Page 1077 - 5G Basics - Core Network Aspects

Transport aspects 2

Appendix V

Retransmission buffer size and the achievable bit-rate
(This appendix does not form an integral part of this Recommendation.)

This appendix presents the relationship between the size of the retransmission memory and the achievable
bit rate. For simplicity, the description relates to downstream transmission only (ACK transmitted in the
upstream direction). The parameters used in the analysis are listed below:
PMD parameters:
• TF: Duration of one TDD frame in ms.

• Tsymb: Symbol period in ms.
• Tack and Tret: Acknowledgement and retransmission latency in ms as defined in clause 9.8.1.
• Mds: Number of symbols allocated for downstream in one TDD frame.
• Mus: Number of symbols allocated for upstream in one TDD frame.
• MEMDS: The memory assigned to the downstream direction.

• NDTU: DTU size in bytes. NDTU = Q × K FEC.
Derived parameters:
• NSYM: Number of symbols (DS+US excluding switching gap) in one TDD frame. NSYM=Mds+Mus
• NSYMack, Retransmission acknowledgement time in symbols. NSYMack = ceiling(Tack/Tsymb)
• NSYMret: Retransmission time in symbols. NSYMret= ceiling(Tret/Tsymb)

Three cases are analysed in the following clauses and an upper bound for memory size is concluded. For
case 1 and case 2 the following assumptions are made:

• NSYMret+NSYMack<NSYM.
• Each symbol holds exactly one DTU.
• The switching times Tg1 and Tg2 are zero.
• The effect of SYNC symbol is ignored.

• The difference in the bit loading between RMC symbols and data symbols is ignored.
The above assumptions are relaxed for case 3 in order to reflect a more realistic configuration. However,
due to the wide dynamic range of parameter configurations for the ITU-T G.9701 framing, it is not easy to
derive a closed form expression for the relationship between net data rate and buffer size. Instead a lower
bound for the achievable net data rate for a given memory is provided as a design guideline.

V.1 Case 1: NSYMret+NSYMack+1 ≤ Mus
In this case, all transmitted DTUs in the downstream direction are acknowledged. Furthermore, the
transmitter has enough time to process the acknowledgement and free the retransmission buffer within
the same TDD frame before the new frame starts.
To optimize the response time, the upstream RMC symbol is shifted from the beginning of the upstream
portion of the TDD frame by a time period greater than or equal to NSYMack. This will allow the
acknowledgment of all downstream DTUs over the same acknowledgment message.
When these conditions are met, the achievable average downstream net data rate can be calculated as:

 (bits/byte 8 )  M EMDS 

Achievable NDRds(kb/s)  min  , MaxAggAchi evableNDR  (V-1)

 TF (ms) 

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