ts the loss of UMD PDUs at lower layers and avoids excessive reordering delays, it reassembles RLC SDUs from the reordered UMD PDUs and delivers the RLC SDUs to the upper layer in ascending order of RLC SN, and it discards received UMD PDUs that cannot be reassembled into a RLC SDU due to loss of a UMD PDU which belonged to the particular RLC SDU at lower layers. At the time of RLC re-establishment, the receiving UM RLC entity reassembles RLC SDUs from the UMD PDUs that are received out of sequence and deliver them to the upper layer, discards any remaining UMD PDUs that could not be reassembled into RLC SDUs, initializes relevant state variables, and stops relevant timers. As shown in Figure 7-24, an AM RLC entity can be configured to deliver RLC PDUs through DL/ UL DCCH or DL/UL DTCH logical channels. When the transmitting side of an AM RLC entity constructs Acknowledged Mode Data (AMD) PDUs from RLC SDUs, it segments and/or concatenates the RLC SDUs SO that the AMD PDUs fit within the total size of the RLC PDU at a particular transmission opportunity. The transmitting side of an AM RLC entity supports re-transmission of RLC data PDUs (i.e., ARQ), if the RLC data PDU to be re-transmitted does not fit within the total size of RLC PDU at a particular transmission opportunity, the AM RLC entity can re-segment the RLC data PDU into AMD PDU segments where the number of re-segmentations is unlimited. E/eNB eNB/UE Radio UE/eNB eNB/UE Radio Interface Interface TM-SAP TM-SAP UM-SAP UM-SAP Transmitting Receiving TM-RLC TM-RLC Entity Entity Transmission Transmission Buffer Reassembly Buffer Transmitting Receiving UM-RLC UM-RLC Entity Entity Segmentation/ Remove RLC Concatenation Header BCCH/PCCH/CCCH BCCH/PCCH/CCCH Add RLC Reception Buffer/HARQ Header Reordering FIGURE 7-23 An illustration of 3GPP LTE TM and UM RLC entities [11] 7.4 3GPP LTE radio link control and medium access control sub-layers AM-SAP Transmission RLC Control SDU Reassembly Buffer Remove RLC Segmentation/ Retransmission Header Concatenation Buffer Recep