What is the purpose of segmentation at RLC?

RAN
1

Hi all 5G / LTE Experts.

I see that RLC at gNB is doing segmentation (SI 11) when GBR data is pumped.

But when I pump NON GBR data then RLC doesnt do any segmentation (SI 00).

My confusion is: what is the purpose of segmentation at RLC? And why it’s happenning?

2

RLC segmentation is done to split RLC SDU in multiple RLC PDUs.

MAC layers inform RLC layer about available transport block size than can be scheduled and transmitted.

If RLC PDU size is higher than available TB size, then RLC layer decides to segment RLC SDU in multiple PDUs.

Another advantage of segmentation is it helps packet processing complete well in advance of packet transmission.

3

Got you.

So why when pumping NON GBR data on NON GBR bearer I don’t see any packet segmentation?

But when doing GBR data on GBR bearer I see segmentation SI and SO.

Maybe if there is segmentation then not enough RBs for GBR bearer?

4

For VoLTE, may be grant is not sufficient to accommodate VoLTE RL PDU.

But this should actually not happen. I am not sure for this…

5

It is data on QCI 3 …so not even VoLTE service.

I see that for GBR data there is no grants for GBR bearer where GBR data is pumping.
What could be the reason?

6

Maybe you have spus activated? (Semipersistent scheduling)

7

But how is it related?

I mean, even when there is only GBR bearer and not pumping any NON GBR data I still see that GBR bearer not getting any grants…

8

Go check scheduler. Why it does not pump any data.

Is it DL or UL issue?

Maybe data doesn’t come from higher layers.

9

It is DL.

10

Then check in gNodeB what’s the buffer status in PDCP layer, in IP layer for this UE.

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