPDU to a plurality of STAs according to the MU-MIMO transmission scheme, the AP includes information, indicating a group ID, in the PPDU, and transmits the information as control information. When an STA receives the PPDU, the STA checks whether it is a member STA of a target transmission STA group by checking a group ID field. If the STA is checked to be a member of the target transmission STA group, the STA may check that what place is a spatial stream set, transmitted thereto, placed from all the spatial streams. Since the PPDU includes information about the number of spatial streams allocated to a reception STA, the STA can receive data by searching for spatial streams allocated thereto. 
       FIG. 3  is a diagram showing an example of a PPDU format used in a WLAN system. 
     Referring to  FIG. 3 , a PPDU  300  may include an L-STF  310 , an L-LTF  320 , an L-SIG field  330 , a VHT-SIG A field  340 , a VHT-STF  350 , a VHT-LTF  360 , a VHT-SIG B field  370 , and a data field  380 . 
     The PLCP sublayer forming the physical layer converts a PSDU, received from the MAC layer, into the data field  380  by adding necessary information to the PSDU, generates the PPDU  300  by adding fields, such as the L-STF  310 , the L-LTF  320 , the L-SIG field  330 , the VHT-SIG A field  340 , the VHT-STF  350 , the VHT-LTF  360 , and the VHT-SIGB field  370 , to the data field  380 , and transmits the PPDU  300  to one or more STAs through the PMD sublayer forming the physical layer. Control information necessary for the PLCP sublayer to generate the PPDU and control information, included in the PPDU and transmitted so that a receiving STA can use the control information to interpret the PPDU, are provided from the TXVECTOR parameter received from the MAC layer. 
     The L-STF  310  is used for frame timing acquisition, Automatic Gain Control (AGC) convergence, coarse frequency acquisition, etc. 
     The L-LTF  320  is used for channel estimation for demodulating the L-SIG field  330  and the VHT-SIG A field  340 . 
     The L-SIG field  330  is used for an L-STA to receive the PPDU  300  and obtain data by interpreting the PPDU  300 . The L-SIG field  330  includes a rate subfield, a length subfield, parity bits, and a tail field. The rate subfield is set to a value indicating a bit rate for data to be transmitted now. 
     The length subfield is set to a value indicating the octet length of a Physical Service Data Unit (PSDU) that the MAC layer requests a physical layer to send the PSDU. Here, an L_LENGTH parameter related to information about the octet length of the PSDU is determined on the basis of a TXTIME parameter related to transmission time. TXTIME indicates a transmission time determined by the physical layer in order to transmit a PPDU including the PSDU, in response to a transmission time that the MAC layer has requested the physical layer to send the PSDU. Since the L_LENGTH parameter is a parameter related to time, the length subfield included in the L-SIG field  330  includes information related to the transmission time. 
     A VHT-SIGA field  340  includes control information (or signal information) necessary for interpreting a PPDU  300  by STAs that receive the PPDU. The VHT-SIGA field  340  is transmitted by using two OFDM symbols. Accordingly, the VHT-SIGA field  340  can be divided into a VHT-SIGA1 field and a VHT-SIGA2 field. The VHT-SIGA1 field includes channel bandwidth information used for PPDU transmission, identification information regarding whether space time block coding (STBC) is used, information indicating either SU or MU-MIMO as a PPDU transmission method, information indicating an AP and a transmission target STA group which is a plurality of MU-MIMO paired STAs if the transmission method is MU-MIMO, and information on a spatial stream allocated to each STA included in the transmission target STA group. Table 2 below can be used for reference as a detailed example of the VHT-SIGA1 field. 
     Information indicating the MIMO transmission method and information indicating the transmission target STA group can be implemented with one piece of MIMO indication information. For example, it can be implemented with a group ID. The group ID can be set to a value having a specific range. A specific value in the range indicates an SU-MIMO transmission method, and values other than the specific value can be used as an identifier for a transmission target STA group when the PPDU  300  is transmitted by using the MU-MIMO transmission method. 
     Table 2 below can be used for reference as a detailed example of the VHT-SIGA1 field. 
     [Table 2] 
     Referring to Table 2 above, in an NSTS part related to information on a spatial stream, a specific bit sequence can be utilized as information indicating a partial AID in case of SU-MIMO transmission. A rule of specifying the partial AID implemented with the specific bit sequence in this case will be described below. 
     A STA, which transmits a PPDU including MPDUs transmitted in group or a null data packet (NDP) PPDU transmitted subsequently to a null data packet announcement (NDPA) frame transmitted in group, sets a value of an information parameter PARTIAL_AID related to a partial AID of a transmission parameter TXVECTOR to “0”. 
     An AP, which transmits a PPDU to an associated STA or a direct link setup (DLS) or tunneled direct link setup (TDLS) peer STA, sets the value of the information parameter PARTIAL_AID related to the partial AID of the transmission parameter TXVECTOR according to Equation 1 below. 
     [Math  FIG. 1 ] 
     Herein, denotes a bitwise exclusive OR operation, and mod X denotes an X-modulo operation. dec(A[b:c]) is the cast to decimal operator where b is scaled by 20 and c by 2c−b. AID[b:c] represents bits b through c inclusive of the AID of the recipient STA with bit  0  being the first transmitted. BSSID[b:c] represents bits b through c inclusive of the BSSID with bit  0  being the Individual/Group bit of a MAC address. 
     The STA which transmits the PPDU to the DLS or TDLS peer STA can obtain an AID of the peer STA from a DLS setup request frame, a DLS setup response frame, a TDLS setup request frame, or a TDLS setup response frame. 
     The STA, which transmits the PPDU to the AP or transmits the NDP after transmission of the NDPA frame, sets the PARTIAL_AID information parameter of the transmission parameter TXVECTOR to LSB 9 bits of a BSSID. 
     An STA which transmits a PPDU to an independent BSS (IBSS) peer STA or an STA which transmits an NDP to the IBSS peer STA after transmission of an NDPA frame sets the PARTIAL_AID information parameter of the transmission parameter TXVECTOR to 0. 
     An STA which transmits a PPDU including MPDUs transmitted individually to a mesh STA sets the PARTIAL_AID information parameter of a transmission parameter TXVECTOR to LSB 9 bits of a MAC address of a recipient STA. 
     The AP sets the PARTIAL_AID information parameter to 0 when the partial AID is not allocated to the STA. 
     If the group ID indicates that the PPDU  300  is transmitted according to the SU-MIMO transmission scheme, the VHT-SIG A2 field includes coding indication information, indicating whether a coding scheme applied to a data field is a Binary Convolution Coding (BCC) scheme or a Low Density Parity Check (LDPC) coding scheme, and Modulation Coding Scheme (MCS) information about a channel between a sender and a recipient. Furthermore, the VHT-SIG A2 field may include the AID of an STA to which the PPDU  300  will be transmitted or a partial AID including some bit sequences of the AID or both. 
     If the group ID indicates that the PPDU  300  is transmitted according to the MU-MIMO transmission scheme, the VHT-SIG A field  340  includes coding indication information indicating whether a coding scheme applied to a data field to be transmitted to reception STAs that are MU-MIMO-paired is the BCC scheme or the LDPC coding scheme. In this case, Modulation Coding Scheme (MCS) information for each reception STA may be included in the VHT-SIG B field  370 . 
     The VHT-STF  350  is used to improve AGC estimation performance in MIMO transmission. 
     The VHT-LTF  360  is used for an STA to estimate a MIMO channel. The VHT-LTF  360  may be set to the number corresponding to the number of spatial streams through which the PPDU  300  is transmitted because the next-generation WLAN system supports MU-MIMO. Additionally, full channel sounding is supported. If the full channel sounding is performed, the number of VHT-LTFs may be further increased. 
     The VHT-SIG B field  370  includes dedicated control information which is necessary for a plurality of MIMO-paired STAs to obtain data by receiving the PPDU  300 . Accordingly, only when common control information included in the VHT-SIG B field  370  indicates that the received PPDU  300  has been subjected to MU-MIMO transmission, an STA may be designed to decode the VHT-SIG B field  370 . On the other hand, if the common control information indicates that the received PPDU  300  is for a single STA (including SU-MIMO), an STA may be implemented not to decode the VHT-SIG B field  370 . 
     The VHT-SIG B field  370  includes information about an MCS and information about rate matching for each STA. The VHT-SIG B field  370  further includes information indicating the length of a PSDU which is included in a data field for each STA. The information indicating the length of the PSDU is information indicating the length of the bit sequence of the PSDU and may be indicated by an octet unit. The size of the VHT-SIG B field  370  may vary an MIMO transmission type (MU-MIMO or SU-MIMO) and a channel bandwidth used for PPDU transmission. 
     The data field  380  includes data intended to be transmitted to an STA. The data field  380  includes a service field for resetting a PLCP Service Data Unit (PSDU) to which an MAC Protocol Data Unit (MPDU) in the MAC layer has been transferred and a scrambler, a tail field including a bit sequence necessary to restore a convolution encoder to a zero state, and padding bits for normalizing the length of a data field. 
     In a WLAN system, such as that shown in  FIG. 1 , if the AP  10  intends to transmit data to the STA  1   21 , the STA  2   22 , and the STA  3   23 , the AP  10  may transmit the PPDU to an STA group including the STA  1   21 , the STA  2   22 , the STA  3   23 , and the STA  4   24 . In this case, the data may be transmitted in such a manner that spatial streams are not allocated to the STA  4   24  and a specific number of spatial streams are allocated to each of the STA  1   21 , the STA  2   22 , and the STA  3   23 , as in  FIG. 2 . In the example of  FIG. 2 , it can be seen that one spatial stream has been allocated to the STA  1   21 , three spatial streams have been allocated to the STA  2   22 , and two spatial streams have been allocated to the STA  3   23 . 
     Meanwhile, if channel sensing is always performed for frame transmission and reception, it causes persistent power consumption of the STA. Since power consumption in a reception state is not much different from power consumption in a transmission state, if the reception state needs to be continuously maintained, relatively great power consumption is generated in an STA that operates by using a battery. Therefore, when the STA senses a channel by persistently maintaining a reception standby state in a WLAN system, ineffective power consumption may be caused without a special synergy effect in terms of a WLAN throughput, and thus it may be inappropriate in terms of power management. 
     To compensate for the problem above, the WLAN system supports a power management (PM) mode of the STA. A power management (PM) mode of a STA is classified into an active mode and a power save (PS) mode in a WLAN system. Basically, the STA operates in the active mode. When operating in the active mode, the STA can operate in an awake state so that a frame can be received all the time. 
     When operating in the PS mode, the STA operates by transitioning between a doze state and the awake state. When operating in the doze state, the STA operates with minimum power, and does not receive a radio signal, including a data frame, transmitted from an AP. In addition, the STA operating in the doze state does not perform channel sensing. 
     The longer the STA operates in a doze state, the less the power consumption is, and thus the longer the STA operates. However, since a frame cannot be transmitted and received in the doze state, the STA cannot operate long unconditionally. If the STA operating in the doze state has a frame to be transmitted to the AP, the STA can transition to an awake state to transmit the frame. However, if the AP has a frame to be transmitted to the STA operating in the doze state, the STA cannot receive the frame and cannot know that there is the frame to be received. Therefore, the STA may need to know whether there is the frame to be transmitted to the STA, and if the frame exists, may require an operation for transitioning to the awake state in accordance with a specific period. According to this operation, the AP can transmit the frame to the STA. This will be described with reference to  FIG. 4 . 
       FIG. 4  shows an example of a power management operation. 
     Referring to  FIG. 4 , an AP  410  transmits a beacon frame to STAs in a BSS in accordance with a specific period (step S 410 ). The beacon frame includes a traffic indication map (TIM) information element. The TIM element includes information for reporting that the AP  410  has buffered traffic for which the STAs associated with and a frame will be transmitted. Examples of the TIM element include a TIM used to report a unicast frame and a delivery traffic indication map (DTIM) used to report a multicast or broadcast frame. 
     The AP  410  transmits the DTIM one time whenever a beacon frame is transmitted three times. 
     An STA 1   421  and an STA 2   222  are STAs operating in a PS mode. The STA 1   421  and the STA 2   422  can be configured such that they can transition from a doze state to an awake state in every wakeup interval of a specific period to receive the TIM element transmitted by the AP  410 . A specific wakeup interval can be configured such that the STA 1   421  transitions to the awake state in every beacon interval to receive the TIM element. Therefore, the STA 1   421  transitions to the awake state (step S 421 ) when the AP  410  transmits a first beacon frame (step S 411 ). The STA 1   421  receives the beacon frame and acquires the TIM element. If the acquired TIM element indicates that there is a frame to be transmitted to the STA 1   421 , then the STA 1   221  transmits to the AP  410  a PS poll frame that requests the AP  410  to transmit a frame (step S 421   a ). The AP  410  transmits the frame to the STA 1   421  in response to the PS poll frame (step S 431 ). Upon completion of frame reception, the STA 1   421  operates by transitioning back to the doze state. 
     When the AP  410  transmits a second beacon frame, a medium is busy, that is, another device accesses to the medium for example. Thus, the AP  410  may not be able to transmit the beacon frame in accordance with a correct beacon interval but may transmit it at a delayed time point (step S 412 ). In this case, the STA 1   421  switches its mode to the wake state in accordance with the beacon interval, but cannot receive the beacon frame transmitted with delay, and thus transitions back to the doze state (step S 422 ). 
     When the AP  410  transmits a third beacon frame, the beacon frame may include a TIM element which is configured as a DTIM. However, since the medium is busy, the AP  410  transmits the beacon frame with delay (step S 413 ). The STA 1   421  operates by transitioning to the awake state in accordance with the beacon interval, and can acquire the DTIM by using the beacon frame transmitted by the AP  410 . The DTIM acquired by the STA 1   421  indicates that there is no frame to be transmitted to the STA 1   421  and there is a frame for another STA. Therefore, the STA 1   221  operates by transitioning back to the doze state. After transmitting the beacon frame, the AP  410  transmits the frame to a corresponding STA (step S 432 ). 
     The AP  410  transmits a fourth beacon frame (step S 414 ). However, since the STA 1   421  cannot acquire information indicating that there is buffered traffic for the STA 1   421  by receiving the TIM element two times, the STA 1   421  may regulate a wakeup interval for receiving the TIM element. Alternatively, if signaling information for regulating a wakeup interval value of the STA 1   421  is included in the beacon frame transmitted by the AP  410 , the wakeup interval value of the STA 1   421  may be regulated. Instead of transitioning an operation state for every beacon interval to receive the TIM element, the STA 1   421  can be configured in the present embodiment such that the operation state is transitioned one time for every three beacon intervals. Therefore, the STA 1   421  cannot acquire a corresponding TIM element since the AP  410  transmits the fourth beacon frame (step S 214 ), and maintains the doze state when a fifth beacon frame is transmitted (step S 415 ). 
     When the AP  410  transmits a sixth beacon frame (step S 416 ), the STA 1   421  operates by transitioning to the awake state, and acquires the TIM element included in the beacon frame (step S 424 ). The TIM element is a DTIM that indicates existence of a broadcast frame, and thus the STA 1   421  receives the broadcast frame transmitted by the AP  410  (step S 434 ) instead of transmitting a PS poll frame to the AP  410 . 
     Meanwhile, the wakeup interval assigned to the STA 2   422  may have a longer period than that of the STA 1   421 . Therefore, the STA 2   422  can receive the TIM element by transitioning to the awake state (step S 425 ) when the fifth beacon frame is transmitted (step S 415 ). The STA 2   422  knows existence of a frame to be transmitted to the STA 2   422  by using the TIM element, and transmits a PS poll frame to the AP  410  to request transmission (step S 425   a ). The AP  210  transmits a frame to the STA 2   222  in response to the PS poll frame (step S 433 ). 
     In order to operate the PS mode of  FIG. 4 , the TIM element includes a TIM that indicates whether there is a frame to be transmitted to the STA or a DTIM that indicates whether there is a broadcast/multicast frame. The DTIM may be implemented by configuring a field of the TIM element. 
     A detailed response procedure of the STA that receives the TIM element can be described below with reference to  FIG. 5  to  FIG. 7 . 
       FIG. 5  shows an example of a response procedure of an AP in a TIM protocol. 
     Referring to  FIG. 5 , an STA  520  switches its operation state from a doze state to an awake state to receive a beacon frame including a TIM from an AP  510  (step S 510 ). The STA  520  interprets a received TIM element and thus can know whether there is buffered traffic to be delivered to the STA  520 . 
     The STA  520  contends with other STAs to access to a medium for transmitting a PS poll frame (step S 520 ), and transmits the PS poll frame to request the AP  510  to transmit a data frame (step S 530 ). 
     Upon receiving the PS poll frame transmitted by the STA  520 , the AP  510  transmits a data frame to the STA  520 . The STA 2   520  receives the data frame, and transmits an acknowledgment (ACK) frame to the AP  510  in response thereto (step S 550 ). Thereafter, the STA 2   520  switches its operation mode back to the doze state (step S 560 ). 
     Instead of immediate response of  FIG. 5  in which the data frame is transmitted immediately after receiving the PS poll frame from the STA, the AP may transmit data at a specific time point after receiving the PS poll frame. 
       FIG. 6  shows another example of a response procedure of an AP in a TIM protocol. 
     Referring to  FIG. 6 , an STA  620  switches its operation state from a doze state to an awake state to receive a beacon frame including a TIM from an AP  610  (step S 610 ). The STA  620  interprets a received TIM element and thus can know whether there is buffered traffic to be delivered to the STA  620 . 
     The STA  620  contends with other STAs to access to a medium for transmitting a PS poll frame (step S 620 ), and transmits the PS poll frame to request the AP  610  to transmit a data frame (step S 630 ). 
     If the AP  610  receives the PS poll frame but fails to prepare for a data frame during a specific time interval such as a short inter-frame space (SIFS), instead of directly transmitting the data frame, the AP  610  transmits an ACK frame to the STA  620  (step S 640 ). This is a characteristic of a deferred response which is different from step S 540  of  FIG. 5  in which the AP  510  directly transmits the data frame to the STA  520  in response to the PS poll frame. 
     The AP  610  performs contending when the data frame is prepared after transmitting the ACK frame (step S 650 ), and transmits the data frame to the STA  620  (step S 660 ). 
     The STA  620  transmits an ACK frame to the AP  610  in response to the data frame (step S 670 ), and switches its operation mode to the doze state (step S 680 ). 
     When the AP transmits a DTIM to the STA, a subsequent procedure of a TIM protocol may differ. 
       FIG. 7  shows a procedure of a TIM protocol based on a DTIM. 
     Referring to  FIG. 7 , an STA  720  switches its operation state from a doze state to an awake state to receive a beacon frame including a TIM from an AP  710  (step S 710 ). The STAs  720  can know that a multicast/broadcast frame will be transmitted by using the received DTIM. 
     After transmitting a beacon frame including the DTIM, the AP  720  transmits the multicast/broadcast frame (step S 720 ). After receiving the multicast/broadcast frame transmitted by the AP  710 , the STAs  720  switch the operation state back to the doze state (step S 730 ). 
     In the power save mode operation method based on the TIM protocol described with reference to  FIG. 4  to  FIG. 7 , STAs can determine whether there is a data frame to be transmitted for buffered traffic by using STA identifying information included in the TIM element. The STA identifying information may be information related to an association identifier (AID) as an identifier assigned when the STA is associated with an AP. The STA identifying information may be configured to directly indicate AIDs of STAs having buffered traffic or may be configured in a bitmap type in which a bit order corresponding to an AID value is set to a specific value. The STAs can know that there is buffered traffic for them if the STA identifying information indicates their AIDs. The STA identifying information is configured in the bitmap type, and if there is buffered traffic for an STA assigned with a specific AID, a bit value of an order corresponding to the specific AID value can be set to ‘1’. 
     One AID is assigned to one STA in one BSS, and the AID may be in the range of 1 to 2007 at present. 14 bits are assigned to indicate the AID, and thus the AID can be assigned with up to 16383. In this case, AID values from 2008 to 16383 are reserved. 
     Meanwhile, machine to machine (M2M) is drawing attention recently as a next generation communication technique. A standardization work is ongoing to support a WLAN communication protocol supported in such a communication environment. The M2M implies a network for exchanging information by using a machine, not a person, as a communication entity. Examples of constitutional elements of the M2M-based network include a sensor for measuring temperature, humidity, or the like, a camera, a home appliance (e.g., TV, etc.), and a large machine (e.g., a processing machine of a factory, an automobile, etc.). Recently, with the introduction of various communication services (e.g., a smart grid, eHealth, ubiquitous, etc.), the M2M technique is drawing attention to support the communication services. An M2M-based network system has the following characteristics. 
     1. Great number of STAs: Unlike the conventional network, it is assumed that the M2M requires a great number of STAs. This is because not only a machine owned by a person but also a sensor installed in a house, an office, etc., can be a target to be considered. Therefore, a significantly great number of STAs can be coupled to one AP. 
     2. Low traffic load per STA: Since an STA constituting an M2M network has a traffic pattern in which information of a surrounding environment is gathered and reported, frequent transmission is not necessary and an amount of the information is relatively small. 
     3. Uplink-focused: The M2M has a structure in which a command is received in a downlink in general, a specific behavior is performed, and then result data is reported in an uplink. Since important data is transmitted mainly in the uplink in general, the M2M is uplink-focused. 
     4. Lifespan of STA: An M2M STA operates mainly by using a battery, and it may be difficult for a user to frequently charge the battery. Therefore, whether the STA supports a power save mode may be an important issue. 
     5. Auto-recovery function: A self-recovery function is necessary in the M2M STA since it is difficult for the user to directly manipulate the STA in a specific situation. 
     There is an ongoing discussion on a standard for one use case in the M2M communication. A remarkable feature of this standard lies in that a significantly wide coverage (up to 1 km) is provided in an unlicensed band of a sub 1 GHz other than a TV white space in comparison with the conventional indoor-based WLAN. That is, unlike the conventional 2.4 GHz or 5 GHz, when the WLAN is used in the sub 1 GHz band represented by 700 to 900 MHz, the coverage of the AP against the same transmit power is increased by about 2- or 3-fold due to a propagation property at the band. In this case, it is characterized that significantly many STAs can be connected to one AP. The use case considered in the standardization work can be summarized as follows. 
     Use Case 1: Sensors and Meters 
     1a: Smart Grid—Meter to Pole 
     1c: Environmental/Agricultural Monitoring 
     1d: Industrial process sensors 
     1e: Healthcare 
     1f: Healthcare 
     1g: Home/Building Automation 
     1h: Home sensors 
     Use Case 2: Backhaul Sensor and Meter Data 
     Backhaul aggregation of sensors 
     Backhaul aggregation of industrial sensors 
     Use Case 3: Extended Range Wi-Fi 
     Outdoor extended range hotspot 
     Outdoor Wi-Fi for cellular traffic offloading 
     The Use Case 1 (sensor and meters) is a use case related to the aforementioned M2M communication. Various types of sensor devices can be connected to an 802.11ah AP to perform M2M communication. Particularly, in case of a smart grid, up to 6,000 sensor devices can be connected to one AP. 
     The Use Case 2 (backhaul sensor and meter data) is a case where an AP supporting M2M provides a wide coverage and takes a role of a backhaul link of a heterogeneous communication system such as 802.15.4g. 
     The Use Case 3 is a use case including a case which aims for outdoor extended range hotspot communication such as extended home coverage, campus wide coverage, and shopping malls and a case which aims for distribution of overflowing cellular traffic when the 802.11 ah AP supports traffic offloading of cellular mobile communication. At present, the number of AIDs supported in the WLAN system may be not enough to be used in a WLAN system supporting an M2M application. When the M2M application is applied to this WLAN environment, the number of STAs associated with one AP may be too many. In such an environment, a situation may occur in which one AID is assigned to two or more STAs. 
     In a WLAN environment in which one AID is assigned to two or more STAs in an overlapping manner, although STAs operating in a power save mode are non-buffered STAs which have no traffic to be actually transmitted to the STAs, a problem may occur in that the STAs are misunderstood as buffered STAs which have buffered traffic and STA identifying information included in the TIM element transmitted by an AP. Therefore, the non-buffered STA persistently maintains an awake state after receiving the TIM element, which results in unnecessary power consumption, thereby decreasing efficiency of the power save mode. In order to solve this problem, a communication method performed by dynamically assigning AIDs to STAs will be described hereinafter. 
     A method of dynamically assigning the AID allocates and changes an old AID of an STA by an AP while the STA performs communication with the AP such as frame exchange, apart from assigning of the AID to the STA when associating with the AP. 
     For example, assume that an STA 1  and an STA 2  are both assigned with an AID of 10, and the two STAs are currently operating in the power save mode. Upon generation of uplink traffic to be transmitted by the STA 1  to the AP, the STA 1  transitions to the awake mode to report to the AP that there is the uplink traffic. The existence of uplink traffic can be reported by transmitting a service period trigger frame or by transmitting an additionally defined specific frame. The AP receives the frame from the STA 1 , and then can know that the same AID is assigned to the STA 1  and the STA 2  in an overlapping manner. Therefore, the AID of the STA 1  can be modified such that the AID does not overlap with AIDs assigned to the STA 2  and other STAs. 
     The present invention proposes an AID assignment management frame for AID assignment. 
       FIG. 8  shows a format of an AID assignment management frame according to an embodiment of the present invention. 
     Referring to  FIG. 8 , an AID assignment management frame  800  includes a category field  810 , an action field  820 , a length field  830 , an AID assignment type field  840 , and an AID field  850 , and may further include a traffic class (TCLAS) field  860 . 
     The category field  810  and the action field  820  are set to a value indicating that a corresponding frame is an AID assignment management frame. The length field  830  indicates a length of the AID assignment management frame  800 . 
     The AID assignment type field  840  indicates a type of the AID assignment management frame  800 . The AID assignment management frame  800  can be transmitted to assign an AID to an STA or to release the AID assigned to the STA. In case of assigning the AID, the AID assignment type field  840  can be set to ‘1’. In case of releasing the AID, the AID assignment type field  840  can be set to ‘0’. However, the value that is set to the field is for exemplary purposes only, and thus can be implemented in various manners. 
     The AID field  850  is set to indicate the AID assigned to the STA or the AID released from the STA. When the AID assignment type field  840  indicates AID assignment, the AID indicated by the AID field  850  is assigned to the STA. When the AID assignment type field  840  indicates AID release, the AID indicated by the AID field  850  is released from the STA. 
     In addition, the AP can assign an AID per traffic. When the AP assigns the AID to a terminal, traffic class (TCLAS) information can be transmitted by being included in the AID assignment management frame  800 , and thus it can be signaled that the AID is used only for a frame corresponding to a specific TCLAS. The TCLAS information can be included in the TCLAS field  860 . The TCLAS can indicate traffic, and can be configured by combining a source MAC address, a destination MAC address, a source IP address, a destination IP address, etc. When assigning an AID per TCLAS, the STA can selectively receive a frame according to an importance level of traffic. That is, regarding traffic having a high importance level, the AP can be more frequently checked for in order to decrease a delay time. Regarding other traffic, instead of using the delay time, the traffic may be stored in the AP for a longer period of time and then is collectively received in order to increase power saving effect. Accordingly, efficiency of the power save mode can be improved. 
     When the AP transmits downlink traffic, STAs assigned with the same AID in an overlapping manner transit from a doze state to an awake state until an AID of a transmission target STA with buffered traffic is changed to avoid overlapping assignment. To solve this problem, the present invention proposes a method for assigning an AID to each STA at a different period, according to which an effective TIM including STA identifying information is transmitted, when the AP assigns the AID. Since the TIM element is transmitted by being included in the beacon frame, the changes in the TIM period can be implemented by changing a beacon period. 
     For example, a WLAN environment in which an AID  10  is assigned to the STA 1  and the STA 2  in an overlapping manner is assumed. When the AP configures a TIM element indicating buffered traffic to be transmitted to the STA 1  and the STA 2 , the AID  10  is used as STA identifying information in a TIM element of a 1st beacon frame. For example, when there is buffered traffic to be transmitted to the STA 1 , a value of a 10th order of a bit sequence constituting the STA identifying information having a bitmap type can be set to 1. 
     Meanwhile, an AID  10  of a TIM element of a 2nd beacon frame is used as STA identifying information for the STA 2 . That is, if there is buffered traffic to be transmitted to the STA 2 , a value of a 10th order of a bit sequence constituting the STA identifying information of a bitmap type can be set to 1. Although the STA 1  confirms the TIM element of the 1st beacon frame which is an effective beacon frame for the STA 1 , the STA 1  may ignore the TIM element of the 2nd beacon frame which is an ineffective beacon frame. 
     As such, in order to support the power save mode operation based on the TIM protocol of STAs assigned with the same AID in an overlapping manner, there is a need for a method for providing different TIM periods when assigning the AID, so that a plurality of STAs can receive the TIM element at different time points. 
       FIG. 9  is a flow diagram showing an example of a communication method of an STA on the basis of an AID assignment method according to an embodiment of the present invention. 
     Referring to  FIG. 9 , an STA 1   921  and an STA 2   922  are STAs supporting a power save mode operation. The STA 1   921  and the STA 2   972  may be in a state in which they are associated with an AP  910  and are assigned with AIDs. The STA 1   921  and the STA 2   972  can support the power save mode operation on the basis of the TIM protocol described above with reference to  FIG. 4  to  FIG. 7 . 
     The AP  910  transmits an AID assignment message to the STA 1   921  (step S 911 ). Transmitting of the AID assignment message may be equivalent to transmitting of an association response frame which is transmitted by the AP  910  in response to an association request frame transmitted by the STA  921  to request the AP  910  to perform association. The association response message includes an AID assignment information element as the AID assignment message. Alternatively, it may be an AID assignment management frame transmitted by the AP  910  to assign an AID to an STA. 
       FIG. 10  shows an AID assignment management frame format and an AID assignment information element format according to an embodiment of the present invention. 
       FIG. 10( a )  shows an AID assignment management frame format. An AID assignment management frame  1000   a  includes a category field  1010   a , an action field  1020   a , a length field  1030   a , an AID assigned beacon offset field  1040   a , an AID assigned beacon interval field  1050   a , an AID assignment type field  1060   a , an AID field  1070   a , and a TCLAS field  1080   a.    
     The category field  1010   a  and the action field  1020   a  are set to a value indicating that a transmitted frame is the AID assignment management frame  1000   a . The length field  1030   a  indicates a length of the transmitted AID assignment management frame  1000   a.    
     The AID assignment type field  1060   a  indicates a type of the AID assignment management frame  1000   a . The AID assignment management frame  1000   a  can be transmitted to assign an AID to an STA or to release the AID assigned to the STA. In case of assigning the AID, the AID assignment type field  1060   a  can be set to ‘1’. In case of releasing the AID, the AID assignment type field  1060   a  can be set to ‘0’. However, the value that is set to the field is for exemplary purposes only, and thus can be implemented in various manners. 
     The AID field  1070   a  indicates an AID to be assigned to or released from a receiving STA. The TCLAS field  1080   a  indicates TCLAS information for the AID. The TCLAS information may refer to the TCLAS field  800  of  FIG. 8 . 
     The AID assigned beacon offset field  1040   a  and the AID assigned beacon interval field  1050   a  include offset information and period information to indicate an effective beacon frame including a TIM element for buffered traffic for the receiving STA. 
     The AID assigned beacon offset field  1040   a  receives several more beacon frames from the currently received AID assignment management frame  1000   a , and thereafter indicates whether transmission of an effective beacon frame including the TIM element for the STA starts. 
     The AID assigned beacon interval field  1050   a  indicates the number of beacon frame intervals according to which the effective beacon frame including the TIM element for the STA is transmitted. 
       FIG. 10B  shows an AID assignment information element format. An AID assignment information element can be transmitted by being included in an association response frame, a probe response frame, and/or a beacon frame which are transmitted by an AP. 
     An AID information element  1000   b  includes an information element (IE) number field  810   b , a length field  1020   b , an AID assigned beacon offset field  1030   b , an AID assigned beacon interval field  1040   b , an AID field  1050   b , and a TCLAS field  1060   b.    
     The IE number field  1010   b  indicates that a corresponding information element included in a frame is the AID information element  1000   b . The length field  1020   b  indicates a length of the AID information element  1000   b . The AID field  1050   b  indicates an AID to be assigned to a receiving STA. The TCLAS field  860   b  indicates TCLAS information for the AID. The TCLAS information may refer to the TCLAS field  860  of  FIG. 8 . 
     The AID assigned beacon offset field  1030   b  and the AID assigned beacon interval field  1040   b  are configured in the same manner as those of the AID assigned beacon offset field  1040   a  and the AID assigned beacon interval field  1050   a , and include offset information and period information to indicate an effective beacon frame for the receiving STA. 
     Referring back to  FIG. 9 , the STA 1   921  receives the AID assignment message (step S 911 ). The STA 1   921  is assigned with ‘10’ as an AID by using the AID assignment message. Further, ‘0’ is assigned as an AID assignment beacon offset, and ‘2’ is assigned as an AID assignment beacon interval. Therefore, the STA 1   921  performs a power save mode operation at a time of receiving the AID assignment message according to a TIM element of a next transmitted beacon frame. 
     The STA 1   921  receives a first beacon frame (step S 912   a ). Since the first beacon frame is a beacon frame effective for the STA 1   921 , the STA 1   921  determines whether there is buffered traffic for the STA 1   921  by using the TIM element included in the beacon frame. If the TIM element includes STA identifying information indicating ‘10’ as an AID of the STA 1   921 , the STA 1   921  receives a data frame for the buffered traffic from the AP  910  (step S 912   b ). Although a second beacon frame is transmitted by the AP  910 , since a beacon interval of the STA 1   921  is 2, the beacon frame is not an effective beacon frame. Therefore, the STA 1   921  does not use the TIM element of the beacon frame. 
     The STA 1   921  receives a third beacon frame (step S 913 ). Although the third beacon frame is an effective beacon frame for the STA 1   921 , the TIM element may not include STA identifying information indicating ‘10’ as the AID of the STA 1   921 . In this case, the STA 1   921  knows that there is no buffered traffic, and performs the power save mode operation according to the TIM protocol. More specifically, the STA 1   921  can operate by transitioning to a doze state until a next beacon frame is transmitted. Since the fourth beacon frame is not an effective beacon frame, the STA 1   921  does not use the TIM element of the beacon frame. 
     The AP  910  transmits an AID assignment message to the STA 2   722  (step S 921 ). The STA 2   922  is assigned with ‘10’ as an AID by using the AID assignment message. Further, ‘0’ is assigned as an AID assignment beacon offset, and ‘2’ is assigned as an AID assignment beacon interval. Although the AID assigned to the STA 2   922  by the AP  910  overlaps with the AID assigned to the STA 1   921 , an offset and an interval can be used to prevent the STA 2   922  from using the same TIM element as that used in the STA 1   921 . 
     The STA 1   921  receives a fifth beacon frame (step S 914   a ). Since the fifth beacon frame is a beacon frame effective for the STA 1   921 , the STA 1   921  determines whether there is buffered traffic for the STA 1   921  by using the TIM element included in the beacon frame. If the TIM element includes STA identifying information indicating ‘10’ as the AID the STA 1   921 , the STA 1   921  receives a data frame for the buffered traffic from the AP  910  (step S 914   b ). On the other hand, since the fifth beacon frame is an ineffective beacon frame for the STA 2   922 , the STA 2   922  does not use the TIM element of the beacon frame. 
     The STA 2   922  receives a sixth beacon frame (step S 922 ). Although the sixth beacon frame is an effective beacon frame for the STA 2   922 , the TIM element may not include STA identifying information indicating ‘10’ as the AID of the STA 2   922 . In this case, the STA 2   922  knows that there is no buffered traffic, and performs the power save mode operation according to the TIM protocol. On the other hand, since the sixth beacon frame is an ineffective beacon frame for the STA 1   921 , the STA 1   921  does not use the TIM element of the beacon frame. 
     The STA 1   921  receives a seventh beacon frame (step S 731   a ). The beacon frame may be an AID assignment message including an AID assignment information element. In order to transmit a data frame for buffered traffic to the STA 1   921  at a higher speed, the AP  910  may assign another AID, which is not assigned in an overlapping manner, to the STA 1   921  in replacement of the AID  10  assigned in an overlapping manner. The STA 1   721  is assigned with ‘20’ as an AID by using the beacon frame. Further, ‘0’ is assigned as an AID assignment beacon offset, and ‘1’ is assigned as an AID assignment beacon interval. Since the STA 1   921  can determine whether there is buffered traffic to be transmitted to the STA 1   921  according to a beacon period interval, the buffered traffic can be received at a higher speed. 
     Since the beacon frame is an effective beacon frame before a new AID is assigned to the STA 1   921 , the STA 1   921  uses a TIM element of the beacon frame. Therefore, if STA identifying information of the TIM element indicates  10  as an AID, the STA 1   921  receives a data frame for buffered traffic from the AP  910  (step S 931   b ). On the other hand, since the seventh beacon frame is an ineffective beacon frame for the STA 2   922 , the STA 2   922  does not use a TIM element of the beacon frame. 
     The STA 1   921  and the STA 2   922  receive an eighth beacon frame (step S 932   a ). The eighth beacon frame is an effective beacon frame for both of the STA 1   921  and the STA 2   922 . Further, since the AID of the STA 1   921  is 20 and the AID of the STA 2   922  is 10, a data frame can be transmitted based on a typical TIM protocol. STA identifying information of a TIM element included in the beacon frame can be configured to indicate the AID  10  and the AID  20 . In this case, the STA 1   921  can receive a data frame from the AP  910  (step S 932   b ), and then the STA 2   922  can receive a data frame from the AP  910  (step S 932   c ). The STA 1   921  and the STA 2   922  can receive the data frame from the AP  910  according to an order which may vary depending on a response of the AP  910  with respect to a poll frame transmitted by the STA. 
     The STA 1   921  receives a ninth beacon frame (step S 933   a ). The ninth beacon frame is an effective beacon frame for the STA 1   921 , and a TIM element included in the beacon frame indicates that there is buffered traffic for the STA 1   921 . Thus, the STA 1   921  receives a data frame from the AP  910  (step S 933   b ). On the other hand, since the ninth beacon frame is an ineffective beacon frame for the STA 2   922 , the STA 2   922  does not use a TIM element of the beacon frame. 
     Meanwhile, a PPDU format generated in a PLCP sublayer in a WLAN system supporting M2M is provided according to  FIGS. 11 and 12 . 
       FIG. 11  is a block diagram representing a PPDU format for SU transmission in WLAN system supporting M2M according to an embodiment of the present invention. 
     Referring to the  FIG. 11 , SU-PPDU for SU transmission  1100  includes STF  1110 , LTF 1   1120 , SIG field  1130 , a plurality of LTFs  1140  and data field  1150 . The STF  1110  is allocated to 2 OFDM symbols. The LTF 1   1120  is allocated to 2 OFDM symbols. The SIG field  1130  is allocated to 2 OFDM symbols. Each of the plurality of LTFs  1140  is allocated each OFDM symbol. 
     Instead of each guard interval (GI) in a each OFDM symbol for LTF, the LTF 1   1120  may includes double guard interval (DGI)  1121  and two long training symbols  1122 ,  1123  in time-domain into which two long training in a frequency-domain is transformed. The DGI is inserted as cyclic prefix (CP), and length of the DGI is two times that of the GI. 
       FIG. 12  is a block diagram representing a PPDU format for MU transmission in WLAN system supporting M2M according to an embodiment of the present invention. 
     Referring the  FIG. 12 , MU-PPDU for MU transmission  1200  includes STF  1210 , LTF 1   1220  SIGA field  1230 , MU-AGC field  1240 , a plurality of MU-LTFs  1250 , SIGB field  1260  and data field  1270 . The STF  1210  is allocated two OFDM symbols. The LTF 1   1220  is allocated two OFDM symbols. The SIGA field  1230  is allocated two OFDM symbols. Each of the plurality of MU-LTFs  1250  is allocated to one OFDM symbol. The SIGB field  1260  is allocated one OFDM symbol. The LTF 1   1220  may include DIG  1221  and two LTSs  1222 ,  1223 . 
     The SU-PPDU  1100  in  FIG. 11  and the MU-PPDU  1200  in  FIG. 12  respectively include STF  1110 ,  1210  and LTF 1   1120 ,  1220 . The corresponding fields implement a function which is similar to that of HT-green field (GF)-STF and HT-LTF 1  in HT-GF PPDU, which enable a STA supporting high throughput HT in HT WLAN system to acquire data by receive and demodulate the PPDU. The HT-GF-STF is used for frame timing acquisition and automatic gain control (AGC) by the HT STA. The HT-LTF 1  is used for channel estimation for demodulating a SIG field and DATA. 
     In spite of receiving the HT-GF PPDU, 
     a Legacy STA, unable to support HT, can&#39;t demodulate and decode the HT-GF PPDU. As shown in the  FIGS. 11 and 12 , in sub 1 GHz band, in case of reusing a PPDU format, based on an OFDM defined in a present HT WLAN system standard, by down-clocking, a present OFDM symbol duration is increased by a multiple number for the down-clocking. It occurs that the OFDM symbol duration at time axis is considerably increased. For an example, down-clocking by 1/10 occurs that the OFDM symbol duration is increased by 10 times. If an OFDM symbol duration is 4 μs in a present WLAN system, the OFDM symbol duration is increased to 40 μs in a sub 1 GHz band. If clock speed is decreased by the down-clocking, an inaccuracy of Time Synchronization Function TSF timer is increased. Furthermore, it is more difficult for a STA, entering an awake state after operating in a doze state for a long time, to maintain timing synchronization with an AP. 
     Meanwhile, if a doze period of the STA increases, a problem may arise in that timing synchronization does not match between the AP and the STA. Especially, in an embodiment in which a period of a beacon frame that uses a specific AID for each STA is assigned differently for each terminal when the AP assigns the AID, it is assumed that correct timing synchronization is maintained between the AP and the STA. Therefore, a general procedure for highly accurate timing synchronization between the AP and the STA is required. 
     In a case where a time-synchronization function (TSF) timer of the STA has an accuracy of +/−0.01%, if the STA has a doze period of 1000 s, then a TSF timer error of the STA becomes +/−100 ms. If a beacon interval of the AP is 100 ms, there is a case where the TSF timer error of the STA is greater than a beacon interval. In such an environment, it may be difficult to normally perform a power save mode operation method in which a period of using the beacon frame is applied differently for each terminal. 
     In low cost devices such as a sensor node, an accuracy of the TSF timer is low, and also battery capacity is also low. By considering the low cost devices, a method for acquiring information capable of determining a next operation state by using a request/response frame such as polling is proposed, instead of determining by the STA an operation state by using the TIM element of the beacon frame. 
     In order to know whether there is buffered traffic for the STA, the STA in the doze state transitions to an awake state at any time and transmits a traffic indication request frame to the AP. The traffic indication request frame is transmitted irrespective of a beacon interval of the AP, and can be transmitted after acquiring a channel access right on the basis of a CSMA/CA mechanism by a transmitter. 
       FIG. 13  shows a traffic indication request frame according to an embodiment of the present invention. 
     Referring to  FIG. 13 , a traffic indication request frame  1300  includes a frame control field  1310 , a duration field  1320 , a receiver address (RA) field  1330 , a transmitter address (TA) field  1340 , an AID field  1350 , and a frame check sequence (FCS) field  1360 . 
     The frame control field  1310  includes information for interpreting the traffic indication request frame  1300 . The duration field  1320  includes information indicating a length of the traffic indication request frame  1300  and information indicating a time for exchanging a request/response frame. The RA field  1330  includes MAC address information of an AP that receives the traffic indication request frame  1300 . The TA field  1340  includes MAC address information of an STA that transmits the traffic indication request frame  1300 . When an effective AID is assigned by the AP to the STA that transmits the traffic indication request frame  1300 , the AID field  1350  is configured to indicate the AID. The FCS field  1360  includes information used to determine whether the traffic indication request frame  1300  is a normal frame. 
     Upon receiving the traffic indication request frame, the AP reports whether there is buffered traffic for the STA. For this, the traffic indication response frame is transmitted to the STA.  FIG. 14  shows a traffic indication response frame according to an embodiment of the present invention. 
     Referring to  FIG. 14 , a traffic indication response frame  1400  includes a frame control field  1410 , a duration field  1420 , an RA field  1430 , a TA field  1440 , an AID field  1450 , a timestamp field  1460 , a traffic type field  1470 , a TCLAS field  1480 , and an FCS field  1490 . 
     The frame control field  1410  includes information for interpreting the traffic indication response frame  1400 . The duration field  1420  includes information indicating a length of the traffic indication response frame  1400  or information indicating a time for exchanging a request/response frame. The RA field  1430  includes MAC address information of an STA that receives the traffic indication response frame  1400 , and this may be a MAC address of an STA that transmits the traffic indication request frame  1300 . The TA field  1440  includes MAC address information of an AP that transmits the traffic indication response frame  1400 . 
     If there is no buffered traffic for the STA that transmits the traffic indication request frame  1300 , the AID field  1450  can be set to a specific value (e.g., 0 or 65535) other than an AID of the STA. If there is buffered traffic for the STA, the AID field  1450  includes information indicating an AID already assigned to a corresponding terminal. In a case where the AID is dynamically assigned as described in the previous embodiment of the present invention, if the STA has no effective AID used to receive a data frame for the buffered traffic from the AP, a new AID is assigned to the STA, and the AID field  1450  may include information indicating the AID. 
     The timestamp field  1460  includes information for timing synchronization between the AP and the STA. 
     The traffic type field  1470  includes information for reporting a type of buffered traffic. Examples of the traffic type may include emergency, real-time, best-effort, background, etc. 
     The TCLAS field  1480  includes information for providing more detailed traffic information. 
     As such, the request frame/response frame may be a control frame or a management frame. When it is transmitted in a control frame format, the response frame can be transmitted at the elapse of interframe space (SIFS) without contention after transmitting the request frame. 
     The aforementioned embodiment of the present invention corresponds to a method for decreasing power consumption of an STA in an IEEE 802.11-based WLAN system that uses a frequency band of 1 GHz or lower. In this case, if an interval in which the STA transitions from the doze state to the awake state increases, a problem occurs in that an error of a TSF timer increases. 
     In a case where an accuracy of the TSF timer of the AP is +/−0.01%, if the STA has a doze period of 1000 s, an error of the TSF timer of the AP becomes +/−100 ms. This value is determined without considering a TSZF timer error of the STA. 
     In a case where the TSF timer error of the AP is +/−0.001%, if the STA has a doze period of 1000 s, then the TSF timer error of the AP becomes +/−10 ms. 
     In order for the STA to receive a beacon frame transmitted by the AP, the STA has to transition to the awake state prior to a target beacon transmission time (TBTT) by 100 ms or 10 ms. 
     If the STA knows TSF timer accuracy information of the AP, the AP&#39;s TSF timer error generated during a doze period can be calculated, and unnecessary power consumption can be decreased by transitioning to the awake state at a more correct time. On the other hand, if there is no information on TSF timer accuracy, the STA needs to determine a time of transitioning to the awake state by considering a minimum requirement for the TSF timer accuracy. 
     In order for an AP to report information on accuracy of a TSF timer of the AP itself to STAs, it is proposed a method of transmitting a frame including a TSF timer information element described below. 
       FIG. 15  shows a TSF timer accuracy information element. The TSF timer information element can be included in a beacon frame, a probe response frame, an association request frame, an association response frame, a re-association request frame, a re-association response frame, and/or a traffic indication response frame. 
     Referring to  FIG. 15 , a TSF timer information element  1500  includes an element ID field  1510 , a length field  1520 , a timestamp field  1530 , a TSF timer accuracy field  1540 , and a TSF timer error limit field  1550 . The element ID field  1510  indicates that an information element included therein is the TSF timer information element  1500 . The length field  1520  indicates a length of the TSF timer information element  1500 . 
     The timestamp field  1530  indicates a current timestamp value of a TSF timer. 
     The TSF timer accuracy field  1540  indicates a margin of error for the TSF timer in a unit of parts per million (PPM). For example, if an accuracy of the TSF timer is 100 PPM, it indicates an error of +/−0.01%, and a TSF timer value after 1000 s may have an error of +/−100 ms. If an AP corrects the TSF timer error after a specific time elapses, a margin of the TSF timer error does not exceed a specific value. For example, the AP can correct the TSF timer by using an external time source. Examples of the external time source may include a network time protocol (NTP), a global positioning system (GPS), etc. 
     A TSF timer error limit field  1550  indicates a threshold for a TSF timer error range. When the field is set to 10 ms, the TSF timer error of the AP cannot exceed +/−10 ms. If the TSF timer error of the AP exceeds the range of +/−10 ms, the AP can correct the error at a specific time point from an external source. That is, since the STA can know a threshold of a maximum TSF timer error range of the AP by using the TSF timer error limit field  1550 , an early wakeup time can be calculated by using the threshold in order not to lose a frame such as a beacon frame. 
     Meanwhile, in order for the AP to report TSF timer accuracy information of the AP itself to the STAs, it is proposed a method of transmitting a revised timestamp field by inserting it into a beacon frame, a probe response frame, an association response frame, a re-association response frame, etc. 
     Since the time stamp field is a field other than an information element, the field itself is transmitted by being included in the beacon frame or the like in general. In a current WLAN system, 8 octets are assigned to the time stamp field, thereby being able to express 264 states in total. In addition, each state is used in such a manner that the state is incremented by one for every 1 micro second (us). It can be indicated by generating a time stamp value during 264 us:=18 trillion sec:=584,942 years. That is, by considering a fact that bits allocated to the time stamps do not have to be excessively long as descried above, the present invention proposes a method of utilizing some MSB or LSB bits of the time stamp field having a size of 8 octets as a part including the TSF timer accuracy information. 
       FIG. 16  shows a revised time stamp field format according to an embodiment of the present invention. 
     Referring to  FIG. 16 , a revised time stamp field  1600  includes a TSF timer accuracy sub-field  1610  and a time stamp sub-field  1620 . In the bit sequence allocated to the conventional time stamp field having a size of 8 octets, MSB 3 bits are configured as the TSF timer accuracy sub-field. The revised time stamp field  1600  may have a size of 8 octets in total. The TSF timer accuracy sub-field  1610  may have a size of 3 bits. The time stamp sub-field  1620  may have a size of 61 bits. 
     Table 3 below can be used for reference as an example of encoding of the TSF timer accuracy sub-field  1610 . 
     
       
         
           
               
               
             
               
                 TABLE 3 
               
               
                   
               
               
                 Value 
                 Description 
               
               
                   
               
             
            
               
                 0 
                 TSF Timer Accuracy is not worse than +/−3 ppm 
               
               
                 1 
                 TSF Timer Accuracy is not worse than +/−6 ppm 
               
               
                 2 
                 TSF Timer Accuracy is not worse than +/−9 ppm 
               
               
                 3 
                 TSF Timer Accuracy is not worse than +/−12 ppm 
               
               
                 4 
                 TSF Timer Accuracy is not worse than +/−15 ppm 
               
               
                 5 
                 TSF Timer Accuracy is not worse than +/−18 ppm 
               
               
                 6 
                 TSF Timer Accuracy is not worse than +/−21 ppm 
               
               
                 7 
                 TSF Timer Accuracy is worse than +/−21 ppm 
               
               
                   
               
            
           
         
       
     
     A bit sequence having a size of remaining 61 bits is used as the time stamp sub-field  1620 . A time period that can be expressed by a sub-field having a size of 61 bits can be indicated by generating a unique time stamp value during a time period of 261 μs:=2 trillion sec:=73,117 years, thereby being able to perform a proper time stamp function. According to the aforementioned format, there is an advantage in that TSF timer accuracy information can be delivered without an additional overhead in the conventional WLAN system operation. 
     Meanwhile, the TSF timer accuracy sub-field  1610  can be configured with MSB 2 bits of a bit sequence constituting a time stamp field having the conventional size of 61 octets. In this case, Table 4 below can be used for reference as an example of encoding the TSF timer accuracy sub-field  1410 . 
     
       
         
           
               
               
             
               
                 TABLE 4 
               
               
                   
               
               
                 Value 
                 Description 
               
               
                   
               
             
            
               
                 0 
                 TSF Timer Accuracy is not worse than +/−5 ppm 
               
               
                 1 
                 TSF Timer Accuracy is not worse than +/−10 ppm 
               
               
                 2 
                 TSF Timer Accuracy is not worse than +/−15 ppm 
               
               
                 3 
                 TSF Timer Accuracy is worse than +/−20 ppm 
               
               
                   
               
            
           
         
       
     
     In addition, in an environment where an OFDM symbol duration becomes significantly long such as a WLAN system supporting M2M, a beacon frame is occupied by a wireless medium for a long time period to perform transmission. It is proposed to simplify the beacon frame to a great extent so that the simplified beacon frame is used to increase efficiency of the wireless medium. 
       FIG. 17  is a block diagram showing a format of a short beacon frame according to an embodiment of the present invention. 
     Referring to  FIG. 17 , a short beacon frame  1700  includes a frame control field  1710 , an SA field  1720 , a compressed SSID field  1730 , a time stamp field  1740 , a change sequence field  1750 , an Info field  1760 , and a CRC field  1770 . If there is a need to transmit additional information elements, the information elements can be additionally included. This format corresponds to a format in which fields for the respective information elements included in the existing beacon frame are compressed. 
     The time stamp field  1740  included in the short beacon frame  1700  can be configured by decreasing its size to 4 octets instead of the conventional size of 8 octets. If the time stamp field  1740  has a size of 4 octets, a time duration that can be expressed by a time stamp consisting of 32 bits in total can be indicated by generating a unique time stamp value during a time of 232 us:=4,295 sec:=72 min. 
     Instead of a unit of us, if a higher time unit is used as a time period expressed by one state of the time stamp, an absolute time consumed for one time stamp circulation can be further increased. The TSF timer accuracy sub-field can be implemented as shown in the example of  FIG. 16  with respect to the time stamp field of the short beacon frame  1700 . That is, in the time stamp field  1740  having a size of 4 octets, 3 bits or 2 bits can be assigned as the TSF timer accuracy sub-field, and a sequence of the remaining bits can be assigned to indicate a time stamp value. In this case, a bit sequence length for the time stamp itself is decreased to 4 octets, and thus the number of expressible states is significantly decreased. Therefore, it may be preferable to allocate MSB 2 bits to the TSF timer accuracy sub-field. 
       FIG. 18  is a block diagram of a wireless apparatus according to an embodiment of the present invention. 
     Referring to  FIG. 18 , a wireless apparatus  1800  includes a processor  1810 , a memory  1820 , and a transceiver  1830 . The transceiver  1830  transmits and/or receives a radio signal, and implements an IEEE 802.11 physical (PHY) layer. The processor  1810  functionally coupled to the transceiver  1830  is configured to transmit and receive an AID assignment message and a TIM element and to implement a MAC layer and/or a PHY layer for implementing the embodiment of the present invention shown in  FIG. 2  to  FIG. 17  in which a data frame is transmitted and received based on information included in the TIM element. The processor  1810  can be configured to interpret the AID assignment message to confirm an AID assigned to the apparatus, and to acquire TCLAS information for the AID. Further, the processor  1810  can be configured to receive information for timing synchronization, to calculate timing of transitioning to an awake state on the basis of the information, and to operate according to the timing. 
     The processor  1810  and/or the transceiver  1830  may include an application-specific integrated circuit (ASIC), a separate chipset, a logic circuit, and/or a data processing unit. When the embodiment of the present invention is implemented in software, the aforementioned methods can be implemented with a module (i.e., process, function, etc.) for performing the aforementioned functions. The module may be stored in the memory  1820  and may be performed by the processor  1810 . The memory  1820  may be located inside or outside the processor  1810 , and may be coupled to the processor  1810  by using various well-known means.