A method performed by a transmitting packet data convergence protocol, PDCP, entity (1i-01) in a wireless communication system, the method comprising: identifying that an uplink data compression, UDC, is configured, based on a radio resource control, RRC, message indicating a UDC configuration; receiving first data and second data, from an upper layer; generating a UDC data block by performing (1i-10) compression on the first data based on the UDC configuration; generating a first UDC packet by adding (1i-15) a first UDC header to the UDC data block; generating a second UDC packet by adding a second UDC header to the second data, wherein the first UDC header indicates that the first UDC packet is compressed and the second UDC header indicates that the second UDC packet is not compressed; based on the identifying that the UDC is configured, determining both of the first UDC header and the UDC data block and both of the second UDC header and the second data, to be ciphered; performing (1i-25) ciphering on both of the first UDC header and the UDC data block and both of the second UDC header and the second data, based on the determining; generating first resulting data by adding (1i-30) a first PDCP header to the first UDC packet; generating second resulting data by adding a second PDCP header to the second UDC packet; and transmitting the first resulting data and the second resulting data to a lower layer; wherein the receiving of the first data and the second data from the upper layer comprises allocating PDCP sequence numbers to the first data and the second data in order of reception, and wherein the transmitting of the first resulting data and the second resulting data to the lower layer comprises transmitting the first resulting data and the second resulting data to the lower layer in order of the PDCP sequence numbers. identifying that an uplink data compression, UDC, is configured, based on a radio resource control, RRC, message indicating a UDC configuration; receiving first data and second data, from an upper layer; generating a UDC data block by performing (1i-10) compression on the first data based on the UDC configuration; generating a first UDC packet by adding (1i-15) a first UDC header to the UDC data block; generating a second UDC packet by adding a second UDC header to the second data, wherein the first UDC header indicates that the first UDC packet is compressed and the second UDC header indicates that the second UDC packet is not compressed; based on the identifying that the UDC is configured, determining both of the first UDC header and the UDC data block and both of the second UDC header and the second data, to be ciphered; performing (1i-25) ciphering on both of the first UDC header and the UDC data block and both of the second UDC header and the second data, based on the determining; generating first resulting data by adding (1i-30) a first PDCP header to the first UDC packet; generating second resulting data by adding a second PDCP header to the second UDC packet; and transmitting the first resulting data and the second resulting data to a lower layer; wherein the receiving of the first data and the second data from the upper layer comprises allocating PDCP sequence numbers to the first data and the second data in order of reception, and wherein the transmitting of the first resulting data and the second resulting data to the lower layer comprises transmitting the first resulting data and the second resulting data to the lower layer in order of the PDCP sequence numbers. <EOS>