ORAN: Introduction of S-Plane (Synchronization Plane)

The synchronization plane (S-plane) in O-RAN is responsible for ensuring that the O-DU and O-RU are synchronized in time and frequency. This is essential for the proper operation of many O-RAN features, such as TDD, carrier aggregation, MIMO, and beamforming.

In cloud RAN deployments, the O-DU and O-RUs may be physically separated by a significant distance. This can make it difficult to achieve the high level of synchronization that is required for O-RAN to operate properly.

•S-Plane refers to traffic between the O-RU or O-DU to a synchronization controller which is generally an IEEE 1588 Grand Master.

•However, Grand Master functionality may be embedded in the O-DU.

•Frequency and time synchronization of O-DUs and O-RUs via Ethernet use SyncE and IEEE 1588-2008 PTP (L2 Ethernet).

•PTP over L3 Ethernet (G.8275.2) has unassured synchronization performance and is not mandatory for the FH.

•Security mechanisms will not be used for this plane – Optional in IEEE 1588 (PTP). However, not feasible at a reasonable cost

The S-plane addresses this challenge by using protocols such as PTP (Precision Time Protocol) and SyncE (Synchronous Ethernet) to synchronize the clocks of the O-DU and O-RUs.

• PTP is a protocol that is used to distribute a precise time reference from a grandmaster clock to slave clocks. The grandmaster clock is typically located at the O-DU side. The slave clocks are located at the O-RU side.
• SyncE is a protocol that is used to synchronize the clocks of multiple Ethernet devices. It is typically used to synchronize the clocks of the O-DU and O-RUs, as well as the clocks of other devices in the RAN, such as switches and routers.

The S-Plane supports a single protocol stack:

• PTP and SyncE over Ethernet: This protocol stack is based on the PTP and SyncE standards, which are used to synchronize the clocks of the O-DU and O-RUs.