A packet sent down to the PDCP layer 300 is referred to as PDCP service data unit (SDU). The PDCP SDU may contain an IP packet (user plane) or RRC control information (control plane). The user plane data may undergo header compression. The header compression operation is taken to reduce the packet size by compressing the header. It may also be possible to undergo ciphering and an integrity protection operation. The ciphering operation is taken for ciphering the packet in order for a specific receiver to receive the packet correctly. The ciphering operation is performed to the payload of the PDCP PDU and MAC-I but not to the PDCP Control PDU. The integrity protection operation is taken for determining whether the packet is corrupted with wrong information. The integrity protection operation is performed to the header and payload of the PDCP PDU before the ciphering. A PDCP header is added to obtain a PDCP PDU.

The RLC layer 310 processes (segments or concatenates) the PDCP PDU (=RLC SDU) received from the PDCP layer into the size indicated by the MAC layer 320 to generate an RLC PDU. The segmentation and concatenation operations being performed in the RLC layer are described in detail with reference to FIG. 4. An RLC header is added to obtain the RLC PDU. The MAC layer 320 generates a MAC PDU composed of a MAC header and MAC payload. The MAC header contains MAC sub-headers corresponding to MAC control elements (CEs) and or MAC SDUs.

FIG. 4 is a diagram illustrating a packet segmentation or concatenation procedure in the LTE layer of the legacy LTE system.