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Transport aspects 2

9 Physical media specific transmission convergence (PMS-TC) sub-layer

9.1 Functional reference model

Figure 9-1 shows the functional reference model of the PMS-TC. DTUs enter the PMS-TC via the α reference
point. The incoming DTUs are scrambled, encoded using a Reed-Solomon forward error correction (FEC)
code, and interleaved using a block interleaver.

The PMS-TC also provides a robust management channel (RMC). The RMC carries acknowledgements for
received DTUs and delay-sensitive management data sourced from the FME and submitted to the PMS-TC
via the PMS-TC_MGMT interface. The RMC message that includes acknowledgement data and
management data communicated over the RMC are formatted in the RMC framer. The RMC message is
encoded using a Reed-Solomon FEC code.
The bytes of encoded DTUs (from the DTU buffer at the DTU interleaver output) are multiplexed with the
bytes of RMC frames (from RMC buffer at the RMC encoder output) by the data frame multiplexer (data
frame MUX) and mapped onto data frames that are further transferred to the PMD via the δ reference
point (see clause 9.5). Each data frame contains an integer number of bytes (from the DTU buffer) to be
loaded onto one symbol (see clause 10.5.2). The RMC frame shall be multiplexed together with encoded
DTUs into the first data frame in a logical frame; this data frame, referred to as an RMC data frame, shall be
further loaded onto an RMC symbol (see clause 10.5.1). All other data frames of a logical frame, referred to
as normal data frames, shall carry DTUs only. The time position of the RMC symbol in a TDD frame is
defined during the initialization (see clause 12.3.2.1). In the L2.1N, L2.1B and L2.2 link states, only RMC data
frames shall be used in both upstream and downstream directions. In the L2.1N and L2.1B link states, RMC
symbols are only transmitted in the assigned TDD frames of each superframe, while other TDD frames
contain only quiet or pilot symbols (see clause 13.4.1). In the L2.2 link state, one RMC symbol is transmitted
in superframes that have been selected at the transition to the L2.2 link state, while other superframes only
contain quiet or pilot symbols (see clause 13.4.2).
The transmitted DTUs are also stored in a retransmission queue buffer. A retransmission multiplexer (RTX
MUX) selects for transmission either a new DTU or a DTU from the retransmission buffer (RTX queue),
depending on the received acknowledgement. The DTU queue allows for prioritization of retransmitted
DTUs and shall support the delay associated with flow control.

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