Unacknowledged Mode Data

Description

Unacknowledged Mode Data (UMD) is the designation for the Protocol Data Unit (PDU) used by the Radio Link Control (RLC) entity when it is configured in Unacknowledged Mode (UM). An RLC PDU is the block of data that is passed between the RLC and the underlying Medium Access Control (MAC) layer for transmission over the radio interface. The UMD PDU has a specific structure defined in the 3GPP specifications that enables the RLC UM functions. The PDU consists of two main parts: the RLC UMD header and the RLC UMD payload. The payload contains either an entire RLC Service Data Unit (SDU) or a segment of an RLC SDU, depending on whether segmentation was applied.

The UMD header is critical for the receiver's operation. Its fields include the Sequence Number (SN), which is used for in-sequence delivery and detection of missing PDUs. The length of the SN field (e.g., 5 bits in some LTE configurations, 6 or 10 bits in others) is configured by the Radio Resource Control (RRC) layer. The header also contains framing information (FI) fields to indicate whether the PDU contains the first and/or last segment of an RLC SDU. This allows the receiver to correctly identify the boundaries of the original SDUs during reassembly. Furthermore, an Extension (E) bit indicates whether a following set of header fields (length indicators) is present. Length Indicators (LIs) are used when the UMD PDU contains more than one RLC SDU segment or a combination of an SDU and a segment; they specify the boundaries between these elements within the payload.

From an architectural perspective, the transmitting RLC UM entity constructs UMD PDUs. It takes RLC SDUs from the upper layer (e.g., PDCP), stores them in a buffer, and may segment and/or concatenate them based on the size of the transport block granted by the MAC layer. It then builds the UMD PDU by adding the appropriate header with the correct SN and framing info. The sequence number is incremented for each new UMD PDU. On the receiving side, the RLC UM entity extracts the SN and LI information. It uses the SN to place the received payload data into a reassembly buffer in the correct order. The LI and FI fields tell the receiver how to reconstruct the original RLC SDUs from the stored segments. Once a complete SDU is reassembled (indicated by the 'last segment' flag), it is delivered in-sequence to the upper layer. The entire process operates without any feedback to the transmitter, making the UMD PDU a 'fire-and-forget' data unit at the RLC level.

Purpose & Motivation

The UMD PDU format was defined to provide a standardized and efficient container for data transferred in Unacknowledged Mode. Without a specific PDU structure, the RLC layer could not support its core functions of in-sequence delivery and segmentation/reassembly. The purpose of the UMD is to encapsulate upper layer data with just enough control information (sequence number, segmentation boundaries) to enable correct reassembly at the receiver, while minimizing overhead to preserve bandwidth for the actual user data. This is especially important for real-time services where header overhead directly impacts spectral efficiency.

It solves the problem of adapting variable-size IP packets (or other upper layer PDUs) to the variable-size transport blocks provided by the physical layer via the MAC. The segmentation and concatenation capabilities, signaled via the UMD header fields, allow for efficient packing of data into radio resources. The sequence number addresses the issue of out-of-order delivery that can occur over the air interface due to HARQ retransmissions at the MAC layer or path diversity. By defining a precise PDU structure, the UMD enables interoperability between transmitters and receivers from different vendors, ensuring that all equipment interprets the data stream correctly. Its creation was an integral part of specifying the RLC protocol, allowing the UM service to be realized in practice.

Key Features

• Defined PDU structure for RLC Unacknowledged Mode operation
• Header contains a Sequence Number for in-order delivery and loss detection
• Includes Framing Information to indicate start/end of RLC SDUs
• Utilizes Length Indicators to delineate multiple data segments within one PDU
• Supports segmentation and concatenation of upper layer SDUs
• Configurable header size based on RRC configuration (SN length)

Evolution Across Releases

Defining Specifications