ss Network (O-RAN) Systems Architecture and Design. DOI: https://doi.org/10.1016/B978-0-323-91923-4.00006-9 © 2022 Elsevier Inc. All rights reserved. 44 Open Radio Access Network (O-RAN) Systems Architecture and Design 3GPP Core Network GTP/IP(Sec) RRC/Control Layer 3 IP packets RRC PDUs SDAP PDUs PDCP Ctrl PDCP PDUs Radio Bearers RLC Ctrl RLC PDUs Logical Channels MAC Ctrl Layer 2 PHY Ctrl MAC PDUs Transport Channels Layer 1 FIGURE 2-1 3GPP protocol split and terminology. GTP-U tunnel (3GPP 29.281) defined as NG-U. CP traffic is carried over Stream Control Transmission Protocol (SCTP) as defined in NG-C standards: In this protocol stack, the NG-C/U Physical Layer can pretty much be any implementa- tion that supports packet (frame)-based transport (3GPP 38.411 standards defined-this stan- dard is near-empty). The Data Link Layer is also open to any implementation, if it supports IPv4 or IPv6 at IP layer (per 3GPP 38.412 for CP, 38.414 for UP). DiffServ marking support is a required feature. The explicit separation of CP and UP is a key aspect of 5G standardization. We delve into the various interfaces that enable this separation later. More on 5G RAN Quality of Service (QoS) is explained later, and we dedicate more space to DiffServ concepts in a separate sec- tion at the end of this chapter. Over the air between Base Station and UE, the protocol stacks for both UP and CP are shown in Fig. 2-4. These protocol stacks are what the O-RAN CU, DU, and RU implement. Within the NG-C, there are (few) UP and (many) CP, with delineation between UP CP being a key design target for 5G standards as compared to the 4G Evolved Packet Core (EPC), to actively support implementing them in different physical locations and software environments, including cloudification (Fig. 2-5). UP functions are implemented in the UP function (UPF): UPF. Packet routing and forwarding and inspection, QoS handling (enforcement), acts as external Protocol Data Unit (PDU) session point of interconnect to data network (DN), Chapter 2 System components