Page 715 - 5G Basics - Core Network Aspects

Transport aspects 2

In the receive direction, data packets are recovered from the DTUs crossing the αO reference point. The
recovered data packets (STREAMus) are conveyed to the DPU's L2+ functional block across the γO reference
point. The RX flow control function receives the RX Enable primitive from the DPU's L2+ functional block,
which together with the FCus primitive prevents upper layer overflow. By setting the value of the RX Enable
primitive to RXoff, the DPU's L2+ functional block indicates to the FTU-O that it is not ready to receive data
packets, in which case, the FTU-O shall set the RX DTU Enable primitive (see clause 8.1.2) to RXoff and the
PMS-TC receiver in the FTU-O (see Figure 9-1) may respond with NACKs to any received upstream normal
DTU when its buffer is full. By setting the value of the RX Enable primitive to RXon, the DPU's L2+ functional
block indicates to the FTU-O that it is ready to receive data packets. In addition to RX flow control, the
FTU-O communicates the setting of the FCus primitive (RXon/RXoff) to the peer FTU-R via the upstream
flow control bit in the RMC (see clause 9.6.4). By setting the value of the FCus primitive to RXoff, the DPU's
L2+ functional block indicates to the FTU-R that it is not ready to receive data packets. In this case the
FTU-R shall set the TX Enable primitive (see clause 8.1.2) to TXoff. By setting the value of the FCus primitive
to RXon, the DPU's L2+ functional block indicates to the FTU-R that it is ready to receive data packets.

Figure 8-2 – Functional reference model of the TPS-TC (FTU-R)

At the FTU-R, in the transmit direction, the data packets (STREAMus) crossing the γR reference point are
encapsulated in data transfer units (DTUs) that are passed to the PMS-TC across the αR reference point. The
TX flow control function generates the TX Enable primitive (TXon/TXoff) towards the NT's L2+ functional
block that prevent TPS-TC overflow. By setting the value of the TX Enable primitive to TXoff, the FTU-R
indicates to the NT's L2+ functional block that either the FTU-R itself or the DPU's L2+ functional block
(if the setting of the upstream flow control bit received from the FTU-O via the RMC is RXoff) is not ready to
receive data packets. By setting the value of the TX Enable primitive to TXon, the FTU-R indicates to the
NT's L2+ functional block that both the FTU-R itself and the DPU's L2+ functional block are ready to receive
data packets. If the setting of the upstream flow control bit primitive received from the FTU-O via the RMC
is RXoff, the FTU-R shall set the value of the TX Enable primitive to TXoff.
In the receive direction, data packets are recovered from the DTUs crossing the αR reference point. The
recovered data packets (STREAMds) are conveyed to the NT's L2+ functional block across the γR reference
point. The RX flow control function receives the RX Enable primitive from the NT's L2+ functional block to

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