Abstract:
The invention proposes a technical scheme for controlling downlink data synchronization in an eMBMS transmission in a radio link control entity in a base station. By determining one length indicator for each service data unit, and meanwhile, each length indicator corresponds to one extension bit, the length indicator and the extension bit of each RLC SDU are byte aligned, and the base station generates different length indicators for the service data units with the different length distribution. By adopting the technical scheme of the invention, the corresponding radio link control header overhead thereof is fixed for each service data unit, and therefore, even in the case where a consecutive packet loss occurs in the service data unit received by a certain base station, synchronization between it and other base stations receiving correctly can still be achieved.

Description:
FIELD OF THE INVENTION  
       [0001]    The present invention is related to a base station in a wireless communication network, in particular, to a method and apparatus for controlling downlink data synchronization in an eMBMS transmission in a radio link control (RLC, Radio Link Control) entity. 
       BACKGROUND OF THE INVENTION 
       [0002]    A requirement is proposed in the SYNC protocol of 3GPP 36.300 as following: in the eMBMS transmission, an eNodeB, namely a base station, is able to detect loss of a radio link control service data unit (RLC SDU, Radio Link Control Service Date Unit, which is also referred to as “service data unit” hereafter), and to maintain synchronization with other base stations in the case of loss of consecutive packets. It is known that an Unacknowledge Mode (UM), in which there is no ARQ error correction mechanism in the radio link control entity, is employed in the eMBMS transmission. 
         [0003]    Since a sequence number of the radio link control service data unit which has been sent in one transmission window and a total length of the service data units which have been sent before them for each packet, the base station knows how many radio link control service data units have been lost and the length of the lost radio link control service data units if loss of consecutive packets occurs in a data transmission from a gateway to eNodeB interface. On the basis of such information, the base station has to know where a service data unit received correctly after the lost service data units is filled in a transmission block, i.e., the radio link control protocol data unit (RLC PDU, Radio Link Control Protocol Date Unit, which is referred to as “protocol data unit” hereinafter), in order to maintain data transmission synchronization with the other base stations which do not lose a packet. In accordance with the requirement above, a radio link control header overhead of the service data unit of a given number of the lost service data units should not depend on the length distribution of the service data units. 
         [0004]    In a radio link control unacknowledge mode data protocol data unit (RLC UMD PDU, referred as to “radio link control protocol data unit” or “protocol data unit”, RLC PDU or PDU) format disclosed in 3GPP TS 36.322 V8.4.0 in December, 2008, as shown in  FIG. 1 , for two consecutive radio link control service data unit, the size of the radio link control header is dependent on the size distribution of the two consecutive radio link control service data units. In the format shown in  FIG. 1 , each protocol data unit includes a radio link control header and a data field, and the data field is used to be filled with one or more radio link control service data units. Wherein, the header of the protocol data unit includes a FI (Framing Info) field, a SN (Sequence Number) and an extension bit of the SN, a length indicator corresponding to one or more radio link control service data units filled in the data field, and an extension bit of the length indicator. Wherein, there are the length indicator and the extension bit of the length indicator if and only if data of more than one radio link control service data units are included in the data field thereof. Moreover, for the last service data unit or segment of a service data unit in the data field, no length indicator or the corresponding extension bit are assigned thereto. 
         [0005]    In accordance with the format of  FIG. 1 , if a first radio link control service data unit is filled into a first radio link control protocol data unit, a second radio link control service data unit has two segments, wherein the first segment is filled into the first radio link control protocol data unit, and the second segment is filled into a second radio link control protocol data unit, there will be a radio link control header overhead of 2 bytes for each radio link control service data unit. 
         [0006]    If a radio link control protocol data unit is just filled with two radio link control service data units, the corresponding radio link control header overhead in the radio link control protocol data unit is 2 bytes. 
         [0007]    If a radio link control protocol data unit is filled with a first segment of a first radio link control service data unit, and a second radio link control protocol data unit is filled with a second segment of the first radio link control service data unit and a second radio link control service data unit, the corresponding radio link control header overhead in the second radio link control protocol data unit is 3 bytes for the two radio link control service data units. 
         [0008]    It can be seen that since the radio link control header overhead of two consecutive the radio link control service data units are dependent on the size distribution of those two, if two consecutive packets are lost, the base station can not know the radio link control header overhead of those two radio link control service data units, and can not accurately fill the radio link control protocol data unit corresponding thereto, either, resulting in a loss of synchronization of downlink data in the eMBMS transmission of multiple base stations. 
       SUMMARY OF THE INVENTION 
       [0009]    For the problem addressed in the Background, a technical scheme for controlling downlink data synchronization in an eMBMS transmission in a radio link control entity in a base station of a wireless communication network is proposed in the present invention. A length indicator and an extension bit of each RLC SDU are byte aligned by determining a length indicator for each service data unit while each length indicator corresponds to one extension bit, and for each service data unit, the corresponding radio link control header overhead thereof is fixed. 
         [0010]    In accordance with a first aspect of the invention, it is provided a method of controlling downlink data synchronization in an eMBMS transmission in a radio link control entity in a base station of a wireless communication network, wherein each radio link control service data unit corresponds to one length indicator, each length indicator corresponds to one extension bit, the length indicator and the extension bit of each radio link control service data unit are byte aligned. The method comprises the steps of: determining whether a last segment of a radio link control service data unit or an entire radio link control service data unit is filled into the end of one radio link control protocol data unit; generating a first specific length indicator for said last segment or said entire radio link control service data unit if the last segment of the radio link control service data unit or the entire radio link control service data unit is filled into the end of one radio link control protocol data unit; filling said first specific length indicator and the corresponding extension bit thereof into the next radio link control protocol data unit. 
         [0011]    In accordance with the second aspect of the invention, it is provided a synchronization control apparatus for controlling downlink data synchronization in an eMBMS transmission in a radio link control entity in a base station of a wireless communication network, wherein each radio link control service data unit corresponds to one length indicator, each length indicator corresponds to one extension bit, the length indicator and the extension bit of each radio link control service data unit are byte aligned. The synchronization control apparatus comprises: a first determining unit for determining whether a last segment of an radio link control service data unit or an entire radio link control service data unit is filled into the end of one radio link control protocol data unit; a generating unit for generating a first specific length indicator for said last segment or said entire radio link control service data unit if the last segment of the radio link control service data unit or the entire radio link control service data unit is filled into the end of one radio link control protocol data unit; a filling unit for filling said first specific length indicator and the corresponding extension bit thereof into the next radio link control protocol data unit. 
         [0012]    By adopting the technical scheme of the invention, the corresponding radio link control header overhead thereof is fixed for each service data unit, and therefore, even in the case where a consecutive packet loss occurs in the service data units received by specific base station, synchronization between it and other base stations receiving correctly can still be achieved. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0013]    The features, objects and advantages of the invention will be more apparent by reading the detailed description made for the unlimited embodiments with reference to the drawings. 
           [0014]      FIG. 1  is a format of a radio link control unacknowledge mode protocol data unit in the related art; 
           [0015]      FIG. 2  is a schematic diagram for a radio link control unacknowledge mode protocol data unit in accordance with an embodiment of the invention; 
           [0016]      FIG. 3  is a flow chart for a method of controlling downlink data synchronization in an eMBMS transmission in a radio link control entity in a base station of a wireless communication network; 
           [0017]      FIG. 4  is another flow chart in accordance with another embodiment of the invention; 
           [0018]      FIG. 5  is one flow chart in accordance with yet another embodiment of the invention; 
           [0019]      FIG. 6  is a schematic diagram for filling a radio link control service data unit into a radio link control protocol data unit in accordance with an embodiment of the invention; 
           [0020]      FIG. 7  is a flow chart for processing the case where packet loss occurs for radio link control service data unit in a base station of a wireless communication network in accordance with an embodiment of the invention; 
           [0021]      FIG. 8  is an example for filling a lost service data unit into a protocol data unit by a base station in the case shown in  FIG. 6 ; 
           [0022]      FIG. 9  is a structural schematic diagram of a synchronization control apparatus  900  for controlling downlink data synchronization in an eMBMS transmission in a radio link control entity in a base station of a wireless communication network; 
       
    
    
       [0023]    wherein identical or similar reference numbers refer to identical or similar step features or apparatuses(modules 
       DETAILED DESCRIPTION OF EMBODIMENTS 
       [0024]    The exemplary description for embodiment of the invention will be made in connection with the accompany drawings hereinafter. 
         [0025]      FIG. 2  illustrates a schematic diagram for the format of a radio link control protocol data unit in accordance with an embodiment of the invention. In  FIG. 2 , the protocol data unit includes two parts: a radio link control header and a data field. The data field is used to be filled with one or more service data units or segments thereof. Specifically, it is understood by the skilled in this art that, in accordance with the size of the data field and the size of the service data unit, one data field can be filled with a plurality of complete service data units, can be filled with one or two segments of the service data unit and a plurality of complete service data units, or can be filled with only one segment of one service data unit, etc. For the convenience of the description, a service data unit or segment of service data unit filled into the data field is referred as to an element. 
         [0026]    Wherein, the radio link control header includes a FI field, a SN field and the extension bit E of SN; a length indicator LI i  and the corresponding extension bit E thereof. The FI field has two bits, in which the first bit indicates whether the first element in the data field is segment of service data unit, and the second bit indicates whether the last element in the data field is segment of service data unit. Each extension bit E is 1 bit. The extension bit E before the SN being “1” means that the length indicator is immediately after the sequence number SN of the protocol data unit, SN is 5 bits, and the extension bit E before LI 1  being “1” means that the length indicator is immediately after LI 1 . The extension bit E before the last length indicator LI k  being “0” means that the data field is right after LI k . The length of the length indicator is 11 bits. Thus, for each length indicator and the corresponding extension bit thereof, they occupy only 12 bits, and for the purpose of byte alignment, four bits are filled for each pair of the length indicator and the corresponding extension bit thereof. 
         [0027]    Generally, the meaning of the length indicator LI i  refers to the length of the i-th element in the data field, i.e., the length of a complete service data unit or the length of a segment of service data unit, and the length is in unit of bytes, and is referred to as a general length indicator. In some particular cases, the meaning of the length indicator does not refer to the length of the element in the data field, and is referred to as a specific length indicator. This will be depicted in detail hereinafter. 
         [0028]    Unlike the format shown in  FIG. 1 , there is a filling vacancy of four bits after each length indicator. Thus, each length indicator and the corresponding extension bit thereof are byte aligned. The meaning of byte-alignment is that each length indicator and the corresponding extension bit thereof occupy a multiple of byte in the radio link control header. In the example shown in  FIG. 2 , each length indicator and the corresponding extension bit thereof occupy two bytes. It should be understood by the skilled in the art that the format shown in  FIG. 2  is merely an example, and the number of the bytes occupied by the length indicator and the corresponding extension bit thereof can be determined in accordance with the practical needs. 
         [0029]      FIG. 3  illustrates a method flow chart for controlling downlink data synchronization in an eMBMS transmission in a radio link control entity in a base station of a wireless communication network. The flow shown in  FIG. 3  will be depicted in connection with the format of the protocol data unit described in  FIG. 2 . 
         [0030]    Firstly, in step S 301 , the base station determines whether the last segment of one radio link control service data unit or one entire radio link control service data unit is filled into the end of one radio link control protocol data unit. 
         [0031]    Next, in step S 302 , the base station generates a first specific length indicator for said last segment or the entire radio link control service data unit if the last segment of one radio link control service data unit or the entire radio link control service data unit is filled into the end of one radio link control protocol data unit. 
         [0032]    Then, in step S 303 , the base station fills the first specific length indicator and the corresponding extension bit thereof into the next radio link control protocol data unit. 
         [0033]      FIG. 4  illustrates another flow chart for another embodiment in accordance with the invention. The flow shown in  FIG. 4  will be depicted in connection with the format of the protocol data unit described in  FIG. 2 . 
         [0034]    Firstly, in step S 401 , the base station determines whether one byte is remained in the radio link control protocol data unit after the last segment of one radio link control service data unit or the entire radio link control service data unit is filled into the radio link control protocol data unit. 
         [0035]    Next, in step S 402 , if one byte is remained in the radio link control protocol data unit after the last segment of one radio link control service data unit or the entire radio link control service data unit is filled into one radio link control protocol data unit, the base station fills the data of the first byte of the next radio link control service data unit into such byte, and generates the second specific length indicator for the last segment or the entire radio link control service data unit. 
         [0036]    Finally, in step S 403 , the base station fills the second specific length indicator and the corresponding extension bit thereof into the next radio link control protocol data unit. 
         [0037]      FIG. 5  illustrates one flow chart in accordance with yet another embodiment of the invention. The flow shown in  FIG. 5  will be depicted in connection with the format of the protocol data unit described in  FIG. 2 . 
         [0038]    Firstly, in step S 501 , the base station determines whether a plurality of bytes are remained in the radio link control protocol data unit after the last segment of one radio link control service data unit or the entire radio link control service data unit is filled into one radio link control protocol data unit. 
         [0039]    Next, in step S 502 , if a plurality of bytes are remained in the radio link control protocol data unit after the last segment of one radio link control service data unit or the entire radio link control service data unit is filled into one radio link control protocol data unit, a general length indicator is generated in accordance with the length of the last segment of the radio link control service data unit or the entire radio link control service data unit. 
         [0040]    Finally, in step S 503 , the base station fills the general length indicator and the corresponding extension bit thereof into the radio link control protocol data unit. 
         [0041]      FIG. 6  illustrates a schematic diagram for filling a radio link control service data unit into a radio link control protocol data unit according to an embodiment of the invention, and an exemplary illustration is made for the flows shown in  FIGS. 3 to 5  in connection with  FIG. 6 . 
         [0042]    Without lose of generality, it is assumed that the transmission capacity of each radio link control protocol data unit is 101 bytes, wherein the FI field and the SN field and the corresponding extension bit thereof occupy 1 byte. The lengths of three service data units to be filled into a protocol data unit are 80, 97 and 97 bytes, respectively. 
         [0043]    It is assumed that 20 bytes out of 100 bytes have been occupied by other service data units or segments of the service data units before the first protocol data unit is filled with the first service data unit. The base station generates a first specific length indicator LI 1  for the first service data unit, and fills LI 1  into a second protocol data unit, as the first service data unit with 80 bytes in length is just filled into the end of the first protocol data unit. Accordingly, 2 bytes have been occupied in the second protocol data unit. As only one byte is remained in the second protocol data unit after the second service data unit with 97 bytes in length is filled into the second protocol data unit, the base station fills the first byte of the third service data unit into the last byte of the second protocol data unit, generates a second specific length indicator for the second service data unit, and fills the second specific length indicator and the corresponding extension bit thereof into the very beginning of the third protocol data unit. Next, the base station fills the remaining segment of 96 bytes in the third service data unit into the third protocol data unit; afterwards, two bytes are remained in the third protocol data unit, then the base station generates a general length indicator, i.e., “00001100000” for the segment of 96 bytes in the third service data unit. Thus, the base station can proceed to fill other service data units into the protocol data unit. 
         [0044]    A schematic table for the value of the length indicator in accordance with an embodiment of the invention is illustrated in Table 1. In Table 1, all zeros value is reserved, “11111111110” is the value for the first specific length indicator, “11111111111” is the value for the second specific length indicator, and “00000000001” to “11111111101” indicates the value for the general length indicator. Of course, the actual value of the general length indicator is determined by the length of the service data unit or the segment thereof. It is to be explained that  FIG. 1  and Table 1 are merely exemplary illustration, and in practice, the value for the general length indicator is determined by the lengths of the actual service data unit and the protocol data unit, the specific value for the specific length indicator is not limited to “11111111110” or “11111111111” as mentioned above, and any values other than the value for the general length indicator can be defined as the values for the specific length indicators. 
         [0045]    A detailed depiction for the case in which the base station fills the service data unit into the protocol data unit when the base station receives the radio link control service data unit correctly is made hereinabove. A user device can unpack the protocol data unit according to the filling manner of the base station to obtain respective service data units. 
         [0000]    
       
         
               
             
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 values and meanings for the length indicator 
               
             
          
           
               
                 Value 
                 Description 
                 Application 
               
               
                   
               
               
                 00000000000 
                 reserved 
                   
               
               
                 00000000001 
                 a general length indicator, 
                 broadcasting 
               
               
                 to 
                 representing the length values for the 
                 service 
               
               
                 11111111101 
                 corresponding service data unit or the 
                   
               
               
                   
                 segment thereof 
                   
               
               
                 11111111110 
                 a first specific length indicator, 
                 broadcasting 
               
               
                   
                 representing that the last element of 
                 service 
               
               
                   
                 the previous protocol data unit is just 
                   
               
               
                   
                 an entire service data unit or the last 
                   
               
               
                   
                 segment of a service data unit, and 
                   
               
               
                   
                 there is no length indicator of the last 
                   
               
               
                   
                 element in the previous protocol data 
                   
               
               
                   
                 unit 
                   
               
               
                 11111111111 
                 a second specific length indicator, 
                 broadcasting 
               
               
                   
                 representing that only one byte is 
                 service 
               
               
                   
                 remained after the last second 
                   
               
               
                   
                 element is filled in the previous 
                   
               
               
                   
                 protocol data unit, the remained one 
                   
               
               
                   
                 byte is filled with the first byte of the 
                   
               
               
                   
                 next service data unit, and there is no 
                   
               
               
                   
                 length indicator of the last second 
                   
               
               
                   
                 element in the previous protocol data 
                   
               
               
                   
                 unit 
               
               
                   
               
             
          
         
       
     
         [0046]    A detailed depiction will be made for the process in the base station in the case where a packet loss occurs in the radio link control service data unit hereinbelow. It is to be clear that the packet loss described herein includes the case where the base station does not receive the radio link control service data unit or the case where an incorrect radio link control service data unit is received. 
         [0047]      FIG. 7  illustrates a flow chart for processing the case where a packet loss occurs in the radio link control service data unit in the base station of the wireless communication network in accordance with an embodiment of the invention. 
         [0048]    Firstly, in step S 701 , the base station obtains the corresponding service data unit sequence number of each radio link control service data unit and the total length of the radio link control service data units before the radio link control service data unit in the transmission window to which the radio link control service data unit belongs. Wherein the transmission window may be a time window or a window of the number of the radio link control service data units. 
         [0049]    For an eMBMS service, the base station obtains the service data unit sequence number and the corresponding service data unit sequence number of the service data unit and the total length of the radio link control service data units before the radio link control service data unit in the transmission window to which the radio link control service data unit belongs through a SYNC protocol header. Specifically, the detailed instruction to the section of the SYNC protocol in the 3GPP organization RAN3 workgroup proposal R3-091303 “SYNC consideration for eMBMS Rel9” (http://www.3gpp.org/ftp/tsg_ran/WG3_Iu/TSGR3 — 64/Docs/) can be referred to, and the description thereof is omitted herein. 
         [0050]    Secondly, in step S 702 , the base station decides whether any radio link control service data unit is lost in accordance with the service data unit sequence number of the obtained radio link control service data units. 
         [0051]    Next, if a radio link control service data unit is lost, the base station determines the number of the lost radio link control service data units and the total length of the corresponding lost radio link control service data units in accordance with the service data unit sequence number of the received radio link control service data units and the total length of the radio link control service data units before the radio link control service data unit in the transmission window to which the radio link control service data unit before and after the lost radio link control service data unit belongs. 
         [0052]    Finally, the base station fills the data of the total length of the lost radio link control service data units and the data of the length of the corresponding length indicators and the extension bits into the corresponding location of one or more radio link control protocol data units. 
         [0053]    Following the example shown in  FIG. 6 , an example in which the base station fills the lost service data unit in the protocol data unit is illustrated in  FIG. 8  in the case shown in  FIG. 6 . 
         [0054]    Taking an eMBMS transmission as an example, in one transmission window, assuming that the fourth to the sixth service data units are all lost, the base station receives the first to the third service data units, the seventh and the subsequent protocol data units. Then, the base station knows that it does not receive the fourth to the sixth service data units in accordance with the received service data unit sequence number in the SYNC protocol header. Assuming that the total length of the first and the second service data units sent before the third service data unit corresponding to the third service data unit is X bytes, the total length of the six service data units sent before the seventh service data unit corresponding to the seventh service data unit is Y bytes, and the length of the third service data unit is Z bytes, the total length of the lost fourth to sixth service data units is M=Y−X−Z. Without loss of generality, it is assumed that M is 274 bytes. Since three service data units are lost, in accordance with the principle that one service data unit corresponds to one length indicator, the radio link control header overhead corresponding to the three service data units is 6 bytes, and accordingly, the base station should fill 280 bytes in the corresponding protocol data units, as shown in  FIG. 8 . It can be seen that the base station is still able to achieve synchronization with the base station receiving correctly. 
         [0055]    It is to be explained that although the radio link control entity of the base station fills the lost service data units in the protocol data unit and the base station still assigns radio resources for those lost service data unit in physical layer, the transmitting of the service data units is not conducted on the corresponding radio resources. At this time, other base stations receiving those service data units correctly send those service data units on the corresponding the radio resources. 
         [0056]      FIG. 9  illustrates a structural schematic diagram of a synchronization control apparatus  900  for controlling downlink data synchronization in an eMBMS transmission in a radio link control entity in a base station of a wireless communication network. The synchronization control apparatus  900  includes a first determining apparatus  901 , a generating apparatus  902 , a filling unit  903 , an obtaining unit  904 , and a second determining apparatus  905 . Wherein the second determining unit  905  is an optional apparatus. 
         [0057]    A detailed depiction for the work process of the synchronization control apparatus  900  in  FIG. 9  is made with reference to  FIG. 2 . 
         [0058]    Firstly, the first determining unit  901  determines whether a last segment of a radio link control service data unit or the entire radio link control service data unit is filled into the end of one radio link control protocol data unit. 
         [0059]    Next, if the last segment of one radio link control service data unit or the entire radio link control service data unit is filled into the end of one radio link control protocol data unit, the generating unit  902  generates a first specific length indicator for the last segment or the entire radio link control service data unit. 
         [0060]    Then, the filling unit  903  fills the first specific length indicator and the corresponding extension bit thereof into the next radio link control protocol data unit. 
         [0061]    Optionally, the first determining unit  901  determines whether one byte is remained in the radio link control protocol data unit after the last segment of one radio link control service data unit or the entire radio link control service data unit is filled into one radio link control protocol data unit. 
         [0062]    If one byte is remained in the radio link control protocol data unit after the last segment of one radio link control service data unit or the entire radio link control service data unit is filled into one radio link control protocol data unit, the generating unit  902  fills the data of the first byte of the next radio link control service data unit into such byte, and generates the second specific length indicator for the last segment or the entire radio link control service data unit. 
         [0063]    Finally, the filling unit  903  fills the second specific length indicator and the corresponding extension bit thereof into the next radio link control protocol data unit. 
         [0064]    Optionally, the first determining unit  901  determines whether a plurality of bytes are remained in the radio link control protocol data unit after the last segment of one radio link control service data unit or the entire radio link control service data unit is filled into one radio link control protocol data unit. 
         [0065]    If a plurality of bytes are remained in the radio link control protocol data unit after the last segment of one radio link control service data unit or the entire radio link control service data unit is filled into one radio link control protocol data unit, the generating unit  902  generates a general length indicator in accordance with the length of the last segment of the radio link control service data unit or the entire radio link control service data unit. 
         [0066]    Finally, the filling unit  903  fills the general length indicator and the corresponding extension bit thereof into the radio link control protocol data unit. 
         [0067]    A further exemplary illustration of the work flow of the first determining unit  901 , the generating unit  902 , and the filling unit  903  will be made in connection with  FIG. 6 . 
         [0068]    Without loss of generality, it is assumed that the transmission capacity of each radio link control protocol data unit is 101 bytes, wherein the FI field and the SN field and the corresponding extension bit thereof occupy 1 byte. The lengths of three service data units to be filled into a protocol data unit are 80, 97 and 97 bytes, respectively. 
         [0069]    It is assumed that 20 bytes out of 100 bytes have been occupied by other service data units or segments of the service data units before the first protocol data unit is filled with the first service data unit. The first determining unit  901  determines that the first service data unit with 80 bytes in length is just filled into the end of the first protocol data unit, and then the generating unit  902  generates a first specific length indicator LI 1  for the first service data unit, and the filling unit  903  fills LI 1  into the second protocol data unit. Accordingly, 2 bytes have been occupied in the second protocol data unit, and then, after the second service data unit with 97 bytes in length is filled into the second protocol data unit, the first determining unit  901  determines that only one byte is remained in the second protocol data unit, and then the filling unit  903  fills the first byte of the third service data unit into the last byte of the second protocol data unit, and the generating unit  902  generates a second specific length indicator for the second service data unit, the filling unit  903  fills the second specific length indicator and the corresponding extension bit thereof into the very beginning of the third protocol data unit. Next, the filling unit  903  fills the remaining segment of 96 bytes in the third service data unit into the third protocol data unit; afterwards, the first determining unit  901  determines that two bytes are remained in the third protocol data unit, then the generating unit  902  generates a general length indicator, i.e., “00001100000” for the segment of 96 bytes in the third service data unit. Thus, the synchronization control apparatus  900  can proceed to fill other service data units into the protocol data unit. 
         [0070]    The values for the length indicators may be referred to the example in Table 1. 
         [0071]    A detailed depiction is made for the case in which the base station fills the service data unit into the protocol data unit when the synchronization control apparatus  900  receives the radio link control service data unit correctly hereinabove. A user device can unpack the protocol data units according to the filling manner of the synchronization control apparatus  900  to obtain respective service data units. 
         [0072]    A detailed depiction will be made for the process of the synchronization control apparatus 900 in the case where the packets loss occurs in the radio link control service data unit hereinbelow. 
         [0073]    Firstly, the obtaining unit  903  obtains the corresponding service data unit sequence number of each radio link control service data unit and the total length of the radio link control service data units before the radio link control service data unit in a transmission window to which the radio link control service data unit belongs. Wherein, the transmission window may be a time window or a window of the number of the radio link control service data units. 
         [0074]    For an eMBMS service, the obtaining unit  903  obtains the service data unit sequence number and the corresponding service data unit sequence number of the service data unit and the total length of the radio link control service data units before the radio link control service data unit in the transmission window to which the radio link control service data unit belongs through a SYNC protocol header. 
         [0075]    The second determining unit  905  determines whether the radio link control service data unit is lost in accordance with the service data unit sequence number in the obtained radio link control service data units. 
         [0076]    If a radio link control service data unit is lost, the second determining unit  905  determines the number of the lost radio link control service data units and the total length of the corresponding lost radio link control service data units in accordance with the service data unit sequence number of the received radio link control service data units and the total length of the radio link control service data unit before the radio link control service data unit in the transmission window to which the radio link control service data unit before and after the lost radio link control service data unit belongs. 
         [0077]    Finally, the filling unit  903  fills the data of the total length of the lost radio link control service data unit and the data of the length of the length indicator and the extension bit in the corresponding location of one or more radio link control protocol data units. 
         [0078]    The specific embodiments of the invention are described hereinabove. It is understood that the invention is not limited to the above particular implementation, and the alternation or modification may be made by the skilled in the art within the scope of the appended claims. The technical scheme of the invention may be implemented by software or hardware.