Radio Link Control Protocol

Description

The Radio Link Control Protocol (RLCP) is a fundamental Layer 2 (Data Link Layer) protocol within the 3GPP wireless access stratum protocol stack, residing between the Packet Data Convergence Protocol (PDCP) above and the Medium Access Control (MAC) protocol below. Its primary function is to ensure the reliable or timely transfer of data (both user plane and control plane) across the inherently unreliable radio link. To achieve this, RLCP operates in three distinct modes, each tailored for specific types of traffic and quality of service requirements. Transparent Mode (TM) is used for delay-critical control signaling like broadcast system information or paging, where data is passed through without adding a protocol header or performing retransmissions. Unacknowledged Mode (UM) is used for delay-sensitive user data such as VoIP or streaming, where it provides segmentation/reassembly and in-sequence delivery but no retransmissions, tolerating some packet loss to maintain low latency. Acknowledged Mode (AM) is used for error-intolerant data like file transfers or TCP-based traffic, where it provides full error correction through an Automatic Repeat Request (ARQ) mechanism, along with segmentation, in-sequence delivery, and duplicate detection.

Architecturally, an RLCP entity is configured per radio bearer. It receives Service Data Units (SDUs) from the PDCP layer. Its core operations include segmentation and concatenation, where it adapts the size of SDUs to fit the transport blocks allocated by the MAC layer. It then adds an RLCP header to create an RLCP Protocol Data Unit (PDU). This header contains crucial information like a Sequence Number (SN), which is used for sequencing and, in AM, for acknowledging received data. The RLCP PDU is then delivered to the MAC layer for transmission over a logical channel. On the receiver side, the peer RLCP entity uses the header information to reassemble SDUs, deliver them in sequence to PDCP, and in AM, generate status reports (ACK/NACK) that are sent back to the transmitter to request retransmission of any missing PDUs, thus implementing the ARQ function.

RLCP works in close coordination with the MAC and physical layers. While the physical layer uses Hybrid ARQ (HARQ) for fast, physical-layer retransmissions, RLCP's ARQ operates at a higher layer to correct any residual errors that HARQ cannot fix, providing a more robust end-to-end data integrity guarantee. This two-layer retransmission scheme is a key design principle. Furthermore, RLCP provides flow control between itself and PDCP and performs protocol error detection and recovery. Its configuration (mode, SN field size, timer values) is managed by the Radio Resource Control (RRC) protocol, allowing the network to optimize performance per service type.

Purpose & Motivation

The Radio Link Control Protocol was created to address the fundamental challenge of providing reliable data transfer over the unpredictable and error-prone wireless radio channel. Early cellular systems were primarily circuit-switched for voice, which has different tolerances for delay and error. With the advent of packet-switched data services in 3G UMTS (where RLCP was formally named and solidified), there was a critical need for a data link layer protocol that could support diverse applications—from real-time voice to web browsing—each with distinct requirements for reliability, delay, and throughput. RLCP solves this by offering multiple operational modes (TM, UM, AM), allowing the network to tailor the link layer service to the specific QoS profile of each data radio bearer.

Its creation was motivated by the limitations of relying solely on physical layer techniques or simpler link protocols. Physical layer coding and HARQ are fast but not infallible. RLCP's ARQ mechanism provides a higher-layer, more robust safety net for critical data. The protocol also solves the problem of mismatched packet sizes between upper-layer applications and the dynamically sized transport blocks of the physical layer through its segmentation and concatenation functions. Historically, RLCP evolved from the link layer protocols of 2G GSM (e.g., the Radio Link Protocol RLP in GPRS) but was significantly enhanced for the higher data rates and lower latencies required by 3G and subsequent generations. It provides a standardized, efficient, and flexible mechanism for error control, which is a cornerstone for enabling the reliable IP-based services that define modern cellular networks.

Key Features

• Three operational modes: Transparent Mode (TM), Unacknowledged Mode (UM), Acknowledged Mode (AM)
• Automatic Repeat Request (ARQ) for error correction in AM
• Segmentation, reassembly, and concatenation of data units
• In-sequence delivery of data to upper layers
• Duplicate detection and discard
• Flow control between RLC and PDCP layers

Evolution Across Releases

Defining Specifications