Why in RLC layer Seq. numbering is given to RLC SDU's and not to RLC PDU's?

Question

As RLC layer Seq. numbering is used to keep a track of packets to ARQ and PDCP, then SN should be given to RLC PDU, otherwise how it will happen in case of Segmentation and Re-segmentation?

Answer 1

Hi,

If we keep SN in PDU then we cannot track the PDC PDU's.

Incase of concatenation or segmentation we have to keep track of PDCP PDU's which is sent and which is received .

Single RLC PDU will have multiple PDCP PDU's, then if any single RLC PDU misses the we cannot track what are all PDCP PDU is missing. RLC will have mapping between RLC SN and PDCP SN, so it is very easy to keep SN's for each PDCP PDU/RLC SDU.