Hi Taichi,
It depends. GNSS at the cell site has its own operational challenges, for example making sure that the antenna has a clear enough view of the sky. A challenge in Asia is that very little of the fiber is in the ground, hence multiple fiber cuts happen on a daily basis which changes the path length when restoration mechanisms kicks in. A change in the path length is not a problem for PTP, but it require that we know the path asymmetry on all possible fiber paths between the master and the slave (we need to use protection on layer 1 in Asia due to the frequent fiber cuts).
Another challenge with GNSS is that we experience that the GNSS is either jammed or even worse, that the GNSS is spoofed.
When we did the PTP design, we also believed that the length of the fiber path length would be equal in both direction. However, in some of our old Metro networks, the line amplifier have embedded Dispersion Compensating Fiber (DCF) to compensate for the chromatic dispersion of different wavelengths. The length of fiber within DCF modules to compensate for the same length of fiber may vary significantly. Other parts in the optical domain can also cause asymmetry, e.g.transponders, software FEC, or FEC in general, and digital signal processors in coherent optical systems. Asymmetry increases with link speed, so we could consider running PTP over 1GE interfaces, but this is a challenge in our Core networks.
We can overcome the DCF (DCF is cheap) issue by either measure the asymmetry of every fiber hop (not practical possible), change the DCP modules to Bragg filters (expensive), or deploy grand masters in the access/aggregation network in order to have less asymmetry impact from the fiber network.
When it comes to the instabilities with PTP implementation, we try to work with the vendors so that they fix failing line cards, port flapping etc. However, its not always easy to get the vendor’s attention on PTP issues.
OAM for PTP is another challenge, i.e. how can we make sure that the clock is healthy? We plan to solve this by deploying GNSS at certain locations in the network and use equipment that can compare the difference between GNSS-input and the received PTP clock.
So key message is that PTP does not work out of the box, it requires significant engineering effort. GNSS has many issues as well, and in certain parts of Europe we cannot rely on GNSS only. So it is not an either-or, - we need both.
best regards,
Geir