What is RLC (Radio Link Control)

The technical details of Radio Link Control (RLC):

Role in Protocol Stack:
- RLC operates at Layer 2 (data link layer) within the Universal Mobile Telecommunications System (UMTS), Long-Term Evolution (LTE), and 5G New Radio (NR) wireless communication standards.
- It sits above the Physical layer and below the Radio Resource Control (RRC) layer in the protocol stacks.
Functions of RLC:
- Segmentation and Reassembly:
  - RLC divides data received from higher layers into smaller units called Protocol Data Units (PDUs) during transmission and reassembles them at the receiving end.
- Error Correction:
  - RLC provides error correction mechanisms to ensure reliable data transmission.
  - It adds sequence numbers to PDUs, allowing the receiver to detect and correct errors.
RLC Modes:
- RLC operates in three modes:
  - Transparent Mode: Used when no error correction or retransmission is required.
  - Unacknowledged Mode (UM): Data transmitted without requiring acknowledgment from the receiver.
  - Acknowledged Mode (AM): Data transmitted with acknowledgment; sender retransmits lost or corrupted packets.
PDU Formats:
- RLC PDUs vary based on the mode:
  - For AM, PDUs include a header with sequence numbers, status information, and the actual data.
  - For UM, PDUs contain only the data without sequence numbers or acknowledgment information.
Flow Control:
- RLC implements flow control mechanisms to prevent sender overload.
- Techniques like window-based flow control manage the rate of data transmission.
Status Reporting:
- In Acknowledged Mode (AM), the receiver sends status reports to the sender, indicating received packets and those needing retransmission.
Timer-Based Retransmission:
- RLC uses timers in AM.
- If acknowledgment isn’t received within a specified time, the sender retransmits the data.
Priority Handling:
- RLC supports priority handling for different data types, allowing differentiated quality of service.