Radio Link Control - Protocol Data Unit

Description

The RLC-PDU is the fundamental data unit processed by the Radio Link Control (RLC) sublayer, which is part of Layer 2 (the data link layer) in the 3GPP protocol stack for both the user plane and control plane. It is the structured packet that traverses the interface between the RLC entity and its peer entity on the other side of the radio link (e.g., from UE to gNB). The format and content of an RLC-PDU are strictly defined by the operational mode of the RLC entity: Transparent Mode (TM), Unacknowledged Mode (UM), or Acknowledged Mode (AM). In TM, the RLC-PDU is essentially the Service Data Unit (SDU) passed through without adding a header, used for delay-sensitive signaling like broadcast system information. In UM and AM, the RLC entity adds a protocol header to the SDU(s) to create the PDU. This header contains critical information such as a Sequence Number (SN) for in-sequence delivery, segmentation indicators, and, in AM, fields for status reporting (ACK/NACK) to support Automatic Repeat Request (ARQ) for error correction.

The construction of an RLC-PDU involves functions like segmentation and/or concatenation of RLC SDUs (which come from the upper PDCP layer) to fit the size of the transport block granted by the lower MAC layer. The RLC entity receives RLC SDUs and may segment a large SDU into multiple RLC-PDUs or concatenate several small SDUs into a single RLC-PDU, optimizing radio resource usage. The header is then appended, and the complete RLC-PDU is delivered to the MAC layer as a MAC SDU. On the receiving side, the peer RLC entity performs the reverse operations: it uses the header information to reassemble the original RLC SDUs from the received RLC-PDUs, deliver them in sequence to PDCP, and in AM, send status reports back to the transmitter to request retransmission of missing PDUs.

Its role is central to providing the data transfer service required by the upper layers. For control plane signaling (e.g., RRC messages), RLC in AM or TM ensures reliable or timely delivery. For user plane data, the mode is configured per data radio bearer to match the QoS requirements—UM for delay-sensitive services like VoIP (tolerating some loss) and AM for error-intolerant data like file downloads. The RLC-PDU is thus the vehicle for implementing key RLC functions: error correction through ARQ (in AM), in-sequence delivery, duplicate detection, and flow control between RLC and PDCP layers. The efficiency and reliability of the entire radio interface data flow depend on the robust design and processing of the RLC-PDU.

Purpose & Motivation

The RLC-PDU exists to provide a standardized, structured format for data exchange over the error-prone radio link, enabling the Radio Link Control protocol to fulfill its mission of reliable data transfer. The radio channel is inherently unreliable due to fading, interference, and mobility, leading to corrupted or lost data packets. Simply passing raw data packets (SDUs) from the upper PDCP layer to the lower MAC layer would be insufficient to guarantee the service quality required by diverse applications. The RLC-PDU, with its mode-specific header, is the mechanism that allows the RLC layer to add the necessary control information (like sequence numbers) to each data unit. This control information enables the receiving RLC entity to perform critical functions such as reassembling packets in the correct order, detecting duplicates, and, in Acknowledged Mode, identifying missing packets and triggering their retransmission via ARQ.

Historically, as cellular systems evolved from 2G GSM (which had simpler link layer protocols) to 3G UMTS, the need for more sophisticated data services beyond voice necessitated a robust Layer 2 protocol. The RLC layer and its PDU format were introduced in UMTS to handle packet-switched data with varying reliability requirements. The concept was refined and carried forward into LTE (Rel-8) and 5G NR. The design of the RLC-PDU addresses the limitations of having no standardized unit for error control over the air interface. It provides a clear separation of concerns: the PDCP layer handles security and header compression, while the RLC layer, via the PDU, handles logical channel-specific error correction and sequencing, independent of the physical layer's channel coding and HARQ processes. This layered approach allows for more efficient and flexible radio resource management.

Key Features

• Mode-specific formatting (TM, UM, AM)
• Contains a header with Sequence Number (SN) for sequencing
• Supports segmentation and concatenation of SDUs
• Enables Automatic Repeat Request (ARQ) in AM via status reporting fields
• Facilitates in-sequence delivery to upper layers
• Allows for duplicate detection and discard

Evolution Across Releases

Defining Specifications