Synchronization
18.3.3 Hold-over Operation
This function will be initiated when the reference source is considered
not usable. See next section.
The function freezes the control values of the internal oscillator at their
present values. This means that the frequency in the RBS is held
constant except for ageing and the temperature drift of the oscillator.
Supervision of the reference will continue. The RBS will still be
considered as synchronized. The function is terminated when the
reference source is considered as usable or after "Hold-over expired
time".
Faults are sent to the BSC after "Hold-over expired time" and
"Hold-over entered time".
18.3.4 Supervision of Reference Source
The function is initiated when synchronization to the reference source is
established.
The reference is supervised relative to the own short-term frequency
source by monitoring the phase deviations between the two sources.
The following criteria are used when evaluating deviations:
PCM interface as reference:
• LOS (Loss Of Signal), LOF (Loss Of Frame alignment) or AIS
(Alarm Indication Signal) detected by the physical interface
• Illegal high wander. The limit is specified in /G.823/
• Illegal high relative frequency deviation. The limit is 0.1 ppm
(parts per million)
Optional reference source:
• Failure of the optional reference source
• Illegal high wander. The limit is specified in /G.823/
• Illegal high relative frequency deviation. The limit is 0.1 ppm
The result of this continuous supervision of the reference can be:
• Usable as reference
• Not usable as reference
This result is used as a basis for entering hold-over mode.
The function terminates when "Hold-over expired time" ends in
hold-over mode.
18.3.5 Locking to the Reference Source
This function limits the time for locking to the reference. The control
values for the internal source must have stabilised to within a range not
including the highest and lowest values, with a margin.
The following criteria are used when evaluating the locking process:
Locking to the PCM interface:
• PCM sync presence (detected by physical interface)
EN/LZT 123 2697 R5A
1998-08-13
127 (306)
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— All Rights Reserved —
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