1E2E3 NACK_SN Octet 7 NACK SN E1E2E3 Octet 7 NACK_SN Octet 8 SO Start Octet 8 NACK_SN Octet 9 SO Start Octet 9 NACK SN E1E2E3 Octet 10 SO End Octet 10 SO Start Octet 11 SO End Octet 11 SO Start Octet 12 NACK range Octet 12 SO End Octet 13 NACK SN Octet 13 SO End Octet 14 NACK SN E1E2E3 Octet 14 NACK range Octet 15 NACK_SN Octet 16 NACK SN Octet 17 NACK SN E1E2E3 Figure 8.16. RLC protocol control message ACK_SN (Acknowledgement SN): this field is coded on 10 bits. It indicates the number of the next expected frame. All transmitted frames, whose number is less than this value, are validated, with the exception of frames whose number is in the NACK_SN field. CPT (Control PDU Type): this field is coded on 3 bits. It indicates the control message type. The only defined message concerns the report relating to the acknowledgement of received RLC frames. E1 (Extension 1): this bit indicates the presence (bit value ONE) or not (bit value ZERO) of NACK_SN, E1 and E2 fields following one another. E2 (Extension 2): this bit indicates the presence (bit value ONE) or not (bit value ZERO) of SO Start and SO End fields following one another. E3 (Extension 3): this bit indicates the presence (bit at ONE) or not (bit at ZERO) of a PDCP sequence following the one received. NACK_range: this field contains the number of consecutive PDCP frames that have been lost. NACK_SN (Negative Acknowledgement SN): this field is coded on 10 bits. It contains the number of the lost frame. NG-RAN and 5G-NR SO End: this field is coded on 15 bits. It indicates the last byte number of the lost segment. SO Start: this field is coded on 15 bits. It indicates the first byte number of the lost segment. 8.4. MAC protocol The Medium Access Control (MAC) protocol provides the following functions: - multiplexing Radio Link Control (RLC) frames from multiple instances in a transport block; - allocation of the resources via a scheduling mechanism for both transmission directions; - management of retransmission in the case of error via the Hybrid Automatic Repea