Hello everyone, I'll introduce you the working mode transition of the eNodeB clock.
The clock has four PLL(phase-locked loop) states: Free running, fast tracking, locked, and holdover. When the BTS is not configured with a reference source, the BTS is in the free running state. After the BTS is configured with a reference source, the BTS will enter the fast-tracking state from the free-running mode. The two states change quickly, generally, which will be changed immediately after the reference source is set.
In time synchronization mode, the time for the BTS enters the locked state from the fast-tracking state is shorter. Generally, the BTS can be locked within 5 minutes.
Free running: The clock algorithm CBB does not adjust the clock crystal oscillator. In free-running mode, the clock is provided by the internal crystal oscillator.
Fast tracking: The local clock changes with the frequency of the reference clock source. When the frequency adjustment amplitude of the reference clock source is larger, the local clock is quickly synchronized with the reference clock source.
Locked: Indicates that the local crystal oscillator is synchronized with the reference clock source. In time synchronization mode, both the frequency and phase of the local clock are traced to the reference clock source.
Holdover: When the reference clock source is abnormal and the PLL is locked, the PLL status will change to hold. At the moment, the local clock uses the average value of the historical DA value when the BTS is locked to control the output frequency of the crystal oscillator.
1. When there are no available reference clocks after the crystal oscillator is warmed up, the clock enters the free running mode.
2. When the eNodeB receives available clock signals or reference clocks are available, the clock transits from the free running mode to the fast tracking mode.
3. When the frequency offset of the reference clocks for the eNodeB is less than the locking threshold, the clock transits from the fast tracking mode to the locked mode.
4. The clock enters the holdover mode when one of the following cases occurs: l Reference clocks become unavailable. l The phase offset exceeds the locking threshold. l The frequency offset exceeds the locking threshold.
5. When the duration for the clock in holdover mode exceeds the specified threshold, the clock transits from the holdover mode to the free running mode.
6. When reference clocks become available and the phase offset is less than the locking threshold, the clock transits from the holdover mode to the locked mode.
7. When reference clocks become available but the phase offset exceeds the locking threshold, the clock transits from the holdover mode to the fast tracking mode
8. When the eNodeB detects that the phase offset reaches the frequency offset threshold, the clock transits from the locked mode to the fast tracking mode.
9. When the reference clocks of the eNodeB become unavailable, the clock transits from the fast tracking mode to the free running mode.
10. When the eNodeB detects that the frequency offset is less than the traceable threshold but reaches the locking threshold, the clock remains in fast tracking mode.
11. When the eNodeB detects that the frequency offset is less than the locking threshold, the clock remains in locked mode.
12. When reference clocks are not recovered within the specified time, the clock remains in holdover mode.
Hope you will enjoy this!
