The clock specifications (e.g., as per ITU-T G.8262 or G.8273.2) are defined at the interfaces of a network element. This means that, in general, the timing performance of a clock not only depends on the oscillator characteristics, but also on the clock’s specific configuration (e.g., PLL, line card, internal clock distribution, etc.) at the time of testing.
However, the timing performance in freerun and holdover clock modes is primarily dominated by the system oscillator characteristics; in . In this case the system may be tested against the clock requirements with less importance given to the specific system configuration at the time of testing.
The Table below summarizes the applicability of clock requirements to the oscillator characteristics for the main clock specifications.
Specification | Clause / Osc.Oscillator Characteristic | Sufficiently independent from system configuration | Specific to system configuration (PLL, Line card, etc.) | Notes |
SyncPHY clocks: *G.812 includes ANSI Stratum 3 (Type IV) Stratum 3E (Type III) Stratum 2 (Type II)
| Frequency Accuracy | X✔ | Sync PHY clocks | |
Pull-in , Hold-In, Pull-out |
| X✔ | Depends on PLL design | |
Noise GenGeneration |
| X✔ | ||
Noise Tolerance | X✔ | |||
Noise Transfer |
| X✔ | ||
Short-Term phase transient response | X✔ | |||
Long-Term phase transient response (Holdover) | X ✔ (see Notes) | X ✔ (see Notes) | Temperature characteristics depend on system design | |
Phase response to input signal interruptions | X✔ | |||
Phase discontinuity | X✔ | |||
Interfaces | X✔ | |||
Packet Clocks (Full timing Support): | SyncE clock | See Notes | See G.8262/G.8262.1 | |
Time Error Noise Gen | X✔ | |||
Noise Transfer | X✔ | |||
Noise Tolerance | X✔ | |||
Transient Response due to PTP and SyncE | See Notes | For further study | ||
Transient Response due to SyncE | X✔ | Strongly related to system design | ||
Transient/ PTP Holdover (loss of SyncE) |
| See Notes | For class C it practically assumes a G.8262.1 clock. | |
Holdover (loss of SyncE and PTP) | See Notes | For further study | ||
Holdover based on physical layer assistance (loss of PTP) | X✔ | |||
Interfaces | X✔ | |||
Packet Clocks (Partial timing Support): G. 8273.4 | Frequency Accuracy | X✔ | ||
SyncE clock (optional) | See Notes | See Notes | See G.8262/G.8262.1 | |
Time Error Noise Gen | X✔ | |||
Noise Transfer | X✔ | |||
Noise Tolerance | X✔ | |||
Transient Response (from GNSS to PTP) | X✔ | Strongly related to system design | ||
Holdover based on oscillator | X✔ | |||
Holdover based on physical layer assistance (loss of PTP) (T-BC-P) | X✔ | Same as per G.8273.2 | ||
Interfaces | X✔ |