In order to answer this question, take a look at the picture below. I have shown RLC protocol both in Control and User plane protocol stack.
Modes of RLC Protocol
The RLC protocol supports three modes of operation
Whether UM or AM is used, it is configured per radio bearer.
For example: UM could be used for VoIP or real time services, while AM is used to carry TCP-based traffic, where delay is not much important but packet loss is not tolerable.
An RLC transparent mode exists as well, but it is only used to send RRC messages when no RLC UM or AM entity is set up yet.
A functional representation of the three modes at both UE and eNodeB side are shown below.
Functionality of RLCThe RLC layer supports segmentation and concatenation of RLC Service Data Units (SDUs). Depending on the scheduler decision, a certain amount of data is selected from the RLC SDU buffer and segmented and/or concatenated depending on the size of the SDUs. This selected data block becomes the RLC PDU to which a sequence number is assigned. This means that one transport block contains only a single RLC PDU per radio bearer, except if an RLC retransmission is required. In this case an RLC PDU containing new data might be multiplexed at the MAC layer with an RLC PDU retransmission. In order to allow the RLC SDU reassembly at the receiver, the RLC header carries the required segmentation, re-segmentation and concatenation information.
Retransmission and Sequence Numbering in RLC