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AM-RLC (Acknowledged Mode Radio link Control)

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AM-RLC (Acknowledged Mode Radio link Control) is a protocol used in wireless communication to ensure reliable delivery of data packets. It is a part of the Radio Link Control (RLC) layer in the LTE (Long-Term Evolution) protocol stack.

The RLC layer is responsible for segmenting the data received from the upper layer into smaller blocks and then transmitting them over the air interface. The RLC layer can operate in three different modes: Transparent Mode, Unacknowledged Mode, and Acknowledged Mode. In this article, we will focus on the Acknowledged Mode.

In Acknowledged Mode, the RLC layer ensures that all the data packets are transmitted successfully by the receiver. If a packet is not received successfully, the receiver sends a negative acknowledgment (NACK) message to the transmitter requesting a retransmission of the lost packet.

AM-RLC adds reliability to the transmission by providing a mechanism for error recovery. The receiver acknowledges the successful receipt of every data packet by sending an acknowledgment (ACK) message to the transmitter. If the transmitter does not receive an ACK message within a specified time interval, it assumes that the packet has been lost and retransmits the packet.

The AM-RLC protocol provides several features to ensure reliable data transmission. These features include:

  1. Segmentation: The RLC layer segments the data into smaller blocks called Protocol Data Units (PDUs) before transmitting them over the air interface. The size of the PDU depends on the Maximum RLC SDU (Service Data Unit) size configured for the RLC layer.
  2. Sequence numbering: Each PDU is assigned a sequence number by the transmitter. The receiver uses these sequence numbers to detect missing packets.
  3. Retransmission: If a PDU is not acknowledged by the receiver within a specified time interval, the transmitter retransmits the PDU.
  4. Duplicate detection: The receiver detects and discards duplicate packets based on the sequence numbers.
  5. Flow control: The AM-RLC protocol provides flow control mechanisms to avoid overwhelming the receiver with too many packets. The transmitter can limit the number of PDUs transmitted without receiving an ACK message.
  6. Status reporting: The receiver sends status reports to the transmitter periodically to indicate the status of the received PDUs. These reports help the transmitter to adjust the transmission rate and avoid congestion.

The AM-RLC protocol can operate in two different modes: the Transparent mode and the RLC mode.

In Transparent mode, the RLC layer does not add any reliability to the transmission. It simply passes the data received from the upper layer to the lower layer without any segmentation or error recovery.

In RLC mode, the RLC layer provides reliability by adding segmentation, sequence numbering, and error recovery mechanisms. The RLC mode can operate in three different sub-modes: Unacknowledged Mode (UM), Acknowledged Mode (AM), and Transparent Mode (TM).

In UM, the RLC layer does not provide any feedback to the transmitter about the status of the transmitted packets. If a packet is lost, the transmitter does not receive any feedback and assumes that the packet has been successfully transmitted.

In AM, the RLC layer provides feedback to the transmitter about the status of the transmitted packets. If a packet is lost, the receiver sends a NACK message to the transmitter requesting retransmission of the lost packet.

In TM, the RLC layer does not add any reliability to the transmission. It simply passes the data received from the upper layer to the lower layer without any segmentation or error recovery.

In conclusion, AM-RLC is a protocol used in wireless communication to ensure reliable delivery of data packets. It provides several features such as segmentation, sequence numbering, retransmission, duplicate detection, flow control, and status reporting to ensure reliable transmission over the air interface. AM-RLC operates in the RLC layer of the LTE protocol stack and can operate in two different modes: Transparent mode and RLC mode. In RLC mode, the AM sub-mode provides feedback to the transmitter about the status of the transmitted packets, making it a more reliable mode of operation.

AM-RLC is an essential component of LTE networks as it ensures that data is transmitted reliably and efficiently over the air interface. It is used in various applications such as voice, video, and data transmission, where reliability and low latency are critical.

The AM-RLC protocol is also used in other wireless communication standards such as 5G NR (New Radio), which is the next-generation wireless communication standard. In 5G NR, AM-RLC is used to ensure reliable delivery of data packets and provide low latency communication for various applications.

In summary, AM-RLC is a protocol used in wireless communication to ensure reliable delivery of data packets. It provides several features such as segmentation, sequence numbering, retransmission, duplicate detection, flow control, and status reporting to ensure reliable transmission over the air interface. It is an essential component of LTE and 5G NR networks, providing reliable and low-latency communication for various applications.