Description
The SUper FIeld (SUFI) is a fundamental element of the Radio Link Control (RLC) protocol, specified for UMTS in 3GPP Release 4 and later. It operates within the RLC control Protocol Data Unit (PDU). The SUFI is not a fixed-format field; rather, it is a TLV (Type-Length-Value) like structure used to convey specific control information between peer RLC entities, primarily in Acknowledged Mode (AM). The transmitting RLC entity includes SUFIs in its control PDUs to signal events like acknowledgements, window updates, or error conditions to the receiver, and vice-versa.
A SUFI consists of three main parts: the SUFI Type, the SUFI Length, and the SUFI Data (or Value). The SUFI Type is a short identifier that defines the meaning and format of the data that follows (e.g., ACK, LIST, BITMAP, WINDOW). The SUFI Length indicates the size of the SUFI Data field in octets. The SUFI Data contains the actual control information, whose interpretation depends entirely on the SUFI Type. For example, an 'ACK' SUFI contains a sequence number acknowledging receipt of all PDUs up to that point, while a 'LIST' SUFI contains a list of specific sequence numbers that are being acknowledged or reported as missing. A 'WINDOW' SUFI is used to advertise the current receiver window size.
The operation of SUFIs is central to RLC AM's ARQ mechanism. When an AM RLC receiver detects missing PDUs (gaps in the sequence number space), it can generate a STATUS PDU containing one or more SUFIs (like LIST or BITMAP) to inform the transmitter exactly which PDUs need retransmission. This enables selective repeat ARQ, which is more efficient than go-back-N. The transmitter processes the received SUFIs, updates its transmission buffer state, and schedules retransmissions accordingly. SUFIs are also used for protocol error recovery and reset procedures. Their variable-length, type-driven design makes the RLC control protocol flexible and efficient, as only the necessary information for a given situation needs to be transmitted, conserving valuable radio resources.
Purpose & Motivation
SUFI was created to provide a flexible and efficient signaling mechanism for the RLC layer's control plane in UMTS. Prior to UMTS, 2G systems like GSM had simpler link layer protocols. The introduction of UMTS, with its wider range of services (from voice to packet data) and more challenging radio conditions, demanded a robust and efficient ARQ mechanism at Layer 2 to ensure reliable data transfer. The RLC Acknowledged Mode needed a way to exchange complex status information—like acknowledging a range of frames, reporting a list of missing frames, or managing the receiver window—without defining a multitude of fixed-format control messages.
The SUFI concept solved this by introducing a generic, extensible container for control information. This design addressed the limitations of having a fixed set of control PDU types, which would be either too simplistic or wasteful. With SUFIs, a single STATUS PDU format could carry any combination of needed control information by concatenating different SUFI elements. For instance, it could carry an ACK for a range, a LIST for specific missing PDUs, and a WINDOW update, all in one efficiently encoded PDU. This flexibility was crucial for optimizing performance over the variable and error-prone WCDMA radio interface.
Furthermore, SUFIs enabled the RLC protocol to adapt to different data flow patterns. For continuous streaming with few errors, simple ACK SUFIs suffice. For bursty errors, detailed LIST or BITMAP SUFIs allow precise retransmission requests. This efficiency in control signaling directly translates to higher user data throughput and lower latency, which were key goals for UMTS packet-switched services. The SUFI mechanism established a design pattern for flexible Layer 2 control that influenced later 3GPP systems, even as the specific RLC protocol evolved in LTE and 5G NR.
Key Features
- Variable-length TLV-style control information element
- Defined by a Type identifier dictating the Data format
- Carried within RLC control PDUs (STATUS PDUs)
- Enables selective repeat ARQ with precise acknowledgement/negative acknowledgement
- Supports various control functions: ACK, LIST, BITMAP, WINDOW, etc.
- Flexible concatenation of multiple SUFIs in a single control PDU
Evolution Across Releases
Introduced the SUFI as part of the UMTS RLC protocol specification for Acknowledged Mode. Defined the initial set of SUFI types (e.g., ACK, LIST, BITMAP, WINDOW, MRW) and their encoding. Established SUFI as the core mechanism for conveying ARQ status and control information between RLC entities over the radio interface.
Enhanced RLC for High-Speed Downlink Packet Access (HSDPA). While the core SUFI concept remained, the introduction of the MAC-hs layer in Node B for HSDPA changed the ARQ paradigm, moving Hybrid ARQ (HARQ) to the MAC layer. RLC AM with SUFIs remained for reliable delivery, but its role evolved to handle residual errors after HARQ.
Further evolution with HSPA+ (Enhanced Uplink and MIMO). Continued refinements to RLC performance and interaction with lower-layer HARQ. The SUFI-based STATUS reporting remained the mechanism for RLC-level error correction, ensuring end-to-end reliability within the radio access network.
With the introduction of LTE, a new RLC protocol was defined. The SUFI concept from UMTS RLC was not carried forward directly. LTE RLC uses different PDU formats and control mechanisms (e.g., RLC STATUS PDUs with a fixed bitmap for AM). Therefore, SUFI is specific to UMTS (and HSPA) RLC and is not part of the LTE or NR protocol stacks.
Defining Specifications
| Specification | Title |
|---|---|
| TS 25.322 | 3GPP TS 25.322 |