 by ARQ and relies on status reports to trigger retransmissions. RLC offers three modes of operations: AM, unacknowledged mode (UM), and transparent mode (TM). The RLC sublayer of 5G differs from the RLC sublayer of 4G on a number of points: no concatenation, no reordering, no sequence numbering, and offset-based segmentation always. Concatenation is a real-time operation which can only be performed once the grant size is known. Removing concatenation from RLC and relying on the lower layers to concatenate PDUs allows the processing of RLC PDUs to become a non-real-time operation. Without a reordering function for RLC PDUs (see Section 7.4.2), the need for SNs in UM mode becomes limited to the reordering of segments of a single PDU, thus reducing overhead. This is especially useful for services with a stable packet size for which the grant can be tailored to accommodate at least one packet always, for instance, a voice service. Finally, using an offset for segmentation always simplifies RLC operations (see Section 7.5.2). The configuration of the RLC sublayer to handle one radio bearer from PDCP is called an RLC entity. When duplication is not configured, there is one RLC entity per radio bearer. When it is configured, there are two. 7.5.2 Segmentation Segmentation occurs when an RLC SDU is too large to fit into the payload allocated for transmission by the lower layers and consists in cutting it into segments in the transmit- ter and putting these segments back together in the receiver through a process called SDU reassembly. In LTE, an offset was only used for re-segmentation, when an already-transmitted PDU would need to be segmented to fit into the payload available for a retransmission. Although it decreases the overhead, it complicates RLC operations by mixing different kinds of PDUs. Thus, for 5G, it was decided to rely on an offset always, but only transmit it from the second segment to help UE processing. Let us explain why. First, the RLC PDU header is dynamic, and the offset is only transmitted when re