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RLC AM (Radio Link Control - Acknowledged Mode)

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RLC AM (Radio Link Control - Acknowledged Mode) is one of the three Transmission Modes used in the Radio Link Control (RLC) sublayer of the UMTS (Universal Mobile Telecommunications System) and LTE (Long-Term Evolution) wireless communication standards. RLC is responsible for ensuring the reliable transmission of data over the radio interface between the User Equipment (UE) and the network's base station (NodeB in UMTS or eNodeB in LTE). RLC AM is characterized by its acknowledgment-based, reliable data transmission mechanism. Here's a detailed technical explanation of RLC AM:

1. Reliable Data Transfer:

  • Reliability: RLC AM ensures reliable data transmission by using acknowledgments to confirm the receipt of data packets.
  • Acknowledgment: Each data packet sent by the transmitter is acknowledged by the receiver.

2. Sequence Numbering:

  • PDU (Protocol Data Unit) Segmentation: RLC AM divides higher-layer data into smaller PDUs for transmission.
  • Sequence Numbers: Each PDU is assigned a unique sequence number by the transmitter.
  • Reordering: The receiver uses sequence numbers to reorder out-of-sequence PDUs.

3. Segmentation and Reassembly:

  • PDUs: RLC AM can segment higher-layer PDUs into smaller PDUs for transmission over the radio interface.
  • Reassembly: The receiver reassembles the PDUs in the correct order before delivering them to higher-layer protocols.

4. Acknowledgment Mechanism:

  • Positive Acknowledgment (ACK): The receiver sends ACKs to confirm the successful reception of PDUs.
  • Negative Acknowledgment (NACK): If a PDU is missing or arrives out of order, the receiver sends NACKs to request retransmissions.
  • Cumulative Acknowledgment: ACKs and NACKs can be cumulative, acknowledging multiple PDUs at once.
  • Selective Acknowledgment (SACK): In some implementations, selective acknowledgment is supported, allowing the receiver to specify which PDUs are missing or out of order.

5. Retransmissions:

  • Retransmission Requests: When the receiver sends a NACK, the transmitter retransmits the requested PDUs.
  • Timers: RLC AM uses timers to manage retransmission requests and prevent excessive delays.

6. Flow Control:

  • Window-Based Flow Control: RLC AM uses a sliding window mechanism to control the number of unacknowledged PDUs that can be in transit at any time.
  • Window Size: The receiver advertises its receive window size to the transmitter, indicating how many unacknowledged PDUs it can buffer.

7. Acknowledgment Mode Transition:

  • Status Reporting: RLC AM can transition to Unacknowledged Mode (RLC UM) for more efficient data transfer when the network conditions are favorable.
  • Recovery Mode: If severe radio link degradation is detected, RLC AM can enter Recovery Mode, which uses additional mechanisms for error recovery.

8. Error Detection and Correction:

  • Header Checksum: RLC AM includes a header checksum to detect errors in the RLC header.
  • Higher-Layer Support: RLC AM relies on higher-layer protocols (e.g., TCP) for end-to-end error detection and correction.

9. Segmentation Timer:

  • Segmentation Timer: RLC AM uses a segmentation timer to trigger the transmission of smaller PDUs when data needs to be sent but the transmitter's buffer is not full.

10. Flow Control Timer:

  • Flow Control Timer: RLC AM may use a flow control timer to pause or resume data transmission based on the status of the receiver's buffer.

RLC AM ensures the reliable transfer of data over the radio link by using acknowledgments, sequence numbering, and error detection mechanisms. It is suitable for applications that require guaranteed data delivery and are tolerant of the added signaling overhead associated with acknowledgment-based communication.