Similar to UMTS technology, Packet Data Convergence Protocol (PDCP) is located above the RLC layer at UE as well as in eNB. However in LTE, all user data goes via the PDCP layer as ciphering is performed in PDCP. The main functions of PDCP are IP header compression (UP), ciphering and integrity protection (Control Plane only). PDCP layer also carries RRC signaling messages as well so it is part of control plane as well.
LTE radio interface consists of 3 layers,
Layer 1 – Physical Layer
Physical layer offers data transport services to MAC layer.
Main functions of physical layer include: Mapping of transport channels on to the physical channels, forward error correction (FEC), Modulation & Coding etc.
Layer 2 Sublayer – Medium Access Control (MAC)
LTE Medium Access Control (MAC) is a layer 2 protocol and provides data transfer and radio resource allocation services to upper layers. It performs scheduling of the data according to priorities, and multiplexing data to physical layer transport blocks. RRC layer controls configuration of MAC sublayer. In E-UTRA, there are two MAC entities, one in the UE and other one in the eNB.
LTE Radio Link Control (RLC) protocol is one of eNB’s Layer 2 sublayers that also consist of Medium Access Control (MAC) and Packet Data Convergence Protocol (PDCP) protocols. RLC layer transfers upper layer (PDCP) PDUs and supports protocol error detection & recovery functions.
With Release 8, 3GPP has introduced a whole new set of protocols as both radio access network and core network have evolved from the earlier 2G & 3G Networks. In LTE architecture, evolved core network includes Mobility Management Entity (MME), Serving Gateway (SGW), Packet Data Network Gateway (PDN GW) where as evolved radio network has only E-UTRAN NodeB (eNB).