Where To Split
I think One of the key idea about OpenRAN is to split the whole RAN protocol stack into bunch of smaller building blocks and let us to combined it like lego block as we like. I know some of you would be angry about this analogy.. especially if you are a development engineer in this area -:). First, splitting and combining the radio protocol stack is not easy as lego block assembly. Second, we are not allowed to combine those building blocks of radio stack as arbitrarily as Lego block. Just take my comment as an analogy..
Just sticking with my lego block analagy, the building blocks of radio protocol would be what we call 'layer' like PHY, MAC, RLC etc. The fundamental idea is to split the whole radio protocol stack into every layers with open interfaces to other radio below and above each layer as shown in the left side of the illustration shown below.
Ideally it would be the best to haven an open interface for each and every layer (i.e, every options in the picture on the left side) without any technical difficulties. However, in practice (at least as of now, 2022) the industry (especially OpenRAN industry) decided to split the radio stack into three major blocks as shown on the right side and distribute all the layers into these three standard blocks. This is the basic idea on 'Split' in Open RAN industry.
Now the question is how to distribute all the layers in the radio stack into three major blocks that are commonly used in modern cellular deployment (e.g, RRU <--> DC <--> CU). Would there be any single best way of distribution ? The simple answer is 'there is no best/single answer that fits all the use-cases'. So there would be a huge set of different combinations (distribution). Just a few most commonly used distributions can be illustrated as below.
One of the questions you may have at this point would be 'why only three split ?' or 'why not split at every layer as suggested in 3GPP ?'. There is no clear answer for this question but we can think of a few possible reasons for it as listed below.
When to use which split is up to each use case of the network deployment. You need to determine the split pattern based on some of important characteristics of each split which is well summarized as illustrated below.
NOTE : Some additional labels and comments are added to the original picture based on my email discussion with Amichai Markovitz who helped me to get better understandings on various solit options.
Source : Courtesy of ParallelWireless WhitePaper - 5G NR LOGICAL ARCHITECTURE AND ITS FUNCTIONAL SPLITS
Digging into further details of Case [C] and [D] of above illustration can be illustrated as follows. This illustration is based on eCPRI specification, O-RAN specification, 3GPP specification.