re 4.4.3 shows the protocol structure for E1. As with any other NG-RAN control-plane interface, it uses IP and SCTP. However, unlike most other NG-RAN Figure 4.4.2 : Overall architecture for separation of gNB-CU-CP and gNB-CU-UP. (Source: Reproduced by permission of © 3GPP). gNB-CU-CP gNB-CU-UP gNB-DU gNB-DU 4 NG-RAN Architecture Control Plane Figure 4.4.3 Interface protocol structure for E1. (Source: Reproduced by permission of © 3GPP). Radio Network EI-AP Layer Transport Network Layer Data link layer Physical layer network interfaces, there is no user plane for E1 as it is purely a control-plane interface. All user-plane interfaces (e.g. NG-U and Xn-U) are terminated in the gNB-CU-UP - E1 is used to control these, but data-plane packets do not go via gNB-CU-CP. gNB-DU is connected to only one gNB-CU-CP, since in the monolithic architecture a gNB-DU is connected to only one gNB-CU. Similarly, a gNB-CU-UP is connected to only one gNB-CU-CP. Such architecture with only one control-plane network node simpli- fies both design and implementation. The number of connections between a gNB-DU and a gNB-CU-UP can be arbitrary, that is, one gNB-DU can be connected to multiple gNB-CU-UPs and vice versa, as long as they all are under control of the same gNB-CU-CP. In Figure 4.4.2, (3GPP TS 38.401) solid lines represent interface instances whereas dashed lines represent F1-U tunnels, for example, carrying data for multiple UEs and/or multiple bearers. A single E1 (and F1-C) interface instance between a pair of gNB-CU-CP and gNB-CU-UP (and a pair of gNB-CU-CP and gNB-DU) can support multiple SCTP asso- ciations, for example, for NG-RAN deployment in virtualized environments (where, e.g., a single instance of a gNB-CU-UP or a gNB-CU-CP network node can be deployed in a virtualized platform with multiple hardware instances having multiple transport network addresses). The E1 standardized interface between gNB-CU-CP and gNB-CU-UP uses the E1AP, which is a control-plane protocol. It generally follows the same design principles as 