e target-gNB during handover and performs integrity protection of RRC messages. The RRC sublayer is responsible for setting up and maintenance of radio bearers. Note that the RRC sublayer in 3GPP protocol hierarchy is considered as layer-3 protocol [16]. Fig. 1.68 shows an example of layer-2 data flow and packet processing, where a transport block is generated by MAC sublayer by concatenating two RLC PDUs from the radio bearer RBX and one RLC PDU from the radio bearer RBy. In this figure, H denotes the layer-specific headers or subheaders of each sublayer. The two RLC PDUs from RBX each corresponds to one of the IP packets n and n + 1, while the RLC PDU from RBy is a seg- ment of the IP packet m. IP packet n IP packet n +1 IP packet m Radio bearerx Radio bearery SDAP SDU SDAP SDU SDAP SDU PDCP SDU PDCP SDU PDCP SDU RLC SDU RLC SDU SDU segment SDU segment MAC SDU MAC SDU MAC SDU MAC SDU MAC PDU - -transport block Figure 1.68 Example of layer-2 packet processing in NR [ 16]. 5G Network Architecture 1.2.2 Core Network The design of 5G core network architecture in 3GPP was based on the following design principles in order to allow efficient support of new service networks such as information- centric networking (ICN)63 [81]: Control- and user-plane separation (CUPS): This was a departure from LTE's vertically integrated control/user-plane network design to the one adopting NFV framework with modular NFs decoupled from the hardware for service centricity, flexibility, and programmability. In doing so, NFs are going to be implemented both physically and vir- tually, while allowing each to be customized and scaled based on their individual requirements, also allowing the realization of multi-slice coexistence. This further allows the introduction of UPF with new control functions, or reusing/extending the existing ones, to manage the new user-plane realizations. Decoupling of RAT and core network: Unlike LTE's unified control plane for access and the core networks, 5GC offers control-plane separation of RAN from the cor