Signalling Data Unit

Description

A Signalling Data Unit (SDU) is a fundamental concept in the layered protocol architecture defined by 3GPP. It represents the data packet as it is passed from a higher protocol layer to a lower protocol layer for transmission. From the perspective of the lower layer, the SDU is the payload to be transported. The lower layer then typically adds its own protocol-specific header and/or trailer to this SDU, transforming it into a Protocol Data Unit (PDU) for that layer. This PDU is then passed further down the stack or transmitted over the physical medium. The process is reversed at the receiving end, where a lower layer processes the incoming PDU, strips its own header/trailer, and delivers the resulting SDU up to the next higher layer.

The SDU/PDU distinction is critical across all 3GPP interfaces and protocol stacks, including the radio interface (Uu), Iu interface between RAN and CN, and core network interfaces. For example, in the Radio Link Control (RLC) layer, an SDU received from the Packet Data Convergence Protocol (PDCP) layer is segmented and/or concatenated, an RLC header is added, and it becomes an RLC PDU. This PDU is then passed to the Medium Access Control (MAC) layer, where it becomes a MAC SDU. The MAC layer may multiplex several MAC SDUs, add a MAC header, and create a transport block (MAC PDU) for transmission over the physical layer.

The integrity and correct processing of SDUs are paramount for signalling reliability. Signalling protocols like Radio Resource Control (RRC), Non-Access Stratum (NAS), and Session Initiation Protocol (SIP) rely on the underlying layers to deliver their SDUs accurately and in sequence. Mechanisms within each layer, such as RLC Acknowledged Mode, ensure the reliable delivery of signalling SDUs. The size and format of an SDU are defined by the service expected by the higher layer and the capabilities of the lower layer, with specifications detailing maximum sizes and handling procedures for segmentation and reassembly.

Purpose & Motivation

The concept of the SDU exists to formalize the data exchange between adjacent layers in a standardized protocol stack, enabling modular design and interoperability. It solves the problem of how information from an application or control process is packaged, transported, and reliably delivered across a complex network by separating concerns. Each layer has a specific function (e.g., error correction, routing, encryption), and the SDU is the well-defined package on which that function operates.

Historically, without a clear layered model and the SDU/PDU abstraction, protocol design was monolithic and inflexible. The Open Systems Interconnection (OSI) model introduced these concepts, which were adopted and refined by 3GPP. The SDU provides a clean service access point for the upper layer, hiding the complexities of the lower-layer transmission. This allows for the independent evolution and optimization of different protocol layers (e.g., introducing a new RLC mode or a new physical layer technology) without disrupting the higher-layer signalling protocols, as long as the SDU delivery service is maintained.