Showing posts with label RIC. Show all posts
Showing posts with label RIC. Show all posts

Friday, 26 February 2021

Network Slicing in NG RAN

I have been asked to explain in a nutshell how network slicing works in NG RAN. The most important facts you will find in the infographic below.

A network slice is virtual part of a network that offers full end-to-end connectivity for particular services and - optionally - for tenants. A tenant is a 3rd-party company that rents a virtual part of a public mobile operator's network. This allows the tenant to run its own private nation-wide mobile network without owning any hardware.

The network slicing is enabled by virtualization and all network functions can be divided into different slices as well. Thus, you can find in the figure the User Plane Function (UPF), gNB Central Unit for User Plane (gNB-CU UP) and gNB Distributed Unit (gNB-DU) all sliced.

It is also possible that a network function is dedicated for a particular network slice as in case of (gNB-) CU UP 2. 

In general - and this is the benefit of the cloudification - the NG RAN is a highly dynamic environment in which additional NW functions can be added (and later released) whenever this is necessary. Mostly this will be triggered by the load on CPU and memory resources. Here comes the automation into the games that deals in large parts with load balancing. Ideally automation enables a zero-touch network management. 

(click to enlarge)

However, the most precious of all RAN resources, the radio resources, cannot be administrated so flexibly and easily. Indeed, there are several automation instances that deal with radio resource management. Open RAN Alliance has defined the RAN Intelligent Controller (RIC) that is split into the Near-Realtime-RIC (RT RIC) that shall operate with a latency between 10 and 500 ms while the Non-Realtime RIC (NRT RIC) deals with non-time critical task, e.g. typical SON functions like Automatic Neighbor Reporting (ANR).

While the RIC can deal with a lot of problems there is one thing it cannot do: adding physical layer radio resources on demand. The physical resources are limited by the number of remote radio heads/antennas and as long as we have only static beamforming the physical resources covering a geographical sector are also limited by hardware and their distribution must be carefully planned. Thus, I think it is fair to say that the RIC (or a similar proprietary automation function) has to deal with the most complex situations in the RAN.

Radio resources can also be sliced in different ways. My figure illustrates a kind of slicing on the physical layer where different physical resource blocks (PRB) are allocated to different network slices. 

However, this is not the only way how the resources of a cell or a beam can be sliced. Beside a split of PRBs it is also possible to slice on the MAC layer where logical channels (slice-specific radio bearers) are mapped onto transport channels or on PDPC layer as it was described and demonstrated by the 5G NORMA project (Chapter 2.1, page 17 ff.).

What in the end will be implemented by the RAN equipment manufacturers is a question I that cannot answer today. 

Friday, 17 July 2020

A Look into 5G Virtual/Open RAN - Part 7: Change of gNB-CU-UP without Handover

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.

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