PDCP: The PDCP protocol supports efficient transport of IP packets over the radio link. It performs header compression, Access Stratum (AS) security (ciphering and integrity protection) and packet re-ordering/retransmission during handover.
RLC: In the transmitting side, the RLC protocol constructs RLC PDU and provides the RLC PDU to the MAC layer. The RLC protocol performs segmentation/concatenation of PDCP PDUs during construction of the RLC PDU. In the receiving side, the RLC protocol performs reassembly of the RLC PDU to reconstruct the PDCP PDU. The RLC protocol has three operational modes (i.e. transparent mode, acknowledged mode and unacknowledged mode), and each offers different reliability levels. It also performs packet (the RLC PDU) re-ordering and retransmission.
MAC: The MAC layer lies between the RLC layer and PHY layer. It is connected to the RLC layer through logical channels, and to the PHY layer through transport channels. Therefore, the MAC protocol supports multiplexing and de-multiplexing between logical channels and transport channels. Higher layers use different logical channels for different QoS metrics. The MAC protocol supports QoS by scheduling and prioritizing data from logical channels. The eNB scheduler makes sure radio resources are dynamically allocated to UEs and performs QoS control to ensure each bearer is allocated the negotiated QoS.
PHY: The physical layer provides the basic bit transmission functionality over air. It is driven by OFDMA in the downlink and SC-FDMA in the uplink. Physical channels are dynamically mapped to the available resources. Towards higher layers the physical layer offers its data transmission functionality via transport channels. Like in UMTS, a transport channel is a block oriented transmission service with certain characteristics regarding bit rates, delay, collision risk and reliability. In contrast to 3G WCDMA or even 2G GSM there are no dedicated transport or physical channels anymore, as all resource mapping is dynamically driven by the cheduler.
GTP-U: GTP-U protocol is used to forward user IP packets over S1-U, S5 and X2 interfaces. When a GTP tunnel is established for data forwarding during LTE handover, an End Marker packet is transferred as the last packet over the GTP tunnel.