Page 753 - 5G Basics - Core Network Aspects
P. 753

Transport aspects                                              2


            The encoding of the ACK bitmap shall be as shown in Table 9-5 (downstream) and Table 9-8 (upstream)
            where each bit in the bit map has the following meaning:
            –       "0" means NACK;
            –       "1" means ACK.
            ACK grouping shall only be used if the number of DTUs per ACK window exceeds the size of the ACK bitmap
            field. Each ACK group represents a number of consecutive DTUs. The number of DTUs per ACK group for a
            given ACK is an ACK group size and shall be communicated together with the ACK bitmap and may vary
            from one RMC message to the next. The valid ACK group size, Gack, is an integer in the range from one to
            three DTUs.
            In the case of ACK grouping, each bit of the ACK bitmap carries the acknowledgement of all DTUs in the ACK
            group and shall be set to 0 if at least one DTU in the group has to be NACKed. The first ACK group coded in
            the  LSB  of  the  ACK  bit  map  shall  contain  the  acknowledgements  for  the  Gack  last  transmitted  DTUs.
            Subsequent groups shall be constructed by taking the acknowledgements for the previous Gack transmitted
            DTUs. The last ACK group in the ACK message may hold acknowledgements for a smaller number of DTUs.

            The upstream and downstream RMC commands shall also include an acknowledgement for the reception
            of  the  RMC  message  associated  with  the  same  ACK  window.  The  RMC  acknowledgement  bit  shall  be
            encoded as shown in Table 9-5 (downstream) and Table 9-8 (upstream) where each bit in the bit map has
            the following meaning:
            –       "0" means NACK;
            –       "1" means ACK.
            Error  detection  for  the  received  RMC  message  is  accomplished  by  using  redundancy  bytes  of  the  RMC
            codeword (see clause 9.6.3).

            9.8     Retransmission function
            All DTUs marked as normal DTUs (see clause 8.2.1.3) that are NACKed or not acknowledged (due to loss of
            the  corresponding  RMC  message)  shall  be  assigned  for  retransmission.  Dummy  DTUs  shall  not  be
            retransmitted.
            The DTUs assigned for retransmission and the new incoming DTUs shall be scheduled for transmission in
            such a way that the number of DTUs, completely mapped on data symbols and RMC symbols sent between
            the end of transmission of a given DTU and the start of its subsequent retransmission, does not exceed Qtx
            DTUs. The value of Qtx is defined as the maximum number of DTUs that can be transmitted between the
            end of the first DTU associated with the ACK window (i.e., the oldest DTU of the ACK window) and the start
            of the last opportunity of this DTU retransmission associated with the given value of Tret defined in clause
            9.8.1 (see Figure 9-9.1).
            NOTE – An upper limit on Qtx is the maximum number of DTUs that can be transmitted fully or partially in the relevant
            direction within a time period of Tqtx_max seconds  starting from the  beginning  of the  first (earliest in time)  symbol
            position within the ACK window up until the end of the last symbol position in the Tret window.
                                                               
                                            _      _      = (⌈        _max _   ⌉ + ⌈        _max _   ⌉ + 2 +    ) ∗            
                                                                                
                                                                 
                                                    
                                                                        
                                                           
                                                               
                                            _      _      = (⌈        _max _   ⌉ + ⌈        _max _   ⌉ + 2 +    ) ∗            
                                                                                
                                                     
                                                                 
                                                           
                                                                        















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