Acknowledged Mode Data

Description

Acknowledged Mode Data (AMD) is a fundamental operating mode of the Radio Link Control (RLC) layer in 3GPP wireless communication systems, specified across UMTS (25.322), LTE (36.322), and 5G NR (38.322) standards. AMD operates as a stateful protocol that provides reliable data transfer services between the User Equipment (UE) and the Radio Access Network (RAN), specifically between the RLC entities in the transmitter and receiver. Unlike Unacknowledged Mode (UM) which offers best-effort delivery, AMD implements Automatic Repeat Request (ARQ) mechanisms to guarantee error-free, in-sequence delivery of Protocol Data Units (PDUs).

The AMD protocol architecture includes several key components: transmission and reception buffers for storing RLC Service Data Units (SDUs) and PDUs, a segmentation and concatenation function that adapts SDUs to available transport block sizes, sequence numbering for all AMD PDUs, and ARQ procedures for retransmission management. Each AMD PDU contains a sequence number in its header, allowing the receiver to detect missing PDUs, reorder received PDUs, and eliminate duplicates. The transmitter maintains a transmission window and retransmission buffer, while the receiver maintains a reception window and reordering buffer to ensure proper sequencing.

AMD operation involves several coordinated processes: segmentation/concatenation of higher-layer SDUs into RLC PDUs, sequence number assignment, transmission with timer management, status reporting from receiver to transmitter, and selective retransmission of missing PDUs. The receiver sends STATUS PDUs containing acknowledgments (ACKs) for successfully received PDUs and negative acknowledgments (NACKs) for missing PDUs. These status reports can be polled by the transmitter or sent periodically by the receiver. The transmitter uses this feedback to retransmit unacknowledged PDUs while maintaining flow control through window-based mechanisms.

In the protocol stack, AMD sits between the Packet Data Convergence Protocol (PDCP) layer above and the Medium Access Control (MAC) layer below. It serves critical signaling bearers (SRB1, SRB2) and data radio bearers (DRBs) requiring reliable delivery, particularly for TCP/IP traffic and control plane signaling. AMD's configurable parameters include maximum retransmission attempts, timer values for status reporting, and window sizes, allowing network operators to balance reliability against latency based on service requirements. The protocol's robustness makes it essential for maintaining connection stability during mobility events and in challenging radio conditions.

Purpose & Motivation

AMD was created to address the fundamental challenge of reliable data transmission over error-prone wireless channels in 3GPP cellular networks. Before standardized ARQ mechanisms at the RLC layer, wireless systems struggled with high packet error rates that degraded user experience and network efficiency. AMD provides a standardized approach to error recovery that operates independently of higher-layer protocols like TCP, offering faster recovery from transmission errors due to its closer proximity to the physical layer.

The primary motivation for AMD development was to support diverse services with varying reliability requirements within a unified framework. While some applications tolerate packet loss (e.g., voice, video streaming), others like web browsing, file transfer, and signaling protocols require guaranteed delivery. AMD enables the network to provide different quality of service levels by configuring reliability parameters per radio bearer. This flexibility allows operators to optimize resource usage while meeting diverse application requirements.

AMD solves several specific problems: it prevents protocol stalling at higher layers by providing local retransmissions, reduces end-to-end latency compared to relying solely on TCP retransmissions, and conserves radio resources by avoiding unnecessary retransmissions of successfully delivered packets. The protocol's duplicate detection and in-sequence delivery features ensure data integrity even during handovers and radio link failures. By standardizing these mechanisms across 3GPP generations, AMD has enabled consistent quality of service management and simplified multi-vendor interoperability in cellular networks.