Abstract:
A wireless data transmitting method through wireless network communications using an access point in an infrastructure mode including a contention-free period. The method includes the access point being set up to operate in the contention-free period, a first data frame including channel occupation information of a second data frame in order to occupy a channel when the access point transmits the first data frame to a designated wireless network apparatus, the wireless network apparatus transmitting an acknowledgement frame of the receipt of the first data frame to the access point, and the access point being released from the contention-free period.

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
       [0001]     This application claims the priority from Korean Patent Application No. 10-2004-0067925 filed on Aug. 27, 2004, the disclosure of which is incorporated herein in its entirety by reference.  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     Apparatuses and methods consistent with the present invention relate to wireless transmission of data. More particularly, the present invention relates to a method for a coordinator to transmit large amounts of multimedia data to a specific multimedia player in a coordinator-based wireless network.  
         [0004]     2. Description of the Related Art  
         [0005]     Recent developments in communications and network technologies have changed the network environment from wired networks using wired media such as coaxial cables or optical cables to wireless networks using signals in various frequency bands. Accordingly, mobile computing apparatuses comprising wireless network interface modules and performing a specific function by processing a variety of data (hereinafter referred to as “wireless networking apparatuses”) have been developed. In addition, wireless network technologies for efficient communication between wireless network apparatuses have also been introduced.  
         [0006]     Wireless networks can be classified into two types. One type is a wireless network comprising an access point, as illustrated in  FIG. 1 , which is referred to as an “infrastructure mode wireless network”, and the other type is a wireless network without an access point, as illustrated in  FIG. 2 , which is referred to as a “wireless network of ad-hoc mode.” In the wireless network depicted in  FIG. 2 , wireless network devices  200   a ,  200   b , and  200   c  are connected to each via the wireless network without the access point coordinating them.  
         [0007]      FIG. 1  depicts an access point  100  coordinating a number of wireless devices  101   a ,  101   b , and  101   c . Moreover, the access point  100  is connected to devices  102   a  and  102   b  of a wired network. In the infrastructure mode, an access point manages a schedule regarding data transmission, so as to connect a wireless network to a wired network or perform communications between wireless network apparatuses belonging to a wireless network. Accordingly, all the wireless network apparatuses can transmit and receive data through a channel allocated by the access point. Hereinafter, the type of network in which a coordinator such as the access point is present will be referred to as a “coordinator-based network.” 
         [0008]     Where a plurality of wireless network apparatuses exist in the coordinator-based network, the coordinator may transmit data to and receive data from each wireless network apparatus by use of a variety of methods for media access.  
         [0009]     The recent increasing demand for transmission of large amounts of multimedia data has prompted research on effective data transmission over wireless local area networks (WLANs), which will be described based on the IEEE 802.11 specification (ISO/IEC 8802-11:1999(E) ANSI/IEEE Std 802.11, 1999 edition), incorporated herein by reference, that defines standards for wireless LANs. In IEEE 802.11 specification, the wireless network apparatus is referred to as a “station” and the coordinator is referred to as a “point coordinator.” 
         [0010]     To transmit a variety of multimedia data, improvement in performance of the wireless LAN can be accomplished by two methods. One method is to guarantee the QoS (Quality of Service) of a MAC (Media Access Control) level to transmit data within a given period of time based on the current wireless LAN where a single channel is shared by a plurality of stations. The IEEE 802.11 e group is making efforts to unify the standards to improve the QoS. The other method is to increase the bandwidth by allowing stations to use multiple channels rather than a single channel in a BSS (Basic Service Set) and to secure a physical channel.  
         [0011]     In the conventional IEEE 802.11 MAC protocol, several nodes share a channel through a mechanism known as a carrier sense multiple access/collision avoidance (CSMA/CA). To share a channel, a distributed coordination function (DCF) using a random backoff algorithm in order to decrease the probability of collision can be used. Another mechanism that can be used to share a channel is a point coordinator function (PCF) that designates the sequence of stations that use the channel based on a polling schedule, in which an access point (AP) serves as a point coordinator.  
         [0012]     In IEEE 802.11 ad-hoc mode, since there is no AP to coordinate and manage nodes, the channel can be shared using the DCF mechanism. In contrast, in IEEE 802.11 infrastructure mode, since an AP serves as a point coordinator, the PCF mechanism (whereby a channel can be used without contention) can be used in addition to the DCF mechanism.  
         [0013]      FIG. 3  illustrates transmission of data between stations according to DCF rules. A transmitting station STA 1   310  sends a Request to Send (RTS) frame  311  to a receiving station STA 2   320  before transmitting data, and ascertains whether the receiving station STA 2   320  present in the same BSS is available for receiving data. Since the STA 2   320  can receive the frame, it sends a Clear to Send (CTS) frame  321 , which is a control frame, to the STA 1   310 , in order to represent that data  312  may be transmitted because the receiving station STA 2   320  is available to receive data. When the STA 1   310  receives the CTS frame  321 , the STA 1   310  transmits the data  312  to the STA 2   320 . In this chain of processes, the stations other than the STA 1   310  and STA 2   320  that exist in the same BSS such as a station STA 3   330 , set their network allocation vector (NAV) and do not transmit data, assuming that the channel is busy during the periods  331  and  332 .  
         [0014]      FIG. 4  illustrates transmission of data between stations according to the rules of the PCF. Generally, the PCF is used with the DCF. A DCF area starts where a PCF area ends, thereby constituting as a whole a single repeated area (a contention-free period (CFP) repeated). In  FIG. 4 , D 1 , D 2  and the like refer to frames transmitted from a point coordinator, and U 1 , U 2  and the like refer to frames transmitted from stations having received polls. A contention-free period (CFP) observing PCF rules begins when the point coordinator first transmits a beacon. A polling operation that the point coordinator present in an AP uses to inquire whether any stations have data to transmit is progressed in a round-robin manner by each station. When the point coordinator conducts a poll, the station having received the poll transmits data and an acknowledgement (ACK) to the point coordinator. Then, the point coordinator transmits the data and the ACK to the target station, and polls the station to receive data. The station having received the poll transmits another ACK to the point coordinator, along with any data to be transmitted. In this manner, data is received and transmitted between stations during a contention-free period. The contention-free period ends when the point coordinator begins transmitting a CF-End frame to the station. The NAV values in the beacon frame are set so that each station does not operate independently but is controlled by the point coordinator during the contention-free period.  
         [0015]     However, a problem can occur when large amounts of multimedia data are transmitted.  
         [0016]     As in the DCF mechanism, using a channel through a contention may cause a collision when data is transmitted between stations, and a binary random backoff algorithm used to prevent this collision may generate a delay for a considerably long period of time depending on the situation. Although it may be possible to maintain the transmission speed, multimedia data such as moving pictures for a high definition television (HTDV) may be unreliable.  
         [0017]     To obviate these shortcomings, the PCF mechanism has been proposed. However, in the PCF mechanism, the polling operations need to be conducted for all the stations according to the created polling list, even for station(s) that do not have data to transmit, thereby generating unnecessary overhead. Further, since it is difficult to make a schedule in proportion to the amount of data to be transmitted, because of the inherent property of polling, it may also be difficult to secure the quality of transmission of large amounts of multimedia data in an environment where several stations operate simultaneously. In addition, because of the inherent complexity of the PCF mechanism, and with the lack of unified standards, the PCF mechanism has not been implemented in commercial products, and stations operate only using the DCF mechanism, which is a common contention mechanism.  
         [0018]     Recently, the IEEE 802.11e specification (draft 6.0) has been proposed to supplement the weak QoS of the IEEE 802.11 wireless LAN specification.  
         [0019]     A method of improving QoS in IEEE 802.11e basically allows a point coordinator to manage the time to use a channel and the order in which nodes transmit data. That is, each node is allocated a priority according to the type of data to be transmitted, and the order of polling is determined according to the priority, or it is determined according to channel contention. Each node that wants to use the channel is allocated time to use the channel, which is known as a transmission opportunity (TXOP), from the point coordinator, and the node transmits data during this TXOP, thereby overcoming the problem that only one frame is transmitted in IEEE 802.11 standards and supporting the transmission of multiple frames. However, the method described in IEEE 802.11 e may increase complexity in transmitting data.  
       SUMMARY OF THE INVENTION  
       [0020]     The present invention has been proposed to solve the problems described above. Accordingly, an aspect of the present invention is to provide a mechanism for transmission of large-sized multimedia data, where the quality of transmission is secured more reliably, in an infrastructure mode wireless network environment, wherein a channel for data transmission is allocated by a coordinator.  
         [0021]     The present invention will not be limited to the technical aspect described above. Other aspects not described herein will be more definitely comprehended by those in the art from the following detailed description.  
         [0022]     According to an aspect of the present invention, there is provided a wireless data transmitting method through wireless network communications using an access point in an infrastructure mode including a contention-free period, comprising the access point being set up to operate in the contention-free period, a first data frame including channel occupation information of a second data frame in order to occupy a channel when the access point transmits the first data frame to a designated wireless networking apparatus, and the wireless network apparatus transmitting an acknowledgement frame on the first data frame to the access point. Then, the access point is released from the contention-free period. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0023]     The above and other features and advantages of the present invention will become more apparent by describing in detail illustrative, non-limiting embodiments thereof with reference to the attached drawings in which:  
         [0024]      FIG. 1  illustrates an infrastructure mode wireless network system;  
         [0025]      FIG. 2  illustrates an ad-hoc mode wireless network system;  
         [0026]      FIG. 3  illustrates data transmission between stations, according to conventional DCF rules;  
         [0027]      FIG. 4  illustrates data transmission between stations, according to conventional PCF rules;  
         [0028]      FIG. 5  illustrates data transmission between stations, according to an exemplary embodiment of the present invention;  
         [0029]      FIG. 6  is a flow chart illustrating an access point setting the value of the DurationID by referencing a CFP flag, according to an exemplary embodiment of the present invention;  
         [0030]      FIG. 7  illustrates channel occupation when a beacon frame is transmitted, according to an exemplary embodiment of the present invention;  
         [0031]      FIG. 8  is a flow chart illustrating the setting of a parameter to occupy a channel when a beacon frame is transmitted, according to an exemplary embodiment of the present invention;  
         [0032]      FIG. 9  illustrates a queuing structure to set a CFP flag, according to an exemplary embodiment of the present invention; and  
         [0033]      FIG. 10  is a flow chart to set a CFP flag, according to an exemplary embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS  
       [0034]     Subject matter of the embodiments will be covered by the detailed description and drawings of the present invention.  
         [0035]     Advantages and features of the present invention and methods of accomplishing the same may be understood more readily by reference to the following detailed description of the exemplary embodiments and the accompanying drawings. The present invention may, however, be embodied in many different forms and should not be construed as being limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete and will fully convey the concept of the invention to those skilled in the art, and the present invention will only be defined by the appended claims. Same reference numerals refer to analogous elements throughout the specification.  
         [0036]     Hereinafter, the wireless data transmitting method according to exemplary embodiments of the present invention will be described, referring to block diagrams and flow charts illustrated in the accompanying drawings. Combinations of each block of the flow chart diagrams and entire flow charts can be executed by computer program instructions. Since these computer program instructions may be installed on general purpose computers, special purpose computers or other programmable data processing equipment, any tool can be constructed so that the instructions performed through computers or processors of other programmable data processing equipment can generate means to perform functions described in the flow charts and/or blocks. These computer program instructions may be stored in computer usable/readable memories to implement the functions.  
         [0037]     The instructions stored in the computer usable or computer readable memories may be manufactured as products including instruction means to perform functions described in the flow charts and blocks. The computer program instructions may be installed on computers or other programmable data processing equipment. The instructions creating processes performed by computers by executing a series of operations on the computers or other programmable data processing equipment and operating the computers, or other programmable data processing equipment may also supply operations to perform the functions described in the flow charts and/or blocks.  
         [0038]     The present invention relates to the continued occupation of a wireless medium for data transmission and reception between specific wireless networking apparatuses, without performing frame fragmentation in a contention-free period of a channel for data transmission and reception that is managed by an access point, in an infrastructure mode wireless network environment. To describe the exemplary embodiment of the present invention more easily, the PCF mode of the IEEE 802.11 specification will be used by way of an example. In describing the exemplary, non-limiting embodiment of the present invention, the terms defined by the IEEE 802.11 specification will be used. A station (STA) controlling the PCF mode will serve as an access point, and the length of a data frame used to compute the value of a DurationID is equivalent to the size of the maximum data frame (2304 bytes), because the data size of the next frame to be transmitted cannot be known.  
         [0039]      FIG. 5  illustrates wireless data transmission according to an exemplary embodiment of the present invention.  
         [0040]     According to an upper layer or a state of the queue (described in greater detail with reference to  FIGS. 9 and 10 ), an access point determines to occupy its own channel and exclusively transmits real-time data such as audio/video streams to a specific station (STA). To transmit the real-time data, the access point sets a CFP flag to “on” and then enters a CFP mode. The CFP flag may be processed as a parameter to indicate whether the access point is in a PCF mode. The CFP flag may be stored in a memory within the access point.  
         [0041]     When the access point is in the CFP mode, the data frame after conversion does not follow the backoff procedure and is transmitted in the unit of SIFS (Short Interframe Space), and the DurationID in the header of the MAC frame is set, as described in greater detail below.  
         [0042]     When transmitting a first data frame (frame_ 1 ) to a station (STA), the access point sets its DurationID to be long enough to secure transmission of the next frame (frame_ 2 +2*Ack+3*SIFS), as if a frame is fragmented. By doing so, stations (STAs) cannot use the channel until transmission of the next data frame is completed. The access point can avoid contention, with holding complete ownership of the wireless medium. The durationID value may be channel occupation information for the access point to continuously occupy a channel.  
         [0043]     When the access point intends to occupy its own channel continuously, it sets its DurationID value in the MAC header as described above during the transmission of a data frame.  
         [0044]     In this process, other stations (STAs) determine, based on a NAV value, that the channel is still busy, and thus, they do not try to use the channel.  
         [0045]     When the access point makes a decision to go far out of the contention-free period, the access point sets the CFT flag to “off” (from “on”). After that, the access point sets the DurationID value only with time (Ack+SIFS) to receive an acknowledgement (Ack) on receipt of the last data frame, and then transmits the data frame.  
         [0046]     When receiving the acknowledgement (Ack) from a station (STA), the access point converts its mode to a contention period (CP) mode and follows the backoff procedure with respect to data frames to be transmitted after the conversion. Accordingly, the DurationID value is set to “Ack+SIFS.” 
         [0047]      FIG. 6  is a flow chart illustrating an access point setting the value of the DurationID by referencing a CFP flag, according to an exemplary embodiment of the present invention.  
         [0048]     First, an access point checks the CFP flag value to ascertain its value, S 610 .  
         [0049]     When the CFP flag value is set to “on,” the DurationID field of the MAC header is set to the value: dur(next frame+2*Ack+3*SIFS) at S 620  and S 640 . Dur(x) refers to the time consumed in transmitting (or processing) x. When the CFP flag is set to “off,” the DurationID field of the MAC header is set to: dur(ACK+SIFS), as detailed in IEEE 802.1 a at S 630  and S 640 . After setting the DurationID, the access point sets another field of the MAC header S 650 .  
         [0050]      FIG. 7  illustrates channel occupation when a beacon frame is transmitted, according to an exemplary embodiment of the present invention.  
         [0051]     A beacon frame refers to a frame periodically broadcasted by the access point for time synchronization in a relevant BSS of an infrastructure mode wireless network environment. To broadcast a beacon frame in the contention-free period, the access point first broadcasts the beacon frame, and then the next data frame. The access point should continuously maintain a channel, secured in advance—prior to broadcasting a beacon frame, so as to transmit the data frames that were not yet transmitted.  
         [0052]     As illustrated in  FIG. 7 , a NAV value set by frame_ 2  is: dur(frame_ 3 +2*Ack+3*SIFS). When an event of a target beacon transmission time (TBTT) occurs, after the STA_ 1  has received frame_ 2 , the access point broadcasts the beacon frame, instead of transmitting frame_ 3 .  
         [0053]     When frame_ 3  is to be transmitted, after having broadcasted the beacon frame, the remaining occupation time will be dur(Ack+α), where α=dur(frame_ 3  having the length of 2304 bytes)−dur(beacon frame having the maximum length of 500 bytes).  
         [0054]     Since the NAV value, that is, dur(Ack+α), is still effective in the current BSS, other stations (STAs) do not enter into the contention period to perform DIFS (Distribute Interframe Space) and Backoff, instead they wait until the NAV value becomes 0.  
         [0055]     However, there is no guarantee that the time consumed when an access point transmits frame_ 3  is always less than dur(Ack+α). If the NAV value becomes 0 before frame_ 3  reaches a destination station (STA), other stations (STAs) enter into the contention period, and may acquire a channel after the backoff.  
         [0056]     To prevent this phenomenon, the access point first checks its CFP flag in order to transmit a beacon frame. When the CFP flag is “on,” the access point sets dur(frame_ 4 +2*Ack+3*SIFS) in the CFP Max Duration field of the beacon frame to thereby compensate the NAV value consumed for transmission of the beacon frame.  
         [0057]     Transmission of frame_ 3  to STA_ 1  is guaranteed during dur(frame_ 4 +2*Ack+3*SIFS) set in the beacon frame, and stations in the current BSS are controlled by the Nav_ 3  value transmitted by frame_ 3 , and thus, the access point can continuously occupy the channel.  
         [0058]      FIG. 8  is a flow chart to set a parameter in order to occupy a channel when a beacon frame is transmitted, according to an exemplary embodiment of the present invention.  
         [0059]     At first, when an event of TBTT occurs, the access point checks the value of the CFP flag S 810 . If the value of CFP flag is “on,” the access point generates a CF parameter set element to be set in a payload of the beacon frame S 820 . Thereafter, the CFP Max Duration field in the generated CF Parameter Set element is set to the value: dur(next frame+2*Ack+3*SIFS) S 830 . The access point sets a value of another field required by the beacon frame S 840 , and broadcasts the beacon frame S 850 . When the value of the CFP is “off” in step S 820 , the access point directly moves to step S 840 , skipping steps S 820  and S 830 .  
         [0060]     There may be various methods whereby the access point sets the value of the CFP. However, a method of setting the CFP flag using a queue memory structure will be described below.  
         [0061]      FIG. 9  illustrates a queuing structure to set a CFP flag according to an exemplary embodiment of the present invention, and  FIG. 10  is a flow chart to set a CFP flag according to an exemplary embodiment of the present invention. Hereinafter, the audio/video multimedia data stream will be referred to as an “AV data” and data other than the AV data will be referred to as an “IT data.” 
         [0062]     It is assumed that the value of the CFP flag is currently “on,” and the access point transmits the AV data to a specific station in the contention-free period S 1010  of  FIG. 10 .  
         [0063]     An OS (Operating System) of the access point receives data through its own medium or another medium, and ascertains the type of data. When the received data is the AV data, it is inserted into an AV queue  910  and when the received data is the IT data, it is inserted into an IT queue  920 . The AV queue  910  and the IT queue  920  may be implemented using software in the MAC layer.  
         [0064]     The OS of the access point or a module to manage the memory checks the state of the AV queue and then checks whether the amount of the AV data in the AV queue  910  is larger than the value of CFP_Enable_Threshold  930  of  FIG. 9 , as depicted in S 1020  of  FIG. 10 . When the value of CFP_Enable_Threshold  930  is too large or small, a transmission of the IT data may be cut off or movement between the AV queue  910  and the IT queue  920  may be frequent, and thus, an appropriate value of CFP_Enable_Threshold  930  may be experimentally determined.  
         [0065]     In step S 1020  of  FIG. 10 , when the amount of data in the AV queue  910  is larger than CFP_Enable_Threshold  930 , the CFP flag is set to “on” S 1050  and the AV data is output from the AV queue S 1060 .  
         [0066]     In step  1020 , when the amount of data in the AV queue  910  is smaller than CFP_Enable_Threshold  930 , the CFP flag is set to “off” S 1030  and the IT data is output from the IT queue S 1040 .  
         [0067]     Thereafter, the outputted data is transmitted to a station (STA) through the MAC layer (embodied with hardware as depicted in  FIG. 9 ). The OS of the access point receives data through its own medium or another medium and performs step S 1020  again S 1070 .  
         [0068]     The exemplary embodiment of the present invention is effective since the access point can transmit large amounts of multimedia data, without performing the frame fragmentation in the contention-free period. By continuously occupying a wireless medium of an infrastructure mode wireless network environment, the quality and reliability of the transmission can be secured.  
         [0069]     It will be understood by those of ordinary skill in the art that various replacements, modifications and changes in form and details may be made therein without departing from the spirit and scope of the present invention as defined by the following claims. Therefore, it is to be appreciated that the above described embodiment is for purposes of illustration only and not to be construed as a limitation of the invention.