andover etc. The mechanism for transparent transfer of NAS messages is also included. The layer 2 of the new radio protocol stack is split into four sublayers: medium access control (MAC), radio link control (RLC), packet data convergence protocol (PDCP), and service data adaptation protocol (SDAP), where each sublayer hosts a number of functions and performs certain functionalities that are con- figurable via radio resource control (RRC) at layer 3. Fig. 2.1 shows the OSI protocol layers and how they map to 3GPP radio protocol architecture. As shown in the figure (dark- shaded boxes), the data link layer of OSI network model maps to these four sublayers that constitute layer 2 of the 3GPP new radio protocols. The layer 2 protocols of NR have some similarities with the corresponding LTE protocols; however, the NR has added more config- uration flexibility and more functionalities to support the new features such as beam man- agement that have no counterpart in LTE. A significant difference in NR RRC compared to LTE RRC is the introduction of a 3-state UE behavior with the addition of the RRC_INACTIVE state. The RRC_INACTIVE provides a state with battery efficiency simi- lar to RRC_IDLE while storing the UE context within the NG-RAN SO that the transitions to/from RRC_CONNECTED are faster and incur less signaling overhead. The other signifi- cant improvements relative to LTE RRC are the support of on-demand system information 5G NR. DOI: : https://doi.org/10.1016/B978-0-08-102267-2.00002-6 © 2019 Elsevier Inc. All rights reserved. Chapter 2 NR protocol structure OSI seven-layer network Non-access stratum (NAS) Internet protocol (IP) model Application layer Radio resource control (RRC) Presentation layer QoS flows Session layer Service data adaptation Control/ protocol (SDAP) configuration Transport layer Radio bearers RRC PDUs Packet data convergence Network layer protocol (PDCP) Control/configuration RLC channels Data-Link layer Radio link control (RLC) Physical layer Control/configuration Logical channels Medium