4.15.3 Time synchronization

Two types of synchronization processes are supported by the 5GS: 5GS synchronization and (g)PTP domain synchronization (see 3GPP TS 23.501 [8]).

For 5GS synchronization, the lower layers provide the 5G internal system clock signalled via the NG-RAN (see 3GPP TS 38.331 [30]) and the UE forwards the 5G internal system clock to the DS-TT(s).

For (g)PTP domain synchronization, the UE supports forwarding (g)PTP messages (see 3GPP TS 23.501 [8], 3GPP TS 23.502 [9], and 3GPP TS 24.535 [19A]). For all (g)PTP domains associated with a PDU session:

• if the UE receives (g)PTP message via the PDU session, the UE forwards the (g)PTP messages to the DS-TT associated with the PDU session; or
• if the UE receives (g)PTP messages from the DS-TT associated with the PDU session, the UE forwards the (g)PTP messages via the PDU session.

Depending on the 5G access stratum-based time distribution or (g)PTP-based time distribution, the network timing synchronization status of the nodes involved in the operation (e.g. NG-RAN nodes, NW-TTs) may change. Using the 5GMM protocol, the network can request a supporting UE to reconnect to the network upon receiving an indication of a change in the RAN timing synchronization status.