The RLC layer is located between the PDCP ( the “top” layer) layer and MAC layer (the “lower” layer). Communicates with the PDCP layer through a service access point ( SAP ) , and the MAC layer via logical channels. The RLC PDU to PDCP layer reformatted in order to fit the size indicated by the MAC layer , that is, the segment of the RLC of the transmitter and / or concatenates the PDCP PDUs , RLC and the receiver reassembles the RLC PDUs to reconstruct PDCP PDUs .
Also, the RLC RLC PDU reordering if received out of sequence due to the HARQ operation is performed in the MAC layer . This is the fundamental difference of UMTS, wherein the HARQ reordering is done in the MAC layer. The advantage of HARQ reordering RLC SN is that additional and receive buffer for HARQ reordering is needed. In LTE, the RLC SN and RLC reception buffer used for both ARQ HARQ reordering and related operations RLC- Leve .
The functions of the RLC layer are performed by ” RLC entities An RLC entity is configured in one of three modes of data transmission . (TM ) Transparent mode, mode unacknowledged (UM ) , and the mode ( AM ) Acknowledged . in AM , special functions are defined to support the broadcasting . When the UM or AM are used , the choice between the two modes is made by the eNodeB during the radio bearer RRC configuration procedure on the basis of the requirements EPS QoS of the bearer .