WHAT IS THE PURPOSE OF THE RLC LAYER?

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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

  • An Unacknowledged Mode (UM)
  • An Acknowledged Mode (AM)
  • Transparent Mode (TM).

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

  • The RLC sequence number is used at the receiver for in-sequence delivery to the RLC SDU reassembly entity.
  • In AM, RLC is responsible for correcting residual HARQ errors by operating another ARQ protocol, since it would be expensive in terms of transmit power, to reach the required residual error rates of 1 over 100000 ( 1/100000) or less in the MAC HARQ protocol.
  • The ARQ retransmission units are RLC Protocol Data Units (PDUs) or RLC PDU segments. If an RLC retransmission is required and the radio quality has changed significantly compared to the original RLC transmission, the RLC protocol is able to perform a re-segmentation. In this case, RLC segments a PDU into smaller PDU segments. The number of RLC re-segmentations of an RLC PDU is unlimited.
  • RLC performs reordering of received RLC PDUs and PDU segments in order to ensure that RLC SDUs are delivered in sequence to higher layers.
  • Re-transmissions are initiated either by status reports sent by the RLC receiver, or by local triggers from MAC layer in case of reaching the maximum number of HARQ transmissions. Status Reports are triggered either by polls sent from the RLC sender, or by detecting missing PDUs after the PDUs have passed the re-ordering entity.
  • Finally, RLC provides means for protocol error detection and recovery (e.g. reset) and duplicate detection.
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