Abstract:
A method of controlling a receiver and a transmitter to handle a transmission window size change procedure is disclosed. The method includes driving the receiver to deliver a control message having a window size parameter to the transmitter for adjusting a transmission window size of the transmitter, and driving the transmitter to output an acknowledgement message to the re- ceiver and adjust the transmission window size according to the window size parameter when the transmitter receives the control message.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]     The application claims the benefit of U.S. Provisional Application No. 60/481,281, which was filed on Aug. 24, 2003 and entitled “RETRANSMISSION OF TRANSMISSION WINDOW SIZE CHANGE COMMAND”. 
     
    
     BACKGROUND OF INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to a method of setting a wireless communication system, and more specifically, to a method of handling a transmission window size change procedure in the wireless communication system.  
         [0004]     2. Description of the Prior Art  
         [0005]     Please refer to  FIG. 1 .  FIG. 1  is a simplified block diagram of a receiver  12  and a transmitter  14  in a prior art wireless communication system  10 . Both the receiver  12  and the transmitter  14  have windows within which they expect to receive the PDUs and transmit the PDUs respectively. The receiver  12  has a receiving window  16  that is delimited by two state variables: VR(R)  18  and VR(MR)  20 . VR(R)  18  marks the beginning of the receiving window  16 , and VR(MR)  20  marks the end of the receiving window  16 . The receiver  14  will only accept PDUs that have sequence numbers that are sequentially on or after VR(R)  18  and sequentially before VR(MR)  20 . The sequence number value held in VR(MR)  20  is not considered to be within the receiving window  16 . Similarly, the peer transmitter  14  has a transmission window  22  that is delimited by two state variables: VT(A)  24  and VT(MS)  26 . VT(A)  24  marks the beginning of the transmission window  22 , and VT(MS)  26  marks the end of the transmission window  22 . The transmitter  14  will only transmit PDUs that have sequence numbers that are within the range of the transmission window  22 , i.e., that are sequentially on or after VT(A)  24 , and sequentially before VT(MS)  26 .  
         [0006]     The receiving window  16  has a receiving window size. The receiving window size is simply the number of sequence number values spanned by the state variables VR(R)  18  and VR(MR)  20 . That is, VR(MR)  20  is always kept a fixed sequence number value distance away from VR(R)  18 , which may be represented mathematically as: 
 
 VR ( MR )= VR ( R )+receiving window size   (1) 
 
         [0007]     Similarly, the transmission window  22  has a transmission window size state variable VT(WS)  28 , which indicates the number of sequence number values spanned by the state variables VT(A)  24  and VT(MS)  26 . The state variable VT(WS)  28  has an initial value that is set to a configured transmission window size, which is supplied by an upper layer. The relation among the state variables VT(MS)  26 , VT(A)  24 , and VT(WS)  28  may be represented mathematically as: 
 
 VT ( MS )= VT ( A )+ VT ( WS )   (2) 
 
         [0008]     As the receiver  12  receives PDUs from the transmitter  14 , the receiver  12  will update the value of the state variable VR(R)  18  to reflect the sequentially earliest sequence number before which all preceding PDUs have been successfully received. Put another way, VR(R)  18  always holds the sequence number of the sequentially earliest PDU that the receiver  12  is waiting to receive. Upon the successful reception of this PDU, the receiver  12  advances the state variable VR(R)  18  to the sequence number value of the next PDU that needs to be received, and the state variable VR(MR)  20  is updated using equation (1) accordingly. In this manner, the receiving window  16  is advanced by the receiver  12  as the PDUs stream in from the transmitter  14 .  
         [0009]     The transmission window  22  is advanced when the transmitter  14  receives an acknowledgment status PDU from the receiver  12 . The acknowledgment status PDU holds the most current value of the state variable VR(R)  18 , and is sent at periodic intervals by the receiver  12 , or in response to an explicit request from the transmitter  14 . The acknowledgement status PDU may also indicate PDUs within the receiving window  16  that are known to have been missed (because, for example, sequentially later PDUs have already been received) and which must consequently be re-transmitted. The transmitter  14  will then set the state variable VT(A)  24  equal to the value held in the acknowledgment status PDU, which in effect sets VT(A)  24  equal to VR(R)  18 . The transmitter  14  updates the state variable VT(MS)  26  using equation (2) accordingly. In this manner, the transmission window  22  and the receiving window  16  move forward with each other.  
         [0010]     It is well-known that the receiver  12  is allowed to change the transmission window size of the corresponding transmitter  14 . As specified in the 3GPP TS 25.322 V4.9.0 (2003-6) Radio Link Control (RLC) protocol specification, a STATUS PDU is used by the receiver  12  to inform the transmitter  14  about the size of the allowed transmission window  22 . The receiver  12  is capable of delivering the STATUS PDU including a control message such as a window size super-field (WINDOW SUFI) to the transmitter  14  for setting the value of the state variable VT(WS)  28 . The purpose of sending this WINDOW SUFI is to facilitate a flow control because of buffer memory size requirement. According to the prior art, the receiver  12  keeps its receiving window size unchanged after the WINDOW SUFI is sent.  
         [0011]     According to the prior art, the receiver  12  deems that the transmitter  14  can always receives the STATUS PDU  54  including the WINDOW SUFI  55  successfully. In other words, by the prior art, the transmission window change procedure terminates when the STATUS PDU  54  including the WINDOW SUFI  55  is sent out by the receiver  12 . However, it is possible that the STATUS PDU  54  carrying the WINDOW SUFI  55  may not be correctly received by the transmitter  14  owing to the external interference over the air. If the STATUS PDU is lost during the wireless transmission, the receiver  12  cannot fulfill the flow control purpose of this WINDOW SUFI.  
         [0012]     Concerning another condition, suppose that the missing WINDOW SUFI outputted from the receiver  12  is used to increase the size of the transmission window  22 . As an example, this may happen when the transmission window size was first asked to shrink to a smaller size and then it is requested to stretch back to its original size. If the STATUS PDU is lost during the wireless transmission, the size of the receiving window  12  will be greater than the size of the unchanged transmission window  22 . It is obvious that all PDUs transmitted from the transmitter  14  according to the transmission window  22  will be accepted by the receiver  12 . However, because the transmission window  22  is not increased as requested by the receiver  12 , the throughput and data rate will be degraded, and the receiving window  16 , which reserves excessive buffers that will never be utilized, wastes the available memory capacity. In a extreme case that the previous transmission window size change (TWSC) procedure requested the transmission window size to zero to temporarily suspend the data transmission because of flow control consideration, if the restoring TWSC procedure to restore the transmission size to original one is lost over air interface, the transmitter  14  will not stop the suspension and a data transmission deadlock will occur.  
       SUMMARY OF INVENTION  
       [0013]     It is therefore a primary objective of the claimed invention to provide a method of handling the transmission window size change (TWSC) procedure to solve problems invoked by the lost of the control message during the wireless transmission over the air.  
         [0014]     Briefly summarized, a method for controlling a transmitter and a receiver in a wireless communication system to handle a TWSC procedure according to a preferred embodiment includes driving the receiver to deliver a control message having a window size parameter to the transmitter for adjusting a transmission window size of the transmitter, and when the transmitter receives the control message, driving the transmitter to output an acknowledgement message to the receiver and adjust the transmission window size according to the window size parameter.  
         [0015]     According to a preferred embodiment, a method for controlling a transmitter and a receiver in a wireless communication system to handle a TWSC procedure includes driving the receiver to deliver a control message to the transmitter for adjusting a transmission window of the transmitter, and driving the receiver to adjust a receiving window size according to the window size parameter after a predetermined period of time, wherein the receiver discards a PDU having a sequence number outside the receiving window.  
         [0016]     According to a preferred embodiment, a method for controlling a transmitter and a receiver in a wireless communication system to handle a TWSC procedure includes driving the receiver to deliver a control message having a window size parameter to the transmitter for reducing a transmission window size of the transmitter, driving the receiver to create an intended receiving window according to the window size parameter, and after a predetermined period of time, driving the receiver to retransmit the control message to the transmitter if the receiver receives a PDU having a sequence number outside the intended receiving window.  
         [0017]     According to a preferred embodiment, a method for controlling a transmitter and a receiver in a wireless communication system to handle a TWSC procedure includes driving the receiver to deliver a control message to the transmitter for increasing a transmission window of the transmitter, driving the receiver to use a counter to count a number of transmission time intervals, called TTI hereafter, in which no new PDU allowed by the receiving window is received after a predetermined period of time, and driving the receiver to retransmit the control message to the transmitter if the TTI counter reaches a predetermined value.  
         [0018]     The claimed method of controlling a transmitter and a receiver in a wireless communication system in a TWSC procedure utilizes the transmitter to output an acknowledgement message to the receiver when receiving a WINDOW SUFI. Therefore, the receiver is capable of determining if the WINDOW SUFI is correctly received by the transmitter. In addition, if the receiver outputs the WINDOW SUFI for reducing the size of the transmission window, the claimed method drives the receiver to detect if the received PDU having a sequence number outside the intended receiving window. To sum up, the claimed method solves the problem of invoked by the lost of the WINDOW SUFI during the wireless transmission over the air. The claimed method improves the radio transmission efficiency between the transmitter and the receiver, and optimizes the buffer memory usage of the receiver.  
         [0019]     These and other objectives of the claimed invention will no doubt become obvious to those of ordinary skill in the art at reading the following detailed description of the preferred embodiments that are illustrated in the various figures and drawings. 
     
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0020]      FIG. 1  is a simplified block diagram of a receiver and a transmitter in a prior art wireless communication system.  
         [0021]      FIG. 2  is a simplified block diagram of a receiver and a transmitter in a wireless communication system according to the present invention.  
         [0022]      FIG. 3  is a flow chart illustrating a first embodiment of adjusting the transmission window according to the present invention.  
         [0023]      FIG. 4  is a flow chart illustrating a second embodiment of adjusting the transmission window according to the present invention.  
         [0024]      FIG. 5  is a flow chart illustrating a third embodiment of adjusting the transmission window according to the present invention.  
         [0025]      FIG. 6  is a flow chart illustrating a fourth embodiment of adjusting the transmission window according to the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0026]     Please refer to  FIG. 2 .  FIG. 2  is a simplified block diagram of a receiver  32  and a transmitter  34  in a wireless communication system  30  according to the present invention. The receiver  32  has a timer  36 , a decision logic  38 , and a receiving window  40 . The transmitter  34  establishes a transmission window  42  corresponding to the receiving window  40 . As mentioned above, the span of the receiving window  40  is controlled by two state variables VR(R)  44  and VR(MR)  46 , and the span of the transmission window  42  is controlled by these state variables VT(A)  48  and VT(MS)  50 . The size of the transmission window  42  is VT(WS)  52 . The receiver  32  is allowed to initiate a transmission window size change (TWSC) procedure. As shown in  FIG. 2 , the receiver  32  outputs a STATUS PDU  54  having a WINDOW SUFI  55  containing a WSN field  58  to the transmitter  34  for modifying the size of the transmission window  42 . That is, the WINDOW SUFI  55  is used to modify the value of the state variable VT(WS)  52  to the value of the WSN field  58 . Therefore, the corresponding state variable VT(MS)  50  is accordingly adjusted, and the state variables VT(A), VT(MS) are capable of defining a new span of the transmission window  42 . In addition, two state variables VR(R)  44  and VR(IMR)  45  can be used to indicate the span of an intended receiving window  43 , where VR(IMR)  45  may be represented mathematically as: 
 
 VR ( IMR )= VR ( R )+ WSN    (3) 
 
         [0027]     Equation (3) is similar to equation (1) except that the receiving window size is replaced by the intended receiving window size WSN  58 .  
         [0028]     In a preferred embodiment, the transmitter  34  gives a notice to the receiver  32  upon successfully receiving the STATUS PDU  54  having the WINDOW SUFI  55 . For example, the transmitter  34  delivers a STATUS PDU  56  carrying an acknowledgement message (WINDOW_ACK SUFI)  57  to the receiver  32  after receiving the STATUS PDU  54 . The receiver  32  is capable of determining whether the transmission window  42  is correctly adjusted through reading the WINDOW_ACK SUFI  57  carried by the received STATUS PDU  56 . The WINDOW_ACK SUFI  57  can optionally contain a WSN_ACK field  60 . The WSN_ACK field  60  is set according to the value of the WSN field  58 . As an example, WSN_ACK field can be set equal to the value of the WSN field  58 . When a new TWSC procedure is triggered before the previous TWSC procedure terminates, the Receiver  32  can utilize the WSN_ACK field to identify if the WINDOW_ACK SUFI corresponds to the newest updated WINDOW SUFI. When the receiver  32  detects that the transmission window  42  is correctly adjusted as desired, the receiver  32  terminates the TWSC procedure.  
         [0029]     As mentioned before, the STATUS PDU  54  may get lost during the wireless transmission over the air. Therefore, the TWSC procedure according to the present invention detects the STATUS PDU  56  for confirming adjustment of the transmission window size  52 . Please refer to  FIG. 3  in conjunction with  FIG. 2 .  FIG. 3  is a flow chart illustrating a first embodiment of the TWSC procedure according to the present invention. The corresponding process is explained as follows. First, the receiver  32  initiates a TWSC procedure by delivering the STATUS PDU  54  having the WINDOW SUFI  55  to the transmitter  34  (step  100 ). After the STATUS PDU  54  is successfully outputted, the timer  36  positioned on the receiver  32  is triggered, and starts clocking (step  102 ). Then, check the timer  36  to see if the timer  36  expires (step  104 ). If the timer  36  does not expire, check the receiver  106  to see if it successfully receives the STATUS PDU  56  having a WINDOW_ACK SUFI  57  corresponding to the WINDOW SUFI (step  106 ). On the other hand, if the timer  36  expires, it is possible that the STATUS PDU  54  outputted from the receiver  32  gets lost during the wireless transmission. Therefore, the receiver  32  is driven to retransmit the same STATUS PDU  54  to the transmitter  34  (step  112 ). The timer  36  is restarted (step  102 ).  
         [0030]     Concerning step  106 , if the receiver  32  does not receive the STATUS PDU  56  that carries a WINDOW_ACK SUFI  57  corresponding to the WINDOW SUFI, the process of the present invention performs step  104  to check the expiration of the timer  36 . On the other hand, if the receiver  32  receives the STATUS PDU  56  that carries a WINDOW_ACK SUFI  57  corresponding to the WINDOW SUFI (step  106 ), the receiver terminates the TWSC procedure (step  110 ).  
         [0031]     In the preferred embodiment, the decision logic  38  is activated to make sure that the transmitter  34  really receives the accurate WINDOW SUFI  55  (step  106 ). As mentioned before, a new TWSC procedure may be triggered before the previous TWSC procedure terminates. The decision logic  38  of the receiver  32  compares the WSN  58  previously sent in the most updated WINDOW SUFI  55  with the WSN_ACK  60  lately received. If the two values match, the receiver  32  terminates the TWSC procedure (step  110 ). Otherwise, if WSN_ACK  60  is different from the most updated WSN  58 , the received WINDOW_ACK SUFI  57  is out of date and the receiver  32  discards it and proceeds to step  104  again.  
         [0032]     In addition, after the receiver terminates the TWSC procedure (step  110 ) when the WSN_ACK  60  matches the most updated WSN  58 , the receiver  32  can optionally adjust its receiving window  40  accordingly (not shown in  FIG. 3 ). In the preferred embodiment, the value of the state variable VR(MR)  46  is set to VR(R)  44 +WSN  58 , wherein WSN  58  is the transmission window size requested by the receiver  32 . In other words, PDUs with sequence numbers on or after VR(R) 44 +WSN  58  are considered to be outside the adjusted receiving window  40 .  
         [0033]     Please note that the period clocked by the timer  36  is defined to be greater than a roundtrip delay, i.e. the expected time between the transmission of the WINDOW SUFI  55  and the reception of a PDU (the STATUS PDU  56  for example) transmitted by the transmitter  34  after receiving the WINDOW SUFI  55 .  
         [0034]     The above-mentioned first embodiment according to the present invention makes use of the acknowledge message (WINDOW_ACK SUFI)  57 , generated from the transmitter  34  to determine if the WINDOW SUFI  55  is correctly received by the transmitter  34 . However, other triggering mechanisms also can be used to achieve the same objective. Please refer to  FIG. 2  in conjunction with  FIG. 4 .  FIG. 4  is a flow chart illustrating a second embodiment of adjusting the transmission window  42  according to the present invention. Please note that the transmitter  34  in this preferred embodiment does not output the acknowledgement message (the WINDOW_ACK SUFI  57  shown in  FIG. 2 ) to the receiver  32  when receiving the STATUS PDU  54 . The process corresponding to the second embodiment is suitable for the case of reducing transmission window size and is explained as follows. First, the receiver  32  is triggered to deliver the STATUS PDU  54  having the WINDOW SUFI  55  to the transmitter  34  for reducing the size of the transmission window  42  (step  200 ). After the STATUS PDU  54  is successfully outputted, the timer  36  positioned on the receiver  32  is triggered, and starts clocking (step  202 ). Please note that the duration of the timer  36  is defined to be greater than a roundtrip delay, i.e. the expected time between the transmission of the WINDOW SUFI  55  and the reception of a PDU transmitted by the transmitter  34  after receiving the WINDOW SUFI  55 . Then, the timer  36  is checked to see if the timer  36  expires (step  204 ). When the timer  36  expires, the receiver  32  creates an intended receiving window  43  (step  206 ) delimited by VR(R)  44  and VR(IMR)  45 , where VR(IMR)  45  is calculated from VT(R)  44  and WSN  58  in the lately sent WINDOW SUFI  55  using equation (3).  
         [0035]     Then, the decision logic  38  is activated to detect if a PDU received from the transmitter  34  has a sequence number outside the intended receiving window  43 . As mentioned above, the WSN  58  is used to reduce the transmission window  42  for the second embodiment. If the transmitter  34  does not correctly receive the STATUS PDU  54 , the size of the transmission window  42  is not changed. However, the size of the intended receiving window  43  is equal to WSN  58  owing to step  206 . In other words, the size VT(WS)  52  of the transmission window  42  is greater than the size of the intended receiving window  43  if the STATUS PDU  54  gets lost. Therefore, if the decision logic  38  detects that a PDU outputted from the transmitter  34  has a sequence number outside the intended receiving window  43  (step  208 ), the receiver  32  can deem that the transmitter  34  had not successfully received the WINDOW SUFI  55 . Therefore, the receiver  32  is driven to retransmit the WINDOW SUFI  55  having the same WSN  58  to the transmitter  34  (step  210 ). The receiver constantly checks if there is different WINDOW SUFI triggered for transmission to adjust the size of transmission window to another value (step  212 ). If this happens, the receiver terminates the current TWSC procedure and starts another TWSC procedure (step  214 ).  
         [0036]     In the above second preferred embodiment, the receiver  32  creates an intended receiving window  43  while the size of the receiving window  40  is kept unchanged. A further embodiment of the present invention is to adjust the receiving window size accordingly. Please refer to  FIG. 2  in conjunction with  FIG. 5 .  FIG. 5  is a flow chart illustrating a third embodiment of the TWSC procedure according to the present invention. The process corresponding to the third embodiment is explained as follows. First, the receiver  32  is triggered to deliver the STATUS PDU  54  having the WINDOW SUFI  55  to the transmitter  34  for changing the size of the transmission window  42  (step  250 ). After the STATUS PDU  54  is successfully outputted, the timer  36  positioned on the receiver  32  is triggered, and starts clocking (step  252 ). As before, the duration of the timer  36  is defined to be greater than a roundtrip delay, i.e. the expected time between the transmission of the WINDOW SUFI  55  and the reception of a PDU transmitted by the transmitter  34  after receiving the WINDOW SUFI  55 . Then, the timer  36  is checked to see if the timer  36  expires (step  254 ). When the timer  36  expires, the receiver  32  adjusts the size of the receiving window (step  256 ) by setting VR(MR)  46  to VR(R)  44 +WSN  58 , wherein WSN  58  stands for the new transmission window size requested by the receiver  32 . Therefore, PDUs with sequence numbers on and after VR(R)+WSN are considered to be outside the adjusted receiving window  40 . In other words, any received PDU having a sequence number outside the receiving window  40  is discarded by the receiver  32  (step  258 ). The receiver  32  constantly checks if there is different WINDOW SUFI triggered for transmission to adjust the size of transmission window  42  to another value (step  260 ). If this happens, the receiver  32  terminates the current TWSC procedure and starts another TWSC procedure (step  262 ).  
         [0037]     The above-mentioned second embodiment according to the present invention detects PDUs having sequence numbers outside the intended receiving window  40  to determine if the WINDOW SUFI  55  used for reducing the size of the transmission window  42  is correctly received by the transmitter  34 . However, the receiver  32  is also capable of requesting the transmitter  34  to enlarge its transmission window  42 . This may happen when the transmission window size was first asked to shrink to a smaller size and then it is requested to stretch back to its original size. The above first and third embodiments apply for both shrinking and stretching transmission window size cases.  
         [0038]     There is a further embodiment applying for the stretching transmission window size case. Please refer to  FIG. 2  in conjunction with  FIG. 6 .  FIG. 6  is a flow chart illustrating a fourth embodiment of a TWSC procedure according to the present invention. Please note that the transmitter  34  in this preferred embodiment does not output the acknowledgement message (the WINDOW_ACK  57  shown in  FIG. 2 ) to the receiver  32  when receiving the STATUS PDU  54 . The process corresponding to the fourth embodiment is explained as follows. First, the receiver  32  is triggered to deliver a STATUS PDU  54  having a WINDOW SUFI  55  to the transmitter  34  for increasing the current size of the transmission window  42  (step  300 ). Then, the receiver  32  is driven to adjust the size of its receiving window  40  to be at least the WSN  58  recorded by the outputted WINDOW SUFI  54  (step  302 ). The WSN  58  is used to enlarge the current size  52  of the transmission window  42 . If the STATUS PDU  54  is not successfully received by the transmitter  34 , the size  52  of the transmission window  42  is not changed, and the adjusted size of the receiving window  40  becomes greater than the currently maintained size of the transmission window  42 .  
         [0039]     After the transmitter  34  transfers all of the PDUs having sequence numbers within the transmission window  42  to the receiver  32 , the transmitter  34  stops outputting PDUs having sequence numbers outside the transmission window  42  until the receiver  32  returns STATUS PDUs acknowledging successfully received data PDUs so that the transmitter can move forward the transmission window  42 . Therefore, during a plurality of transmission time intervals (TTIs), the receiver  32  receives no new PDU with sequence number in the receiving window. The decision logic  38  is enabled to count the TTIs for computing a corresponding count value (step  304 ), and detects if the count value is equal to a predetermined value (step  306 ). In this preferred embodiment, the count value is calculated according to the TTIs in which no new PDU allowed in the receiving window is received. For example, each of the TTIs is counted to compute the count value. In addition, each group of the contiguous TTIs is capable of being used to compute the count value. That is, each group of the contiguous TTIs when no new PDU allowed in the receiving window is received is counted as one when computing the count value. When a new PDU allowed in the receiving window is received, the count value is reset to zero. When the count value equals the predetermined value, the receiver  32  deems that the size of the receiving window  40  is greater than the size of the transmission window  42 . In other words, the receiver  32  believes that the transmitter  34  does not correctly receive the previously sent WINDOW SUFI  55 . Therefore, the receiver  32  retransmits the WINDOW SUFI  55  having the same WSN  58  to the transmitter  34  for enlarging the transmission window  42  (step  308 ) and the count value is reset to zero (step  310 ).  
         [0040]     It is possible that the size of the transmission window  42  has been correctly set, but the count value still equals the predetermined value. For example, suppose that the size of the transmission window  42  is correctly adjusted. The transmitter  34  has transferred all PDUs having sequence numbers within the transmission window  42  to the receiver  32 , and utilizes a polling function to request information of the receiver  32 . However, the corresponding STATUS PDUs outputted from the receiver  32  may get lost during the wireless transmission. Therefore, the transmission window  42  is not moved, and the count value is continuously computed. Once the count value equals the predetermined value, the same WINDOW SUFI  55  is transmitted again to define the size of the transmission window  42  that has been correctly set. In this preferred embodiment, the number of transmissions of the same WINDOW SUFI  55  is limited to a certain number to solve this problem.  
         [0041]     In contrast to the prior art, the transmitter according to the present invention is controlled to output an acknowledgement message to the receiver when receiving a WINDOW SUFI. Therefore, the receiver is capable of determining if the WINDOW SUFI is correctly received by the transmitter. If the acknowledgement message is not received within a pre-determined time bound, the receiver retransmits the same WINDOW SUFI. In addition, if the receiver outputs the WINDOW SUFI for reducing the size of the transmission window, the receiver according to the present invention is capable of detecting if the received PDU having a sequence number outside the intended receiving window. If the receiver receives any PDU with a sequence number outside the intended receiving window, the receiver retransmits the same WINDOW SUFI. Concerning another embodiment, whether the WINDOW SUFI is to reduce or increase the transmission window size, the receiving window is adjusted accordingly after a predetermined time bound so that any received PDU outside the adjusted receiving window is discard to fulfill the flow control or buffer size control purpose of the TWSC procedure.  
         [0042]     To sum up, the claimed method solves the problem invoked by the lost of the WINDOW SUFI  55  during the wireless transmission over the air. The claimed method improves the radio transmission efficiency between the transmitter and the receiver, and optimizes the buffer memory usage of the receiver.  
         [0043]     Those skilled in the art will readily observe that numerous modifications and alterations of the device may be made while retaining the teachings of the invention. Accordingly, the above disclosure should be construed as limited only by the metes and bounds of the appended claims.