What is PDCP-RLC Packet Data Convergence Protocol–Radio Link Control
PDCP (Packet Data Convergence Protocol) and RLC (Radio Link Control): A Technical Breakdown
PDCP and RLC are two critical protocols that work together within the Radio Link Control (RLC) layer of LTE (Long-Term Evolution) and 5G NR (New Radio) cellular networks. They play distinct but complementary roles in ensuring efficient and reliable data transmission over the air interface.
PDCP (Packet Data Convergence Protocol):
- Function: Prepares user plane data packets for transmission over the air interface.
- Key functionalities:
- Header Compression: Optimizes data transmission by compressing headers of user plane data packets (typically IP headers) using techniques like RFC 2507 or RFC 3095. This reduces the number of bits transmitted, improving network efficiency.
- Decapsulation (For Control Plane Data): Strips unnecessary headers from control plane PDUs (Protocol Data Units) received from upper layers before handing them over to the RLC layer.
- Ciphering: Encrypts user plane data packets to protect user data confidentiality during transmission.
- Integrity Protection: Adds a cryptographic checksum to data packets, allowing the receiving entity to verify data integrity and detect any potential tampering or errors during transmission.
- Packet Sequencing: Maintains the in-order delivery of data packets, ensuring they arrive at the receiver in the same order they were sent.
RLC (Radio Link Control):
- Function: Handles reliable data transfer over the air interface.
- Key functionalities:
- Segmentation: Segments large data packets into smaller units suitable for transmission over the radio link.
- Reassembly: Reassembles received data segments back into the original data packets at the receiver.
- Error Correction: Employs error correction mechanisms (e.g., Automatic Repeat Request - ARQ) to detect and correct errors that might occur during transmission.
- Flow Control: Regulates the data flow between the network and the User Equipment (UE) to prevent buffer overflow at the receiver.
- Delivery Guarantee: Offers different service options with varying levels of delivery guarantees (e.g., guaranteed delivery, non-guaranteed delivery) to cater to different types of traffic.
PDCP vs. RLC:
| Feature | PDCP | RLC |
|---|---|---|
| Function | Prepares data for transmission | Handles reliable data transfer over the air interface |
| Data Units | PDCP PDUs (processed data packets) | RLC SDUs (segmented data units) |
| Focus | User data preparation, security | Reliable data delivery, error correction |
| Key Operations | Header compression, ciphering, integrity protection, packet sequencing | Segmentation, reassembly, error correction, flow control, delivery guarantee |
Interaction between PDCP and RLC:
- Upper layers (e.g., IP) deliver data packets to PDCP.
- PDCP performs header compression (if configured), ciphering, and adds integrity protection to the data packets.
- PDCP encapsulates the processed data (PDCP PDU) and hands it over to the RLC layer.
- RLC segments the PDCP PDUs into smaller units suitable for transmission (RLC SDUs).
- RLC transmits the RLC SDUs over the air interface.
- At the receiving end, RLC reassembles the received SDUs back into the original PDCP PDUs.
- RLC delivers the PDCP PDUs to PDCP.
- PDCP performs decryption and integrity verification on the PDCP PDUs.
- PDCP decapsulates the PDUs (if control plane data) and delivers the user plane data packets to upper layers.
Conclusion:
PDCP and RLC work in tandem to ensure efficient and reliable data transmission in cellular networks. PDCP prepares user data for transmission, while RLC handles reliable data transfer over the air interface. By understanding their distinct functionalities and interactions, you gain a deeper insight into the data flow within the Radio Link Control layer of LTE and 5G NR networks.