Evolved Packet System (EPS): The LTE and SAE Evolution of 3G UMTS

4.1 Overall Architecture

Figure 4.1 describes a simplified view of the EPS architecture. Not all the network nodes and interfaces are represented here (as this was already done in Chapter 2). This picture rather focuses on E-UTRAN/EPC interactions and user signalling and data connectivity and architectural aspects which will be developed in this chapter.

Starting from the E-UTRAN part, the X2 interface shall be seen as a meshed interface rather than a point-to-point between two specific E-UTRAN nodes. This optional interface has been defined for the main purpose of forwarding packets between eNodeB so as to limit packet loss for intra E-UTRAN user mobility.

The S1 interface shall also not be seen as a simple interface between one eNodeB and one MME/Serving gateway (represented above as a single box for simplicity), since an eNodeB can possibly be connected to more than one MME. This flexibility is known as S1-flex (an equivalent of the Iu-flex 3G/UMTS option) described later in this chapter.

When MME and Serving GW are deployed as two separate physical boxes, the S1 interface is split into two parts:

• The S1-U (for User plane) – which carries user data between eNodeB and Serving GW.
• The S1-C (for Control plane) – which is a signalling-only interface between the eNodeB and the MME.

On the S5 side, a serving GW may also be linked to different PDN GW, corresponding to the fact that EPC may provide user connectivity to several different and separated IP networks.

Figure 4.1 EPS architecture ...