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?

Admin note: this post was updated with image below.
segmentation at RLC

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.

1 Like
11

In 5G NR, “RLC segmentation” refers to the process where the Radio Link Control (RLC) layer breaks down large data packets (called Protocol Data Units - PDUs) into smaller segments for transmission over the air interface, ensuring they fit within the limitations of the lower layer, and then reassembling them at the receiving end once received; essentially managing data size for efficient transmission by dividing it into smaller, manageable chunks.

image

Key points about RLC segmentation:

  • Function: RLC is responsible for segmenting and reassembling data to optimize transmission over the radio link.
  • Layer in 5G protocol stack: RLC sits between the PDCP (Protocol Data Convergence Protocol) layer and the MAC (Medium Access Control) layer.
  • Why segmentation is needed: Large data packets might exceed the capacity of the lower layers, so segmentation ensures data can be transmitted in smaller, manageable pieces.
  • Reassembly at receiver: The receiving device reassembles the segmented data back into its original form using sequence numbers included in the RLC header.

Read more::poi

12

RLC segmentation in 5G NR is the process where the RLC layer (Radio Link Control) breaks down large pieces of data into smaller chunks so they can be sent over the network more efficiently. These smaller chunks are called RLC PDUs (Protocol Data Units).

Why is segmentation needed?

Imagine you’re sending a large package, but the delivery truck can only carry smaller boxes. You have to divide your package into smaller boxes that fit. Similarly, in 5G:

  1. The RLC SDU (the big piece of data) might be too large to fit in the “space” allocated for it in the network.
  2. The network has a limit on how much data can be sent at once, depending on conditions like bandwidth and signal strength.

To handle this, the RLC layer:

  • Cuts the data into smaller pieces (segments).
  • Labels each piece so they can be put back together in the correct order at the destination.

How does it work in simple steps?

  1. RLC receives a large SDU from the layer above (PDCP layer).
  2. It checks how much data can fit in the transport block (the “box” used to send data).
  3. If the SDU is too big, it breaks it into smaller segments that fit.
  4. Each segment is wrapped into an RLC PDU with a header to track its position and size.
  5. The PDUs are sent to the MAC layer, which delivers them over the air.

Example Analogy

Think of sending a large painting:

  • The painting is too big for a regular delivery truck.
  • You cut it into smaller sections (segments), label them (e.g., “Part 1 of 3”), and pack them in smaller boxes (PDUs).
  • At the destination, the receiver opens the boxes and reassembles the painting using the labels.

Why does 5G use segmentation?

  1. Adapt to Network Conditions:
  • The available space (transport block size) varies depending on signal strength and bandwidth. Smaller segments fit better.
  1. Efficient Use of Resources:
  • Segmentation ensures no data is wasted or left behind.
  1. Reliability:
  • If one segment is lost, only that small piece needs to be resent, not the entire data.

RLC segmentation ensures data is sent efficiently, reliably, and in a way that adapts to the network’s capacity.