Transparent Mode Data

Description

Transparent Mode Data (TMD) is one of the three data transfer services (alongside Unacknowledged Mode and Acknowledged Mode) offered by the Radio Link Control (RLC) sublayer in the 3GPP protocol stack for UMTS (TS 25.322), LTE (TS 36.322), and NR (TS 38.322). When operating in Transparent Mode (TM), the RLC layer provides a pass-through service for upper layer Protocol Data Units (PDUs). The defining characteristic of TMD is that the RLC layer performs no manipulation on the data; it does not add any RLC header, does not perform segmentation or concatenation, and does not execute any retransmission or error correction procedures.

Architecturally, the TMD service is established between peer TM RLC entities, typically configured for specific logical channels that carry control information or delay-sensitive user data. In the transmitting entity, the RLC receives a Service Data Unit (SDU) from the upper layer (e.g., the Packet Data Convergence Protocol (PDCP) layer or the RRC layer). For TMD, this SDU is passed directly to the lower layer (the Medium Access Control (MAC) layer) as an RLC PDU without any modification. On the receiver side, the TM RLC entity receives the PDU from the MAC layer and delivers it as an SDU to the upper layer, again without processing. The service is truly transparent.

How it works is intrinsically simple but critical for specific traffic types. Because there is no RLC header, TMD offers the lowest possible overhead and the minimal processing delay. However, it also provides no guarantees. Data can be lost due to radio errors, as there is no Automatic Repeat Request (ARQ) mechanism within RLC. Similarly, out-of-sequence delivery can occur if the lower layers reorder packets. Therefore, TMD is suitable only for applications where such losses are acceptable or are handled by application-layer protocols, or for information that is broadcast periodically where a missed packet can be ignored. Its key role in the network is to provide an efficient, low-latency conduit for time-critical signaling (like some RRC messages broadcast on the BCCH) and for certain types of user plane traffic, such as Voice over IP (VoIP) where jitter buffers can handle some loss, or for multimedia broadcast services.

Purpose & Motivation

TMD was introduced from the earliest 3GPP RLC specifications to fulfill the need for an ultra-low latency and minimal overhead data transfer service within the radio protocol stack. Not all data requires the robust, in-order delivery guarantees provided by Acknowledged Mode (AM). Some traffic, particularly real-time control signaling and certain types of streaming media, prioritizes timeliness over absolute reliability.

The problem TMD addresses is the avoidance of unnecessary protocol processing delay and overhead for delay-sensitive or loss-tolerant information. Using AM or even Unacknowledged Mode (UM) for such traffic would add header overhead and processing latency for segmentation/reassembly and sequence numbering, which could degrade the performance of real-time applications. For example, system information broadcast by the cell must be received quickly by all UEs in its coverage; waiting for retransmissions of lost blocks is not practical, and the information is often repeated in subsequent broadcasts anyway.

Its creation and persistence through 2G, 3G, 4G, and 5G reflect a fundamental design principle in layered communication protocols: providing multiple service qualities to match diverse application requirements. TMD represents the 'lightest' possible service. It solves the problem of efficiently transporting data where the cost of reliability mechanisms (in latency and overhead) outweighs their benefit. This design allows network architects to map different types of logical channels to the most appropriate RLC mode, optimizing overall system performance and resource utilization for a mix of traffic profiles.

Key Features

• Zero RLC header overhead, maximizing spectral efficiency for supported traffic
• Minimal processing delay as no segmentation, concatenation, or retransmission is performed
• Suitable for delay-sensitive signaling and real-time user plane traffic
• No in-sequence delivery guarantee; packets may be lost or delivered out-of-order
• Configured per logical channel based on the QoS requirements of the carried data
• Maintains consistency across UMTS, LTE, and NR RLC protocol specifications

Evolution Across Releases

Defining Specifications