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2                                                 Transport aspects


            Interleaved  versions  of  the  OTU,  ODU  and  OPU  overhead  may  be  present  on  OTUCn  interfaces.  This
            interleaving is interface specific and specified for OTN interfaces with standardized application codes in the
            interface  specific  Recommendations  (ITU-T  G.709.x  series).  Within  the  other  clauses  of  this
            Recommendation an OTUCn, ODUCn and OPUCn are presented in an interface independent manner, by
            means of n OTUC, ODUC and OPUC instances that are marked #1 to #n.


                                                        Column #
                                         . . .                          . . .
                              1                        7  8                             14 15  16
                           1       Frame alignment area         OTU specific overhead area
                         Row #  2 3               ODU specific overhead area                 OPU   specific  overhead area


                           4
                                                                                     G.709-Y.1331(12)_F5-1


                                         Figure 5-1  OTU, ODU and OPU overhead


            6       Optical transport network interface structure
            The optical transport network as specified in [ITU-T G.872] defines two interface types, each with two sub-
            types:
            –       Single optical transport unit interfaces:
                    •   without optical layer overhead (SOTU);
                    •   with optical layer overhead (SOTUm).

            –       Multi optical transport unit interfaces:
                    •   without optical layer overhead (MOTU);
                    •   with optical layer overhead (MOTUm).
            The  OTN  interfaces  without  optical  layer  overhead  are  defined  with  3R  processing  at  each  end  of  the
            interface.
            The interfaces consist of digital and management information defined in this Recommendation as carried
            by a physical interface, the specifications of which are outside the scope of this Recommendation.

            Each  interface  may  be  associated  with  one  or  more  application  identifiers.  Each  application  identifier
            represents  either  a  standardized  application  code  or  a  vendor  specific  identifier.  An  interface  with  an
            application code may be used to interconnect (OTN) equipment from different vendors. An interface with a
            vendor  specific  identifier  is  to  be  used  to  interconnect  (OTN)  equipment  from  the  same vendor. When
            considering interconnection over an interface between user and provider administrations or provider to
            provider  administrations,  3R  termination  shall  be  used  on  either  side  of  the  interface,  unless  mutually
            agreed otherwise.

            6.1     Basic signal structure
            The basic structure is shown in Figures 6-1 and 6-2 and consists of a digital and an optical structure.

            6.1.1   OTN digital structure
            The OTN digital structure (see Figure 6-1) consists of two classes of optical transport units (OTU); the OTUk
            and  OTUCn.  The  OTUk  signal  consists  of  a  4080  column  by  4  row  frame,  which  includes  256 columns
            allocated to contain a forward error correction code (FEC) and is operated at various bit rates that are
            represented by the value of k. The OTUCn signal consists of n interleaved 3824 column by 4 row frames,
            which do not include a FEC area and is operated at n times a basic rate that is represented by OTUC. FEC for
            the OTUCn signal is interface specific and not included in the OTUCn definition.




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