Patent Publication Number: US-11038610-B2

Title: OTN adaptation for support of subrate granularity and flexibility and for distribution across multiple modem engines

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
     The present application/patent is a continuation of U.S. patent application Ser. No. 16/137,977, filed Sep. 21, 2018, and entitled “OTN ADAPTATION FOR SUPPORT OF SUBRATE GRANULARITY AND FLEXIBILITY AND FOR DISTRIBUTION ACROSS MULTIPLE MODEM ENGINES,” which is a continuation of U.S. patent application Ser. No. 14/467,769, filed Aug. 25, 2014, and entitled “OTN ADAPTATION FOR SUPPORT OF SUBRATE GRANULARITY AND FLEXIBILITY AND FOR DISTRIBUTION ACROSS MULTIPLE MODEM ENGINES,” the contents of each are incorporated by reference herein. 
    
    
     FIELD OF THE DISCLOSURE 
     The present disclosure relates generally to networking systems and methods. More particularly, the present disclosure relates to Optical Transport Network (OTN) adaptation for distribution to one or more optical modem engines. 
     BACKGROUND OF THE DISCLOSURE 
     In Optical Transport Network (OTN), there is momentum regarding flex modulation and flex line rates. There are different ways to achieve a flex rate. For example, some vendors have created proprietary Optical channel Data Unit Group (ODUG) Super High Order (SHO) wrappers to handle cases of 50G, 100G, 200G, etc. for modem technologies. In ITU-T, there have been discussions about defining OTUCn, a byte interleaved scheme for flexibility in increments of 100G. OTUCn stands for Optical channel Transport Unit Cn where C means 100 and n is a multiplier of 100, e.g. OTUC2 is 200 GB/s, OTUC4 is 400 GB/s, etc. The current ITU-T OTUCn standards are planning on defining a modular (not necessarily flexible) architecture for 100G slices and granularity. The problem is that this architecture does not give enough granularity on next-Gen devices for bandwidth versus performance/reach tradeoffs. It also does not cover some modulation rates (e.g., 8-Quadrature Amplitude Modulation (8QAM) at 150G) that are not aligned to 100G boundaries. Other initiatives have proposals to turn off single or groups of physical, virtual, or logical lanes in a Physical Medium Dependent (PMD) layer to achieve a desired rate. There are some significant implementation and logic complexities when designing a protocol to support multiple different rates at the physical layer. 
     This flexible line rate is becoming a hot topic in the industry and recent activities by end users include a desire for sub-100G granularity (25G or 50G). Again, some conventional schemes address flexibility by turning off physical or virtual lanes, but complexity and logic cost is significant. Resizing using lanes scheme is also a challenge. The 25G/50G granularity does not line up well to existing 10G traffic. Also, scaling conventional techniques for mux/mapping rates of 10G up to 500G requires large logic complexity. 
     As optical transmission systems start approaching the Shannon limit for non-linear noise and demand for increased data rates continues, Digital Signal Processing (DSP)/modem engines can get implemented in parallel devices or multiple engines get integrated to create super-channels with optical or electrical mixing. Low complexity and flexible schemes are needed for inverse multiplexing (“muxing”) and distributing signals across these different channels at the physical layer. To minimize line-side penalties, an equal and symmetrical bandwidth split is required across the multiple engines, and there is high complexity involved to support flexible rate bandwidth splitting. For example, to split 340G across two devices would be 2×170G channels, 930G across three devices would be 3×310G channels, etc. 
     There are different conventional techniques developed to handle the breakup and inverse muxing of signals across multiple channels. For example, IEEE has defined Link Aggregation Groups (LAG) and ITU has been using Virtual Concatenation (VCAT) type of schemes of standard defined containers Low Order (LO)/High Order (HO) Optical channel Transport Unit-k (OTUk). LAG is a higher layer protocol utilizing smaller-sized channels to carry a super-channel. The protocol is implemented at Layer 2 (Ethernet) and adds huge complexity and memory requirements. It is typically implemented using a Network Processing Unit (NPU) and other types of devices; LAG is not an appropriate approach to be integrated into optical DSP/modem devices. Standard ITU-defined VCAT schemes include grouping smaller sized standard containers, which would be Optical channel Data Unit-2 (ODU2) to get 10G granularity on the line side. There is a large logic complexity to map a signal (i.e. 240G) to nxODU2 (i.e. 24) and then switch and distribute these ODU2 signals across multiple optical DSP/modem devices. The extra mapping complexity can add to wander and decrease network performance. 
     BRIEF SUMMARY OF THE DISCLOSURE 
     In an embodiment, a method for Optical Transport Network (OTN) transmission includes receiving an OTN client signal for transmission via a plurality of line side modems; segmenting the OTN client signal into a plurality of flows; and providing the plurality of flows to the plurality of line side modems, wherein the OTN client signal is a single client which is transmitted optically via the plurality of line side modems which are each different line interfaces. The OTN client can be an Optical channel Transport Unit (C=100)×n (n=1, 2, 3, . . . ) OTUCn. The segmenting can determine allocated tributary slots in the OTN client in specified increments for the providing. Each modem rate of the plurality of line side modems can be disassociated from a rate of the OTN client signal. The segmenting can create cells associated with the OTN client signal and the providing can include switching the cells to the plurality of line side modems. The plurality of line side modems can include a first plurality of line side modems, and the method further includes receiving the plurality of flows with a second plurality of line side modems; and reassembling the plurality of flows into the OTN client signal. The segmenting can be determined based on an Optical channel Path Unit Multiframe Identifier (OMFI) and Multiplex Structure Identifier (MSI) associated with the OTN signal. The segmenting can be based on tributaries in the OTN client signal. 
     In another embodiment, a circuit for Optical Transport Network (OTN) transmission includes cell adaptation circuitry configured to receive an OTN client signal for transmission via a plurality of line side modems and to segment the OTN client signal into a plurality of flows; and line adaptation circuitry configured to provide the plurality of flows to the plurality of line side modems, wherein the OTN client signal is a single client which is transmitted optically via the plurality of line side modems which are each different line interfaces. The OTN client can be an Optical channel Transport Unit (C=100)×n (n=1, 2, 3, . . . ) OTUCn. The cell adaptation circuitry can be configured to determine allocated tributary slots in the OTN client in specified increments for the line adaptation circuitry. Each modem rate of the plurality of line side modems can be disassociated from a rate of the OTN client signal. The cell adaptation circuitry can be configured to create cells associated with the OTN client signal and the line adaptation circuitry can be configured to switch the cells to the plurality of line side modems. The plurality of line side modems can include a first plurality of line side modems, and wherein a second plurality of line side modems receive the plurality of flows from the first plurality of line side modems and associated circuitry coupled to the second plurality of line side modems can be configured to reassemble the plurality of flows into the OTN client signal. The cell adaptation circuitry can utilize an Optical channel Path Unit Multiframe Identifier (OMFI) and Multiplex Structure Identifier (MSI) associated with the OTN signal for segmentation. 
     In a further embodiment, a system for Optical Transport Network (OTN) transmission includes a first adaptation circuit communicatively coupled to a first plurality of line side modems; and a second adaptation circuit communicatively coupled to a second plurality of line side modems; wherein the first adaptation circuit is configured to receive an OTN client signal for transmission via the first plurality of line side modems, segment the OTN client signal into a plurality of flows, and provide the plurality of flows to the plurality of line side modems, wherein the OTN client signal is a single client which is transmitted optically via the first plurality of line side modems which are each different line interfaces. The second adaptation circuit can be configured to receive the plurality of flows from the second plurality of line side modems, and reassemble the plurality of flows into the OTN client signal. The OTN client can be an Optical channel Transport Unit (C=100)×n (n=1, 2, 3, . . . ) OTUCn. The first adaptation circuit can determine allocated tributary slots in the OTN client in specified increments for the plurality of flows. The OTN client signal can be segmented based on an Optical channel Path Unit Multiframe Identifier (OMFI) and Multiplex Structure Identifier (MSI) associated with the OTN signal. 
     In an embodiment, a method for Optical Transport Network (OTN) line side adaptation to provide sub-rate granularity and distribution includes segmenting an OTN signal into N flows of cells with associated identifiers, based on tributary slots of the OTN signal, wherein N≥0, and wherein the cells do not include unallocated payload from the OTN signal; and switching the cells, with a scheduler, to one or more line side modems. The segmenting can be determined based on an Optical channel Path Unit Multiframe Identifier (OMFI) and Multiplex Structure Identifier (MSI) associated with the OTN signal, and N is based on a number of allocated tributary slots in the OTN signal. When N=0, only overhead is provided in the cells to the one or more line side modems. The OTN signal can be an Optical channel Transport Unit (C=100)×n (n=1, 2, 3, . . . ) OTUCn or a High Order or Super High Order OTN signal with tributary slots. A signal provided to the one or more line side modems can be a sub-rate or a full-rate of the OTUCn or the High Order or Super High Order OTN signal with tributary slots. Optionally, the OTN signal can be from a client and the N flows of cells from the client are sent to different line side modems. Alternatively, the OTN signal can be from two or more clients and the N flows of cells from the two or more clients are sent to a same line side modem. The method can further include inserting a framing cell to enable recovery from the one or more line side modems. The scheduling can include switching some of the cells to a first line side modem and a second line side modem. 
     In another embodiment, a circuit for Optical Transport Network (OTN) line side adaptation to provide sub-rate granularity and distribution includes cell adaptation circuitry configured to segment an OTN signal into N flows of cells with associated identifiers, based on tributary slots of the OTN signal, wherein N≥0, and wherein the cells do not include unallocated payload from the OTN signal and switch the cells to a scheduler; and line adaptation circuitry configured to schedule, from the scheduler, the cells for one or more line side modems. The N flows can be determined based on Optical channel Path Unit Multiframe Identifier (OMFI) and Multiplex Structure Identifier (MSI), and N is based on a number of allocated tributary slots in the OTN signal. When N=0, only overhead is provided in the cells to the one or more line side modems. The OTN signal can be an Optical channel Transport Unit (C=100)×n (n=1, 2, 3, . . . ) OTUCn or a High Order or Super High Order OTN signal with tributary slots. A signal provided to the one or more line side modems can be a sub-rate or a full-rate of the OTUCn or the High Order or Super High Order OTN signal with tributary slots. The OTN signal can be one of a) from a client and the N flows of cells from the client are sent to different line side modems and b) from two or more clients and the N flows of cells from the two or more clients are sent to a same line side modem. The line adaptation circuitry can be further configured to switch some of the cells to a first line side modem and a second line side modem. 
     In a further embodiment, a system for Optical Transport Network (OTN) line side adaptation to provide sub-rate granularity and distribution includes a first adaptation circuit communicatively coupled to a first line side modem; and a second adaptation circuit communicatively coupled to a second line side modem; wherein each of the first adaptation circuit and the second adaptation circuit include cell adaptation circuitry configured to segment an OTN signal into N flows of cells with associated identifiers, based on tributary slots of the OTN signal, wherein N≥0, and wherein the cells do not include unallocated payload from the OTN signal and switch the cells to a scheduler, and line adaptation circuitry configured to schedule, from the scheduler, the cells for one or more line side modems. The N flows can be determined based on Optical channel Path Unit Multiframe Identifier (OMFI) and Multiplex Structure Identifier (MSI), and N is based on a number of allocated tributary slots in the OTN signal. The OTN signal can be an Optical channel Transport Unit (C=100)×n (n=1, 2, 3, . . . ) OTUCn or a High Order or Super High Order OTN signal with tributary slots, and wherein a signal provided to the one or more line side modems is a sub-rate or a full-rate of the OTUCn or the High Order or Super High Order OTN signal. The first adaptation circuit and the second adaptation circuit can be communicatively coupled to one another and configured to provide one or more of the cells to one another. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present disclosure is illustrated and described herein with reference to the various drawings, in which like reference numbers are used to denote like system components/method steps, as appropriate, and in which: 
         FIG. 1  is an atomic function for an adaptation process of an OTUCn; 
         FIG. 2  is an atomic function for an adaptation process where a single client gets split among two or more different line interfaces; 
         FIG. 3  is a diagram atomic function for an adaptation process where two or more clients are provided to a same line interface; 
         FIG. 4  is a diagram of front-end adaptation with the adaptation process; 
         FIG. 5  is a diagram of cell adaptation of a plurality of cells; 
         FIG. 6  is a diagram of cell adaptation with unallocated tributary slots; 
         FIG. 7  is a block diagram of cell adaptation for different flows; 
         FIG. 8  is a block diagram of the line adaptation and a scheduler to service multiple flows; 
         FIG. 9  is a block diagram of two 500G adaptation circuits; and 
         FIG. 10  is a flow chart of a process for Optical Transport Network (OTN) line side adaptation to provide sub-rate granularity and distribution. 
     
    
    
     DETAILED DESCRIPTION OF THE DISCLOSURE 
     The following acronyms are utilized herein: 
     
       
         
           
               
               
               
             
               
                   
                   
               
             
            
               
                   
                 ASIC 
                 Application Specific Integrated Circuit 
               
               
                   
                 BIP 
                 Bit Interleaved Parity 
               
               
                   
                 CI 
                 Characteristic Information 
               
               
                   
                 COMMS 
                 Communications channel 
               
               
                   
                 DSP 
                 Digital Signal Processing 
               
               
                   
                 FAS 
                 Frame Alignment Signal 
               
               
                   
                 FEI 
                 Forward Error Indication 
               
               
                   
                 FPGA 
                 Field Programmable Gate Array 
               
               
                   
                 GFEC 
                 Generic Forward Error Correction 
               
               
                   
                 HEC 
                 Header Error Control 
               
               
                   
                 HO 
                 High Order 
               
               
                   
                 LAG 
                 Link Aggregation Group 
               
               
                   
                 LO 
                 Low Order 
               
               
                   
                 MS 
                 Multiplex Section 
               
               
                   
                 MSI 
                 Multiplex Structure Identifier 
               
               
                   
                 PMD 
                 Physical Medium Dependent 
               
               
                   
                 OAM 
                 Operations, Administration, Maintenance 
               
               
                   
                 ODU 
                 Optical channel Data Unit 
               
               
                   
                 ODUflex 
                 Optical channel Data Unit flexible 
               
               
                   
                 ODUCn 
                 Optical channel Data Unit 
               
               
                   
                   
                 (C = 100) × n (n = 1, 2, 3, . . .) 
               
               
                   
                 OH 
                 Overhead 
               
               
                   
                 OMFI 
                 OPU Multiframe Identifier 
               
               
                   
                 OPU 
                 Optical channel Path Unit 
               
               
                   
                 OPUCn 
                 Optical channel Path Unit 
               
               
                   
                   
                 (C = 100) × n (n = 1, 2, 3, . . .) 
               
               
                   
                 OTN 
                 Optical Transport Network 
               
               
                   
                 OTUk 
                 Optical channel Transport Unit level k 
               
               
                   
                   
                 (k = 1, 2, 3, or 4) 
               
               
                   
                 OTUCn 
                 Optical channel Transport Unit 
               
               
                   
                   
                 (C = 100) × n (n = 1, 2, 3, . . .) 
               
               
                   
                 PCS 
                 Physical Coding Sub-layer 
               
               
                   
                 PM 
                 Path Monitoring 
               
               
                   
                 QAM 
                 Quadrature Amplitude Modulation 
               
               
                   
                 RS 
                 Regenerator Section 
               
               
                   
                 SAR 
                 Segmentation and Reassembly 
               
               
                   
                 SD-FEC 
                 Soft Decision Forward Error Correction 
               
               
                   
                 SDN 
                 Software Defined Networking 
               
               
                   
                 SHO 
                 Super High Order 
               
               
                   
                 SM 
                 Section Monitoring 
               
               
                   
                 SV-IaDI 
                 Single Vendor Intra-Domain Integration 
               
               
                   
                 TDM 
                 Time Division Multiplexing 
               
               
                   
                 TS 
                 Tributary Slot 
               
               
                   
                 VCAT 
                 Virtual Concatenation 
               
               
                   
                   
               
            
           
         
       
     
     In various embodiments, OTN line adaptation systems and methods are described. In an embodiment, an OTUCn line adaptation layer for a proprietary line side (SV-IaDI) splits or segments the OTUCn into 10G tributary slots (i.e., 10× per OTUC1), and only transmits on the line side the allocated tributary slots and OTUCn overhead. The process removes extra unused capacity of a standard OTUCn 100G container. This adapts a standard OTUCn frame (or any HO or SHO OTN signal), but utilizes the tributary slot structure within an OPUCn payload to get to 10G granularity. LO ODUk (k=0, 1, 2, 3, 4, flex) can get mapped into OTUCn tributary slots with 10G granularity and these will be adapted on the line. On the receive side of the line, the m×10G signal is reconstituted and put back into an OTUCn standard frame format. Ethernet clients can be sub-rate groomed, then mapped to an appropriately sized ODUflex, and then mapped to m×10G tributary slots in the OTUCn structure. At the receive side of the line, the ODUflex signal is recovered from the equivalent set of allocated tributary slots. 
     In another embodiment, an OTUCn cell adaptation layer for a proprietary line side (SV-IaDI) creates fixed-sized cells from multiple traffic streams. These cells are switched using scheduling algorithms across a cell switch which is distributed outside or inside multiple DSP/modem devices or engines. The switch distributes cells to the multiple DSP/modem devices or engines, which achieves flexible bandwidth split. Cells having taken different paths are deskewed and aligned on the receiver to recreate the original signal. The cells contain unique IDs for switching and path selection within the modem/DSP device scope (and the line adaptation). This provides a process for addressing the complexities of providing flexible bandwidth across multiple modem/DSP devices. 
     Advantageously, the OTN line adaptation systems and methods align to the ITU-T OTUCn structure and models, but extends the protocol (OTN frame format) to achieve flexible rates at small increments below 100G. This allows adaptation of a fixed rate interface to a flexible rate line interface. ITU standards are not planning on defining SV-IaDI adaptation for OTUCn. Note, SV-IaDI is an adaptation between standard client interfaces at OTUCn and optical modems. This allows flex line and bandwidth splits among multiple devices/wavelengths. Additionally, the OTN line adaptation systems and methods result in a small logic implementation in comparison to other muxing schemes. Also, the mapping into cells can be protocol agnostic, and does not have to be limited to 10G granularity. This approach provides flexibility of cells (or packets) to switch and assign different path to the desired Time Division Multiplexing (TDM) (OTUCn) traffic. 
     The OTN line adaptation systems and methods use OTUCn as SHO (or HO OTUk) to avoid proprietary schemes and align with future 100G OTN standardization. The OTN line adaptation systems and methods prevent unnecessary mapping/demapping stages by carrying OTUCn OH+payload into cells and only sending used (allocated) tributary slots in 10G increments (or any other increments). The OTN line adaptation systems and methods carry tributary slots independently whether the traffic source includes a single Cn, groups of Cn (e.g., C5) in single chip or groups of Cn across multi-chips. In this manner, the OTN line adaptation systems and methods disassociate modem rate to OTUC1 and tributary slot structure. 
     Referring to  FIG. 1 , in an embodiment, an atomic function diagram illustrates an adaptation process  10 . The adaptation process  10  is based on the ITU-T standard OTUCn frame structures and tributary slots. Note, because the adaptation process  10  operates between a client  12  and a line  14 , the adaptation of OTUCn for SV-IaDI is not subject to standardization. That is, the adaptation process  10  is used for a vendor&#39;s line-side transmission. The adaptation process  10  is illustrated from the top, at a client_CI  12 , down to a Line_CI  14 . The adaptation process  10  takes in an OTUCn frame (OTUCn/CI  20 ), breaks-up the 64-byte overhead (per multi-frame OMFI) and 10G tributary payload into 10×10G streams (Cn10G/OTUCn, cell adaptation  22 ). The individual 10×10G streams are SARed (create cells/packets), with associated identifiers (IDs) for the streams. The OPUCn MSI OH contains the allocation and structure of the 10G tributary slots and can distinguish which slots are filled with data and which are empty. The cell adaptation  22  also includes OTUCn BIP compensation, a cell ID for each cell, and timing information. 
     Subsequent to the cell adaptation  22 , line adaptation  24  is performed on the line TX side. On the line TX side, a scheduler can be used and configured to service and interleave only the cell streams that are filled with data (as reflected by the OPUCn MSI). The cell adaptation  22  provides Cn10G (n×10G cells) (n≥1), and the line adaptation  24  provides m10 Gcells (not necessarily carrying a multiple of 100G). The OTUCn/ODUCn/OPUCn OH is distributed across cell streams based on the OMFI. This results in removing unused capacity in the OTUCn structure by only transmitting the used 10G streams. This also results in 10G granularity on the line side, but still utilizing the OAM, section and functions of the SHO OTUCn. In the line adaptation  24 , since cell streams are interleaved, the OTUCn frame FAS cannot be used for alignment. A special cell can be used for alignment, with fixed occurrence in order to prevent the need for a PCS layer. After the line adaptation  24 , SD-FEC  26  can be added and the line_CI  14  can be provided to a modem. 
     In the opposite direction after transmission on the line RX side, cells are received and reassembled for the used streams and tributary slots. The cell ID is used to distinguish the different streams. A standard OTUCn frame is recreated and empty streams (which have been omitted on the line) are filled in and set as unallocated in the OTUCn frame (MSI). The overall adaptation process  10  includes receiving a standard OTUCn in, performing the adaptation process  10  from the client_CI  12  to the line_CI  14 , transmission (between two modems), performing the adaptation process  10  from the line_CI  14  to the client_CI  12 , and providing the OTUCn as the output. 
     Referring to  FIGS. 2 and 3 , in an embodiment, flow diagrams illustrate an adaptation processes  10 A,  10 B. The adaptation processes  10 A,  10 B are similar to the adaptation process  10 , but are described to illustrate disassociation of the client_CI  12  from the line_CI  14 . On the line TX side, cells are switched (via cell switch) using IDs to redirect a path to an appropriate optical modem/DSP device. Again, the granularity of this switching cell flow can be 10G; although other rates are possible. Client rates, interfaces and OTUCn frame format is disassociated to the actual line rate and engine instance. A scheduler is used in the line adaptation  24  and modem/DSP device to service the different cell flows destined for its line. On the line RX side, cells received then switched (via cell switch) back to desired OTUCn processing logic and group. Suppose the layer that OTUCn is adapted to is called Cn10G and the layer below (not carrying a multiple of 100G) is called m10 Gcell. The number of client interfaces is disassociated from number of line interfaces. For example,  FIG. 2  illustrates the adaptation process  10 A where a single client, client_CI  12 , gets split among two different line interfaces, line_CI  14 A,  14 B.  FIG. 3  illustrates the adaptation process  10 A where two clients, client_CI  12 A,  12 B, are provided to a same line interface, line_CI  14 . 
     Referring to  FIG. 4 , in an embodiment, a diagram illustrates the front-end adaptation with the adaptation process  10 .  FIG. 4  illustrates an example of converting an OTUCn to a modem bus. The OTUCn includes OPUCn payload  30 , OPUCn OH  32 , ODUCn payload  34 , ODUCn OH  36 , OTUCn payload  38 , and OTUCn OH  40 . The OPUCn payload  30  is split into 10 tributary flows or TSs and each of the 10 tributary payloads is mapped to one of the 10× flows per OTUC1, based on OMFI. Note,  FIG. 4  illustrates four OMFIs, but there may be 10 in this embodiment for 10G tributary slots. The OPUCn OH  32 , ODUCn OH  36 , and the OTUCn OH  40  is split amongst the 10× tributary flows based on OMFI. 
     As described herein, there can be N flows of cells, where N≥0, and each of the N flows represents an allocated tributary slot in the OTN signal, where the sub-rate equals the overall OTN signal rate divided by N. In this example, N=10 and the OTUCn=100, so each flow is 10G. In the cell adaptation  22 , before the cell switch, OTUCn frames (OTUCn/ODUCn/OPUCn OH+tributary payload) are converted into 10× tributary flows and cells. Where N=0, no allocated payload, only overhead is provided from the OTN signal, i.e. the systems and methods contemplate a sub-rate signal where there is no payload, only overhead. 
     Referring to  FIG. 6 , in an embodiment, cell adaptation is illustrated with unallocated tributary slots. In this example, tributary slots are occupied for OMFI=0, 3, but unallocated for OMFI=1, 2. Note, the OTN OH is still sent when there are unallocated tributary slots. A framer is needed to hunt for OTN FAS in cell stream (after reassembly), and no alignment of cells is needed. The framer looks for the normal frame position, or back to back OH cells if unused tributary. 
     Referring to  FIG. 7 , in an embodiment, a block diagram illustrates the cell adaptation  22  for different flows. Here, the TS from multiple frames of OTUCn  20  are adapted into the cells via a cell adaptation function  60  (SAR). The cell adaptation function  60  can be implemented in circuitry with various queues or buffers. The cell adaptation function  60  connects to a cell switch  62  which is configured to provide the cells to the line adaptation  24 . 
     Referring to  FIG. 8 , in an embodiment, a block diagram illustrates the line adaptation  24  and a scheduler  70 . The line adaptation  24  includes circuitry after the cell switch  62 , and the line adaptation  24  is configured to combine cells from all used and allocated flows using the scheduler  70 . The scheduler  70  can utilize round-robin, a calendar, or the like. OH circuitry  74  and framing circuitry  76  also connects to the scheduler  70 . Subsequent to the scheduler  70 , scrambling circuitry  78  is configured to scramble the data. 
     Referring to  FIG. 9 , in an embodiment, a block diagram illustrates two 500G adaptation circuits  80 . The 500G adaptation circuits  80  perform the adaptation processes described herein and physically reside between client interface and optical modem. Note, for illustration purposes, two of the 500G adaptation circuits  80  are illustrated to show scheduling between the 500G adaptation circuits  80 . The 500G adaptation circuits  80  include OTU4/ODU4/OTUC1/ODUflex framers  82  which is communicatively coupled to the client. The framers  82  are configured to operate at a client rate−OTU4/ODU4/OTUC1/ODUflex. The framers  82  are communicatively coupled to ODTUC1.j PT=0x22 multiplexers  84  (payload type=0x22). 
     The multiplexers  84  are configured to interface at tributary slots with the framers  82 . The multiplexers  84  are communicatively coupled to OTUC1 framers  86  which are configured to interface the tributary slots. Subsequent to the OTUC1 framers  86 , the cell adaptation function  60  (SAR) is configured to interface to the OTUC1s from the framers  86  with the 10× tributary slots therein. The cell adaptation function  60  (SAR) connection to the cell switch  62  which is communicatively coupled to the scheduler  70  and a scheduler  90  for cells between the 500G adaptation circuits  80 . Finally, the scheduler  70  is configured to interface to a modem for optical transmission of the cells. 
     Referring to  FIG. 10 , in an embodiment, a flow chart illustrates a process  100  for Optical Transport Network (OTN) line side adaptation to provide sub-rate granularity and distribution. The process  100  includes segmenting an OTN signal into N flows of cells with associated identifiers, based on tributary slots of the OTN signal, wherein N≥0, and wherein the cells do not include unallocated payload from the OTN signal (step  102 ). The process  100  also includes switching the cells to a scheduler (step  104 ). Finally, the process  100  includes scheduling, from the scheduler, the cells for a line side modem (step  106 ). Note, the steps  104 ,  106  can include switching the cells, with a scheduler, to one or more line side modems. The OTN signal can be one of a) from a client and the N flows of cells from the client are sent to different line side modems and b) from two or more clients and the N flows of cells from the two or more clients are sent to a same line side modem. 
     The segmenting can be determined based on the OMFI and MSI associated with the OTN signal, and N is based on a number of allocated tributary slots in the OTN signal. When N=0, only overhead is provided in the cells to the line side modem. The OTN signal is an Optical channel Transport Unit (C=100)×n (n=1, 2, 3, . . . ) (OTUCn) or a High Order or Super High Order OTN signal with tributary slots. The signal provided to the line side modem is a sub-rate of the OTUCn or the High Order or Super High Order OTN signal with tributary slots or a full-rate of the same signal. The scheduling can utilize round robin or a calendar. The method can further include inserting a framing cell to enable recovery from the line side modem. The scheduling can include scheduling some of the cells to a second line side modem. 
     In the various embodiments described herein, reference has been made to OTUCn for illustration purposes. Those of ordinary skill in the art will recognize the systems and methods can also be used on High Order (HO) OTN signals with corresponding tributary slots. For example, a HO OTU4 has 80 TSs of 1.25G each, and the systems and methods described herein can enable transmission of less than the 80 TSs, i.e. a sub-rate, to provide a composite signal of less than 100G to the line side modem when there are unallocated TSs. The systems and methods also contemplate operations with any Super High Order (SHO) OTN signal that may be developed. For example, OTUCn has been described herein where C=100 and n=1, 2, 3 . . . , and there may be other variants of this such as OTULn where L=50 and n=1, 2, 3, . . . for increments of 50G (whereas the OTUCn has increments of 100G). Any such embodiments are contemplated herein. 
     It will be appreciated that some embodiments described herein may include one or more generic or specialized processors (“one or more processors”) such as microprocessors, digital signal processors, customized processors, and field programmable gate arrays (FPGAs) and unique stored program instructions (including both software and firmware) that control the one or more processors to implement, in conjunction with certain non-processor circuits, some, most, or all of the functions of the methods and/or systems described herein. Alternatively, some or all functions may be implemented by a state machine that has no stored program instructions, or in one or more application specific integrated circuits (ASICs), in which each function or some combinations of certain of the functions are implemented as custom logic. Of course, a combination of the aforementioned approaches may be used. Moreover, some embodiments may be implemented as a non-transitory computer-readable storage medium having computer readable code stored thereon for programming a computer, server, appliance, device, etc. each of which may include a processor to perform methods as described and claimed herein. Examples of such computer-readable storage mediums include, but are not limited to, a hard disk, an optical storage device, a magnetic storage device, a ROM (Read Only Memory), a PROM (Programmable Read Only Memory), an EPROM (Erasable Programmable Read Only Memory), an EEPROM (Electrically Erasable Programmable Read Only Memory), Flash memory, and the like. When stored in the non-transitory computer readable medium, software can include instructions executable by a processor that, in response to such execution, cause a processor or any other circuitry to perform a set of operations, steps, methods, processes, algorithms, etc. 
     Although the present disclosure has been illustrated and described herein with reference to preferred embodiments and specific examples thereof, it will be readily apparent to those of ordinary skill in the art that other embodiments and examples may perform similar functions and/or achieve like results. All such equivalent embodiments and examples are within the spirit and scope of the present disclosure, are contemplated thereby, and are intended to be covered by the following claims.