Abstract:
A power saving method in a wireless LAN system is disclosed for estimating at a terminal station whether or not data destined thereto is buffered in a base station, and temporarily stopping a downlink data receiving operation when it is estimated that no such data is buffered, thereby preventing the consumption of a bandwidth for needless wireless frame sequences. Upon receipt of a beacon management frame Beacon (TIM) from the base station, the terminal station stores TIM information in a memory. The TIM information indicates whether or not downlink data destined to the terminal station is buffered in the base station (indicating that the data is buffered when it is at “1” and that the data is not buffered when it is at “0”). At a PS-Poll transmission timing, the terminal station references the memory and stops the transmission of PS-Poll when the two latest values of TIM are “00.”

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to a power saving method in a wireless LAN system.  
         [0003]     2. Description of the Related Art  
         [0004]     In a wireless LAN system which operates asynchronously to a beacon management frame and has a power saving function, a terminal station can receive downlink data at an arbitrary timing without being fixed at a beacon period. Therefore, a power saving operation can be accomplished without largely increasing a data delay time even during the power saving operation.  
         [0005]     However, in the prior art described above, since the terminal station operates asynchronously to a beacon management frame, the terminal station attempts to receive downlink data at an arbitrary timing irrespective of whether a base station is buffering data destined to the terminal station. As such, when there is not data destined to the terminal station in the base station, the terminal station consumes a bandwidth for unnecessary PS-Poll control frames and continuous wireless frame sequences.  
       SUMMARY OF THE INVENTION  
       [0006]     It is an object of the present invention to provide a power saving method in a wireless LAN system, which prevents a terminal station from consuming a bandwidth for unnecessary wireless frame sequences.  
         [0007]     To achieve the above object, in a power saving method in a wireless LAN system according to the present invention, as a terminal station receives a beacon signal from a base station to which the terminal station is subordinate, the terminal station extracts TIM information from the beacon signal, and stores the TIM information in a memory. The TIM information indicates whether or not the base station contains data destined to this terminal station. Then, at a timing of transmitting a PS-Poll signal to the base station for prompting the same to transmit data destined to the terminal station, asynchronous to the beacon signal reception timing, the terminal station references a log of the TIM information stored in the memory to determine based on the log whether the PS-Poll signal should be transmitted or stopped.  
         [0008]     By thus estimating whether or not the base station contains data destined to the terminal station using the log of the TIM information, the power saving method according to the present invention can save the power consumed by the terminal station because the terminal station can reduce the transmission of useless PS-Poll management frames and a wireless frame sequences associated therewith when downlink data to be received by the terminals station is not buffered in the base station. As a result, it is possible to avoid useless consumption of wireless resources to increase the transmission capacity, as viewed in the whole system.  
         [0009]     In another power saving method, as a terminal station receives a beacon signal from a base station to which the terminal station is subordinate, the terminal station extracts TIM information from the beacon signal, and stores the TIM information in a memory. The TIM information indicates whether or not the base station contains data destined to the terminal station. The terminal station further stores data reception information, which indicates whether or not the terminal station has received data destined thereto, in the memory as a response to a PS-Poll signal transmitted to the base station at a transmission timing for prompting the same to transmit data destined to the terminal station. The transmission timing is asynchronous to a timing at which the beacon signal is received. Then, the terminal station references a log of the TIM information or data reception information stored in the memory to determine based on the referenced log whether the PS-Poll signal is transmitted or stopped.  
         [0010]     In this way, the terminal station estimates whether or not the base station contains data destined to the terminal station, using the log of the data reception information or TIM information, thereby preventing the transmission of useless PS-Poll signals.  
         [0011]     The above and other objects, features and advantages of the present invention will become apparent from the following description with reference to the accompanying drawings which illustrate examples of the present invention. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]      FIG. 1  is a diagram illustrating an exemplary frame sequence for a power saving method in a wireless LAN system according to one embodiment of the present invention;  
         [0013]      FIG. 2  is a diagram illustrating another exemplary frame sequence for a power saving method in a wireless LAN system according to one embodiment of the present invention; and  
         [0014]      FIG. 3  is a block diagram illustrating an exemplary configuration of a terminal station in one embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0015]     A terminal station of the present invention can know whether or not downlink data destined thereto has been buffered in a base station by analyzing TIM (Traffic Indication Map) included in a beacon management frame transmitted from the base station. The terminal station can also know whether or not downlink data destined thereto has been buffered in the base station by determining the presence or absence of downlink data for a PS-Poll management frame transmitted by the terminal station to the base station.  
         [0016]     The terminal station stores a log as to whether or not downlink data destined thereto is buffered in the base station.  
         [0017]     For transmitting a PS-Poll management frame at an arbitrary timing asynchronous to the beacon management timing, the terminal station references the log stored in a memory as to whether or not past downlink data destined thereto is buffered in the base station.  
         [0018]     Specifically, when there has not been downlink data destined to the terminal station beyond a certain time, the terminal station temporarily stops the transmission of the PS-Poll management frame, as well as a sequence of subsequent downlink data receiving operations. On the other hand, when the terminal station has been informed by a preceding beacon management frame that downlink data destined to the terminal station has been buffered in the base station, as indicated by the log, the terminal station transmits a PS-Poll management frame, and performs a sequence of subsequent link data receiving operations without fail.  
         [0019]     By thus applying a power saving function, which operates asynchronously to the beacon management frame, while referencing the log stored in the memory as to whether or not downlink data destined to the terminal station has been buffered in the base station in the past, it is possible to reduce the transmission of useless PS-Poll management frames and wireless frame sequences associated therewith when downlink data to be received by the terminal station is not buffered in the base station.  
         [0020]     Referring to  FIG. 1 , an exemplary frame sequence is illustrated for a power saving method in a wireless LAN system in accordance with one embodiment of the present invention. The illustrated example shows that a terminal station stops the transmission of PS-Poll when the value of TIM is “0” N times in succession (“0” indicates that downlink data destined to the terminal station is not buffered in a base station), and resumes the transmission of PS-Poll when the terminal station detects that the value of TIM is “1” (“1” indicates that downlink data destined to the terminal station has been buffered in the base station). N is set to two, by way of example.  
         [0021]     In  FIG. 1 , Beacon (TIM) is a signaling information of the base station. Beacon (TIM)  1  has the value “1” which indicates that downlink data destined to the terminal station has been buffered in the base station. The terminal station stores the value of TIM in a memory. At PS-Poll transmission timing  11 , the terminal station references a TIM log from the memory, and transmits PS-Poll because the two latest TIM&#39;s are not “0” in succession. Subsequently, at PS-Poll transmission timings  12 ,  13 , the terminal station references the TIM log from the memory, and transmits PS-Poll because the two latest TIM&#39;s are not “0” in succession.  
         [0022]     Next, the terminal station receives Beacon (TIM)  2 . Beacon (TIM)  2  has the value “0” indicating that downlink data for the terminal station is not buffered in the base station. The terminal station stores the value of TIM in the buffer. At PS-Poll transmission timing  14 , the terminal station references the TIM log from the memory, and transmits PS-Poll because the two latest TIM are not “0” in succession. Subsequently, at PS-Poll transmission timings  15 ,  16 , the terminal station also references the TIM log from the memory, and transmits PS-Poll because the two latest TIM are not “0” in succession.  
         [0023]     Next, the terminal station receives Beacon (TIM)  3 . Beacon (TIM)  3  has the value “0” which indicates that downlink data for the terminal station is not buffered in the base station. The terminal station stores the value of TIM in the memory. At PS-Poll transmission timing  17 , the terminal station references the TIM log from the memory, and stops the transmission of PS-Poll because the two latest TIM&#39;s are “0” in succession. Subsequently, at PS-Poll transmission timings  18 ,  20 , the terminal station also references the TIM log from the memory, and stops the transmission of PS-Poll because the two latest TIM are “0” in succession.  
         [0024]     Next, the terminal station receives Beacon (TIM)  4 . Beacon (TIM)  4  has the value “1” which indicates that downlink data for the terminal station has been buffered in the base station. The terminal station stores the value of TIM in the memory. At PS-Poll transmission timing  21 , the terminal station references the TIM log from the memory, and transmits PS-Poll because the two latest TIM&#39;s are not “0” in succession.  
         [0025]     Referring to  FIG. 2 , there is illustrated another exemplary frame sequence for a power saving method in a wireless LAN system according to one embodiment of the present invention. This example shows a scenario in which a terminal station stops transmitting PS-Poll when no data is transmitted from a base station for M successive PS-Polls from the terminal station, and resumes the transmission of PS-Poll when the terminal station detects that the value of Beacon (TIM) is “1.” M is set to three, by way of example.  
         [0026]     Beacon (TIM) is signaling information of the base station. Beacon (TIM)  1  has the value “1” which indicates that downlink data for the terminal station has been buffered in the base station. The terminal station stores the value of TIM in a memory. At PS-Poll transmission timing  11 , the terminal station references a TIM log from the memory, and transmits PS-Poll because the latest TIM is “1” in the log. The base station transmits downlink data to the terminal station in response to PS-Poll  11  when it is buffering the data. The terminal station stores in the memory the presence or absence of data transmitted from the base station in response to PS-Poll  11  (PRESENCE).  
         [0027]     At PS-Poll transmission timing  12 , the terminal station references a base station data transmission log from the memory, and transmits PS-Poll because the three latest transmissions are not “ABSENCE” in succession in the log. The terminal station stores in the memory the presence or absence of data transmitted from the base station in response to PS-Poll  12  (PRESENCE).  
         [0028]     At PS-Poll transmission timing  13 , the terminal station references the base station data transmission log from the memory, and transmits PS-Poll because the three latest transmissions have been performed (PRESENCE) in succession in the log. The terminal station stores in the memory the presence or absence of data transmitted from the base station in response to PS-Poll  13  (ABSENCE).  
         [0029]     At PS-Poll transmission timings  14 ,  15 , the terminal station also references the base station data transmission log from the memory, and transmits PS-Poll because the three latest transmissions have been performed (PRESENCE). The terminal station stores in the memory the presence or absence of data transmitted from the base station in response to PS-Poll (ABSENCE).  
         [0030]     At PS-Poll transmission timing  16 , the terminal station references the base station data transmission log from the memory, and again references a TIM log from the memory to confirm the latest TIM log because the three latest transmissions have not been performed (ABSENCE) in succession. Since the latest TIM has the value “0” of Beacon (TIM)  2 , the terminal station stops the transmission of PS-Poll. Since the terminal station does not transmit PS-Poll, no data will be transmitted from the base station. In this event, therefore, the terminal station does not store in the memory the presence or absence of data transmitted from the base station.  
         [0031]     At PS-Poll transmission timing  17 , the terminal station references the base station data transmission log from the memory, and again references the TIM log from the memory to confirm the latest TIM log because the three latest transmissions have not been performed (ABSENCE) in succession. Since the latest TIM has the value “0” of Beacon (TIM)  3 , the terminal station stops the transmission of PS-Poll. Since the terminal station does not transmit PS-Poll, no data will be transmitted from the base station. In this event, therefore, the terminal station does not store in the memory the presence or absence of data transmitted from the base station. The terminal station performs similar operations as well at PS-Poll transmission timings  18  to  20 .  
         [0032]     At PS-Poll transmission timing  21 , the terminal station references the base station data transmission log from the memory, and again references the TIM log from the memory to confirm the latest TIM log because the three latest transmissions have not been performed (ABSENCE) in succession. Since the latest TIM has the value “1” of Beacon (TIM)  4 , the terminal station transmits PS-Poll. The terminal station stores in the memory the presence or absence of data transmitted from the base station in response to PS-Poll  21  YES).  
         [0033]     In the example illustrated in  FIG. 2 , the terminal station transmits PS-Poll or stops the transmission of PS-Poll depending on the number of times data has not been transmitted from the base station in response to PS-Poll. Alternatively, instead of the number of times, the terminal station may transmit PS-Poll or stop the transmission of PS-Poll depending on whether or not data has been transmitted within a preceding particular time period. Specifically, the terminal station may stop the transmission of PS-Poll when no data has been communicated from the base station for preceding P msec.  
         [0034]     In another example, when the terminal station determines to stop the transmission of PS-Poll, the terminal station may stop the transmission of PS-Poll the subsequent X times and resume the transmission of PS-Poll at an (X+1)th PS-Poll transmission timing. In this event, the terminal station may not reference the Beacon (TIM) log. The number of times X may vary depending on the time or on the number of times the terminal station has stopped the transmission of PS-Poll.  
         [0035]     The foregoing frame sequence may be executed by reading a computer program or a computer-readable recording medium which has recorded thereon such a computer program.  
         [0036]     Referring to  FIG. 3 , there is illustrated an exemplary configuration of the terminal station in the foregoing embodiment.  
         [0037]     The terminal station comprises CPU (central processing unit)  221  having central functions for a variety of controls; memory  222  for temporarily storing a control program, TIM log (“0” and “1”), base station data reception log (“PRESENCE” and “ABSENCE”); first interface circuit  223 ; and bus  224  which interconnects these components. Connected to first interface circuit  223  is one end of a series circuit composed of MAC processing unit  225  for processing a MAC (Media Access Control) address of the wireless LAN; baseband circuit  226  for processing a baseband signal; and RF (Radio Frequency) unit  227 . RF unit  227  is connected to antenna  228  for transmitting/receiving radio waves (including the reception of the beacon management frame, and the transmission of the PS-Poll management frame). First interface circuit  223 , MAC processing unit  225 , baseband circuit  226 , and RF unit  227  are each powered by a power supply unit  230 , to which battery  229  is connected, such that the power supply can be controlled on/off by switch circuit  231 .  
         [0038]     CPU  221  is also connected to input/output (I/O) circuit  223  and DSP (Digital Signal Processor)  234  in addition to bus  224 . Input/output circuit  233  is connected to a variety of devices such as keyboard  235  for entering characters and the like; vibrator  236  for informing through vibrations; illumination LED (Light Emitting Diode)  237  for illuminating a display, not shown, during a communication and the like, and so on. DSP  234  in turn is connected to speaker  242 , microphone  243 , and ringer  244  through second interface circuit  241 . Here, ringer  244  is a circuit for generating sound when a communication partner is called. CPU  221 , memory  222 , input/output circuit  233 , DSP  234 , and second interface circuit  241  are directly coupled to power supply unit  230  through power supply line  246 , such that the respective components are powered at all times unless the power output of power supply nit  230  is turned off by a main switch, not shown.  
         [0039]     While preferred embodiments of the present invention have been described using specific terms, such description is for illustrative purposes only, and it is to be understood that changes and variations may be made without departing from the spirit or scope of the following claims.