Abstract:
A wireless network communication method and apparatus for enhancing a data transfer rate by using a direct link protocol (DLP) and multi channels during a point coordination function (PCF) period in wireless network communications in which an access point is employed in an infrastructure mode using both a contention-free period and a contention period. The wireless network communication method of the present invention including transmitting/receiving data among stations supporting a direct link, during a given duration, through the direct link using an independent channel; transmitting/receiving data among stations other than the stations supporting the direct link, during the duration, in a specific mode corresponding to the contention-free or contention period; switching the DLP stations to a primary channel after the given duration; and transmitting/receiving data among all stations including the DLP stations, during the remaining duration, in a specific mode corresponding to the contention-free or contention period.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     This application claims the priority of Korean Patent Application No. 10-2003-0056595 filed on Aug. 14, 2003, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference.  
         [0002]     1. Field of the Invention  
         [0003]     The present invention relates to a method of enhancing a transfer rate in wireless communications, and more particularly, to a wireless network communication method and apparatus for enhancing a data transfer rate by using a direct link protocol (DLP) and multi channels during a point coordination function (PCF) period in wireless network communications in which an access point (AP) is employed in an infrastructure mode using both a contention-free period and a contention period.  
         [0004]     2. Description of the Related Art  
         [0005]     Recently, as networks are increasingly being constructed in a wireless manner, and as requests for the transfer of large volumes of multimedia data continue to rise, there is a need for an effective transfer method in wireless local area networks (LANs). There are two methods for improving the performance of wireless LANs with regard to the transfer of various multimedia data. The first is a method of ensuring the quality of service (QoS) in a Media Access Control (MAC) level in order to effectively transmit data within a given time period over conventional wireless LAN schemes in which a plurality of stations share a single channel. In this regard, the IEEE 802.11e group makes an effort to unify standards for improving QoS. The second is a method of increasing bandwidth by allowing stations to physically acquire channels using multi channels rather than a single channel in a basic service set (BSS).  
         [0006]     A conventional IEEE 802.11 MAC protocol employs a carrier sense multiple access with collision avoidance (CSMA/CA) protocol in which a plurality of nodes share a single channel. The method of sharing a single channel includes a distributed coordination function (DCF) scheme in which a random back-off algorithm is employed to reduce collision probability. In addition, there is a point coordinator function (PCF) scheme in which an AP serving as a point coordinator is operated to specify a channel sequence of stations according to polling scheduling.  
         [0007]     In IEEE 802.11 ad-hoc mode, a channel can be shared among the nodes through contention in DCF mode since there is no AP for managing and controlling nodes. On the other hand, in IEEE 802.11 infrastructure mode, not only the DCF mode but also the PCF mode in which an AP serving as a point coordinator enables the use of a channel without contention can be used.  
         [0008]      FIG. 1  illustrates a process of transferring data among stations based on DCF rules. A sending station STA 1   110  sends a Request to Send (RTS) frame  111  to a receiving station STA 2   120  present in the same BSS before transferring data  112  to STA 2   120 , in order to determine whether STA 2   120  can receive data  112 . STA 2   120  sends a clear-to-send (CTS) frame  121 , i.e. a control frame, which notifies STA 1   110  that STA 2   120  can receive the data  112  and allows STA 1   110  to transfer the data. Then, the station STA 1   110  sends the data to STA 2   120 . In this process, Network Allocation Vectors (NAVs) are set up in the remaining stations STA 3   130  except for STA 1   110  and STA 2   120  present in the same BSS, and stations STA 3   130  do not send data by considering the channel as being busy during NAV periods  131  and  132 .  
         [0009]     Meanwhile,  FIG. 2  illustrates a process of transferring data among stations according to PCF rules. In general, such a PCF is used along with DCF. If a PCF period is completed, a DCF period is started. Both the PCF and DCF periods become a single repetition period. In this figure, D 1 , D 2 , and the like indicate frames sent by a point coordinator, while U 1 , U 2 , and the like indicate frames sent by each station that has received a poll. The point coordinator transmits a beacon, which initiates a contention-free period complying with the PCF rules. Polling through which the point coordinator asks whether a station has data to send is performed in a round-robin mode for each station. If the point coordinator performs the polling, a station that received the polling sends data and acknowledgement (ACK) to the point coordinator. Then, the point coordinator transmits the data and ACK to a station that will receive them and polls the station that will receive the data. The polled station sends an ACK together with data, if any, back to the point coordinator. In such a manner, data are transmitted/received among stations during the contention-free period.  
         [0010]     IEEE 802.11e has been proposed to supplement a wireless LAN standard that is weak in the provision of QoS, as in IEEE 802.11. In IEEE 802.11e, the AP basically manages channel use time and the transfer sequence of nodes to enhance QoS therein. That is, a priority is assigned to each node according to the type of data that each node will send, so that a polling sequence is determined based on priority. Otherwise, priority is determined through channel contention. Further, each node using a channel is assigned the channel use time called transmission opportunity (TXOP) by the AP channel, and transfers data during this period. Thus, a disadvantage that only a single frame was transmitted in the IEEE 802.11 standard can be overcome and multi-frame transmission can be supported.  
         [0011]     Even though network throughput was improved through the multi-frame transmission, there is a problem of network performance efficiency because the frames still pass through the AP in infrastructure mode. A direct link protocol (DLP) has been proposed to improve network performance through direct communication among the nodes without intervention of the AP. According to the DLP specified in IEEE 802.11e, stations perform data communication using an independent link without the intervention of the AP while transmitting/receiving data, in a case where the infrastructure mode is used in a BSS. Further, the DLP corresponds to a method of stably managing channels using the AP and allowing the maximum throughput to be provided by causing direct communications to be made among the stations. According to this DLP, since data does not have to pass through the AP while being transmitted, it is possible to enhance transfer efficiency by reducing transmission time, propagation time and AP MAC processing time.  
         [0012]     To perform communication using DLP, a DLP setup process is first required. This setup process will be now explained with reference to  FIG. 3 . QSTA- 1   310  that is a DLP requesting station sends a DLP request frame to an AP  320  (S 1   a ). At this time, the DLP request frame contains information on a data transfer rate, the capability of the station, and the like. Next, the AP simply forwards the DLP request frame to QSTA- 2   330  that is a receiving station (S 1   b ). QSTA- 2   330  confirms the DLP request frame received from the AP  320  and then transmits a DLP response frame, which contains information on whether to participate in a direct link  340 , to the AP  320  (S 2   a ). The DLP response frame contains information on the status code informing the results of the DLP request, the data transfer rate, the capability of the station, and the like. Finally, the AP  320  simply forwards the DLP response frame to QSTA- 1   310  (S 2   b ). A series of these four processes is called a four-handshake process of DLP. For reference, the structures of the DLP request frame and the DLP response frame in the related art are shown in  FIG. 4 .  
         [0013]     In conventional techniques by which a plurality of stations share a single channel, a critical point is how the plurality of stations efficiently share the maximum transfer rate of the single channel (e.g., 54 Mbps in case of 802.11a). In the transfer of large volumes of multimedia data, however, QoS cannot be adequately ensured by using only conventional technology. Accordingly, there have been developed many MAC algorithms in view of QoS so as to transfer data within a given period of time. DLP is one of these methods, which directly transfers data through a direct link without passing through an AP under the condition that peer to peer (P2P) communications should be made after a DLP is set up. Even through DLP is used, however, it is difficult to make use of the advantages of the direct link if contention is increased due to the presence of many stations in a BSS.  
         [0014]     Therefore, there is a need for a method that enables efficient communication as well as makes use of the advantages of DLP in a case where a plurality of stations are present in a wireless LAN. To this end, there is proposed a new mechanism for a method of using an independent DLP channel within a BSS in which PCF and DCF are used.  
       SUMMARY OF THE INVENTION  
       [0015]     The present invention addresses the aforementioned problems. An aspect of the present invention is to provide an apparatus and method for reducing contentions among stations using PCF and DCF.  
         [0016]     Another aspect of the present invention is to provide a compatible wireless environment in which stations operate either according to PCF or DCF rules by using a suitable independent direct link.  
         [0017]     A further aspect of the present invention is to provide a new DLP frame format necessary for a compatible wireless environment.  
         [0018]     Consistent with an aspect of the present invention, there is provided a wireless network communication method, which comprises (1) transmitting/receiving data among stations supporting a direct link, during a given duration, through the direct link using an independent channel; (2) transmitting/receiving data among stations other than the stations supporting the direct link, during the duration, in a specific mode corresponding to the contention-free or contention period; (3) switching the stations supporting the direct link to a primary channel after the given duration; and (4) transmitting/receiving data among all stations including the stations supporting the direct link, during the remaining duration, in a specific mode corresponding to the contention-free or contention period.  
         [0019]     Consistent with another aspect of the present invention, there is provided a communication station, which comprises a channel-switching module that switches an existing channel to an independent channel by writing a new channel number into a DLP request frame and a MAC frame-generating module that generates a predetermined MAC frame including the DLP request frame.  
         [0020]     Consistent with a further aspect of the present invention, there is provided an access point, which comprises a polling list-managing module that provides sequential polling to the stations based on a polling list, a channel list-managing module that manages a list of available channels through periodical channel condition analysis and allocates an independent channel to a station which perform communications through a direct link, a channel number-writing module that determines whether there are available channels based on the channel list and writes the available channels into a DLP request frame, and a point coordinator that receives frames to be sent to the DLP stations from stations present in a primary channel and performs buffering and management for the received frames.  
         [0021]     The present invention operates according to PCF/DCF of a BSS. In a case where the BSS uses only the DCF, a DLP performs a direct link to the BSS using the DLP and then contends with other stations in the BSS. If the DLP station has lost the contention, it does not wait for a NAV period but transmits and receives data to and from the DLP stations using an independent channel. Alternatively, if the DLP station has won the contention, the DLP station broadcasts a duration, which will be used to transmit and receive data among the DLP stations in the independent channel, to other stations and then transmits and receives the data through the independent DLP channel during the duration. During the duration (DLP NAV), other stations operate according to the DCF rules. After the duration (DLP NAV), the DLP stations also return to a primary channel and all the stations operate according to DCF rules.  
         [0022]     On the other hand, in the event that the BSS uses both PCF and DCF, the DLP stations communicate with one another via independent DLP channels in a PCF period and then again return to the primary channel. If the time point when the DLP stations return to the primary channel is within the PCF period, the DLP stations operate according to PCF rules during the remaining PCF period and operate according to DCF rules during the DCF period. Alternatively, if the time point when the DLP stations return to the primary channel is within the DCF period, the DLP stations operate according to the DCF rules since then. If there are any data to be sent among the DLP stations in a period when they operate according to the DCF rules, the DLP stations operate in the remaining DCF period according to the same manner as the case where the BSS uses only the DCF.  
         [0023]     The direct link communications in the present invention means a method for transmitting and receiving data directly among stations without passing through an AP in wireless communications in infrastructure mode using the AP. The direct link communications include communications using DLP specified in IEEE 802.11e. Hereinafter, communications using the DLP will be described as an example of the direct link communications. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0024]     The above and other aspects, features and advantages of the present invention will become apparent from the following description of exemplary embodiments given in conjunction with the accompanying drawings, in which:  
         [0025]      FIG. 1  illustrates a process of transferring data among stations according to DCF rules in the related art;  
         [0026]      FIG. 2  illustrates a process of transferring data among stations according to PCF rules in the related art;  
         [0027]      FIG. 3  illustrates a four-handshake process corresponding to a DLP setup process;  
         [0028]      FIG. 4  shows the structures of various DLP MAC frames in the related art;  
         [0029]      FIG. 5  is a block diagram illustrating the configuration of a DLP station for implementing an exemplary embodiment of the present invention;  
         [0030]      FIG. 6  shows the structures of various DLP MAC frames consistent with the present invention;  
         [0031]      FIG. 7  shows the structure of an association request frame;  
         [0032]      FIG. 8  is a block diagram illustrating the configuration of an AP for implementing an exemplary embodiment of the present invention;  
         [0033]      FIG. 9  is a flowchart illustrating a modified four-handshake process for implementing an exemplary embodiment of the present invention;  
         [0034]      FIG. 10  is a graph showing a data transfer process for each station with the passage of time in a state where only DCF is used;  
         [0035]      FIG. 11  is a flowchart illustrating the steps of the process shown in  FIG. 10 ;  
         [0036]      FIG. 12  shows a data transfer process in which a time point where a BSS returns to a primary channel after using a DLP channel that is within a PCF period, in a case where both PCF and DCF are used;  
         [0037]      FIG. 13  shows a data transfer process in which a time point where the BSS returns to the primary channel after using the DLP channel that is within a DCF period, in a case where both PCF and DCF are used;  
         [0038]      FIG. 14  is a flowchart illustrating the steps of the process shown in  FIG. 12 ; and  
         [0039]      FIG. 15  is a flowchart illustrating the steps of the process shown in  FIG. 13 . 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0040]     Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings.  
         [0041]      FIG. 5  shows the configuration of a DLP station for implementing the present invention. As shown in this figure, the DLP station  500  may comprise a MAC frame-generating module  510 , a channel-switching module  520  and a MAC frame-transmitting/receiving module  530 . The MAC frame-generating module  510  functions to generate a DLP request frame, a DLP response frame, a DLP probe frame, a DLP start frame, an association request frame and a data frame to be transmitted/received. The structures of the frames will be described later with reference to  FIGS. 6 and 7 .  
         [0042]     The channel-switching module  520  functions to switch a channel by writing a new channel number into a channel number field of the DLP request frame when it is necessary to switch from a primary channel to a new channel assigned by an AP for direct link communications, and vice versa.  
         [0043]     The MAC frame-transmitting/receiving module  530  functions to transmit/receive a variety of the frames generated in the MAC frame-generating module  510 .  
         [0044]      FIG. 6  shows the structure of a DLP MAC frame consistent with the present invention. As compared with the structure of the DLP MAC frame in the related art shown in  FIG. 4 , the external and general structure of the DLP MAC frame shown in  FIG. 6  is the same as shown in  FIG. 4 . A MAC header section consists of a frame control field, a duration/ID (Dur/ID) field, a destination address (DA) field, a source address (SA) field, a basic service set ID (BSSID) field, and a sequence control (Seq Ctrl) field. A subsequent frame body section has a variable length and contains information on frame category and variables. Codes representing various kinds of frames to be described later are written in this category. Field values contained in various frames are stored in the variables. Further, a frame check sequence (FCS) field has IEEE 32-bit Cyclic Redundancy Check (CRC) information.  
         [0045]     However, the kind of the category contained in the frame body section and the constituent fields of the DLP frame shown in  FIG. 6  are different from those shown in  FIG. 4 . Category  410  will be first considered. It can be seen that a “DLP start” field  413  indicating the DLP start frame has been added to the category. Next, the DLP start frame  450  may consist of a MAC address field  451  of a destination station (receiving station), a MAC address field  452  of a source station (sending station), and a channel number field  453  of a channel through DLP communications are made.  
         [0046]     The format of a DLP probe frame  440  is the same as a conventional one. This frame serves to check whether a direct link connection works well. This frame is not an indispensable one but an optional one.  
         [0047]     A DLP request frame  420  is a frame by which a sending station requests a direct link before it transmits/receives data to/from a receiving station. If the DLP request frame is sent to the AP, the AP forwards this frame to the receiving station. Fields added to a conventional DLP request frame include a channel number field  425  that determines a channel through which direct link communications will be made, and a duration field  426  that determines the duration of the connection state established through the direct link. When the sending station initially transmits the DLP request frame to the AP, it cannot know an available channel number. Thus, the channel number is assigned a “NULL” value. Then, the AP finds an available channel number and then writes the value of the channel number in the channel number field  425  before forwarding the DLP request frame to the receiving station.  
         [0048]     A DLP response frame  430  is a frame that is forwarded to the sending station by the AP when the receiving station receives the DLP request frame, determines whether to join the DLP direct link, and then transmits the DLP response frame to the AP. The results of determination on whether to join the direct link are shown in a status code field  431 . A field added to a conventional DLP response frame is a channel number field  437  containing the channel number allocated by the AP to the channel number field  425  of the DLP request frame. The sending station can know the channel number to be connected through the direct link by referring to the channel number field  437  of the DLP response frame. Accordingly, both the sending and receiving stations can communicate with each other through a single channel.  
         [0049]      FIG. 7  shows the structure of an association request frame. The association request frame  700  is constructed such that its head section includes a frame control field, a Dur/ID field, a DA field, an SA field, a BSSID field and a Seq Ctrl field, in the same manner as the DLP frame. The header section is followed by a frame body field  710  and a FCS field. Contrary to the DLP frame, the frame body field  710  consists of a capability information field  720 , a listen interval field, an SSID field and a supported rates field. Further, the capability information field  720  includes sub-fields each of which contains bit information (0 or 1). The sub-fields further includes a CF Poll Request field  730  and a DLP Capable field  740 .  
         [0050]     In a case where an infrastructure mode is used, a station becomes a member of a BSS through association and can thus perform communications within the BSS. The station requests the association by transmitting the association request frame  700  to the AP. Then, the AP gives a chance for each station to transmit data through polling. While requesting the association, the station sets a DLP Capable field added to implement the present invention, i.e. the bit  740  informing whether the station supports a DLP, as well as the bit  730  informing whether the station can receive a poll, i.e. whether the station is CF Pollable, as a value of “1” or “0”, into the capability information field  720  of the association request frame  700 . Then, the station informs the AP of the set results. Here, “1” indicates a TRUE value, and “0” indicates a FALSE value.  
         [0051]      FIG. 8  illustrates the configuration of an AP  800  for implementing the present invention. As shown in this figure, the AP  800  may comprise a channel list-managing module  810 , a polling list-managing module  820 , a channel number-writing module  830 , a point coordinator  840  and a MAC frame-transmitting/receiving module  850 .  
         [0052]     The polling list-managing module  820  manages a polling list table such as Table 1 to provide sequential polling. Here, a bit value of “1” indicates a TRUE value, while a bit value of “0” indicates a FALSE value.  
         [0053]     It is first determined from the polling list table whether a DLP is supported. Then, only when the DLP is supported, a channel use list is confirmed. Thus, if a DLP station uses a channel other than the existing channel, the polling is not performed.  
                           TABLE 1                                   Station   CF Pollable/DLP Capable                           STA1   1/1           STA2   1/0           STA3   1/1           STA4   1/0                      
 
         [0054]     The channel list-managing module  810  manages a list of available channels through periodical channel condition analysis and distributes the list. Since channels are limited resources, the AP cannot distribute channels without restriction. The following table shows an example of a list of available channels existing in the AP. In such a way, the channel list-managing module  810  can manage a list of channels used in the BSS, including the primary channel, according to channel number. The AP manages and distributes the available channels in the channel list, excluding the primary channel used in the BSS, according to the order of less noise based on the received signal strength indication (RSSI).  
                                   TABLE 2                                   Channel Number   Completion Time   Station List   RSSI                           CH1   Tch1   S1, S2   10           . . .    . . .    . . .    . . .              CHn       Tchn       S3, S4       5                        
 
         [0055]     The channel number-writing module  830  checks whether there are any distributable DLP channels when receiving a DLP request frame via the MAC frame-transmitting/receiving module  850 , and then writes the checked distributable DLP channel into the DLP request frame.  
         [0056]     If a frame that needs to be sent from another station to a DLP station is sent to the AP when the DLP station uses the other channel, the point coordinator  840  performs the buffering of the frame by considering the DLP station in the other channel as a sleeping station. Then, if the DLP station again uses the existing channels, the AP sends the buffered frame to the DLP station.  
         [0057]     The MAC frame-transmitting/receiving module  850  receives a data frame transmitted via a primary channel from a transmitting station and forwards the received data frame to a receiving station. Further, the MAC frame-transmitting/receiving module  850  forwards a DLP request frame received from a DLP sending station to a DLP receiving station and forwards a DLP request frame received from the DLP receiving station to the DLP sending station.  
         [0058]      FIG. 9  illustrates a modified four-handshake process of implementing the present invention. If there is a station that intends to transmit data through a direct link, a DLP sending station creates a DLP request frame and then transmits the DLP request frame to an AP (S 910 ). The AP periodically scans available channels and manages a list of the available channels. Upon distribution of the available channels, the AP distributes available channels except channels that are currently being used in a BSS. The AP writes one channel number of the available channels into the channel number field of the DLP request frame and then forwards the DLP request frame to a DLP receiving station (S 920 ). The DLP receiving station determines whether to receive the DLP request (S 930 ). Next, the DLP receiving station sends a DLP response frame including the determination results, to the AP (S 940 ). The AP forwards the DLP response frame to the DLP sending station (S 950 ). Finally, the DLP sending station checks the status of the DLP response, i.e., whether the DLP receiving station has rejected or accepted the direct link, based on the received DLP response frame (S 960 ).  
         [0059]      FIG. 10  shows a data transfer process for each station with the passage of time in a state where a BSS uses only DCF. If a station has lost contention against other stations in the BSS after the station joins the direct link using a DLP, the station does not wait for an NAV period but enhances the transfer rate in a DLP station by using a DLP channel. If the station does not transmit data to the DLP station but should communicate with other stations in the BSS, the station communicates with the other stations via a primary channel according to DCF rules. The other stations in the BSS also have more chances to use a channel since the chance of the DLP station to use the primary channel is reduced. On the other hand, if the DLP station has won the contention, the DLP station performs communications through the DLP channel without using the primary channel. The other stations in the BSS again contend with one another and comply with a basic contention algorithm of the DCF.  FIG. 10  shows both cases where the DLP station has won and lost the channel contention. This method is advantageous in that communications between the DLP stations and general stations in a BSS can be made, the advantages of the DLP can be utilized, and an overall channel efficiency in the BSS can also be enhanced.  
         [0060]      FIG. 11  is a flowchart illustrating the operating process when a BSS uses only a DCF. A four-handshake process as shown in  FIG. 9  is first executed (S 1100 ). Then, all stations contend with one another for a channel (S 1110 ). The process is divided into two cases where a DLP station has won or lost primary channel contention (S 1120 ). When the DLP station has won the channel contention, a receiving station may be either a DLP station that is connected through a direct link or a general station that is not connected through a direct link. For this reason, the case where the DLP station has won the channel contention will be divided into two cases according to whether the receiving station is a DLP station or not (S 1130 ).  
         [0061]     First, in the case where the DLP station has lost the primary channel contention, the sending station that has won the channel contention sends a RTS frame to a receiving station (S 1140 ) and the remaining stations except for the DLP station set up their NAV values (S 1141 ). During the period corresponding to the set NAV value, the DLP stations communicate with one another using a DLP channel (S 1142 ). The receiving station transmits a CTS frame to the sending station (S 1143 ). Then, the sending station transmits data to the receiving station (S 1144 ) and the receiving station sends an ACK frame to the sending station (S 1145 ).  
         [0062]     Second, in the case where the DLP station has won the primary channel contention and the receiving station is a DLP station, the DLP sending station first broadcasts a DLP start frame to inform all the remaining stations that DLP communication has started (S 1150 ). The remaining stations set up NAV values (hereinafter, referred to as “DLP NAV”) during the period that is reserved for communications by the DLP station and thus are in a state where communications cannot be made through the DLP channel (S 1151 ). The DLP stations communicate with one another using a DLP channel (S 1152 ). Meanwhile, since the primary channel is still empty, the remaining stations can contend with one another for the channel (S 1153 ).  
         [0063]     As a result of the contention, a sending station that has won the channel contention sends a RTS frame to a receiving station (S 1154 ). The stations other then the DLP sending/receiving stations and the sending/receiving stations established through the channel contention set up their NAV values (S 1155 ). Thereafter, the receiving station sends a CTS frame to the sending station (S 1156 ) and the sending station sends data to the receiving station accordingly (S 1157 ). Then, the receiving station transmits an ACK frame to the sending station (S 1158 ). During the period where the DLP NAV is set up, the above process of S 1153  to S 1158  is repeated (S 1159 ).  
         [0064]     Finally, in the case where a DLP station has won the primary channel contention and the receiving station is not a DLP station, the process is the same as the channel contention scheme of the general station other than the DLP station (S 1160  to S 1164 ).  
         [0065]     If desired data are completely transmitted in the last steps of the three cases, the process is terminated. If desired data are not completely transmitted, the process is repeated from the first step in which all the stations contend with one another for a channel (S 1170 ).  
         [0066]      FIGS. 12 and 13  show a data transfer process for each station with the passage of time in a case where a BSS uses both PCF and DCF. In particular,  FIG. 12  shows a data transfer process in which a time point when a station returns to the primary channel after using a DLP channel is within a PCF period and  FIG. 13  shows a data transfer process in which a time point when the station returns to the primary channel after using the DLP channel is within a DCF period. When the BSS uses both PCF and DCF, a DLP setup process, i.e. a DLP four-handshake process is first performed. Then, DLP stations exchange data with one another during a DLP NAV period. Such a DLP NAV period is determined by the value of the duration field  426  ( FIG. 6 ) that determines the DLP NAV period in the four-handshake process.  
         [0067]     During a Contention Free Period (CFP) period, an AP sequentially sends a poll from a polling list. At this time, if a station is not CF Pollable, the AP does not send a poll. If the station is CF Pollable, the AP checks whether the station is DLP Capable. If the station is DLP Capable, the AP checks a channel list of the AP and sends the poll to the station after confirming that the DLP station uses an existing primary channel other than a DLP channel. Therefore, when the DLP station uses the DLP channel, general stations have more chances to take a poll and thus to transmit data.  
         [0068]     The stations attempt to contend with one another for a channel according to PCF/DCF. According to the PCF, the AP transmits a beacon to all stations in a BSS every target beacon transmission time (TBTT) period. Further, as the beacon starts its broadcast, the PCF and DCF periods are performed in a super frame according to information contained in the beacon. The DLP NAV period, i.e. a period of communication through the DLP channel, is informed to all the stations through the beacon. During this period, the DLP stations are switched to DLP channels to exchange data with one another. At this time, a mechanism for switching the DLP station to an existing channel is determined by comparing the DLP NAV period with a CFP period (CFPDurRemaining) value of a beacon frame representing the CFP period. If the DLP NAV value is less than the CFPDurRemaining value, the DLP stations will be switched to the existing channel in the PCF period. However, if the DLP NAV value is greater than the CFPDurRemaining value, the DLP stations will switch to the existing channel in the DCF period.  
         [0069]     If the DLP stations are switched to the existing channel within the PCF period complying with the PCF rules as shown in  FIG. 12 , all the stations including the DLP stations comply with a PCF mechanism in which the stations receive polls from the AP and communicate with one another during the remaining PCF period. Then, during the DCF period, all the stations communicate with one another while contending with one another according to the DCF rules. Otherwise, they switch to DLP channels through the channel contention in a manner such as the case where only the DCF is used as shown in  FIGS. 10 and 11 , and then perform data communications.  
         [0070]     On the other hand, if the DLP stations are switched to the existing channel in the DCF period as shown in  FIG. 13 , all the stations communicate with one another while contending with one another according to DCF rules during the remaining DCF period. Otherwise, they are switched to the DLP channel through channel contention in a manner such as the case where only DCF is used as shown in  FIGS. 10 and 11 , and then perform data communications.  
         [0071]      FIG. 14  is a flowchart illustrating the operating process in which DLP stations are switched to an existing primary channel in a PCF period in a state where a BSS uses both PCF and DCF. In the PCF period, the DLP stations are switched to an independent channel during a DLP NAV period according to a beacon indicating the start of a super frame. If the DLP NAV period is ended, all the stations operate according to a PCF polling mode during the remaining PCF period. Thereafter, during the DCF period, the stations switch to a DLP channel through channel contention and then perform data communications, in the same manner as the case where only DCF is used (refer to  FIGS. 10 and 11 ).  
         [0072]     A four-handshake process such as shown in  FIG. 9  is first performed (S 1400 ). Then, DLP stations perform synchronization for channel switching through a beacon. The DLP stations switch to an independent DLP channel and then perform data communications (S 1410 ). The channel switching process corresponds to a process in which the channel-switching module  520  ( FIG. 5 ) switches the DLP station to a channel allocated by the channel list-managing module  810  ( FIG. 8 ) of the AP. The period during which data are transmitted/received via the DLP channel among DLP stations corresponds to the duration  426  ( FIG. 6 ) written into the DLP request frame.  
         [0073]     In the PCF period, the AP causes the polling list-managing module  810  ( FIG. 8 ) to determine a polling sequence and whether it polled the stations, based on a polling list. The polling list-managing module  810  ( FIG. 8 ) finds out whether a station associated through the CF Pollable bit  730  of the association request frame  700  ( FIG. 7 ) can receive a poll and whether the associated station can use a DLP through the DLP Capable bit  740 , and then writes the results into the polling list.  
         [0074]     The polling list-managing module scans the polling list (S 1420 ) and determines whether a relevant station can use a DLP (i.e., “DLP Capable”) (S 1430 ). If it is determined in S 1430  that the relevant station can use DLP, the module will determine whether the relevant station exists in a primary channel (S 1440 ). If the relevant station exists in the primary channel, the AP transmits a poll frame to the relevant station (S 1450 ). A relevant station that receives the poll sends a data frame to the AP, which in turn forwards the received data frame to a receiving station (S 1460 ). In such a case, a station that receives the poll frame or data frame sends an ACK frame to a sending station so that it can be confirmed whether the poll frame or data frame has been correctly received. If it is determined in S 1440  that a relevant station is not present in the primary channel, the AP does not poll the relevant station because the station uses an independent DLP channel.  
         [0075]     If it is determined in S 1430  that the relevant station cannot use DLP, the AP determines whether the relevant station can receive the poll (i.e., “CF Pollable”) (S 1431 ). If the relevant station cannot receive the poll, the AP does not poll the relevant station. Meanwhile, if the relevant station can receive the poll, the above steps S 1450  and S 1460  are performed for the relevant station. Then, the steps S 1420  to S 1460  are repeated until the PCF period is ended (S 1470 ). If a DCF period is started after the PCF period is ended, it is determined whether there are any data to be sent among DLP stations (S 1480 ). If there are data to be sent among DLP stations, the same operation as the case where only the DCF is used ( FIGS. 10 and 11 ) is performed (S 1490 ). If there are no data to be sent, all the stations operate while contending with one another according to common DCF rules (S 1491 ).  
         [0076]      FIG. 15  is a flowchart illustrating the operating process in which DLP stations are switched to an existing primary channel in a DCF period in the case where a BSS uses both PCF and DCF. In the PCF period, DLP stations are switched to an independent channel during a DLP NAV period according to a beacon indicating the start of a super frame. During the remaining DCF period, the stations switch to a DLP channel through the channel contention as in the case where only the DCF is used ( FIGS. 10 and 11 ) and perform data communications. The steps S 1500  to S 1560  of  FIG. 15  are the same as the steps S 1400  to S 1460  of  FIG. 14 . However, it is determined after the step S 1560  whether the duration of DLP communications specified in the four-handshake process has expired (S 1570 ).  
         [0077]     Until the duration of DLP communications has expired, the steps S 1520  to S 1560  are repeated. On the other hand, if it is determined that the duration of DLP communications has expired, all the stations operate according to a common PCF polling mode during the remaining PCF period (S 1580 ). Then, it is determined in the DCF period whether there are any data to be sent among DLP stations (S 1591 ). If there are data to be sent among DLP stations, the same operation as the case where only the DCF is used ( FIGS. 10 and 11 ) is performed (S 1592 ). If there are no data to be sent, all the stations operate while contending with one another according to common DCF rules (S 1593 ).  
         [0078]     Consistent with the present invention, there is an advantage in that compatible wireless environments can be provided such that stations use either a DCF or PCF or an independent direct link suitable for their operating conditions.  
         [0079]     Further, there is another advantage in that high bandwidth can be obtained by reducing contentions among the stations using DCF and increasing the chances to take a poll among the stations using PCF.  
         [0080]     In addition, there is a further advantage in that QoS can be enhanced since a stable throughput can be ensured when P2P communications are needed among stations in a BSS.  
         [0081]     Although the embodiments of the present invention have been described with reference to the accompanying drawings, it can be understood by those skilled in the art that the present invention can be implemented in the other specific forms without modifying or changing the technical spirit and essential features thereof. Therefore, it should be understood that the aforementioned embodiments are not restrictive but illustrative in all aspects. The scope of the present invention should be defined by the appended claims, and all changes or modifications made from the spirit and scope of the invention and equivalents thereof should be construed as falling within the scope of the invention.