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PDCP Sequence Number

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The Packet Data Convergence Protocol (PDCP) Sequence Number is a crucial component in mobile communication networks, particularly in LTE (Long-Term Evolution) and 5G. PDCP is one of the protocol layers responsible for ensuring the reliable and efficient transmission of user data over the air interface. The PDCP Sequence Number plays a vital role in achieving this goal. Here's a detailed technical explanation:

1. PDCP Layer in the Protocol Stack:

  • PDCP is a protocol layer within the Radio Protocol Control Plane (RRC) of LTE and 5G networks. It sits above the Radio Link Control (RLC) layer and below the Radio Resource Control (RRC) layer.

2. Role of PDCP:

  • The PDCP layer is primarily responsible for data compression, header compression, encryption, integrity protection, and sequencing of user data packets.
  • It ensures the efficient transmission of data over the radio interface while maintaining data integrity and security.

3. PDCP Sequence Number:

  • The PDCP Sequence Number (SN) is a 12-bit field (in LTE) that is included in the PDCP header of each PDCP protocol data unit (PDU) or data packet.
  • In 5G NR, the PDCP Sequence Number can be either 11 bits or 18 bits, depending on the configuration.

4. Purpose of the Sequence Number:

  • The PDCP Sequence Number serves several critical purposes:
  • Sequencing: It allows the receiver to reassemble received PDUs in the correct order, ensuring the proper reconstruction of user data.
  • Duplicate Detection: The Sequence Number helps the receiver identify and discard duplicate PDUs, which may occur due to retransmissions or errors in the network.
  • Out-of-Order Detection: It assists in detecting and handling out-of-order PDUs, which may occur due to packet reordering in the network.

5. Sequence Number Management:

  • The Sequence Number is managed independently for each radio bearer (RB) established between the UE and the base station (eNodeB in LTE or gNodeB in 5G).
  • The sender increments the Sequence Number for each PDU it sends within a specific RB, ensuring that each PDU has a unique and monotonically increasing Sequence Number.
  • The receiver uses the Sequence Number to identify missing, duplicated, or out-of-order PDUs and processes them accordingly.

6. Handling of Missing or Out-of-Order PDUs:

  • If the receiver detects missing PDUs (gaps in the sequence numbers), it may request retransmissions from the sender or employ techniques like selective repeat to recover lost data.
  • In the case of out-of-order PDUs, the receiver buffers and reorders them before delivering them to the higher layers.

7. Detecting Duplicate PDUs:

  • To detect duplicate PDUs, the receiver compares the Sequence Number of incoming PDUs with the Sequence Numbers of previously received PDUs. Duplicate PDUs are discarded.

8. Security Considerations:

  • The PDCP Sequence Number also plays a role in the security of the data transmission. It ensures that replayed or reordered packets do not disrupt the integrity of the data transfer.

9. Sequence Number Wrapping:

  • The Sequence Number is a finite field, which means it will eventually wrap around after reaching its maximum value.
  • Wrapping of the Sequence Number is handled by the protocol, and the receiver can still correctly identify out-of-order or duplicate PDUs even when the Sequence Number wraps.

In summary, the PDCP Sequence Number is a critical element within the PDCP layer of LTE and 5G networks. It is used for sequencing, detecting duplicate and out-of-order packets, ensuring data integrity, and maintaining the reliability and order of data packets over the wireless radio interface. This contributes to the overall efficiency and robustness of data transmission in mobile communication networks.