GNSS
The SR Linux routers with network synchronization support can run frequency synchronization using a Layer 1 interface such as synchronous Ethernet. In cases where these methods are not possible, or where accuracy cannot be ensured for the service, you can deploy a GNSS receiver as a synchronous timing source. GNSS data is used to provide network-independent frequency and ToD synchronization.
The following table lists the support by platform for GPS and Galileo references using an integrated GNSS RF port.
| SR Linux platform | Integrated GNSS timing reference | |
|---|---|---|
| GPS | Galileo | |
| 7730 SXR | ✓ | ✓ |
An SR Linux chassis equipped with a GNSS receiver and an attached GNSS antenna can be configured to receive frequency traceable to Stratum-1 (PRC/PRS). The GNSS receiver provides a synchronization clock to the SSU in the router with the corresponding QL for SSM. This frequency can then be distributed to the rest of the router from the SSU as configured with the following commands:
- system sync freq-references instance priority
- system sync freq-clock ql-selection
The GNSS reference is qualified only if the GNSS receiver is in a position hold state and has a frequency successfully recovered.
If GNSS signal loss or jamming result in the unavailability of timing information, the GNSS receiver automatically prevents output of clock or synchronization data to the system, and the system can revert to alternate timing sources.
GNSS failure with APTS
When the G.8275.2 profile is used for GNSS-enabled 7220 IXR platforms, the APTS capability frequently measures and stores the delay offset between the GNSS time and a backup PTP session time. If a GNSS failure occurs, the backup PTP session automatically becomes the selected reference for time and frequency, and the stored delay offset value is added to or subtracted from the backup PTP session time to keep time and phase for the router as accurate as possible.
When GNSS has recovered and is stable, the system automatically switches back to GNSS for time and frequency reference, and backup PTP monitoring and delay measurement resumes.