icator (SI) bits are used to indicate whether this PDU is a segment of an RLC SDU (or not), and if it's a segment, whether it's the start, middle or end of a packet. D/C indicator bits that are used to indicate whether this PDU contains RLC SDU data or is used (instead) to transfer RLC layer control signaling (such as acknowledgment information) The Polling (P) field is used to trigger transmission of a STATUS report from the RLC peer. Additional bits associated with RLC STATUS reports that are used to indicate correct (or incorrect) reception of RLC PDUs by the receiving peer entity, triggering retransmission if required. These fields are relevant for AM mode operation only. Control PDU Type (CPT) that indicates whether this is a STATUS PDU (or, alternatively, reserved type) Acknowledgment SN (ACK_SN) field indicates the SN of the next not received RLC SDU which is not reported as missing in the STATUS PDU. The original transmitter interprets that all RLC SDUs up to but not including the RLC SDU with SN = ACK_SN have been received by its peer AM RLC entity Extension 1 bit to indicate there is a NACK_SN and set of E2/E3 bits following, and Negative Acknowledgment (NACK_SN) field that indicates the SN of the RLC SDU (or RLC SDU segment) that has been detected as lost at the receiving side of the AM RLC entity. Extension 2 bit to indicate there is a n SO Offset set of fields following SO start and end (SOstart, SOend). The SOstart field (together with the SOend field) indicates the portion of the RLC SDU with SN = NACK_SN (the NACK_SN for which the SOstart is related to) that has been detected as lost at the receiving side of the AM RLC entity. Chapter 6 User-plane application components 221 8 bits FIGURE 6-6 RLC header example (acknowledged mode, 18b SN). Extension 3 bit to indicate there is a NACK range field following and the NACK range field itself. This NACK range field is the number of consecutively lost RLC SDUs starting from and including NACK_SN An example of an AM mode PDU with 18 bits SN is shown below i