10.2 MAC/RLC Overview 363 10.2 MAC/RLC Overview As there is close interaction between MAC and RLC sublayers [5, 6], we discuss them together in this section. The RLC layer performs segmentation and/or concatenation on PDCP PDUs based on the size indicated by the MAC. RLC also reorders the RLC PDUs once they are received out of order possibly due to H-ARQ processes in the MAC layer. The RLC layer also supports an ARQ mechanism, which resides on top of the MAC layer H-ARQ and is used only when all the H-ARQ transmissions are exhausted and the RLC PDU has not yet been received without errors. As mentioned previously, at the transmitter and the receiver there is one RLC entity per radio bearer. The MAC layer performs multiplexing and demultiplexing of the various logical channels on to the transport channels. At the eNode-B, this includes multiplexing and prioritizing various UEs that are being served by the eNode-B as well as multiplexing and prioritizing the various radio bearers within a given UE. At the UE, the MAC layer only performs the task of multiplexing and prioritizing the various radio bearers associated with the UE. The MAC layer provides services to the RLC layer through logical channels, while it accesses the data transfer services provided by the PHY layer through transport channels. 10.2.1 Data Transfer Modes Functions of the RLC layer are performed by RLC entities. Each RLC entity can be operated in three different modes: the Transparent Mode (TM), the Unacknowledged