Unacknowledged Mode

Description

Unacknowledged Mode (UM) is one of the three operational modes of the Radio Link Control (RLC) layer in 3GPP wireless systems, alongside Acknowledged Mode (AM) and Transparent Mode (TM). The RLC layer resides in both the User Equipment (UE) and the radio access network (RAN) node (e.g., NodeB, eNodeB, gNB) and is responsible for the transfer of upper layer protocol data units (PDUs). In UM, the primary function is the unacknowledged transfer of data. This means the transmitting RLC entity sends RLC PDUs without expecting a positive or negative acknowledgment from the receiver. Consequently, there is no automatic repeat request (ARQ) mechanism for error correction at the RLC level in this mode. The protocol handles segmentation and concatenation of RLC service data units (SDUs) from the upper layers into RLC PDUs for transmission, and performs reassembly of these PDUs back into SDUs at the receiver.

Architecturally, an RLC entity configured in UM has a transmitting side and a receiving side. The transmitting side includes a transmission buffer and functionality for segmentation/concatenation and PDU construction. A key component is the sequence numbering. Each RLC PDU is assigned a sequence number within a finite range, which is included in the PDU header. This sequence number is essential for the receiver's operation. The receiving side utilizes a reassembly buffer. It uses the sequence numbers to detect any missing PDUs (due to errors or loss over the air interface) and to ensure in-sequence delivery of RLC SDUs to the upper layer. Since delivery is not guaranteed, detected missing PDUs are not requested for retransmission; instead, the RLC may deliver a partially reassembled SDU or indicate an error, depending on configuration and the nature of the upper layer service.

The role of UM in the network is to support services that are sensitive to delay and jitter but can tolerate a certain level of packet loss. The absence of ARQ retransmissions avoids the introduction of variable and potentially large delays that come with waiting for acknowledgments and performing retransmissions. This makes UM ideal for real-time services carried over packet-switched networks, such as Voice over IP (VoIP) using the Internet Protocol (IP) Multimedia Subsystem (IMS), or streaming video. The in-sequence delivery feature is still important to maintain the order of packets, as out-of-order delivery could severely degrade the quality of these real-time applications. The configuration of UM (e.g., sequence number field size) is determined by the Radio Resource Control (RRC) layer based on the requirements of the established radio bearer.

Purpose & Motivation

Unacknowledged Mode was created to efficiently support real-time, streaming, and broadcast/multicast services over 3GPP packet-switched networks. Prior to widespread packet data, circuit-switched voice had guaranteed, constant bitrate channels. With the move to IP-based services, a mechanism was needed to carry delay-sensitive IP packets over the unreliable wireless link without introducing the latency overhead of retransmission protocols. Pure, connectionless IP delivery could result in out-of-order packets causing poor application performance. UM solves this by providing a lightweight, connection-oriented link layer service that adds sequence numbering for in-order delivery but deliberately omits acknowledgment and retransmission to keep latency minimal.

The problem it addresses is the fundamental trade-off between reliability and latency. Acknowledged Mode (AM) with ARQ provides high reliability but variable, potentially high latency, which is suitable for data traffic like web browsing or file transfer. For real-time conversational or streaming traffic, this latency is unacceptable. Transparent Mode (TM) offers even lower processing overhead but does not provide segmentation or in-sequence delivery, limiting its use. UM occupies the critical middle ground, adding necessary sequencing and reassembly functions for packet-based real-time services while avoiding the delay penalty of reliability mechanisms. Its creation was motivated by the need to define quality of service (QoS) differentiation at the RLC layer, enabling the network to optimize transport for different traffic types, a cornerstone of the Universal Mobile Telecommunications System (UMTS) and subsequent 4G and 5G architectures.