S1 – Control/User Plane

On the S1 reference point the physical layer L1 will in most cases be realized by Gigabit Ethernet cables. L2 in this case will be Ethernet. On top of Ethernet we find IP, but used as a transport protocol between two network nodes: eNB and MME. This lower layer IP does not represent the user plane frames.

Instead, the user plane IP frames (higher layer IP) are carried by the GTP Tunneling Packet Data Unit (T-PDU). The GTP is responsible for the transport of payload frames through the IP tunnels on S1-U. The transport layer for GTP-U is the User Datagram Protocol (UDP). As IP this protocol may be found twice in the user plane stack: lower UDP for transport between the eNB and MME and higher UDP (not shown in Figure 1) that is transparently routed through the mobile network as the transport protocol for real-time application data. The higher layer IP on top of GTP-U as well as all application data on top of this higher layer IP are identical with the user plane information.

 
Figure 1: Protocol stack S1 control/user plane

On the control plane side, the Streaming Control Transport Protocol (SCTP) provides reliable transport functionality for the very important signaling messages. S1AP is the communication expression between MME and S-GW while NAS

LTE Network Protocol Architecture

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.

 
Figure 1: Protocol stack LTE Uu interface

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.