What is UMD (Unacknowledged Mode Data)

UMD (Unacknowledged Mode Data) in 5G NR

UMD, standing for Unacknowledged Mode Data, is a specific data transfer mode within the Radio Link Control (RLC) protocol employed 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 delivery over the air interface. Let's delve into the technical details of UMD in 5G NR RLC:

RLC Modes in 5G NR:

• 5G NR RLC offers three operational modes, each tailored for specific data transmission needs:
  • Unacknowledged Mode (UMD): Explained in detail below.
  • Acknowledged Mode (AM): Provides reliable data transfer with error detection and retransmission mechanisms.
  • Transparent Mode (TM): Offers a basic service with minimal RLC overhead for specific scenarios.

Function of UMD:

UMD prioritizes throughput and minimizes latency by sacrificing guaranteed delivery. It's suitable for applications that can tolerate some data loss, such as:

• Real-time streaming services (e.g., low-latency video)
• Sensor data transmissions with relaxed timing constraints
• Background file downloads

Key Characteristics of UMD:

• No Acknowledgements (ACKs/NACKs): Unlike AM mode, UMD doesn't involve sending acknowledgement (ACK) or negative acknowledgement (NACK) signals after receiving data packets from the User Equipment (UE). This eliminates the overhead associated with retransmission requests and simplifies the protocol.
• Error Detection: While UMD forgoes retransmissions, it can still detect errors in received data using techniques like Cyclic Redundancy Check (CRC). However, the responsibility for error correction lies with higher layer protocols or applications.
• Buffering: Both the UE and the base station employ buffers to temporarily store data packets before transmission. This buffering helps smooth out bursty traffic patterns and improve efficiency.
• Segmentation/Reassembly: UMD performs segmentation and reassembly of data received from the PDCP layer. Large data Protocol Data Units (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.

Benefits of UMD:

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

Drawbacks of UMD:

• Unreliable Delivery: Since UMD 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 (UMD, AM, or TM) depends on the specific application requirements:

• For applications demanding high throughput and low latency with some tolerable data loss, UMD 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.

Comparison of UMD with AM:

Conclusion:

UMD plays a vital role in 5G NR by offering a high-performance option for data transfer in applications that prioritize speed over absolute reliability. Understanding the characteristics and trade-offs of UMD is essential for optimizing RLC operation and network performance for diverse 5G NR services.