or compressed user-plane data. As we stated earlier, the header compression only applies to user-plane data. The MAC-I field carries a message authenti- cation code. For SRBs the MAC-I field is always present; however, if integrity protection is not configured, the MAC-I field is still present in PDCP PDU but is padded with zeros. New Radio Access Layer 2/3 Aspects and System Operation 233 For DRBs, the MAC-I field is present only when the DRB is configured with integrity protection, which is unique to NR. The D/C field indicates whether the corresponding PDCP PDU is a PDCP data PDU or a PDCP control PDU. The PDU Type identifies the type of control information included in the corresponding PDCP control PDU, which can be a status report, interspersed ROHC feedback, or reserved. The first missing COUNT indicates the COUNT value of the first missing PDCP SDU within the reordering window. The Bitmap field indicates which SDUs are missing and which SDUs have been correctly received in the receiving entity. The interspersed ROHC feedback has a variable length and contains one ROHC packet with only feedback, that is, a ROHC packet which is not associated with a PDCP SDU. When an interspersed ROHC feedback is generated by the header compression protocol, the transmitting PDCP sends the corresponding PDCP con- trol PDU to the lower layers without associating a PDCP SN or performing ciphering. The receiving PDCP entity delivers the corresponding interspersed ROHC feedback to the header compression protocol without performing deciphering [11]. 2.2.4 Service Data Adaptation Protocol Sublayer The main services and functions of SDAP sublayer include mapping between a QoS flow and a DRB and marking QFI in downlink and uplink IP packets. A single-protocolocol entity of SDAP is configured for each individual PDU session. The SDAP sublayer was introduced in NR because of the new QoS framework compared to LTE QoS management when con- nected to the 5G core. However, if the gNB is connected to the EPC, which is the case for non-stand