Provides segmentation/reassembly (payload units, PU) and retransmission
service for both user(Radio Bearer) and control data (Signalling Radio bearer).
• Transparent mode (Tr): no overhead is added to higher layer data.
• Unacknowledged mode (UM): no retransmission protocol is used and data delivery is not guaranteed.
• Acknowledged mode (AM): Automatic Repeat reQuest (ARQ) mechanism is used for error correction.
