Uu – Control/User Plane
The protocol stack used on radio interface Uu is shown in Figure 1. The physical layer in this stack is represented by OFDM in the DL and SC-FDMA in the UL. Then we see the MAC protocol that is responsible for mapping the transport channels onto the physical channels, but also for such important tasks as packet scheduling and timing advance control. RLC provides reliable transport services and can be used to segment/reassemble large frames. The main purpose of PDCP is the compression of larger IP headers as well as ciphering of user plane data and integrity protection of both user plane and control plane data.
On top of PDCP the stack is split into the user plane and control plane parts. On the control plane side we see RRC protocol, that is, the expression for the communication between the UE and eNB. RRC provides all the necessary functions to set up, maintain, and release a radio connection for a particular subscriber.
RRC also serves as a transport protocol for NAS signaling messages. NAS is the expression for the communication between the UE and MME in which MME represents the core network.
On the user plane side we see IP as the transport layer for end-to-end applications. On the Uu stack the IP is always end-to-end IP, which means that all these IP packets are transparently routed, often tunneled through the mobile network. The user plane IP frames we see on Uu are the same IP frames that can be monitored at SGi reference points before or behind the PDN-GW.
The IP version can be Internet Protocol version 4 (IPv4) or Internet Protocol version 6 (IPv6). In the case of VPN (Virtual Private Network) traffic, IPsec will be used.
The applications on top of IP in the user plane stack are all protocols of the TCP/IP suite, such as the File Transfer Protocol (FTP), HTTP (web-browsing), and POP3/SMTP (for e-mail), but also Real-Time Transport Protocol (RTP) and SIP for real-time services like VoIP.