PDCP

PDCP is an abbreviation for Packet Data Convergence Protocol. It is one of the layers of the Radio Traffic Stack in LTE, UMTS and performs IP header compression and decompression, transfer of user data and maintenance of sequence numbers for Radio Bearers which are configured for lossless serving radio network subsystem (SRNS) relocation.

The compression technique can be based on either RFC 2507 or RFC 3095. RFC 1144 can also be used for some background information, and although the techniques in the RFC are not used in modern TCP/IP implementations, it still shows what the compression/decompression technique looks like.

If PDCP is configured for No Compression it will send the IP Packets without compression; otherwise it will compress the packets according to its configuration by upper layer and attach a PDCP header and send the packet. It uses the service provided by a lower layer called Radio Link Control (RLC) that uses the Radio Link Protocol.

PDCP header consists of two fields: PID and PDU TYPE.

PDU Type field indicates whether the PDU is Data PDU or Sequence Number PDU.
PID field value indicates header compression protocol type used and packet type or CID.

References

ETSI TS 125 323 V5.0.0; Universal Mobile Telecommunications System (UMTS); Packet Data Convergence Protocol (PDCP) specification (3GPP TS 25.323 version 5.0.0 Release 5); Mar. 2002; pp. 1–21; XP-002259036.
Wu, Chih-Hsiang, Method for determining triggering of a PDCP sequence number synchronization procedure, September 2007, US Patent 7266105, Innovative Sonic Limited

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