(gNB-CU-UP initiated). Bearer Context Setup Procedure The purpose of the Bearer Context Setup procedure is to allow the gNB-CU-CP to establish a bearer context in the gNB-CU-UP. The bearer can either connect the gNB-DU and the gNB-CU-UP, or the UPF/S-GW and the gNB-CU-UP. Mes- sages include Bearer Context Setup Request, Bearer Context Setup Response, and Bearer Context Setup Failure. Among the numerous IEs defined for these messages, uplink or downlink TNL address information of F1-U, S1-U/NG-U play a vital role, as they represent either end of a bearer. From that perspective, there are two somewhat different procedures defined that are used to admit a new UE in a gNB-CU-UP, for example, during an initial access or a handover. The key difference between these two procedures is which node (gNB-CU-CP or gNB-CU-UP) allocates the TNL address (which can be a F1-U UL TNL address or NG-U/S1-U DL TNL address for a particular bearer) at the gNB-CU-UP side for a UE. These procedures are illus- trated in Figure 4.4.4. In the former case, shown in Figure 4.4.4a, in a "one-shot" procedure the gNB-CU-CP selects TNL addresses. 4.4 Control-User Plane Separation gNB-DU gNB-CU-UP gNB-CU-CP gNB-DU gNB-CU-UP gNB-CU-CP 0. Bearer context setup is 0. Bearer context setup is triggered triggered 1. Bearer Context Setup Request 1. F1 UE context setup procedure 2. Bearer Context Setup Response 2. Bearer Context Setup Request Uplink user data 3. F1 UE context setup procedure 3. Bearer Context Setup Response Downlink user data 4. Bearer Context Modification Request Uplink user data 5. Bearer Context Modification Response Downlink user data (a) "One-shot" procedure (b) Two-step procedure Figure 4.4.4 Two options to admit a new UE in a gNB-CU-UP. For the sake of brevity we only explain the two-step procedure shown in Figure 4.4.4b, which is performed as follows: 0. Bearer context setup (e.g. following the Initial UE Context Setup Request from the core network) is triggered. 1. The gNB-CU-CP sends a Bearer Context Setup Request message containi