Page 754 - 5G Basics - Core Network Aspects

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

744