lable spectrum and therefore throughput. Distributed unit Given that DC uses a single PDCP entity to serve both MeNB and SeNB links, it is "natural" to make the split between CU and DU along the PDCP/RLC interface as is done in the "Option 2" logical split. This also eliminates the CP components to reside in the DU, which reduces the DU implementation complexity and allows for mix-and-match of CU and DU components in the network. F1 termination The DU needs to terminate the standardized F1-C and F1-U interfaces to and from the CU unit. This includes flow control as we described for the CU. Flow control limits payload buff- ering (and thus, DDR dimensioning) in the DU, keeping system cost and complexity down. Radio Link Control and Medium Access Control The 5G Layer-2 protocol stack is responsible for converting IP packets that are carried in QoS flows to transport blocks (TBs) that can be transmitted over the air, and vice versa. This protocol stack consists of four layers: SDAP and PDCP (associated with the CU) and RLC and MAC (associated with the DU). The latter is being discussed here. In these discussions, we use 3GPP terminology for the frames that go in and come out of the RLC and MAC layers (Fig. 2-18). Frames ingress to the RLC from the PDCP layer are called RLC SDUs and frames egress from the RLC layer are called RLC PDUs. These RLC PDUs that are ingress to the MAC layer are also called MAC SDUs and MAC output frames are called MAC PDUs. These MAC PDUs are the same as what is ingress to the Physical Layer: TBs. Per 3GPP 38.322, RLC functions are defined as: transfer of upper layer PDUs according to transmission modes (transparent mode/TM, unacknowledged mode/UM and acknowledged mode/AM) 60 Open Radio Access Network (O-RAN) Systems Architecture and Design 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 Channels FIGURE 2-18 3GPP Layer-2 pro