What is UM (Unacknowledged mode (RLC configuration))

UM (Unacknowledged Mode) in 5G NR RLC

Unacknowledged Mode (UM) is one of the three Radio Link Control (RLC) configurations defined in 5G New Radio (NR) technology. RLC acts as a crucial layer between the Packet Data Convergence Protocol (PDCP) and the Medium Access Control (MAC) layer, responsible for reliable data transfer over the air interface. Here's a detailed explanation of UM mode and its characteristics in 5G NR:

RLC Modes in 5G NR:

• 5G NR RLC offers three operational modes, each catering to different data transmission requirements:
  1. Unacknowledged Mode (UM): Explained in detail below.
  2. Acknowledged Mode (AM): Provides reliable data transfer with error detection and retransmission mechanisms.
  3. Transparent Mode (TM): Offers a basic service with minimal RLC overhead, suited for specific scenarios.

Function of UM Mode:

• UM mode is designed for data transmissions where reliable delivery is not critical, and maximizing throughput is the primary objective. This mode is often used for applications that can tolerate some data loss, such as:
  • Real-time streaming services (e.g., low-latency video)
  • Sensor data transmissions
  • Background file downloads

Key Characteristics of UM Mode:

• No Acknowledgements (ACKs/NACKs): Unlike AM mode, the network does not send any acknowledgement (ACK) or negative acknowledgement (NACK) signals to the UE after receiving UM data PDUs (Protocol Data Units). This eliminates the overhead associated with retransmission requests and simplifies the protocol.
• Error Detection: While UM mode doesn't involve retransmissions, it can still detect errors in the received data using techniques like Cyclic Redundancy Check (CRC). However, the error correction is left to higher layer protocols or applications.
• Buffering: Both the UE and the base station employ buffers to temporarily store data packets before transmission on the air interface. This buffering helps to smooth out bursty traffic patterns and improve efficiency.
• Segmentation/Reassembly: UM mode performs segmentation and reassembly of data received from the PDCP layer. Large data PDUs are fragmented into smaller units suitable for transmission over the radio channel. Upon reception at the network, these fragments are reassembled into the original PDU.

Advantages of UM Mode:

• Low Latency: The absence of acknowledgement procedures significantly reduces latency compared to AM mode. This is beneficial for real-time applications where delays are critical.
• High Throughput: By eliminating the overhead of ACK/NACK and retransmissions, UM mode achieves higher throughput compared to AM mode, maximizing data transfer rate.
• Simplified Protocol: The lack of acknowledgement mechanisms makes UM mode simpler to implement compared to AM mode, reducing processing overhead.

Disadvantages of UM Mode:

• Unreliable Delivery: Since UM mode doesn't involve retransmissions, there's no guarantee that all transmitted data will be received correctly by the network. This might be unsuitable for applications requiring high data integrity.
• Error Handling at Higher Layers: The responsibility for handling errors falls upon higher layer protocols or applications, which might require additional processing and error correction mechanisms.

Choosing the Right RLC Mode:

The selection of the appropriate RLC mode (UM, AM, or TM) depends on the specific application requirements:

• For applications demanding high throughput and low latency with some tolerable data loss, UM mode is a suitable choice.
• For applications requiring guaranteed delivery and high data integrity, AM mode is preferred.
• TM mode finds use in specific scenarios where minimal RLC overhead is essential.

Conclusion:

Unacknowledged Mode (UM) plays a crucial role in 5G NR RLC by offering a high-throughput, low-latency data transfer option. It caters to applications where maximizing speed is more critical than absolute reliability. Understanding the characteristics and trade-offs of UM mode is essential for optimizing RLC operation and network performance for diverse 5G NR services.