An apparatus for transmitting a client signal in an optical transport network, OTN, comprising: a receiving unit, adapted to acquire the client signal, and extract a client signal clock from the client signal; a client signal byte number Cn generating unit, adapted to generate a client signal byte number Cn according to the client signal clock and a system clock; characterized by further comprising: a determining unit, adapted to determine whether the client signal byte number Cn exceeds a range of client signal byte number transported in an OTN frame or not; and a setting unit, adapted to set and/or fill an area of an optical channel payload unit-k, OPUk, of the OTN frame; a mapping unit, adapted to map client signal bytes to an OPUk payload area of a next OTN frame; and a transmitting unit, adapted to transmit an optical channel transport unit, OTU;wherein if the determining unit determines that the Cn falls in the range, the setting unit is adapted to: identify a predetermined area in an overhead field of the OPUk of the OTN frame as normal, that means a client signal type remains unchanged, and fill Cn filled in the previous OTN frame in the overhead field of the OPUk of the OTN frame, reverse values of a first series of bit positions of the overhead area of the OPUk if the Cn transported in the OTN frame needs to be increased; and reverse values of the second series of bit positions of the overhead area of the OPUk if the Cn transported in the OTN frame needs to be decreased; the mapping unit is further adapted to map Cn client signal bytes to the OPUk payload area of the next OTN frame, and the transmitting unit is adapted to transmit the OTU comprising the OPUk to the OTN network. a receiving unit, adapted to acquire the client signal, and extract a client signal clock from the client signal; a client signal byte number Cn generating unit, adapted to generate a client signal byte number Cn according to the client signal clock and a system clock; characterized by further comprising: a determining unit, adapted to determine whether the client signal byte number Cn exceeds a range of client signal byte number transported in an OTN frame or not; and a setting unit, adapted to set and/or fill an area of an optical channel payload unit-k, OPUk, of the OTN frame; a mapping unit, adapted to map client signal bytes to an OPUk payload area of a next OTN frame; and a transmitting unit, adapted to transmit an optical channel transport unit, OTU;wherein if the determining unit determines that the Cn falls in the range, the setting unit is adapted to: identify a predetermined area in an overhead field of the OPUk of the OTN frame as normal, that means a client signal type remains unchanged, and fill Cn filled in the previous OTN frame in the overhead field of the OPUk of the OTN frame, reverse values of a first series of bit positions of the overhead area of the OPUk if the Cn transported in the OTN frame needs to be increased; and reverse values of the second series of bit positions of the overhead area of the OPUk if the Cn transported in the OTN frame needs to be decreased; the mapping unit is further adapted to map Cn client signal bytes to the OPUk payload area of the next OTN frame, and the transmitting unit is adapted to transmit the OTU comprising the OPUk to the OTN network. a determining unit, adapted to determine whether the client signal byte number Cn exceeds a range of client signal byte number transported in an OTN frame or not; and a setting unit, adapted to set and/or fill an area of an optical channel payload unit-k, OPUk, of the OTN frame; a mapping unit, adapted to map client signal bytes to an OPUk payload area of a next OTN frame; and a transmitting unit, adapted to transmit an optical channel transport unit, OTU;wherein if the determining unit determines that the Cn falls in the range, the setting unit is adapted to: identify a predetermined area in an overhead field of the OPUk of the OTN frame as normal, that means a client signal type remains unchanged, and fill Cn filled in the previous OTN frame in the overhead field of the OPUk of the OTN frame, reverse values of a first series of bit positions of the overhead area of the OPUk if the Cn transported in the OTN frame needs to be increased; and reverse values of the second series of bit positions of the overhead area of the OPUk if the Cn transported in the OTN frame needs to be decreased; the mapping unit is further adapted to map Cn client signal bytes to the OPUk payload area of the next OTN frame, and the transmitting unit is adapted to transmit the OTU comprising the OPUk to the OTN network. identify a predetermined area in an overhead field of the OPUk of the OTN frame as normal, that means a client signal type remains unchanged, and fill Cn filled in the previous OTN frame in the overhead field of the OPUk of the OTN frame, reverse values of a first series of bit positions of the overhead area of the OPUk if the Cn transported in the OTN frame needs to be increased; and reverse values of the second series of bit positions of the overhead area of the OPUk if the Cn transported in the OTN frame needs to be decreased; the mapping unit is further adapted to map Cn client signal bytes to the OPUk payload area of the next OTN frame, and the transmitting unit is adapted to transmit the OTU comprising the OPUk to the OTN network.