Payload Unit

Description

The Payload Unit (PU) is a critical data structure within the 3GPP Universal Mobile Telecommunications System (UMTS) and evolved Radio Link Control (RLC) protocol architecture. It resides in the Data Link Layer (Layer 2) of the radio interface protocol stack. The PU is the output of the RLC transmitting entity and the input of the RLC receiving entity for a given logical channel. Its structure and handling depend on the RLC mode of operation: Transparent Mode (TM), Unacknowledged Mode (UM), or Acknowledged Mode (AM).

In terms of architecture, the RLC layer receives Service Data Units (SDUs) from the upper layer (e.g., the Packet Data Convergence Protocol (PDCP) or RRC). The RLC entity then processes these SDUs to create PUs for transmission. This processing involves key functions: For AM and UM, the RLC adds a header to the SDU (or a segment thereof). This header contains crucial information like the Sequence Number (SN), which is used for in-sequence delivery and retransmission management, and segmentation indicators. The data field contains the actual SDU or a segment. This header+SDU combination forms the PU. In Transparent Mode, no header is added; the PU is essentially the SDU itself, passed through without modification.

The operational workflow is as follows: The transmitting RLC entity performs segmentation and/or concatenation on incoming SDUs to fit them into the size of available Transport Blocks (TBs) indicated by the MAC layer. It then constructs the PU by attaching the necessary RLC header. This PU is delivered to the Medium Access Control (MAC) layer as a MAC Service Data Unit. The MAC layer multiplexes PUs from different logical channels into a Transport Block for physical layer transmission. On the receiver side, the process is reversed. The MAC delivers the PU to the appropriate RLC entity. The RLC entity strips the header, uses the sequence number for reordering, and performs reassembly of SDUs from multiple PUs (if segmentation occurred) before delivering the complete SDU upwards.

The role of the PU is central to RLC's reliability functions. In Acknowledged Mode, the PU is the unit of retransmission. If a PU is lost, the receiving RLC can request its retransmission using status reports that reference the PU's sequence number. The PU structure enables the peer RLC entities to maintain a synchronized view of what data has been successfully transferred. It is the atomic unit of data transfer for which the RLC layer provides its characteristic services: transparent transfer, unacknowledged data transfer, or acknowledged data transfer with error correction.

Purpose & Motivation

The Payload Unit concept was established from the earliest 3GPP UMTS specifications (Release 99) to provide a structured and efficient mechanism for data transfer over the error-prone radio link. The core problem it addresses is the mismatch between the size of data packets from higher layers (which can be large, e.g., an IP packet) and the size of physical layer transmission units (Transport Blocks), which are constrained by radio conditions and scheduling grants.

Before the RLC processing embodied by the PU, there was no standardized way to handle segmentation and reassembly reliably across the radio interface for packet-switched services. The PU, with its header containing sequence numbers and segmentation info, provides this capability. It allows large higher-layer packets to be broken down for transmission and reliably reassembled in order at the receiver, even if the individual segments (PUs) arrive out of order due to HARQ retransmissions or variable delay.

Historically, its creation was motivated by the need to support diverse services—from voice (using small, delay-sensitive TM PUs) to packet data (using larger, reliable AM PUs)—over a single, flexible radio link control layer. The PU is the embodiment of the RLC layer's service to the upper layers, abstracting the complexities of the radio link. It solves the fundamental issue of adapting arbitrary higher-layer data flows to the granular, variable, and unreliable resource of the physical radio channel, enabling efficient and reliable data transport that is foundational to all 3GPP cellular data services.

Key Features

• The formatted data block transferred between peer RLC entities
• Contains an RLC header (for UM/AM modes) and a data field
• The data field carries an upper layer SDU or a segment of an SDU
• Sequence Number in the header enables in-order delivery and retransmissions
• Serves as the unit of segmentation, concatenation, and error correction
• Structure varies between Transparent Mode (no header), Unacknowledged Mode, and Acknowledged Mode

Evolution Across Releases

Defining Specifications