eed to use the LTE radio protocols as a baseline for 5G and to enhance them for the support of very high data rates with low latency, dynamic spectrum usage, and flexible Quality of Service (QoS). This chapter describes the 5G radio protocols, with a focus on the modifications and enhancements brought to the LTE baseline. Unless otherwise mentioned, it can be assumed that the descriptions apply to both 5G with dual connectivity with LTE toward Evolved Packet Core (EPC) (non-standalone), and 5G with connection to 5GC (standalone). 5G Technology: 3GPP New Radio, First Edition. Edited by Harri Holma, Antti Toskala, and Takehiro Nakamura. © 2020 John Wiley & Sons Ltd. Published 2020 by John Wiley & Sons Ltd. 7 5G Radio Protocols --NG-C- NG-U- QoS Flows- Radio Bearers RLC Channels Logical Channels Transport Channels Control Plane User Plane Figure 7.1 3GPP radio protocols. 5G Radio Protocol Layers The 5G radio protocols consist of a user plane (UP) and a control plane (CP). The UP is located between the IP layer and the physical layer: in the OSI model of computer net- working, the UP corresponds to the Data Link Layer and is therefore commonly referred to as Layer 2. Layer 2 of New Radio (NR) consists of four different sublayers, as depicted in Figure 7.1: Service Data Adaptation Protocol (SDAP, specified in 3GPP TS 37.324), which offers QoS flows to the UPF in 5GC (see Chapter 5). Packet Data Convergence Protocol (PDCP, specified in 3GPP TS 38.323), which offers radio bearers to SDAP. Radio Link Control (RLC, specified in 3GPP TS 38.322), which offers RLC channels to PDCP. Medium Access Control (MAC, specified in 3GPP TS 38.321), which offers logical channels to RLC. Note that the SDAP layer is only needed when interfacing with 5GC. In other words, SDAP is not needed for non-standalone 5G with dual connectivity with LTE toward EPC but is required for standalone 5G with connection to 5GC. The CP consists of the Radio Resource Control (RRC) protocol, as shown in Figure 7.1. On top of RRC is the Non-Access Stratum (NAS