What is RLC Radio Link Control (protocol)

The technical details of the Radio Link Control (RLC) protocol.

1. Overview:
   • RLC is a layer-2 protocol within the Radio Access Network (RAN) of cellular networks, including UMTS, LTE, and 5G NR.
   • It is specified by 3GPP in different standards: TS 25.322 for UMTS, TS 36.322 for LTE, and TS 38.322 for 5G NR.
   • RLC operates between the MAC (Medium Access Control) layer and the PDCP (Packet Data Convergence Protocol) layer.
2. Functions and Features:
   • The main tasks of the RLC protocol include:
     • Segmentation and Reassembly: RLC divides data into smaller segments for efficient transmission and reassembles them at the receiver.
     • Flow Control: RLC manages the flow of data between the User Equipment (UE) and the base station (eNodeB).
     • Error Correction: It provides error detection and correction mechanisms to ensure reliable data transfer.
     • Acknowledgments (ACK/NACK): RLC uses acknowledgments to confirm successful reception of data segments.
     • Timer-Based Retransmissions: If a segment is not acknowledged within a specified time, RLC retransmits it.
     • Different Modes:
       • Transparent Mode ™: Used for circuit-switched services. No segmentation or reassembly.
       • Unacknowledged Mode (UM): Segments data but doesn’t wait for acknowledgments.
       • Acknowledged Mode (AM): Provides reliable, in-sequence delivery with acknowledgments.
3. Entities:
   • RLC architecture involves the following entities:
     • Transmitting TM RLC Entity: Segments and transmits data.
     • Receiving TM RLC Entity: Reassembles segments and delivers data to higher layers.
     • UM RLC Entity: Handles segmentation and reassembly for UM mode.
4. References:
   • For detailed specifications, refer to the 3GPP documents: TS 25.322, TS 36.322, and TS 38.322.