The transmission protocol of LTE is divided into several layers.

At the top, the Packet Data Convergence Protocol (PDCP) performs functions related to data integrity (like enciphering) and IP header compression.

The PDCP hands its packets, called Service Data Units (SDUs) to the Radio Link Control (RLC).

The RLC segments and/or concatenates the SDUs into packets that are more suitable for transmission over the radio channel, the Protocol Data Units (PDUs).

Due to the large dynamic range of the transmission data rates, the size of a PDU can be adjusted dynamically, but in any case, a PDU can contain bits from one or more SDUs; conversely one SDU might be segmented, and its segments transmitted in multiple PDUs.

The RLC also makes sure that at the receiver (RX), all PDUs arrive (and arrange for retransmission if they do not) and hand them to the PDCP in their correct order.

The Medium Access Control (MAC) handles the scheduling of the PDUs, as well as the Hybrid Automatic Repeat reQuest (HARQ) for retransmissions on the PHY.2.

Finally, the PHY handles all the processes of actually transmitting data over the air, including coding and modulation.

Note that the PHY not only interfaces with the MAC layer (layer 2), but also the Radio Resource Control (RRC) of layer 3.MAC layer and PHY.