 channels. The SAP between the MAC sublayer and the RLC sublayer provides the logical channels. The physical layer provides transport channels to the MAC sublayer. From the physical layer perspective, the MAC sublayer provides and receives services in the form of transport channels. The data in a transport channel is organized into transport blocks. By Open systems interconnection is a standard description or a reference model for the computer networks which describes how messages should be transmitted between any two nodes in the network. Its original pur- pose was to guide product implementations to ensure consistency and interoperability between products from different vendors. This reference model defines seven layers of functions that take place at each end of a communication link. Although open systems interconnection is not always strictly adhered to in terms of grouping, the related functions together in a well-defined layer, many if not most products involved in tele- communication make an attempt to describe themselves in relation to the open systems interconnection model. Open systems interconnection was officially adopted as an international standard by the International Organization of Standards and it is presently known as Recommendation X.200 from ITU-T. The layers of open systems interconnection model are classified into two groups. The upper four layers are used whenever a message passes from or to a user. The lower three layers (up to the network layer) are used when any mes- sage passes through the host computer. Chapter 1 NR protocol structure Non-access stratum OSI Seven-layer network (NAS) Internet protocol (IP) model Application layer Radio resource control (RRC) Presentation layer QoS flows Session layer Service data adaptation protocol Control/ (SDAP) configuration Transport layer Radio bearers RRC PDUs Packet data convergence Network layer Control/configuration protocol (PDCP) Data-link layer RLC Channels Radio link control (RLC) Physical layer Control/configuration Logical channels Medi