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2                                                 Transport aspects



                                                           Qtx DTUs
                                                              DS ACK window
               T symb
                                          ACK window              Shift



                   Oldest DTU of        RMC                                  RMC               Last opportunity to
                   ACK window                                                                  start retransmission
                                                                     Tack             Tret     of the "Oldest DTU
                                                                                               of ACK window"
                            TDD frame #N                        TDD frame #N+1
                                                                                            G.9701(14)-Cor.1(15)_F9-9.1


                                        Figure 9-9.1 – Illustration of definition of Qtx

            Both the transmitter and the receiver shall discard aged DTUs. The age of the DTU shall be computed in
            symbol periods, as the difference between the symbol count at which the symbol carrying the first bit of
            the DTU appears at the U-interface and the TS value of this DTU.

            A transmitter shall discard a DTU assigned for retransmission if the age of the DTU is older than delay_max
            expressed in symbols (delay_max × fDMT). Accordingly, the receiver shall discard a received DTU if the age of
            this DTU is older than delay_max expressed in symbol periods.
            9.8.1   Acknowledgement and retransmission latency requirements
            The FTU shall be able to decode a received DTU and shall respond with an ACK message within Tack µsec.
            Tack shall be measured at the U reference point from the time the last symbol of the ACK window has ended
            at the receiver port until the beginning of the RMC symbol carrying the acknowledgement for that ACK
            window  at  the  transmitter  port.  The  value  of  Tack  shall  not  exceed  400  µsec  for  the  FTU-O  (denoted
            Tack_max_O) and shall not exceed 300 µs for the FTU-R (denoted Tack_max_R).

            The FTU shall be able to decode an ACK message and respond with retransmission of the relevant DTUs
            within Tret µs. Tret shall be measured at the U reference point from the time the RMC symbol (carrying the
            acknowledgement information) has ended at the receiver port until the symbol carrying the first bit of the
            retransmitted  DTU  associated  with  this  acknowledgement  information  has  started  over  the  transmitter
            port. The value of Tret shall be smaller than 400 µsec for the FTU-O (denoted Tret_max_O) and smaller than 300
            µsec for the FTU-R (denoted Tret_max_R).
            Figure  9-10  presents  the  definition  of  Tack  and  Tret.  It  relates  to  retransmission  in  the  downstream  data
            direction. The same definitions apply to the upstream direction.


                            DownstreamACK window   ACK bitmap relates to the last ACK window
                                                                               T ret
               Downstream  Data Data Data RMC  Data  Data                         Data Data Data RMC  Data Data
               transmission   sym  sym  sym  sym   sym   sym                      sym  sym  sym  sym   sym  sym
                                           DS ACK window shift
                                                                                  First retransmitted DTU shall be
                                                                                      sent in this symbol
                 Upstream                                  Data Data Data  RMC Data
               transmission                                sym  sym  sym   sym  sym
                                                              DRMC
                                                                  ds
                                                         T ack

                                              TDD frame #N                             TDD frame #N+1
                                                                                                 G.9701(14)_F9-10

                                           Figure 9-10 – Definition of Tack and Tret







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