the present invention will be described in detail hereinafter with reference to accompanying drawings and embodiments to make the technical solution and merits therein clearer . embodiments of the present invention provide a method for transmitting uplink data in an epcn system . the main idea relies in that : a cnu determines a transmission path loss between a clt and the cnu , determines a transmission level for transmitting uplink data according to the transmission path loss , then transmits the uplink data by using the determined transmission level . fig2 is a flowchart illustrating a method for transmitting uplink data in an epcn system according to an embodiment of the present invention . as shown in fig2 , in the embodiment of the present invention , the method for transmitting the uplink data in the epcn system includes the following processes . block 201 : when transmitting downlink data , a clt carries a transmission level adopted for transmitting the downlink data in the downlink data . at this block , the procedure of the clt carrying the transmission level adopted for transmitting the downlink data in the downlink data may be as follows . the clt adds a field in the downlink data in advance ; when transmitting the downlink data , the clt carries the transmission level adopted for transmitting the downlink data in the field added in advance in the downlink data . or when transmitting the downlink data , the clt may carry the transmission level adopted for transmitting the downlink data in an existing field , e . g ., a padding field , in the downlink data . block 202 : the clt transmits the downlink data to a branch . block 203 : the downlink data are transmitted to cnus of different users through the branch . block 204 : the cnu detects a receiving level adopted for receiving the downlink data . at this block , the cnu may detect the receiving level adopted for receiving the downlink data by using an adaptive receiving technique . the detailed process belongs to the prior art and will not be described herein . block 205 : after receiving the downlink data , the cnu determines the transmission level adopted by the clt according to the transmission level carried in the downlink data . through the blocks 201 to 204 , the cnu determines the transmission level adopted by the clt . in practical service implementations , the cnu may determine the transmission level adopted by the clt through other methods . for example , it is possible to pre - configure on the cnu the transmission level adopted by the clt , and the cnu determines the transmission level adopted by the clt according to the configuration . block 206 : the cnu subtracts the detected receiving level from the transmission level adopted by the clt , and determines the difference as a transmission path loss between the cnu and the clt . at this block , for example , it is determined that the transmission level adopted by the clt is 30 dbv and the receiving level of the cnu is 8 dbv . 30 dbv minus 8 dbv is 22 dbv . thus it is determined that the transmission path loss between the cnu and the clt is 22 dbv . through the above blocks 201 to 206 , the cnu is able to determine the transmission path loss between the cnu and the clt . block 207 : the cnu determines a transmission level for transmitting uplink data according to the determined transmission path loss . in practical service implementations , it is only required that when arriving at the branch , the level of the uplink data transmitted by the cnu is not higher than the level isolating amplitude of the branch . thus , the branch can physically isolate the uplink data among the cnus , thereby ensuring the security of the uplink data . it can be known from the conventional data transmission procedure that the level isolating amplitude of the branch is higher than the transmission path loss between the clt and the branch . thus , if there is no additional path loss between the cnu and the branch , the transmission path loss between the clt and the cnu determined by the cnu is equal to the transmission path loss between the clt and the branch . if the uplink data is transmitted by using this transmission path loss , it can be ensured that the level of the uplink data is not higher than the level isolating amplitude of the branch when arriving at the branch . for example , when the cnu 1 mentioned above in the background is equipped at the doorway of the user 1 &# 39 ; s home , i . e . there is no additional loss between the cnu 1 and the branch , the cnu 1 may determine that the transmission path loss is 22 dbv which is lower than the level isolating amplitude 25 dbv of the branch . thus , if the cun 1 transmits the uplink data by adopting a transmission level higher than 22 dbv but lower than 25 dbv e . g . the minimum receiving level of the clt is 1 dbv and the transmission level of the cnu 1 is 23 dbv , it can be ensured that the uplink data can arrive at the clt but not arrive at other user terminals connected with the branch in the corridor . thus , isolation of the uplink data is realized . if there is additional path loss between the cnu and the branch in the corridor , when the cnu transmits the uplink data by adopting a transmission level which equals to the transmission path loss between the clt and the cnu determined by the cnu according to the method provided by the embodiments of the present invention , after passing the branch in the corridor , the transmission level is still able to ensure that the uplink data can arrive at the clt but not arrive at other user terminals connected with the branch . thus , the isolation of the uplink data is also ensured . for example , as to the cnu 2 which is equipped behind the 3 - distributor in the user 2 &# 39 ; s home mentioned above in the background , there is the additional loss between the cnu 2 and the branch . the transmission path loss determined by the cnu 2 according to the method of the present invention is 28 dbv . when the cnu 2 transmits the uplink data by adopting a transmission level higher than 28 dbv but lower than 31 dbv , for example , the transmission level adopted by the cnu 2 is 29 dbv and the minimum receiving level of the clt is 1 dbv , although the transmission level is higher than the level isolating amplitude of the branch , i . e . 25 dbv , it will become lower than 25 dbv but higher than 22 dbv , i . e . 23 dbv after the uplink data undergoes 6 dbv attenuation of the 3 - distributor in the user 2 &# 39 ; s home . thus , it is also ensured that the uplink data can arrive at the clt but not received by other user terminals connected with the branch in the corridor . it can be seen from the above that , at this block , the cnu may directly take the transmission path loss between the cnu and the clt as the transmission level for transmitting the uplink data . in addition , since there may be burst interferences in the network , on basis of implementing the uplink data isolation , in order to further ensure the uplink data transmitted by the cnu can arrive at the clt , it is required that the transmission level of the uplink data is slightly higher than the sum of the receiving level of the clt and the transmission path loss determined by the cnu . therefore , at this block , the cnu may preferably add the receiving level of the clt and an adjusting level to the transmission path loss , and determines the sum as the transmission level for transmitting the uplink data . the adjusting level meets a following condition : the difference between the transmission level calculated by using the adjusting level and the transmission path loss between the cnu and the branch is not higher than the level isolating amplitude of the branch . block 208 : the cnu transmits the uplink data by adopting the determined transmission level . accordingly , the embodiments of the present invention also provide a cnu . fig3 is a schematic diagram illustrating a structure of the cnu according to an embodiment of the present invention . as shown in fig3 , in the present invention , the cnu may include a path loss determining unit , a transmission level determining unit and a transmitting / receiving unit . the path loss determining unit is adapted to determine a transmission path loss between the cnu and a clt and send the transmission path loss to the transmission level determining unit . the transmission level determining unit is adapted to determine a transmission level for transmitting uplink data according to the received transmission path loss and pre - determined receiving level of the clt , and send the determined transmission level to the transmitting / receiving unit . the transmitting / receiving unit is adapted to transmit the uplink data by adopting the transmission level . the path loss determining unit may determine the transmission path loss between the cnu and the clt as follows . the transmitting / receiving unit sends downlink data carrying a transmission level of the downlink data from the clt to the path loss determining unit . the path loss determining unit determines the transmission level adopted by the clt according to the transmission level carried in the downlink data , detects a receiving level for receiving the downlink data , subtracts detected receiving level from the transmission level adopted by the clt , and takes the difference as the transmission path loss between the cun and the clt . fig4 is a schematic diagram illustrating a preferable structure of a cnu according to an embodiment of the present invention . in this embodiment , the cnu may further include a configuration information storing unit , adapted to store the pre - configured transmission level of the clt . in this embodiment , the path loss determining unit may determine the transmission path loss between the cnu and the clt as follows : the path loss determining unit determines the transmission level adopted by the clt according to the information stored in the configuration information storing unit , detects the receiving level for receiving the downlink data , subtracts the detected receiving level from the transmission level adopted by the clt , and takes the difference as the transmission path loss between the cnu and the clt . accordingly , the embodiments of the present invention also provide a clt . fig5 is a schematic diagram illustrating a structure of a clt according to an embodiment of the present invention . as shown in fig5 , the clt includes a downlink data processing unit and a transmitting / receiving unit . the downlink data processing unit is adapted to carry a transmission level for transmitting downlink data in the downlink data , and send the downlink data to the transmitting / receiving unit . the transmitting / receiving unit is adapted to transmit the downlink data received from the downlink data processing unit to the cnu . the foregoing descriptions are only preferred embodiments of this invention and are not for use in limiting the protection scope thereof . any changes and modifications can be made by those skilled in the art without departing from the spirit of this invention and therefore should be covered within the protection scope as set by the appended claims .