Hybrid Networks

The Precise Time Protocol (PTP) synchronizes the network by transferring the master clock information in the form of timestamps in the PTP messages (Sync/FollowUp/DelayReq/DelayResp). In the slave clock, the clock offset is computed through the reception of PTP messages that carry master clock as timestamps.

In practice, a network could employ multiple synchronization methods in the same network. SyncE transfers the frequency of the reference clock through the ethernet‘s physical layer. The frequency recovered from the Synchronous Ethernet is highly accurate when compared to the frequency recovered through PTP messages. However, SyncE does not carry the Time-of-Day (TOD), or the Phase information of the clock as PTP does. Networks that employ SyncE and PTP for synchronization can leverage the accuracy of time transfer through PTP by using SyncE. Such Hybrid networks use SyncE for frequency transfer, and PTP for Phase/Time-of-Day transfer.

PTP Time-source

Each node in the Hybrid network recovers and transfers primary reference frequency using SyncE and phase/ToD using PTP. The PTP implementation usually includes a local reference clock at each boundary and ordinary clocks. The PTP protocol synchronizes this local reference clock to the Grandmaster clock by correcting its offset. In Hybrid networks, since frequency of the Primary reference is already available through clock recovered from SyncE, this recovered clock is used as a local reference clock. The timestamps received through the PTP event messages are processed and the Phase/ToD information is recovered.

The PTP protocol does not specify the source of time (frequency and/or Phase/ToD), or the method of clock recovery. This is left to the implementation. For boundary and ordinary slave clocks, the source of time is through PTP messages. However, since PTP messages carry timestamps that can be used to recover both frequency and phase/ToD, the implementations separate this aspect of recovery, and perform only phase/ToD recovery on the received PTP messages. The frequency recovery from the received PTP messages is not performed. The PTP implementation uses the recovered clock from SyncE as a frequency time-source.