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Layer 2: Radio Link Control – RLC

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Radio Link Control (RLC) is a layer-2 protocol within the Radio Access Network (RAN) of a cellular network, especially within the Universal Mobile Telecommunications System (UMTS) and Evolved UMTS Terrestrial Radio Access Network (E-UTRAN or LTE). Its primary function is to ensure that data is reliably and efficiently transferred between the mobile device and the base station (Node B for UMTS, eNodeB for LTE).

Let's dive into the technical details of RLC:

1. Functionality of RLC:

  • Segmentation and Reassembly: RLC breaks down the higher-layer PDUs (Protocol Data Units) into smaller RLC SDUs (Service Data Units) for transmission over the radio interface. At the receiving end, it reassembles these smaller units back into complete PDUs.
  • Concatenation and De-concatenation: Apart from segmentation and reassembly, RLC can also concatenate smaller PDUs into larger RLC SDUs for transmission efficiency, and the reverse process is called de-concatenation.
  • Error Correction: RLC provides error correction through techniques like Automatic Repeat reQuest (ARQ). If an RLC PDU is found to have errors at the receiving end, it can request the sending end to retransmit that specific PDU.
  • Flow Control: RLC ensures that data flows at an appropriate rate, preventing congestion and ensuring efficient utilization of the radio resources. It uses various mechanisms like windowing to manage the flow.

2. Modes of RLC Operation:

Depending on the specific requirements of the application and the network configuration, RLC can operate in different modes:

  • Transparent Mode (TM): In this mode, RLC simply transmits the data units as they are without adding any specific headers or performing segmentation and reassembly.
  • Unacknowledged Mode (UM): UM is a mode where the RLC layer doesn't require acknowledgment for the data units it sends. It's faster but might not guarantee delivery.
  • Acknowledged Mode (AM): This is the most reliable mode where every RLC PDU is acknowledged by the receiving entity. If a PDU is not correctly received, it will be retransmitted.

3. PDU Formats:

  • AM Mode PDUs: In Acknowledged Mode, the RLC PDU contains several fields like the Sequence Number (to order PDUs), Status (to convey acknowledgment or other control info), and the actual data or RLC SDU.
  • UM and TM PDUs: For Transparent and Unacknowledged Modes, the PDU might be simpler, primarily consisting of the RLC SDU or its fragments.

4. Radio Bearer Types:

In the context of LTE, different types of RLC bearers exist:

  • AM Bearer: Used for the data that requires reliable transfer.
  • UM Bearer: Used for data that can tolerate some loss, usually due to its real-time nature or redundancy at higher layers.

5. QoS (Quality of Service):

RLC also plays a crucial role in maintaining the Quality of Service. By managing segmentation, retransmissions, and flow control, RLC ensures that the QoS parameters like latency, throughput, and reliability are met.

Conclusion:

The Radio Link Control (RLC) layer serves as a critical link in ensuring the reliable and efficient transmission of data over the radio interface. By handling segmentation, reassembly, error correction, and flow control, RLC ensures that the data exchange between the mobile device and the base station is robust, efficient, and meets the desired quality parameters.