RLC Functions

As shown in Figure 6-3, the RLC layer interfaces the RRC, PDCP, and BMC layers on one side and the MAC layer on the other. Its main functions are

• Data transfer

• Segmentation and reassembly

• Error detection and correction

• Flow control

• Ciphering for the purpose of providing security

If the PDUs coming from the higher layers to the RLC sublayer are too long, they are first segmented into smaller units of equal size and then transferred to the MAC layer. If the higher-layer PDU or a segment thereof is smaller than the given size, it may be concatenated with the first segment of the next incoming higher-layer PDU. Alternatively, it may be extended by adding padding bits. A header is added to each segment that includes, among other things, the sequence number and the length of the PDU, and the resulting packet is passed to the MAC layer.

There are three modes in which the data transfer can take place between peer RLC layers (that is, the transmit and receive RLC layers of a communication link)-acknowledged, unacknowledged, and transparent. In the acknowledged transfer mode, the RLC provides guaranteed, error-free delivery. On receiving a packet from the MAC layer, the RLC layer checks to see if its sequence number is correct. If it is not, the receiver discards the packet and attempts to recover from that error in one of the following ways. It may use an automatic repeat request (ARQ) whereby the sender is...