First, you need to have understanding of LTE Air Interface protocols. Once you have that understanding, it is easy to trace an IP packet from user plane perspective, passing through each of the protocols all the way down to physical layer to understand the role of each protocol layer.
For your convenience, I have listed the the user plane protocol stack on the air interface here below.
FUNCTIONALITY OF EACH LAYER IN THE PROTOCOL STACKA picture is worth thousand words. You can read the description of the protocol stacks. Before you do so, take a look below. The user plane data flow for an IP packet transmitted in downlink is illustrated in the figure below.
I have listed the explanation for each protocol below.
Can you see the above picture? The headers are added at each of the protocol layers to the data packet, as shown vividly in the figure above.
Robust Header compression (RoHC) is (optionally) performed in the PDCP layer. In addition, the PDCP Service Data Unit (SDU) is ciphered. The PDCP header carries the required information for RoHC decompression and deciphering.
Each PDCP PDU corresponds to an RLC SDU. RLC performs segmentation and concatenation of those SDUs and adds an RLC header. The RLC PDUs form MAC SDUs.
MAC SDUs from several radio bearers may be multiplexed in MAC. Depending on the amount of scheduled resources, more or less bits are selected for each transport block. The scheduling decision affects the concatenation and segmentation in RLC. Similarly, scheduling decisions affect the multiplexing in MAC.
In order, delivery is proposed to be performed per radio bearer (logical channel) by means of RLC sequence numbers. In contrast to current MAC headers, MAC in LTE supports multiplexing of MAC SDUs from different radio bearers into the same MAC PDU. Therefore, a MAC PDU includes an RLC sequence number per RLC PDU, but no sequence number per MAC PDU. It is assumed that the RLC retransmission unit is an RLC PDU or an RLC PDU segment while the HARQ retransmission unit is the MAC PDU.
Finally, MAC delivers the transport block to the physical layer, where a Cyclic Redundancy Check (CRC) is added.
PDU = Protocol Data Unit
SDU = Service Data Unit