# *Open Frontahul Interface* Important function for Fronthaul - Transporting data between DU and RU - Splits functionality between O-RU and O-DU - Enables multivendr RU and DU interoperability - Increase transmission bandwidth through Split Option 7-2x - Downlink and uplink bits processing Architecture of O-RAN Fronthaul Interface ![](https://i.imgur.com/fh6icWK.png) From image above, we can see that O-RAN fronthaul interface used split option 7-2x. ### Split Option 7-2x Split Option 7-2x is a spesification for functional splittig between O-RAN Distributed Unit (O-DU) and O-RAN Radio Unit (O-RU) adopted by O-RAN fronthaul specifications. #### Split Option 7-2x in The Downlink 1. Implements function up to Resource Element (RE) mapping in O-DU. 2. Supports an O-RU that implements - digital beamforming (easy to implement): Category A - digital beamforming + precoding: Category B #### Split Option 7-2x in The Uplink 1. Implements resource element mapping and higher functions in the O-DU and digital BF and lower functions in th O-RU. #### Open Fronthaul Interface Stacks ![](https://i.imgur.com/744SxwO.png) From image above, open fronthaul interface consist C/U-Plane, S-Plane, and M-Plane. - **C/U-Plane**: transport using eCPRI or Radio Over Ethernet - **S-Plane** is a synchronization protocol that use between O-DU and 0-RU that required to achive linking control that assumes inter o-RU synchronization for Time Division Duplex (TDD), Carrier Aggregation (CA) using O-RUs, MIMO, and other processes. As an S-Plan, O-RAN fronthaul sepesifications support protocol such as Precision Time Protocol (PTP) or SyncE over Etherner. - **M-Plane** is used for configuration in network managementof O-RU. It will be related to O1 interface. M-Plane consist NETCONF protocol over SSH/TCP/IP. #### Fronthaul Network Topologies ![](https://i.imgur.com/5JBW36S.png) **Point to Point** It means each O-DU is connected to O-RU with individual fiber. **L2 Switch** In this picture consist three case. 1. Case 1 In case 1, between O-DU and first L2 Switch is connected with low speed fiber. Between the first L2 Switch and the second L2 Switch is connected with trunk using a fiber. From that case, bandwidth to be used is smaller than the bandwidth in case 3 2. Case 2 There are have two path, so if the first one is fails, the traffic can move to another path. 3. Case 3 Case 3 is a simplified case for point to point. O-DU connected to L2 Switch using a fiber. In this case, the bandwith to be used is higher, so making cost high.