5.3.2 RLC Services and Functions
25.3013GPPRadio interface protocol architectureTS
This subclause provides an overview on services and functions provided by the RLC sublayer. A detailed description of the RLC protocol is given in [8].
5.3.2.1 Services provided to the upper layer
– Transparent data transfer. This service transmits upper layer PDUs without adding any protocol information, possibly including segmentation/reassembly functionality.
– Unacknowledged data transfer. This service transmits upper layer PDUs without guaranteeing delivery to the peer entity. The unacknowledged data transfer mode has the following characteristics:
– Detection of erroneous data: The RLC sublayer shall deliver only those SDUs to the receiving upper layer that are free of transmission errors by using the sequence-number check function.
– Immediate delivery: The receiving RLC sublayer entity shall deliver a SDU to the upper layer receiving entity as soon as it arrives at the receiver.
– Duplication avoidance and reordering: Alternative to immediate delivery, the RLC sublayer shall deliver SDUs to the receiving upper layer entity in the same order as the transmitting upper layer entity submits them to the RLC sublayer without duplication. SDUs may be delayed by this procedure to ensure in-sequence delivery.
– Out-of-sequence SDU delivery: Alternative to immediate delivery, the RLC sublayer shall deliver SDUs to the upper layer receiving entity as soon as they can be recovered from PDUs, without waiting for earlier in-sequence SDUs to be recovered.
– Sequence-number delivery to JBM.
– Acknowledged data transfer. This service transmits upper layer PDUs and guarantees delivery to the peer entity. In case RLC is unable to deliver the data correctly, the user of RLC at the transmitting side is notified. For this service, both in-sequence and out-of-sequence delivery are supported. In many cases a upper layer protocol can restore the order of its PDUs. As long as the out-of-sequence properties of the lower layer are known and controlled (i.e. the upper layer protocol will not immediately request retransmission of a missing PDU) allowing out-of-sequence delivery can save memory space in the receiving RLC. The acknowledged data transfer mode has the following characteristics:
– Error-free delivery: Error-free delivery is ensured by means of retransmission. The receiving RLC entity delivers only error-free SDUs to the upper layer.
– Unique delivery: The RLC sublayer shall deliver each SDU only once to the receiving upper layer using duplication detection function.
– In-sequence delivery: RLC sublayer shall provide support for in-order delivery of SDUs, i.e., RLC sublayer should deliver SDUs to the receiving upper layer entity in the same order as the transmitting upper layer entity submits them to the RLC sublayer.
– Out-of-sequence delivery: Alternatively to in-sequence delivery, it shall also be possible to allow that the receiving RLC entity delivers SDUs to upper layer in different order than submitted to RLC sublayer at the transmitting side.
– Maintenance of QoS as defined by upper layers. The retransmission protocol shall be configurable by layer 3 to provide different levels of QoS. This can be controlled.
– Notification of unrecoverable errors. RLC notifies the upper layer of errors that cannot be resolved by RLC itself by normal exception handling procedures, e.g. by adjusting the maximum number of retransmissions according to delay requirements.
For AM RLC, there is only one RLC entity per Radio Bearer. For UM and TM RLC, there is one or two (one for each direction) RLC entities per Radio Bearer.
5.3.2.2 RLC Functions
– Segmentation and reassembly. This function performs segmentation/reassembly of variable-length upper layer PDUs into/from smaller RLC PDUs. The RLC PDU size is either fixed (a semi static variable configured by upper layers) or flexible (automatically adjusted to the incoming SDU size) depending on the configuration. Flexible RLC PDU size is only supported when mapped on HS-DSCH or E-DCH.
– Concatenation. If the contents of an RLC SDU cannot be carried by one RLC PDU, the first segment of the next RLC SDU may be put into the RLC PDU in concatenation with the last segment of the previous RLC SDU.
– Padding. When concatenation is not applicable and the remaining data to be transmitted does not fill an entire RLC PDU of given size, the remainder of the data field shall be filled with padding bits.
– Transfer of user data. This function is used for conveyance of data between users of RLC services. RLC supports acknowledged, unacknowledged and transparent data transfer. QoS setting controls transfer of user data.
– Error correction. This function provides error correction by retransmission (e.g. Selective Repeat, Go Back N, or a Stop-and-Wait ARQ) in acknowledged data transfer mode.
– In-sequence delivery of upper layer PDUs. This function preserves the order of upper layer PDUs that were submitted for transfer by RLC using the acknowledged data transfer service. If this function is not used, out-of-sequence delivery is provided.
– Duplicate Detection. This function detects duplicated received RLC PDUs and ensures that the resultant upper layer PDU is delivered only once to the upper layer.
– Flow control. This function allows an RLC receiver to control the rate at which the peer RLC transmitting entity may send information.
– Sequence number check. This function is used in unacknowledged mode and guarantees the integrity of reassembled PDUs and provides a mechanism for the detection of corrupted RLC SDUs through checking sequence number in RLC PDUs when they are reassembled into a RLC SDU. A corrupted RLC SDU will be discarded.
– Protocol error detection and recovery. This function detects and recovers from errors in the operation of the RLC protocol.
– Ciphering. This function prevents unauthorised acquisition of data. Ciphering is performed in RLC layer for non-transparent RLC mode. Details of the security architecture are specified in [15].
– SDU discard. This function allows an RLC transmitter to discharge RLC SDU from the buffer.