PTP
- Dec 04, 2017 -

PTP is other method for synchronsation. It is unlike SyncE did sychronisation over physical layer. It is packet based timing which relies on time stamp packets inserted in the data stream. Comparing with SyncE, it has several advantages:

  • Advantage 1: Notes between Master and Slave clocks can remain asynchronous;

  • Advantage 2: Avoid additional upgrade for existing equipment;

  • Advantage 3: Be able to transmit both frequency accuracy and phase information.


Basic PTP synchronisation steps:

  • Step 1: Time stamps are inserted at master node where a stratum 1 traceable reference is available;

  • Step 2: Then, extracted at the slave clock where the synchronised frequency is required.


PTP has defined three types of clock:

  1. Ordinary Clock (OC): has a single PTP port in a domain and maintains the timescale used in domian. It can be served as a source of time, treated as master clock, also can be synchronised to another clock, treated as slave clock.

  2. Boundard Clock (BC): has multiple PTP ports (single slave port) in a domain and maintains the timescale used in the domain. It can be served as a source of time, treated as master clock, also can be synchronised to another clock, treated as slave clock.

  3. Transparent Clock (TC): a device which measures the time taken for a PTP event message to transit the device and provides this information to clocks which are receiving this PTP event messages.


PTP has two synchronisation mechanism:

Delay request - response mechanism:

  • One step clock: sync egress time stamp is carried in Sync message, and Follow_Up is not sent;

  • Two step clock: sync egress time stamp is carried in Follow_Up message.

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Peer delay measurement mechanism:

  • One Step clock: sync egress time stamp is carried in Sync message, and Follow_Up is not sent; Pdelay_Resp message carries turnaround time t5-t4, and Pdelay_Resp_Follow_Up is not sent.

  • Two Step clock: sync egress time stamp is carried in Follow_Up message; turnaround time t5–t4 is carried in Pdelay_Resp_Follow_Up; or t5 is carried in Pdeay_Resp_Follow_Up, and t4 is carried in Pdelay_Resp.

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Two PTP hierarchical topology:

Peer to peer TC (P2P TC): providing PTP event transit time information, also provides corrections for propagation delay of the link connected to the port receiving the PTP event message. Its delay measurment uses Peer Delay Measurement Mechanism.

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End to end TC (E2E TC): only measures the time taken for a PTP event message to transit the device and provides this information to clocks receiving this PTP event message. Its delay measurment uses Delay Request-Response Measurement Mechanism.

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