Feedback COUNT IP packet BEARER IP packet "gNB-UP Encryption Decryption IP packet IP packet Adding PDCP header Removing PDCP header IP packet IP packet Figure 8.7. PDCP operations relating to the DRB bearer 8.2.3. Protocol structure The PDCP defines headers to encapsulate the RRC signaling data, the data traffic and the control messages associated with the traffic data. The structure of NR-PDCP frames is described in Figure 8.8. NR SDB PDCP NR DRB PDCP NR Control PDCP PDCP SN PDCP SN Type PDU PDCP SN (cont) PDCP SN (cont) FMC (Cont) FMC (Cont) MAC-I MAC-I (optionnel) FMC (Cont) MAC-I (cont) MAC-I (cont) (optionnel) Bitmap (optionnel) MAC-I (cont) MAC-I (cont) (optionnel) MAC-I (cont) MAC-I (cont) (optionnel) Bitmap (cont) (optionnel) Figure 8.8. NR-PDCP frame structure. For a color version of this figure, see www.iste.co.uk/launay/5g.zip 218 NG-RAN and 5G-NR PDCP SN: this field is coded on 12 bits for RRC signaling data and on 12 or 18 bits for traffic data. It indicates the PDCP frame sequence number. The sequence numbering allows recovery of the PDCP frames lost during the handover. MAC-I: this field is coded on 4 bytes. It contains the authentication code to control the integrity of the PDCP frame containing RRC signaling data and optionally the IP traffic. Data/Control (D/C): this bit indicates whether the frame contains traffic data (bit value ONE) or control messages specific to the PDCP (bit value ZERO). Packet Data Unit (PDU) type: this field is coded on 3 bits. It shows the control message type associated with traffic data: - the Status Report (value 000) is used in the reestablishment process after a cell change. It indicates a list of PDCP frame numbers not received during the handover; - the ROHC PDCP feedback (value 001) message is linked to the ROHC header compression mechanism. First Missing (FMS): this field is coded on 12 bits. It contains the first sequence number of the missing PDCP frames. Bitmap: this field is a collection of bits indicating whether the PDCP frame was received correctly (bit 