# Patent

## Title
Method and apparatus for mapping and de-mapping in an optical transport network

## Abstract

The embodiments of the present invention disclose method and apparatus for mapping and de-mapping in an optical transport network, where the mapping method includes: constructing an Optical Channel Data Tributary Unit (ODTU) according to an amount M of time slots of a High Order Optical Channel Payload Unit (HO OPU) to be occupied by a Low Order Optical Channel Data Unit (LO ODU); mapping the LO ODU to a payload area of the ODTU in a M-byte granularity; encapsulating overhead information to the overhead area of the ODTU; and multiplexing the ODTU, which has been mapped the LO ODU and encapsulated with the overhead information, to the HO OPU, so as to provide a high-efficient and universal mode for mapping the LO ODU to the HO OPU.

## Background
CROSS-REFERENCE TO RELATED APPLICATIONS
This application claims the benefit of the Chinese Patent Application No. 200910106028.2 filed on Mar. 9, 2009, which is incorporated herein by reference in its entirety.
FIELD OF TECHNOLOGY
The present invention relates to the field of communication technologies, and particularly, to a mapping technique in an optical transport network.
BACKGROUND OF THE INVENTION
With the quick development of communication technology, the Optical Transport Network (OTN) with advantages of flexible scheduling and management of large capacity services is increasingly becoming a major technology of the backbone transport network. In the OTN, the client data is encapsulated into an Optical Channel Payload Unit (OPU), some overhead (OH) is added to the OPU to constitute an Optical Channel Data Unit (ODU), some OH and Forward Error Correction (FEC) is added into the ODU to constitute an Optical Channel Transport Unit (OTU), and is finally transmitted in the form of OTU.
With the rapid development of data service, more and more clients adopt Ethernet technology as the physical interface at the client side. It is foreseeable that in the coming years, the Ethernet service will keep a continuous high-speed growth. However, the current OTN technology is designed based on speech service such as synchronous digital hierarchy (SDH), and cannot well support the development trend of data service like Ethernet, thus studies are gradually carried out on the next generation of OTN network (NG OTN). The NG OTN is required to not only meet the requirements of services newly appear, but also bear the current OTN. Thus how to map a Lower Order Optical Channel Data Unit (LO ODU) to a Higher Order Optical Channel Data Unit (HO ODU) is a focus being discussed in the industry. The LO ODU may be ODUk (k=0, 1, 2, 2e, 3, 3e) existing in the current OTN, and herein represented as LO ODUk (k=0, 1, 2, 2e, 3, 3e); the HO ODU may be regarded as a data transmission unit of higher rate, which belongs to the category of the NG OTN and is used for bearing the LO ODU, the corresponding OPU is represented as HO OPUk (k=1, 2, 3, 3e, 4).
The conventional technical solution maps the standard ODUj (j=1, 2) (for 20 ppm bit tolerance) into the OPUk (k=2, 3) in an asynchronous manner. The asynchronous method maps the ODUj signal into the OPUk through an adjustment policy of −1/0/+1/+2. With the asynchronous manner, a maximum bit error tolerance between ODU1 and OPU2 is −113 to +83 ppm, a maximum bit error tolerance between ODU1 and OPU3 is −96 to +101 ppm, and a maximum bit error tolerance between ODU2 and OPU3 is −95 to +101 ppm.
During the process of implementing the present invention, the inventor finds that the prior art has at least the following problems:
The method is not suitable for mapping the LO ODU into the HO ODU, e.g. for ODU2c (100 ppm bit tolerance), etc. ODUflex of higher bit tolerance may occur in the future, and the adjustment policy of −1/0/+1/+2 of the prior art cannot meet the requirement for mapping the LO ODU into the HO ODU.
BRIEF SUMMARY OF THE INVENTION
The embodiments of the present invention provide method and apparatus for mapping and de-mapping in an OTN, so as to map the LO ODU into the HO ODU universally and efficiently.
The embodiments of the present invention provide a mapping method in an OTN, including: (1) constructing an ODTU according to an amount M of time slots of an HO OPU to be occupied by an LO ODU; (2) mapping the LO ODU to a payload area of the ODTU in an M-byte granularity; encapsulating overhead information to the overhead area of the ODTU; and (3) multiplexing the ODTU, which has been mapped the LO ODU and encapsulated with the overhead information, to the HO OPU.
The embodiments of the present invention provide a de-mapping method in an OTN, including: (1) parsing an HO OPU to determine an ODTU and an amount M of time slots occupied by the ODTU; and (2) de-mapping an LO ODU from a payload area of the ODTU in an M-byte granularity.
The embodiments of the present invention further provide a mapping apparatus in an OTN, including: (1) a constructing unit, configured to construct an ODTU according to an amount M of time slots of an HO OPU to be occupied by an LO ODU; (2) a mapping unit, configured to map the LO ODU to a payload area of the ODTU in an M-byte granularity; (3) an encapsulating unit, configured to package overhead information to the overhead area of the ODTU; and (4) a multiplexing unit, configured to multiplex the ODTU, which has been mapped the LO ODU and encapsulated with the overhead information, to the HO OPU.
The embodiments of the present invention further provide a de-mapping apparatus in an OTN, including: a parsing unit, configured to parse an HO OPU to determine an ODTU and an amount M of time slots occupied by the ODTU; and a de-mapping unit, configured to de-map an LO ODU from a payload area of the ODTU in an M-byte granularity.
The technical solutions of the embodiments of the present invention bring the following beneficial effect: providing a high-efficient and universal mode for mapping LO ODU to HO OPU.



BRIEF DESCRIPTION OF THE DRAWINGS
In order to describe the technical solutions of the embodiments of the present invention more clearly, the drawings needed for describing the embodiments are introduced briefly as follows. It is apparent that the following drawings are just some embodiments of the present invention, and a person skilled in the art can obtain other drawings based on these drawings without paying a creative effort.
FIG. 1 is a flowchart of a mapping method in an OTN according to an embodiment of the present invention;
FIG. 2 is a structural schematic view illustrating dividing an HO OPU into eight 1.25 G time slots according to an embodiment of the present invention;
FIG. 3 is a schematic view illustrating a mapping mode according to an embodiment of the present invention;
FIG. 4 is a schematic view illustrating another mapping mode according to an embodiment of the present invention;
FIG. 5 is a schematic view illustrating a mapping from LO ODU to HO OPU2 according to an embodiment of the present invention;
FIG. 6 is a schematic view illustrating another mapping from LO ODU to HO OPU2 according to an embodiment of the present invention;
FIG. 7 is a schematic view illustrating another mapping from LO ODU to HO OPU2 according to an embodiment of the present invention;
FIG. 8 is a schematic view illustrating information encoding of “C8M-base+C8M-delta+C8-delta” according to an embodiment of the present invention;
FIG. 9 is a flowchart of a de-mapping method in an OTN according to an embodiment of the present invention;
FIG. 10 illustrates a mapping apparatus in an OTN according to an embodiment of the present invention;
FIG. 11 illustrates a mapping apparatus in an OTN according to another embodiment of the present invention; and
FIG. 12 illustrates a de-mapping apparatus in an OTN according to an embodiment of the present invention.



DETAILED DESCRIPTION OF THE EMBODIMENTS
The technical solutions of the embodiments of the present invention are clearly described as follows in conjunction with the drawings of the embodiments. It is apparent that the described embodiments are just a part of embodiments of the present invention, instead of all embodiments thereof. Based on the embodiments of the present invention, other embodiments, which are obtained by a person skilled in the art without paying a creative effort, all fall within the protection scope of the present invention.
At the data transmitting side, as shown in FIG. 1, a mapping method in an OTN according to an embodiment of the present invention includes: (1) constructing an ODTU according to an amount M of time slots of an HO OPU to be occupied by an LO ODU; (2) mapping the LO ODU to a payload area of the ODTU in an M-byte granularity; encapsulating overhead information to the overhead area of the ODTU; and (3) multiplexing the ODTU, which has been mapped the LO ODU and encapsulated with the overhead information, to the HO OPU, so as to provide an efficient and universal mode for mapping the LO ODU to the HO OPU.
For the convenience of understanding the embodiments of the present invention, the implementation processes of those embodiments during specific applications are described in detail as follows.
The mapping method in an OTN according to an embodiment of the present invention includes:
S1: constructing an ODTU according to an amount M of time slots of an HO OPU to be occupied by an LO ODU.
The step includes:
S11: determining the amount M of the time slots of the HO OPU to be occupied by the LO ODU.
Specifically, the amount M of the time slots of the HO OPU to be occupied by the LO ODU may be determined according to the rate of the LO ODU and the rate of a single time slot of the HO OPU, i.e. M=upper rounding of (the rate of the LO ODU/the rate of a single time slot of the HO OPU). For example, HO OPU2 is divided into eight 1.25 G time slots, and eight frames of HO OPU2 forms a large multi-frame, as shown in FIG. 2. If the rate of a certain LO ODU is assumed as 6 G, then five time slots shall be occupied, and M is 5. We can allocate five time slots to the current LO ODU according to the usage conditions of the time slots in the HO OPU, herein the five time slots allocated to the current LO ODU are assumed as time slots 2, 3, 5, 7 and 8.
Of course, other methods may also be adopted to determine M and allocate the time slots to be occupied. Meanwhile, they are not limited by the embodiments of the present invention, and fall within the protection scope of the embodiments of the present invention.
S12: constructing the ODTU.
The ODTU includes M time slots, and further includes Justification Control Overhead (JC OH) corresponding to JC OH position of the HO OPU. As shown in FIG. 2, the ODTU includes the time slots 2, 3, 5, 7 and 8 in the multi-frame formed of eight HO OPUs, and further includes JC OH position of HO OPU frames to which the respective time slots correspond. The shaded portions in FIG. 2 form corresponding ODTU.
S2: mapping the LO ODU to a payload area of the ODTU in an M-byte granularity.
The step includes:
S21: determining an amount of LO ODU(s) of M-bytes to be mapped when mapping in the M-byte granularity, according to an amount X of the LO ODU(s) to be transmitted during each multi-frame period, and herein is represented as C8M; in another embodiment of the present invention, determining clock information according to the amount X of the LO ODU(s) to be transmitted during each multi-frame period, and herein is represented as C8-delta. The above two information is represented as “C8M+C8-delta.”
In which, the method for acquiring X is existing, and hence is not described in the embodiments of the present invention.
Specifically, the embodiment of the present invention may use the following method to determine “C8M−C8-delta”:

C8M,MAX=(LO ODU rate*LO ODU maximum frequency deviation)/(M*TS rate*TS minimum frequency deviation)*15232

C8M,MIN=(LO ODU rate*LO ODU minimum frequency deviation)/(M*TS rate*TS maximum frequency deviation)*15232

Where C8M is an integer value with a range of [C8M, MIN lower rounding, C8M, MAX upper rounding]. C8-delta is X−M*C8M, indicating the clock information, where X is C8, and C8 is an integer value with a range of [C8, MIN lower rounding, C8, MAX upper rounding].

C8,MAX=(LO ODU rate*LO ODU maximum frequency deviation)/(TS rate*TS minimum frequency deviation)*15232

C8,MIN=(LO ODU rate*LO ODU minimum frequency deviation)/(TS rate*TS maximum frequency deviation)*15232

Assuming a certain LO ODU has its X=76111 and occupies M=5 time slots, then C8M=(X/M) lower rounding=15222, C8-delta=X−M*C8M=1. Or C8M=(X/M) lower rounding+1=15223, C8-delta=X−M*C8M=−4. The mapped data information and clock information can be completely reflected by transmitting the information “C8M+C8-delta”, i.e. (15222, 1) or (15223, −4). The receiving side may perceive, according to (15222, 1) or (15223, −4), that the transmitting side needs to transmit client data of 76111 bytes during one multi-frame period, so as to accurately recover the client clock at the receiving side.
The present invention may also use other methods to determine “C8M+C8-delta”. Meanwhile, they are not limited by the embodiments of the present invention, and all fall within the protection scope of those embodiments.
S22: mapping the amount of the LO ODU(s) of M-byte to the payload area of the ODTU in the M-byte granularity.
Mapping in the M-byte granularity means performing one time of mapping operation of M bytes of client data as a whole; as the above example, mapping the amount of the LO ODU(s) of M-byte to the payload area of the ODTU in the M-byte granularity means mapping 15222 or 15223 LO ODUs of 5-byte to the payload area of the ODTU, performing the mapping operation every 5 bytes of LO ODU, and totally mapping for 15,222 or 15,223 times.
Specifically, the sigma-delta algorithm or other Generic Mapping Procedure (GMP) mapping methods may be used to map the LO ODU(s) to the payload area of the ODTU, the other GMP mapping methods meet the following characteristics:

    
    
        1. being capable of automatically determining a filling amount according to the mapped signals and the rate of a target container;
        2. being capable of automatically determining distribution positions of the filling and mapping signals in the target container according to the mapped signals and the rate of the target container; and
        3. transporting position information carrying the filling and mapping signals in the overhead of the target container.
    
    


The characteristic information of the GMP mapping method in the embodiments of the present invention is further described with the following two mapping modes, but is not limited by the two mapping modes.
Mapping mode 1: evenly distributing filling data and mapping signal data to the payload area through the sigma-delta algorithm. Information such as positions of the filling and mapping signals is carried and transported by the overhead of the target container; the effect after the mapping is shown in FIG. 3, where S is filling data and D is mapping signal data; the filling data and the mapping signal data are evenly distributed to the payload area.
Mapping mode 2: concentratedly placing the filling data in a fixed position of the payload area, and determining which portions in the payload area are for the filling data and which portions in the payload area are for the mapping signal data according to the filling amount. Information such as positions of the filling and mapping signals is carried and transported by the overhead of the target container; the effect after the mapping is shown in FIG. 4, where S is the filling data and D is the mapping signal data.
S3: encapsulating overhead information to the overhead area of the ODTU;
In an embodiment of the present invention, the overhead information includes the amount information of the LO ODU(s) of M-byte, and encapsulating the overhead information to the overhead area of the ODTU includes: (1) encapsulating the amount information of the LO ODU(s) of M-byte to the overhead area of the ODTU; (2) Specifically, encapsulating the amount information of the LO ODU(s) of M-byte to the overhead area of the ODTU includes:
directly encapsulating the amount information of the LO ODU(s) of M-byte to the overhead area of the ODTU, and a form of “C8M” may be used; or (3) dividing the amount information of the LO ODU(s) of M-byte into a fixed portion of the amount information of the LO ODU(s) of M-bytc and a variable portion of the amount information of the LO ODU(s) of M-byte, and encapsulating the fixed portion of the amount information of the LO ODU(s) of M-byte and the variable portion of the amount information of the LO ODU(s) of M-byte to the overhead area of the ODTU; transporting in a form of this overhead information may also achieve the same effect. Specifically, a form of “C8M-base+C8M-delta” may be adopted, where C8M-base+C8M-delta equivalent to C8M, indicating an amount of M-bytes in the LO ODU(s) mapped to the payload area of the ODTU; C8M-base is an amount of M-bytes in the fixed portion, and C8M-delta is an amount of M-bytes in the variable portion; or (4) determining amount information of the LO ODU(s) of M-byte to be filled according to the amount information of the LO ODU(s) of M-byte, and encapsulating the amount information of the LO ODU(s) of M-byte to be filled to the overhead area of the ODTU; Transmitting in a form of this overhead information may also achieve the same effect; specifically, a form of “S8M” may be adopted, where S represents the filling data, S8M indicates an amount of filled bytes in the ODTU after the LO ODU(s) are mapped to the ODTU, and S8M=15232−C8M.
In another embodiment of the invention, the overhead information includes the amount information of the LO ODU(s) of M-byte and the clock information, encapsulating the overhead information to the overhead area of the ODTU includes: encapsulating the amount information of the LO ODU(s) of M-byte and the clock information to the overhead area of the ODTU.
Specifically, encapsulating the amount information of the LO ODU(s) of M-byte and the clock information to the overhead area of the ODTU includes: (1) directly encapsulating the amount information of the LO ODU(s) of M-byte and the clock information to the overhead area of the ODTU, and a form of “C8M+C8-delta” may be used; or (2) dividing the amount information of the LO ODU(s) of M-byte into a fixed portion of the amount information of the LO ODU(s) of M-byte and a variable portion of the amount information of the LO ODU(s) of M-byte, and encapsulating the fixed portion of the amount information of the LO ODU(s) of M-byte and the variable portion of the amount information of the LO ODU(s) of M-byte to the overhead area of the ODTU; transporting in a form of this overhead information may also achieve the same effect; specifically, a form of “C8M-base+C8M-delta+C8-delta” may be used, where C8M-base+C8M-delta equivalents to C8M, indicating an amount of M-bytes in the LO ODU(s) mapped to the payload area of the ODTU; C8M-base is an amount of M-bytes in the fixed portion, and C8M-delta is an amount of M-bytes in the variable portion; or (3) determining amount information of the LO ODU(s) of M-byte to be filled according to the amount information of the LO ODU(s) of M-byte, and encapsulating the amount information of the LO ODU(s) of M-byte to be filled to the overhead area of the ODTU; transporting in a form of this overhead information may also achieve the same effect; specifically, a form of “S8M+S8-delta” may be adopted, where S represents the filling data, S8M indicates an amount of filled bytes in the ODTU after the LO ODU(s) are mapped to the ODTU, S8M=15232−C8M, and S8-delta indicates the clock information.
Encapsulating the overhead information to the overhead area of the ODTU includes: encapsulating the overhead information to an overhead corresponding to a first time slot or a last time slot in the ODTU.
The overhead information indicates clock information and an amount of the LO ODU(s) of M-byte mapped to the ODTU in next n multi-frames, or indicates clock information and an amount of the LO ODU(s) of M-byte mapped to the ODTU in next n frames, where n is a natural number.
In an embodiment of the present invention, the “C8M+C8-delta” information indicates conditions of the clock information and an amount of M-bytcs in the LO ODU(s) mapped to the ODTU in the next multi-frame.
If the “C8M+C8-delta” information is encapsulated to the JC OH corresponding to the first time slot in the ODTU, i.e. the JC OH position of the 2nd frame of HO OPU corresponding to the time slot 2 in the current multi-frame, then the mapping process is shown in FIG. 5. If the “C8M+C8-delta” information is encapsulated to the JC OH corresponding to the last time slot in the ODTU, i.e. the JC OH position of the 8th frame of HO OPU corresponding to the time slot 8 in the current multi-frame, then the mapping process is shown in FIG. 6.
In another one embodiment of the invention, the “C8M-base+C8M-delta+C8-delta” information indicates conditions of the clock information and an amount of M-bytes in the LO ODU(s) mapped to the ODTU in the next frame of HO OPU.
Also taking HO OPU2 as an example, in the embodiment, as shown in FIG. 7, encapsulating “C8M-base+C8M-delta+C8-delta” to the JC OH positions of the 2nd, 3rd, 5th, 7th and 8th frames of HO OPU2 corresponding to time slots 2, 3, 5, 7 and 8 in the current multi-frame, respectively. The information “C8M-base+C8M-delta+C8-delta” at the 2nd, 3rd, 5th, 7th and 8th frames of HO OPU2 indicate clock information and amounts of M-bytes in the LO ODU(s) mapped to the payload area of the ODTU in the next frame of HO OPU2, respectively. The amount of M-bytes in the LO ODU(s) mapped to the payload area of the ODTU in other frames of HO OPU2 is C8M-base; i.e. indicating mapping C8M-base+C8M-delta M-byte data of the LO ODU(s) in an M-byte granularity to the ODTU payload areas in the 3rd, 4th, 6th and 8th frames of HO OPUs in the current multi-frame and the ODTU payload area in the 1st frame of HO OPU in the next multi-frame. Mapping C8M-base M-byte data of the LO ODU(s) in an M-byte granularity to the ODTU payload areas in the 2nd, 5th and 7th frames of HO OPUs.
In the embodiment of the present invention, the information “C8M-base+C8M-delta C8-delta” may be encapsulated in the following mode, but not limited thereby, as shown in FIG. 8.
In which, C8M-base occupies 13 bits, C8M-delta occupies 3 bits, C8-delta occupies 8 bits, and FEC occupies 8 bits; herein the FEC error correcting function is added, and an encoding mode of BCH (16, 12) may be used to achieve an effect of correcting an error of lbit, and improve the reliability of the information “C8M-base+C8M-delta+C8-delta” during transportation. In addition, the FEC may also be replaced by CRC, and the information “C8M-base+C8M-delta+C8-delta” received at the receiving side is ensured as correct through a CRC verification.
S4: multiplexing the ODTU, which has been mapped the LO ODU and encapsulated with the overhead information, to the HO OPU.
The mapping method in an OTN according to the embodiment of the present invention not only provides a high-efficient and universal mode for mapping the LO ODU to the HO OPU, compatible with processes of mapping LO ODU to HO OPU in different granularities for the convenience of interconnection, but also separates the data information from the clock information to mapping in a large granularity, and carries the clock information with the byte granularity to solve the problem of poor performance of clock recovered at the receiving side caused by mapping only with large granularity.
Correspondingly, at the data receiving side, as shown in FIG. 9, the embodiment of the present invention further provides a de-mapping method in an OTN, for parsing an HO OPU to determine an ODTU and an amount M of time slots occupied by the ODTU; and de-mapping an LO ODU from a payload area of the ODTU in an M-byte granularity.
De-mapping the LO ODU from the payload area of the ODTU in the M-byte granularity includes: (1) acquiring information of an amount of the LO ODU of M-byte from the overhead of the ODTU; and (2) de-mapping the amount of the LO ODU of M-byte from the payload area of the ODTU in the M-byte granularity.
Dc-mapping the LO ODU of M-byte from the payload area of the ODTU in the M-bytc granularity further includes: acquiring clock information from the overhead of the ODTU, and recovering the clock of client service according to the clock information.
As shown in FIG. 10, the embodiment of the present invention further provides a mapping apparatus in an OTN, including: (1) a constructing unit, configured to construct an ODTU according to an amount M of time slots of an HO OPU to be occupied by an LO ODU; (2) a mapping unit, configured to for map the LO ODU to a payload area of the ODTU in an M-byte granularity; (3) an encapsulating unit, configured to encapsulate overhead information to the overhead area of the ODTU; and (4) a multiplexing unit, configured to multiplex the ODTU, which has been mapped the LO ODU and encapsulated with the overhead information, to the HO OPU.
As shown in FIG. 11, another embodiment of the present invention provides a mapping apparatus in an OTN, including the constructing unit, the mapping unit, the encapsulating unit and the multiplexing unit as shown in FIG. 10, where the mapping unit includes: (1) a determining module, configured to determine an amount of the LO ODU of M-byte to be mapped when mapping in an M-byte granularity, according to an amount X of the LO ODU to be transmitted in each multi-frame period; (2) a mapping module, configured to map the amount of the LO ODU of M-byte to the payload area of the ODTU in the M-byte granularity; and (3) the encapsulating unit is configured to encapsulate the amount information of the LO ODU of M-byte to the overhead area of the ODTU.
In another embodiment of the present invention, the determining module is further adopted for determining clock information according to the amount X of the LO ODU to be transmitted during each multi-frame period; and the encapsulating unit is further configured to encapsulate the clock information to the overhead area of the ODTU.
As shown in FIG. 12, the embodiment of the present invention further provides a de-mapping apparatus in an OTN, including: (1) a parsing unit, configured to parse an HO OPU to determine an ODTU and an amount M of time slots occupied by the ODTU; and (2) a de-mapping unit, configured to de-map an LO ODU from a payload area of the ODTU in an M-byte granularity.
The detailed contents about signal processing and executions among the components of the above apparatuses are based on the same concept of the method embodiments of the present invention, please refer to the descriptions of the method embodiments of the present invention, and herein are not described.
The above descriptions are just some exemplary embodiments of the present invention, and the protection scope of the present invention is not limited thereby. Any modification or substitution that can be easily conceived by a person skilled in the art within the technical scope disclosed by the present invention shall be covered by the protection scope of the present invention. Therefore, the protection scope of the present invention shall be subject to that of the claims.

## Claims

1. A mapping method used in an Optical Transport Network (OTN), comprising:
constructing an Optical Channel Data Tributary Unit (ODTU) according to an integer value M larger than 1, wherein the integer value M equals the number of time slots of a High Order Optical Channel Payload Unit (HO OPU) that are to be occupied by data of a Low Order Optical Channel Data Unit (LO ODU) via the ODTU;
mapping the data of the LO ODU into a payload area of the ODTU by mapping the data of the LO ODU in units of M bytes, such that multiple units of M bytes are mapped into the payload area of the ODTU;
encapsulating overhead information to an overhead area of the ODTU; and
multiplexing the ODTU, into which the data of the LO ODU has been mapped and to which the overhead information has been encapsulated, to the HO OPU.

2. The method according to claim 1, wherein the overhead information comprises information indicating an amount Y of M-byte data units of the LO ODU which are transmitted during a multi-frame period.

3. The method according to claim 2,
wherein the overhead information further comprises clock information that is determined according to an amount of data of the LO ODU required to be transmitted during the multi-frame period and the amount Y.

4. The method according to claim 3, wherein the encapsulating the overhead information to the overhead area of the ODTU comprises at least one of the group consisting of:
(a) encapsulating information of the amount Y to the overhead area of the ODTU;
(b) dividing information of the amount Y into information of a fixed portion of the amount Y and information of a variable portion of the amount Y, and encapsulating the information of the fixed portion of the amount Y and the information of the variable portion of the amount Y to the overhead area of the ODTU; and
(c) determining an amount of M-byte filling data to be filled according to the amount Y, and encapsulating information of the amount of the M-byte filling data to be filled to the overhead area of the ODTU.

5. The method according to claim 2, wherein the encapsulating the overhead information to the overhead area of the ODTU comprises at least one of the group consisting of:
(a) encapsulating information of the amount Y to the overhead area of the ODTU;
(b) dividing information of the amount Y into information of a fixed portion of the amount Y and information of a variable portion of the amount Y, and encapsulating the information of the fixed portion amount Y and the information of the variable portion of the amount Y to the overhead area of the ODTU; and
(c) determining an amount of M-byte filling data to be filled according to the amount Y, and encapsulating information of the amount of the M-byte filling data to be filled to the overhead area of the ODTU.

6. The method according to claim 1, wherein after the multiplexing,
the overhead information is presented in an overhead area corresponding to the first time slot or the last time slot among time slots occupied by the ODTU in a multi-frame.

7. The method according to claim 1, wherein the overhead information is multiplexed to a current multi-frame comprising multiple HO OPU frames and comprises amount information indicating an amount of M-byte data units of the LO ODU which are mapped to an ODTU in next n multi-frame, wherein n is a natural number.

8. The method according to claim 1, wherein mapping the data of the LO ODU into the payload area of the ODTU is realized by using a sigma-delta algorithm.

9. The method according to claim 1, wherein mapping the data of the LO ODU into the payload area of the ODTU is realized by using a Generic Mapping Procedure (GMP) mapping method, and said GMP mapping method meets the following characteristics: automatically determining a filling amount according to mapped signals and a rate of target container; automatically determining distribution positions of filling and mapping signals in the target container according to the mapped signals and the rate of target container; and transporting position information carrying the filling and mapping signals in the overhead of the target container.

10. The method according to claim 1, wherein the overhead information comprises first overhead information indicating a fixed portion of an amount Y of M-byte data units of the LO ODU and second overhead information indicating a variable portion of the amount Y;
wherein the number of M-byte data units which are multiplexed in a first HO OPU frame allocated to the LO ODU is indicated by a combination of the first overhead information and the second overhead information;
wherein the number of M-byte data units which are multiplexed in a second HO OPU frame allocated to the LO ODU is indicated by the first overhead information without the second overhead information.

11. A mapping apparatus used in an Optical Transport Network (OTN), comprising:
a constructing unit, configured to construct an Optical Channel Data Tributary Unit (ODTU) according to an integer value M larger than 1, wherein the integer value M equals the number of time slots of a High Order Optical Channel Payload Unit (HO OPU) that are to be occupied by data of a Low Order Optical Channel Data Unit (LO ODU) via the ODTU;
a mapping module, configured to map the data of the LO ODU into a payload area of the ODTU by mapping the data of the LO ODU in units of M bytes, such that multiple units of M bytes are mapped into the payload area of the ODTU;
an encapsulating unit, configured to encapsulate overhead information to an overhead area of the ODTU; and
a multiplexing unit, configured to multiplex the ODTU, to which the data of the LO ODU has been mapped and to which the overhead information has been encapsulated, to the HO OPU.

12. The apparatus according to claim 11, wherein the overhead information comprises information indicating an amount of M-byte data units of the LO ODU which are transmitted during a multi-frame period.

13. The apparatus according to claim 11,
further comprising: a determining module, configured to determine clock information according to an amount of data of the LO ODUs to be transmitted during the multi-frame period; and
wherein the encapsulating unit is configured to encapsulate the clock information to the overhead area of the ODTU.

14. The apparatus according to claim 11, further comprising:
a determining module, configured to determine an amount Y of M-byte data units of the LO ODU during a multi-frame period;
wherein the encapsulating unit is configured to encapsulate information indicating the amount Y to the overhead area of the ODTU.

15. The apparatus according to claim 11, wherein the overhead information comprises first overhead information indicating a fixed portion of an amount Y of M-byte data units of the LO ODU and second overhead information indicating a variable portion of the amount Y;
wherein the number of M-byte data units which are multiplexed in a first HO OPU frame allocated to the LO ODU is indicated by a combination of the first overhead information and the second overhead information;
wherein the number of M-byte data units which are multiplexed in a second HO OPU frame allocated to the LO ODU is indicated by the first overhead information without the second overhead information.