hannels FIGURE 2-6 3GPP Layer-2 protocol stack and interfaces (replicated from 3GPP 38.300). The PDCP layer includes (optional) Robust Header Compression (RoHC) to compress IP/ UDP and/or other header types, reducing the effective payload size of the packet that is transferred over the air. After header compression, packet ciphering and (optional, for con- trol bearers) integrity protection are applied to the packet, using AES, SNOW, or ZUC algo- rithms. The packet is then routed to the appropriate RLC/MAC layer (see the discussion on dual/multiconnectivity later). On the receive side, the sequence number is used to detect reception of duplicate packets as well as to ensure in-order delivery of the packets to the next layers. There are two types of radio bearers: DRBs for data channels and signaling radio bearers (SRBs) for control channels. The RRC protocol is responsible for signaling and data radio bearer establishment, configuration, maintenance, and release. This includes the same steps for additional carriers associated with dual connectivity (DC, more about that later) operation. The RRC is also responsible for broadcasting system information (SI) and handling mobility. 50 Open Radio Access Network (O-RAN) Systems Architecture and Design Control plane user plane split and central unit/distributed unit interface Let's look at the CU with regards to its interfaces first, as shown in Fig. 2-7. We discussed both the CP/UP split and the NG network interfaces and summarized in the following figure. Relevant interfaces are shown in the figure. We will quickly discuss them here and point to the appropriate 3GPP standards for further knowledge. If this comes across as a complex spiderweb of standards, that is because it is! Read through the individual standards docu- ments to make sense of it all. 3GPP 38.460 defines the El general aspects and principles-this is a high-level summary of the El interface including links to the related more detailed interfaces. These include the El interface Layer 1 and signaling tran