ystem Overview Unlike LTE, PDCP is responsible for reordering and duplicate detection continuously rather than just during a handover. Duplicate detection and reordering based on PDCP SN is built into PDCP protocol itself at the receiving end and operates continuously without the need for additional configuration. Each DRB has exactly one PDCP entity and one or more RLC bearers. The interface between PDCP and RLC is well specified to allow the PDCP and RLC to be in different nodes such as in the case of dual connectivity or split CU-DU architecture. The RLC layer (specified in 3GPP TS 38.322) provides the required reliability for the data transmission. Similar to LTE, it supports three transmission modes - Transparent Mode (TM) used for data that are broadcast, Unacknowledged Mode (UM) used for services that can tolerate data loss such as voice, and Acknowledged Mode (AM) with retransmission mechanism (ARQ) for services such as TCP/IP and RRC signaling (Signaling Radio Bearers [SRBs]) that require reliable delivery of data. Both RLC UM and AM modes also support seg- mentation and resegmentation of data at the transmitter to fit into the transport block size and reassembly at the receiver. RLC status PDUs provide positive and/or negative acknowl- edgments of RLC SDUs (or portions of them) for ARQ functionality. RLC AM also provides duplicate detection. Figure 3.4.5 shows one of the RLC PDU structures. SI indicates the Segmentation Infor- mation on whether the data are segmented and whether it is the first, last, or an intermedi- ate segment. SN is the RLC Sequence Number, and the Segment Offset (SO) field indicates the position of the RLC SDU segment in bytes within the original RLC SDU. P is the polling bit, which can be used to trigger status reporting from the peer AM RLC entity. D/C indi- cates whether the PDU is a Data or Control PDU. Unlike LTE, RLC in NR does not do reordering of data as PDCP offers continuous (i.e. not just during handovers) reordering. RLC also does not perform concatenation of multiple R