![Symmetricom TimeHub 5500 User Manual Download Page 35](http://html.mh-extra.com/html/symmetricom/timehub-5500/timehub-5500_user-manual_1419592035.webp)
097-55501-01 Revision M – January 2009
TimeHub 5500 User’s Guide
35
Chapter 1 Description
Passthrough Mode
The TimeHub 5500 uses the quality level associated with the Clock card in the
following situations:
After warming up, the Clock card enters the Freerun mode (indicated by an LED
on the front panel) while input references are being qualified. During this time the
internal oscillator on the active Clock card is the active timing reference.
If for some reason all external timing references become disqualified (for
example, signal is lost), the TimeHub 5500 enters either Freerun or Holdover
mode (indicated by an LED on the front panel). In either mode, the active Clock
card becomes the active timing reference. If external timing references later
become re-qualified, the TimeHub 5500 returns to normal operation and the
output SSM quality level changes to match that of the selected input.
You can manually set the output SSM quality level on DS1 ESF outputs using the
OMANSSM (Output Manual SSM) keyword. This allows you to select a specific SSM
quality level to be output on all channels in an Output card. By enabling OMANSSM
(set to “Y”) for a particular output card, any DS1 ESF outputs associated with that
card outputs the quality level set via the OQLEVEL (Output Quality Level) keyword.
A primary reason for providing this capability is to allow information such as “DON'T
USE for synchronization” (quality level 9) to be sent to downstream equipment during
maintenance operations. Using a signal marked DUS for timing purposes is a
violation of the BITS concept and Telcordia general requirements.
Passthrough Mode
The TimeHub 5500 can provide a usable clock in a situation where both Clock cards
are removed from the shelf and a usable input signal is present. The capability is
called Passthrough and resides on the TimeHub 5500 Management card. The
Management card has a connection to the timing inputs, which it continually
monitors for signal presence. An LED on the Management card indicates when the
Passthrough mode is active. An automatic TL1 message is also provided on both
communication ports whenever Passthrough operation begins or ends.
Passthrough is available only when a Clock card has qualified at least one input
since power was applied to the Management card. The Management card monitors
the Clock cards for qualified inputs; if no qualified input has been detected, or if the
signal is not above a minimum level, Passthrough will not enable upon removal of
Clock cards. When the Management card has determined that Passthrough is ready,
an automatic message, PTAVAIL, is issued. You can remotely check if Passthrough
is available by using the TL1 keyword RTRV-COND. See the
TL1 Reference Guide
for more information.
When Passthrough mode is activated, the following hierarchy determines which
input is selected as the reference:
The first choice is the most recently selected input reference prior to removal of
the Clock card.