A method for transmitting client signals in an Optical Transport Network, OTN, comprising: obtaining the client signals; mapping the client signals into Optical Channel Payload Unit-k Tributary Slot, OPUk TSs, of an OPUk frame, wherein the OPUk frame includes an overhead; sending the OPUk frame with the added overhead to the OTN; the mapping the client signals into the OPUk TSs of the OPUk frame comprises: mapping a first client signal among the client signals into the OPUk TSs corresponding to the first client signal in an agnostic CBR mapping mode; and mapping a second client signal among the client signals into the OPUk TSs corresponding to the second client signal in a GFP mapping mode or an ATM cell mapping mode; wherein the step of mapping the first client signal among the client signals into the OPUk TSs corresponding to the first client signal in the agnostic CBR mapping mode comprises: mapping a value that indicates the number of bytes to an overhead of a current Optical Channel Data Tributary Unit, ODTU, frame, wherein the value that indicates the number of bytes is determined by comparing a clock of the first client signal with a local clock and indicates the number of bytes to be inserted into a payload area of an ODTU frame next to the current ODTU frame; mapping bytes of the first client signal to the payload area of the ODTU frame next to the current ODTU frame according to the value that indicates the number of bytes in the overhead of the current ODTU frame; mapping bytes of each ODTU frame to at least one OPUk TS. obtaining the client signals; mapping the client signals into Optical Channel Payload Unit-k Tributary Slot, OPUk TSs, of an OPUk frame, wherein the OPUk frame includes an overhead; sending the OPUk frame with the added overhead to the OTN; the mapping the client signals into the OPUk TSs of the OPUk frame comprises: mapping a first client signal among the client signals into the OPUk TSs corresponding to the first client signal in an agnostic CBR mapping mode; and mapping a second client signal among the client signals into the OPUk TSs corresponding to the second client signal in a GFP mapping mode or an ATM cell mapping mode; mapping a first client signal among the client signals into the OPUk TSs corresponding to the first client signal in an agnostic CBR mapping mode; and mapping a second client signal among the client signals into the OPUk TSs corresponding to the second client signal in a GFP mapping mode or an ATM cell mapping mode; wherein the step of mapping the first client signal among the client signals into the OPUk TSs corresponding to the first client signal in the agnostic CBR mapping mode comprises: mapping a value that indicates the number of bytes to an overhead of a current Optical Channel Data Tributary Unit, ODTU, frame, wherein the value that indicates the number of bytes is determined by comparing a clock of the first client signal with a local clock and indicates the number of bytes to be inserted into a payload area of an ODTU frame next to the current ODTU frame; mapping bytes of the first client signal to the payload area of the ODTU frame next to the current ODTU frame according to the value that indicates the number of bytes in the overhead of the current ODTU frame; mapping bytes of each ODTU frame to at least one OPUk TS. mapping a value that indicates the number of bytes to an overhead of a current Optical Channel Data Tributary Unit, ODTU, frame, wherein the value that indicates the number of bytes is determined by comparing a clock of the first client signal with a local clock and indicates the number of bytes to be inserted into a payload area of an ODTU frame next to the current ODTU frame; mapping bytes of the first client signal to the payload area of the ODTU frame next to the current ODTU frame according to the value that indicates the number of bytes in the overhead of the current ODTU frame; mapping bytes of each ODTU frame to at least one OPUk TS.