Selected REJect frame

Description

The Selected REJect (SREJ) frame is a supervisory frame defined in data link layer protocols such as LAPD (Link Access Procedure on the D-channel) and its mobile derivative LAPDm, used in GSM and early 3GPP signaling. It operates within the context of a balanced, error-corrected data link connection, typically between a mobile station and the network (e.g., over the Um or Abis interface). The primary function of the SREJ frame is to implement a selective reject ARQ (Automatic Repeat reQuest) mechanism. When a receiver detects a gap in the sequence of incoming I-frames (Information frames)—meaning it has received frame N+1 but is missing frame N—it can send an SREJ frame specifically requesting the retransmission of the missing frame N. This is in contrast to a REJ (Reject) frame, which requests retransmission starting from frame N and all subsequent frames.

Architecturally, the SREJ frame is generated by the receiving side of the data link layer entity. It contains a control field with the SREJ command identifier and the send sequence number N(R), which indicates the sequence number of the specific I-frame that needs to be retransmitted. Upon receiving a valid SREJ frame, the transmitting entity must retransmit the requested I-frame with sequence number N(R). The transmitter can continue sending frames with sequence numbers higher than N(R) while it processes the retransmission, as the receiver is capable of buffering out-of-sequence frames. This is a key aspect of its operation, enabling higher link utilization.

The protocol ensures that only one SREJ condition can be outstanding at a time per direction of transmission. This prevents ambiguity. The SREJ mechanism works in conjunction with other supervisory frames like RR (Receiver Ready) and RNR (Receiver Not Ready), and the timer-based recovery procedures defined in the link layer protocol. Its role is to enhance the efficiency of the error recovery process over potentially noisy radio links. By requesting only the missing frame, it minimizes unnecessary retransmissions, conserves radio resources, and reduces latency compared to go-back-N ARQ schemes triggered by a standard REJ frame. This efficiency was crucial for signaling channels where message delivery must be reliable but also timely.

Purpose & Motivation

The SREJ frame was introduced to optimize error recovery in the signaling links of digital telecommunications systems, specifically addressing the limitations of simpler ARQ schemes. In early digital mobile systems like GSM, the radio interface was (and remains) a primary source of transmission errors. The data link layer needed a robust yet efficient method to ensure the reliable delivery of critical signaling messages (e.g., for call setup, handover, and mobility management). A simple Go-Back-N ARQ, initiated by a REJ frame, would force the retransmission of all frames from the point of error, wasting bandwidth and increasing delay, especially if the window size was large or the link was moderately error-prone.

The selective reject mechanism provided by SREJ solved this problem by enabling the receiver to request only the specific frame that was lost or corrupted, assuming it could buffer subsequent correctly received frames. This approach, known as Selective Repeat ARQ, maximizes throughput on channels with non-bursty errors. Its creation was motivated by the need to make the most efficient use of the scarce and expensive radio spectrum allocated for signaling channels. Lower latency in signaling translation directly improved user experience metrics like call setup time.

Historically, SREJ was part of the ISDN-based protocol suite (Q.921/LAPD) adapted for mobile use (LAPDm). Its inclusion reflected the engineering trade-offs of the late 1980s and 1990s: implementing selective repeat required more complex receiver logic (for buffering and re-sequencing) than go-back-N, but the benefits in performance were deemed worth the added complexity for the critical control plane. As 3GPP evolved with UMTS and LTE, the underlying link layer protocols changed (e.g., to RLC in the radio stack), which use different ARQ mechanisms, making SREJ specific to the GSM/EDGE Radio Access Network (GERAN) and certain core network signaling links.

Key Features

• Implements Selective Repeat ARQ for efficient error recovery
• Requests retransmission of a single, specifically identified frame
• Allows continued transmission of subsequent frames while retransmission is pending
• Defined within LAPD/LAPDm control field formats
• Only one SREJ condition allowed per transmission direction at a time
• Reduces signaling delay and bandwidth waste compared to Go-Back-N

Evolution Across Releases

Defining Specifications