An Introduction to 5G

19Inter‐operation with the Evolved Packet Core

The evolved packet core (EPC) will operate alongside the 5G core network for several years, so that network operators can migrate from one system to the other, and so that LTE devices can continue using the older system. Such migration will be gradual: for example, a network operator is unlikely to add the NG reference point to all of its LTE base stations at the same time. That can lead to a situation in which a mobile is registered with the 5G core network, but moves to a geographical area in which the base stations can only communicate with the EPC. At that point, the system has to transfer the mobile from one core network to the other.

In this chapter, we will discuss these issues by looking at the inter‐operation architectures that the 5G system supports and the signalling procedures that result. We will assume some previous knowledge of the evolved packet core, but we will review its most important features as we go along.

19.1 Inter‐operation Architectures

19.1.1 Migration Architecture

The specifications define two inter‐operation architectures, the first of which is illustrated in Figure 19.1 [1]. In this architecture, the two core networks share a single subscriber database, which acts as unified data management (UDM) towards the 5G core network, and as a home subscriber server (HSS) towards the EPC. The radio access networks can overlap in the usual way, with a base station lying in the next‐generation radio access ...