An Introduction to 5G

18Mobility Management in RRC_INACTIVE

This chapter addresses the last of 5G's mobility management states, RRC_INACTIVE. This state is designed for mobiles that are communicating with a low data rate. It is only available if the mobile is controlled by the 5G core network and the next‐generation radio access network (NG‐RAN), although the mobile can be served by either a gNB or an ng-eNB.

It is easiest to understand the state of RRC_INACTIVE by thinking about what happens if the nodes are split into central and distributed units. As part of this chapter, we will take the opportunity to investigate the signalling procedures inside a split gNB, both on the F1 reference point between the gNB central unit (gNB-CU) and the gNB distributed unit (gNB-DU), and on the E1 reference point between the gNB central unit control plane (gNB-CU-CP) and the gNB central unit user plane (gNB-CU-UP).

The main specifications for this chapter overlap with the ones used in the states of RRC_CONNECTED and RRC_IDLE [1-4]. In addition, TS 38.401 [5] contains some valuable examples of the signalling flows inside a split gNB.

18.1 Introduction to RRC_INACTIVE

18.1.1 Principles

The state of RRC_INACTIVE is an optimization for mobiles that are communicating with a low data rate, such as machine‐type devices. It uses the most appropriate features of RRC_CONNECTED and RRC_IDLE, so as to limit the mobile's power consumption in between bursts of data, and to limit the signalling overhead once data eventually ...