Acknowledged Mode

Description

Acknowledged Mode (AM) is a fundamental operational mode of the Radio Link Control (RLC) layer in 3GPP radio access networks (UTRAN, E-UTRAN, NG-RAN). It is also a mode used in the Packet Data Convergence Protocol (PDCP) layer, particularly for control plane data and for data radio bearers requiring reliability. Its primary function is to ensure the reliable, in-sequence delivery of upper layer protocol data units (PDUs) over the potentially error-prone air interface.

Architecturally, AM operates within the RLC entity established for a specific logical channel. An RLC entity in AM consists of a transmitting side and a receiving side. The transmitting side segments or concatenates Service Data Units (SDUs) from higher layers into RLC PDUs, which are then transmitted. Each PDU is assigned a sequence number. The receiving side uses these sequence numbers to detect missing PDUs and to reassemble SDUs in the correct order for delivery upward. Crucially, the receiving side sends status reports (ACK/NACK) back to the transmitter, indicating which PDUs were received correctly.

The mode works through a selective repeat Automatic Repeat Request (ARQ) mechanism. When the transmitting AM RLC entity receives a status report indicating a missing PDU, it retransmits that specific PDU. The transmitter maintains a transmit buffer to hold PDUs until they are positively acknowledged. Window-based flow control is used to manage the sequence number space and prevent buffer overflows. This process ensures that all data is delivered, albeit with variable latency due to retransmissions.

Key components include the sequence numbering scheme, the retransmission buffer, the status reporting mechanism (using RLC control PDUs), and the timers that trigger status reports and poll for acknowledgments. Its role is critical for signaling messages (RRC, NAS) and for user plane data that cannot tolerate loss, such as TCP/IP traffic. It provides the reliability that the underlying physical layer cannot guarantee, forming a crucial link in the end-to-end data integrity chain.

Purpose & Motivation

Acknowledged Mode was created to address the inherent unreliability of wireless communication channels. The physical layer alone, with its forward error correction (FEC), cannot guarantee 100% error-free packet delivery, especially at cell edges or under poor radio conditions. AM introduces a link-layer retransmission scheme to recover from these residual errors, solving the problem of packet loss for critical data.

Its motivation stems from the need to support a wide range of services with different reliability requirements. For applications like web browsing, file transfer, or signaling, packet loss leads to severe performance degradation at the transport layer (e.g., TCP timeouts). By providing reliable delivery at the RLC layer, AM improves overall network efficiency and user experience. It addresses the limitations of Unacknowledged Mode (UM), which offers no retransmissions and is unsuitable for error-sensitive traffic. The creation of AM was essential for making cellular data networks a viable platform for reliable IP-based services.