Radio Link Control is the interface between Radio Resource Control and Logical channels. As you can see from the diagram.
The primary function of RLC is to transfer user data and signaling between the upper layers and the MAC layer. Data is transferred into the RLC in data blocks called Service Data Units (SDU). Data is transferred out of the RLC in data blocks called Protocol Data Units (PDU).
Three data transfer modes are defined:
RLC Transparent Mode FunctionsThe primary function of RLC is to transfer user data and signaling between the upper layers and the MAC layer. Data is transferred into the RLC in data blocks called Service Data Units (SDU). Data is transferred out of the RLC in data blocks called Protocol Data Units (PDU).
Three data transfer modes are defined:
- Transparent Mode (TM) – PDUs are transferred with no additional header information.
- Unacknowledged Mode (UM) – A header containing length and sequence number fields are added to the PDU before it is transmitted.
- Acknowledged Mode (AM) – PDUs are delivered according to a bidirectional protocol that includes in-sequence delivery, flow control, error detection, and duplicate detection.
- Segmentation and Reassembly – If the SDU size is too large to fit in a single PDU, it may be segmented on the transmitting side and reassembled on the receiving side.
- SDU Discard – The transmitting TM entity may be configured to perform timer-based SDU discard. An SDU that is not transmitted after a configurable period of time is discarded. Note that ciphering for Transparent Mode Logical Channels is performed by the MAC layer. Voice and some signaling channels use transport mode.
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| RLC Transparent Mode Functions |
In Unacknowledged Mode (UM), RLC provides slightly more reliable service than in Transparent Mode. A small header, which contains information about segmentation, concatenation, and the sequence number, is attached to each PDU. The functions performed by UM RLC are:
- Segmentation and Reassembly – If the SDU size is too large to fit in a single PDU, it may be segmented on the transmitting side and reassembled on the receiving side.
- Concatenation/Padding – Multiple SDUs may be combined into a single PDU to fill the available PDU size, and padding bits may be added to fill the available PDU.
- Sequence Number Check – The RLC header contains a sequence number, which is used during reassembly to detect corrupted SDUs. • Ciphering – If enabled, ciphering of UM Logical Channel data streams is performed by RLC.
- SDU Discard – The transmitting UM entity may be configured to perform timer-based SDU discard. An SDU that is not been transmitted after a configurable time is discarded.
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| RLC Unacknowledged Mode Functions |
The Acknowledged Mode (AM) protocol is a complex protocol that provides reliable service. Both positive and negative acknowledgment are supported. The functions performed by AM RLC are:
- Segmentation and Reassembly – If the SDU size is too large to fit in a single PDU, it may be segmented on the transmitting side and reassembled on the receiving side.
- Concatenation/Padding – Multiple SDUs may be combined into a single PDU to fill the available PDU size, and padding bits may be added to fill the available PDU.
- Error Correction – PDUs received in error are retransmitted. The receiver may poll the transmitter to prompt it to send an acknowledgment status report. In-sequence Delivery – PDUs are delivered to the upper layers in the same order as they were submitted to the transmitting side RLC.
- Duplicate Detection – A PDU is delivered only once to the upper layers.
- Flow Control – Configurable transmit and receive window sizes perform flow control.
- Ciphering – Ciphering of AM Logical Channel data streams is performed by RLC.
- Protocol Error Detection and Recovery – AM RLC may be reset to recover from protocol errors.
- SDU Discard – Several mechanisms are defined for SDU discard in AM RLC, including timerbased and maximum data retransmissions.
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| Acknowledged Mode |
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