Patent Publication Number: US-2022232300-A1

Title: Service data transmission method, related device, and digital processing chip

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of International Application No. PCT/CN2020/118198, filed on Sep. 27, 2020, which claims priority to Chinese Patent Application No. 201910980016.6, filed on Oct. 15, 2019. The disclosures of the aforementioned applications are hereby incorporated by reference in their entireties. 
    
    
     FIELD 
     The embodiments relate to the field of optical signal transmission, and in particular, to a service data transmission method, a related device, and a digital processing chip. 
     BACKGROUND 
     As shown in  FIG. 1 , as a broadband optical access technology, a passive optical network (PON)  101  is characterized by a point-to-multipoint physical topology structure. The PON  101  includes an optical line terminal (OLT)  102 , an optical distribution network (ODN)  103 , and a plurality of optical network units (ONUs)  104 . The PON needs to be used in cooperation with an appropriate transport mechanism, to complete end-to-end transmission of a service. An optical transport network (OTN)  105  is an ultra-reliable and interoperable high-speed optical network, includes a plurality of OTN devices, and can be used as a transport network that cooperates with the PON  101 . 
     The PON  101  and the OTN  105  are independent of each other. To implement interconnection between the PON  101  and the OTN  105 , a switching device  106  is needed. The switching device  106  may be a switch or a router. 
     SUMMARY 
     Embodiments provide a service data transmission method, a related device, and a digital processing chip, to reduce a transmission latency of service data and networking costs. 
     According to a first aspect, an embodiment provides a service data transmission method. In a transmission process of service data in an upstream transmission direction, a first device is an ONU, and a second device is a convergence device. In a transmission process of service data in a downstream transmission direction, a first device is a convergence device, and a second device is an ONU. 
     The method shown in this embodiment includes: first, a first device encapsulates a channel frame in a transmission frame, where the channel frame is used to carry service data, a transmission manner of the channel frame is a non-decapsulation manner between an optical transport network and an access network, and the transmission frame includes indication information used to indicate a channel frame; and then, the first device sends the transmission frame to a second device. 
     In this implementation, the channel frame is transmitted between the optical transport network and the access network in the non-decapsulation manner. This effectively reduces a transmission latency of the service data between the two networks. In addition, because the channel frame does not need to be forwarded between the optical transport network and the access network by using a switching device, network construction costs are effectively reduced. 
     Based on the first aspect of the embodiments, in an optional implementation, before the first device encapsulates the channel frame in the transmission frame, the first device processes the channel frame to form an intermediate frame, where a byte quantity corresponding to the intermediate frame is less than or equal to a first byte quantity, and the first byte quantity is a byte quantity corresponding to a payload area of a 10-gigabit-capable passive optical network encapsulation method XGEM frame. 
     In this implementation, when a byte quantity corresponding to the channel frame does not match the byte quantity corresponding to the XGEM frame, the first device may process the channel frame into the intermediate frame. In addition, the byte quantity corresponding to the intermediate frame is less than or equal to the byte quantity corresponding to the XGEM frame, thereby improving a success rate of encapsulating the intermediate frame in the payload area of the XGEM frame and avoiding a packet loss in the channel frame. This effectively reduces a latency in encapsulating the channel frame in the payload area of the XGEM frame. 
     Based on the first aspect of the embodiments, in an optional implementation, that the first device processes the channel frame to form an intermediate frame includes: if a second byte quantity is greater than the first byte quantity, the first device divides the channel frame to form a plurality of intermediate frames, where the second byte quantity is a byte quantity corresponding to the channel frame. 
     In this implementation, when the second byte quantity is greater than the first byte quantity, the first device may encapsulate the intermediate frame in the payload area of the XGEM frame by dividing the channel frame to form the plurality of intermediate frames. This improves a success rate of encapsulating the channel frame in the payload area of the XGEM frame and avoids a packet loss in the channel frame. 
     Based on the first aspect of the embodiments, in an optional implementation, that the first device divides the channel frame to form a plurality of intermediate frames includes: the first device obtains a division parameter, where the division parameter is a quotient between the second byte quantity used as a dividend and the first byte quantity used as a divisor; and the first device evenly divides the channel frame based on the division parameter, to form the intermediate frames, where a quantity of intermediate frames is equal to the division parameter. 
     In this implementation, the channel frame is divided by obtaining the division parameter. This effectively ensures that a byte quantity corresponding to the intermediate frame obtained after the division can be less than or equal to the first byte quantity corresponding to the XGEM frame, improves a success rate of encapsulating the channel frame in the payload area of the XGEM frame, and avoids a packet loss in the channel frame. 
     Based on the first aspect of the embodiments, in an optional implementation, that the first device processes the channel frame to form an intermediate frame includes: if a second byte quantity is less than or equal to the first byte quantity, the first device combines a plurality of channel frames to form the intermediate frame, where the second byte quantity is a byte quantity corresponding to the channel frame. 
     In this implementation, when the second byte quantity is less than or equal to the first byte quantity, the first device may combine the plurality of channel frames to form the intermediate frame, so that a byte quantity corresponding to the obtained intermediate frame is less than or equal to the first byte quantity corresponding to the XGEM frame. This effectively improves a quantity of channel frames encapsulated in the payload area of the XGEM frame and improves utilization efficiency of the payload area of the XGEM frame. 
     Based on the first aspect of the embodiments, in an optional implementation, a quotient between the first byte quantity used as a dividend and the second byte quantity used as a divisor is a positive integer greater than 1. 
     In this implementation, when determining that the quotient between the first byte quantity and the second byte quantity used as the divisor is greater than 1, the first device may combine the plurality of channel frames to form the intermediate frame. This effectively improves bandwidth utilization of the payload area of the XGEM frame. 
     Based on the first aspect of the embodiments, in an optional implementation, that a first device encapsulates a channel frame in a transmission frame includes: the first device encapsulates the intermediate frame in the payload area of the XGEM frame; and the first device encapsulates the XGEM frame in the transmission frame, where the transmission frame is a 10-gigabit-capable passive optical network transmission convergence XGTC frame. 
     In this implementation, the byte quantity corresponding to the intermediate frame obtained by the first device matches the first byte quantity corresponding to the XGEM frame, so that the first device can directly encapsulate the intermediate frame in the payload area of the XGEM frame. This effectively improves a success rate of encapsulating the intermediate frame in the payload area of the XGEM frame, and avoids a packet loss 
     Based on the first aspect of the embodiments, in an optional implementation, a first intermediate frame is used to carry first service data, a second intermediate frame is used to carry second service data, and that a first device encapsulates a channel frame in a transmission frame includes: the first device encapsulates a first service slice and a second service slice in the payload area of the XGEM frame in an interleaving manner, where an overhead of the XGEM frame carries the indication information, the first service slice belongs to the first intermediate frame, the second service slice belongs to the second intermediate frame, and a ratio between a quantity of bytes included in the first service slice and a quantity of bytes included in the second service slice is equal to a bandwidth ratio between the first service data and the second service data; and the first device encapsulates the XGEM frame in the transmission frame, where the transmission frame is an XGTC frame. 
     In this implementation, the first device encapsulates, in the XGEM frame in the interleaving manner, the first intermediate frame and the second intermediate frame that carry different service data. Therefore, a transmission latency and jitter in service data transmission can be effectively reduced, utilization of a bandwidth allocated to the first device is effectively improved, and complexity of dynamic bandwidth allocation (DBA) in a service data transmission process can be effectively reduced. 
     Based on the first aspect of the embodiments, in an optional implementation, that a first device encapsulates a channel frame in a transmission frame includes: if a second byte quantity is less than or equal to a first byte quantity, the first device encapsulates the channel frame in a payload area of an XGEM frame, where the second byte quantity is a byte quantity corresponding to the channel frame, and the first byte quantity is a byte quantity corresponding to the payload area of the XGEM frame; and the first device encapsulates the XGEM frame in the transmission frame, where the transmission frame is an XGTC frame. In this implementation, the first device can directly encapsulate the channel frame in the payload area of the XGEM frame. This effectively simplifies a process of matching the channel frame and the XGEM frame, thereby reducing a transmission latency. 
     Based on the first aspect of the embodiments, in an optional implementation, a quotient between the first byte quantity used as a dividend and the second byte quantity used as a divisor is 1. 
     Based on the first aspect of the embodiments, in an optional implementation, if the first device is an optical network unit ONU, before the first device encapsulates the channel frame in the transmission frame, the method further includes: the first device receives the service data; and the first device encapsulates the service data in the channel frame. 
     Based on the first aspect of the embodiments, in an optional implementation, if the first device is a convergence device, and the convergence device is configured to connect the optical transport network and the access network, before the first device encapsulates the channel frame in the transmission frame, the method further includes: the first device receives the channel frame from the optical transport network. 
     According to a second aspect, an embodiment provides a service data transmission method. The method includes: a second device receives a transmission frame from a first device, where the transmission frame includes indication information used to indicate a channel frame; and the second device obtains the channel frame carried in the transmission frame, where the channel frame is used to carry service data, and a transmission manner of the channel frame is a non-decapsulation manner between an optical transport network and an access network. For beneficial effects shown in this aspect, refer to those shown in the first aspect. Details are not described again. 
     Based on the second aspect of the embodiments, in an optional implementation, that the second device obtains the channel frame carried in the transmission frame includes: the second device obtains an intermediate frame carried in the transmission frame, where a byte quantity corresponding to the intermediate frame is less than or equal to a first byte quantity, and the first byte quantity is a byte quantity corresponding to a payload area of an XGEM frame; and the second device processes the intermediate frame to obtain the channel frame. 
     Based on the second aspect of the embodiments, in an optional implementation, that the second device obtains an intermediate frame carried in the transmission frame includes: the second device obtains the XGEM frame carried in the transmission frame, where the transmission frame is an XGTC frame, and an overhead of the XGEM frame carries the indication information; the second device obtains a first service slice and a second service slice from the XGEM frame, where the first service slice belongs to a first intermediate frame, the second service slice belongs to a second intermediate frame, the first intermediate frame is used to carry first service data, the second intermediate frame is used to carry second service data, and a ratio between a quantity of bytes included in the first service slice and a quantity of bytes included in the second service slice is equal to a bandwidth ratio between the first service data and the second service data; the second device obtains the first intermediate frame based on the first service slice; and the second device obtains the second intermediate frame based on the second service slice. 
     According to a third aspect, an embodiment provides a digital processing chip. The chip includes a processor and a memory, the memory and the processor are interconnected by using a line, the memory stores instructions, and the processor is configured to perform the service data transmission method according to any one of the first aspect, the second aspect, or the implementations of the first aspect or the second aspect. 
     According to a fourth aspect, an embodiment provides a first device, including a processor, a memory, and an optical transceiver. The processor, the memory, and the optical transceiver are interconnected by using a line. The processor invokes program code in the memory to perform the processing function performed by the first device according to any one of the first aspect, the second aspect, or the implementations of the first aspect or the second aspect. The optical transceiver is configured to perform the sending/receiving function performed by the first device according to any one of the first aspect, the second aspect, or the implementations of the first aspect or the second aspect. 
     According to a fifth aspect, an embodiment provides a second device, including a processor, a memory, and an optical transceiver. The processor, the memory, and the optical transceiver are interconnected by using a line. The processor invokes program code in the memory to perform the processing function performed by the second device according to any one of the first aspect, the second aspect, or the implementations of the first aspect or the second aspect. The optical transceiver is configured to perform the sending/receiving function performed by the second device according to any one of the first aspect, the second aspect, or the implementations of the first aspect or the second aspect. 
     According to a sixth aspect, an embodiment provides a computer-readable storage medium, including instructions. When the instructions run on a computer, the computer is enabled to perform the service data transmission method in any one of the first aspect, the second aspect, or the implementations of the first aspect or the second aspect. 
     According to a seventh aspect, an embodiment provides a computer program product including instructions. When the computer program product runs on a computer, the computer is enabled to perform the service data transmission method in any one of the first aspect, the second aspect, or the implementations of the first aspect or the second aspect. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is an example diagram of a networking architecture of an existing communication system; 
         FIG. 2  is an example diagram of a networking architecture of a communication system; 
         FIG. 3  is a step flowchart of an embodiment of a service data transmission method; 
         FIG. 4  is an example diagram of a protocol stack of a communication system; 
         FIG. 5  is an example diagram of a protocol stack of a communication system according to an existing solution; 
         FIG. 6  is a step flowchart of another embodiment of a service data transmission method; 
         FIG. 7  is an example diagram of service data encapsulation; 
         FIG. 8  is another example diagram of service data encapsulation; 
         FIG. 9  is a step flowchart of another embodiment of a service data transmission method; 
         FIG. 10  is another example diagram of service data encapsulation; 
         FIG. 11  is an example diagram of a frame structure of an XGTC frame; 
         FIG. 12  is a step flowchart of another embodiment of a service data transmission method; and 
         FIG. 13  is an example diagram of a structure of a first device or a second device. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     The solutions in the embodiments are clearly described in the following with reference to the accompanying drawings. 
     To better understand the service data transmission method, the following describes, with reference to  FIG. 1 , a defect of implementing service data transmission in an existing communication system. 
     In a communication system shown in  FIG. 1 , a PON  101  and an OTN  105  are of different network types, and consequently the two networks cannot be directly connected to implement end-to-end interconnection. For example, the OTN  105  uses an OTN network type, and the PON  101  uses a 10-gigabit-capable passive optical network (10 GPON) network type. Failure to implement end-to-end interconnection indicates that service data cannot be directly exchanged between the OTN  105  and the PON  101 . This causes an increase in a transmission latency of the service data. 
     The following describes a process of transmitting service data between communication networks that cannot implement end-to-end interconnection. A direction in which service data is transmitted from the OTN  105  to the PON  101  is defined as a downstream transmission direction, and a direction in which service data is transmitted from the PON  101  to the OTN  105  is defined as an upstream transmission direction. 
     For example, service data is transmitted in an upstream transmission direction. A user-side device (such as a computer, a telephone set, or a television) sends service data to an ONU  104 . The ONU  104  encapsulates the service data as a gigabit-capable passive optical network (GPON) encapsulation method (GEM) frame. The ONU  104  encapsulates the GEM frame in an upstream optical burst packet. The ONU  104  sends the upstream optical burst packet to an OLT  102 . 
     To implement end-to-end interconnection, the PON  101  and the OTN  105  need to exchange service data by using a switching device  106 . An example in which the OLT  102  is connected to the switching device  106  by using an Ethernet interface is used for description. When there is a plurality of OLTs  102 , the switching device  106  needs to be separately connected to different OLTs by using different Ethernet interfaces. That is, each Ethernet interface of the switching device  106  corresponds to only one unique OLT  102 . 
     Ethernet is a local area network communication manner that is currently most widely used and is also a protocol. The Ethernet interface is a port that uses an Ethernet protocol and that is used for data connection. The Ethernet interface may be configured to receive or send a signal that uses the Ethernet protocol, such as an Ethernet frame. 
     When the OLT  102  receives the upstream optical burst packet from the ONU  104 , the OLT  102  needs to terminate transmission of the upstream optical burst packet. The OLT  102  obtains the service data by parsing the upstream optical burst packet. The OLT  102  sends the service data to the switching device  106  by using the Ethernet interface. 
     An OTN device  107  receives an Ethernet frame from the switching device  106  by using an Ethernet interface. The OTN device  107  obtains the service data by parsing the Ethernet frame. The OTN device  107  sends the service data to a network server. Alternatively, the OTN device  107  sends the service data to another OTN device in the OTN  105 . The network server may be the Internet, a public switched telephone network (PSTN), or the like. 
     Because point-to-point communication is implemented between the PON  101  and the OTN  105  by using the Ethernet interface, a relatively large quantity of transmission media are used, costs are relatively high, and a transmission latency in a service data transmission process is high. A service data transmission technology may effectively reduce a transmission latency in a service data transmission process while implementing end-to-end interconnection between a PON and an OTN. In addition, no switching device needs to be introduced between the PON and the OTN, thereby effectively reducing network construction costs. 
     The following first provides, with reference to  FIG. 2 , example descriptions of a networking architecture of a communication system to which a service data transmission method is applied. 
     As shown in  FIG. 2 , the communication system provided in this embodiment includes a convergence device  201 . The convergence device  201  has functions of the OLT and the OTN device that are shown in  FIG. 1 . Because the OTN device is a device located in an optical transport network, and the OLT is a device located in an access network, it can be learned that the convergence device is configured to connect a PON  202  used as an access network and an OTN  203  used as a transport network. 
     The PON  202  shown in this embodiment includes an optical distribution network (ODN)  204 . The ODN  204  is connected to a plurality of optical network units (ONUs)  205 . The convergence device  201  shown in this embodiment has a PON interface. The convergence device  201  exchanges data with the ODN  204  based on the PON interface. 
     In this embodiment, for example, a network type of the PON  202  is a 10 GPON. It should be noted that a network type of the PON  202  is not limited in this embodiment. For example, the PON  202  may alternatively use a gigabit-capable passive optical network (GPON) network type. The PON  202  may alternatively use a subsequent higher-rate network type, such as a 25 G passive optical network (25 GPON), a 50 G passive optical network (50 GPON), or a 100 G passive optical network (100 GPON). Different network types of the PON  202  may use different PON interfaces described above. 
     The convergence device  201  shown in this embodiment further has an optical transport network (OTN) interface. The convergence device  201  exchanges data with an OTN device  206  in the OTN  203  based on the OTN interface. 
     The following describes, with reference to  FIG. 3  and  FIG. 4 , an embodiment of a service data transmission method.  FIG. 3  is a step flowchart of an embodiment of a service data transmission method.  FIG. 4  is an example diagram of a protocol stack of a communication system. In this embodiment, an example in which service data is transmitted in an upstream transmission direction is used as for description. 
     Step  301 : An ONU receives service data from a user-side device. 
     As shown in  FIG. 4 , an ONU  401  receives upstream service data from a user-side device by using a user network interface (UNI). A service type of the upstream service data is not limited in this embodiment. For example, the service type may be a packet service, a time division multiplexing (TDM) service, or the like. 
     Step  302 : The ONU encapsulates the service data in a channel frame. 
     The ONU may preset a value of a byte quantity corresponding to the channel frame, such as 8 bytes, 16 bytes, 32 bytes, 64 bytes, 128 bytes, 192 bytes, 256 bytes, or 512 bytes. 
     Optionally, the ONU may reduce the value of the byte quantity corresponding to the channel frame, to reduce a latency in encapsulating the service data in the channel frame. To simplify a matching process of encapsulating the service data in the channel frame, the ONU may increase the value of the byte quantity corresponding to the channel frame. 
     A frame structure of the channel frame shown in this embodiment may include bytes or bits whose quantity is an integer multiple. The channel frame may include an overhead area and a payload area. The overhead area includes but is not limited to a service frame header indication, a trail trace identifier (TTI), bit interleaved parity-X (X bit-interleaved parity, BIP-X), a backward error indication (BEI), a backward defect indication (BDI), a status indication, a timestamp, a sequence identifier, a mapping overhead, a tributary port number (TPN), or the like. The payload area is used to carry the service data. 
     The ONU may pre-determine a value of a rate of the channel frame. For example, the rate of the channel frame is 2 megabits per second (Mbps), 10 Mbps, 100 Mbps, 200 Mbps, 500 Mbps, 1 gigabit per second (Gbps), or the like. A value of the rate of the channel frame is not limited in this embodiment, provided that the rate of the channel frame is greater than or equal to a rate of the service data. 
     As shown in  FIG. 4 , an ONU-side channel layer  402  may be added to the ONU. The ONU-side channel layer  402  is configured to: determine the byte quantity corresponding to the channel frame, and asynchronously encapsulate the service data in the channel frame. A disposition manner of the ONU-side channel layer  402  is not limited in this embodiment. For example, a printed circuit board or the like configured to implement a function of the ONU-side channel layer may be disposed in the ONU. 
     Step  303 : The ONU encapsulates the channel frame in an XGEM frame. 
     The byte quantity corresponding to the channel frame is fixed, and the ONU may set a frame rate of the channel frame to carry service data with different bandwidths. A frame rate of the 10-gigabit-capable passive optical network encapsulation method (10 GPON encapsulation method, XGEM) frame is fixed, and a byte quantity corresponding to a payload area of the XGEM frame is variable. Step  303  shown in this embodiment aims to encapsulate the channel frame in the payload area of the XGEM frame. There may be two manners: 
     Manner  1   
     When the ONU determines that a second byte quantity corresponding to the channel frame does not match a first byte quantity corresponding to the payload area of the XGEM frame, the ONU may process the channel frame to form an intermediate frame. A byte quantity corresponding to the formed intermediate frame matches the first byte quantity. That a second byte quantity corresponding to the channel frame does not match a first byte quantity corresponding to the payload area of the XGEM frame may indicate that the second byte quantity is greater than the first byte quantity or indicate that the second byte quantity is less than the first byte quantity. That a byte quantity corresponding to the intermediate frame matches the first byte quantity may indicate that the byte quantity corresponding to the intermediate frame is less than or equal to the first byte quantity. 
     A manner of forming the intermediate frame is not limited in this embodiment. For example, the channel frame may be divided to form a plurality of intermediate frames. For another example, a plurality of channel frames may be combined to form the intermediate frame. When the ONU obtains the intermediate frame, the ONU can synchronously encapsulate the intermediate frame in the payload area of the XGEM frame. 
     Manner  2   
     If the ONU determines that a second byte quantity corresponding to the channel frame is less than or equal to a first byte quantity corresponding to the payload area of the XGEM frame, the ONU may directly synchronously encapsulate the channel frame in the XGEM frame. 
     As shown in  FIG. 4 , the ONU-side channel layer  402  is disposed on an XGEM layer  403 . The XGEM layer  403  may perform the foregoing process of encapsulating the channel frame in the XGEM frame. 
     Step  304 : The ONU encapsulates the XGEM frame in a transmission frame. 
     The transmission frame shown in this embodiment may be a 10-gigabit-capable passive optical network transmission convergence (XGPON transmission convergence, XGTC) frame. The ONU may encapsulate the XGEM frame in a payload area of the XGTC frame. Indication information is set in an overhead of the XGEM frame shown in this embodiment. It can be learned that the indication information is used to indicate that data carried in the payload area of the XGEM frame is the channel frame. 
     As shown in  FIG. 4 , the XGEM layer  403  is disposed on an XGTC layer  404 . The XGTC layer  404  is configured to encapsulate the XGEM frame in the XGTC frame. 
     Step  305 : The ONU sends the transmission frame to a convergence device. 
     In this embodiment, the ONU may send the transmission frame to the convergence device by using a PON interface. 
     As shown in  FIG. 4 , the XGTC layer  404  is disposed on a passive optical network physical layer (PON-PHY)  405 . The PON-PHY  405  is configured to send the transmission frame to the convergence device by using the PON interface. 
     Step  306 : The convergence device receives the transmission frame from the ONU. 
     In this embodiment, the convergence device performs de-capsulation processing on the transmission frame, to obtain the XGEM frame in the payload area of the transmission frame. 
     As shown in  FIG. 4 , a PON-PHY  407  of a convergence device  406  receives the transmission frame by using the PON interface. 
     Step  307 : The convergence device obtains the channel frame carried in the transmission frame. 
     First, when receiving the transmission frame, the convergence device can extract the XGEM frame from a payload area of the transmission frame. 
     As shown in  FIG. 4 , an XGTC layer  408  of the convergence device  406  is disposed on the PON-PHY  407 . The XGTC layer  408  obtains the transmission frame from the PON-PHY  407 , so that the XGTC layer  408  can extract the XGEM frame from the transmission frame. The XGTC layer  408  may lock a frame header of the XGEM frame based on bandwidth map information of the XGTC frame, and then extract the XGEM frame. 
     Next, the convergence device extracts the channel frame from the payload area of the XGEM frame. After obtaining the XGEM frame, the convergence device can obtain the indication information carried in the overhead of the XGEM frame, so that the convergence device can determine, based on this information, that the channel frame is carried in the payload area of the XGEM frame. 
     As shown in  FIG. 4 , an XGEM layer  409  of the convergence device  406  is disposed on the XGTC layer  408 . The XGEM layer is configured to obtain the channel frame carried in the XGEM frame. 
     Step  308 : The convergence device sends the channel frame to an OTN device. 
     In this embodiment, the convergence device determines, based on the indication information, that the data carried in the payload area of the XGEM frame is the channel frame. The convergence device determines that a transmission manner of the channel frame is a non-decapsulation manner between an optical transport network and an access network. 
     It can be learned from the descriptions shown in  FIG. 2  that, the convergence device shown in this embodiment has a function of an OLT and a function of an OTN device. For example, as shown in  FIG. 4 , that the convergence device  406  has a function of an OLT indicates that the convergence device  406  has the PON-PHY  407 , the XGTC layer  408 , and the XGEM layer  409 , and that the convergence device  406  has a function of an OTN device indicates that the convergence device  406  has an LO ODU layer  411 , an HO ODU layer  412 , and an OTN-LO layer  413 . The LO ODU layer  411  represents a lower-order optical data unit (ODU) layer. The HO ODU layer  412  represents a higher-order ODU layer. In another example, the convergence device  406  may alternatively have only one ODU layer, and a convergence-device-side channel layer  410  is configured to encapsulate the channel frame in the ODU layer. 
     As shown in  FIG. 4 , the convergence-device-side channel layer  410  is disposed in the convergence device  406  shown in this embodiment. The convergence-device-side channel layer  410  is located on both the XGEM layer  409  and the LO ODU layer  411 . The convergence device  406  may implement a function of connecting the optical transport network and the access network based on the convergence-device-side channel layer  410 . 
     When the convergence-device-side channel layer  410  obtains the channel frame from the XGEM layer  409 , the convergence-device-side channel layer  410  may directly encapsulate the channel frame in a low-order ODU of the LO ODU layer  411 . The convergence-device-side channel layer  410  can send the channel frame from the access network to the optical transport network without decapsulating the channel frame. 
     To better understand a difference between the method shown in this embodiment and an existing solution, the following provides comparison descriptions with reference to  FIG. 4  and  FIG. 5 .  FIG. 5  is an example diagram of a protocol stack of a communication system according to an existing solution. 
     As shown in  FIG. 5 , when an ONU  501  receives service data from a user-side device, the ONU  501  may sequentially perform processing based on an XGEM layer  503 , an XGTC layer  504 , and a PON-PHY  505 , to send, to an OLT  506 , an XGTC frame that carries the service data. As shown in  FIG. 5 , because no channel layer is disposed on an ONU  501  side, the XGEM layer  503  directly encapsulates the service data in an XGEM frame. For descriptions of the XGTC layer  504  and the PON-PHY  505 , refer to the corresponding descriptions in  FIG. 4 . Details are not described again. 
     A PON-PHY  507  of an OLT  506  shown in  FIG. 5  is configured to receive a transmission frame by using a PON interface. An XGTC layer  508  is configured to decapsulate the transmission frame to obtain the XGEM frame. An XGEM layer  509  is configured to obtain the service data by decapsulating the XGEM frame. The XGEM layer  509  is further configured to convert the service data into an Ethernet frame and send the Ethernet frame to a switching device  510 . The switching device  510  forwards the Ethernet frame to an OTN device  511 , so that the OTN device  511  can obtain the service data by parsing the Ethernet frame. It can be learned that based on the communication system shown in  FIG. 5 , decapsulation processing needs to be performed on the XGEM frame to obtain the service data, and then the service data is transmitted from an access network to an optical transport network by using an Ethernet interface. 
     As shown in  FIG. 4 , the XGEM layer  409  shown in this embodiment performs decapsulation processing on the XGEM frame to obtain the channel frame. The convergence device does not further perform decapsulation processing on the channel frame. The convergence-device-side channel layer  410  may directly encapsulate the channel frame in the LO ODU layer  411 . After the HO ODU layer  412  and the OTN-LO layer  413  sequentially perform processing, an optical transport network (OTN) frame that carries the channel frame is sent to an OTN device  414 . 
     An OTN-side channel layer  415  is disposed in the OTN device  414 , and the OTN-side channel layer  415  is configured to obtain the service data by decapsulating the channel frame. The OTN-side channel layer  415  is further configured to send the service data to a network server by using a service network interface (SNI). 
     It can be learned that, in this embodiment, a hard pipeline for transmitting the channel frame may be created between the convergence device and the ONU, to implement end-to-end interconnection between the optical transport network and the access network. As shown in  FIG. 4 , the hard pipeline for transmitting the channel frame may be created between the access network and the optical transport network by using the ONU-side channel layer  402  and the convergence-device-side channel layer  410 . The hard pipeline is a pipeline that can effectively ensure that transmission is not interrupted, and no packet loss occurs in a process of transmitting the channel frame. Transmission performance of the channel frame can be directly monitored by using the hard pipeline, thereby effectively reducing monitoring complexity. In a process in which the channel frame is transmitted between the access network and the optical transport network in the non-decapsulation manner, a transmission latency in transmitting the service data between the access network and the optical transport network is effectively reduced. 
     The following describes, with reference to  FIG. 6 , another embodiment of a service data transmission method. The embodiment shown in  FIG. 6  describes how to process a channel frame to form an intermediate frame, and how to implement a transmission process of service data in an upstream transmission direction based on the intermediate frame. 
     Step  601 : An ONU receives service data from a user-side device. 
     Step  602 : The ONU encapsulates the service data in a channel frame. 
     For execution processes of step  601  and step  602 , refer to the related descriptions of step  301  and step  302  that are shown in  FIG. 3 . Details are not described herein again. 
     Step  603 : The ONU processes the channel frame to form an intermediate frame. 
     In this embodiment, the ONU needs to process the channel frame to form the intermediate frame, and then encapsulate, in a payload area of an XGEM frame, the intermediate frame that carries the service data. A manner of processing the channel frame is not limited in this embodiment, provided that a second byte quantity corresponding to the intermediate frame formed after the processing is less than or equal to a first byte quantity. The first byte quantity is a byte quantity corresponding to the payload area of the XGEM frames. The following describes several optional manners of forming the intermediate frame. 
     Manner  1   
     In this manner, if a second byte quantity corresponding to the channel frame is greater than the first byte quantity, the ONU may divide the channel frame to form a plurality of intermediate frames. A detailed process is as follows: 
     The ONU obtains a division parameter. The division parameter is a quotient between the second byte quantity used as a dividend and the first byte quantity used as a divisor. 
     The ONU evenly divides the channel frame based on the division parameter, to form the intermediate frames. A quantity of intermediate frames is equal to the division parameter. If the division parameter is N, in this manner, for example, the determined division parameter N is a positive integer greater than 1. The ONU may evenly divide the channel frame into N parts. Each part obtained by evenly dividing the channel frame is an intermediate frame shown in this embodiment. 
     In this embodiment, an example in which the channel frame is evenly divided is used for example description. In another example, the channel frame may be alternatively unevenly divided, provided that a byte quantity corresponding to each intermediate frame is less than or equal to the first byte quantity. 
     To better understand this manner, the following describes this manner with reference to an example. 
       FIG. 7  is an example diagram of service data encapsulation. The following first describes preconditions of this example: 
     Precondition a: An ONU sends a transmission frame in a slot specified by a convergence device, and a frame rate of the transmission frame is 8000 times per second. 
     Precondition b: A bandwidth of service data is 3 megabits per second (Mbps). 
     Precondition c: A second byte quantity corresponding to a channel frame is 192 bytes. 
     First, an ONU encapsulates service data  701  in a channel frame  702 . For an execution process, refer to step  601  and step  602 . 
     Next, the ONU may calculate a first byte quantity based on the precondition a and the precondition b. 3 Mbps=3072 kbps (kilobits per second)=3072000 bits (bits per second). If the first byte quantity is set to X, X*8000*8=3072000 bit/s. X=48 bytes can be derived. 
     Then, the ONU may determine a division parameter based on the first byte quantity and the precondition c. When the ONU determines that a second byte quantity used as a dividend=192 and the first byte quantity used as a divisor=48, the ONU determines that the second byte quantity/the first byte quantity=192/48=4. In this case, the ONU determines that the division parameter=4, so that the ONU can evenly divide the channel frame to form four intermediate frames  703 . In addition, a byte quantity corresponding to each intermediate frame  703 =192/4=48. 
     It can be learned that, in this example, if the second byte quantity used as the dividend can be exactly divided by the first byte quantity used as the divisor, the ONU can directly evenly divide the channel frame based on the division parameter. 
     The following example is described by using an example in which the second byte quantity used as the dividend cannot be exactly divided by the first byte quantity used as the divisor. The following first describes preconditions of this example: 
     Precondition a: An ONU sends a transmission frame in a slot specified by a convergence device, and a frame rate of the transmission frame is 8000 times per second. 
     Precondition b: A bandwidth of service data is 10 Mbps. 
     Precondition c: A second byte quantity corresponding to a channel frame is 350 bytes. 
     An ONU may calculate a first byte quantity based on the precondition a and the precondition b. When the first byte quantity is set to X, X*8000*8=10240000 bits. X=160 bytes can be derived. 
     Then, the ONU may determine a division parameter based on the first byte quantity and the precondition c. When the ONU determines that a second byte quantity used as a dividend=350 and the first byte quantity used as a divisor=160, the ONU determines the second byte quantity/the first byte quantity=350/160=2.18. In this case, the ONU determines that the division parameter=2, so that the ONU can evenly divide a channel frame to form two intermediate frames. In addition, a byte quantity corresponding to each intermediate frame=350/2=175. 
     In this example, if the second byte quantity used as the dividend cannot be exactly divided by the first byte quantity used as the divisor, the ONU can directly evenly divide the channel frame based on the division parameter. 
     Manner  2   
     In the foregoing manner  1 , an example in which the channel frame is divided into the intermediate frames is used for description. In this manner, an example in which a plurality of channel frames is combined to form the intermediate frame is used for description. 
     In this manner, if a second byte quantity is less than or equal to the first byte quantity, the ONU combines a plurality of channel frames to form the intermediate frame. In this manner, a quantity of channel frames included in one intermediate frame is not limited, provided that a byte quantity corresponding to the intermediate frame is less than or equal to the first byte quantity. The following describes how the ONU combines the plurality of channel frames to form the intermediate frame. 
     First, the ONU may determine a combination frame quantity, where the combination frame quantity is a quotient between the first byte quantity used as a dividend and the second byte quantity used as a divisor. 
     Next, if the ONU determines that the channel frame meets a combination condition, the ONU combines channel frames whose quantity is the combination frame quantity, to form the intermediate frame. The combination condition is that the second byte quantity is less than or equal to the first byte quantity. The combination condition may further include that the combination frame quantity is a positive integer greater than 1. 
     For better understanding, the following provides descriptions with reference to an example.  FIG. 8  is another example diagram of service data encapsulation. In this manner, for example, a second byte quantity corresponding to each channel frame  802  that carries service data  801  is 24. In an example in which an ONU determines that a first byte quantity is 72, the ONU can determine that a combination frame quantity is 72/24=3, so that the ONU can determine that the channel frame meets the combination condition, and therefore the ONU can combine three channel frames to form one intermediate frame  803 . In this case, a byte quantity corresponding to the intermediate frame  803 =24*3=72. 
     Step  604 : The ONU encapsulates the intermediate frame in the payload area of the XGEM frame. 
     After the ONU obtains the intermediate frame, the ONU can encapsulate the intermediate frame in the payload area of the XGEM frame. 
     In this embodiment, an example in which the intermediate frame is encapsulated in the payload area of the XGEM frame is used for example description. In another optional example, the ONU may directly encapsulate the channel frame in the payload area of the XGEM frame without processing the channel frame. 
     When determining that the channel frame meets a direct encapsulation condition, the ONU can directly encapsulate the channel frame in the payload area of the XGEM frame. The direct encapsulation condition is that a second byte quantity is less than or equal to the first byte quantity. The direct encapsulation condition may further include that a combination frame quantity is equal to 1. For descriptions of the combination frame quantity, refer to step  603 . Details are not described again. For example, if the second byte quantity corresponding to the channel frame is 192, and the first byte quantity is 200, the ONU can directly encapsulate the channel frame in the payload area of the XGEM frame. 
     Step  605 : The ONU encapsulates the XGEM frame in a transmission frame. 
     In this embodiment, the transmission frame is an XGTC frame. The ONU may encapsulate the XGEM frame in a payload area of the XGTC frame. For better understanding, the following still uses those shown in  FIG. 7  as an example for description. 
     As shown in  FIG. 7 , because a frame rate of a transmission frame shown in this embodiment is 8000 times per second, a slot occupied by each transmission frame is 125 microseconds (μs). The ONU can encapsulate an XGEM frame  704  in a payload area of an XGTC frame  705 . 
     Step  606 : The ONU sends the transmission frame to a convergence device. 
     Step  607 : The convergence device receives the transmission frame from the ONU. 
     For descriptions of step  606  and step  607 , refer to the related descriptions of step  305  and step  306  that are shown in  FIG. 3 . Details are not described herein again. 
     Step  608 : The convergence device obtains the channel frame carried in the transmission frame. 
     First, when receiving the transmission frame, the convergence device can extract the XGEM frame from a payload area of the transmission frame. 
     Next, the convergence device extracts the intermediate frame from the payload area of the XGEM frame. After obtaining the XGEM frame, the convergence device can obtain indication information carried in an overhead of the XGEM frame, so that the convergence device can determine, based on the indication information, that data carried in the payload area of the XGEM frame is the intermediate frame. 
     Then, the convergence device processes the intermediate frame to form the channel frame. The convergence device may execute an inverse process of step  603  shown in  FIG. 6 , to implement a process of obtaining the channel frame based on the intermediate frame. For better understanding, the following describes, with reference to step  603 , a process of obtaining the channel frame based on the intermediate frame. 
     Manner  1   
     If the ONU divides the channel frame to form the plurality of intermediate frames (for details, refer to the manner  1  shown in step  603 ), the convergence device may combine a plurality of intermediate frames that carry a same service into a channel frame in a framing manner. 
     The example shown in  FIG. 7  is still used. When the convergence device receives four XGTC frames  705  shown in  FIG. 7 , the convergence device may decapsulate the four XGTC frames  705  to obtain XGEM frames  704 . The convergence device determines, based on indication information included in overheads of the XGEM frames  704 , that data carried in the XGEM frames  704  is intermediate frames. The convergence device determines, based on the overheads of the XGEM frames  704 , intermediate frames that carry same service data, that is, four intermediate frames  703  shown in  FIG. 7 , so that the convergence device can combine the four intermediate frames  703  in a service data encapsulation sequence, to form a channel frame. 
     Manner  2   
     If the ONU combines the plurality of channel frames to form the intermediate frame (for details, refer to the manner  2  shown in step  603 ), in this case, the intermediate frame carries the plurality of channel frames. The convergence device may divide the intermediate frame to form the plurality of channel frames. 
     The example shown in  FIG. 8  is still used. When the convergence device obtains an XGEM frame  804  from an XGTC frame, the convergence device determines, based on an overhead of the XGEM frame  804 , that data carried in the XGEM frame  804  is an intermediate frame. The convergence device determines, based on the overhead of the XGEM frame, a plurality of channel frames included in the intermediate frame  803 , so that the convergence device can divide the intermediate frame  803  to form three channel frames  802  shown in  FIG. 8 . 
     Step  609 : The convergence device sends the channel frame to an OTN device. 
     For an execution process of step  609 , refer to step  308  shown in  FIG. 3 . Details are not described herein again. 
     In this embodiment, when a frame rate of the XGEM frame does not match a frame rate of the channel frame, and the second byte quantity corresponding to the channel frame is fixed, the ONU can process the channel frame to form the intermediate frame, so that the ONU can successfully encapsulate the intermediate frame in the payload area of the XGEM frame, thereby improving a success rate of encapsulating the channel frame in the payload area of the XGEM frame, and avoiding a packet loss in the channel frame. This effectively reduces a latency in encapsulating the channel frame in the XGEM frame. 
     In the embodiment shown in  FIG. 6 , how to transmit, from an access network to an optical transport network, channel frames that carry same service data is used as an example for description. The following describes, with reference to  FIG. 9 , how to transmit, from an access network to an optical transport network, channel frames that carry different service data. 
     A quantity of pieces of service data transmitted by an ONU to a convergence device is not limited in this embodiment. For better understanding, in this embodiment, an example in which the ONU transmits two pieces of service data to the convergence device is used for example description. If the ONU needs to transmit more than two pieces of service data to the convergence device, a person skilled in the art may obviously obtain a method, based on the method shown in this embodiment, without creative efforts. This is not described in this embodiment in detail. 
     Step  901 : The ONU receives first service data and second service data. 
     Service types of the first service data and the second service data are not limited in this embodiment. For example, a service type of the first service data is a video on-live service, and a service type of the second service data is a video on demand service. 
     Step  902 : The ONU encapsulates the first service data in a first channel frame. 
     Step  903 : The ONU encapsulates the second service data in a second channel frame. 
     For processes shown in step  902  and step  903 , refer to the process, of encapsulating the service data in the channel frame, shown in the foregoing method embodiments. Details are not described herein again. There is no execution time sequence limitation between step  902  and step  903 . 
     Step  904 : The ONU processes the first channel frame to form a first intermediate frame. 
     Step  905 : The ONU processes the second channel frame to form a second intermediate frame. 
     For processes shown in step  904  and step  905 , refer to the process, of processing the channel frame to form the intermediate frame, shown in the foregoing method embodiments. Details are not described herein again. The formed first intermediate frame is used to carry the first service data, and the formed second intermediate frame is used to carry the second service data. There is no execution time sequence limitation between step  904  and step  905 . 
     Step  906 : The ONU obtains a first service slice that belongs to the first intermediate frame. 
     Step  907 : The ONU obtains a second service slice that belongs to the second intermediate frame. 
     The ONU shown in this embodiment needs to separately extract at least some bytes from the first intermediate frame to form the first service slice. The ONU further needs to extract at least some bytes from the second intermediate frame to form the second service slice. For better understanding, the following describes a manner in which the ONU obtains the first service slice and the second service slice. 
     First, the ONU determines a target bandwidth ratio. The target bandwidth ratio is a ratio between a bandwidth occupied by the first service data and a bandwidth occupied by the second service data. For example, if the bandwidth occupied by the first service data is 5 Mbps, and the bandwidth occupied by the second service data is 4 Mbps, the ONU can determine that the target bandwidth ratio is 5/4. 
     Next, the ONU extracts the first service slice and the second service slice based on the target bandwidth ratio. A ratio between a quantity of bytes included in the first service slice and a quantity of bytes included in the second slice is equal to the target bandwidth ratio. 
     Still referring to the foregoing example, that is, the example in which the target bandwidth ratio is 5/4, the ONU may extract 5 bytes from the first intermediate frame to form the first service slice, and the ONU may further extract 4 bytes from the second intermediate frame to form the second service slice. It can be learned that the ONU only needs to separately extract the first service slice from the first intermediate frame and the second service slice from the second intermediate frame based on an integer multiple of the target bandwidth ratio. 
     Step  908 : The ONU encapsulates the first service slice and the second service slice in a payload area of an XGEM frame in an interleaving manner. 
     To better understand the method shown in this embodiment, the following describes the method with reference to  FIG. 10 . In this example, a service bandwidth occupied by first service data  1010  is 4 Mbps, and a service bandwidth occupied by second service data  1020  is 6 Mbps. 
     As shown in  FIG. 10 , the ONU forms first intermediate frames for a first channel frame  1011 , for example, a first intermediate frame  1012 , a first intermediate frame  1013 , and a first intermediate frame  1014  that are shown in  FIG. 10 . The ONU forms second intermediate frames for a second channel frame  1021 , for example, a second intermediate frame  1022  and a second intermediate frame  1023  that are shown in  FIG. 10 . 
     The ONU may determine, based on the service bandwidth occupied by the first service data  1010  and the service bandwidth occupied by the second service data  1020 , that a target bandwidth ratio=4/6=2/3. 
     The ONU may extract 2 bytes from the first intermediate frame  1012  to form a first service slice  1015 . The ONU may further extract 3 bytes from the second intermediate frame  1022  to form a second service slice  1024 . 
     Therefore, the ONU can encapsulate the first service slice  1015  and the second service slice  1024  in a payload area of an XGEM frame  1030 . The process is performed through polling until the first service data  1010  and the second service data  1020  are completely encapsulated in the XGEM frame. 
     Step  909 : The ONU encapsulates the XGEM frame in a transmission frame. 
     Still as shown in  FIG. 10 , the ONU may encapsulate the XGEM frame in a transmission frame  1031 . For a process, refer to step  605  shown in  FIG. 6 . Details are not described again. 
     Step  910 : The ONU sends the transmission frame to the convergence device. 
     Step  911 : The convergence device receives the transmission frame from the ONU. 
     For execution processes of step  910  and step  911  that are shown in this embodiment, refer to step  606  and step  607  that are shown in  FIG. 6 . The execution processes are not described again. 
     Step  912 : The convergence device obtains the first channel frame and the second channel frame that are carried in the transmission frame. 
     First, when receiving the transmission frame, the convergence device can extract the XGEM frame from a payload area of the transmission frame. 
     Next, the convergence device extracts the first service slice and the second service slice from the payload area of the XGEM frame. After obtaining the XGEM frame, the convergence device can obtain indication information carried in an overhead of the XGEM frame, so that the convergence device can determine, based on the indication information, that data carried in the payload area of the XGEM frame is the first service slice and the second service slice. 
     Then, the convergence device combines all obtained first service slices to form the first intermediate frame. The convergence device may further combine all obtained second service slices to form the second intermediate frame. 
     When the convergence device receives the first service slice and the second service slice, the convergence device may perform intermediate frame combination based on a quantity of bytes included in each service slice. The foregoing example is still used. The convergence device may determine that first service slices each having 2 bytes come from a same first intermediate frame, and second service slices each having 3 bytes come from a same second intermediate frame. 
     Then, the convergence device processes the first intermediate frame to form the first channel frame. The convergence device may further process the second intermediate frame to form the second channel frame. For a process, refer to step  608  shown in  FIG. 6 . Details are not described again. 
     Step  913 : The convergence device sends the channel frames to an OTN device. 
     For an execution process of step  913  shown in this embodiment, refer to step  609  shown in  FIG. 6 . Details are not described again. 
     A plurality of hard pipelines may be disposed in the ONU shown in this embodiment. Each hard pipeline is configured to encapsulate one piece of service data in a channel frame. A transmission manner of the channel frame is a non-decapsulation manner between an optical transport network and an access network. Because the ONU shown in this embodiment needs to transmit a plurality of pieces of service data, the plurality of pieces of service data may be transmitted by using different hard pipelines. 
     In this embodiment, an example in which the ONU allocates a bandwidth by considering all hard pipelines as a whole is used for description. That the ONU allocates a bandwidth by considering all hard pipelines as a whole indicates that the ONU encapsulates, in a payload area of a same XGEM frame in an interleaving manner by considering all channel frames that carry different service data as a whole, all the channel frames that carry the different service data. 
     Optionally, in another example, the ONU may separately allocate bandwidths to hard pipelines. That is, the ONU encapsulates, in payload areas of different XGEM frames, channel frames that carry different service data. 
     In the method shown in this embodiment, the ONU allocates a bandwidth by considering all hard pipelines as a whole, so that a transmission latency and jitter in service data transmission are reduced, utilization of a bandwidth allocated to the ONU is effectively improved, and complexity of dynamic bandwidth allocation (DBA) in a service data transmission process can be effectively reduced. 
     The following describes, with reference to  FIG. 11 , a frame format of a transmission frame used to transmit service data from an access network to an optical transport network. In this embodiment, an example in which the access network is an XGPON is used for example description. 
     As shown in  FIG. 11 , an upstream overhead of an XGTC frame  1101  may include physical layer overhead upstream (PLOu), physical layer operation, administration, physical layer OAM upstream (PLOAMu), dynamic bandwidth report upstream (DBRu), and the like. 
     An XGEM frame  1103  is carried in an XGTC frame payload area  1102  of the XGTC frame  1101 . A quantity of XGEM frames carried in the XGTC frame payload area  1102  is not limited in this embodiment, that is, the XGTC frame payload area  1102  may carry a plurality of XGEM frames. 
     A payload area of the XGEM frame shown in this embodiment carries a channel frame  1104 . In  FIG. 11 , an example in which the payload area of the XGEM frame  1103  carries the channel frame  1104  is used for example description. In the foregoing method embodiments, the ONU may alternatively process the channel frame to form an intermediate frame. In this example, the payload area of the XGEM frame  1103  carries the intermediate frame. For descriptions, refer to the foregoing method embodiments. Details are not described again. 
     An XGEM frame header  1105  shown in this embodiment includes a payload length indicator (PLI) field, a Port ID field used to support multi-port multiplexing, a key index field, a last fragment (LF) field, and a header error check (HEC) field. 
     In this embodiment, the XGEM frame header  1105  further includes a reserved options field. The options field is used to carry indication information, and the indication information is used to indicate the channel frame. For descriptions of the indication information, refer to the foregoing method embodiments. Details are not described again. 
     The foregoing embodiments describe transmission of service data in an upstream transmission direction, and the following describes transmission of service data in a downstream transmission direction with reference to  FIG. 12 . 
     Step  1201 : An OTN device receives service data from a network server. 
     As shown in  FIG. 4 , the OTN device receives the service data from the network server by using a service network interface. For descriptions of the network server, refer to that shown in  FIG. 1 . Details are not described again. 
     Step  1202 : The OTN device encapsulates the service data in a channel frame. 
     For a process in which the OTN device encapsulates the service data in the channel frame, refer to the descriptions (such as step  302  in  FIG. 3  and step  602  in  FIG. 6 ), of encapsulating, by the ONU, the service data in the channel frame, shown in the foregoing method embodiments. Details are not described herein again. 
     As shown in  FIG. 4 , an OTN-side channel layer  415  may be added to the OTN device. The OTN-side channel layer  415  is configured to: determine a byte quantity corresponding to the channel frame and encapsulate the service data in the channel frame. For descriptions of the OTN-side channel layer  415 , refer to the descriptions of the ONU-side channel layer  402  in the embodiment shown in  FIG. 3 . Details are not described again. 
     Step  1203 : The OTN device encapsulates the channel frame in an XGEM frame. 
     For an execution process of step  1203  shown in this embodiment, refer to the process, in which the ONU encapsulates the channel frame in the XGEM frame, in the foregoing method embodiments, such as step  303  shown in  FIG. 3 , step  603  and step  604  that are shown in  FIG. 6 , and step  904  to step  908  that are shown in  FIG. 9 . 
     Step  1204 : The OTN device encapsulates the XGEM frame in a transmission frame. 
     Step  1205 : The OTN device sends the transmission frame to a convergence device. 
     Step  1206 : The convergence device receives the transmission frame from the OTN device. 
     Step  1207 : The convergence device obtains the channel frame carried in the transmission frame. 
     Step  1208 : The convergence device sends the channel frame to an ONU. 
     For execution processes of step  1204  to step  1208  that are shown in this embodiment, refer to step  304  to step  308  that are shown in  FIG. 3 , refer to step  605  to step  609  that are shown in  FIG. 6 , or refer to step  909  to step  913  that are shown in  FIG. 9 . Details are not described again. 
     An ONU-side channel layer  402  of the ONU is configured to obtain the service data by de-capsulating the channel frame. The ONU-side channel layer  402  is further configured to send the obtained service data to a corresponding user-side device. 
     A difference between the transmission frame, in the downstream transmission direction, shown in this embodiment and the transmission frame, in the upstream transmission direction, shown in the foregoing method embodiments lies in that the transmission frame in the upstream transmission direction is transmitted in a burst manner, and the transmission frame in the downstream transmission direction is transmitted in a continuous manner. For descriptions of a frame format of the transmission frame shown in this embodiment, refer to those shown in  FIG. 11 . Details are not described again. 
     For descriptions of beneficial effects of the method shown in this embodiment, refer to those shown in the foregoing method embodiments. Details are not described in this embodiment again. 
     The following describes, with reference to  FIG. 13 , a first device and a second device. In transmission of service data in an upstream transmission direction, the first device is the ONU in the foregoing embodiments, and the second device is a convergence device. In transmission of service data in a downstream transmission direction, the first device is the convergence device in the foregoing embodiments, and the second device is an ONU. The following describes structures of the first device and the second device with reference to different transmission directions of service data. 
     In upstream transmission of service data: 
     In a possible implementation,  FIG. 13  may be a schematic diagram of a structure of a first device. The first device includes a processor  1301 , a memory  1302 , and an optical transceiver  1303 . The processor  1301 , the memory  1302 , and the optical transceiver  1303  are interconnected by using a line. The memory  1302  is configured to store program instructions and data. 
     In a possible implementation, the memory  1302  stores program instructions and data that support the steps performed by the ONU in the steps shown in  FIG. 3 ,  FIG. 6 , and  FIG. 9 , and the processor  1301  and the optical transceiver  1303  are configured to perform the method steps, performed by the ONU, shown in any one of the embodiments shown in  FIG. 3 ,  FIG. 6 , and  FIG. 9 . 
     In  FIG. 13 , the optical transceiver  1303  is configured to perform step  301  and step  305 , and the processor  1301  is configured to perform step  302  to step  304 . In  FIG. 6 , the optical transceiver  1303  is configured to perform step  601  and step  606 , and the processor  1301  is configured to perform step  602  to step  605 . In  FIG. 9 , the optical transceiver  1303  is configured to perform step  901  and step  910 , and the processor  1301  is configured to perform step  902  to step  909 . 
     In another possible implementation,  FIG. 13  may be alternatively an example diagram of a structure of a second device. Therefore, in this implementation, a memory  1302  stores program instructions and data that support the steps performed by the convergence device in the steps shown in  FIG. 3 ,  FIG. 6 , and  FIG. 9 , and a processor  1301  and an optical transceiver  1303  are configured to perform the method steps, performed by the convergence device, shown in any one of the embodiments shown in  FIG. 3 ,  FIG. 6 , and  FIG. 9 . 
     In  FIG. 3 , the optical transceiver  1303  is configured to perform step  306  and step  308 , and the processor  1301  is configured to perform  307 . In  FIG. 6 , the optical transceiver  1303  is configured to perform step  607  and step  609 , and the processor  1301  is configured to perform  608 . In  FIG. 9 , the optical transceiver  1303  is configured to perform step  911  and step  913 , and the processor  1301  is configured to perform  912 . 
     In downstream transmission of service data: 
     In a possible implementation,  FIG. 13  may be a schematic diagram of a structure of a first device. A memory  1302  shown in  FIG. 13  stores program instructions and data that support the steps performed by the convergence device in the steps shown in  FIG. 12 , and a processor  1301  and an optical transceiver  1303  are configured to perform the method steps, performed by the convergence device, shown in  FIG. 12 . In  FIG. 12 , the optical transceiver  1303  is configured to perform step  1206  and step  1208 , and the processor  1301  is configured to perform  1207 . 
     In another possible implementation,  FIG. 13  may be alternatively an example diagram of a structure of a second device. A memory  1302  shown in  FIG. 13  stores program instructions and data that support the steps performed by the ONU in the steps shown in  FIG. 12 , and a processor  1301  and an optical transceiver  1303  are configured to perform the method steps, performed by the ONU, shown in  FIG. 12 . For descriptions of the method steps performed by the ONU, refer to those shown in  FIG. 12 . Details are not described again. 
     An embodiment further provides a digital processing chip. A circuit and one or more interfaces that are configured to implement functions of the foregoing processor  1301  are integrated into the digital processing chip. When a memory is integrated into the digital processing chip, the digital processing chip may complete the method steps in any one or more of the foregoing embodiments. When a memory is not integrated into the digital processing chip, the digital processing chip may be connected to an external memory through an interface. The digital processing chip implements, based on program code stored in the external memory, the actions performed by the ONU or the convergence device in the foregoing embodiments. 
     A person of ordinary skill in the art may understand that all or some of the steps of the embodiments may be implemented by a program instructing related hardware. The program may be stored in a computer-readable storage medium. The foregoing storage medium may be a read-only memory, a random access memory, or the like. For example, the foregoing processing unit or processor may be a central processing unit, a general-purpose processor, a digital signal processor (DSP), an application-specific integrated circuit (ASIC), a field programmable gate array (FPGA) or another programmable logic device, a transistor logic device, a hardware component, or any combination thereof. Whether the functions are performed by hardware or software depends on particular applications and design constraint conditions of the solutions. A person skilled in the art may use different methods to implement the described functions for each particular application, but it should not be considered that the implementation goes beyond the scope. 
     When software is used to implement the embodiments, the method steps in the foregoing embodiments may be implemented completely or partially in a form of a computer program product. The computer program product includes one or more computer instructions. When the computer program instructions are loaded and executed on a computer, the procedures or functions according to the embodiments are completely or partially generated. The computer may be a general-purpose computer, a special-purpose computer, a computer network, or another programmable apparatus. The computer instructions may be stored in a computer-readable storage medium or may be transmitted from a computer-readable storage medium to another computer-readable storage medium. For example, the computer instructions may be transmitted from a website, computer, server, or data center to another website, computer, server, or data center in a wired (for example, a coaxial cable, an optical fiber, or a digital subscriber line (DSL)) or wireless (for example, infrared, radio, or microwave) manner. The computer-readable storage medium may be any usable medium accessible by a computer, or a data storage device, such as a server or a data center, integrating one or more usable media. The usable medium may be a magnetic medium (for example, a floppy disk, a hard disk, or a magnetic tape), an optical medium (for example, a DVD), or a semiconductor medium. 
     Finally, it should be noted that, the foregoing descriptions are merely implementations, but are not intended to limit the scope of the embodiments. Any variation or replacement readily figured out by a person skilled in the art shall be envisioned to be within the scope of the embodiments.