nto two strata (Figure 2.1): - the access stratum AS includes air interface and provides functions related the to data link layer and the lower part of OSI layer 3. The access stratum deals with the radio protocols on the Uu interface and with the protocols managed on the NG (Next Generation) terrestrial interface; - the non-access stratum (NAS) deals with the exchange of traffic and data between the mobile and the core network. As a result, NAS is concerned with all the protocols allowing the exchange of signaling between the mobile UE (User Equipment) and the AMF (Access and Mobility Management Function) entity of the 5G core network. The NAS protocol refers to the procedure related to mobility, authentication and management of service requests (data session or Voice over IP call). CM, SM NON ACCESS STRATUM CM, SM Trafic Radio Radio proto protocol Protocol Protocol ACCESS STRATUM Radio ng-eNB, gNB Terrestrial Interface NG-RAN Interface Figure 2.1. Protocols on the 5G interface For a color version of all the figures in this chapter, see www.iste.co.uk/launay/5g.zip NG-RAN and 5G-NR: 5G Radio Access Network and Radio Interface, First Edition. Frédéric Launay. C ISTE Ltd 2021. Published by ISTE Ltd and John Wiley & Sons, Inc. NG-RAN and 5G-NR The protocols on the Uu and NG interface concern: - the user plane protocols used to transport the traffic data of the PDU (Protocol Data Unit) session; - the control plane protocols allowing us to control the PDU sessions, the radio connection and the mobility during a session (handover). 2.1.1. Protocol stack on the Uu interface The Uu radio interface is the interface between the mobile and the NG-RAN access node. The NG-RAN access network includes two types of radio entities: - the ng-eNB (next generation evolved Node Base station) radio station offering 4G radio access; - the gNB radio station (next generation Node Base station) offering 5G radio access. The Uu interface supports: - the RRC (Radio Resource Control) signaling exchanged between the mobile and the NG-RAN nod