LC entity UM RLC entity AM RLC entity logical channel lower layers (i.e. MAC sublayer and physical layer) radio interface lower layers (i.e. MAC sublayer and physical layer) logical channel receiving transmitting receiving transmitting TM RLC entity UM RLC entity UM RLC entity AM RLC entity TM RLC entity SAP between upper layers upper layer (i.e. RRC layer or PDCP sublayer) Figure 3.17 Overview model of the RLC sublayer. Reproduced with permission from C 2009 3GPP [17] Next Generation Mobile Communications Ecosystem 3.6.2.2 Services and Functions The following services are provided by RLC to upper layer (i.e., RRC or PDCP): TM data transfer. UM data transfer. AM data transfer, including indication of successful delivery of upper layers PDUs. The following services are expected by RLC from lower layer (i.e., MAC): Data transfer. Notification of a transmission opportunity, together with the total size of the RLC PDU(s) to be transmitted in the transmission opportunity. The main functions of the RLC sublayer include: Transfer of upper layer PDUs. Error Correction through ARQ (only for AM data transfer). Concatenation, segmentation and reassembly of RLC SDUs (only for UM and AM data transfer). Re-segmentation of RLC data PDUs (only for AM data transfer). In sequence delivery of upper layer PDUs (only for UM and AM data transfer). Duplicate detection (only for UM and AM data transfer). Protocol error detection and recovery. RLC SDU discard (only for UM and AM data transfer). RLC re-establishment. 3.6.2.3 PDU Structure Figure 3.18 depicts the RLC PDU structure. The PDU sequence number carried by the RLC header is independent of the SDU sequence number (i.e., PDCP sequence number): The dotted lines indicates the occurrence of segmentation within [n] and [n+3]. Because segmentation only occurs when needed and concatenation is done in sequence, the content of an RLC PDU can generally be described by the following relations: {0; 1} last segment of SDUi + [0; n] complete SDUs + {0; 1} first segment of SDUi+n+1; or 1 segment