In typical telecom networks, synchronization works in a hierarchal manner where the core network is connected to a stratum-1 clock. The stratum-1 clock is then distributed along the network in a tree-like structure. However, with a GNSS receiver, clocking is changed to a flat architecture, where access networks can directly take clock from satellites in sky by using an on-board GPS chip.

To optimize the GNSS system, it requires all the systems to share a common time scale and coordinated system. If all the systems do not have a common time, the receiver sees a time offset and then the receiver will have to select only one constellation having common time scale. Then there will be a requirement to add more satellites to increase the coverage of the constellation itself.

This capability simplifies network synchronization planning, provides flexibility and resilience in resolving network synchronization issues in the hierarchical network.

These Cisco IOS XR routers now support on board GNSS receiver to recover time.

The following routers support the GNSS receiver:

• NCS-55A2-MOD-S

• NCS-55A2-MOD-HD-S

• NCS-55A2-MOD-HX-S

• NCS-55A2-MOD-SE-S

Starting from Cisco IOS XR Release 7.5.1, the following 57B1 variants of NCS 5700 fixed chassis support GNSS receiver:

• NCS-57B1-6D24-SYS

• NCS-57B1-5DSE-SYS

No license is required to enable the GNSS module. The GNSS LED on the front panel indicates the status of the module. The following table describes the different status of GNSS LED:

NCS-57B1-6D24-SYS and NCS-57B1-5DSE-SYS can also receive 1PPS, 10 MHz, and ToD signals from an external clocking and timing source. However, the timing signals from the GNSS module (when enabled) take precedence over those of the external source. For more information, see the Cisco NCS 5700 series: NCS-57B1 Fixed Chassis Data Sheet.

Note	NCS-55A2-MOD-SE-S can also receive 1PPS, 10 MHz, and ToD signals from an external clocking and timing source. However, the timing signals from the GNSS module (when enabled) take precedence over those of the external source.

Note	We do not recommend that you configure both the front panel (10M, 1PPS and ToD) input configuration and the GNSS input configuration.

By default, anti-jamming is enabled on the GNSS module.

The GNSS module has the following stages of acquiring and providing timing signals to the Cisco router:

• Self-survey mode - When the router is reset, the GNSS module comes up in self-survey mode. It tries to lock on to a minimum of four different satellites and computes approximately 2000 different positions of the satellites to obtain a 3-D location (Latitude, Longitude, and Height) of it current position. This operation takes about 35 to 40 minutes. During this stage also, the module is able to generate accurate timing signals and achieve a Normal or Phase-locked state.

• Over determined clock mode - The router switches to over determined (OD) mode when the self-survey mode is complete and the position information is stored in non-volatile memory on the router. In this mode, the module only processes the timing information based on satellite positions captured in self-survey mode.

The router saves the tracking data, which is retained even when the router is reloaded.

The GNSS module stays in the OD mode unless one of the following conditions occur:

• A position relocation of the antenna of more than 100 meters is detected. This detection causes an automatic restart of the self-survey mode.

• A manual restart of the self-survey mode or when the stored reference position is deleted.

• A worst-case recovery option after a jamming-detection condition that cannot be resolved with other methods.

You can configure the GNSS module to automatically track any satellite or configure it to explicitly use a specific constellation. However, the module uses configured satellites only in the OD mode.

Note	GLONASS and BeiDou satellites cannot be enabled simultaneously.

When the router is reloaded, it always comes up in the OD mode unless:

• The router is reloaded when the self-survey mode is in progress.

• The physical location of the router is changed to more than 100 m from its pre-reloaded condition.

When the system restarts GNSS self-survey by using the default gnss slot R0/R1 command in config mode, the 10MHz, 1PPS, and ToD signals are not changed and remain up.

To use GNSS, the antenna must see as much as possible from the sky. For proper timing, a minimum of four satellites must be locked. For more information, see the Cisco NCS 5500 Series Router Hardware Installation Guide or Cisco NCS 5700 Series Router Hardware Installation Guide.

• The GNSS module is not supported through SNMP; all configurations are performed through commands.

• The GNSS holdover performance is one microsecond in two hours of holdover after twelve hours of GNSS lock time.

• TDEV fails marginally on NCS-55A2-MOD-SE-S with GNSS input.