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TimeHub 5500 User’s Guide
097-55501-01 Revision M – January 2009
Chapter 1 Description
Synchronization Status Messaging
1. Quality Level, which is either read directly from the SSM on the input signal, or is
assigned using the TL1 keyword QLEVEL. There is no preference given to an
input that is actually extracting SSM values (requires enabling SSMENB)
compared with one that is using an assumed SSM quality level (SSMENB
disabled).
2. SSM Priority level, which you assign to each input using the TL1 keyword
SSMPRI. For information on using these keywords and the syntax, see the
TL1
Reference Guide
.
3. The location of the reference signal on the Input panel; inputs on lower-numbered
inputs have higher priority when the other two factors have the same value.
If the CURRSSM quality value for an otherwise qualified input is numerically higher
than the quality level of the internal reference oscillator in the Clock card, that input
will be disqualified and hence cannot be selected as the active timing reference.
The rationale for this is that the internal reference oscillator will provide a better
timing reference than any input with a lower quality level. The quality levels for
TimeHub 5500 internal oscillators are 3 for any rubidium type clock (090-55513-01
and 090-55514-02) (Stratum 2 traceable) and 5 for any quartz type clock
(090-55511-01 and 090-55512-02) (Stratum 3E traceable). The QCUTOFF keyword
can be used to set an additional quality value that will disqualify any input whose
CURRSSM value is higher than the selected QCUTOFF value.
Output SSMs
SSMs are automatically encoded onto any DS1 output set for ESF framing. Normal
operation automatically encodes an appropriate SSM quality level on such outputs;
you can override the automatic setting and manually set the quality level to be
encoded on DS1 ESF outputs. Here are some details regarding how the SSM
quality level is set for ESF outputs:
During normal operation, where a qualified input is being used as the active timing
reference, the SSM quality level of that input is included on all DS1 ESF outputs.
This quality level can be read by retrieving the CURRSSM value for the currently
active timing reference. If the active reference changes to a different input, the SSM
output changes to that of the newly selected input. Note that the SSM value is
output on DS1 ESF outputs regardless of the selected synchronization mode (see
SYNCMDE discussion above). It isn't necessary for SYNCMDE to be set to “SSM”
for outputs to encode SSM information.
When the TimeHub 5500 is using an internal oscillator as the timing reference
source, the output SSM quality level is the quality level associated with the
performance level for that internal oscillator. For the rubidium clocks (090-55513-01
and 090-55514-02) (Stratum 2 traceable), the quality level is 3. For the quartz clocks
(090-55511-01 and 090-55512-02) (Stratum 3E traceable), the quality level is 5.