Reject Frame

Description

A Reject (REJ) frame is a control frame employed in data link layer protocols within 3GPP systems, particularly in protocols like the Radio Link Control (RLC) in UMTS and LTE, or the Link Access Protocol (LAP) used in signaling. It functions as a mechanism for error control, where a receiving entity sends a REJ frame to inform the transmitter that one or more frames have been received incorrectly or are out of sequence, and thus need retransmission. This frame contains information such as sequence numbers to identify the problematic frames, enabling selective retransmission and ensuring data integrity.

In the context of RLC, which operates in acknowledged mode (AM), the REJ frame is part of the Automatic Repeat Request (ARQ) procedure. When an RLC receiver detects a gap in the sequence numbers of received Protocol Data Units (PDUs)—indicating a missing or corrupted frame—it can send a REJ control PDU to the transmitter. This REJ PDU specifies the sequence number of the first missing PDU, prompting the transmitter to retransmit that PDU and any subsequent ones if necessary. This process helps maintain in-order delivery and reliability over the radio interface, which is prone to errors due to fading and interference.

The REJ frame is also used in older protocols like LAP for circuit-switched connections, where it serves a similar purpose in ensuring reliable frame transmission over the air interface or wired links. In modern 3GPP systems, while RLC AM uses REJ frames, there are also other control frames like Selective Reject (SREJ) for more efficient retransmission in some implementations. The use of REJ frames is critical for minimizing latency and optimizing throughput by quickly addressing errors without requiring full window retransmissions, thus supporting the quality of service requirements for various applications.

Purpose & Motivation

The Reject (REJ) frame was introduced to address the need for efficient error control in data link layer protocols within 3GPP networks, with roots in earlier telecommunication standards. In pre-3G systems, error management often relied on simpler mechanisms like stop-and-wait ARQ, which could be inefficient for high-speed data services. The REJ frame, as part of protocols like LAP and later RLC, provided a more dynamic way to handle frame errors by enabling selective retransmission, thus improving throughput and reliability.

REJ frames solve the problem of how to quickly signal transmission errors without resorting to full retransmission of all outstanding frames, which would waste bandwidth and increase latency. By allowing the receiver to explicitly request retransmission of specific frames identified by sequence numbers, REJ frames facilitate faster recovery from errors, which is crucial in wireless environments where packet loss is common. This is especially important for real-time services and data applications that require low latency and high reliability.

The adoption of REJ frames in 3GPP protocols like RLC AM from Release 5 onwards supported the evolution toward all-IP networks and higher data rates in UMTS and LTE. They addressed limitations of previous error control methods by integrating with other ARQ mechanisms, such as positive acknowledgments and polling, to create a robust framework for data integrity across the radio interface.

Key Features

• Control frame used for error indication in data link layer protocols
• Contains sequence numbers to identify frames requiring retransmission
• Supports selective retransmission in ARQ procedures
• Used in RLC acknowledged mode for reliable data transfer
• Facilitates quick error recovery to minimize latency and improve throughput
• Integrates with other control frames like ACK and SREJ for comprehensive flow control

Evolution Across Releases

Defining Specifications