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2                                                 Transport aspects



















































                                       Figure 10-1 – PMD functional reference model


            In  the  receive  direction,  the  incoming  signal  from  the  line  (U-O  interface)  during  the  time  of  reception
            (controlled by the TDD switch) is demodulated. The recovered constellation points of the received symbol
            are post-cancelled, to mitigate FEXT accumulated in the line, and decoded to recover the data frame, which
            is then passed to the PMS-TC via the  interface. The post-canceller uses demodulated symbols received
            from  all  other  lines  of  the  vectored  group  that  are  submitted  via  the  η(k,n)  interface;  post-canceller
            coefficients  are  provided  by  the  VCE  via  η-c  interface.  The  ε(k,n),  η(k,n),  ε-c  and  η-c  are  vendor
            discretionary interfaces (see clause 10.3).
            The functional reference model of the FTU-R is the same as that of the FTU-O, except it does not include a
            precoder (Zi' = Zi) and post-canceller; the signal constellation points obtained from the symbol encoder are
            directly passed to the modulator. Similarly, in the receive direction, the demodulated constellation points
            are passed to the data symbol decoder.
            10.1.1  U interface

            The U reference point describes both a physical and a logical interface of the data plane between the PMD
            and the transmission medium (twisted wire-pair). The data at the U reference point in both transmit and
            receive directions is a stream of symbols organized into TDD frames, as defined in clause 10.5. The TDD
            frame format provides time separation between upstream and downstream transmission, so symbols are
            never transmitted and received simultaneously. Groups of subsequent TDD frames form superframes; each
            superframe carries a sync symbol used for TDD frame synchronization and channel estimation. Each symbol
            is  a  time-domain  object  generated  using  inversed  Fourier  transformation,  as  defined  in  clause  10.4.3.



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