Chapter 5

Radio Link Control (RLC)

3GPP provides the RLC specification in [1]. The RLC layer performs framing of RLC SDUs to put them into the size indicated by the lower MAC layer. The RLC transmitter segments and/or concatenates RLC SDUs to construct RLC PDUs, and the RLC receiver reassembles RLC PDUs to reconstruct RLC SDUs.

The RLC layer is connected to the lower MAC layer via logical channels. Each logical channel transports different types of traffic, and the name of the logical channel reflects the characteristic of the traffic it transports. In Release 8, a total of five types of logical channel are defined, as explained in Section 6.3.2.

The layer above the RLC layer is typically the PDCP layer, but in some cases it is the RRC layer. RRC messages transmitted on the logical channels PCCH, BCCH, and CCCH do not require security protection, and thus go directly to the RLC layer, bypassing the PDCP layer. Other than those RRC messages, all control plane and user plane traffic goes through the PDCP layer and RLC layer.

RLC functions are performed by the RLC entity. An RLC entity is established when a radio bearer is set up, and removed when the associated radio bearer is released. When established, the RLC entity is configured in one of three operating modes: Transparent Mode (TM), Unacknowledged Mode (UM), and Acknowledged Mode (AM). Depending on the operating mode, different RLC functions are performed by the RLC entity.

An RLC entity is used for only one radio bearer. However, ...

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