I have been struggling a whole day to understand the real differences of Precise Time Protocol (IEEE 1588) and Synchronous Ethernet (syncE) and their role in the network. Greatly appreciated if you help me.

So, my questions:

1- Does the PTP synchronize both time and frequency over the network? I've read somewhere that PTP is able to synchronize both time and frequency, but I can't understand how frequency get synchronized by packets?

2- In case synchronization is necessary, can one of these protocols synchronize network? for example, just using SyncE suffices for synchronization? Or both protocols are necessary? Maybe each protocol works in a specific and separate part of a network?

3- Is Grand Master clock (PTP term) the same as Primary Reference Clock ( SyncE term)? Are they one device in a network? Equivalently, can we have this mapping for all PTP devices and SyncE devices?

Thanks, Amir

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    I think you are confusing clocking with time. – Ron Maupin Nov 12 '18 at 16:04
  • You mean that clock adjustment isn't a PTP responsibility? – A.A Nov 12 '18 at 16:22
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    There is a difference between signal clocking and a real-time clock. The terms can be confusing. That happens with other things, too. For example, PCs originally didn't have a real-time clock, but CPUs use clocking to coordinate when to try to read memory, execute commands, etc., but that hs nothing to do with the time. Data communications use clocking to determine when to measure the voltage on the line to see if it is a one or zero, but that, also, has nothing to do with the time. PTP is about the time, not signal clocking. – Ron Maupin Nov 12 '18 at 16:27

When they say PTP synchronises "clocks", they mean what we usually call "time-of-day clocks" but which normally are something more like "time since epoch X", often beginning of 1970, so it also tells you absolute time. PTP is akin to NTP but more precise; which is akin to RFC 868 time protocol, which is akin to "tell me the time of day please". If your time sync is close enough, you can use it for all kinds of things. PTP does picoseconds; NTP does milliseconds; RFC 868 time protocol does about 1/10 sec; operator pressing return at the sound of the tone perhaps 1/4 sec.

When they say SyncE synchronises "clocks", they mean what we'd normally call a "bit clock", which often just a square wave which controls the moment of sampling the data lines. The most common one of these (to network engineers) is probably the X.21 interface, ubiquitous on older Cisco routers, optional on modern ones, where one of the signals ("Signal Timing") is the bit clock. SyncE is a variety of ethernet with additional bit clock.

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Let's understand this from real-world analogy:

just imagine a Parade .. scout cadets have to take every step in rhythm and perfect time.

Here this rhythm can go out of sync in two ways:
1. Rise and fall of the foot of all cadets are not exactly at the same time. 2. Cadet step distance is not correct .. taller man taking longer footstep and shorter one taking a shorter step.

Case 1 is where sync comes into the picture and lets nodes to join in a rhythm however for perfect counter(footstep) sync you need 1588 PTP.

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