e to dedicated control data, and for traffic Internet Protocol (IP) packets. The PDCP provides the following functions: - compression of traffic data headers using the ROHC (Robust Header Compression) mechanism; - the security of RRC traffic and signaling data (integrity and confidentiality); - sequence numbering and sequential delivery of RRC messages and IP packets; - the recovery of PDCP frames lost during the handover; - reestablishment of PDCP frames in the event of a loss of the RLC frame in acknowledged mode; - duplication of PDCP PDUs; - detection of duplication; NG-RAN and 5G-NR - deletion of PDCP frames (discard); - routing of PDCP frames in the case of dual connectivity (split-bearer). Several PDCP instances can be activated simultaneously: - two instances for SRB1 (Signaling Radio Bearer) and SRB2: - the SRB1 is used for the transmission of an RRC message that can support a NAS (Non-Access Stratum) message, - SRB2 is used for transmission of a NAS message only; - one instance for each DRB (Data Radio Bearer) relating to traffic data. 8.2.1. Procedures 8.2.1.1. Header compression The header compression is based on the ROHC mechanism, for which multiple algorithms have been defined by the Request For Comment (RFC) specifications of the Internet Engineering Task Force (IETF) standards body (see Table 8.1). Profile identifier Compressed header References 0x0000 no compression RFC 4995 0x0001 RTP/UDP/IP RFC 3095, RFC 4815 0x0002 UDP/IP RFC 3095, RFC 4815 0x0003 ESP/IP RFC 3095, RFC 4815 0x0004 RFC 3843, RFC 4815 0x0006 TCP/IP RFC 4996 0x0101 RTP/UDP/IP RFC 5225 0x0102 UDP/IP RFC 5225 0x0103 ESP/IP RFC 5225 0x0104 RFC 5225 Table 8.1. Compression protocols 5G-NR Radio Interface - Data Link Layer The header compression is based on the observation that in a session, a number of fields are invariant, such as IP addresses or port numbers. The header compression is particularly effective when the size of the payload is relatively low, which is the case for voices (Figure 8.5). 12.2 kbps 12.2 kbps 244 bits every 2