This will be the last part of my series about Virtual/Open RAN signaling procedures. In this final post (although not the last one on this blog) I would like to present a very unique procedure that emerges from the facts of virtualization and automation of the RAN. And again I would like to present the big picture overview of the scenario that is called “Change of gNB-CU UP” (without handover). The full message flow (ladder diagram) can be found in 3GPP 38.401, chapter 8.9.5.
In the same chapter one can read that the trigger point for starting a change of the gNB-CU UP is quite vague. 3GPP writes: “e.g. a measurement report”. However, which particular measurement event should trigger such a procedure? Even when looking into the Rel. 16 versions of 3GPP 38.331 (NR RRC) it becomes evident that all measurement events that are not dealing with NR sidelink or V2X connectivity are triggered by changing reference signal strength or rising interference.
However, in case of a gNB-CU UP change without handover the UE does not move to a different cell. This makes me think – correct me if I am wrong – the true trigger points for this procedures come form a different entity, e.g. from the AI-driven policies and algorithms of the RAN Intelligence Controller (RIC) that is a fundamental element of the Open RAN architecture.
So what is necessary from a signaling perspective to change the gNB-CU UP during an ongoing connection?
There are new transport network resources aka GTP/IP-Tunnels required to steer the user plane traffic to and through the RAN. A new F1-U tunnel is necessary as well a a new NG-U tunnel, because also the user plane traffic between RAN and the UPF in the 5G core network must be exchange using a new route.
When it is clear which new UP transport tunnels need to be established (and which old ones need to be deleted) it is really simple to understand the overall scenario.
A F1AP UE Context Modification procedure is performed to switch the F1-U tunnel. NGAP Path Switch procedure is performed to switch the NG-U tunnel. And an E1AP Bearer Context Modification procedure is the prerequisite, because it delivers the new UL GTP-TEID for the F1-U tunnel as well as the new DL GTP-TEID for the NG-U tunnel.
Unfortunately the authors of 3GPP 38.401 are not very precise when mentioning protocol procedures defined in other specs. Thus, they speak about “bearer modification” when looking at F1AP and “Path Update” for NGAP.
It is not a big deal, but something you just need to know if you want to analyze real-world message flows of this scenario.