yer-2 (L2) switching functions executing in the DP and higher level CP functions, allowing for separation of DP control and monitoring. By defining a dedicated DP component, the OpenFlow concept accepts the notion that certain functions are indeed best executed on an optimized hardware platform such as a network processor. The OpenFlow concept introduces centralization of the CP using the SDN principles, allowing (but not forcing) for the CP to move to a remote (centralized/cloud) environment. Central unit As we discussed in the chapter 1, 3GPP (38.801) defines multiple functional splits between functionality in the CU and DU. Referring to Fig. 2-6, we assume what is defined as Option 2 which defines the Radio Link Control (RLC)/Medium Access Control (MAC) to be allocated to the DU and the PDCP and Radio Resource Control (RRC) to the CU. In addition, following CP/UP split principles, the RRC CP piece is separated from the Packet Data Convergence Protocol (PDCP, UP). The Service Data Adaptation Protocol (SDAP) function is associated with the PDCP and defined in 3GPP 37.324. This component is responsible for mapping between QoS flows and radio bearers. More information about this is present later. The PDCP layer (defined in 3GPP 38.323) is responsible for transfer of user data between SDAP layer and RLC. In the transmit direction, the PDCP defines a unique sequence number to each payload packet and adds its own header to the packet that includes this sequence number to be used by the remainder processing steps in the PDCP layer. For one, a "SDU discard timer" is associated with the packet, allowing the PDCP layer to track successful delivery of the packet to the receiving entity after which the associated buffer can be returned to its pool. Chapter 2 System components, requirements, and interfaces 49 QoS Flows QoS Flow Handling QoS Flow Handling Radio Bearers Securtiy Security Securtiy Security Channels TM/UM/ TM/UM/ TM/UM/ TM/UM/ Logical Channels Scheduling / Priority Handling Multiplexing Multiplexing Transport C