SDUs and PDUs in 5G(NR)

In the 5G(NR) system, two types of data units, PDU and SDU, are passed between the terminal and the network respectively, and usually the terminal (UE) provides end-to-end user-plane connectivity between the UPF (User-Place Function) and the DN (Specific Data Network) through the PDUSession; this is because the SDU is passed from the OSI layer or sublayer to the lower layer in the OSI-based (Open System Interconnection) system, and the SDU has not been encapsulated into the PDU (Protocol Data Unit) by the lower layer. OSI (Open System Interconnection) based systems SDUs are data units passed from the OSI layer or sub-layer to the lower layers, which have not yet been encapsulated into PDUs (Protocol Data Units) by the lower layers, whereas the SDUs are encapsulated into the lower layer's PDUs and the process continues until the PHY (Physical Layer) of the OSI stack. Regarding SDU and PDU in 5G(NR), 3GPP defines them as follows;

1、 SDU(Service Data Unit)

• Definition:A Service Data Unit (SDU) is a unit of data that is passed from the upper layer to the lower layer in the network protocol stack; the SDU contains the payload or the data that needs to be transmitted, and the upper layer expects the lower layer to be able to transmit this data.
• Role:SDUs are essentially data that a service (application or process) wishes to transmit using the underlying network. When the SDU is passed to the lower protocol layer for transmission, it may be combined with other information (e.g., header or tail) to convert it into a Protocol Data Unit (PDU) appropriate for that layer.

2、The PDU (protocol data unit)

• Definition:A PDU (Protocol Data Unit) is a combination of SDUs and protocol-specific control information (e.g., header and tail). Each layer in the network can add or remove its own PDU header or tail, thus encapsulating or decapsulating the SDU as it passes through the layers.
• Role:A PDU represents a packet with SDUs (raw service data) and control information required for the network to process the data correctly. This control information can include error checking, segmentation, identification, and other control mechanisms to ensure that the data can be properly routed and transmitted.

3、SDUs and PDUs

The use of SDUs and PDUs in 5G(NR) networks is critical to ensure that data is properly formatted and processed at different layers, where Layer2 in 5G(NR) handles PDUs and SDUs as follows:

• PDCP Layer:Handles PDCP PDUs, which encapsulate upper layer SDUs (from RRC or user data) with control information (e.g., sequence numbers and header compression) for efficient transmission.
• RLC Layer:Manages RLC PDUs, segments and reorganizes RLC SDUs to ensure reliable transmission of data over the network.
• MAC Layer:Utilizes the MAC PDU aspect of formatted data units containing primarily MAC headers and payloads to ensure that data is efficiently scheduled and transmitted by the physical layer.

4、The data processing process 5G (NR) system data processing specific process is shown in the following figure: