Transfer Mode Entity

Description

The Transfer Mode Entity (TME) is a functional component within the Radio Resource Control (RRC) protocol layer of the UMTS Terrestrial Radio Access Network (UTRAN), as specified in 3GPP TS 25.331. It operates in both the User Equipment (UE) and the UTRAN (specifically the Radio Network Controller, RNC). Its primary role is to manage the transfer mode configuration for radio bearers, which defines how data is transferred over the radio interface. The TME is responsible for the mapping between logical channels (the service access points offered by the MAC layer) and transport channels (the characteristics of the data transfer over the radio interface). This mapping is a fundamental aspect of radio bearer establishment, reconfiguration, and release procedures.

During radio bearer setup or reconfiguration, the RRC layer, utilizing the TME, configures the appropriate transfer mode based on the required quality of service (QoS). The TME handles the configuration of parameters such as the transmission time interval (TTI), the coding scheme, and the rate matching attributes. It supports different transfer modes, including transparent mode (TM), unacknowledged mode (UM), and acknowledged mode (AM), which correspond to different Radio Link Control (RLC) protocol modes. The TME ensures that the lower layers (RLC and MAC) are correctly configured to support the chosen mode, directly impacting the reliability and characteristics of the data flow.

The TME's operation is tightly coupled with the Radio Bearer Control functions. When the network decides to establish a new radio bearer for a service (like a voice call or packet data session), the RRC layer generates a RADIO BEARER SETUP message. This message contains configuration information derived from the TME's management, detailing the logical channel identity, the RLC mode, and the associated transport channel parameters. The receiving entity's TME interprets these instructions and configures its local protocol stacks accordingly. This centralized management of the transfer mode abstraction simplifies the control logic and ensures consistent configuration between the UE and the network for reliable communication.

Purpose & Motivation

The TME was introduced to provide a structured and standardized method for managing the complex mapping and configuration required for data transfer over the UMTS radio interface. Prior to its formal definition, the handling of logical-to-transport channel mapping and RLC mode configuration could have been ad-hoc or implementation-specific. The TME abstracts these details into a manageable entity, simplifying the RRC protocol's control procedures for radio bearer management.

It solves the problem of efficiently and reliably establishing communication channels with diverse QoS requirements. Different services (e.g., circuit-switched voice, streaming video, web browsing) require different transfer characteristics regarding delay, error tolerance, and throughput. The TME provides the mechanism to translate these service requirements into specific, actionable configurations for the RLC and MAC layers. Its creation was motivated by the need for a clear architectural separation of concerns within the RRC layer, promoting interoperability between equipment from different vendors by ensuring a common understanding and handling of transfer mode configurations.

Key Features

• Manages the mapping between logical channels and transport channels
• Configures the Radio Link Control (RLC) mode (Transparent, Unacknowledged, Acknowledged)
• Handles parameters for transmission time interval (TTI) and rate matching
• Integral part of Radio Bearer establishment, reconfiguration, and release procedures
• Operates in both the User Equipment (UE) and the Radio Network Controller (RNC)
• Abstracts lower-layer configuration details for the RRC control logic

Evolution Across Releases

Defining Specifications