What is SDU and PDU in LTE?
Today, let me help you understand two fundamental concepts in LTE data transmission: SDU (Service Data Unit) and PDU (Protocol Data Unit). These two are like building blocks in the LTE protocol stack, and once you get the hang of how they work, many other topics in LTE start making more sense.
So, let’s start with a simple perspective. When you send data over an LTE network — whether it’s browsing, streaming, or messaging — that data doesn’t just travel in one big chunk. Instead, it goes through multiple protocol layers, and at each layer, data is processed, packed, or unpacked. This is where SDUs and PDUs come in.
What is an SDU (Service Data Unit)?
You can think of an SDU as a piece of raw data coming from an upper layer and passed down to a lower layer. It’s like when you’re sending a letter — the message you write is the SDU. This data hasn’t been processed yet by the current layer; it’s just coming in to be handled.
For example, the data that the RLC (Radio Link Control) layer receives from the PDCP (Packet Data Convergence Protocol) layer is called the RLC SDU. The same RLC layer will process it and prepare it for transmission over the air.
What is a PDU (Protocol Data Unit)?
Now, once that layer processes the SDU, it wraps it with its own header or applies segmentation or concatenation, and this newly formed unit is called a PDU. So using our letter analogy, the SDU is your letter, and the PDU is the envelope containing that letter, stamped and ready to be sent.
So to put it simply:
| Term | Description | Example |
|---|---|---|
| SDU | Data received from the upper layer, not yet processed by the current layer. | Letter message before placing into envelope. |
| PDU | Processed data with headers, ready to be sent to the lower layer or over the air. | Letter inside an envelope with a stamp. |
How SDU and PDU Flow Through LTE Layers
Let me give you a clearer flow. Imagine data is moving from the top of the LTE protocol stack down to the physical layer:
- Application Layer: Data is created (e.g., a video stream).
- IP Layer: Passes IP packets as SDUs to PDCP.
- PDCP Layer: Takes the SDU (IP packet), adds headers to form a PDU, and sends it to RLC.
- RLC Layer: Takes the PDCP PDU as an SDU, processes it, forms a new PDU, and passes it to MAC.
- MAC Layer: Does similar handling and sends it to the physical layer for transmission.
So as you see, each layer treats the received data as an SDU, processes it, and hands over a PDU to the next layer. It’s a layered approach where SDUs and PDUs continuously transform as data flows down (and up) the LTE protocol stack.
Earlier, we discussed how the PDCP and RLC layers handle data buffering and integrity — now you can connect how SDU and PDU roles fit into that workflow. Understanding this flow is also essential when we get into deeper discussions like packet loss handling, segmentation, or even handover-related buffering.
Once you’re comfortable with SDUs and PDUs, it’ll be easier to understand how features like QoS (Quality of Service) and header compression actually work within LTE. So keep this structure in mind — it will support you as we explore more technical areas ahead.