My friend: Hello Ibrahim, I have a question for you.
- Me: Please, go ahead.
My friend: I am confused with the split options and which one of them is really used in 5G and Open RAN? As we are not using all of them, right?
- Me: So, let’s clarify it. 3GPP offered 8 split options where option 2 was adopted in Rel.15 for 5G, so we have gNB-CU and gNB-DU definitions in 3GPP where the gNB-CU has the RRC, SDAP and PDCP layers, while gNB-DU has the RLC, MAC and Physical layers. Here option 2 is considered as higher layer split (HLS).
My friend: So, what about Open RAN?
- Me: O-RAN Alliance adopted a lower layer split (LLS) option which is 7.2x in addition to what 3GPP offered, so we have here O-CU, O-DU and O-RU (here O refers to Open), where option 2 is used between the O-CU and the O-DU, while option 7.2x is used between the O-DU and the O-RU. In this case, the O-CU has the RRC, SDAP, and PDCP layers. While the O-DU has the RLC, MAC and higher functions of the physical layer such as encoding, scrambling, etc., then the O-RU has the lower functions of the physical layer such as IFFT, beamforming, etc. But it is not stopping here
My friend: Why? Do we use any other options?
- Me: Actually, the SCF (Small Cell Forum) adopted a LLS option which is option 6 to allow for small cells using the Open RAN solution in addition to what 3GPP offered, so we have S-CU, S-DU and S-RU (here S refers to Small), where option 2 is used between the S-CU and the S-DU, while option 6 is used between the S-DU and the S-RU. In this situation, the S-CU has the RRC, SDAP, and PDCP layers. While the S-DU has the RLC and MAC layers, then the S-RU has only the physical layer. The interface between the S-DU and the S-RU is called n-FAPI (network- functional application platform interface).
My friend: Thank you very much. You made it very clear.
- Me: You are most welcome.